Brillouin scattering from polyurethane gels

The hypersonic sound speed and attenuation in a series of PPO polyurethane gels show a strong crosslink dependence in highly crosslinked samples. The temperature of maximum loss for the α-relaxation is raised from ∼370 K to ∼400 K in a highly crosslinked gel whilst the β-loss maximum is unaffected. This result indicates cooperative motion at the transition with a rearranging unit which appears to involve some 3 or 4 monomer units. The hypersonic properties were also found to depend upon the relative proportions of polymer and solvent in the gel. In the dry network hydrogen bonding between the urethane links and the interposed PPO dominates the dynamics. As the proportion of solvent increases, the effect of this hydrogen bonding is reduced, lowering the relaxation temperatures.