Novel orientation techniques for the preparation of high-performance materials from semi-rigid polymers, and, Mechanical properties of thermoreversible gels

The first goal of this study is to devise a novel orientation technique for semi-rigid polymers for the preparation of highly-ordered materials having excellent high-performance characteristics. The orientation method consists of four steps: (1) selecting polymer chains with enough stiffness to give liquid-crystalline mesophases, (2) cross linking chains, either by chemical reagents or by high-energy radiation, (the cross-links conferring sufficient solidity for the polymer to remain in a deformed for any length of time), (3) deforming the swollen networks to induce segmental orientation, and (4) removing the solvent at constant length or stress, causing an isotropic-to-anisotropic phase transition, and thus yielding a highly-ordered material.;Liquid-crystalline phase separation in networks of cellulose acetate and hydroxypropylcellulose has been investigated. The networks cross linked in the liquid-crystalline phase (relatively concentrated solution) were found to preserve the original liquid-crystalline structure. The gels prepared in the isotropic state (dilute solution) were also observed to retain the anisotropic mesophases.;Theoretical and experimental studies on the elastic and photoelastic properties of networks having semi-rigid chains have been carried out. Theoretical work was performed, using a recently proposed lattice model, to quantitatively study the stress, strain, orientation, and isotropic-to-nematic phase transformation for deformed swollen networks with semi-rigid chains. Another application of this model involved investigation of the relations of strain and stress to optical birefringence, and strain to stress-optical coefficient. Of particular interest have been the effects of volume fraction of polymers on the properties of the swollen networks in the isotropic and nematic states, and networks undergoing phase transitions. Results show exceptional orientability and an isotropic-to-nematic phase transition for the networks under macroscopic deformation. Experimental work has been performed on the gels from hydroxypropylcellulose, using stress-strain and strain-birefringence isotherms. The results from experiments are in good agreement with the theoretical predictions.;Mechanical properties such as tensile modulus and strength were measured for uniaxially and biaxially oriented films of cellulose acetate and hydroxypropylcellulose processed with this novel orientation technique, which showed dramatic increases.;The second part of the research involved use of mechanical property measurements to characterize gels and gelation processes. Thermoreversible gels of ethylene hexene-1 copolymers and n-hydrocarbons were prepared, and their shear moduli upon compression were measured with U-type glass gel-tubes. The measurement could be done by pressure pulses instantaneously, so information on gelation kinetics has been obtained by study of the evolution of the modulus of gels with time. Also investigated were the effects of concentration, gelation temperature and time, molecular weight, and branch content of copolymers on gel modulus.