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QLIR1I.Adl. FILE COP2I
AP' 0 M2~75-6
02P4O3t)3 01c,
BEST AVAILABLE COPY
CONTRACT F61052-69-C-0026 30TH JUNE 1973.
FINAL SCIENTIFIC REPORT
DEGRADATION REACTIONS IN POLYMERS
70 FEBRUARY 01 - 73 JUNE 30
N. GRASSIE, R. JENKINS, J. CUNNINGhAM and I.G. MACFARLANE
DEPARTMENT OF CHEIISTRY, UNIVERSITY OF GLASGOW,
GLASGOW G12 8QQ, SCOTLAND.
THIS RESEARCH HAS BEEN SPONSORED IN PART BY THE AIR FORCE MATERIALS
LABORATORY, WRIGHT-PATTERSON AIR-FORCE BASE, OHIO 45433, THROUGH
THE EUROPEAN OFFICE OF AEROSPACE RESEARCH, OAR, UNITED STATES AIR
FORCE UNDER CONTRACT F61052-69-C-0026.
FOREVORD
The research work described in this report was carried
out during the period 70 February 01 - 73 June 30 under
Contract No. F61052-69-C-0026 sponsored by Dr. I.J. Goldfarl
and Dr. H. Rosenberg, Air Force Materials Laboratory, Wright-
Patterson Air Force Base, Ohio 45433*
Acknowledgement is made of the many useful discussions
with and assistance of members of the Macromolecular Chemistry
Research Group of the University of Glasgow and of the clerical
work of Miss Ann Matheson.
ABSTRACT
This report is concerned with three distinct aspects
of polymer degradation and stability with the general aim
of the production of materials of improved stability through
a more complete understanding of their degradation mechanism
and its relationship to chemical structure*
The photothermal degradation of copolymers of methyl
methacrylate and n-butyl acrylate covering the whole composi-
tion range has been studied at 165'C, The gaseous and liquid
products, which consist of hydrogen, carbon monoxide, methane,
methyl methacrylate, n-butyl acrylate, n-butanol and n-butyr-
aldehyde were analysed quantitatively using a gas chromato-
graph technique and the yields related to copolymer composition.
Zip lengths, calculated from molecular weight and volatilisation
data, decrease with increasing acrylate content of the copolymer
but are higher than those observed during thermal degradation.
Infra-red spectral changes in the residue are attributed to
lactone formation and associated with the formation of butanol.
The "cold-ring" or chain fragment fraction of the products
becomesincreasingly more abundant as the acrylate content of
the copolymer is increased and spectral differences from the
original polymer have been accounted for. All the products
and principal features of the reaction are explained in terms
of a radical process initiated by scission of pendant acrylate
units and propagated by a combination of d.epropagation, intra
and intermolecular transfer processes the relative importance
of which depends upon copolymer composition. Differences
from the thermalreaction and the corresponding reaction in
mi
copolymers of methyl methacrylate and methyl acrylate are
discussed.
It is shown that a number of aromatic compounds, both
carbocyclic and heterocyclic, may be copolymerized with
dimethoxy-p-xylene with stannic chloride as catalyst.
Purification of materials and preparations of copolymers are
described.
The preparations of a number of poly(dimethyl siloxanes)
are described. Thermal analysis using TGA and TVA have shown
that thermal decomposition starts at approximately 3600C but this
is depressed somewhat by reprecipitation. Neither heating to
130 0 C to destroy residual catalyst nor the molecular weight of
the starting polymer have any significant influence on the
threshold degradation temperature or the general characteristics
of the thermal degradation reaction. End-blocking lowers the
degradation temperature threshold although the subsequent
build up in degradation rate is slower. The principal products
of degradation are cyclic trimer and tetramer g.l.c. and infra-
red methods are being devised for their analysis.
iii
TABLE OF CONT-NITS
Page
FOREMTORD.. .. ... 0..... 0... ... .. ..... ....... ....... i
ABSTRACT ......... 0 0 0 00 00.0 0. 0.. 0 0 1LIST OF ILLUSTRATIONS ** 0 000000 ..0..... v
LIST OF TABLES ... 0..0..0........
I. INTRODUCTION. 00.000.00.
Introduction ...... oo00........ o... ... . . 0
Photothermal Degradation of Copolymers ofMethyl Methacrylate and n-Butyl Acrylate... 2
Synthesis of Friedel-Crafts Polymers....... 3
Thermal Degradation of Polysiloxanes ...... 4
II. PHOTOTHERMAL DEGRADATION OF COPOLYMERS OF
METHYL METHACRYLATE AND n-BUTYL ACRYLATE ..... 6
Experimental 0 °°°.°.. ...00 o...o ........ 0 o 6
20
Discussion 045
III. SYNTHESIS OF FRIEDEL-CRAFTS POLYMERS ........ o,53
Introduction..... . ..000000000000.0..... .. .. . 53
Experimental and Results.. 0 ............... . 56
IV. THERMAL DEGRADATION OF POLYSIXOLANES ....... o 63
Preparation of Polymerso.00000....0........ 63
Measurement of Molecular Weight .......... 67
Gas-Liquid Chromatography..0 .....o.*oo..... 67
Thermal Analysis of Polymers ............... 75
Infra-Red Spectra of Products ofDegradation ....................... • ........ 86
The "Sealed Tube" Degradation Technique0.. 0 97
Conclusions 099
REFERENCES ..0................... 0.00*000 00001(1. . ~ e ~ om o ~ oe •oo oo oe oooo-V°
LIST OF ILLUSTRATIONS*
Page
1. Photolysis cell and associated vacuumequipment. *o.,,ooo,...o ... e.o.°.... ........... * 10
2. Output of Hanovia Lp°°°........°..13
3. Absorption spectrum of fused silica (2mm) ...... 14
4. Inlet system of thermal conductivity g.l.c....o 17
5, Influence of film thickness on rate of volati-lisation of a 50 mole % n-butyl acrylatecopolymer photothermally degrading at 165 0 C .... 21
6. Infra-red spectrum of liquid condensiblesfrom a 16.3 mole % n-butyl acrylate copolymerphotothermally degraded at 1650C for 23 hours.. 22
7. G.L.C. trace of the liquid condensibles froma 50.0 mole % n-butyl acrylate copolymer photo-thermally degraded at 165 0 C (Column tempera-ture, 85°C)oo.°o. . .. ... .. ..oa* s . . . . ..... 24
8. Infra-red spectrum and GL.C. trace of thepermanent gas products from a 50 mole % n-butyl acrylate copolymer photothermallydegraded at 1650C for 72 hours. (G.L.C.column at room temperature)..*..,..a...*.... 27
9. Weight loss, cold ring and liquid condensiblesproduced during photothermal degradation ofcopolymers of methyl methacrylate and n-butylacrylate at 165 0C. Copolymer composition,mole % n-butyl acrylate, 0, 1.0; 0, 3.9;
16.3; w , 50; A, 82; A, 93.4; V, 1I0 .... 28
100 Mblecular weight changes in copolymers ofmethyl methacrylate and n-butyl acrylate photo-thermally degrading at 1651C. Copolymer com-position, mole % n-butyl acrylate, 0, 1.0;0, 3.9; o , 16.3; •, 50; A, 82; A, 93.4... 34
11. Chain scissions per chain unit in copolymersof methyl methacrylate and n-butyl acrylatephotothermallyegadn at l650 c~ C(oponlyecomposition, mole % n-butyl acrylate, 0, 1.0;r, 3.9; 0 , 16.3; i , 5 , A, 934... On
12. Relationship between chain scissions per chainunit and volatilisation in copolymers of methylmethacrylate and n-butyl acrylate phototherm-ally degrading at 1650C. Copolymer composi-tion, mole % n-butyl acrylate, 0, 1.0; 0, 3.9;J, 16.3; t, 50.
V
LIST OF ILLUSTRATIONS (cont'd)
Page
13. Relationship between scissions per polymermolecule and volatilisation in a 50 mole % n-butyl acrylate copolymer photothermallydegrading at 1650C. ... .... oo .. 0 . °°.. 39
14. Infra-red spectra of A, undegraded and B,photothermally degraded (64 hr) 93.4 mole %n-butyl acrylate copolyrner. .. o..o ...... ... o 42
15. Ultra-violet spectra of progressively photo-thermally degraded 50rmole % n-butyl acrylatecopolymer. 1, undegraded; 2, preheated to1651C- residual benzene removed; 3, irradia-ted for 40 hr; 4, 62 hr; 5, 80 hr ............ 43
16, Comparison of infra-red spectra of A, undegraded50 mole % n-butyl acrylate copolymer and B, thecold ring fraction after photothermal degrada-tion for 48 hro°,.oo.ooo.° ....... 44
17. A typical osmotic plot.oooooooo....o ..... 68
18. G°L.Ce on DC 560 on Gas-chrom Q. column showingtetramer and pentamer.ooo°o°...°.oo°....°..°°o. 69
19. G.L.C. on 1% SE 30 column showing trimer, tetra-mer and pentamer ... .0oocoo.°.o00 oo .. . .00 o.0.00 70
20. G.LoC. on 1% SE 30 column showing tetramer,pentamer and hexamer . 0.o..0 .*.. oo.° ....00 .... * 71
21, G.L.Co of siloxane dimer on DC 560 on Gas-chrom Q column ... 73
22. G.L.C. of trimer on DC 560 on Gas-chrom Qcolumn°°°00 0 0 .. 0000o0oo0o°oooooo.oo0° . ... oe00 74
23. T.G.A. of polymer 1. (reprecipitated)......... 76
24. T.G.Ao of polymer 2. (reprecipitated)....... .. 77
25. T.GA. of polymer 3. (reprecipitated and heatedto 13000).,o~ooooooo..,.ooo0000o.ooooooooo~oo° 78
26. T.G.A. of polymer 4o (reprecipitated)...o......°o 79
27. T.G.A. of polymer 4o (repr3cipitated and heatedto 1 80
28. T.G.A° of polymer 4. (reprecipitated)....0....0 81
vi
LIST OF ILLUSTRATIONS (cont'd)
Page
29. T.G.A. of polymer M .(reprecipitated)°......o.... 82
30. T.G.A. of polymer M (reprecipitated and heated
to 130 0 )..o...°... ........... o....o 83
31. T*G*A. of polymer 4 (end blocked). °.°.°°....... 84
32. T.V.A. of polymer M (as prepared)..°°.......°.. 87
330 T.V.A. of polymer M (reprccipitated)o..°°.....° 88
34. T°V.A. of polymer M (heated to 1500) ......... • 89
35. Infra-red spectrum of polymer M (as prepared).. 90
36,; Infra-red spectrum of trimer..6.oooo4 .... oo0oo 91
37. Infra-red spectrum of tetrameroo ...... ..... . 92
38. Infra-red spectrum of condensible products frompolymner M (heated to 1500C)o ................. °.o 93
39. Infra-red spectrum of chndensible products frompolymer M (reprecipitated) .............. ** ..... 94
40. Infra-red spectrum of condensible products frompolymer M (as prepared)oooooooooo....o....oooo e 95
41. Infra-red spectrum of chain fragments ("coldring") from polymer M (as prepared)........... 96
420 The "sealed tube'l ........................ .. . 98
vii
LIST OF TABLES*
Table Page
1. P6lymer Composition and MolecularWeight data ...... o ..... . * .... • .... . 8
2 G .L .C .Dat a.° 00e 18
3. Assignment of Infra-red Absorptions ofLiquid Condensibleso0.o0o8o8 o°0oo°.0oo8oo00 25
4. GC-MS Identification of Product Peaks inFigure o 29
5o Ouantitative Analysis of Liquid Products
(Millimoles/gms of initial polymers) ....... 31 and 31a
6. GoLoC. Peak Ratios for Permanent Gases.'*.. 32
7. Molecular Weight/Volatilisation Data forMethyl Methacrylate/n-Butyl AcrylateCopol37mers * * * .. .... .. . oo. 0 ... .o °o00 000 .... °° 35 and 35a
8. Zip Lengths for Photothermal .Degradation... 41
9. Molar Ratios of Monomeric Products.o......, 51
10. Preparations of Poly(dimethyl siloxanes)... 66
11. Details of T.G.A. Thermograms - Figures23 31 ooo oooo~oo~ooooooooeo°° cooo@ o°°o eo e , 9 8
viii
I
CHAPTER I
INTRODUCTION.
