Stress-Strain Behavior of Randomly Crosslinked Polydimethylsiloxane Networks

We have demonstrated by means of our small-strain data that suppression of junction fluctuations cannot solely account for the discrepancy between experimental modulus values and the predictions of the phantom-network theory. The good agreement between the intercepts in Figures 3 and 4 and the value of GN0 leaves little doubt regarding the relation between the two and the validity of the model represented by Equation (12). Further experiments should be carried out on materials with higher GN0 values than the PDMS chains used here. This will magnify the contribution of trapped entanglements and will demonstrate more clearly the effects discussed here. Further study is also required in order to understand the role played by the polymer backbone on the amount of junction suppression. The question raised by Dossin and Graessley as to whether differences in h values for different networks are due to differences in structure between randomly crosslinked and end-linked networks or to differences in the relative magnitude of topological contributions for different polymers was answered by this work. The agreement of the h value obtained here with those obtained for endlinked PDMS networks indicates that no inherent differences in structure exist between endlinked and crosslinked networks and that differences in polymer backbone are responsible for the values of h obtained. Objections that radiation crosslinked networks are somehow not suitable for testing rubber elasticity theories should also be laid to rest by the good agreement of our results with those of Langley and Polmanteer. The large-strain data obtained here show the ability of Flory's strain energy function to correctly model tension-compression data over the range of crosslink densities covered by this work. Edwards' model did not agree well with our data for low degree of crosslinking samples. Further work is still required since our data exhibited relatively small deviations from Mooney-Rivlin behavior. Finally, the extreme importance of the careful analysis of the materials used, the reaction employed, and the resulting networks was demonstrated. The simplest available method for the verification of the network structure is by the determination of the sol fraction. The extraction of solubles in the case of highly crosslinked networks was found to be susceptible to weighing uncertainty and the presence of unreactive material. The former can be avoided by the use of larger samples, while the latter could be removed by vacuum stripping for our material.