Crystallization of polydioxolan. III. Microscopy and dilatometric analysis of the crystallization kinetics

Crystallization of Polydioxolan. 111. Microscopy and Dilatometric Analysis of the Crystallization Kinetics

PIERRE ARCHAMBAULT and ROBERT E. PRUD'HOMME," Groupe de Recherches sur les MacromolBcules, Chemistr-y Department , Lava1

Uniitersity, Quebec 10, P.Q., Canada G l K 7P4

Synopsis

Crystallization kinetics has heen studied for a polydioxolan (PDOL) sample. over a wide temperature range. by dilatometry and microscopy. The dilatometry results can be analyzed using the Avrami equation. At temperatures higher than 22OC, the crystallization data must he analyzed in two steps: the first part of the curve corresponds to PDOI, with a very disordered morphology (Phase I 1 while the second par t of the crystallization curve is related to a spherulitic morphology (Phase 11). The passage from the low to the high crystallization temperature region is associated with a change in the Avrami exponent from 3 to 4. The crystal surface free-energy product oo', was found to he 18 X lo" erg"/cm-', very close to that of polyoxymethylene. The crystallization kinetics wa\ studied by microscopy over the temperature range -18 to 95°C. Gro\vth and nucleation rates were recorded. Two phases are ftiund only at temperatures higher than 22°C. The appearance of Phase I1 is related to a decrease in the growth rate of the sample. From the growth rates. the crystal surface free-energy product on<, was found equal to 17 X 10' erg2/cm'. The detailed analysis of the crystallization of the two phases reveals a complicated process which can he divided into four different steps: ( a ) growth of a disordered phase. Phase I: ( h ) nucleation of a higher birefringence structure: ( c ) propagation of a high hirefringence phase; and ( d ) spherulitic gnnvth. Phase 11. T h e analysis of PDOI, crystallization strongly suggests the presence of a hedrite + oval spherulite transition: the hedrite formation corresponds to step ( a ) . the oval formation to steps (b) and (c). and the spherulite formation to step ( d ) .

INTRODUCTION

In previous papers'.' it has been shown that polydioxolan (PDOL) has two phases when observed optically. These phases are associated with different crystal structures and different melting points. They are only obtained when the crystallization is conducted a t low degrees of supercooling. An example of this type of morphology is presented in Figure 1 for a sample crystallized at 25OC. It is observed that the inner phase, hereafter called Phase I, does not have the Maltese-cross appearance characteristic of a spherulite. Phase I is composed of disordered lamellas with a large number of extinction positions distributed almost randomly. However, the outer phase, hereafter called Phase 11, is clearly spherulitic. The melting points of Phases I and I1 were found to be 63 and 59"C, respectively. Their crystals have been shown to be modification I11 and Modi- fication I1 crystals, respectively.' On the other hand, PDOL crystallized a t large degrees of supercooling has a spherulitic morphology, a melting point around 57OC, and is made of Modification I1 crystals.

The reason why Phase I appears is not known. But the temperature conditions where it is found have been determined' and its thermal behavior has been es-

To \\horn all corre5pondence 5hould he addre5sed

,Journal of Polymer Science: c 1980 .John LViley & Sons. Inc.

Polymer Physics Edition. Vol. 18, 45-.50 (1980) 0098- 1 X 3 / 8 O / O O 18-00:35501 .OO

36 ARCHAMRAULT AND PKLJII’HOMME

Fig. 1. Photomicrograph of polytlioxolan crystallized a t 25°C

tablished.’ We believe that the knowledge of the crystallization kinetics of PDOL may help to answer this question. Using a DSC apparatus, a crystallization kinetics study of PDOL was conducted over the temperature range 0 to 21°C.’ However, the two phases appear only a t temperatures above 22°C. It is then the purpose of the present paper to conduct a crystallization kinetics study of PDOL by dilatometry and microscopy, over a wider temperature range, in order to obtain more information on the two phases of PDOL.

EXPERIMENTAL

The sample used in this study was prepared in our laboratory, by cationic polymerization, in bulk under vacuum a t 50°C. using triphenylmethyl hexaflu- orophosphate as initiator. The polymer was precipitated in a methanol-di- chloroethane (10:l) mixture, filtered, and dried in a vacuum oven.