A. Introduction
A great deal of effort has been expended around the world
in recent years in the study of the reactions which occur in
polymers when they are subjected to a variety of degradative
agencies. Although individual polymers in specific applica-
tions may require to be resistant to, for example, ozonisation,
hydrolysis, high energy radiation, mechanical stress, etc., it
is almost certain that in most applications of nearly every
polymer it will be important to have the maximum amount of
information about their stability to heat and visible or near
ultraviolet light.
This report describes work carried out on three different
aspects of polymer chemistry which have the common theme of
polymer degradation mechanism and stability. Thus Chapter 2
deals with a comprehensive study of the photothermal degradation
of copolymers of methyl methacrylate and n-butyl acrylate cover-
ing the entire composition range. This is followed, in
Chapter 3, by a description of some preliminary investigations
into the synthesis of polymers by the reaction of dimethoxy-p-
xylene (DIPX) with various aromatic compounds using stannic
chloride as catalyst. This was part of a continuing programme
of research which is being carried out in this Department and
which is concerned with the synthesis of materials of improved
thermal stability° It was superceded, as far as the present
contract is concerned and as a result of discussions with the
Materials Department, by a programme of study of the degrada-
tion reactions of simple polysiloxanes of which progress to
date is described in Chapter 4 and which is currently being
pursued.
B. Photothermal Dearadation of Copolp ers of Methyl Meth-acrylate and n-BuL .crylate.
A series of investigations into the thermal degradation
of acrylate and methacrylate homopolymers and copolyrners has
recently culminated in a description of the thermal degradation
reactions which occur in copolymers of methyl methacrylate (1A4A)
and n-butyl acrylate (n-BuA) covering the whole composition
1range. Currently, in these laboratories, photodegradation
studies of some ,of these systems are being pursued. 2 In order
to have relevance to an understanding of the deteriorative
processes which occur in polymeric materials in their commercial
applications these photodegradation processes are being studied
at ambient temperatures in polymer films and solutions.
Although the overall effects of heat on the one hand and
o1 light at. ambient temperatures on the other are often super-
ficially different it is clear that there are many close simi-
larities in the basic mechanisms, especially in the sense that
the same radicals are usually involved. The differences in
overall characteristics thus follow from the different secondary
reactions in which these primary radicals participate in the
melt and in solid filmso In order to assist in a clarification
of the relationships which exist between the purely thermal and
purely photo reactions it is interesting to study the effects of
-2 --
ultra-violet radiation on polymers at elevated temperatures
such that they are molten, as are thermally degrading materials,
but below their threshold for thermal degradation. To empha-
sise the links with purely thermal and purely photo processes
we describe these as photothermal degradations.
Studies of this kind have previously been made on poly
(methyl methacrylate) 3 and copolymers of methyl methacrylate
with acrylonitrile4 and methyl acrylate. 5 The work described
in Chapter 2 is a sequel to this as well as to the thermal
degradation studies on the same system which have recently been
published.1
C. S nthesis of Friedel-Crafts__Poly)erso
It has been shown in this laboratory that polymers may
be synthesised by the condensation of di(chloromethyl)benzene
(DCMB) with a wide variety of aromatic and heterocyclic compounds
in presence of Friedel-Crafts catalysts such as SnCl4.6
Although these materials are structurally comparable with the
phenolic resins some of them prove to have superior electrical
properties and thermal stability* In the past, work in this
laboratory has been specifically directed towards the investiga-
tion of relationships between structure and stability in these
materials and detailed investigations have been carried out in
the DCNB/benzene 7 and DCMB/thiophen8 systems in order to discover
in structural terms why the latter is at least 1000 more stable
than the former.
Polymers may be similarly prepared by condensation of
aromatic compounds with the corresponding ether, dimethoxy-p-
xylene (DMPX)
CH -0 -C2-,U 2 -0-C_ 3
3 2 -3-L
and these materials are beginning to be applied commercially
for electrical applications, being preferred because methanol
is the volatile by-product rather than hydrogen chloride from
DCMB.
It was intended to synthesise a number of these polymers
based on DMPX with the ultimate aim of defining the structural
factors necessary for optimum stability and the preparative,
means of achieving them. It was only possible to carry out
some very preliminary experiments along these lines, however,
before the contract was redirected towards silicone polymers.
It is these preliminary experiments which are described in
Chapter 3.
D. Thermal Degradation of Polyiloxanes.
The immediate objective of this work is to characterise
the degradation and stability properties of the relatively
simple siloxane polymers. When this is achieved emphasis
will move towards some of the more sophisticated silicon con-'
taining materials which are currently of interest to the
Materials Laboratory.
Since polymer structure impurities and additives should
be expected to influence degradation and stability properties
it was considered. desirable to prepare silicone polymers under
carefully controlled conditions rather than to depend upon
com..•al materials. Thus work to date has been concerned
with the preparation, handling and characterisation of suitable
samples of poly(dimethyl siloxane) and this is being followed
by a fundamental study of their thermal degradation using the
wide variety of appropriate analytical techniques available to
use
-4-
I
Degradation studies, described in Chapter 4, have been
principally concerned with thermal analysis using thermal
gravimetric analysis (TGA) and thermal volatilisation analysis
(TVA), separation and analysis of volatile products using gas
liquid chromatography and infra-red spectroscopy and the
influence of polymer preparation and structure on these proporties.
-5
CHAJPTEl? II
PILOTOTHERMAL DEGRADATION OF COPOLYDIERS OF
METHYL METIACRYLATE AND n-BUTYL ACHYLATE
A. Ee rimental
1. rjparaion of Copoalners. Methyl-methacrylate
(BoDo11. Limited) and n-butyl acrylate (Koch Light Laboratories)
were washed three times with 5M sodium hydroxide to remove the
inhibitor, and three times with distilled water to remove traces
of alkali. The purified monomer was dried for 24 hours- over
calcium chloride, followed by 24 hours over freshly dried
calcium hydride. Finally, the monomer was filtered into a
reservoir containing some calcium hydride and stored, until
required, in a refrigerator at -18'C. Before use it was
degassed twice in a reservoir, attached to a vacuum system, by
the usual freezing and thawing method, The first 5 per cent
was distilled off and discarded,, after which the required quan-
tity was distilled into a graduated reservoir, and finally into
the dilatometer.
The initiator, 2,.2' Azobisisobutyronitrile (Kodak Limited),
was purified by recrystallisation from methanol (m.pto 1041C.).
The initiator was introduced into the dilatometer as a freshly
prepared solution in Analar toluene, the solvent being removed
on the vaciumr line0 The dilatomoie-t- r- was then, pumped for several
hours under a high vacuum, light being excluded to prevent the
decomposition of the initiator.
Pyrex glass dilatometers of approximately 100 ml. capacity
with a 5 ml. graduated stem were washed with cleaning solution,
distilled water, and finally Analar acetone before being
attached to the vacuum line. The dilatometers were then
flamed intermittently for several hours, prior to the addition
of the initiator solution.
After degassing and vacuum distillation, pure methyl
methacrylate and monomer mixtures of known composition were
polymerized in bulk in dilatometers at 601C + 0.50u. Poly
n-butyl acrylate homopolyirer was polymerized in solution in
n-butyl acetate (BoDoHo Ltd.) at 400C. The initiator concen-
trations are given in table 1. Polymerization was carried to
"8 per cent where possible, but with copolymers of high acrylate
content, polymerization could only be carried to 3 or 4 per cent
because of the high viscosity of the medium. For the calcula-
tion of the copolymer composition the reactivity ratios used
were as follows. 9
rI (MoM.A.) = 1.80
r2 (nBu.A.) = 0.37
After precipitation of the.homopolymers and copolymers by
addition to methanol, they were dissolved in Analar toluene and
precipitated by running the solution slowly into 5 litres of
Analar methanol, with constant stirring* This procedure was
repeated three times. Those with a high methacrylate content
precipitated as a fine powder, while acrylate rich samples were
oroduced as rubberv solids. All the polymors were -f .... P__
from solution in benzeneo Initiator concentrations, copolymer
compositions and molecular weights are given in table 1.
-7-
Table 1
Copolymer Composition and Molecular Weight Data
Copolymer Composition Initiator Conc. Molecular(mole n_-BuA.) (% W r) Weight
1.0 0.17 260,000
3.9 0o17 347,000
16.3 0.042 1,300,000
50 0.084 230,000
82,2 0.084 506,000
93.4 0.002 1,400,000
100 0.l1 2,800,000
=8
2. Photothermal DepzraJation -. Photothermal
degradation must be carried out in a s:ilica vessel which is
transparent to the ultraviolet light. The disposition of
the photolysis cell is sho-un in figure 1, The two halves
of the cell are joined by a ground glass joint.
The photolysis cell was heated to the required tempera-
ture (16500) by means of a Woods metal bath controlled by a
proportional temperature controller and an isolating trans.-
former. When the reaction vessel was immersed to a depth
of half an inch, a temperature control of + 0.50C. was attained
inside the vessel for long operating periods. The inside
temperature was calibrated using a copper-constantan thermo-
couple to record the temperature of the I-Toods metal bath, and
a chromel-alumel thermocouple to record the temperature of the
silica disc inside the photolysis cell,
The invariable experimental procedure was to immerse
the photolysis cell to a depth of half an inch in the Woods
metal bath for a period of 20 minutes, after which time the
required temperature of 1651C. was attained. The ultra-
violet lamp and voltage regulator were switched on 5 minutes
prior to use, and the radiation was excluded., using a shield,
from the sample until the temperature was attained.
3. olgymer Form. In photodegrada-
tion studies it is important to use as thin films of polymer
as posbebu.t to PXPOSe a m:1inarea so tas -mitA
material as possible may be available for chemical analysis.
In choosing a suitable film thic'ness, three possible sources
of error must be considered,
-- 9--
L
>00
Ccr
m> 0
Co
-> 0
Prn
C: C*~ C 70. -MCfm <=0
A>m~j
_ IIC
C) i 03 )
ii> _
-,nn
cn >f] mo
-U 40
0 _
0>l-
ao Rates of diffusion of volatile products from the de-raa-
iing polymerr
b . Possible temperature gradients in the polymer°
Co 0 :possible skin effects such that the ratdiation. completely
absorbed in the surface layers so that the reaction is not hcmo-
genous throughout the polymer.
The use of powdered polymer would, be unsatisfactory on.
account of the large number of uncontrollable varixiables associ,,a-
ted with such a system, for example, surfiace arrea of the povder,
particle size, layer thickness and so onr
Several standard methods are available for preparing
poly•mer, films, for example, mle.ting, moulding, pressing, e0c..
but each suffers from the disadvantage that the polymer may
become partially degraded, or oxidised during the formation of
the film.
Alternatively, the homopolymers end copolymers were
dissolved in Analar benzene and an aliquot (calculated to
produce a film thickness of 1001) was allowed to evaporate on
an optically flat silica disc. The disc and film were then
placed in a vacuum oven at 401C. for 16 hours to remove residual
solvent. The silica disc which supported the film was then
placed inside the photolysis reaction cell. The average
weight of the polymer films was 0.085 - 0.100 gins.
