Model networks of end‐linked polydimethylsiloxane chains. XIV. Stress‐strain, thermoelastic, and birefringence measurements on the bimodal networks at very low temperatures

Bimodal networks consisting of very short and relatively long poiydimethylsiloxane (PDMS) chains were studied from 30 to −52°C in an attempt to elucidate the anomalous increases in modulus [f*] exhibited by such elastomeric materials at high elongations. Temperature was found to have very little effect on (i) the elongation at which the upturn in [f*] becomes discernible, (ii) the elongation at which rupture occurs, and (iii) the total increase in [f*] up to the rupture point. The standard force‐temperature (“thermoelastic”) plots were linear, but gave values of the energetic contribution to the total force which were significantly smaller than those universally obtained on unimodal, long‐chain PDMS networks. Birefringence‐temperature relations were also found to be linear, and yielded values of the optical‐configuration parameter and its temperature coefficient which were in satisfactory agreement with the corresponding values reported for unimodal PDMS networks. These results indicate that even at very low temperatures the observed increases in modulus (and consequent improvements in ultimate strength) are due to non‐Gaussian effects arising from limited chain extensibility, rather than from intermolecular reinforcing effects such as strain‐induced crystallization.