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 (Mg) and the end-reactive precursor PDMS. Two host networks with different mesh characters and sizes, i.e., Mx ≈ Me (trapped entanglement dominant) and Mx < Me (cross-link dominant), were prepared by endlinking the sufficiently longer and shorter precursor PDMS relative to entanglement spacing (Me), 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 Mg. The characteristic time τg, defined by the inverse of the frequency at the tan δ maximum, varies with Mg in good agreement with the predictions of the reptation theories, regardless of Mx. Interestingly, the reptation of the guest chains in the cross-link-dominant network of Mx < Me is markedly retarded relative to that in the entanglement-dominant network of Mx ≈ Me: τg in the crosslink-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 Mx, which suggests that the effects of the high concentration of cross-link in the network of Mx < Me on the dynamics of guest chains must be considered.

Nonlinear Mechanical Behavior of Scarcely Crosslinked Poly(dimethyl siloxane) Gel: Effect of Strand Length Polydispersity

“…This behavior is qualitatively similar to usual crosslinked rubbers/gels. For example, Figure 3 shows the E data for the Gel-U sample having monodisperse strands with trapped entanglements (and containing 15 wt % of the guest sol chains of M n ¼ 138 Â 10 3 ) measured in this study at 20 C. These data, normalized by the G e data in literature (¼ $ 1:4 Â 10 5 Pa) 27 and plotted against ", clearly indicate the nonlinear hardening/ rupture after the linear elastic behavior at small ".…”

“…Figure 1 of Ref. 27). The corresponding molecular weight of the effective gel strands, M c ¼ ð1 À f guest ÞRT=G e ¼ $ 15 Â 10 3 ( = PDMS density, R = gas constant, and T = absolute temperature), was considerably smaller than the prepolymer molecular weight (84 Â 10 3 ) and rather close to the entanglement molecular weight (M e ¼ $ 8:1 Â 10 3 for PDMS 28 ), indicating that Gel-U prepared through the end-linking reaction in bulk contained densely trapped entanglements.…”

“…This gel, identical to the NL-138 sample in Ref. 27 and referred to as Gel-U in this study, was generously supplied from Prof. Urayama at Kyoto University. He synthesized the gel through tetrafunctional end-linking of monodisperse PDMS prepolymer (M n ¼ 84 Â 10 3 ) in bulk in the presence of unreactive, guest homo-PDMS chains (M w ¼ 138 Â 10 3 ; f guest ¼ 15 vol % in the system).…”

“…He synthesized the gel through tetrafunctional end-linking of monodisperse PDMS prepolymer (M n ¼ 84 Â 10 3 ) in bulk in the presence of unreactive, guest homo-PDMS chains (M w ¼ 138 Â 10 3 ; f guest ¼ 15 vol % in the system). 27 Gel-U had the equilibrium shear modulus of G e ¼ $ 1:4 Â 10 5 Pa at 30 C (cf. Figure 1 of Ref.…”

“…The corresponding molecular weight of the effective gel strands, M c ¼ ð1 À f guest ÞRT=G e ¼ $ 15 Â 10 3 ( = PDMS density, R = gas constant, and T = absolute temperature), was considerably smaller than the prepolymer molecular weight (84 Â 10 3 ) and rather close to the entanglement molecular weight (M e ¼ $ 8:1 Â 10 3 for PDMS 28 ), indicating that Gel-U prepared through the end-linking reaction in bulk contained densely trapped entanglements. 27 This Gel-U sample, supplied in a form of thick disk of the thickness ¼ $ 5 mm, was carefully sliced with a razor into rectangular specimens of the thickness ¼ $ 1 mm, length ¼ $ 20 mm, and width ¼ $ 10 mm, and these specimens were utilized in the elongational test.…”