Makromol. Chem., Rapid Commun. 14, 703 - 706 (1993) 703

Polyimide film modified with polymeric copper ph thalocyanine

Subhaiya Venkatachalam, Thekkenezhattu M. Vijayan, Shanmugam Packirisamy*a)

Polymer and Special Chemicals Division, Vikram Sarabhai Space Center, Trivandrum-695022, India

(Received: June 1, 1993; revised manuscript of July 16, 1993)

Introduction

Polyimides are known to have excellent mechanical and thermal properties. The properties of polyimides were modified by many techniques. The conventional polyimi- des were modified with siloxane’) to produce materials with excellent adhesion to microelectronic devices. Modifications were also carried out with dispersed metals 2) ,

metal ions and metal complexes 3-7). Wohrle et al. incorporated 2,3,9,10,16,17,23,24- phthalocyanineoctacarbonitrile in a polyimide film and studied the electrochemical behavior of the modified film8).

In the present communication, we report on the modification of a polyimide film based on pyromellitic dianhydride and bis(4-aminophenyl) ether through the incor- poration of polymeric copper phthalocyanine containing peripheral carboxyl groups. The thermal stability and electrical properties of the virgin and modified polyimide films and the effect of heat treatment of the films on the electrical properties have been investigated.

Experimental part

Materials

Pyromellitic dianhydride, bis(4-aminophenyl) ether and N,N-dimethylacetamide were purified by standard procedures.

Polyamic acid

Polyamic acid solution (solid content 10 wt.-%, qinh 1,8 dL/g (c = 0,5 g/dL N,N-dimethylacet- amide)) was prepared by the condensation of pyromellitic dianhydride and bis-(4-aminophenyl) ether in N,N-dimethylacetamide following the procedure reported elsewhere ’).

Polymeric copper phthalocyanine (PCuPc)

Polymeric copper phthalocyanine containing peripheral carboxyl groups (Fig. 2) was prepared by the procedure of Achar et al. lo) The details of the purification and characterization of PCuPc are reported elsewhere“-”).

a) Present address: Michigan Molecular Institute, 1910 West St. Andrews Road, Midland, Michigan 48640, USA.

0 1993, Hiithig & Wepf Verlag, Base1 CCC 0173-2803/93/$02.00

704 S. Venkatachalam, T. M. Vijayan, S. Packirisamy

Polyimide film (Film I)

The polyamic acid solution was spread on a glass plate uniformly and cured in nitrogen atmosphere at 100°C for 1 h, at 150°C for 1 h, and then at 300°C for 1 h.

PCuPc-modified polyimide film (Film II)

Finely powdered PCuPc was mixed with the polyamic acid solution in the ratio 1 : 5 (w/w) based on the solids and the mixture was stirred for 30 min. The resulting solution was centrifuged to separate undissolved phthalocyanine polymer. This solution was used for the preparation of the film following the procedure described above. The metal content of the film was estimated by decomposing a known weight of the film with concentrated sulfuric acid followed by careful estimation of the solution by atomic absorption spectroscopy. The copper content in the film was found to be 0,19% which corresponds to 2% (by weight) of the anhydride-terminated polymeric copper phthalocyanine in the polyimide film.

Thermal treatment of Films I and II

The film was rolled and kept in a furnace. It was evacuated to mmHg and heated to 520 "C over a period of 1 h, maintained at this temperature for 1 h and then allowed to cool slowly under vacuum.

Electrical conductivity measurements

Electrical conductivity of the Films I and I1 (thickness 35-40 pm) was measured by the standard procedures 14). As the heat-treated films were brittle in nature, film conductivity could not be measured, and hence these films were finely powdered and the conductivity was measured by the method reported earlier ").

Results and discussion

The structure of the polyimide and polymeric copper phthalocyanine (PCuPc) are shown in Figs. 1 and 2, respectively. The conductivity data of the Films I and I1 are shown in Tab. 1. The incorporation of PCuPc results only in a marginal improvement in electrical conductivity.

We reported earlier ' 9 12) that carboxyl-containing polymeric nickel phthalocyanine on thermal treatment a t 480 "C shows a large increase in electrical conductivity due to decarboxylative coupling reactions leading to extended conjugated structures. As PCuPc is solubilized in the polyamic acid solution it is likely that the polymer is uniformly distributed in the polyimide base material when the film is formed. On thermal treatment of Film 11, PCuPc incorporated in the film is expected to undergo a decarboxylative coupling reaction leading to extended conjugated structures causing an improvement in electrical conductivity. Hence, we have attempted to improve the electrical conductivity of the Film I1 by thermal treatment a t 520 "C under vacuum

mmHg). It is observed that the thermal treatment improves the electrical conductivity of the polyimide by 5 orders of magnitude (i. e. from 4,s * S/cm to 3,9 - l ow3 S/cm; Tab. 1). As the amount of PCuPc incorporated in the film is only 2'70,

it is likely that the formation of extended conjugated structures from phthalocyanine moieties is not alone responsible for the increase in conductivity. Possibly, PCuPc in addition to undergoing the above reaction facilitates the condensation of the aromatic

Polyimide film modified with polymeric copper phthalocyanine 705

Fig. 1. Structure of polyimide

HOOC

H O O C

Fig. 2. Structure of polymeric copper phthalocyanine HOOC COOH H O O C COOH

Tab. 1. Electrical conductivity of the polymeric copper phthalocyanine, polyimide film and PCuPc-modified polyimide film measured at room temperature

Material Electrical conductivity in S/cm Weight loss during heat treatment

before heat after heat in % treatment treatment a)

PCUPC i,8.10-7 9,1-lo-' 41 Film I 3 s . 1 0 - ~ 1,4. lo-* 34 Film I1 4 3 * 10-8 3,9. 1 0 - ~ 39

a) Heat treatment was carried out at 520°C under vacuum mmHg).

706 S. Venkatachalam, T. M. Vijayan, S. Packirisamy

rings in the polyimide base material to form extended conjugated structures. The above prediction is supported by the observation that the modified film undergoes a weight loss of 39% and the virgin film gives a weight loss of 34% on thermal treatment. The conversion of PCuPc to biphenylene-type units containing phthalocyanine alone cannot account for the increased weight loss of 5070, indicating that PCuPc induces some structural change in the polyimide during heat treatment. Unlike the Film 11, the Film I shows only a marginal improvement in electrical conductivity on thermal treatment (Tab. I). From this observation it may be concluded that the polymeric phthalocyanine plays a vital role in improving the electrical conductivity of the polyimide film on heat treatment.

The improvement in electrical conductivity of Film I1 has been realized at the cost of film properties. The film has become brittle on thermal treatment. On similar thermal treatment the virgin film (Film I) also became brittle. It was reported15) that heat treatment of the polyimide coating at high temperatures reduces the flexibility and makes the coating brittle. The formation of brittle films on heating indicates that the temperature of heat treatment should be lowered to retain the mechanical properties of the films. However, lowering the heat treatment temperature reduces the extent of formation of highly conjugated structures and affects the improvement in electrical conductivity. We are currently working on alternative methods for improving the electrical conductivity of the PCuPc-modified polyimide film.

The authors are thankful to their colleagues in Standard and Calibration Luboratory, Testing and Evaluation Division, VSSC for their help in electrical conductivity measurements and to Raghavan Asari for laboratory assistance. The authors also wish to thank T. S. Ramasubrama- nian, K. Venugopal, K. Krishnan and G. Viswanathan for analytical and thermal analysis data.

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