Gelation and dynamic mechanical behavior of liquid crystalline networks

The dynamic mechanical behavior of polyurethane networks based on liquid cr ystalline (LC) diol, 6,6'-[ethylenebis(1,4-phenyleneoxy)]dihexanol (D), 2(4 )-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) a t the stoichiometric molar ratio of isocyanate (NCO) and hydroxy (OH) group s was studied. Samples were prepared at various initial molar ratios of the reactive groups [OH](T)/[NCO](DI)/[OH](D) from 1/1/0 to 1/40/39. The gelat ion studies during the curing reaction in the LC and isotropic states showe d that the critical gel structure at the sol/gel transition exhibits a powe r-law mechanical behavior; the relaxation exponent in the LC state is alway s higher than that in the isotropic state. From viscoelastic results, it fo llows that formation of the LC mesophase enhances the connectivity of the m olecular structure at the gel point. Introduction of chemical cross-links i n fully cured networks reduces the flexibility of the elastically active ne twork chains (EANCs) and inhibits conformational arrangements required for LC ordering. Increasing the content of LC diol in networks, which leads to an increase of the length of EANCs, increases the concentration of permanen t physical interactions (trapped entanglements) in the systems; the intensi ty of the slow relaxation process in the rubbery region also increases.