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CHAPTER 1: INTRODUCTION
1.1 THERMALLY STABLE POLYMERS
Thermally stable / High performance polymers (HPPs) are resistant to oxidative
degradation at elevated temperature, resistant to thermolytic process and stable to
radiation and chemical reagents. These polymers are able to provide long-term service at
elevated temperatures. The development of different thermally stable polymers is
continued due to the large requirement of high temperature polymers.
Thermally stable polymers are widely used in electrical, electronics, automotive
and aerospace industries. The aerospace industry was a significant driving force behind
the development of new materials. The research and development of HPPs was started in
the year 1960. The most prolific decade for HPPs was 1960 -1970 when thermally stable
heterocyclic rings were incorporated into polymers.
A large number of thermally stable polymers have been synthesized. In these the
thermal stability is imparted by the high regularity and rigidity of the backbone. These
types of polymers exhibit poor processing characteristics due to insolubility in common
organic solvents and high melting temperature. Many efforts have been taken to improve
the processability without decreasing the thermal stability. Among the class of thermally
stable polymers a few are technically important and they are polyoxadiazole,
polyquinoxaline, polyquinolines, hexafluoroisopropylidene containing polymers,
hexfluroisopropoxy group containing polymers and polyimides. Of these polyimides have
been the most widely used.
2
1.2 POLYIMIDES
Aromatic polyimides (Figure 1.1) are one of the most important classes of high
performance polymers due to their excellent thermal, mechanical and electrical
properties.1,2
Polyimides are polycondensation products prepared from derivatives of
tetracarboxylic acids and primary diamines3-7(Scheme 1.1). Polyimides are also formed
by the self-polycondensation of amiodicarboxylic acid derivatives8,9
(Scheme 1.2).
N Ar N Ar'
O O
OO
Figure 1.1: Structure of polyimides
OH
OCH3
HO
H3CO
O
O
O
O
+ H2N (CH2)2 NH2HO
H3CO OCH3
ONH3
O
O O
O
(CH2)2 NH2
NN (CH2)2
n
O
O O
O
derivatives of tetracarboxylic acids
primary diamines
polyimides
Scheme 1.1: Synthesis of polyimide
3
H2N
O N
n
+ H2O
O
O
O
O
aminodicarboxylic acid polyimides
Scheme 1.2: Self-polycondensation of aminodicarboxylic acid derivatives
Polyimides backbone contains heterocyclic imides unit. Among imides containing
polymers, polyimides derived from aromatic tetracarboxylic acid and aromatic diamines
are of primary importance and represent typical high performance speciality polymers
that are commercially employed in various applications.
Historically the first report concerning the polyimides was made by Bogert and
Renshaw in 1908 which was synthesized starting from 4-aminophthalic anhydride.
Edwards and Robinson first prepared high molecular weight polyimides by the
polymerization of salts formed from diamines and pyromellitic acid or related diester.10
Endrey used a two step method to prepare the first commercial aromatic
polyimide (Scheme 1.3) from pyromellitic dianhydride and 4,4’-diaminodiphenyl ether
in the year 1960.11-15
This polymer was patented by Du Pont and is known by the trade
name Kapton. In the two-step method, a soluble polyamic acid was prepared first and
then converted into polyimide.
Vinogradova and coworkers developed a one-step method to prepare soluble
aromatic polyimides.16
This method involved the direct polymerization of diamines and
dianhydrides at an elevated temperature in the presence of tertiary amines.
4
O O
O
O O
O
+ H2N O NH2
RT, DMAc
NN
O O
OO
On
HN
HO
NH
OH
O
O
O
O
O
n
-H2O
poly(amic acid)
PMDA ODA
Kapton
Scheme 1.3: Synthesis of kapton polyimide
1.2.1 Synthesis of polyimides
Various synthetic methods for the polyimides have been described. A successful result
for each method depends on the nature of the chemical components involved in the
system, including monomers, intermediates, solvents and the polyimides products, as
well as on the physical condition during the synthesis. Properties such as monomer
reactivity, solubility, glass - transition temperature (Tg), crystallinity (Tm) and melt
viscosity of the polyimides products ultimately determine the effectiveness of each
process. Accordingly proper selection of synthetic method is often critical for the
preparation of polyimides.
5
1.2.1.1 Two-step method
In the two-step method, a soluble polyamic acid is prepared by the condensation of a
tetracarboxylic acid dianhydride and diamine in polar approtic solvent such as NMP,
DMAc, or DMF at or below room temperature17-19
(Scheme 1.4). The resulting polyamic
acid is then converted into the polyimides via imidization.
O Ar O NH2Ar'H2N
O O
OO
ArHO NH
OH
O O
OO
NH
Ar'
-2H2ON Ar N Ar'
O O
OO
+
dianhyride diamine polyamic acid
polyimide
Scheme 1.4: Synthesis of polyimides by two-step method
1.2.1.1.a Formation of polyamic acid
The mechanism of formation of polyamic acid involves the nucleophilic attack of the
amino group on the carbonyl carbon of the anhydride group, as shown in Scheme 1.5. In
this equilibrium reaction, the forward reaction is much faster than the reverse reaction.