The intrinsic viscosity of the unfractionated PDOI, sample was determined in chlorobenzene a t 25°C. A value of 0.164 dl 9-l was found which corresponds, according to Pravikova et aL;j to a molecular weight of 6.7 X 10:’ g mole-’. The unfractionated sample was used for the dilatometry. The polymer was then

CRYSTALLIZATION OF POLYDIOXOLAN 37

fractionated by allowing the PDOL to precipitate from a methanol-dichloro- ethane solution by slow cooling. The intrinsic viscosity of this fraction, which was used for the microscopy work, was 0.195 dl g-l (in chlorobenzene, a t 25°C). I t corresponds to a molecular weight of 9.4 X loz3 g mole-'.'

The thermal stability of the polymer was checked by measuring its intrinsic viscosity and by recording its gel permeation chromatograph?; (GPC) curve before and after keeping a sample a t 80°C for 24 hr. No detectable change was observed.

Dilatometric measurements were conducted by the method described by Bekkedahl." A U-shaped dilatometer, with a 1.25-mm-diam capillary, was used. Mercury was the confining liquid. About 2.6 g of PDOL was necessary. Usual corrections were applied to the data."; Before each crystallization measurement, the sample was allowed to melt for 4 hr a t 80°C in order to completely destroy its crystallinity. The bath temperature was regulated a t *O.l"C.

Crystallization kinetics measurements were conducted on a Zeiss polarizing microscope equipped with a Polaroid or a 35-mm camera. The latter camera makes it possible to record photographs every second. The sample was main- tained a t the desired temperature using a Mettler hot stage, or a homemade stage.

RESULTS

Dilatometric Results

In Figure 2, the specific volume-temperature curve of PDOL is presented. The heating curve was recorded a t a rate of 2"C/min for a sample crystallized by slow cooling to 0°C. Melting occurs between 48 and 58°C which is in agreement with the differential scanning calorimetric (DSC) melting curves reported previously for a sample crystallized in similar conditions.' The cooling curve was also re-

COOLING CYCLE 0 HEATING CYCLE

.wo-

h

'w .820- U

v

W I ,800- 3 -I

I 9 I g .780- I Y 0 w a v) ,760-

I I

0 2 0 40 60 I 3 Temperature e0

Fig. 2. Specific volume-temperature curve for polydioxoian. ( m ) Cooling cycle; ( 0 ) heating cvcle.

38 ARCHAMBAULT AND PRUII'HOMME

corded a t a rate of 2"C/min. From Figure 2, the specific volume of PDOL can be written

un (5") = 0.796 + (0.764 X 10--')T (1)

where u, (7 ' ) is the specific volume of' the amorphous melt a t the temperature T expressed in "C. Similarly

Ll, ( T ) = 0.734 + (0.620 x 10-,')T ( 2 )

where u, (7') is the semicrystalline specific volume of PDOL having a degree of' crystallinity of 58%. Equations (1) and (2) are, of course, only valid in the temperature range covered in Figure 2. Assuming that the density of the amorphous solid polymer is given by eq. ( I ) , and taking the density of the pure crystal to be 1.41 g ern-,' (ref. 3 ) (only Modification I1 crystals are found under the crystallization conditions followed'), one calculates on the basis of eqs. (1) and (2) a degree of crystallinity of 58571, a t 25"C, for the PDOL sample. This value is slightly larger than the values previously obtained from isothermal crystallizations,2 but it is similar to the value reported by Clegg and Melia.8 The above equations are in general agreement with those reported by Binet and Leonard for the specific volumes of' PDOL in solution in its own monomer a t 18.34% (w/w) concentration.!'

As shown in Figure 3, the isothermal crystallization of PDOI, was followed dilatometrically over a temperature range of 19.5 to 35OC. This is the range where two phases are observed optically.' However, the dilatometric mea-

0.820

0.810

0.800 - 4 0 - 0.790

2 A

9

8 0 !& 0.780

P fn

0.770

0.760

1.0 2 .o 3.0 4.0 5.0

Log t (S)

Fig. 3. Crystallization isotherms o1)serveti by dilntomet ry.

