Relaxation behavior of polyurethane networks with different composition and crosslinking density

The relaxation behavior of a series of solvent free polyurethane model networks with variable cross-link density prepared based on different commercial diols and a diisocyanate containing component are studied by differential scanning calorimetry (DSC), dynamic-mechanical analysis (DMA) and dielectric relaxation spectroscopy (DRS). A systematic decrease of the calorimetric glass temperature T-g as well as of the softening temperatures T-alpha(DMA) and T-alpha(DRS) from relaxation methods is observed with increasing length of the diol sequences between neighbored diisocyanate units acting as cross-linker. This trend is explained based on an internal plasticization of the polymeric network by long, highly mobile diol units containing an increasing fraction of methylene sequences. Cold crystallization effects are only indicated for the longest diol sequence under investigation. This is understood as a consequence of a large fraction of methylene sequences in combination with weaker geometrical constraints. Two secondary relaxations, beta and gamma, are observed in the glassy state for all amorphous samples at low temperatures by dielectric spectroscopy indicating the existence of localized motions in the polyurethane networks. Below T-g these relaxation processes are practically unaffected by changes in the length of the diol units and the softening behavior of the polymeric model networks. Interrelations between secondary beta relaxation and cooperative alpha dynamics are indicated. An onset of the dielectric alpha relaxation strength Delta epsilon(alpha) is observed for all amorphous polyurethane networks. A linear extrapolation of Delta epsilon(alpha) vs. 1/T gives onset temperatures T-on which are in good agreement with alpha beta crossover temperatures T-alpha beta being the temperature where the difference between alpha and beta relaxation times tau(alpha)-tau(beta) approaches a minimum. This finding supports an onset of the cooperative alpha motions in the alpha beta crossover region as reported in the previous literature for many other glass forming materials. (C) 2017 Elsevier Ltd. All rights reserved.