The acylation reaction of amine is an exothermic reaction.20
The forward reaction in a
dipolar approtic solvent is a second order reaction and the reverse reaction is a first order
6
reaction. Therefore, the forward reaction is favoured at low temperature and high
monomer concentration to form high molecular weight polyamic acid.21
O
O
O
+ NH2O
O
NH2O
O
NH2
O
O
OH
NH
O
O
Scheme 1.5: Mechanism of formation of polyamic acid
1.2.1.1.b Monomer reactivity
The reactivity of the monomer is an important factor in deciding the rate of polyamic acid
formation. The nucleophilicity of the amino nitrogen atom of the diamine and the
electrophilicity of the carbonyl group of the dianhydride are important factors in
determining the rate of the reaction. There is close relationship between the reaction rates
and the electron affinity Ea of dianhydrides. Dianhydrides with electron withdrawing
group enhance the Ea value and reactivity, where as electron donating substituents
rreduce reactivity. The nucleophilicity of the amino nitrogen atom of diamines enhances
their reactivity toward a given dianhydride. Electron-donating substituents increase the
reaction rate but electron withdrawing substituents, particularly located at para position to
the amino group reduce the reactivity. The diamines with basicity pKa of 4.5-6.0 are
more suited for the acylation reaction.22
7
1.2.1.1.c Effect of solvents
The most commonly used solvents are dipolar approtic solvents such as DMAc, DMF,
DMSO and NMP. The use of polar approtic solvents that form strong hydrogen bond
with the carboxyl group and prevent the reverse reaction favours the amic acid
formation.23
Esterification or neutralization of carboxylic acid group with tertiary amine
can also prevent the reverse reaction. The resulting polyamic acid is then converted into
the polyimide by the extended heating at elevated temperature called thermal imidization
or by treatment with chemical dehydrating agents called chemical imidization.
1.2.1.1.d Thermal imidization
The polyamic acid is processed into desired shapes such as films or fibre and then heated
at 250 to 400 0C to remove the solvent and water. This process is called thermal
imidization. One of the popular heating cycles is 1 h at 100 0C, followed by 1 h at 200
0C
and 1 h at 300 0C.
24, 25 The imidization is accompanied through nucleophilic attack of the
amide nitrogen on the acid carbonyl carbon with elimination of water. Two possible
reaction pathways can occur during the thermal imidization26
(Scheme 1.6).
In the first pathway (a), the proton is lost after cyclization, while it is removed
prior to or during the ring closure in the second pathway (b). Ring closure occurs
considerably faster in second pathway since the nucleophilicity of the conjugate base of
the amide is much stronger than that of the amide. Therefore, fully ionized amine salts of
polyamic acids imidize up to ten times faster than the corresponding polyamic acid.
Thermal imidization is particularly effective for the preparation of thin materials such as
8
films, coatings, fibres and powders because it allows the diffusion of the byproducts and
the solvent without forming bristles and voids.
NH
OH
O
O
N
O
OHO
NH
O
OHHO
N
O
O
(a)
(b)
NH
OH
O
O
NH
O
OO
N
OH
O
O
N
O
O OH
1.+ H
2 .- H2ON
O
O
-H
H
Scheme 1.6. Mechanism of thermal imidization of polyamic acid
1.2.1.1.e Chemical imidization
Imidization of polyamic acid can also be performed by means of chemical dehydration at
ambient temperatures. Commonly used reagents are dehydrating agent and a basic
catalyst.27
Among the basic catalysts used are tertiary amines, such as pyridine, methyl
pyridine, trimethylpyridine, triethylamine, and isoquinoline. Among the dehydrating
9
agents used are acetic anhydride, propionic anhydride, butyric anhydride and benzoic
anhydride, etc.
O
NH
O O CH3
O
R3N + (CH3CO)2O H3C C NR3
O+ CH3COO
NH
OH
O
O
R3NNH
ONHR3
O
O
CH3 C NR3
O
NH
O CH3
O
O
O
NH
O CH3
O
O
O
route a
route b
NH
O O
O
CH3
O
CH3COOHN
O
O
CH3COOH
O
N
O
Scheme 1.7: Mechanism of chemical imidization of polyamic acid
The mechanism of chemical imidization28
is shown in Scheme 1.7. The tertiary
amine plays two important roles in the conversion of the acid hydroxyl group to a better
leaving acetate group. The tertiary amine converts the acetic anhydride to its salt form,
which is more susceptible to nucleophilic attack. Secondly, it converts the aromatic
carboxylic acid to its ionic conjugate base, which is a better nucleophile. Ring cyclisation
can proceed via a second nucleophilic substitution reaction by two different pathways. In
route “a” attack of the nitrogen atom generates an imide ring, while attack of the oxygen
10
atom generates an isoimide ring in rout “b”. At first, both products can be formed.
However, since the isoimide is kinetically favoured product, it then rearranges to
thermodynamically stable product imide. This reaction is catalyzed by the acetate ion
present in the reaction mixture. The mechanism is shown in Scheme 1.8.29
The reverse
reaction cannot occur since the carboxylic acid proton is eliminated in the chemical
imidization. The degree of imidization of the final polymer depends on the solubility. In
general, polymer that is more soluble will remain in solution longer and reach high
degree of imidization. Even if the polymer is soluble throughout the polymerization
reaction, it will contain more amic acid than obtained by thermal imidization. Therefore,
the polyimide obtained by chemical imidization usually needs to be heated to elevated
temperature to increase the degree of imidization.
O
N
O
CH3COOO
N
O O CH3
O
O
O CH3
OO
N
N
O CH3
OO
O
-CH3COON
O
O
Scheme 1.8: Catalization of acetate ion in chemical imidization of polyamic acid
1.2.1.1.f High temperature solution imidization of polyamic acid
Polyimides resulting from thermal imidization often demonstrate insolubility, infusibility
and thus poor processability.30
To overcome these drawbacks, high temperature solution
11
imidization has been successfully utilized.31,32
Cyclodehydration is conducted by heating
a polyamic acid solution in a high boiling solvent at temperature of 160-200 0C, in the
presence of an azeotropic agent. Compared with bulk thermal imidization, the lower
process temperature and greater mobility in the solution ensures the avoidance of
degradation and side reaction. Several studies were carried out investigating the kinetics
and reaction and mechanism of the imidization process.33
Second order kinetics were
demonstrated.
