Dynamic mechanical behavior of ordered off-stoichiometric polyurethane systems at the gel point threshold

Dynamic mechanical and thermal behavior of ordered off-stoichiometric polyu rethane (PU) systems, before and after the gel point, based on the mesogeni c diol 6,6'-[ethylenebis(1,4-phenyleneoxy)]dihexan-1-ol (D), 2(4)-methyl-1, 3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) were studied . Polymer samples were prepared at various initial molar ratios of the reac tive groups, r= [OH](T)/[NCO](DI)/[OH](D), ranging from 1/10/9 to 5/10/9 (t he ratio [NCO](DI)/[OH](D) = 10/9 was constant); the total mole ratio of hy droxy (OH) and isocyanate (NCO) groups, r(OH) = [OH]/[NCO] = ([OH](D))/[NCO ](DI), changed from 1 to 1.4. Dynamic mechanical measurements during the cu ring reaction showed that the power law parameters that characterize the cr itical gel state (gel strength S and relaxation exponent n) are dependent o n the initial composition (the ratio r(OH)). The gel-point critical ratio o f reactive groups r(OH)(c), found during curing in the ordered state of the diol (at low curing temperature), has revealed that the critical gel (CC) structure is determined by a contribution of strong physical interactions a s well as chemical junctions and does not correspond to pure chemical gelat ion (CG structure formed at low temperature exhibits flow at elevated tempe ratures in the isotropic state). This fact suggests that formation of the m esophase enhances the connectivity of the molecular structure at the gel po int. Dynamic mechanical behavior of fully cured chemical networks (r(OH) < r(OH)(c)) and un-cross-linked (r(OH)> r(OH)(c)) samples (and a CG sample) h as also been investigated. Decreasing the r(OH) ratio (increasing concentra tion of chemical cross-links in the systems) inhibits conformational rearra ngements required for ordering at the same time, the intensity of the slow relaxation process in the rubbery region decreases.