Viscoelastic behavior of scarcely crosslinked poly(dimethyl siloxane) gel

Abstract

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 ≅ 35×103. 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. © 2006 The Society of Rheology, Japan.