1.2.1.2 One - step high temperature solution polymerization
Soluble polyimide can also be prepared via a one-step high temperature solution
polycondensation of tetracarboxylic acid anhydride and diamines. In this process, the
dianhyride and diamine monomers are heated in a high boiling solvent or a mixture of
solvents at a temperature of 140-250 0C, which permits the imidization reaction to
proceed rapidly. Commonly used solvents are dipolar approtic amide solvents,
nitrobenzene, benzonitrile, α–chloronaphthalene, o-dichlorobenzene, trichlorobenzene,
phenolic solvents such as m-cresol and chlorophenols. In some cases toluene, xylene, o-
dichlorobenzene are used as co-solvent in order to facilitate the removal of water formed
by condensation via azeotropic distillation.34-36
When a mixture of diamine, dianhydride and solvent is heated, a viscous solution
is formed at an intermediate temperature of approximately 30-100 0C. At this point, the
composition of the products is mainly polyamic acid and phase separation is usually
observed in nonpolar solvents such as chlorinated aromatic hydrocarbons. However, on
raising the temperature to 120–160 0C, a vigorous evolution of water occurs and the
reaction mixture becomes homogeneous. At this stage the, product is essentially a low
12
molecular weight polyimides having o-dicarboxy and amino end groups. Thereafter a
slow stepwise polycondensation reaction takes place according to the reaction between
the end groups. The rate is slower in basic approtic solvents, and faster in acidic solvents
such as m-cresol. In general, the imidization reaction has been shown to be catalyzed by
acid37
. Kreuz et al., however observed thermal imidization of polyamic acid could be
catalyzed by tertiary amines.26
High temperature solution polymerization in m-cresol is
often performed in the presence of high boiling tertiary amine such as quinoline as
catalyst.
1.2.1.3 Polyimides via derivatized polyamic acid precursors
Polyamic acids in solution are unstable and are susceptible to hydrolytic degradation.
This process breaks down the molecular weight of the amic acid and result in
polyimides.38
It is believed that hydrolysis occurs through the acid catalyzed formation
of an anhydride as shown in Scheme 1.9.
OH
O
N
HO
O
O
N
HOH
O
N
HHO
O + H2N
1
2
3
OO
O
Scheme 1.9: Mechanism for amic acid back reaction to anhydride and amine.
13
To prevent this, efforts have been made to exclude the proton transfer from the
acid groups. The simplest way to eliminate the proton transfer step is to neutralize the
acid group with a base, such as a tertiary or a secondary amine, to form a polymeric
salt.28
But this method increases the viscosity of the solution due to the presence of ionic
polymer chain. Alternative method involves converting the acid group into either amide
or ester. The ortho-carboxylic group in polyamic acid can be chemically modified to
either an ester or an amide moiety. The ester or amide derivatives of polyamic acid are
unable to form carboxylate anion, which prevents the creation of degradation
intermediates. Polyamic ester can be stored for an indefinite period at ambient
temperatures. Conversion of esters of poly(amic acid) to polyimides proceeds thermally,
but at slower rate, and generally requires a temperature significantly higher than 200 0C.
1.2.1.4 Polyimides via polyisoimide precursors
In general, polyisoimides are more soluble and possess lower melt viscosities and lower
glass–transition temperatures than the corresponding polyimides.39
Polyisoimides are
formed from the corresponding polyamic acid using a dehydrating agent such as
trifluroacetic anhydride in conjunction with triethylamine, N,N’-dicyclohexylcarbo
diimide (DCC) and acetyl chloride.40
A polyisoimide can easily be converted to the
corresponding polyimide via thermal treatment at temperature >250 0C. Alternatively,
polyisoimides have been reacted with alcohol to produce polyamic ester, which could
then be thermally converted to polyimides.41
On treatment with amines, polyisoimides
likewise give polyamic amide quantitatively which can be converted to polyimides
thermally (Scheme 1.10).
14
N
OH
O
O H
(C2H5)3N / (CF3CO)2O
or DCC
O
O
N
N
O
O
O
N
O
ROH
O
O
OR
N
H
N
O
O
O
N
O
R2NH
N
NR2
O
O H
N
O
O
heating
heating
heating
Scheme 1.10: Polyimides via polyisoimide precursors
1.2.1.5 Polyimides from diester-acids and diamines (ester-acid routes)
Synthesizing polyimides via the ester-acid route involves derivatizing the anhydrides to
ester-acid and subsequently allowing diamines to react which yields the desired polyamic
acid and polyimides.42
In the initial stage of esterification, the dianhydride is simply
15
refluxed with excess of alcohol. Once the excess alcohol has been evaporated, the
resulting diester diacid is then reacted in solution with the suitable diamine to form a
polyamic acid. The polyimide is obtained by thermal or high temperature solution
imidization. Johnston et al. discovered that the anhydride functional group was formed at
elevated temperatures in situ from the ortho ester acid.43,44
The anhydride then reacts with
the diamine to yield a polyamic acid.
