Formation, structure, thermal and dynamic mechanical behaviour of ordered polyurethane networks based on mesogenic diol

Dynamic mechanical and thermal behaviour of ordered linear and crosslinked polyurethane systems based on a mesogenic diol, 4,4′-bis(11-hydroxyundecyloxy)biphenyl (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI) and poly(oxypropylene)triol (T) was investigated during network formation and in fully cured samples. The networks were prepared at various stoichiometric initial molar ratios of the reactive groups, [OH]T/[NCO]DI/[OH]D ranging from 1/1/0 to 1/12/11; for comparison, linear sample with the ratio 0/1/1 was also investigated. From our measurements it followed: (a) The power-law parameters, which are characteristic of the critical structure at the gel point (the gel strength S and relaxation exponent n), are dependent on the initial ratio of the reactive groups. With increasing content of mesogenic diol in network chain (increasing length of elastically active network chain (EANC)) both, the gel strength S and relaxation exponent n decrease. (b) Introduction of chemical junctions reduces flexibility of the EANCs in fully cured networks and inhibits conformational rearrangements required for ordering. Increasing the content of mesogenic diol in EANCs increases the concentration of permanent physical interactions (trapped entanglements) in the systems and the intensity of the slow relaxation process in the rubbery region.