Viscoelastic behaviour of polymethyl methacrylate networks with different crosslinking degrees

The influence of the cross-linking degree on the dynamics of the segmental motions close to the glass transition of poly(methyl methacrylate), PMMA, networks was investigated by three different mechanical spectroscopy techniques: thermally stimulated recovery (TSR), dynamic mechanical analysis (DMA), and creep. The application of the time-temperature superposition principle to isothermal DMA and creep results permitted to successfully construct master curves for PMMA networks with distinct cross-linking degrees. The former results were fitted to the KWW equation. The obtained variation of âKWW for the distinct networks indicated that the relaxation curves tend to broaden as the cross-linking degree increases. TSR results clearly revealed a significant shift of the R-relaxation to longer times and a broader relaxation as the cross-linking degree increases, what was also observed by DMA and creep. A change from a Vogel to an Arrhenius behavior was detected by the three techniques with the decrease of temperature below Tg. The temperature dependence of the apparent activation energies (Ea) was calculated from DMA, creep, and TSR experiments; above Tg a good agreement was seen between the Ea values for all the techniques. Furthermore, the effect of the cross-linking degree on the fragility of PMMA networks was evaluated. For these materials an increase of fragility with increasing cross-linking degree was observed

An experimental and analytical study of the elasticity of model polyurethane networks crosslinked by tri- and quadriisocyanate

The original publication is available at www.springerlink.com : http://link.springer.com/article/10.1007%2Fs00396-009-2145-6?LI=truePolyurethane networks have been prepared from a mix of tri- and quadriisocyanate and from two types of diols, polyether-based (with molar masses of 1,000, 2,000, and 4,000 g/mol) and polyester-based (1,035 g/mol). The weight fraction of sol has been measured, as well as the elastic shear modulus of the gels. It has been found that the statistical theory of network formation predicts a weight fraction of sol in agreement with the experimental results, but its standard combination with the theory of rubber elasticity disagrees significantly with the elastic modulus measured. This suggests a discrepancy between theory and experiment in terms of elastically active chains. In contrast, the assumption that all nodes in the gel, or even in the system, are elastically active gives much better predictions for the system considered.ANR JCJC MELA