1.2.1.6 Polyimides from tetracarboxylic acids and diamines
The aromatic tetracarboxylic acids and aliphatic diamines combine to form salts, similar
to the synthesis of nylon via nylon salts. The salts are thermally imidized under high
pressure at temperature above 200 0C to form polyimides (Scheme 1.11). It should be
pointed out that the intermediate polyamic acids are not detected during the
polycondensation stage. Rather, it appears that the imidization and formation of polyamic
acid take place at the same time. This means that the imidization rate is very fast.45
HO2C CO2H
CO2RRO2C
+ H2N (CH2)x NH2
R= H, CH3
CO2
CO2RRO2C
O2C+ H3N (CH2)x NH3
NN (CH2)x
n
+ H2O + R OH
O
O
O
O
Scheme 1. 11: Polyimides from tetracarboxylic acids and diamines
16
1.2.1.7 Polyimides from dianhydrides and diisocyanates
The reaction of aromatic diisocyanate with dianhydride has been utilized to synthesize
polyimides46 - 49
(Scheme 1.12).
O
O Ar
O
O
O
O
+ O C N Ar N C O-CO2
N Ar N Ar
O O
OO
Scheme 1.12: Polyimides from dianhydrides and diisocyanates
O O
O
O
O
O
+ COCN
H
H
NCO
heat
N
O N
O
C
O
O
O
O
O
O
H
H nheat
NN
O
O
O
O
C
H
H n
+ 2nCO2
PMDA methylenediphenyl diisocyanate
polyimide
Scheme 1.13:Proposed route to polyimides from dianhydride and
diisocyanate via seven-membered rings
17
The reaction takes different pathways depending on the conditions. In the absence
of catalyst the reaction has been claimed to proceed through a seven membered
polycyclic intermediate that finally gives rise to polyimides with separation of carbon
dioxide (Scheme 1.13). Spectroscopic evidence of the seven-membered rings has been
found in the preparation of polyimides from pyromellitic dianhydride and
methylenediphenyldiisocyanate.48
On the other hand polyimides of very high molecular
weight have not been reported by this method. The mechanism is different when the
catalysts accelerate the reaction. Catalytic quantity of water or alcohol facilitate imide
formation and intermediate ureas and carbamates seem to form, which then react with
anhydrides to yield polyimides.48-50
It was also reported that high molecular weight polyimides were obtained using
mixtures of anhydride and their corresponding tetracarboxylic acids with diisocyanates in
the presence of tertiary amines.51
In Volksen’s review28
a mechanism has been proposed
to address the role of the hydrolyzed species (Scheme 1.14).
In the presence of water, the anhydride and isocyanate hydrolyse simultaneously
to dicarboxylic and carbamic acids, respectively (reaction 1 and 2). Some of the carbamic
acid reacts with isocyanate to form urea (reaction 3). It has been suggested that the urea is
capable of reacting slowly with anhydride to form imide, so the presence of urea would
not limit molecular weight. Additionally, either products of the hydrolysis, carbamic acid
or diacid, is capable of reacting to form a mixed carbamic carboxylic acid anhydride
(reaction 4 and 5 respectively). Subsequent heating causes the mixed anhydride to cyclize
into imide with the loss of carbon dioxide and water.
18
OH2O OH
OH
(1)
N C OH2O
N C OH
H O (2)
N C OH
H O N C O
N C N
H O H(3)
N C OH
H O
+ O
OH
O C N
O
O O H
(4)
(5)
OH
OH
O
O
+ NCO
OH
O C N
O
O O H
O
O
O
OO
O
Scheme 1.14: Mechanism of formation of polyimides from dianhydride and
diisocyanate
1.2.1.8 Polymerization via nucleophilic aromatic substitution reactions
Aromatic nucleophilic substitution reaction of bishalo-and bisnitro– substituted aromatic
ketones and sulfones with bisphenolates can produce polyetherketones52
and
polyethersulfones53
respectively. Aromatic halo-and nitro-groups are also strongly
activated by imide groups toward nucleophilic aromatic substitution54
with anhydrous
bisphenol salt in polar approtic solvents. The polymer chain is generated by the formation
of successive aromatic ether bonds. The synthetic pathway is given in Scheme 1.15 a.
Halo- and nitro- substituted imides are more reactive than the corresponding sulfones and
19
ketones. This is due to the fact that the imide ring is not only activated by the additional
carbonyl group, but the two carbonyl groups are locked in a coplanar conformation with
the phenyl ring, providing more effective resonance. Because of the favourable carbonyl
conformation, the Meisenheimer type of transition is stabilized by the effective
delocalization of the negative charge (Scheme 1.15 b).
N Ar N
X
+O Ar'
X
N Ar N
OO Ar'
+ X
(a)
Ar' O
X
N Ar
NX
OAr'
Ar NX
OAr'
Ar
(b)
O
X=NO2 or Cl
OO
O O
O
O O
O
O
O
O
O
O
O
Scheme 1.15: Mechanism of polymerization via nucleophilic aromatic substitution
reactions
Alternatively, ether-containing dianhydrides bis(etheranhydride)s are prepared by
a nitro substitution reaction, followed by hydrolysis of the product bis(etherimide)s
according to the Scheme 1.16.55
Unlike commercial dianhydride such as PMDA and
20
BTDA, bis(etheranhydride)s possess moderate reactivity toward nucleophile because of
the electron-donating property of the ether groups. Bis(etheranhydride)s are hardly
affected by atmospheric moisture. The stability and solubility of bis(etheranhydride)s
provide significant advantages in the manufacturing operation.
R-NO Ar O
N R
OO Ar O
O
bis(etheranhydride)
bis(etherimide)
2 R-N
NO2
H2O
O
O
O
O
O
O
O
O
O
O
HO Ar OH
Scheme 1.16: Synthesis of bis(etheranhydride)s
1.2.1.9 Polymerization by transimidization reaction
High molecular weight poly(etherimide)s have been synthesized via one-step imide-
amine exchange reaction between bis(etherimide)s and diamines56
according to the
Scheme 1.17. The Scheme 1.17 eliminates the process of converting bis(etherimide)s to
bis(etheranhydride)s. When poly(etherimide)s are fusible the polymerization is
performed in the melt, allowing the monoamine to distill off. Bisimides derived from
heteroaromatic amines such as 2-aminopyridine are readily exchanged by common
aromatic diamines.57,58
High molecular weight poly(etherimide)s have been synthesized
from various N, N’-bis(heteroaryl)bisetherimides.
