Linear viscoelastic behavior was investigated for a poly(dimethyl siloxane) (PDMS) gel formed through a bulk double-liquid crosslinking reaction of two types of vinyl-terminated PDMS prepolymers of the molecular weights M pre ≅ 35×10 rapidly decreased to its equilibrium plateau at the modulus G e = 2800 Pa. On a further decrease of ω well in the plateau regime of G', G" decreased in proportion to ω 0.3 . Thus, the gel exhibited the fast and slow relaxation processes characterized with these types of power-law behavior of G". The molecular weight between the crosslinks evaluated from the G e data (as well as the equilibrium swelling ratio in toluene), M c ≅ 340×103 , was about ten times larger than M pre . The crosslinking reaction was made in the bulk state but still gave such a scarce gel network (with M c ≅ 10 M pre ) possibly because a large amount of sol chains and dangling chains had diluted the trapped entanglements during the reaction. From the analysis of the G' and G" data on the basis of the above M c value and the intrinsic Rouse relaxation time, the fast relaxation process was assigned as the Rouse-like constraint release (CR) process of individual gel strands. The polydispersity of the strands was found to be essential for the power-law behavior (G"∝ω n with n ≅ 0.6) to be observed in the plateau regime of G'. The slow relaxation process was related to fluctuation of the crosslinking points, which is equivalent to cooperative Rouse-CR motion of many gel strands connected at these points.

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Linear viscoelastic behavior was examined for poly(dimethyl siloxane) (PDMS) gels scarcely crosslinked through the double-liquid reaction. The gel sample after the sol extraction exhibited the fast and slow relaxation processes characterized with the power-law behavior of the dynamic loss modulus, G" ∝ω 0.5 and G" ∝ω 0.8 at high and low angular frequencies ω. The fast relaxation was essentially the same as that in the gel before the sol extraction and attributed to the constraint release (CR)-Rouse relaxation of individual gel strands having a considerable length distribution. In contrast, the slow relaxation, being related to cooperative CR involving neighboring gel strands, appeared to become faster after the sol extraction than before. This puzzling result may have reflected a spatial heterogeneity of the crosslinking density, although this hypothesis has not been proven yet. For the as-prepared PDMS gels containing the sol chains, an increase of the crosslinking density was found to extend the CR-Rouse relaxation tail to low frequencies compared to the more scarcely crosslinked strands after the sol extraction. Thus, the CR motion of the strands appeared to be more strongly affected by the crosslinking density than by the sol chains.

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Nonlinear mechanical behavior was examined for a scarcely crosslinked poly(dimethyl siloxane) gel (referred to as Gel-1/1) under constant-rate elongation and large step shear strains. The average molecular weight of the gel strands evaluated from the equilibrium modulus in the linear viscoelastic regime was M c ¼ 190 Â 10 3 , and the strands had a significantly broad molecular weight distribution, M w =M n ¼ $ 600 as estimated by fitting the linear viscoelastic moduli with a Rouse network model. In the elongational test at constant elongational rates _ " " (¼ _ = ; = elongational ratio), the Gel-1/1 sample exhibited _ " "-insensitive strain hardening followed by rupture at max ¼ 4:5. This max was significantly smaller than the max nominally expected for a gel composed of monodisperse strands having M c ¼ 190 Â 10 3 ; max ¼ 53 and max = max ¼ $ 0:08 for those strands. In contrast, a reference experiment made for a Gel-U sample composed of monodisperse strands (M c ¼ 15 Â 10 3 ; including densely trapped entanglements) indicated that max of this gel was close to max ; max ¼ $ 14, max ¼ 16, and max = max ¼ $ 0:9 for Gel-U. These results suggested that the low-M fractions of the strands in the Gel-1/1 sample were highly stretched and broken at much smaller than the max defined for the average M c , thereby governing the nonlinear elongational behavior/rupture of Gel-1/1. Under large step shear strains (> 2), Gel-1/1 exhibited nonlinear decay of the shear stress with time. Analysis of the linear viscoelastic moduli of Gel-1/1 after imposition of large strains indicated that the stress decay under large strains reflected scission of the low-M fractions of the gel strands as well as the motion of scissionformed long strands occurring with time. This behavior was qualitatively similar to the nonlinear elongational behavior, although a delicate difference related to time-dependent cessation/motion of the scission-formed long strands remained between the nonlinearities under the large shear and elongation.KEY WORDS: Scarcely Crosslinked Siloxane Gel / Polydisperse Gel Strands / Large Deformation / Strand Scission / The rubber elasticity has been one of the most important subjects in the field of polymer physics, and the relationship(s) between the mechanical properties and the network structure of rubbers/gels has been studied over several decades. In the early studies, Flory 1 and James and Guth 2 showed that the fluctuation of the crosslinking points reduces the equilibrium modulus compared to the modulus expected for the affine displacements of these points. Langley 3 and Dossin and Graessley 4 demonstrated that the modulus is enhanced by the trapped entanglement (permanent knot) between the network strands, and Murakami et al. 5 suggested a chemo-rheological method for determining the molecular weight between crosslinks. After these pioneering studies, several new aspects have been revealed for natural rubbers (crosslinked cis-polyisoprenes). For example, Toki et al. 6 examined X-ray diffraction from natural rubbers e...