CRYSTALLIZATION OF POLYDIOXOLAN 3 9

surements were mainly designed to study Phase I which is not spherulitic since Phase I1 crystallizes in a manner very similar to other spherulitic polymers, as has been shown.' A t 19.5 and 22OC, only one spherulitic phase is observed by microscopy and the crystallization isotherms appear, after a very short induction period, in the time interval 100-1000 sec. A t 25, 28, and 35OC, the induction period becomes longer, but the crystallization process on a log time scale is similar to the crystallization a t lower temperatures.

The initial crystallization step is however followed by a knee, then another rapid increase in density and finally a slow crystallization step similar to that usually observed for secondary crystallization. The knee is well defined for the 35°C isotherm. It was also apparent a t 25 and 28°C on the original graphs but these details were lost in the reduction of the graph necessary for publication. I t is indicated by arrows in Figure 3. The time where it appears is only approximate since it does not correspond exactly to the observations made in microscopy for reasons explained in the section on microscopy results of this paper.

By comparing the dilatometry curves with the microscopy results, we now wish to postulate that the knee corresponds to the Phase I - Phase I1 transition since it only appears a t temperatures where two phases are observed in microscopy. On this basis, the rapid increase in density following the knee corresponds to the crystallization of Phase 11. A t 25°C. only a small fraction of the sample is spherulitic since the increase in density corresponding to Phase I1 crystallization is small. This conclusion is also reached from direct microscopic observation. However this fraction increases with increasing crystallization temperature. A t all temperatures, the increase in density due to Phase I1 crystallization is small compared to the change in density induced by Phase I crystallization.

These results can be analyzed using Avrami's equation

[I - cb(t)] = exp(-Ztr7) (3)

where d ( t ) is the fraction of the polymer already crystallized a t time t ; n is a constant, the Avrami exponent; and Z is a second constant related to the crystal growth rate. In terms of specific volumes, d ( t ) can be written

(4)

where ~ l ( t ) , u ( 0 ) , and u ( m ) are the specific volumes of the sample a t times t , 0, and a. As usual, eq. ( 3 ) can be written

(5) where it is observed that the plot of the left-hand side of eq. (.5) as a function of logt will give a straight line of slope n . The constant 2 is usually obtained from the relation

Z = lna/(to,)r7 (6) where to ;r is the time of half-crystallization.

The results of Figure 3 are plotted in Figure 4 in agreement with eq. (5). In order to draw eq. (5), we have postulated, in the higher-temperature range, that L I ( ~ ) is given by the plateau observed a t the end of the crystallization of Phase I. This procedure is similar to the usual procedure where the primary and secondary crystallization processes are separated since the Avrami equation is only applicable to the primary process.]').'

( b ( t ) = [ u ( t ) - L J ( O ) ] / [ U ( W ) - O(0)l

log{-ln[l - 4( t ) ] l = n logt + log2

40 ARCHAMBAULT AND PRUD'HOMME

I . 2.0 2.6 3.2 3.0 4.4 5.0

LOG t (s)

Fig. 4. Avrami equation applied to polydioxolan.

6

Each isotherm can be represented by a straight line. As shown in Table I, Avrami exponents of 2.8,2.8,4.1,4.1, and 3.0 are found at 19.5,22,25,28, and 35"C, respectively. The increase in Avrami exponent from 22 to 25°C suggests a change in nucleation mode in that temperature range. Indeed, an Avrami exponent of 3 is usually associated with a predetermined mode, while a value of 4 is related to a sporadic mode of nucleation.1° On the other hand, the value of 3.0 found at 35°C suggests that a t low degrees of supercooling a second change in nucleation mode might occur. The microscopy observations also indicate a change in nucleation mode between 22 and 25°C although a pure sporadic mode is not obtained. It rather seems that there is a transition from a predetermined mode to a mixed mode of nucleation which returns to predetermined at 35°C. The increase in Avrami exponent at 25 and 28°C might also be explained by the growth of a disordered structure since increases in Avrami exponents are expected for sheaflike growth^.'^-'^ A t 35"C, the Phase I structure is certainly more or- dered than it was at lower temperatures, indicating the possibility of obtaining a single phase again at higher crystallization temperatures, as well as a lower Avrami exponent. A t the present time, our observations do not allow us to distinguish between these two possibilities.

