Viscoelastic behavior of scarcely crosslinked poly(dimethyl siloxane) gel: 3. Examination of gelation process

Abstract

Linear viscoelastic behavior was investigated during a gelation process of two poly(dimethyl siloxane) prepolymers A and B of molecular weight Mw,pre = 3.5×104. The gelation occurred mainly through a reaction of monomethylsilyl groups (0.7 mol% in the backbone of the prepolymer B) and vinyl groups at the chain ends of A and B, giving a scarcely crosslinked gel having a large sol fraction (wsol = 0.58 on completion of gelation) and exhibiting the equilibrium modulus much smaller than a nominal modulus calculated from Mn,pre, (1-wsol)r RT/Mn,pre with ρ = density. The reaction was conducted at a relatively low temperature, 50 °C, so that this reaction proceeded slowly and the viscoelastic changes during the gelation process were well resolved. The terminal flow behavior was observed at a reaction time tr ≤ 54 min (pre-gel stage), while the elasticity at low angular frequencies ω became prominent at tr ≥ 60 min (post-gel stage). The critical gelation behavior characterized with a power-law relationship between the storage/loss moduli and ω,G' ∝ G" ∝ ωn (0 < n < 1), was not observed at tr between these stages, suggesting that the gelation did not occur through formation of a huge, self-similarly hyper-branched critical gel network. For further examination of this gelation process, the prepolymer mixture was collected at several reaction times tr and the molecular characteristics of toluene-soluble sol component therein and a number fraction of the remaining (unreacted) monomethylsilyl group nMeSi were examined. It turned out that the high-M tail of the sol became enriched with densely branched chains of M reaching 1×107 (≅ 300Mw,pre) and nMeSi rapidly decreased to ~ 0.3 with increasing tr up to 65 min while the branch content/molecular weight of the sol decreased and nMeSi gradually approached 0 on a further increase of tr. These results suggested a gelation mechanism that densely branched pre-gel chains involving up to 300 prepolymer chains were first formed on consumption of the majority of the monomethylsilyl groups at tr ≤ 65 min and then these pre-gel chains were scarcely linked through a small amount of remaining monomethylsilyl groups to form the 3-dimensional gel at tr > 65 min. The lack of critical gelation behavior as well as the small equilibrium modulus (