21
RNO Ar O
NR + H2N Ar' NH2
NO Ar O
N Ar' + RNH2
O
O
O
O
O
OO
O
bis(etherimide)sdiamines
poly(etherimide)s
Scheme 1.17: Polymerization by transimidization
1.2.1.10 Other routes to polyimide formation
Many other polyimides preparation methods have been reported in addition to the above
mentioned routes. Due to the improved solubility and stability of derivatized PAAs, a
number of techniques have been developed to form alkyl esters,59
silyl esters60
and
ammonium salts61, 62
of PAAs, all of which can be thermally cyclized to form polyimides.
Polyimides can also be prepared by Diels-Alder and Michael cycloaddition reaction,
63
palladium64
and nickel65
catalyzed carbon-carbon coupling reactions.
1.3 COPOLYIMIDES
The most important copolyimides are poly(etherimide)s, poly(esterimide)s and
poly(amideimide)s. The most important methods for the preparation of copolyimides are
given here.
22
1.3.1 Poly(etherimide)s
Poly(etherimide)s are a class of polymers containing ether and heterocyclic imide unit in
the polymer backbone. Such a structural modification leads to lower glass transition
temperature and crystalline melting temperature as well as significant improvement in
solubility and other characteristics without sacrificing thermal stability.
1.3.1.1 Synthesis of poly(etherimide)s
Poly(etherimide)s can be synthesized by i) Displacement polymerization method similar
to the method discussed in Section 1.2.1.8. ii) The cyclization polymerization method66
:
Ether containing diamine monomer is cyclized with dianhydride or ether containing
dianhydride is cyclized with diamines to form the imide ring. The cyclization process is
similar to the one described for polyimides. iii) Poly(etherimide)s can also be prepared by
transimidization reaction similar to the method discussed in Section 1.2.1.9.
1.3.2 Poly(esterimide)s
Poly(esterimide)s are the class of polymers containing ester and rigid heterocyclic imide
unit in the polymer backbone. The structural modification improves solubility
significantly and other characteristics without sacrificing thermal stability.
1.3.2.1 Synthesis of poly(esterimide)s
Poly(esterimide)s are usually synthesized by i) The imide forming reaction using ester
containing monomers 67
: Ester containing amine monomer is cyclized with anhydride to
form the imide ring. In another approach ester containing anhydride is cyclized with
diamine monomers to form the imide ring. The cyclization process is similar to the one
23
described for polyimides. ii) Ester forming reaction using imide containing monomers68
:
Imide containing diacid monomers polymerized with diol monomers or the imide
containing diol monomers polymerized with diacid monomers or self condensation of
hydroxyl acid containing imide.
1.3.3 Poly(amideimide)s
Poly(amideimide)s are the class of polymers containing amide and heterocyclic imide
unit in the polymer backbone. Poly(amideimide)s combines the thermal stability property
of polyimides and the ease of processability of polyamides and are intermediate in
properties between polyimides and polyamides.
1.3.3.1 Synthesis of poly(amideimide)s
O
O
OXOC
+ H2N Ar NH2 N
O
O
Ar
HN-OC
-HX
-H2O
X= OH, Cl, etc
Scheme 1.18: Synthesis of poly(amideimide)s by amide-imide forming reaction
Poly(amideimide)s are usually synthesized by i) Amide-imide forming reaction69
:
An acid chloride such as trimellitic anhydride can be considered as an ideal monomer.
Both the carboxylic acid and anhydride groups can react with diamines to make up
poly(amideimide)s. The overall reaction is depicted in Scheme 1.18. ii) Imide forming
reaction using amide containing monomers70
: Amide containing diamine monomer is
cyclized with anhydride to form the imide ring. In another approach amide containing
24
anhydride is cyclized with diamine monomers to form the imide ring. The cyclization
process is similar to the one described for polyimides. iii) Amide forming reaction using
imide containing monomers71
: Imide containing diacid is polymerized with diamine
monomers or the imide containing diamine is polymerized with diacid monomers
(Scheme 1.19). They have been also successfully synthesized by phosphorylation method
from diamines and diacids (Scheme 1.20).72
N
O
O
Ar N
O
OClOC COCl
+ H2N Ar' NH2
N
O
O
Ar N
CO-NHOC
O
OAr' NH
-HCl
Scheme 1.19: Synthesis of poly(amideimide)s by amide forming reaction
N
O
O
Ar N
O
OHOOC COOH
+ H2N Ar' NH2
TPP/NMP/PY/CaCl2
N
O
O
Ar N
CO-NHOC
O
OAr' NH
Scheme 1.20: Synthesis of poly(amideimide)s by phosphorylation method
25
1.4 STRUCTURE PROPERTY RELATIONSHIP IN POLYIMIDES
1.4.1 Chain-chain interaction
It has been proposed that aromatic polyimide chains interact with each other via charge
transfer or electronic polarization mechanism.73
These electronic interactions affect the
neighbouring polymer molecule and influences the polyimides colour, glass transition
temperature, crystallinity, mechanical properties and chemical résistance. Research has
been directed towards elimination or lessening these interactions in order to improve the
thermoplastic flow and to increase solubility without sacrificing thermal stability.