TABLE I Avrami Analysis of Polydioxolan

Crystallization temperature ("C) n t0.s (mid z

19.5 2.8 4.0 1.1 x 10-2 min-3 22 2.8 5.7 3.7 x 10-3 min-3 25 4.1 10.5 5.7 x 10-5 min-4 28 4.1 22 2.8 X min-4 35 3.0 1.4 x 103 2.1 x 10-10 min-3


The values of the Avrami exponent found a t 19.5 and 22'C are in agreement with the value o f 2 observed in the same temperature range by calorimetry' if one takes into account that the calorimetry experiment only allows the growth of' two-dimensional structures because of sample size limitations while the dilatometry experiment generates three-dimensional morphologies. A value of unity must be added to the two-dimensional result to transform it in a three- dimensional va1ue.l"

The equilibrium melting point of PDOL, Tji2, is 352°K and its enthalpy of f'usion, AH::, is 17 kJ Using these values, as well as those of to,, reported in Table I, it is possible to determine fit, and 0, which are, respectively, the crystal surface free energies in the directions parallel and perpendicular to the chain direction. As a matter of fact, t(1.5 can be related to AF*, A(/)*. and T,, which are, respectively, the transport free activation energy at the liquid-solid interface, the free energy of formation of a nucleus of critical size and the crystallization temperature, by the relation]" I ;

( 7 )

where k and R are the Boltzmann and gas constants, respectively. The to.; values reported in Table I include the induction time. Strictly speaking, the t ( l . 5 used in eq. ('7) should exclude the induction time. However in practice, both set of values lead to the same slope within the experimental error. The Williams- Landel-Ferry equation can he used for AF*:

l11(t~~.;) - AF*/RT,. = A0 + Aqb"/hT,.

AF" 2.303 C'ICI

RT, __- -

C L + ( T , - T ? ) where C1 and C j are "universal" constants equal to 17.4 and 31.6 K , respectively,]' and T: is the glass-transition temperature. As shown in Figure 3, T, is equal to 218 K. This curve was obtained on a Perkin-Elmer DSC-2 apparatus, a t a heating rate of lO"/min. The quantity A(b* is given by1'

A4* = Ybonn, T):,/AHljAT (9)

where bo is the length of the surface nucleus, A T is the degree of supercooling o f the sample, and Y usually equals 4. It has been shown for polyethylene that, a t large degrees of supercooling, Y should be replaced by a value of 2.lY In this instance, the low crystallization temperature region is where the spherulitic morphology is found, while at high crystallization temperatures, the spherulitic morphology is replaced by an axialite morphology. I t is tempting to apply the same argument to PDOL and to analyze the Phase I results in the same

180 200 220 240 260

TEMPERATURE ( K )

Fig. 5. I)SC trace of polydionolan in the glass transition temperature range.


manner as the polyethylene axialite structure ( Y = 4) while analyzing the Phase I1 crystallization range in a different manner ( Y = 2). However, for polyoxymethylene, a different result is observed: a t low degrees of supercooling, the growth rate-temperature dependence decreases while hedrites and ovoids are formed instead of axialites.'"." I t is then difficult to predict the exact value of the numerical coefficient of eq. (9) for PDOL without further analysis. In the meantime, the value of Y = 4 will be retained.

In Figure 6, a graph of [In(to 5) - aF*/RT, ] vs. T!,/T, AT is given. A straight line can be drawn and a least-square analysis leads to a value of 18 x 10" erg'/cm4 for rrr,, using a value of 0.779 for bo." One can also write the relation

(T = tuAH1)bo (10)

where (Y is a constant whose value is 0.10 for most polymers.22,2:1 Values of 25 and 72 erg/cm" are then obtained for (T and rc>, respectively.

