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 ≅ 35×10 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.

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

“…These prepolymers were from the same batches as those utilized in the previous work. 25,26 The prepolymer-A, a neat PDMS having vinyl groups at the ends, had the weight-average molecular weight and polydispersity index of M w ¼ 34:7 Â 10 3 and M w =M n ¼ 2:6. 25 The prepolymer-B, a chemically modified PDMS with a small fraction (0.7 mol %) of the dimethyl siloxane units being replaced by monomethyl siloxane units, had M w ¼ 35:1 Â 10 3 and M w =M n ¼ 3:1.…”

“…25,26 The prepolymer-A, a neat PDMS having vinyl groups at the ends, had the weight-average molecular weight and polydispersity index of M w ¼ 34:7 Â 10 3 and M w =M n ¼ 2:6. 25 The prepolymer-B, a chemically modified PDMS with a small fraction (0.7 mol %) of the dimethyl siloxane units being replaced by monomethyl siloxane units, had M w ¼ 35:1 Â 10 3 and M w =M n ¼ 3:1. 25 The prepolymers A and B were mixed at a weight ratio of w A =w B ¼ 1=1 and allowed to react at 120 C for 3 h to give the as-prepared gel referred to as Gel-1/1.…”

“…25 The prepolymer-B, a chemically modified PDMS with a small fraction (0.7 mol %) of the dimethyl siloxane units being replaced by monomethyl siloxane units, had M w ¼ 35:1 Â 10 3 and M w =M n ¼ 3:1. 25 The prepolymers A and B were mixed at a weight ratio of w A =w B ¼ 1=1 and allowed to react at 120 C for 3 h to give the as-prepared gel referred to as Gel-1/1. As shown in the top part of Scheme 1, the heat (at 120 C) and a catalyst contained in the commercial prepolymers allowed the vinyl groups at the ends of prepolymers A and B to react with the monomethyl siloxane group in the prepolymer B to form the crosslinks.…”

“…As shown in the top part of Scheme 1, the heat (at 120 C) and a catalyst contained in the commercial prepolymers allowed the vinyl groups at the ends of prepolymers A and B to react with the monomethyl siloxane group in the prepolymer B to form the crosslinks. 25 In addition to this major reaction, two vinyl groups can also react with each other to give the crosslinks (bottom part of Scheme 1), though this minor reaction occurred less frequently. 25 The Gel-1/1 thus prepared contained a large amount of sol chains.…”

“…25,26 The Gel-1 sample contained a large fraction f sol of sol chains ( f sol ¼ 0:58; mostly unreacted prepolymers), and the sol chains appeared to have suppressed formation of densely trapped entanglements during the crosslinking reaction. Thus a very scarcely crosslinked network structure was formed in Gel-1.…”