A 'rt I •A J OAl
?Chrola, tolitel low pressure mercury resonance lamp was used
as shown in figure 1. This produces a typical mercury
emission spectrum, the intensities of the two resonance lines0 0t .-o A and A being very much g.. eater tan those of all
- -d 3 -
the other wavelengths present. The output of this lamp is
shown in figure 2. The mercury discharge tube was placed
directly above the cell, at a distance of 18 cm. from the
polymer film. The lamp was connected to a LTH Transistorised
lkVA Voltage Regulator which ensured that any variations in
the mains output did not affect the lamp emission.
The output of the lamp transversed 3 cm. of air, 2 mm. of
fused silica and 15 cm. of vacuum inside the photolysis cell
before finally impinging on the films. The nature of the
radiation eventually -reaching the film will thus be modified
to 'some extent by the absorbing characteristics of silita and
air media and it was therefore important to examine these
characteristics.
The transmission of fused silica is shown in figure 3.
0This shows that for light of wavelength 2537 A, the percentage
transmission is 86 per cent.
The only component in air which is capable of absorbing
the ultraviolet radiation produced is oxygen, the other con-
stituents being completely transparent in this region of
spectrum. The oxygen absorption spectrum consists of two0
sets of bands, the one system converging at 2400 A, and the
more important Schumann-Runge system having a threshold wave-0
length at 2000 A and converging towards 1761 A.
Figure 2 shows that 93 per cent of the Outrplt of the
lamp is composed of mercury resonance radiation with wave-0 0
lengths of 1849 A and 2537 A. The intensity of the former
is reduced to an extremely low value in traversing the arc to
polymer path. Since the intensities of the longer wavelengths
QUANTUM OUTPUT (Olo)
Eli
-{d(4n0K
cn
09-1/0
CT)T
133
ABSORPT'JI0N SPE~CTRUM OF FUSED STLICA (2mm.)
-80
-70 z0~
50
-50
14.00 l6oo 1800 2000 2200 2o00
14
are negligible, the radiation reaching the polymer is virtually0 0
monochromatic (2537 A.). It is obvious then that the 1849 A
line has an energy associated with it which exceeds the threhold
energy for the photo°-dissociation of oxygen. This line is
thus absorbed by the air, and only 1 cm. of air is sufficient
to absorb it completely. 1 1
5. Measurement of Molecular Weight Number average
molecular weights were measured by means of a Hewlett-Packard
High Speed Membrane Osmometer, Model 501, with cellophane
membranes*
6. Product Anal].si The terms used
to describe the four kinds of products are self-explanatory,
namely, permanent gases, liquid condensibles, 'cold ring'
(short chain fragments) and residue0 For the analysis of
the degradation products the silica degradation cell was
attached to two small traps immersed in liquid nitrogen. With
the traps at liquid nitrogen temperature the permanent gases
were removed for analysis by means of a Topler pump. The
liquid products retained in the traps were either distilled into
an infra-red cell or into a weighed capillary and. stored at
-18 0 C. until required, The 'cold ring' formed on the sides
and window of the photolysis cell 0 It was dissolved in chloro-
form and after evaporation of the solvent on a salt plate, an
i.r. spectrum was obtained. Residual polymer on the silica
disc was dissolved in toluene for molecular weight measurement 0
7. S ectroscopic Measurements. Infra-red
spectra were obtained using a Perkin Elmer 257 Grating Spectro-
photometer. The soluble residues were run as films deposited
on Naci plates from chloroform solution, the insoluble residues
-15 -
by means of the standard KC1 disc technique.
For ultra-violet" spectral meassuremenrts ea Unlicar
S.P. 800 instrument was used with the polymer supported on
the silica disc on which degradation., was carried out.
.8. Ga-iudChromatoýru~ G.L.C. data
for liquid products were obtained using a Microtek G.C. 2000R
Research Gas Chromatograph equipped. with dual columns, flame
ionization detector, with isothermal and linear temperature pro-
gramming. A modified Gallenkamp chromatograph, with a thermal
conductivity detector, was used for the analysis of the per-
manent gases. A list of the columns employed is given in
table 2. The 5% D.No.P, 5% B34. column proved to be most
effective in separating the liquid products.
ao Permanent gas sampling apparatus. The gases were first
introduced into the sample bulb using a Topler pump i.ith the
traps at liquid nitrogen temperature. The sampling system
shown in figure 4 was constructed so that air could be excluded.
The sample bulb was attached to the socket with taps T1 and
T 2 open and taps T4 and T5 set as shown. The argon carrier gas
thus by-passes the volume V1 . This volume was filled with gas
by closing tapT1 and Opening tapsT 2 and T By shutting T2
and reversing the positions of T4 and T5 the sample was carried
to the detector.
b. Quantitative gas-liquid chromatography. An internal
standard (cyclohexane) was chosen which did not coincide with
the retention time of any constituent. A known weight of this
standard was added to a weighed quantity of liquid degradation
product, and a g.l.co trace was obtained. Pure samples of
_ 16-
INLET SYSTENI OF TflERI'4AL CONDUCTIVITY, G.L.C.
sc-ample bulb
to Vacuum(pumnp. ___ ___
carrier gas.
-17 -
Table 2
GoL.Coo Data
COLUIJTN PR0GRAMMiE2 USE DETECT0R
10 f-t I inch Isothermal at Separation of Filame
diameter, 5%DoNoP.. 600C, then all the liquid ionization
5% B.34. on 100- Isothermal at products.
120 mesh embacel. 800C.
10 ft. ¼ inch Isothermal at Liquid vola- Flame
diameter. I%SoEo30. 400oC. then tiles ionization
on 100-120 mesh programme at
embacel. 10°C/mino to
(SoEo 30 is a 2500C.
10 ft. ¼- inch Isothermal at Liquid vola- Flame
diameter 10OD.N.P. 800C. tiles ionization
on 1001120 mesh
embacel.
20 ft. ¼ inch Isothermal at Permanent Thermal
diameter 30-60 1 room tempera- gases iconductivity
mesh silica gel. !ture
each product and the standard were mixed in knowin amounts
and run on the chromat-ograph indicating the sensitivity of
each product relative to the internal standard. Since the
peak area of a product is proportional to its weight. the
peak areas of each component were measured by planirmetry.
If the sensitivity factor, k, for any product Y is defined as
the ratio of the peak areas of product to standard, when
equal weights of both are considered, then the percentage Y
by weight is given by
peak area of Y10
peak area of standard o of s
Co Combined gas chromatography-mass spectrometry. Until
recently, the standard method of analysing complex organic
mixtures has been to separate the components by chromatography,
to collect each product and analyse it by the normal. techniques
of I °R., U oVP, and N °MoR o to supplement the retention time data
from the gas chromatograph. The small samples obtained from
degradation experiments very often made it necessary for products
from several runs to be combined to give enough of a single
component for analysis. The combination of gas chromatography
with mass spectrometry has eliminated much of the tedious
separation and collection of components. The instrument used
was an L°K.B1. 9000 (LoK.B. - Produkter, Stockholm)'.
•t~ rc -.'i a , su~r momnt oTwo p r es .su r e
measuring devices, a constant volume manometer and a McLeod
gauge for use in different pressure ranges were attached to the
vacuum line. Since the actual pressure of gases found after
degradation were negligible no detailed pressure measurements
were undertaken.
- i9 -
B i Results.
Choice of Reac;ion Temnera.ture. A temperature
of 165 0 C was arbitrarily chosen since it was believed that
this would be sufficiently above the glass transition and
melting points of all, the copolymers such that the medium in
which reaction was occurring would be reasonably comparable
with that of thermal degrardationo It was also to be hoped
that there would be free enough diffusion of volatile products
such that the reaction would be homogeneous throughout the
mass of the polymer. At the same time, by reference to
thermal studi.esI 1651C. is clearly well below the thermal
degradation threshold.
In order to test for diffusion effects, samples of
various thicknesses (weight/unit area) were degraded under
standard conditions. A typical result is illustrated in
figure 5, which demonstrates that the rate of volatilisation
is independent of sample thickness. Similar results were
obtained throughout the copolymer composition range.
2. Identification of Products.
a. Liquid condensibleso A typical infra-red spectrum
of products condensible in. liquid nitrogen is illustrated in
figure 6. Assignments of the principal absorptions are
given in table 3. Although the polymer films were preheated
in a vacuunrn ov.ren at 4 01C , prior to irradia-t-in- +.hev clear~ly
still contaidn residual -benzene (solvei-duX• Ih s
suggest that the major friction of the volatile products is
one or other or both of the monomers.
Pi o
INFLUENCE OF FILM THICKNESS ON RATE OF VOLATILISATION OF
A 50 MOLE, % n-BIUTY.L A.Cl.YT,ArE COPOLYMER PIHOTOTHERMALLY
DEGRADING AT 1650 C.
.II
0
2 810 12
-21
I%IO
Tran rnitai-ice ppr ce-OH
Ila C1f)O0
1-tD-
Z-d
((D
- HoI
- - ---------- 0
ONr4
;zW
Ch
________________0 td
22-
The absorption at 2138 cm 1 is interesting.
-C-C- stretch would give rise to absorption in the region
2140-2100 cm . If the structure were H-CC--, a peak at
3,300 cm 1 due to C-11 stretching would appear. Since this
is absent, then if absorption at 2138 cm is due to alkyne,
it can only arise from R-.CC-fl A reference spectrum of
CHI3-C=-C-CH3 showed absorption at 2135 cm-I but this was not
the strongest absorption so it is unlikely that the observed
absorption is due to an alkyne. A reference spectrum of
'ketene (H2 C=C=0) is included in figure 6. This has similar
absorption and although the shape is rather different it-i
seems likely that the absorption at 2138 cm is due to a
substituted ketene (RCII:C=0).
Carbon dioxide and but-l-ene w'ere prominent products
in the thermal degradation of copolYMers of methyl methacrylate
and n-butyl acrylate. Neither of these was detected in the
photothermal reaction throughout the entire copolymer composi-
tion range.
Figure 7 illustrates a typical GoL.C. trace of the
liquid degradation products. By the application of combined
gas chromatography and mass spectrometry (G.C - M.S) and the
use of reference spectra these were identified as in table 4.
Components 1 49 5, 8 and 9 are obviously genuine degradation
products. The benzene is residual solvent- xylene an impurity
dri i C' ...... ....
ducts of photolysis o:; the tap grease. No peak corresponding
to a substituted ketene was observed.
-23-
t- -j
zo
viJ0
0
0
J _i
h-3
Table 3
Assi.gnFenE of I L:n r- red A]Isor-ruions o a Li id Condensi.bles
Fr ci ueny ( cmI) As smInm ent
700 - 650 benzene C-H
1750 \C 0 stretch
164-44 )C 0 stretch
3000 - 2900 C-H stretlch in -CU3 or Z CHI
2855 C-H stretch in -0-CHl 3
3150 - 3000 -- C-H
2138 ke tne
b. iPermanent gases. A typical. infra-red spectrum and-L anent gasnrdut
GLC .,ace of the small proportion of permanen, gas products
are illustrated in figure 8. These were identified as hydro-
gen, carbon monoxicde and. methaneo
3• Ana]nvsis of Degr dat ion Products. The
degradation' of the copolymers as represented by- the weitght;
loss and the formation of products vol.astile and. involatile
(cold ring) at room temperature is illustrated in figure 90
As the acrylate content of the copoly-er is increased the
total rate of decomposition and of production of volatile
products decreases ,hile the rate of production of 'cold ring'
fraction increases with acrylate content up to .6.3 mole%
thereafter decreasing . However the ratio of cold ring to
volatiles increases continuously with acrylate content.
