Viscoelastic Relaxation of Guest Linear Poly(dimethylsiloxane) in End-Linked Poly(dimethylsiloxane) Networks

Abstract

Dynamics of guest linear polymer trapped in permanently cross-linked network is investigated using the end-linked poly(dimethylsiloxane) (PDMS) networks with well-defined mesh sizes ( M x ) containing unattached linear PDMS. Viscoelastic relaxation behavior of the PDMS networks with the unattached PDMS has been examined as a function of the molecular mass of the guest PDMS ( M g ) and the end-reactive precursor PDMS. Two host networks with different mesh characters and sizes, i.e., M x ≈ M e (trapped entanglement dominant) and M x < M e (cross-link dominant), were prepared by end-linking the sufficiently longer and shorter precursor PDMS relative to entanglement spacing ( M e ), respectively. The dynamic mechanical measurements reveal that the networks containing the guest chains show the definite maxima in loss factor tan δ at certain frequencies which are attributed to the viscoelastic relaxation of the guest chains, and in addition, the location of the tan δ peak shifts to the lower frequencies with increasing in M g . The characteristic time τ g, defined by the inverse of the frequency at the tan δ maximum, varies with M g in good agreement with the predictions of the reptation theories, regardless of M x . Interestingly, the reptation of the guest chains in the cross-link-dominant network of M x < M e is markedly retarded relative to that in the entanglement-dominant network of M x ≈ M e: τ g in the cross-link-dominant network is larger than that in the entanglement-dominant network by a factor of nearly 10 5 . Such remarkable slowing-down of reptation is not simply explained in terms of only the reduction in M x, which suggests that the effects of the high concentration of cross-link in the network of M x < M e on the dynamics of guest chains must be considered.