ADVANCED COMPOSITES BASED ON NOVEL POLY (AMIDE-IMIDE)S

Novel poly (amide-imide)s were prepared using polycondensation reaction of an amino terminated oligoimide with various aliphatic diesters. The resultants PAIs were characterized for preliminary tests and number average molecular weight estimation. PAI-epoxy resin was used as a matrix material for the preparation of carbon and glass fiber reinforced composites and they were characterized for mechanical and electrical properties and chemical resistances by ASTM methods.

been reported [5][6][7][8][9] . Homopolymerization of bismaleimides has been shown to possible only either at elevated temperature or in the presence of an initiator 10,11 . Patel at el. reported the synthesis of amino terminated oligoimides based on Michael addition reaction of bismaleimide with 4,4'-diamino diphenyl sulfone 12,13 . Such oligoimides having terminal amino groups that make them suitable for further reactions such an amidation. The amidation of such amino terminated oligoimide with various diesters can afford novel poly(amide-imide)s possessing improved properties. The presence of the structural features of both amide and imide in the PAIs was expected to imbibe almost all the essentials and remove the shortcomings of neat polyimides. Much less, work is done in this field of PAIs and several anticipated applications prompted us to undertake the research work reported in the present research paper. The research

INTRODUCTION
Polyimides with heteroatoms as links in the polymer chain have attained outstanding position in the field of polymeric materials 1

Synthesis of oligoimide
A reported Michael addition reaction procedure 12,13 has been modified for the synthesis of the amino terminated BBMDDS oligoimide as follows: Benzidine bismaleimide (0.01 mole) was added gradually to a well-stirred solution of 4,4'-diamino diphenyl sulfone (0.02 mole DDS in 50 mL THF) with continuous stirring over a period of a half hour. The homogeneous viscous was obtained that was poured into ether (25 mL) and cooled by ice. The obtained powdered form of the oligoimide was filtered and washed with ether (10 mL) to remove unreacted DDS, followed by washing with warm DMF (10 mL) to remove unreacted bismaleimide. The resultant oligoimide was dried in vacuum oven. Preliminary characterisation of prepared oligoimide is presented in Table-1.

Synthesis of poly (amide-imide)s
PAIs were prepared by polyconden-sation reaction using an equimolar ratio of prepared oligoimide with various aliphatic diesters (listed in Table 2) by following method: The amino terminated BBMDDS oligoimide (0.01 mole), an aliphatic diester (0.01 mole) and potassium hydroxide (catalytic amount) in 70:30 (v/v) THF-ethanol mixture (50 mL) was mixed in three necked round bottom flask equipped with mechanical stirrer and water condenser and reaction mixture was refluxed for 4-5 h on a water bath at 70-80°C. The obtained product was then cooled at room temperature and poured into 75:25 (v/v) water-methanol mixture (80 mL) and filtered off. The resultant product was washed with THF (10 ml) and then with DMF (10 ml) to remove unreacted reactants and dried in vacuum oven. Thus, poly (amide-imide)s were obtained in the form of yellow to dark brown amorphous powder. The details of all PAIs prepared in a similar method are furnished in Table 2. #

Composite fabrication
The glass and carbon fiber reinforced composites based on PAI-epoxy system were prepared by the reported procedure 12 . A mixture of equimolar proportion of PAI and epoxy resin (40% weight of total glass cloth) in dioxane was stirred well for 2 to 5 min. The suspension was then applied with a brush on to a 150mm x 150mm polyimide compatible fiberglass cloth, and the solvent was allowed to evaporate. Once dried, the ten prepregs were stacked one on top of another, pressed between steel plates coated with a. All the mechanical and electrical properties and chemical resistances were deter-(12K), where in the fibers were allowed to pass through the suspension (mentioned above) and were then compression molded under similar molding conditions to those used for glass fiber composites. The composites have 40% (w/w) PAI-epoxy resin content.

RESULTS AND DISCUSSION
Preliminary characterisation of amino terminated oligoimide (Table 1) indicates that it is brown coloured amorphous powder and is insoluble in almost all the common organic solvents (except formic acid). A positive red azo dye test confirms the presence of the terminal aryl amino groups in the oligoimide. Its average molecular weight reveals that it has 3-4 repeating units in the polymer chain.
All the PAIs presented in Table 2 are yellow to dark brown coloured amorphous powder. They show the qualitative amide and imide tests to be positive. They are insoluble in almost all common organic solvents (except formic acid) and therefore the study of colligative properties could not be possible for the average molecular weight estimation.
The average molecular weights of all PAIs (Table 2) reveal that average molecular weight of PAI increases with increasing molecular weight of an aliphatic diester.
PAI-epoxy contents of the prepared composites are found in the range of 35-40%w/w. Bismaleimide produce a highly cross-linked and brittle product 7 , they can not be applied for preparing advanced composites.
Modifications to the bismaleimide system by addition of diamines are examined to improve the toughness and mechanical properties 1 . Addition of the epoxy resin to PAI having -CO-NH-and -CO-NH-CO-groups increases the toughness of the final products. The mechanical properties of the produced composites (    the concentrated alkali (25% w/v NaOH) affects the composites in weight and thickness.

CONCLUSION
The presence of terminal amide (-CO-NH-) and imide (-CO-NH-CO-) groups in the chain, novel PAIs possess the anticipated properties of polyamides and polyimides. The extraordinary thermal stability arises out of the resonance stabilisation due to aromatic rings near sulfone (SO2) group. The PAI-epoxy system shows good adhesion to glass and carbon fibers. Prepared void free sheets are insulated and have good mechanical properties. GFRCs have better chemical resistances and mechanical properties, but they are heavier than CFRCs.
The specific gravities of the composites (Tables 3 & 4) suggest that CFRCs are lighter in weight than GFRCs. The mechanical properties of GFRCs are better than that of CFRCs, due to its texture structure.
The lower values of electric breakdown potential of composites (Tables 3 & 4) could imply the presence of a charred path, over which subsequent discharge could take place more rapidly. Additionally, minute leakage of current may arise from surface contamination. The lower values of dielectric break-down strength for the present composites indicate that the composites are insulated.
The high chemical resistances of both types of composites (Tables 3 & 4) indicate that the imide moity might contribute to a high level of cross-linking of epoxy resin with PAI during composite fabrication. However,