Viscoelastic Behavior of Scarcely Crosslinked Poly(dimethyl siloxane) Gel

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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 Mpre ≈ 35×103. Time-temperature superposition worked, and master curves at 20 °C were constructed for the storage and loss moduli, G' and G, in a wide range of angular frequency ω (= 103 −10−5 s−1) by combining the data obtained from dynamic oscillatory tests and creep tests. With decreasing ω to 10−1 s−1, G decreased in proportion to ω0.6 and G' rapidly decreased to its equilibrium plateau at the modulus Ge = 2800 Pa. On a further decrease of ω well in the plateau regime of G', G decreased in proportion to ω0.3. 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 Ge data (as well as the equilibrium swelling ratio in toluene), Mc ≈ 340 ×103, was about ten times larger than Mpre. The crosslinking reaction was made in the bulk state but still gave such a scarce gel network (with Mc ≈ 10 Mpre) 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 Mc 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∝ωn 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.