Relaxation behavior of polyurethane networks with different composition and crosslinking density

- A decrease of Tg occurs in polyuretanes with increasing diole length between crosslinks.
- Explanation is internal plastizication due to an increasing number of alkyl carbons n.
- Crystallization is found for the longest diole sequences under investigation (n = 16.6).
- αβ crossover is defined based on minimum difference between α and β relaxation times.
- The dielectric α relaxation strength Δεα shows an onset close to the αβ crossover.

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 Tg as well as of the softening temperatures TαDMA and TαDRS 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, β and γ, 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 Tg 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 β relaxation and cooperative α dynamics are indicated. An onset of the dielectric α relaxation strength Δεα is observed for all amorphous polyurethane networks. A linear extrapolation of Δεα vs. 1/T gives onset temperatures Ton which are in good agreement with αβ crossover temperatures Tαβ being the temperature where the difference between α and β relaxation times τα−τβ approaches a minimum. This finding supports an onset of the cooperative α motions in the αβ crossover region as reported in the previous literature for many other glass forming materials.

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