Polyimides.pdf

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POLYIMIDES

Introduction

Polyimides are a class of high performance, highly versatile resins that can be tai-

lored for application through the use of chemistry, processing, molecular weight

control, and reaction conditions. Polyimides display excellent thermal, mechani-

cal, and physical properties that place them in a general use category as material

systems that can compete with other structural materials, mechanical fasteners,

higher voltage connectors, and chemically resistant parts. As the requirements for

lighter weight, broader temperature performance, increased strength, durability,

and inertness continue to drive technology, polyimides will play an increasing role

in the innovations of the future.

The ﬁrst reported synthesis of an aromatic polyimide was in 1908 (1). How-

ever, much of the credit for the development and commercialization of polyimide

products goes to DuPont, who benchmarked this endeavor in the 1960s with the

release of Kapton H ﬁlm, Vespel molded parts, and Pyre-ML wire varnish (2).

This effort inspired other researchers in academia, industry, and government lab-

oratories to pursue the chemistry, fabrication, and applications of polyimides not

envisioned several decades ago. There are several excellent books that address

the extensive topic of polyimides (3–6). This article provides an overview of the

ﬁeld of polyimides from synthesis and basic kinetic behavior to fabrication and

articles of manufacture.

Polyimide Synthesis

The synthetic approach to the formation of polyimides is divided into three groups.

The ﬁrst group involves the synthesis of the polyimide by increasing the molecular

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weight of the monomer(s) through the formation of the imide ring via the amic

acid/ester route. The second approach involves increasing the molecular weight

of a precursor already containing the imide ring. The last method involves the

conversion of a polymer into a polyimide.

Polyimides via Imide Ring Formation. The two-step approach is the

most general approach used in the synthesis of linear polyimides (7–9). The ﬁrst

step involves the treatment of a diamine with either a dianhydride (the most pop-

ular), a tetracarboxylic acid, a diester diacid, a diester diacid-chloride or a trimel-

litic anhydride chloride (TAC) as in the case of polyamide-imides (Amoco’s Torlon)

(10,11). These reaction schemes are generally carried out in a high boiling polar

protic (phenolic) or aprotic (NMP, DMAc, DMF, etc) solvent at low to moderate

temperatures, resulting in the formation of the polyamic acid (or polyamide acid),

amic ester, amic acid ester, etc via nucleophilic attack by the amine on the car-

bonyl carbon (12). Next the formation of the imide ring is brought about through

thermal treatment, chemical dehydration, or both by a second nucleophilic at-

tack, occurring at the adjacent carbonyl carbon, by the amide nitrogen, followed

by the elimination of a condensate (typically water or alcohol). Removal of this

condensate forces the equilibrium toward the right, driving the polymerization to

completion (Le Chatelier’s principle), thus forming the imide ring (8,13). Alter-

natively, a “one-step” method or direct conversion is used where a diamine and a

dianhydride are placed in a solvent and heated to a temperature where the amic

acid moiety formed dehydrates rapidly to the resulting imide. Thus, the polyimide

is formed in “one step” (14).

Polyamic Acid Formation. To successfully carry out this synthetic ap-

proach, the following requirements must be met: chemical purity; the proper

type and percent solids of reaction medium (facilitating solubility of the resulting

polyamic acid); reaction temperature; and a method allowing for the removal of

any by-products of polymerization (heat, condensates, etc) (13,15). Most com-

monly, a bifunctional compound is treated with a diamine to afford the amic acid,

followed by imide ring formation during the ﬁnal polymerization step. The routes

to achieving a high molecular weight polyimide starting from the monomers via

the polyamic acid are shown in Figure 1 (16). When a diamine ( 1 ) is treated with

either a dianhydride ( 2 ), or a tetraacid or diacid-ester ( 3 ), alternative routes may

occur depending on the reaction kinetics, where k 1 and k 3 are desirable and k 2 and

k 4 are not. Table 1 summarizes the approximate kinetic “ k x ” values. The pathway

to k 4 depends on the amount of water or alcohol in the reaction medium; the

reactivity (electrophilicity) of the dianhydride or tetraacid/diacid-ester; and the

Table 1. Typical Rate Constants for Polyamic Acid

Synthesis a

Reaction

Rate constants

Propagation ( k 1 )

0.5–6.01 mol − 1

· s − 1

Depolymerization ( k 2 )

10 − 5 –10 − 6 s − 1

Imidization ( k 5 )

10 − 8 –10 − 9 s − 1

Anhydride hydrolysis ( k 4 )

0.1–0.41 mol − 1

· s − 1

Amine hydrolysis ( − k 3 )

0–10 − 6 s − 1

a Refs. 5,17, and 18.