As seen in Table 11, similar values are obtained for PDOL from the analysis of the results presented in ref. 1, or from the analysis of the crystallization kinetics of PDOL followed by microscopy (presented in the next subsection of this paper). The values of rrr, for PDOL are high compared to those of other polymers. Since PDOL is the alternating copolymer of methylene oxide and ethylene oxide, a

2.00 2.10 2.22 2.23 2.24 2.25

(Tm /Tc A T ) X lo2

2.00 2.10 2.22 2.23 2.24 2.25

(Tm /Tc A T ) X lo2 Fig. 6. I'olydioxolan growth rate-temperature dependence analysis.

TABLE 11 Interfacial Free Rnergies of I'olydioxolan arid of' Some Other Polymers

l ' I~OI,-Dilat~~rnetry ~'I)OI,-Microscop?, Phase I I'I)OI,~Microscopq. Phase I1 I'DOI,-DS(I (ref. 1) I'olyethylene oxide (refs. 25 and 266) I'olyox?methylene (rel's. 21 and ? 2 )

( a ) low temperature ( 1 ) ) high temperature

Polyethylene (ref . 20)

0.779 + lil6 18 0.807 - I ; { ] 17 0.779 -146 1 9 0.579 -145 19 . . . . . . 6.40

0.446 . . . 2 1 .oo 0.446 . . . 15.55 0.415 . . . 12.6?

7 2 26 66 25 83 2 .i 78 8 80

'7 .- d)

23 9 5 17 91 14 90


comparison with the an,, of these two homopolymers seems appropriate. From Table 11, it is seen that the no,, value of PDOL is of the same order of magnitude as those found for polyoxymethylene (POM),"-.":' but quite different from those found for polyethylene oxide (PEO).24,2r) Similar behavior was found for PDOL by Riande and Mark by measuring conformation-dependent properties, such as dipole moment.'6 The conformational properties of PDOL cannot be obtained by averaging the values of the same properties for POM and PEO. The PDOL property is generally close to that of POM.2fi

The nuG2 value of PDOL is large compared to that obtained for polyethylene (PE), but this behavior is often found in the polyoxide A t the same time, a, is slightly smaller and CT is larger than the corresponding value of PE. However, a is much larger than that of PEO in agreement with other published data.2x These results indicate that it is slightly easier to form a fold in polyoxides than in P E because of the -C-0-C- linkages; but a t the same time, the energy associated with the regular crystal plane (a) is much higher for PDOL compared to other polymers.

Reported values of an,,, n, and n, are of course dependent upon the value chosen for bo. In fact, bo is probably overestimated. Lowering bo, however, would further increase oa,, and would strengthen our conclusions. On the other hand, the use of Y = 2 (Regime 11) in eq. (2) instead of Y = 4 (Regime I) would lead to unreasonably large an,, values. Hoffman and his co-workers have pro- posed a criterion which permits one19.":i to distinguish between these two possibilities. The application of this criterion to our results also leads to a Re- gime I crystallization. I t then seems clear that one must choose Y = 4 for PDOL whatever morphology is found.

Microscopy Results

The crystallization behavior of PDOL was followed by microscopy in a temperature range between -3 and 35°C. A t low temperatures (22°C or less), a classical crystallization behavior is observed with the growth of one-phase spherulites having dimensions of the order of 1 mm. A t 19"C, larger spherulites are formed but the Maltese cross appears less defined. In some spherulites, the crystallization does not seem to have given rise to a well-defined morphological order. Often, the Maltese cross is still seen for each spherulite, but it is faint, indicating a low degree of order as compared to the spherulites obtained a t larger degrees of supercooling.

A t 25"C, the morphological growth follows a peculiar path leading clearly to two optically distinct structures (Fig. 7). The whole process can be separated into four different steps: (a) initial growth; (b) birefringence nucleation: (c) birefringence propagation; and (d) spherulitic growth:

(a) Initially, very disordered structures grow radially [Figs. 7(a) and 7(b)]. This step seems to be similar to that seen a t larger degrees of supercooling except that the spherulitic order is not present anymore.

(b) Then, a t about 5000 sec, some of the structures rapidly acquire a high value of birefringence compared to others [see the bottom left-hand structure in Fig. 7(c)]. This change in superstructure is surprisingly not related to a change in the degree of crystallinity of the sample because it does not correspond to a significant change in the specific volume of the sample as seen in the previous subsection.

