Flexibility of aromatic polyimides and polyamidoacids

EuropeanPolymer

Journal, Vol 13. pp 375 to 378. Pergamon Press 1977 Printed in Great Britain

FLEXIBILITY OF AROMATIC POLYIMIDES AND POLYAMIDOACIDS T. M. BrRSHTErN, V. A. ZUBKOV, I'. S. MILEVSKAYA,V. E. ESKIN, I. A. BARANOVSKAYA, M. i . KOTON, V. V. KUDRYAVTZEV and V. P. SKLIZKOVA Institute of High-Molecular-Weight Compounds, Academy of Sciences of USSR. Leningrad, 199004, USSR

(Received 28 July 1976) Abstract--A theoretical study is carried out on the flexibility of aromatic polyamidoacids and polyimides containing planar cycles connected by different types of joints. It is shown that such polymer chains may be considered as a sequence of linear links joined at some angles with free rotation about the links. An expression for the mean square end-to-end distance for some types of these chains is given. An experimental investigation of the flexibility of some polyamidoacids obtained by polycondensation is also performed. The intrinsic viscosity is measured in 0-solvent. The molecular weight is determined by light scattering. The agreement between experimental and theoretical values of (ho2/M)1'2 for polyaminoacids is satisfactory thus justifying the use of the suggested theoretical approach for study of the flexibility of insoluble polyimides.

be represented as

INTRODUCTION

Valuable technological applications of aromatic polyimides have stimulated investigation of their properties [1]. Physical properties such as thermostability, impact strength, elasticity etc. are markedly dependent on the presence in the polymer chain of some bridging groups; both the number of these bridging groups and their arrangement in the chain are of significance. The bridging groups may influence the properties of the bulk polymer in two ways. Firstly, the presence of flexible joints in the chain increases the number of allowed conformations and the flexibility of individual chains, thereby directly affecting the bulk properties. Secondly, the structure of joints is essential for a specific arrangement of chains in the bulk polymer so affecting the intermolecular interactions of neighbouring chains, believed to be of primary importance for the bulk polymer properties. It is apparent that study of conformational flexibility of individual chains in solution is very desirable for better understanding of the relation between chemical structure and physical properties of polyimides (PI). Unfortunately, the limited solubility of polyimides makes direct study of molecular flexibility practically impossible. However the study may be carried out by combining theoretical and experimental approaches, as reported here. In the theoretical part, we consider conformations and the flexibility of polymer chains containing planar cycles connected by different types of joints. The analysis shows that such chains become practically free to rotate about the bonds adjoining the bridging groups. Formulae derived for the mean square end-to-end distance (ho2) allow us to calculate this quantity for different PI types and also for polyamidoacids (PAA) which are prepolymers for polyimides and are soluble. The chemical structures of these macromolecules may

1

i

o-co

c-o, o

j1 J°I

g PAA where

R = -,-~0-'~'-

( P I t, PAA 1)

(F~ a, PAA z) (1~,

PAA 3)

The results of the study of PAA in solution by light scattering and viscometrv are presented in the experimental part of this paper. The experimental data on the average dimensions of PAA confirm the occurrence of free rotation about the bonds adjoining bridging groups and are in complete agreement with the theoretical predictions. The agreement between theoretical and experimental data for PAA justifies the theoretical predictions about the flexibility of polyimides. THEORETICAL As typical examples of polymers under consideration, structures of PAAt and PI~ are shown in Fig. 1. From geometrical reasons these chains may be described by extended linear links, each of them containing several chemical bonds including planar cycles. The links join each other through one-atom bridging groups (e.g.-43---), planar cycle or amide group (Fig. 1). Detailed analysis of internal rotation in such chains has been carried out [3-5] and we only briefly recount here some of the salient features.

