| Abstract
| - Amorphous poly(diethylsiloxane) (PDES) elastomers undergo a transition to an alignedmesomorphic state when subjected to uniaxial tension. The structural changes associated with thistransition and the kinetics of its formation have been investigated by in-situ synchrotron wide-angle andsmall-angle X-ray scattering. In the mesomorphic state, the PDES elastomers are biphasic, consisting ofaligned mesophase domains and amorphous material. Because of the well-defined structure of the networksused, we were able to determine that the mesophase domain size is governed by the precursor chainlength and is unaffected by trapped entanglements. The observed increase in mesophase content withincrease in extension ratio in a fully necked sample is caused by an increase in the number of mesophasedomains rather than an increase in domain size. In the extended state, PDES elastomers attain a veryhigh degree of segment orientation comparable to that of mesogen-containing liquid crystalline elastomers.
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