Quantitative analysis of the liquid degradation products
-was carried out using the G+LoC. technique and results are
recorded -in table 5. All the residual solvent, benzene, was
always liberated during the 15 minute period required for the
polyimer films to attain the operating temperature of 1651C.
The total weight of residual benzene, calculated from GoLoC.
data was subtracted from the initial polymer weight before
calculating the data in table 5. Methyl methacrylate is the
predominating product followed in order by butyraldehyde, butyl
a....y.at e, u-ta ld a- u i.yl .et.c•±yiu±±-•to The O.-der of
accuracy for the trace products is obviously low but the results
do show interesting general trends.
The total -pressure of the permanent gases was very small
(approximately 1 torr as measured on a Nacleod gauge) so that
-26,-
01Z
0
L- 0
0,0
-d0
CCD
tjzF-3
27z
F o•_9°
WEIGHT LOSS, COLD RING AND LIQUID CONDENWSIBLES PPRODUCED
DURING PHOTOTHERI4AL DEGRADATION OF COPOLXIMERS OF METIHYL
METHACRYLATE AND in-BUTYL ACRYLATE AT 1651C. COPOLYMER
COMPOSITION, MOLE % n-BITTYL ACRYLATE, 0, 1.0; 09 3.9;
j, 163; ,3 50; A, 82; A, 93.4.; 7, 100.
-2 o
,,,,-'• u • - - -
(0
S-60o-4-.0 - - ----- ----- - -
"" ---0 o
28 -
0
Table 4
G.C4-.S. dnifcto of the Product PeaksinEiyire 3.
Peak No. Product
I n-butyraldehyde
2 benzene
3 siloxane trimer
4 methyl methacrylate
5 n-butanol
6 siloxane tetramer
7 xylene
8 n-butyl acrylate
9 n-butyl inethacrylate
-29-
quantitative measurements were not possible. However the
GoLOCo peak ratios for two copolymers, recorded in table 6,
represent an approximate measure of their relative
importance.
These results demonstrate that, as expected, the amount
of methyl methacrylate produced increases with the methacrylate
content of the copolymer while the proportion of butyralde-
hyd.e decreases0 However butyraldehyde is produced even from
copolymer containing only 3.9 mole% n-butyl acrylate in which
the percentage of acrylate-acrylate linkages, calculated from
sequence distribution data, is only 0oi.1 Thus butyralde-
hyde production must be associated with single isolated
n-butyl acrylate units. While butyl alcohol production. also
increases with acrylate content it is not detected in the
products until the acrylate content of the copolymer is as
high as 50 mole % that is until a significant proportion (22%l)
of acrylate units are present in adjacent sequences. More
n-butyl acrylate is formed from 16.3 and 50 mole% copolymer
than from 82.2 mole % copolymer. A similar result was observed
in the thermal reaction. Acrylate monomer must be produced
in the depolymerization reaction initiated and propagated
principally through the methacrylate segments of the polymer
chain. Apparently a certain proportion of isolated acrylate
units can be liberated in this way but adjacent sequences
prefer to take part in transfer reactions leading to the forma-
tion of large chain fragments at the expense of both monomers.
As in thermal degradation, the production of n-butyl meth-
acrylate is areatest in 50 mole % copolymer. The limited
-30-
C+-
H~ 0O0 a PCD H
C+ CT
H CIL
ý-j~~~~~9-.- w N4N --. NIjH0 (
ýln-. t..I l\s ~
I.-
00 N)Y \0J 0 '\0-J4-1 C\ 'u.f N 0N2Hj.
0 0 .'a 0 H Q H H 0- -o 0 0 10 0- 0 0 0 a- t- 0~ l 0 0 00
w w W ) ý- - 0 P
0__ _ __ _ _ _ ____% F-9-O L~c77 C7"1ciH-
00000 Z0a) 0 a a) a) -I H F3--0000 0- CDH- K'0 H- IQ 0 F1C)O\ H NI 0) --411
H -j
pci-CD 0
t 4 4
I- I-- I ji
cirH HH CD
I p-
_ _ _ _ _ I _ _ _ D
C2-01-d0
0'9~ ~ %..nl ON-
000 00001-100 WO-
0o
0.•
000 007,0,
0000
r" (0
C+0
0 0 0 0 -
000 00
Pa~0 -11 <O- 0-O0 0
•oo °000 0000000 0 i 0 0- *• 1" n00,,+0
- 3ia -
Table 6
GoL.CO Peak Ratios for Permanent Gases
Coo oloe x"Peak Ratio
t!2 CO 0114112 co H
50 mole % acrylate 0.9 2.4 1
82.2 " " 15.6 1.3 1
- 32 -
data for permanent gases indicate that hydrogen evolution
increases with acrylate content but the methane/carbon
monoxide ratio remains roughly constant.
4. Molecular e All
copolymers containing up to 50 mole % n-butyl acrylate
remain soluble at all stages of photothermal degradation.
Copolymers richer in n-butyl acrylate become insoluble in the
later stages of reaction but in all copolymers there is a
rapid decrease in molecular weight in the initial stages of
reactiono Data are presented in table 7. Molecular weight
measurements by themselves can not give a quantitative picture
of relative rates of chain scission since starting molecular
heights and extents of volatilisation vary. Nevertheless
figure 10 demonstrates qualitatively that the rate of chain
.scission increases with acrylate content for low acrylate
copolymers but that for high acrylate copolymers the situation
is rather more .complicated°
A more quantitative picture may be obtained by calcula-•
ting the number of scissions per polymer molecule (N) and per
chain unit (n) using the formulae 1 2
CL (l-x)
CL
1-x 1
CL CL
in which CL and CL are the chain lengths initially and after0
an average of N scissions per polymer molecule have occurred
.33-
V11.3GHT ChlANGEJ',S IN COP(OLY-MERS OPJ TMETHYLi METBACHYLATE ANDn--3tTTYL ACRYLrATE, PHilOT jT IIRMLY E R AD N, G AT T6 '11 CopCLM R CCVOTION5 MOLE 1% -n-BLUTYL A.CRYLAV,",0 1.0; 39;u,16.;,50
oo
AA1
X-1~
Timcell
a'- 1-3) H' H'H
0 8 (D
P -~
CCD
a 0 0 0 0 0 0 0 0 CF2 C
-. 8 ODO )--\G D
o00 C) 00HH.
Cc4-
004sN)N N) U.0,H'~Ho C) H' VIOD i\0)Q 0D 0 ' H'c 4 C-- o -- \- I- FA 8 0 0 0
0 ~ In
04 N) '8I '8' \'8 'w w8 ' '80 H. '4 0 '0 ýl0
Go QO Q 000 00 0 C) 'o o P ,o00000\-q00 000 0"H coo0 ý000
0H'IC o" )
\AO04\K 0 N) 0 0 0 0 0 0 0 0 08H' C
C ' I- CD__________ U ~ c-i'
0
6 0 0 0 C-0'V00 E0(
ul Vf. l0znr
0
4 0 0 . ID
C+
0
0~ c o Cr 0 -1u CD wGF- 1 \;0
01 0CDC)00 0--1 ) 0d H'(D ~ ~ ~ *"' 0 l D - C ) 0 0" 0
6-L
I-I F- H" 0 01H -O H H~ F-,U -~~ w (D F-lF4.0 iC
GkA D w 0 vi Go 00 p. 0o 0 . e * C, 0 0 -, 0 0 0 CQ H
iQ~~ ~ ~ ~ 0l) CDjCOC a0\0w oF1\
0 0
. 00 00 0 0 a 4I)0 '0 0 Hz 0
0 1)'~0 4>. HQ ~ n j- H1 ýýjt. P
w_____ _______ I__V1_________co__o_-1 __4___CO
and x is the fraction of polymer volatiliseo It should be
noted that in deriving these formulae the only important
assumption is that no polymer molecules have bWen lost by
complete unzipping to volatile fragments Calculated volues
of N and. n are included in table 7° Plotting n against time
as in figure III it is clear that while the rate of chain
scission increases with copolymer acrylate content up to 1.6o3
mole 0, thereafter the rate is constant within exper imental
erroro A similar result was previously observed for the
methyl methacrylate/methylacrylate copolymer system.9 On the
other hand figure 12 demonstrates that the volatile material
produced per chain scission is progressively reduced by
increasing the n-butyl acrrylate content of the copolymer.By plotting volatilisation against N, as in figure 13,
for the four methacrylate rich copolymers, the zip length for
each copolymer was calculated as in table 8. These values
compare with 1,200 and 9 in the thermal degradation at 3131C.
of 0o25% and 5051 copolymieri, calculated from the data of
Grassie and Fortune. I Lower zip lengths were also observed
in the thermal13 compared with the photothermal5 degradation
of copolymers of methy! methacrylate and methyl acrylateo
5. _Spectrosconic Examination of the Residueo Infra-red
spectra of undegraded and degraded 93.4 mole ¶ n-butyl acrylate
cOpOlywuei are shown in figure 14 The only difference is that
the spectrum of the degraded material has a shoulder at 1770 cm
on the carbonyl peak. There were no significant spectral
changes on degradation of any of the other copolymers . Similar
-36
CHAIN SCI PER CHAI TN IT IN COl 0120 iT•S OF WEB....
ITET.AM IAT ) AL)) ND TY -.rT,,T TT n-BUrYL A7R1 .) D A ING
AT 1650C. COPOLYMER. 0..... TIO, ONM.LE S• '• -BI-TYL ACRYLAT ,I,'
O to; of 3.9; LO 16.3; 0 , 50; As 82; A?• 93.4.
09
Tin cv fr 137
Time,
RETMtTT.ONSHITI BETWPE:N CuIN 1>~sON 80E8> CHINUNTA,
VOLA.TILISATION IN C7OPOLYMERS OF lIMTIYL MLIIIACRYLATL AND)
.n¼3U*'TjTh.! x0P.YIKTE PTETOTHEFINAlLY DEGRAPINQC AT 1 6500
C OP OLYMELR COMPOST ITON, P OLE '/ n -BU1T YL A CRY LATE, 0,NO
02
00
00 Al 50f..... . - -----
VUL~kZti~tlaobH cant,
-38 -
RJ1LATIO N SUIP BEi<TWEEN SCISSIONGS PER MUM M OLECULL AND
VOLATT I.S/TIOT IŽ N A 50 HOLE 5! n-B~UTYL A GEItATZ CQPOLY>EU
I HOT OMM21ALLY DECRADI KG AT 1 650CG
CDC
IN
-39 -
absorption at 1760 cmI dur in the rmui de rati.n was
attributed to ].actone formatlion but absorptions at 1605 and
1560 cm- during thermal degradation were not observed Or-
ing the phototherina] reaction• These had previously been.
assigned to carboxyl.ate ion and conjugated carbon-carbon
bonds respectively.There was never any visible change in colour in any of
the copolymers du.r.inng piototh ermal degradation unlike thermal
degradation during which the colour deepened through yellow
to brown with incrcasing decomposition. Uv.o-visible spectra
were run on photothermally degraded copolymers and a typical
result is presented in figure 15. It is important to note
that after 40 hours irradiation approximately 50i of the
original material had been lost so that the true absorption
per unit mass is very much greater than is shown in figure 15.
As photothermal degradation proceeds there :is a general increase
in absorption in the region 4£00-255 mpe which can be attributed
to increasing ethylene conjugation in the polym er chain back-
bone as in the thermal reaction.
6 The Cold Rin:. It has already
been demonstrated that the 'cold ringt, which consists of
short c]han fragments, becomes increasingly more abundant as
the acrylate content of the copolymers is increased. Compar-
spectrum of its cold ring as in figure 16, new absorptions are-l
clearly present. A shoulder at about 1780 cm ppears, new
absorption at 1635 cm-I emerges and the C-0 single bond peak
- 4 0-
'<I F-11
o c U) F2 ~0 P.