375

376

T.M. BIRSHTEINet al. When considering chains containing flexible joints with free or nearly free rotation about non-parallel bonds (X = - - - O - - , ---CH~--, )C---~O), it seems reasonable to replace the group P h C O N H P h (or P h C O O P h ) by an extended link with free rotation about it (for more details see [5]). Thus polymer chains of the types shown in Fig. 1 may be considered as sequences of linear links joined at some angles with free rotation about the links. General expressions for the mean square endto-end distance of such chains with any number of links in a monomer unit have been derived [5]. In the simplest case when all the backbone valence angles are equal, the following expression may be used

Fig. I. Geometrical representation of portions of chains of PAAt (1) and PIt (If) with links joining each other through planar cycle (I), one-atom bridging group (2) and amide group (3). The internal rotation about link i is characterized by a set of possible mutual orientations of links i - 1 and i + 1. Due to the significant lengths of links, the direct interaction between links i - 1 and i + 1 is negligible and cannot be a source of hindrance to the rotation about link i. In these circumstances, only the interactions of the cyclic group of link i with links i - 1 and i + 1 may be a source of hindrance. Consequently analysis of internal rotation about links containing cyclic groups (specifically phenyl groups) joined through a bridging group - - X - - reduced to an analysis of internal rotation in molecules Ph~X--Ph. Such analysis was carried out [2-4] by semi-empirical quantum chemical E H T method for bridging groups X with one atom (X = - - - - O - - , --)-CH2--, C------O) and two atoms (X = - - C O N H - - , ------COO---) of the main chain. In all the cases considered, it was shown that, due to the symmetry of benzene rings, there are several symmetrically arranged energy minima in the conformational space. Positions of these minima are determined by the chemical structure of bridging groups. In all cases (except that with benzene ring adjoining carbonyl group in PhCOOPh), energy minima correspond to rotation of the ring plane out of the backbone plane. Previous results [2-4] show that the average values of (cos ¢p) and (cos ~b) are zero (~0, ~b are i n t e r n a l rotation angles). This means that such a rotation is free, i.e. unhindered in a thermodynamical sense. Correspondingly the rotations around the skeletal bonds joining bridging groups are also unhindered. For diatomic bridges, the configuration of bridges is also essential. E H T calculations have shown that a transconfiguration is much preferred. The energy difference between trans and cis forms reaches 5-7 kcal/mole and rotations about bonds C - - N and C---O are practically forbidden. Therefore the presence in the chain of groups - - - ~ O N H - - and - - C O O - - - allows only rotations about the bonds adjoining the bridging groups. Two bonds adjoining each of these groups are almost parallel because the valence angles in these groups are approximately equal. Hence the joints such as P h C O N H P h or P h C O O P h do not increase substantially the flexibility of the polymer chain.

h2 --

n

~ ~- 1 =

zL

E

(cos "~,)~+ (cos T)"- ~ l l

== t ~= o ~ t~+61

. . . . .

1 -- (cos 7)"

(1)

where n is the degree of polymerization, v is the number of bonds in a monomer unit, l= is the length of the bond ct, n - ) , is the valence angle between the links of a chain, and [ c t + < 5 ] = ~ + 3 or ct + 6 - v if correspondingly ct + 3 ~< v or ~t + <5 > v. We used expression (I) for calculation of ~ / M and the persistent length ~o/2L ( L - the contour length) of some polyamido-acids and polyimides. Analysis of the results will be carried out in the last section after discussion of experimental data. EXPERIMENTAL

Polymer preparation PAAt and PAA, were obtained by the polycondensation of pyromellitic anhydride with 4,4'-diamino diphenyl ether or bis-(4-aminophenyl) ether of hydroquinone in N,Ndimethylacetamide (DMAA) solution at 10L In order to obtain samples with different values of M, the concentration of solutions was varied from 5 to 15~o w/w. All reagents were purified by the usual methods. Solutions for measurement of light-scattering and viscosity were prepared from powdered polymers obtained by precipitation in toluene from the mixture dimethylacetamide/tetrahydrofuran (1/1). The final washing of powdered PAA by ethyl ether was followed by vacuum-drying at 20 ° to constant weight.

Measurements Light scattering was measured by a photogoniodiffusometer Sofica at 21 ° and ~. = 5460 A with calibration on benzene (R =2.32 x l0 -s cm-l). The dependence of cH/19o on concentration c(19o is excess intensity at scattering angle 90°, H is an optical constant) was obtained for values c between 0.05 x 10 -2 and 0.5 x 10 -2 g.cm -3. The solutions were purified by eentrifugation at thousands revolutions per minute for 40--50 rain. The intrinsic viscosity was measured in an Ostwald-type viscometer (time of flow approximately 100 see).