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Fig. 1. Reaction Scheme for thermal imidization via polyamic acid/ester.

nucleophilicity of the diamine (Table 2). Steric hindrance and isomerization;

polarity and autocatalytic effects of the solvent; and the time/temperature proﬁle

during the polyamic acid formation also affect the pathway to k 4 (17,18,20). Con-

version of the anhydride moiety, k 4 , has the effect of introducing another chemical

functionality with a different reactivity that can affect both k 1 and k 3 via reso-

nance. As shown on the reaction coordinate (Fig. 2), the effect of k 2 competes with

k 1 and k 3 when the reaction temperature is raised and the thermal dehydration ( k 5

conversion) to imide is occurring. Once the imide ring is formed, depolymerization

is severely limited through removal of the condensate by-product. The choice of di-

amine and dianhydride affects the rate of polyamic acid formation both sterically,

and through the electronic interactions of the monomers (afﬁnity and basicity).

PMDA is the dianhydride with the highest E a value in Table 2. Any diamine that

is treated with PMDA should polymerize faster than if it is treated with a dianhy-

dride having a lower afﬁnity. However, once one of the anhydride units of PMDA

has reacted, the second anhydride moiety will display a lower reactivity because

the former anhydride group has less electronic withdrawing ability since it ceases

to exist with the progress of the reaction. This effect on other dianhydrides is less

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Table 2. Electron Afﬁnity and Basicity Values of Several Common Dianhydrides and

Diamines Used in the Synthesis of Polyimides a

Abbreviated E a ,

Abbreviated pK a

Dianhydride

name

Diamine

name

(H 2 O)

PMDA

1.9

pDA

6.08

BPDA

1.38

mDA

4.08

BTDA

1.55

DAB

4.60

DSDA

1.57

DABP

3.10

ODPA

1.30

DDSO 2

2.15

6FDA

ND b

ODA or

5.20

DAPE

BPADA or 1.12

Bis-DA

ND b

ULDA

a Ref. 19.

b ND = not determined.

Fig. 2. Qualitative reaction coordinate diagram for polyimde synthesis via polyamic acid.

From Ref. 21.

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pronounced as the anhydride groups are separated by additional moieties

that decrease the impact of the reacted anhydrides (20). A similar trend is seen

with diamines. The more basic the amine, the faster it can react with a moiety

that is susceptible to nucleophilic attack. Additionally, depending on type, the

larger the number of chemical groups separating the amines, the less of an effect

the conversion of one amine will have on the reactivity of the other (22).

Solvent effects depend on several factors, including the polarity of the

medium, the percent solids, and the solubility of the reactants. As the polarity

of the medium changes with conversion to polyamic acid, the viscosity and poten-

tial autocatalytic effects (kinetic conversion to polyamic acid) are observed (23).

These effects are caused by the increasing molecular weight of the polyamic acid,

which results in the buildup of carboxylic acid. The hydroxyl portion of the amic

acid can form additional hydrogen bonds to adjacent polyamic acid groups, artiﬁ-

cially increasing the observed viscosity (13). These additional carboxyl acid pro-

tons can assist nucleophilic attack (on the carbonyl carbon) via coordination with

the anhydride carbonyl oxygen, increasing the step growth kinetics to polyamic

acid formation. An accurate kinetic proﬁle has been difﬁcult to obtain as a result

of the solvent effects and the changing reaction medium. Some investigators claim

that irreversible second-order kinetics is followed, while others have observed re-

versible autocatalytic kinetics when THF is the solvent. Typically, polymerizations

carried out in amide solvents (polar aprotic) do not display autocatalytic behav-

ior (23,24). These solvent molecules closely associate with the hydroxyl protons,

effectively isolating them from interacting with other anhydrides and amic acids.

The reaction rate generally increases as the solvents become more polar and more

basic (25,26). A model compound study showed that the acylation rate increased

in the order THF

acetonitrile

DMAc

m -cresol. The polar protic solvent m -

cresol was claimed to increase the rate because it functions as an acidic catalyst

(23,24). Similar observations were noted when acids (benzoic acid) were intro-

duced into a polyamic acid medium employing an amide solvent. THF, not being

very basic, does not associate with the polyamic acid and some reversal occurs.

This “retro” reaction ( k 2 ) occurs when a proton is transferred from a carboxylic

acid to an amide nitrogen (or any other basic species), followed by the subsequent

attack of the carboxylate oxygen on the adjacent carbonyl carbon to re-form the

anhydride or the rare isoimide (25,27). This reverse reaction can be prevented

by replacing the hydroxyl proton of the polyamic acid with amine salts or esters,

causing a marked decrease in conversion to monomeric species (28–30).

The effects of adventitious water on both the starting anhydride and polyamic

acids have been shown to decrease their molecular weight over time via conversion

to diacid and hydrolytic cleavage, respectively (16,31–33). This drop in molecular

weight appears to be more dramatic the more dilute the solution, which is caused

by the apparent increase in water content as the percent polyamic acid decreases.

Additionally, the conversion from polyamic acid to monomer moieties is unimolec-

ular, whereas the reverse reaction is bimolecular, shifting the equilibrium to the

left toward the reactants (8). Evidence has indicated that further imidization of

the polyamic acid occurs upon standing over time, having the effect of releasing

water into the solution, resulting in further hydrolysis of the polyamic acid (16,34).

Lastly, monomer addition (sequence and rate) has been shown to affect

both thermodynamic equilibrium, and the resulting molecular weight of the

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