44 ARCHAMBALJLT AND PRUD'HOMME

U

b

C

d

e

f

Fig. 7. I'hotomicrographs i j f ' t h e growth process of polydioxc~lan at X 0 C : crystallization tinie (sc'c): (a) 316. ( b ) 1585. (c ) 501'. td , 5315. ( e ) 3675. (1') 79.1:;.

(c) When a structure of high birefringence impinges upon one of low birefringence, the high birefringence structure crystal nucleates a high birefringence nucleus on the latter structure and this process is followed by a birefringence propagation step which is extremely rapid [Fig. 7(d)]. The growth rate of this process is of the order of 30 X lo-,? cm sec-' a t 25°C compared to a growth rate of 2.55 X lo-: cm sec-' for the normal structure a t the same temperature (Table 111). The structures where the nucleation step has not occurred remain un- changed as in step (a) and they continue to lead to a disordered morphology while those where the transformation has already occurred grow spherulitically [Fig.

(d) Finally, most structures transform into high birefringence structures and their growth leads to normal spherulites although the maltese-cross pattern is not very well defined.

Similar observations have been made for samples crystallized between 25 and 35°C.

Quantitatively, the increase in diameter of all morphological structures was followed, a t different temperatures, as a function of time. These results are presented in Figures 8-10. At temperatures lower than 22"C, the growth rate

7(e)J.


' r m w 111 Polydioxolan Nucleatirm and Growth Rates

(; x 10' c: x 10' t I\' (cm sec-') (cm sec-')

("C) (mm-') (Phase I ) (Phase 11)

-18.0 -9.8 -3.0

8.n 14.0 16.0 17.0 19.0 22.0 25.0 28.0 31.0 :34.0

11 11 7.0 0.95 0.77 0.27 0.24 0.:30 0.18 0.25 0.24 0.15 . . .

18.4 :19.8 46.8 56.5 54. I 24.5 2 6 2 17.2 3.85 2.55 0.945 0.291 0.021

. . .

. . .

. . .

. . .

. . .

. . .

. . . ( X 7 P (2.4);' 0.900 0.100 0.009

I' Approximate values

is linear over the complete time interval. A t temperatures of 25,28,31, and 34"C, a well-defined break is observed in the growth rate curve. I t is important to realize tha t the diameters reported in Figures 8-10 are values obtained by following the growth rate of a single structure. The 25OC curve of Figure 9 does

0 295 -

0 210 - - - 5 z

E a 0 1 2 6 -

- E

4 0 0 800 1200

0 0 4 2

0

Time (s)

Fig. 8. Polydioxolan growth rates a t -9.8. 8.5. 17 , and 19" r

46 ARCHAMBAULT AND PRUU’HOMME

I

1 I

0 295 c

= I 280”C, 1 9’ e ,

a W

7000 11000 15000 3000

TIME (s)

FIX 9 Pol \ t i ioxc i l~ i i growth r , i t c i ‘it 2 2 , 2 i , ant i 2 8 ” r

not correspond exactly to the particular example shown in Figure 7. The break seen in Figure 9 occurs a t long times while the example presented in Figure 7 was chosen to illustrate the different steps of the two-phase structure growth. This latter example is one where Phase I1 is exceptionally important a t that temperature. Rut its growth rate is not very different from that of Phase I. In fact,

10000 30000 50000

0.2C5

I

c

0.210 h

E, v

a W c W 5 0.126 - E

0.042 1 J

0 100000 30000@ 500000 TIME ( s )

Fig. 1 0 . I’~ilydiosolan growth ratvs a t : i l and :l-to(’.

CKYSTAL,I,IZA'I'ION OF POLYDIOXO1,AN 47

as shown in Table 111, the difference between the growth rates of Phases I and I1 increases with t h e crystallization temperature. Similarly, it is clear from Figure 8 tha t t he growth of t h e morphological structures follows an induction period which increases with the crystallization temperature. This induction period is related t o t he primary nucleation rate which f'ollows the same trend. In all cases. t he growth rate of Phase 11 is slower than tha t of Phase I.

