Formation, structure, thermal and dynamic mechanical behavior of polyurethane networks based on a diethanolamine derivative with mesogenic group

Dynamic mechanical, differential scanning calorimetry and X-ray scattering behavior of ordered polyurethane systems, based on a diol with rigid (mesogenic) group in side chain (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI) and two triols (T)--stiff trimethylolpropane (TMP) or flexible poly(oxypropylene)triol (PPT), was investigated during crosslinking and on the networks. The networks were prepared at various stoichiometric initial molar ratios of the reactive groups, [OH]_T/[NCO]_DI/[OH]_D ranging from 1/2/1 to 1/20/19. From our measurements it follows that: (a) Power-law parameters, which are characteristic of the structure at the gel point (the gel strength S and the relaxation exponent n), are dependent on the initial ratio of the reactants. With increasing content of mesogenic diol in the system (increasing length of elastically active network chains, EANCs), the gel strength S increases and the relaxation exponent n decreases; higher S and lower n are found for stiffer TMP networks in comparison with more flexible PPT ones. (b) Introduction of crosslinks reduces the flexibility of the network chains in fully cured samples and inhibits conformational rearrangements required for ordering. A more complex thermal behavior was found for networks based on TMP in comparison with those based on PPT. (c) Strong physical interactions between the mesogens promote cyclization in the course of crosslinking; the fraction of bonds lost in intramolecular cycles is ∼15% for fully cured networks.