Glass-transition dynamics of a polyurethane gel using ultrasonic spectroscopy, dynamic light scattering, and dynamical mechanical thermal analysis

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Abstract
The glass-transition dynamics of a polyurethane gel were studied over a wide temperature and frequency range using ultrasonic spectroscopy (US), dynamic light scattering (DLS), and dynamical mechanical thermal analysis (DMTA). DMTA showed both an α and a β relaxation, while with DLS only the α relaxation could be observed. The α relaxation measured by DLS and DMTA was analyzed in terms of a continuous relaxation time distribution. This analysis method is compared to an analysis in terms of the Kohlrausch-Williams-Watts function for the DLS results and the Havriliak-Negami function for the DMTA results. The shape of the relaxation time distribution is temperature independent over the temperature range covered and identical for both techniques. The temperature dependence of the characteristic relaxation rates is well described by the so-called Vogel-Fulcher-Tamman-Hesse equation. Characteristic relaxation rates measured by DLS were about a factor 10 smaller than interpolated from the US and DMTA measurements. Since DLS measures a compliance and the two other techniques a modulus, lower values are expected. If the compliance data are converted into corresponding values for the modulus, the DLS results are compatible with the US and DMTA results.