Results Previous studies of PAA in solution were carried out [7,8] only in thermodynamically good solvents, mostly in DMAA, and the data on an equilibrium flexibility of PAA macromolecules were derived from It/] values in a good solvent. We have found that 0-solvents for PAA t and PAA2 are DMAA-dioxane (DO) mixtures with volume fractions 1:2.5 and 1:7.5 for PAAt and PAA2 respectively. In 0-solvents the second virial coefficients A2 are zero for all investigated PAA samples.

377

Flexibility of aromatic polyimides and polyamidoacids Table 1, Molecular weights and intrinsic viscosities of samples of PAAz and PAA2 in DMAA and mixed 0-solvent

Sample

M x 10- 3 ( + 10° o)

[r]] o cm 3 g - i ( + 5?0)

[r/] DMAA cm 3 g - 1 ( _+57:o)

A2 104 cm 3 mole g - 2 ( _+ 10°o)

PAA l

1 2 3 4

350 150 130 90

111 74 64 59

205 120 94 68

9.5 10.5 10.0 11.0

PAA2

1 2 3 4

90 80 50 20

62 60 46 27

144 122 96 45

20.0 15.0 17.0 20.0

Values of M and [r/] for four samples of PAAI with M , from 90 x 103 to 350 x 103 and for four samples of PAA 2 with Mw from 20 x 103 to 90 x 103 were measured in DMAA and 0-solvent. The M , values in DMAA for all samples considered were equal with those obtained in 0-solvent and hence there are no associative phenomena in 0-solvent. All the measured data are shown in Table 1. ~ l , and [r/] values obtained in DMAA and 0-solvent were used for analysing the dependence of lg[q] on lgM~ (Fig. 2). The K u h n - M a r k - H o u w i n k equations are obtained : PAAI

in DMAA [r/] =0.77 x 1 0 - 2 M °'a° in 0-solvent =2.0 x 10 - j M °'~° PAA2 in DMAA =2.8 × 10 -2 M °Ts in 0-solvent =2.0 × 10 -1 M °~°. The exponents 0.80 and 0.75, signifying considerable expansion of the random coil in DMAA solutions, are in accord with the large values of the second virial coefficients A2 in DMAA. In a D M A A / D O mixture, the exponent is 0.50 as expected for a 0-solvent where there are no volume effects. The unperturbed dimensions (ho~t1'2 of random coiled macromolecules PAA l and PAA2 were determined by the Stockmayer-Fixman extrapolation method i-9]. The extrapolation carried out both for DMAA and 0-solvent measurements gives for K0 = ~0 (h'~o/M)3/2 the same value of 0.20g-3/2 cm3molel:2 for all cases. Using 4~o = 2.66 1023 mole- ~, we get (h~/'M~ 1 2 = (0.9 + 0.1)A for both PAA1 and PAA2.

i(3)

.~(2l

2 g-.

I 40

4.5

I

I

5 0

5 5

tg M Fig. 2. Dependence of lg[r/'] on lg M for PAA 1 (O) and for PAA2 (®) in 0-solvent (curve 1), for PAAI in DMAA (curve 2) and for PAA2 in DMAA (curve 3).

DISCUSSION W e c o m p a r e n o w experimental values o f e n d - t o end distance with calculated values. T h e values (h02/M) 1:2 for PAA1 and PAA2, calculated by m e a n s of Eqn. (1) with a s s u m e d t r a n s - c o n f i g u r a t i o n o f a m i d e g r o u p s a n d free r o t a t i o n a b o u t b o n d s C---O, are given in T a b l e 2. As there is n o direct evidence o n the m a n n e r in which skeletal a m i d e g r o u p s are joining the b e n z e n e rings, the calculation was carried out both for the m e t a - and p a r a - p o s i t i o n s o f a m i d e

Table 2. Experimental and theoretical values of (ho2,/M)12 and persistent length a (both in A) for polypyromellitic acids and polypyromellitic imides (h2/M)l'2

a

Theoretical (for different positions of amide groups) Groups adjoining atom N

Theoretical (for different positions of amide groups)