UC)
U,, C)uiI
\0 0 4- [1 H A
4-1 10 0 0 L" (r,
cc C) C)t~ C)4Il- H 0
0 ~
H H4
l-~ 'Ji -
~.a C'.) 0 C) 7-W
-~ '-0 0 4
C) a
0000
tz
1>
.0
CD -
IzI4
Abso iK bF.rnce
-01
Pt..
Pi a
0 0
DO
f .... l
t4
C)
t'.4
'* 0
43 a
ON
Ivancrnittan ce, per enlo
CT)C
CiLi
i-I
K;
CD-
~ HU
at 1160 cm-I becomes more diffuse.
These chain fragEments must be formed in intra-
molecular transfer processes.
H H T HI I _ _ I-C-- CHH C CH- C.
2 2 "2
COOCH 3 C00CHI3 COOCIT COOCH 3
H H H
-C + 2 C h2 C CW-2- C - H
COOCH 3 COOCH 3 COOCH C00CL3
n
which must introduce terminal carbon--carbon unsaturation which
may account for the new absorption at 1635 cm 1 As in the
residue the shoulder at 1780 cm 1 may be attributed to lactone
formation as may also the changes around 1160 cm 1
C. Discussion
It is obvious from the foregoing results that the princi-
pal features of the photothermal degradation of copolymers of
methyl methacrylate and n-butyl acrylate are similar to those
of the thermal reactionoI Thus chain terminal radicals are
formed somehow in the system. These depropagate readily
throlgh methacrylate segments of the polymer molecule and ra. ther
less readily uhrough acrylate units, Acrylate ter-±uinated.
chain radicals prefer to undergo transfer processes, especially
,intramolecular transfer which results in short chain fragments.
-45 -
Thus methacrylate monomer and short chain fragments are
by far the most dominant products of decomposition of this
copolymer system. Other minor products, butanol, n-butyl
methacrylate, methane, hydrogen as well as the appearance of
unsaturation and- lactone ring formation in the residue and
short chain fragments may also be explained as in thermal
degradationI by reactions of chain side radicals formed by
intermolecular transfer
H
--. + CH 2- C .... 1)• + ... CH2-
COOBu COOBu
CHS IiC
o0 OBu
Grassie and Fortune proposed two possible mechanisms for the
production of n-butyl methacrylate in the thermal reaction.
One of them involved the simultaneous production of carbon
dioxide. Since carbon dioxide is not a product of photo-
thermal degradation, the sccond of Grassie and Fortune's
mechanisms, which they favoured, would appear to be confirmed.
H
-CH - C -HH- (C -- + C2C C1 CH2 2 C 2 ~''I 2 2: 0112
cnoO: COOr,, Mu COOBu COOBu COOBu COO•,
CH _ 1R- H C < " -ICH C CH2 +,CH--i
COOBu COOBu COOBu
-46,
There are however some -very significant differences in
the pattern of products of the photothermal reaction compared
with the thermal reaction. Thus butyraldehyde, which is
completely absent from the products of thermal degradation,
is produced in the photothermal reaction while but-l-ene and
carbon dioxide, which are significant products of thermal
degradation, are not produced photothermally.
It is possible to formulate reactions of chain terminal
or chain side radicals, of the type believed to be present in
this degrading system, which would result in butyraldehyde,
but ,it is difficult to justify their occurrence in the photo-
thermal process and their exclusion from the thermal reaction.-
Instead, it is suggested that the presence and absence of
"butyraldehyde is a manifestation of d'ifferent initiation steps
in the two reactionso It was proposed that thermal initiation
consists of random scission of the polymer chain backbone, On
the other hand, there is strong evidence that the primary effect
of uov. radiation on acrylate and. methacrylate polymers is to
cause scission reactions in the pendant ester groups. ItS 15
has been suggested that butyraldehyde is produced in the photo-
degradation of poly(n.-butyl methacrylate) by disproportionation
of butoxy radicals,
CH• CHi I, I 3
c I I°° ' 0I 2
-- 't2 C =- + C
2 4292 49 ~C 4 - 9 H 2 + CO0H
2~~~~~~~ C- H -__ O H
This reaction sequence would account for the production of
carbon monoxide but would result in equimolar yields of
n-butanol and n-butyraldehydeo However the characteristics
of formation of butyraldehyde and butanol are found experio-
mentally to be quite different, Butyraldehyde formation
increases with increasing acrylate content of the copolymers
and is a degradation product even from 3.9 mole % acrylate
cojolymero Thus the evolution of butyraldehyde is a function
of single acrylate units. On. the other hand, butanol is not
a product until the acrylate content of the copolymer reaches
50 mole % and is associated with acrylate sequences rather
than with single units,
It seems more likely that butyraldehyde is produced
by fragmentation of butoxy radicals. It is known that alkoxy
radicals fragment to form a carbonyl function,1 6 the formation
of the carbonyl group providing the driving force for cleavage.
Primary and secondary alkoxy radicals have been studied mainly
in the gas phase and under-these conditions loss of an alkyl
radical predominates over loss of a hydrogen atom. Thus
butyraldehyde formation would be preferred to formaldehyde
formation in the decomposition of butoxy radicals.
H CH 20 + C31H70 preferred
H-C - 0.
T-TC HCF + H.3.7
In the previous thermal degradation studies of copolymers
of methyl methacrylate and n-butyl acry]ate1 and of polykn-butyl
acrylate) 1 7 mechanisms proposed for the production of butwl-ene
and carbon dioxide involved the same types of chain side radicals
- 48-
as are believed to participate in the phototfhermal reaction
under discussion. An independent molecular reaction seemed
out of the question in view of the fact that the characteristics
of the formation of but-l-ene and carbon dioxide were sim:ilar
to those of the other products. It is therefore somewhat
surprising that but-l-ene and carbon edioxide are completely
absent from the products of photothecrmal degradationso Perhaps.
the formation of .e'i steric configuration necessary for the
production of but-l-ene in a radical process is inhibited by
the very much ,greater viscosity of the medium in which the
photothermal reaction is occurring due to the very much lower
temperature at which it is being carried outo The absence of.
the butyl, group decomposition products, ethane, ethylene,
propane, propylene and butane in the p•otothermal reaction may
also be an indication that they were formed as a by-product of
the reaction in i,,hich but-l-ene was formed in the thermal
reaction.
In addition to these differences in the products of the
thermal and photothermal reactions there are also minor differences
concerning the zip lengths and the relative amounts of acrylate
and methacrylate monomers produced o The. values of 1200 and 9
for the zip lengths in 0.25 and 50 mole % acrylate copolymers
calculated from Grassie and Fortune's thermal degradation data1
clearly indicate a tendency to lower zip lengths than those
quoted for the photothermal reaction in table 8 Bearing in
mind the method of calculation, these zip lengths are a measure
of the number of monomer units lost by depropagation and intra-
molecular transfer between each act of intermolecular transfer.
Thus it may be concluded that intermolecular transfer becomes
- 49 -
relatively more importan.t compared with depro'pagation and
intramolecular transfer at the hi ghelr temperature of thermal
degrad.ation (3130C), all the photothermal degradation daota
having been obtainoed at 1650C. This is reasonable since it
is to be expected, that intermolecular interactions should be
favoured by the greater mobility of the polymer molecules at
the higher temperature,
Molar ratios of the two monomers, calculated from the
present and previous data]" are presented in table 9 which
demonstrates that approximately one in forty of the n-butyl
acrylate units is liberated as monomer in the photothermnal
reaction compared with one in five in the thermal reaction
indicating the greater tendency at the higher temperature to
deprop'agation of acrylate units at th.e expense of intra-
molecular transfer.
These zip length and. monomer ratio data thus give a
qualitative indication of the effect of temperature on the
relative tendencies for depropagation, intra and intermolecular
transfer'at acrylate terminated chain radicals.
It may also be worthy of note that zip lengths for n-butyl
acrylate copolymers are verst much less than those for methyl
acrylate copolymers which demonstrates a lower relative tendency
for n-butyl acrylate units to depropagateo This will undoubtedly
also be influenced, however, by the different viscosities of
the two polymers at the temperature at which the photothermal
degradations were carried out.
Bearing in mind the various mechanisms which have been
proposed, the overall photothermal degradation process may be
represented qualitatively by the following reaction sequence in
which the various observed products are underlined.
- 50 -
Table 9
Molar Ratios of.Monomeric Products
Copolymer Monomer.Ratio (M.M.A.7n-Bu )
(mole %n-BuA.) Photothermal Thermal
16.3 215 23
50 304
82.2 10 1
- 51.-
Copolymer Molecule
I ester group scission
Butyraldehyde + Carbon Monoxide
+ Chain Side Radical
chain scission
Terminal Chain Radical
depropagation to nearest
acrylate unit-
Methyl Methacrylate
+
Acrylate Terminated Radical
depropagation intra olecular intermoleculartrt nsfer transfer
n-Butyl Acrylate Chain Fragments Chain Side Radical
n-Butyl Methacrylate Methane
n-Butanol Hydrogen
Unsaturation
52-
CHAPTER III
SYNTHESIS OF FRIEDEL-CRAFTS POLYMERS
A. Introduction
.18Phillips, in,1964, showed that polymers prepared by
the condensation of p-di(chloromethyl)benzene with benzenoid
aromatic substances, in presence of Friedel-Crafts catalysts,
can be considerably more thermally stable than conventional
phenolic resins and hence might have considerable potential
commercial application. Grassie and Meldrum subsequently
demonstrated that the range of polymers could be further
extended by replacing the benzenoid aromatic component by
heterocyclic aromatic components. They used thiophen,
pydridine, indole, quinoline and pyrrole with stannic chloride
as catalyst and found that the resulting polymers had widely
differing stabilities. In particular, the thiophen polymer
was considerably more stable than the benzene polymer.
To investigate the reasons for these differences in
stability and with the long term objective of defining condi-
tions for optimum stability, a comprehensive investigation
of the DCMB/benzene/SnCl 4 system was undertaken. 7 Each
individual product, up to a molecular weight of approximately
1000, was separated using g.l.c. or g.p.c., identified, and
rate constants for its formation and further reaction deduced.
Since material of molecular weight 1000 represents more than
90% reaction as defined by acts of condensation and measured
by HC1 librated, it is clear that the principal structural
- 53 -
features of the final insoluble, infusible, polymer already
exist and it is a relatively simple matter to extrapolate
to 100% reaction.
The ultimate picture of the DCMB/benzene polymer was
of a highly branched structure of the type,
-0 _ CH2 ,0 - CH 2- 0 - CH2- 0-
1CHI
0- c•P 2 - CH2-0 - CH2 - 0 - CH2- 0
CH2 CH12
CH2
the finer structure being defined by the ratio of mono: di:
tri: tetra substituted benzene rings.
A similar investigation of the DCMB/thiophen/SnCl 4
system was subsequently reported. 8 Differences from the
DCMB/benzene/SnCl 4 system were immediately obvious. The
first two steps in the reaction are strictly comparable,fT presents a th ophen nucleus)
Cl - OH2- 0 - CH 2 - Cl
Cl- CH2 - 0 - CH2 - T
4T
T CH 2 - , CII2 -T
-54 -
The thiophen nucleus is very much more reactive than the
benzene nucleus, however, so the next product in the thio-
phen system is almost exclusively
T -CH 2 - H- T2- - CH2-0- CH2 - Cl A.
rather than a mixture of linear and branched isomers as in
the benzene system. In the thiophen, but not in the benzene
system the stereochemistry of A is such that the two ends
of the molecule react to form
T
CH CH22 2
0 B.
CH 2 CH22 T 2
Thiophen nuclei, being more reactive than benzene nuclei
further reaction with DCMB occurs preferentially at the former
so that what is in effect a partial ladder structure is rapidly
built up and it is to this that the superior stability of the
thiophen polymer has been attributed.