T h e breaks observed in Figures 9 and 10 corresjxmd to the appearance of Phase I1 by direct observation. I t however appears tha t t he t ime a t which the transformation occurs does not correspond tci t he characteristic breaks previously obtained in dilatometry. One can see tha t t he crystallization i n dilatometry occurs much more rapidly than in microscopy. In some instances ( a t 22OC), the

tallization in dilatometry is completed before it is observed in microscopy. lie must then be very careful in making comparisons between the t w o sets of

results, particularly concerning the times of cr llization. This difference in hehavior is partly related to differences in polydispersity and molecular weighk of'the two samples, and partly to the different environments used in the two sets of' experiments, mercury in dilatometry, and glass in microscopy.

T h e growth rates recorded as a function of temperature are given in Table 111. A bell-shaped behavior is apparent even if t h e low-temperature portion is not well defined. T h e maximum crystallization rate is found at about 8°C. T h e nucleation rates N recorded at different temperatures are also presented in Table 111. A s expected, they increase rapidly with increasing degrees of supercooling finally giving rise to small spherulites. ' ; As has been shown before for polyethylene a n d polychlorotrif'luoroeth\ilene.'~ the general tendency of t he nucleation ra te is similar t o tha t of the growth rate. except tha t t he maximum in the curve occurs at lower temperatures for N. Consequently, even if t he maximum in C is seen in Table 111, it is still not apparent for N. Finally, in Figure 10, a graph of (InC + 1 F / R T , ) vs. T,7z/(7', 17') is presented in accordance with eq. ( 7 ) . From the slope of this graph, and using the Lrarious parameters already mentioned in the previous subsection, one ohtains values of 17 X 109 and 19 X 10' erg'/cm' [ ( I 7 a n d 19) X .J/cmlj, for nn,,, for Phase I and Phase 11: respectively, according to a least -square analysis. As discussed before. these values

en t with those found by dilatometry atid calorimetry (Table rence between the two quantities obtained for Phases I and

I1 cannot be considered as significant in view of' the different errors made in the measurement of' t he growth rate. and considering also the :issumptions made i n t he computation of o m ( , .

DISCUSSION AND C ONCLU SI 0 N

A microscopy study of the crystallization kinetics of PDOl, confirms previous measurements: two phases can he grown at temperatures higher than 22°C. Moreover, Phase I grows more rapidly than Phase 11. and the tliff'erence in growth rate increases with crystallization temperature, Even i f the difference between the d u e s of' obtained f o r the t w o phahes is very small and prolmbly not significant, the existence of the two phases is certainly related to a dil'f'erence in crystal structure since i t has been s h o n n bef'ore that different melting points and different x-rah- patterns are found for t he two phases.'

However. since the melting points of' the two samples are close, and since their

48 ARCHAMBAULT AND PRLJD'HOMME

'1 CENTRAL PHASE I ' \ ,. \ < \

0.019 0.024 0.029

EXTERNAL PHASE 0

Fig. 11. Polydiosolnii growth r;rle-!emperature dependence analysis. ( 0 ) Central phase: ( 0 ) external pliaw.

cr) rstal structure dimensions are also very close, it is possible that the major difference between the two phases is of a morphological nature. As a matter of fact, the behavior of Phase I is similar to that ohserved for hedrites2x while Phase I1 is clearly spherulitic. I t has been reported that, hedrites, when viewed between crossed polars, show a very low value of' birefringence as we have observed for PDOL in Figure 7 . Rut a t the same time, the hedrites are generally made of lamellae, and have a relatively high value of crystallinity. These two observations indicate tha t the molecules in the hedrites are oriented normal to the surface of the film. The observed birefringence is due t,o the imperfect alignment of the laniellae.2s For PDOI,. the dilatometric results indicate that the growth of Phase I is accompanied by a significant increase in the degree of crystallinity of the sample. In fact, the increase in crystallinity observed during the growth of Phase I is larger than that occurring during the growth of the spherulitic phase. This observation is certainly in agreement with the presence of hedrites in PDOL.

In gutta percha,"!' polq.oxymeth~lene,:"'~'~' and polv-l-methyl-peitene-l,"2 it has been shown that the formation of' hedrites is favored at high temperatures. bu t that, regular spheruiites are found when the crystallization is done a t low temperatures. Similar ohservations have I)tvii made for PI)OI, in previous studies' as well as in the present one.

