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-hydroxyundecylo xy)biphenyl (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI) and poly(oxypr opylene)triol (T) was investigated during network formation and in fully cu red samples. The networks were prepared at various stoichiometric initial m olar 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 al so investigated. From our measurements it followed: (a) The power-law param eters, which are characteristic of the critical structure at the gel point (the gel strength S and relaxation exponent n), are dependent on the initia l ratio of the reactive groups. With increasing content of mesogenic diol i n network chain (increasing length of elastically active network chain (EAN C)) both, the gel strength S and relaxation exponent n decrease. (b) Introd uction of chemical junctions reduces flexibility of the EANCs in fully cure d networks and inhibits conformational rearrangements required for ordering . Increasing the content of mesogenic diol in EANCs increases the concentra tion of permanent physical interactions (trapped entanglements) in the syst ems and the intensity of the slow relaxation process in the rubbery region. (C) 2001 Elsevier Science Ltd. All rights reserved.