1.4.2 Glass transition temperature (Tg)
The Tg of a polymer depends upon several factors. Increasing interchain interaction as
well as chain stiffness and crosslinking raises Tg values. The Tg values decrease as the
polymer goes from all para catenation to meta.74
This is due to the flexibility in the meta
catenation polymers. Higher dipolar bridging groups such as carbonyl, sulfonyl impart
higher Tg than do the groups such as oxy or methylene. By introducing kinks or bulky
substituent along the chain, the regularity of the chain is disrupted which results in
decreasing Tg value.75
1.4.3 Thermo-oxidative stability
Accordding to the rule of thermo-oxidative stabilities in the polyimides, if the
dianhydrides and diamines are in a higher oxidation state, polyimides should exhibit
better oxidation resistannce than the other polymers of similar molecular weight.
Common polyimides based on PMDA, BTDA, ODPA, and 6FDA have dianhydrides that
are in high oxidation states; then the thermo-oxidative instability in general must be
26
related to the more oxidation susceptibility of the diamine derived units. In the process of
preparing polyimides one generally tries to maximize molecular weight by using exactly
the same number of moles of the dianhydride and diamine or else the stoichiometry will
be upset. High molecular weight species generally exhibit better thermo-oxidative
stability than their corresponding low molecular weight species.
1.4.4 Crystallinity
Crystallinity76
of polyimides depends on several factors such as regularity in the chain,
chain flexibility and rigidity of the chain. Presence of polar groups along the backbone
increases the inter chain interaction and the crystallinity.77
The presence of bulky pendant
groups prevents ordering of molecules and prevent the crystallinity.78
Crystallinity of
polyimides has direct bearing on their physical properties and applications as fibres or
plastics. Generally fibres possess high degree of crystallinity and tensile strength. Plastics
are amorphous and un-oriented.
1.4.5 Solubility
Systematic variations in the structure of polyimides have led to improved solubility.
Changlu Gao et al. reported79
soluble biphenyl-based polyimides from asymmetric
bis(chlorophthalimides). When compared with polyimides derived from symmetric 4,4’-
BCPIs and 3,3’-BCPIs, the polyimides derived from asymmetric 3,4-BCPIs showed
better solubility. Shuqing Wu et al.80
prepared polyimides with copolymerization of
bis(chlrophthalimide)s with 2,5 dichlorobenzophenone. The insertion of benzophenone
group into the polyimides increases the solubility. Any structural modification in the
polyimides which decreases the chain-chain interaction leads to increase in solubility.
27
1.4.6 Dielectric constant
Linear aromatic polyimides which contain the hexafluoroisopropyl group have dielectric
constant lower than more conventional polyimides. More flexible meta-linked diamine
gives lower dielectric constant values than the corresponding para-linked system. This
may be related to free volume in the polymer, the meta-substituted systems have a higher
degree of entropy. The 6F based polyimides have the lowest overall dielectric constant
values which are lowered further when the 6F moiety is in both the dianhydride and the
diamine. Tomoko et al. 81
reported a dielectric constant of 2.40 at 10GHz for fluorinated
polyimides. The incorporation of fluorinated substituents into polymers reduces the
dielectric constant because of the mutual repulsion of the outermost shell electrons of
fluorine atoms and large free volume of trifluoromethyl groups resulting in low
intermolecular cohesive energy and low chain packing efficiency.
1.5 APPROACHES TO IMPROVE THE PROCESSABILITY OF POLYIMIDES
The fabrication process for a polymeric material requires that the polymers be either
soluble in organic solvents for a casting process or molten below their decomposition
temperature for a moulding extrusion process. However, aromatic polyimides are
insoluble and infusible and very difficult to fabricate into desired articles.82
Many
significant synthetic efforts have been made to improve the processability without
decreasing thermal stability. In order to increase the processability the main approach
used for structural modifications include:
(i) Incorporation of flexible linkages
(ii) Incorporation of Kinks in the polyimides chain
(iii) Incorporation of large non-polar and polar pendant groups
28
(iv) Attachment of flexible side chain
(v) Soluble copolyimides derived from two dianhydrides or two diamines
1.5.1 Incorporation of flexible linkages
A common approach to increase the flexibility of the polymer chain is to introduce a
flexible unit into either a diamine or dianhydride monomers.83
Polyimides with aliphatic
linkages, such as those derived from MDA have lower thermal stability. To maintain
good thermal stability, it is common to use flexible units containing thermally stable
groups such as O, C=O, O=S=O, S and C(CF3)2.
N N O
O O
OO
n
N N
O
O
O
O
OO
n
ON
ON
O
O
O
O
H3C CH3
n
Kapton, Tg = 377' C
LaRC-TPI, Tg = 264' C
Ultem, Tg = 220' C
Figure 1.2: Example of some commercial polyimides
The incorporation of the 6F group into the polymer chain usually increases the
solubility, thermal stability, flame resistance and oxidation resistance, while decreasing
29
the crystallinity.84
The high thermal stability of the 6F containing polyimides is due to
strong C-F bond. The Tg value decreases with increasing number of flexible groups in
the polyimide chain and the solubility also increases (Figure 1.2). Comparing with the
polyimide derived from PMDA and p-phenylenediamine, the commercially available
polyimides kapton, laRC-TPI and ultem have much lower Tg’s value.
1.5.2 Incorporation of kinks in the polyimides chain
The solubility of the polyimides can be increased by the incorporation of a kink in the
polyimides backbone through ortho or meta catenation. Para catenation results in the
least soluble polyimides because of linearity. The ortho catenation polyimide is more
soluble than the meta catenation polyimides.85 Polyimides derived from m-
phenylenediamine are generally more soluble than the polyimides derived from p-
phenylenediamine. Comparing the polyimdes derived from 3,3’,4.4’-bipheny
tetracarboxylivc anhydride with the polyimides derived from its isomer 2,2’,3,3’
biphenyltetracarboxylic dianhydride, the latter is more soluble due to non-planar
structure86
(Figure 1.3).