Experimental

para

meta

para

meta

0.9 _-_+0.1

1.55

1.10

32

16

0.9 __ 0. l

1.34

1.05

21

14

Polyamidoacids

-0-o653C)-o-C, o-CPolyimides

C>C)-o-O-

1.58

31

1.34

20

1.26

18

378

T.M. BIRSHTEINel al.

groups adjoining the ring. The valence angle Cph-----O-----Cphwas assumed to be 120°. As follows from Table 2, the values (h~/M) 1/2 calculated for PAA with para-position of amide groups exceed considerably the experimental values. Moreover, for PAA2 which has an additional Ph----O group as compared with PAAI, mean square dimensions (h~/M) l/: must be significantly less than that for PAAI and this inference does not agree with the experimental results. The values of (h~/M) ~/2 calculated for PAA chains with metapositions of amide groups are close to experimental results both for PAA1 and PAA2. Hence in chains of PAAt and PAA2, amide groups at least preferably join the benzene ring in the meta-position. Comparison of theoretical and experimental results confirms our conclusion that free rotation takes place about bonds O----Ceh. The experimental values of (h~/M) 1/2 do not exceed the theoretical, calculated assuming free rotation, whereas a hindered rotation leads to an increase in (ho2/M)1/2. Some difference between (ho2/M)~/2 calculated for the meta position of amide groups and experimental values may be explained by approximations inherent in our calculations (groups PhCONHPh were replaced by effective links). However it is possible also that the difference reflects the presence of small amount of cis configurations of amide groups in real chains. On the whole, the agreement between theoretical and experimental values confirms the validity of our theoretical approach and justifies its use for the evaluation of the flexibility of polyimide chains. The validity of the suggested approach is supported also by the results obtained recently [10] on the flexibility of soluble polyimides with the bridging group ---C(CH3) (C6H5}---. By experiment [10] Kuhn's segment is 21-24 .A, which is close to the calculated 21 A. The values of (h:o/M) ~/2 and persistent length a calculated according to (1) for polyimides PI~, PI2, PI3,

i.e. for polymers with increasing number of phenylenoxide groups, are given in Table 2. It is necessary to remember the difference between PAA and PI geometrical structures. As was noted, the existing data on h~/M leads to the conclusion that in PAA amide groups mostly have meta-positions so creating an additional kink in every monomer unit (Fig. 1). In polyimides such kinks are absent and hence the measured flexibility of PAA is greater (the persistent length is smaller) than that of PI. At the same time the more rigid structure of PI is more sensitive to the inclusion of additional flexible joints (cf. PI1 and PI2) than it is in the case of PAA. As follows from Table 2, calculated persistent length decreases from 31A for PI~, to 20A for PI2 whereas for PAA corresponding values are 16 and 14 A.

REFERENCES

1. N. A. Adrova, M. I. Bessonov, L. A. Layus and A. P. Rudakov, Polyimides: A New Class of Thermostable Polymers. Nauka, Leningrad (1968). 2. V. A. Zubkov, T. M. Birshtein and 1. S. Milevskaya, Vysokomolek. Soedin., Ser. A, 16, 2438 (1974). 3. V. A. Zubkov, T. M. Birshtein and I. S. Milevskaya, J. molec. Struct. 27, 139 (1975). 4. V. A. Zubkov, T. M. Birshtein and I. S. Milevskaya.. Vysokomolek. Soedin., Set. A, 17, 1955 (1975). 5. T. M Birshtein, Vysokomolek. Soedin. Ser. A, 18, N2 (1977). 6. V. E. Eskin, Light Scattering by Polymer Solutions, Nauka, Moscow (1973). 7. M. L. Wallach, J. Polym. Sci. A2, 653 (1967). 8. B. Vollmert and A. Howath, Angew. Makromolek. Chem. 23, 117 (1972). 9. W. H. Stockmayer and M. Fixman, J. Polym. Sci. CI, 137 (1963). 10. N, A. Glukhov, T. I. Garmonova, V. S. Skazka, S. V. Bushin, M. G. Vitovskaya and L. M. Scherbakova, Vysokomolek. Soed., Ser. B, 17, 579 (1975).