Main-Chain Lyotropic Liquid-Crystalline Elastomers. 2. Orientation and Mechanical Properties of Polyisocyanate Films

Novel types of main-chain, lyotropic liquid-crystalline polymer networks consisting of polyisocyanate chains were prepared by hydrosilation cross-linking. The characteristic liquid-crystalline band texture generated by shearing these networks in the swollen state was found to persist 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 suitable stresses and then dried in the strained state, in an orientation technique developed to improve the properties of networks consisting of such semiflexible polymers. The mechanical properties of the oriented films were then carefully examined as a function of their processing history: curing temperature, curing time, sol fraction, equilibrium swelling ratio, cross-link length, and polymer concentration during deformation. In the case of uniaxial extension, the tensile strengths and moduli of these oriented films increased 5-7 fold in the orientation direction. As expected, however, their mechanical properties in the perpendicular direction were diminished. Equibiaxial extension was found to have the advantage of giving mechanical properties that were uniform in the plane of the films and gave more than 2-fold increases in tensile strength and modulus at extension ratios during drying as small as 20%! The conclusion that these improvements were due to long-range ordering of the chains was verified by birefringence measurements.