Novel interphases: Synthesis, molecular orientation and grafting of liquid crystal polymers on carbon fibers.

摘要

The objective of this research has been to study composite materials containing liquid crystal polymers either as matrices or in interfacial boundary zones. The fundamental point of interest here is the interfacial response that fiber surfaces can potentially induce in self ordering polymers. This investigation includes studies in three broad categories: molecular orientation of nematic melts containing dispersed carbon fibers, the chemical grafting of a liquid crystal polymer to these fibers by in-situ polymerization of monomeric melts, and the synthesis and anchoring on fibers of specially designed liquid crystal monomers and polymers containing pendant chemical functions on their mesogenic groups.;The matrix polymer used in orientation studies is a chemically disordered thermotropic copolyester synthesized from the monomers p-acetoxybenzoic acid, diacetoxyhydroquinone and pimelic acid. Broadline proton NMR studies revealed enhanced rates of magnetic orientation in the polymer melt when carbon fibers are dispersed in the medium. Polarized optical microscopy studies of these composite samples revealed that fiber surfaces influence the orientation of mesomorphic molecules in their immediate surroundings. Inspite of this orienting influence of fibers on the matrix molecules, fracture surface analysis of composite specimens by SEM revealed poor wetting of the fibers by the matrix polymer. It was possible to induce excellent wetting of this polymer on carbon fibers by in-situ polymerization of the monomers in the presence of functionalized fibers which result in covalent grafting of mesogenic chains on fiber surfaces. New hydroxy-functionalized side chain liquid crystal polymers were synthesized which could be useful in the molecular design of interfacial boundary layers in polymer-based composites. Optical observations reveal that the phenolic monomer has an exceptionally strong tendency for orthogonal anchoring on glass surfaces. However, this anchoring by glass is disturbed around carbon fibers dispersed in the monomeric melt. From studies with a compensator, it is inferred that these "boundary zones" contain arrays of molecules oriented in two different directions. In contrast to the monomer, the orienting behavior of the polymer or either glass or carbon surfaces is less defined. However, this property of the polymer could be improved significantly by replacing some phenolic groups by bulky, nonpolar silyl ether groups.