O O
O
O
O
O
biphenyl- 3,3', 4,4'- tetracarboxylic dianhydride
O
OO
O
O
O
biphenyl- 2,2', 3,3'- tetracarboxylic dianhydride
Figure 1.3: Rigid and kinked dianhydride (BPDA)
30
1.5.3 Incorporation of large non-polar and polar pendant groups
Polyimides solubility can be increased by the introduction of large non-polar and polar
pendant groups along the backbone. The polar groups increase the affinity between the
polymer and the solvent and hence the solubility increases. Polyimides which contain
benzamide groups have been prepared. 87
The polyimides with benzamide groups showed
more solubility than the corresponding unsubstitted polyimides (Figure 1.4).
N N
O
O
O
O R
R= H
R=NHCOPh
Figure 1.4: Polyimides derived from PMDA and diphenylamine
The increase in solubility depends upon the nature of non-polar pendant groups.
The polyimides (Figure 1.5) derived from PMDA and m-phenylenediamine is an
insoluble material. However once the isopropyl group is introduced the resulting
polyimides is soluble in DMF, with the introduction of large anthracene as a pendant
group, the polyimides become more soluble in DMF, NMP, DMAc, DMSO etc.88
N N
O
O O
O
R
R= CH(CH3)2
R=
R= H
Figure 1.5: Polyimides derived from PMDA and m-phenylenediamine
31
1.5.4 Incorporation of twisted biphenyl structure
A series of twisted diamines 2,2’-disubstituted -4,4’diaminophenyl (Figure 1.6) such as
2,2’-bis(trifluoromethyl)-4,4’diaminophenyl,89
and 2,2’-dimethyl-4,4’diaminophenyls90
have been synthesized to produce high strength polyimides.
H2N NH2
R
R
R = -CF3
R = -CH3
Figure 1.6: 2, 2’-disubstituted -4.4’-diaminobiphenyls
The substituted group at the 2,2’-position of biphenyl sterically forces the two
aromatic rings into a non-planar conformation. Such non-planarity causes a decrease in
the polymers crystallinity which leads to enhanced solubility, while the high temperature
properties associated with rigid-rod polyimides remain.
Several twisted dianhydrides, containing twisted biphenyltetracarboxylic
dianhydride structure have been developed91
(Figure 1.7). The polyimides obtained from
a twisted dianhydrides and some of the twisted diamines contain segmental rigid-rod
structure while being soluble in common organic solvents like acetone and THF.
R
RO
O
O
O
O
OR = Br
R = Ph
R = CF3
Figure 1.7:2, 2’-disubstituted-4, 4’, 5, 5’-biphenyltetracarboxylic dianhydrides
32
1.5.5 Soluble copolyimides derived from two dianhydrides or two diamines
The copolymerization of two dianhydrides or diamines has been used to disrupt chain
symmetry and regularity and improve solubility. For example, polyimides derived from
PMDA and 4,4’-(dibenzeneamine)-9-fluorene completely insoluble and polyimides
derived from BPDA and 4,4’-(dibenzeneamine)-9-fluorene only soluble in 1,1,2,2-
tetrachloroethane. However, the copolyimides derived from PMDA, BPDA and 4,4’-
(dibenzeneamine)-9-fluorene prepared from a 50:50 ratio of each dianhydride is soluble
in amide solvents as well as 1,1,2,2-tetrachloroethane.92
1.5.6 Attachment of flexible side chain
Polyimides with flexible side chains are prepared and are exhibiting high solubility in
common organic solvents. Polyimides containing n-dodecyl side chains were prepared by
Schmidt and coworkers93,94
(Figure 1.8).
NN
R
R C12H25
C12H25C12H25
C12H25C12H25
C12H25
O
O O
O
Figure 1.8: Polyimides containing n-dodecyl side chains
The high molecular weight polymers had good solubility in common solvents.
However, the polymers exhibited a loss of weight at 380 0C due to the presence of
aliphatic moieties.
33
1.6 APPLICATIONS OF POLYIMIDES
The list of applications of polyimides is unending and keeps growing with the increasing
demand of growing technologies. Polyimides are widely used in electrical, electronics,
automotive, semiconductor and aerospace industries. Polyimides are used in the form of
films, fibres, foams, plastics, adhesives and coatings.95
1.6.1 Films
Polyimides films are used as insulation in electrochemical items, cables, generators,
electric motors, for the parts operating at elevated temperatures. Polyimides film can
also be used as electrical insulation material for systems operating at lower temperatures.
Polyimides film considerably extends service life and ensures reliable protection in the
case of emergency overheating. Films based on fluorinated polyimides96
are promising
dielectrics; their dielectric constant is within 2.3-3.0 and moisture absorption within 0.50-
0.85 %. As the temperature is raised to 300 0C, the dielectric constant increases by not
more than 1.0-1.5 %. Fully fluorinated polyimides film exhibit high optical transparency
in the visible and UV ranges and used in optical telecommunication technology.
1.6.2 Fibres
Arimid T polypyromellitimide fibre is the most heat resistant synthetic fibre. Polyimides
fibres fully preserve the elasticity and strength at the liquid nitrogen temperature.
Polyimides fibres and fabrics completely restore elastic deformation at elevated
temperature. Combinations of unique properties allow the polyimides fibre to be used in
equipment operating for a long time at increased level of radiation and temperature.