Polymers may be similarly prepared by replacing DCMB
by a,,a'-dimethoxy-p-xylene (DMPX) in which case methanol,
rather than HCM is the volatile condensation product. By
the time the work described above was complete it was clear
that materials based on DMPX rather than DCMB were more
appropriate for commercial exploitation and it was wi+h this
knowledge and background that a programme of research was
proposed, "to study the products and mechanism of polymeriza-
tion of DMPX with benzene and thiophen as a first step towards
the elucidation of the relationships between structure and
stability in this class of polymers so that the factors for
the achievement of optimum stability may ultimately be
determined".
It was only possible to carry out some very preliminary
experiments before the contract, was directed towards the
work described in Chapter 4. These preliminary experiments
are described in the following section.
B. Experimental and Results.
l.- Preparation of Polymers under Nitrogen. For precise
kinetic measurements it is, of course, necessary to use
highly purified materials, but in order to establish suitable
experimental conditions for the preparation of these materials
as well as to get some feel for the range of aromatics which
may be copolymerized with DMPX, a number of preliminary experi-
ments were carried out under nitrogen using stannic chloride
as catalyst. Using the general experimental method, which
follows, the following carbocyclic and heterocyclic aromatic
compounds were. copolymerized with DfPX and satisfactory
copolymers were obtained in each case.-
Benzene thiophene
dimethylbenzene DCMB
pyridine styrene
furan phenol
toluene p-xylene
2-methyl thiophen m-xylene
2-chloro thiophen dimethyl terephthalate
terephthalic acid.
The preparations were carried out in a round-bottomed
flask fitted with a thermometer, nitrogen outlet and reflux
condenser fitted with a drying tube.
The aromatic compound, solvent (1,2-dichlorethane)
and DMPX were added to the flask and the apparatus flushed
out with dry oxygen-free nitrogen while it was heated to
just below the reflux temperature (831C). The stannic
chloride, in solution in 1,2-dichlorethane, was added.
The usual recipe was,
4-10 gm aromatic compound
2-4 ml DMPX
0.2 ml SnCl 4
40-50 ml DCE
Solutions were refluxed for periods of 2 to 24 hours
from the time of addition of SnCl 4 , most runs being of
6 hours duration. In some of the copolymerization reactions,
especially those with benzene, dimethyl benzene, thiophene
and DCMB, several molar feed ratios were used. Otherwise
the most common were 1:4 and 1:5, DMPX: aromatic. After
the desired polymerization time the reaction was stopped by
adding water, the organic layer was separated and dried, and
the polymer was precipitated in an excess of Analar methanol.
The precipitated polymer was filtered, dried, and tvice
reprecipitated from Analar chloroform solutions. The resul-
tant pure polymer was dried under vacuum and stored. In a.
few cases the polymer in solution was not precipitated by
methanol. In these cases the solvent was removed by distilla-
tion under vacuum.
2. Preparation of Polymers under Vacuum. Having
established, in general, that a variety of polymers can be
prepared by replacing DCMB by DMPX it was then important to
devise suitable methods of purification of reagents and
-57-
techniques for carrying out the reactions under vacuum for
kinetic purposes. The following methods were found to be
generally applicable.
ao Purification and drying of reagents. Since small
quantities of water or other impurities could have a con-
siderable effect upon the polymerizations, great care was
taken to insure dryness and absence of contamination.
i. Dimethoxy-p-xylene
ca' Dimethoxy-p-xylene (DMPX) (Albright and Wilson)
was purified by passing a 100 ml aliquot twice through a
large basic alumina column (diameter 3.5 cm, height 40 cm)
using diethyl ether as the elutant. This removes acid
impurities formed by oxidation and any peroxides that may be
present. A stream of nitrogen was directed into the receiving
flask to keep out oxygen and to help evaporate the ether.
The purified DMPX could be stored for several days under nitro-
gen.without the reappearance of impurity peaks (1270, 1310,
1580, 1610, 1690, 1710 cmJ1 ) in the I.R. spectrum but should
be kept under vacuum to avoid oxidation.
Diethyl ether was removed from the DMPX by distillation
under vacuum. The apparatus was constructed as follows.
Freshly ground calcium hydride was added to a 250 ml flask
with a break-seal tube and side-arm attached. The DMPX-Et 2 0
solution was added. The apparatus was attached to the
vacuu.m line- and de-assed at least three Iimes by the freezing
and thawing technique. The diethyl ether was then distilled
from the mixture using ice/water for the cold trap rather
than liquid nitrogen so as to minimise bumping. The distilla-
tion was completed with liquid nitrogen, however, in order to
- )
remove the last traces of diethyl ether. The DMPX was
again degassed several times, sealed off and stored for at
least a week in the dark with periodic shaking. The CaH2
was removed by filtering under vacuum. The filtered DMPX
was sealed off and the flask used as a storage ampoule.
ii. Benzene Benzene (Hopkins and Williams Analar)
was pre-dried over freshly ground calcium hydride. It was
attached to the vacuum line, degassed three times, distilled
onto fresh CaHl2 , and stored for at least a week.
This process was repeated, the benzene was degassed
three times, distilled under vacuum in all glass apparatus
and redistilled two more times. Extra degassing was often
necessary since the drying process resulted in the production
of hydrogen and the CaHl2 also seemed'to degas slowly. The
pure, dry benzene was stored in a sealed ampoule under vacuum
until used.
iii. Stannic chloride Phosphorus pentoxide
(BDH laboratory reagent) was first sublimed under vacuum into
an ampoule which was sealed off. Stannic chloride, anhydrous
(BDH Reagent Grade) was transferred quickly from its ampoule
to a flask, degassed several times under vacuum and distilled
onto the freshly sublimed phosphorus pentoxide. The flask
was sealed off and stored in the dark for at least one week.
After redistillation under vacuum onto freshly sublimed
phosphorus pent.xid^, followed by storage for one more week in
the dark, the stannic chloride was twice vacuum distilled in"
all glass apparatus and stored under vacuum in a sealed ampoule
wrapped in a black plastic bag in the dark.
59-
iv. l,2-Dichloroethane The purification and
drying of 1,2-dichloroethane (DCE) (BDH Reagent G.-rade,,) was
similar to that for stannic chloride, DCE was vacuum distilled
three times in all glass apparatus before being stored.
b Preparation of Glassware. Pyrex glass
was used throughout. It was cleaned before use by washing
in a solution of Pyroneg, rinsing with distilled water,
Analar acetone, and Analar chloroform, then dried in air at
1000 C. Before use glassware was evacuated on the vacuum line-A
to a pressure of 10 torr or less and flamed, out to remove all
traces of moisture.
c. Preparation and Distribution of Solutions. Several tyres
of solutions were made up and distributed. They included the
"standard solutions" for polymerizations in which the same
initial reagent concentrations were used but the length of.
time of reaction was varied. A single "standard solution"
was used for a whole series of polymerizations0 These solutions
were employed in runs to determine the rate of reaction and the
time of appearance of various products. Other reactions such
as those used for kinetic determinations, involved the use of
different initial reagent concentrations. Thus individual
reagent solutions had to be prepared.
i. "Standard Solutions" "Standard solutions" of
benzene and stannic chloride in ID- were prepared as fllows....
Exact amounts of each reagent were poured under vacuum from
their storage flasks into previously calibrated graduated
ampoules fitted with break.-seals. 1 ml ampoules were used for
SnC1 4, 10 ml or 25 ml for benzene, and 100 ml for DGE. The
graduated ampoules containing the reagents were then attached
to a large storage flask, the break- seals broken and the
reagents distilled into the flask.
A typical "standard solution" .recipe would be 0.2 ml
SrCI 4 , 10,0 ml benzene, 84.8 ml DCE.
The "standard solution" was then thoroughly shaken and
distributed under vacuum into 10 ml calibrated ampoules* The
apparatus used consisted of a 250 ml central reservoir surroun-
ded by a periphery of ten calibrated 10 ml graduated ampoules
and one inlet tube. The storage flask containing the standard
solution was sealed onto the inlet tube and the apparattis was
evacuated and sealed off. The standard solution was intro-
duced into the central reservoir via the break-seal and poured
into each ampoule in turn, After each addition the ampoule
was sealed off at a constriction.
it. D7DIX Approximate amounts of DMPX were distri-
buted from the storage flask into a number of small ampoules
(-'-10 ml) using apparatus similar to that mentioned above.
Since DINMPX could not be distilled and proved impossible-to pour
accurately, the small ampoule had to be broken open and. the
necessary amount pipetted into the reaction flasks, Five ali-
quots (usually 0.50 ml) of DMPX at a time were pipetted into
10 ml ampoules having stems 6-8 in. long. Care was taken to
vi,,- *... ......•, P, on .e.p upp. r part- of the stem as this
would have been oxidised on heating. A break-seal tube witlh
side-arm w,%as then attached to the stem without blowing (to
prevent possible oxidation) and a constriction formed at this
pint. This =ppartus w-s attached to the vacuum line, dgse
several times, pumped down to a pressure -. 10 tor.r or less
•ow• pessur Of ,
and sealed off. The sample was then stored in the dark until
needed*
iii. Individual Samples of Stannic Chloride, Benzene,
and DCE. In kinetic determinations the initial
concentrations of reagents would have -to be varied from run to
run. Thus standard solutions as defined above could not be
used. The most straight-forward 'method of doing these poly-
merizations would be to distil the reagents directly onto the
DMPX. This worked well enough for benzene and DOE but the
exceedingly small amounts of stannic chloride needed for a
given reaction could not be measured with any accuracy. Thus
a solution of SnCl 4 in DOE was prepared. leasured amounts of
the two reagents were transferred to calibrated ampoules. The
contents of these ampoules were then distilled into a flask
and the re.sulting solution was distributed into calibrated
ampoules as described abovee These ampoules were stored in
the dark until needed. Measured quantities of benzene and
DCE were transferred and distributed in a similar fashion.
CHAPTER IV
THEPMIJAL DEGRADATION OF POLYSILOXANES
A. Preo aration of Polwmers.
1. Introduction. Poly(dimethyl siloxanes) are
prepared by the hydrolytic conversion of dimethyldichloro-
silane to the cyclic tetramer, octamethylcyclotetrasiloxane,
followed by polymerization of the tetramer in presence of
suitable catalysts to form the linear polymer.
According to Patnode and. Wilcock 1 9 the relative pro-
portions of the compounds formed by hydrolysis of dimethyl-
dichlorosilane depend upon the conditions of hydrolysis but
there is a. very strong tendency toward the formation of the
cyclic tetramer which is generally formed in the greatest
amount. Their method was used with variations in. the
silane/water ratio. Since the hydrolysis reaction is exo-
thermic, the reaction vessel must be cooled. Water to silane
ratios of 2:1 to 3:1 were used without noticably-affecting the
yield of short chain cyclic polymer. The products of this
hydrolysis are principally the cyclic trimer (bhp. 1340C),
tetramer (bopo 1750C) and pentarher (b.po 2100C) with the tetra-
mer predominating. The tetramer may thus be separated in high
purity by distillation0
Cyclic tetramer was polymerized by the method of Gilbert
and Kantor20 usinng tetramethylammonium hydroxide as caatalysto
This system was chosen since the catalyst may be deactivated
simply by heating to 1300C when it decomposes to trimethylamine
and methyl alcohol.^
2. EHydrlsi2 s of dimetthyLzdichlorosil ane. Distilled water
(750 ml) in an R.Bo flask equipped with magnetic stirrer and
thermometer was cooled in an ice/water bath. Dichlorodimethyl-
silane (Merck) (250 ml) was added dropwise, the temperature of
the reaction mixture being maintained below 200 C. The reaction
mixture was vigorously stirred throughout the hydrolysis.
