MAIN-CHAIN LYOTROPIC LIQUID-CRYSTALLINE ELASTOMERS .2. ORIENTATION AND MECHANICAL-PROPERTIES OF POLYISOCYANATE FILMS

Novel types of main-chain, lyotropic liquid-crystalline polymer networ ks consisting of polyisocyanate chains were prepared by hydrosilation cross-linking. The characteristic liquid-crystalline band texture gene rated by shearing these networks in the swollen state was found to per sist much longer than in the case of solutions of the same polymer in the un-cross-linked state, suggesting preservation of increased chain orientation. A series of these gels were deformed by application of su itable stresses and then dried in the strained state, in an orientatio n technique developed to improve the properties of networks consisting of such semiflexible polymers. The mechanical properties of the orien ted films were then carefully examined as a function of their processi ng history: curing temperature, curing time, sol fraction, equilibrium swelling ratio, cross-link length, and polymer concentration during d eformation. In the case of uniaxial el:tension, the tensile strengths and moduli of these oriented films increased 5-7 fold in the orientati on direction. As expected, however, their mechanical properties in the perpendicular direction were diminished. Equibiaxial extension was fo und to have the advantage of giving mechanical properties that were un iform in the plane of the films and gave more than 2-fold increases in tensile strength and modulus at extension ratios during drying as sma ll as 20%! The conclusion that these improvements were due to long-ran ge ordering of the chains was verified by birefringence measurements.