Moreover. gutta percha hedrites melt at .58"(: while its spherulites melt at ,X"C.~+' This result is again very similar to that found for PDOI, where Phase I presents a slightly larger melting temperature than Phase 11. In view of'the ohservation5 quoted above. it is then suggested that Phase I o f PDOI, is hedritic while Phase IJ is spherulitic.

It W'W seen At temperatures higher than Z 0 C . h d r i t e s develop i i i PI>OI,.


in Figure 7 that a t one point the hedrites become highly birefringent. Normally, the hedrites have a low value of birefringence when observed under crossed polars because the crystal c axis is oriented along the sample normal. In PDOL, the sudden appearance of high birefringence suggests a hedrite - oval transition where the crystal c axis would change from normal to tangential to the film surface although t,he driving force for such a change is unknown. Indeed, the ovals are known to be structures which are quite similar to the hedrites except that they have a high degree of birefringence.28 The degree of crystallinity of the two structures is usually of the same order of magnitude's and consequently a hedrite - oval transition does not need to be related to a change in the degree of crystallinity of the sample. For PDOL, the suggested hedrite - oval transition would not correspond to any significant change in the degree of crystallinity, as reported in the dilatometry analysis.

Once the high birefringence structure has been obtained, the additional crystal growth leads to a spherulitic structure with its crystal c axis i n the tangential position (a negative sign of birefringence has been noted). This observation suggests that spherulitic growth can only be obtained from ovoids but not from hedrites. I t is then suggested that the higher crystallization temperatures favor the occurrence of screw dislocations from which hedrites are known to grow.'8 As long as hedrites are present, the spherulites do not appear. When the hedrite - oval transition occurs, the appearance of the tangential c axis orientation of the lamellae favors the growth of spherulites a t subsequent times. On the other hand, the lower crystallization temperatures allow the growth of spherulitic structures.

The spherulite - hedrite transition around 22°C is finally quite similar to the spherulite - axialite t,ransition of polyethylene (PE)'" and to the spherulite -+ hedrite transition of polyoxymethylene (POM).'0.21 In all cases, the more irregular morphology (axialities or hedrites) is found a t low degrees of supercooling. However, for P E and POM. the growth rate-temperature dependence changes significantly in the transition region, even if it does not change in the same sense in both cases. For PDOL, the results obtained in this paper, as well as those obtained by dilatometric measurements. do not indicate any significant change in growth rate-temperature dependence a t the spherulite - hedrite transition temperature even if the growth rate of the spherulite is slightly different from that of the hedrite a t a given temperature. I t can also be mentioned that in PE the spherulite - axialite transition is only seen for samples of molecular weights between 2 X 10' and 2 X 10" g/mole.I9 For samples having lower molecular weights (as that of the PDOL used herein), the transition does not occur and the morphology is mainly axialitic. Higher-molecular-weight PE's have a spherulitic morphology.

The dilatometric analysis of the crystallization kinetics of PDOI, also confirms that a t temperatures larger than 2ZoC, the nucleation and growth processes are

rent from those occurring at lower temperatures. In microscopy, a second phase is observed optically; i n dilatometry, breaks appear in the specific volume-time curves, and an increase in Avrami exponents is found. This increase is related to a change in the nucleation mode of the sample and to the develop- ment of' a complex morphology, since an increase in the number of nuclei with time as well as a special morphology of the sample are observed in the microscope.


The growth rate-temperature dependence of PDOL leads to high values of mmlt and c which are of the same order of magnitude as those found for polyoxymethylene, but much larger than those of polyethylene oxide. Even if PDOL is an alternating copolymer of POE and YOM, its crystallization properties seem to be closer to those of POM than to those of POE.

The authors thank the National Research Council of' ('iuiada and the Ministry of' Education of the Province of QuBhec ( W A C program) for the research grants that supported this work.

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Received January 3,1979 Revised April 17, 1979 Accepted May 4,1979

Documents

Crystallization of polydioxolan. III. Microscopy and dilatometric analysis of the crystallization kinetics