Terminal relaxation of model poly(dimethylsiloxane) networks with pendant chains

Stress relaxation of model poly(dimethylsiloxane) networks with pendant cha ins and nearly constant cross-linking density was studied. The networks wer e obtained by end-linking a mixture of long chains bearing terminal reactiv e groups with a trifunctional cross-linker. Long poly(dimethylsiloxane) cha ins in the initial mixture contain 90 wt % of difunctional molecules with r eactive groups at both ends and 10 wt % of monofunctional chains with a sin gle reactive terminal group located in one of their ends. Difunctional chai ns will be mainly elastically active chains after cross-linking while monof unctional chains will remain as long pendant molecules. The fitting of the experimental stress relaxation values to the classic Chasset-Thirion equati on shows a strong dependence of the exponent on the molecular mass distribu tion of the pendant chains. From these results, a new model for the termina l relaxation of elastomers is proposed, taking into account the molecular m ass distribution of pendant chains. The dynamics of polymer networks in the terminal relaxation zone is modeled considering the reptation theory. In t he terminal zone, the proposed model behaves similarly to the Chasset-Thiri on equation. The model adequately describes the behavior of networks synthe sized by end-linking. On the other hand, when applied to different networks , it leads to similar conclusions as previous theories developed for networ ks obtained by random cross-linking.