34
1.6.3 Foamed polyimide plastics
Foamed plastics have long been studied as materials for insulation of various apparatus,
devices, units etc.97,98
Polyimides attracted particular attention of researchers working on
development of protective structures for high-speed apparatus, primarily for air-and
spacecrafts. The main requirements to such protection are enhanced heat and fire
resistance, low density, and flexibility. Foamed materials are used for manufacturing
walls, floors, seats, ceilings, and other elements of not only air and space craft, but also
ships, submarine, high speed train and cars. In some cases, to enhance the rigidity and
impact strength foamed polyimides are formed in combination with reinforcing materials
such as fibre glass, fibrous carbon etc.
1.6.4 Polyimide membrane
Polyimide used as a membrane for separation of gases, vapors, and liquids99-104
.Their
high heat resistance allows separation of gases for long time at elevated temperature.
Polyimides membranes showed the best performance in separation of gas mixture
especially simple gases such as hydrogen, helium, carbon dioxide and some other gases
from petrochemical industry. Polyimide membrane used by Ube (Japan) for separation
and purification of hydrogen operate at 150 0C and a pressure of 15 MPa. Polyimides
membrane is used for production of ultra pure hydrogen widely used in semiconductor
industry, refining of some metals, and chemical reactions involving fine catalytic
processes. Another gas purified on polyimide membrane is helium. Helium is recovered
both from mixtures of natural gases and from artificial mixtures containing air and
industrial gases. Polyimide membrane is successful in recovery of CO2 from various gas
mixtures in gas and oil-refining industry.
35
1.6.5 Plastic
Polyimides plastics105,106
are used in aerospace parts, such as tips of nose cones and front
edges of wings of space shutters, and part of gas pipes of rocket and aviation engines.
Thermoplastic polyimide Aurum was developed by Mitsui Tooatsu (Japan). It is stable
without weight loss up to approximately 500 0C, radiation resistant, and inert to acids,
oils, organic solvents, and other chemicals. It is used in the form of composite reinforced
with fibre glass or carbon fibres. Polyimide materials are suitable in engine construction
for carburetors, high-strength motor parts, reducers, and other parts. Carbon-reinforced
polyimides plastics materials are of particular interest for development of reliable
spacecraft and hypersonic aircraft.
1.6.6 Varnishes, adhesives and coatings
Polyimide adhesives are available as liquids or films. Polyimides are superior to most
other adhesive types with regard to long term strength retention at elevated
temperatures.107-111
Polyimide varnishes are successfully used both as solution and as
adhesives formulations, for preparing enameled copper, aluminium, and steel wires. This
is useful for multi-layered printed circuit boards due to the excellent adhesive properties,
good electrical properties and reliability under the hot / wet condition. Microelectronics
field requires readily soluble materials forming elastic, flexible, hydrophobic, low-
shrinking, and transparent coatings. These requirements are met by partially or fully
fluorinated polyimides. Along with fluorinated imides, siloxane–containing polyimides
are used as adhesives in electronics.
36
1.7 OBJECTIVES OF THIS WORK
Polyimides are one of the most useful classes of high performance polymers.112-114
However, their applications are limited due to processing difficulties like insolubility in
common organic solvents and their extremely high softening or melting temperature,
because of the rigidity and strong inter chain interaction. So, polyimides can be
synthesized by structural modifications, that can be melt and / or solution processed. The
structural modifications that disturb the chain-chain interaction increase the solubility and
decrease the glass transition temperature and melting point. In recent years in the area of
polyimides synthesis efforts have been focused on the design and synthesis of
processable polymers with the purpose of increasing properties such as electrical
insulation, adhesion, gas permeability etc. These properties of the polymers depend to a
large extent on the monomers structure and hence the choice of monomers is of prime
importance in the design of novel polymers.
Research work has been directed to synthesize polymers with structural
modification, which disturb the regularity and chain packing thus providing better
processability. The prime objective of this research was to synthesize monomers
containing flexible groups (-O-, -SO2-, and -C=O), isopropylidene groups, pendant
groups and 1, 3-substituted phenyl ring. The structurally modified monomers were
polymerized into polyimides, poly(etherimide)s, poly(esterimide)s and
poly(amideimide)s and were characterized by IR, 1
H-NMR, X-ray diffraction,
thermogravimetry, differential scanning calorimetery, gel permeation chromatography,
solution viscosity and solubility behavior.
37
With these above objectives the following problems were chosen for the present work.
1 To synthesize new aromatic diamine monomers containing flexible groups (-O-,
SO2-, and -C=O) and isopropylidene groups and to study the structure-property
relationship of polyimides derived from these diamine monomer.
2 To synthesize new bis(nitrophthalimide)s monomers containing ether, sulfonyl
and carbonyl groups and to study the effect of flexible groups (-O-, -SO2-, and -
C=O) on crystallinity, solubility and thermal properties of the poly(etherimide)s
derived from these bis(nitrophthalimide)s.
3 To synthesize AB type monomers and to develop poly(etherimide)s from these
AB monomers and to study the crystallinity, solubility and thermal properties of
the poly(etherimide)s.
4 To synthesize new imide containing aromatic diols monomers and to study the
effects of ether groups, isopropylidene groups and meta substituted phenyl rings
on the solubility, crystallinity and thermal properties of the poly(esterimide)s
derived from these diols monomer.
5 To synthesize novel diimide-diacids monomers containing ether and pendant
hexafluoroisopropylidine unit and to develop poly(amideimide)s. To study the
effect of flexible ether and pendant hexafluoroisopropylidine units on the
solubility, crystallinity and thermal properties of poly(amideimide)s.
6 To study the applications of selected polymers as insulation materials for high
temperature electrical insulation.