The oil formed by hydrolysis was separated from the
excess water and dried over magnesium sulphate. The liquid
was separated by filtration and distilled under reduced pressure
to separate the low molecular weight cyclic siloxanes from the
non-volatile products of hydrolysis. The distillate, twhich
was contaminated with HCI, was washed with aqueous sodium
bicarbonate, separated and dried over magnesium sulphate.
The liquid was then distilled through a 10" Vigreux
column at ,ordinary pressure giving approximately 50 ml of
material distilling at 168-1731C, and. redistilled a number of
times to give progressively purer cyclic tetramer (bopo 170-
174 0 C).
3. kl•erization of octameylcvelotetrasiloxaneo A typi-
cal polymerization of the cyclic tetramer was carried out as
follows. Octamethylcyclotetrasiloxane (30 ml) in an RoBo
flask was heated to 115-1160 C in an oil bath, Several drops
of a 25% aqueous solution of tetramethylammonium hydroxide were
ng, itl -ri-uit' -s IidS r-s s fn"aaa•. w'±• ....... g Aft1er 25 ' "^i .
obviously more viscous. Heating was continued for a further
20 minutes and. the mixture allowed to cool to a tacky gum.
Benzene and anhydrous toluene are suitable solvents for
the pOolyme vith iethanol ti suitable precipita-ing agent.
- 64-
4. Poler La a leso The material des-
cilibed above has been. designated polymer I1 Further sampoles
have been prepared at different temperatures using pure cata-
lyst rather than an aqueous solution (polynmers 2, 3 and 4)
and, a large sample has been prepared from tetramer of very
high purity (confirmed by goi°co) obtained by five successive
fractional distillations (polymer M)0 Full details are given
in table 10o
The catalyst decomposes to trimethylamine and methanol
at 1300 and. briefly heating to this temperature after reaction
is complete has been used to destroy residual catalyst ivlhich
can influence the stability of the polý7mero Gilbert and
Kantor 2 0 have reported that the amine has no effect on polymer
stability. The residual amine may be easily removed by
methanol.
All these polymerizations have been carried to high con-
version and probably, in most cases, to equilibrium. Poly-
merizations have been in air. The molecular weight is
obviously an inverse function of polymerization temperature
but there is insufficient data to indicate possible effects of
impurities and catalyst concentration on molecular weight.
5o L _.eal,-,t-ion of End-Blocked Ioleia:o An attempt has
been made to block the active end groups of a sample of polymrer
It j05 by ref-luxing w-iluh excess 31 1 3,3hxanty-
disilazane (1.2 ml) in toluene (20 ml) for 24 hours. The poly-
mer was reprecipitated from methanol. No characterisation
of this material has been carried out but its thermal degrada-
tion characteristics have been observed using TGA (see below)
- 65 -
Table 10
Preparations of Pol-dimethylsiloxanes)
Polymer Tetramer Temp(OC) Catalyst I Subsequent
n Treatment
impure 115 few drops of25% aqueous 258,000 Nonesolution
2 impure 110-130 dry, 0.02% 94,500 INone
3 impure 108-110 dry, 0o01% 141,000 Heated to 13000
4 im-pure 97-98 dry. 0.02% 183,000 (a-None(b-heated to 130 0C
M pure 100 dryý 0o015 269,000 (a-None(b--heated to 1300Cand reprecipitated
- 66 -
Bo Measurement of MoleculJarPj- PiLht
Molecular weights were measured in toluene solution
using a Hewlett-Packard 501 High Speed. Membrane Osmometer
with cellophane 300 membrane. The osmotic plot for polyTmer1, which is juite ty-pical, is illustrated in figure 17 from which
a molecular weight of 258,000 was deduced. This measurement
was perfectly normal and straight-forward and. it is clear that
polymers can be prepared with molecular weights in a range
ideally suitable for the use of molecular weight as a tool for
the investigation of degradation reactions.
C. Gas-LiauidChromatL22rh
Gas-liquid chromatography has been vital in two phases
of this work. First in checking the purity of the cyclic
tetramer formed in the first stage of the preparation of poly-
mer. Second in the analysis of degradation products. In
both cases the principal constituent of the mixture to be
analysed is the cyclic tetramer and the minor constituents are
expected to be principally smaller and larger cyclic analogues
and closely related compounds o Thus the problems associated
with the application of goo.c. to the two phases of the work
are almost identical and are most conveniently considered to-
gether.
All g.l.co data have been obtained using a Microtek
G.Co 2000P instrument with flame ionization detector. Two
columns have proved suitable, namely, 1% S.E. 30 and 20% DOC.560
on gas chrom Q and- their separation cbgracteristics have been
established. Typical chromatograms, shown in figures 18-20,
demonstrate the high quality of separation attainable. The
- 67 -
o17
A TIPICAL OSMOTIC PLOT.
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assignment of tet-ramer peaks presents no difficulty since it
is the major product of hydrolysis of dichliorod.imethylsiiane
in the first stage of the preparation of polymer. The other
peaks were preliminarily assigned according to the order of
molecular weights of the species which are known to be present
in the tetramer as impurities, namely trimer (0.55,), pentamer
(6.7%) and hexamer (1.6%). The 1% S.E. 30 column is generally
the more useful and convenient but the D.C. 560 column is
better for the separation of trimer.
In order to calibrate the instrument for the quantitative
estimation of degradation products pure samples of as many of
the potential volatile degradation products as possible will
be necessary. Thus a pure sample of the dimer, hexamethyl-
disiloxane,
(H 3H ) 3-s i-°-si- (C c3 ) 3
has been prepared by hydrolysis of trimethylchlorosilaneo The
chromatogram in figure 21 illustrates its high purit±y and a
retention time of 2.4 minin on a standard column.
Cyclic trimer has been prepared by pyrolysis, at tempera-
tures in excess of 4001C, of the residues from the preparation
of cyclic tetramer, Figure 22 demonstrates a retention time
of 9.2 min. on a standard column.
Pure tetramer is available as the intermediate in the
prepa.-.0ionn of polyrner nnd pure pentamer and hexamer are
*• - ,T I r.
the residues remaining after fractional distillation of impure
tetramer. Progress towards pure pentamer and hexamer are
illustrated in figure 20 which represents some of the residual
products after two d.istililations.
- 72 -
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Vi-41 I,
Do Thermal Analysis of PolIerso
1. Introduction. Thermal analysis can be used to provide
a rapid preliminary picture of the principal characteristics
and products of the thermal degradation of polymers. Thermal
Gravimetric Analysis (TGA) and Thermal Volatilsation Analysl 2 1
have been applied in the present instance.
TGA demonstrates the progressive weight loss as the
sample is heated through a linear temperature programmeo The
Du Pont 950 instrument has been used for this purpose, degrada-
tions being carried out, unless otherwise mentioned, in a
nitrogen atmosphere (80 ml/min.) at a heating rate of 10ý1o/miný
to 4850C.
TVA demonstrates the evolution of volatile products
as the sample is heated through a linear temperature programme.
In the apparatus, as devised by McNoeill2 2 the stream of volatile
products is divided into four, each passing through a trap at
a different temperature (normally 00C, -451C, 750C and 1lO00C)
A Pirani gauge measures the pressure of products passing through
each trap. Finally the gas streams are recombined and passed
through a trap at -1961C which is also associated with a Pirani
gauge° The Pirani gauge responses are fed into a pen recorder sc
that the thermogram displays a qualitative picture ,of the products
volatile at each of these temperatures. A further trace provides
a check of the linearity of the temperature programme.
2 heza "ra11i01tric A naIyvS is Tcogam _rc, present
in figures 23-31 and additional details in table l. It 1is
clear from these thermogram. that neither heatingto 1300C to
destroy the catalyst nor the molecular -weight of the starting
polymer have any significant influence on the threshold degra, datic:_
- 75
I Lg' I °"y
K- .__ ____ <%.
| E.•
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Table 11
Details of TGA Thermograms-Figures 23-31
Fig. No. Polymer Pretreatment Threshold Degrad.Atmoshere
__Ter perature Atmosphere
23 1 reprecipitated 342 nitrogen
24 2 reprecipitated 335 nitrogen
25 3 reprecipitated 342 nitrogen
and heated to 1301C
26 4 reprecipitated 325 nitrogen
27 4 reprecipitated 345 nitrogenand heated to 1300
28 4 reprecipitated 335 static air
29 M reprecipitated 320 nitrogen
30 M reprecipitated 342 nitrogen
and heated to 1300
31 4 End-blocked 270 nitrogen
-- 83 --
temperature or the general characteristics of the thermal
degradation reaction* It is surprising thatthe end-
blocked polymer appears to have a lower degradation threshold
although the subsequent build up in' degradation rate is slower.
3. Thermal Volatilisation Analysis. it is known that
polymers, as prepared, are in equilibrium with the starting
material, cyclic tetramer. This shows up as a peAk in the
TVA thermogram around 175-190'C as shown in figure 32 for
polymer M -but is eliminated by reprecipitation as shown in
figure 33. However, the reprecipitated polymer is very much
less thermally stable, beginning to degrade at 343 0 C compared
with 3980C for the polymer as prepared.
A further sample of polymer M was heated to 150*C, to
destroy residual catalyst, and cooled. The thermogram in
figure 34 demonstrates that degradation once again occurs at
the lower temperature. Further experiments will be required
to clarify these observations.
E. Infra-Red Spectra of Products of Degradation
Further preliminary information about the products of
degradation have been obtained by comparing the infra-red spectra
of polymer M (figure 35), cyclic trimer (figure 36) and cyclic
tetramer (figure 37) with those of the TVA condensibles obtained
from polymer M preheated to 1500C (figure 38), reprecipitated
(figure 39) and as prepared (figure 40) and the "cold ring" or
chain fragment fraction from polymer M as prepared (figure 41)
All the spectra are similar. The information available
is not sufficient for a detailed analysis but it is clear that
the main products are cyclic trimer and tetramer or closely
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-, ý. ý - - --------- ý - ---.- I -- . - - _---- -!-;-*-. ;. . : :ý t,.''!
ý
I
._j I .
,ý,
vt- , . .. -,--,;*:-- -- ,-;-. ---:-I-ýý ,-,- -l --ý 2 .I ---*-, -, - F-,-, -
L I -- ---- - --- ý-,-ý- -- :--Lz-ý-t. i-jr- : ;! ;..-.-ý ý -- :-.-l--- .
. i I -- ý ý-7-7-----":,.:ý-- .1 .I ! -, ., -1 d, ; -: -: ;.- -- ý.!-i-;.-,--: ! -.'.'- ! i -L i
I-E .- I- I I . . , I . !-ý . . ; 'i . I .- , - .; -- -:-L-,.;- -, I i ý , , . . ý-. I . ; , ,-
- , ---.-.
- -
,
- ' ' '- - ý-ý'71-----.---
----,..
--- , : ;ý , - I :ý -- , - . : ---
,-- I -- i - .-----.- ý-'ý;---lLL:-ý-, -dj I . - 1! ;: - -- -. , , I - i - : - -.
!! I - -ýý , . - - . ; .- I , :-.--. _;_ --- !- -: ý ; - ý 1* ---- ý--. , ý ! _ -- __-.L, :-..---t ---- --
ý
ý
-I.,------ -- ,----. I - -I- -:.ý-!-!- i-i- . . , - ---.- - - - ý- ý2 , . -- I--
--- . 1..---. ---.. - , - , . i. I. .-- ---
.1 1: I . - - . -1.!.".. -J-!j.LI --.-.-. .. -- ::::. ': . y .
