Preparation and Properties of Poly(Phenyleneterephthalamide)-Silica Ceramers with Interphase Bonding from Aminophenyltri
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* Permanent address: Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistan. 127 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
hybrid material having much-improved thermal and dimensional stability. Syntheses of ceramers using high-temperature polymers such as the poyimides1 6 ,17 have been carried out recently, and there has been a report' 8 of the preparation of silica-Aramid hybrid gels and films which were thermally stable up to 450 oC. In the present study, copolymers of Kevlar@ were prepared for use in hybrid inorganicorganic materials synthesized through this sol-gel technique. For preparation of mechanicallystrong ceramers, Kevlarg itselfwould have been ideal. However, to carry out the sol-gel process, a suitable solvent having compatibility with the polymer and the metal alkoxide is necessary. Unfortunately, there is no known solvent that will keep Kevlar® in solution as its molecular weight increases. To overcome this difficulty, a few kinks were placed into the otherwise linear Kevlar® chain, through incorporation of some meta phenylene groups. Specifically, a 35:65 molar mixture of 1,3 and 1,4 phenylene diamine with terephthaloyl chloride was stoichiometrically reacted in the solvent dimethyl acetamide (DMAC). During the polymerization reaction, the solvating power of DMAC increases due to the formation of a
complex with the HC1 being produced as the molecular weight of the polymer builds up. 19 Its molecular weight is dependent on the type of polymer, and on the concentration of the reaction mixture as well. 20 A mixture of solvents using hexamethylphosphorictriamide (HPT) and Nmethylpyrrolidine (NMP) was also tried, in an attempt to achieve higher molecular weights. The high molecular weight Aramid thus prepared was used to prepare a series of hybrid materials. In these composites, a silica network was developed within the polymer matrix by incorporating and then hydrolyzing various proportions of tetramethoxysilane (TMOS). Aramid chains having carbonyl chloride end groups were then prepared by adding a slight excess of terephthaloyl chloride (TPC) at a later stage in the polymerization. The polymer chains were then end-capped with aminophenyltrimethoxysilane (APTMOS). The polymer thus prepared was used to synthesis a hybrid material in which the chains were chemically bonded to the silica network produced in-situ by the hydrolysis of TMOS. The mechanical properties of the chemically-bonded "ceramers" were compared with those of the corresponding materials having an unbonded structure, as described in the following sections. EXPERIMENTAL Materials The monomers used for the preparation of the desired polyamide, 1,4- and 1,3-phenylene diamines, were analytical-grade reagents from Aldrich. They were kept in a vacuum oven at 55 oC overnight before use. The TPC used was of analytical grade and the anhydrous DMAC was 99+ % pure, and the anhydrous NMP and HPT were both 99 % pure; all were obtained from Aldrich and used as received. The TMOS was 99 % pure and the
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