I..: --- - I ý ; .,- ; : -1 -ý , - . !;,. .,i i -.- - --. , -- ---- Vý-- ------.- -- -,----:--- -- - -- ,.--.--
.-
--
:J
-.
1 ... !-;--!-.:--
-I-,-
-1--
.ý,:---.----:
::
;
.:
ý
-, !
1
;
,
-
----
ý-t-ýýý-.,-....,.----,-------------,.;---.--:-----!
.---- ---- :.I -- . - . , . .: 1. - - i.;.., , i - ; - ,- : -: , . . -----.ýiI,.. h. ,?--., . ý-. I 1. . . ! . . i - - -,d i i- - ý- ... %] ý .--! "-: _..-ý -, - - -,.- . , - : : : I -, I -. ..
. - " --- - --- - .---- ý--- ---- '
I-j ) -a . I . - -. ý- .: ,
I-- .- . ,--, : .- -- -- - - . =--A --
-"--:-,-- -,-------. - -.- - 1, -7-= --ý- -.- ---- ____ ---- I-
l-!---:--- i - - -!-I! Ll , - ý..; - !-
Cý ýý 1 *- -, -1 .- - - - -.. - - ; :! '' * - - -:.-:4-,-!-'---,-!- ,:I- -.- -. , I- .. , I ,
related compounds and. that the chain fragments are closely
related to the initial polymer. It seems also that the
Si-C bond is remaining intact since there is no indication
of the production of methane.
F. The "Sealed Tube" Degradation Technique,
Having completed all the preliminary investigations
described in earlier sections of this chapter it is now
appropriate to make a'more systematic and detailed study of
the thermal degradation of poly(dimcthyl siloxane)° It is
proposed to use the "sealed tube" technique which has been
successfully applied in the past to similar investigations. 2 2 ' 2 3
The equipment necessary is being constructed and assembled.
The "sealed tube" is represented in figure 42.
A sample of polymer (=--50-00 mg) is placed in an
aluminiumboat, inserted at B and pushed along to point A. The
apparatus is then evacuated and sealed off at Bo Tube A is
inserted into a thermostatted furnace and C immersed in liquid
nitrogen* After a predetermined reaction time the volatile
degradation products may be exafmined by attaching D to a vacuum
system, breaking the break seal and collecting the products
in a manner consistent with the method of analysis contemplated,
i.r., g.l.c., etc.
Less volatile large chain fragments (the "cold ring"
r a aCioY, accumiulaut in A just outside the furnace an" m1ay be
recovered by breaking the tube at an appropriate point and
washing out with solvent. Residual polymer is also available
for inspection.
-97-
Figo 42.
THE "SEALED TUBE".
.c.
- 98 -
G. Conclusions.
Progress in studying the thermal degradation of poly
(dimethyl siloxane) may thus be summarised as follows:
a* Satisfactory methods for the preparation of suitable
samples have been established and samples prepared.
b. Suitable methods and conditions for the measurement of
molecular weight have been determined.
c° The main features of the degradation process as revealed
by two thermal analysis methods, namely TVA and TGA, have been
established*
d. Thus the reaction starts around 3501Co Tetramer and
trimer are the main products of degradation in vacuum.
Oxygen has a profound effect on the reaction.
eo Gas-liquid chromatographic methods have been devised for
the separation and analysis of products.
fo It has been established that neither residual catalyst
nor the molecular weight of the starting polymer have a sig-
nificant influence on the threshold degradation temperature or
the general characteristics of the thermal degradation reaction.
End blocking by 1,1,1,3,3,3-hexamethyldisilazane surprisingly
leads to a lower degradation temperature threshold although the
subsequent build up in degradation rate is slower.
Using the "sealed tube" technique described in section.F
of this chapter ania. ap.ropr•a•t analv]ical miethods• a sytrH
now be undertaken with a view to establishing the detailed mec-
hanism of degradation and the structural features upon which
-99 -
stability depends. The following topics seem to be relevant.
a. Further studies of thermal analysis using TGA and TVA.
b. Molecular weight changes during degradation.
c. Detailed analysis of volatile products.
d. Changes in the involatile residue.
e. Influence of molecular weight of the starting material.
f. Effect of end-stopping stabilisers.
g. Effect of polymer preparation - temperature, initiator,
etc.
It is anticipated that these studies could later be
extended to other polysiloxanes, for example, poly(diphenyl-
siloxane) and appropriate copolymers.
- 100 -
"^REFERENTCES
1. N. Grassie and JXD. Fortune, Makromol. Chem. 162, 93
(1972); 168, 1, 13 (1973); 169, 117 (1973).
2. N. Grassie, A. Scotney, R. Jenkins and T°I. Davis,
Chem. Zvesti, 26, 208 (1972).
3. P°RoEoJo Cowley and H.Vo Melville, Proc. Roy. Soc., A,
210, 461 (1952); 211, 320 (1952).
4. N. Grassie and E. Farish, European Polymer Jo, , 627
(1967).
5. N. Grassie, B.J.D. Torrance and J.B. Colford, J. Polymer
Sci., A-l, 7, 1425 (1969).
6. N. Grassie and I.G. Meldrum, European Polymer J., 4,
571 (1968).
7. N. Grassie and I°G. Meldrum, European Polymer J., 5,
195 (1969); 6, 499, 513 (1970); 7, 17, 613, 629, 645,
1253 (1971).
81 N. Grassie, JoB. Colford and I.G. Meldrum, J. Polymer Sci.,
A-i, 29 2817, 2835 (1971)..
9. N. Grassie, B.J.D. Torrance, JoD, Fortune and J.Do Gemmel,
Polymer, 6., 653 (1965).
10. J. Ditchburn, Proc. Roy. Soc., 236, 217 (1956).
E. Massey and F. Potter, RoI.C. Monograph, I, 2 (1961).
11. A.A. Noyes and P.A. Leighton, Photochemistry of Gases,
S(l .*j ... lA.2 ... ..
4 -, 1-~J -1t 11LA A~
12. N. Grassie and E.M, Grant, European Polymer J. 2, 255 (1906).
13. N. Grassie and B.JoD° Torrance, J. Polymer Sci°, Al, 6, 3303,
3315 (1,968).
14. R.Bo Fox, Progr. Polymf. Sci., 1, 45 (1967)o
15. LoG. Isaacs and R.B. FOx, U.S. Naval Research Lab. Report
No. 6339 (1965).
16. C.J.Mo Stirling, "Radicals in Organic Chemistry",
Oldbourne, London, 1965.
17. N. Grassie, JoG. Speakman and T.I.Davis, J. Polymer Sci.,A-i, 2 931 (1971).
18. L.N. Phillips, Trans. Plastics Inst., 3-2, 298 (1964).
19. 11. Patnode and D. Wilcock, J. Amer. Chem. Soc., H, 358
(1946).
20. A.R, Gilbert and S.N: Kantor, J. Polymer Sci., 40, 35
(1959)0
21c I.C. McNeill, European Polymer J. 6, 373 (1970).
22, N. Grassie and J.D° Fortune, Makromol. Chem., 168,
i (1973).
23. N. Grassie and D.Ro Bain, J. Polymer Sci., A-1, s,
2653, 2665, 2679 (1970).
""02 -
Unclassifiedoecuritv Classification
DOCUMENT CONTROL DATA- R & D(Security classification of title, body of abstract and indcxin- •nnototicn must be entered when the overall report is le~ssified)
I. ORIGINATING ACTIVITY (Corporate author) 2a. REPORT SECURITY CLASSIFICATION
Department of Chemistry _fUniversity of Glasgow, 2b. GROUP
G1 aSJ3G1 2 8 0Q0 Snn, and..3. REPORT TITLE E
Degradation Reactions in Polymers
4. DESCRIPTIVE NOTES (Type of report and inclusive dates)
Final Scientific 70 Feb 01 - 73 Jun 305. AUTHOR(S) (First name, middle initial, last name)
Norman Grassie, Roy Jenkins, John Cunningham and Ian G. MacFarlane
6. REPORT DATE 7a2. TOTAL NO. OF PAGES 7b. NO. OF REFS
73 Auguýs t 3 1 1123___ ____
Be. CONTRACT OR GRANT NO. 9a. ORIGINATOR'S REPORT NUMBER(S)
F61052-69-C-0026b. PROJECT NO. None
7342C. 9b. OTHER REPORT NO(S) (Any other numbers that may ba essigned
this report)
d. None10. DISTRIBUTION STATEMENT
11. SUPPLEMENTARY NIOTES 112. SPONSORING MILITARY ACTIVITY
Air Force Materials LaboratoryTech., Other I Wright-Patterson Air Force Base,
_ Ohio 4543313. ABSTRACT
This report is concerned with three distinct aspects of poly_5erdegradation and stability with the general aim of the production ofmaterials of improved stability through a more complete understandingof their degradation mechanism and its relationship to chemicalstructure.
The photothermal degradation of copolymers of methyl methacryla:and n-butyl acrylate covering the whole composition range has beenstudied at 1651C. The gaseous and liquid products, which consist ofhydrogen, carbon monoxide, methane, methyl methacrylate, n-butylacrylate, n-butanol.and n-butyraldehyde were analysed quantitativelyusing a gas chromatographic technique and the yields related tocopolymer composition. Zip lengths, ca.cu•at-ed. from molecular
-'1- and atii4t4'--on. . data. decrease with increasin- acrvlate
content of the copolymer but are higher than thoseobserved duringthermal degradation. Infra-red spectral changes in the residue areattributed to lactone formation and associated with the formationof butanolo The "cold-ring" or chain fragm.ent fraction of theproducts becomes increasingly more abundant as the acrylate contentof the copolymer is increased and. spectral differences - UOLO
original polymer have been accounted for. All the uroducts andprincipal features of the reaction are explained in terms of a radicaiprocess initiated by scission of pendantI acryae s and proragrLe:_
DD' No .1473
Security Classification
TISecurity Cl1assification
1 4. LINK A LINK B LKEY WORDS
ROLE WT ROLE WT ROLE T
Degradation of PolymersPhotothermal DegradationThermal Degradation
Stability of Polymers
Copolymers, Degradation of MethylMethacrylate -
n-Butyl Acrylate
Gas-Liquid Chromatography . -
T.G.Ao
ToV°A.
Products of Degradation of Polymers
Friedel-Crafts Polymers
Poly(Dimethyl Siloxanes)
Infra-Red Spectroscopy
Pyrolysis of Polymers .
Mechanism of Degradation of Polymers
Thermal Analysis
i C I
.Security Classification
Abstract (cont'd)
by a combination of depropagation, intra and intermolecular transferprocesses the relative importance of which depends upon copolymer
.composition. Differences from the thermal reaction and thecorresponding reaction in copolymers of methyl methacrylate and methylacrylate are discussed.
It is shown that a number of aromatic compounds, both carbo-cyclic and heterocyclic, may be copolymerized with dimethoxy-p-xylene-with stannic chloride as catalyst. Purification of materials andpreparations of copolymers are described.
The preparations of a number of poly(dimethyl siloxanes) aredescribed. Thermal analysis using TGA and TVA have shown thatthermal decomposition starts at approximately 360 0 C but this isdepressed somewhat by reprecipitation. Neither heating to 130'C todestroy residual catalyst nor the molecular weight of the startingpolymer have any significant influence on the threshold degradationtemperature or the general characteristics of the thermal degradationreaction. End-blocking lowers the degradation temperature thresholdalthough the subsequent build up in degradation rate is slower. Theprincipal products of degradation are cyclic trimer and tetramer andg.l.c. and infra-red methods are being devised for their analysis.
