Glass Transition Temperature and Heat Capacity Change at the Glass Transition Region of Polymethylsiloxanes with Phenyl Groups

Abstract

Glass transition temperature (Tg) and heat capacity change WO at the glass transition region of two kinds of quenched polymethylsiloxanes partially substituted with phenyl groups, polydimethyldiphenylsiloxane (PDMDPS) and polydimethylmethylphenylsiloxane (PDMMPS), were studied using a differential scanning calorimeter.
The T_g and the δC, were related to the weight fraction of the siloxane unit with phenyl groups.
The T_g was expressed by the equation according to Pochan et al. and the Tg's of polydiphenylsiloxane (PDPS) and polymethylphenylsiloxane (PMPS) were determined as 388 K and 330 K, respectively.
By assuming that the δC, is proportional to the free energy change (δG) at the glass transition, δ_p∝ δG=RT in f(x) was derived. Here f(x) represents a function of state depending on the content of the siloxane unit with phenyl groups.
According to the above equation, δC_p⋅T^-1 were related to the logarithmic weight fraction of siloxane unit with phenyl groups, and the δC_ps of PDPS and PMPS were determined as 0.291 and 0.371 ⋅g^-1⋅K^-1, respectively.
For both PDMDPS and PDM MPS, the δC, have a minimum values at 10 mol% of the phenyl-bearing siloxane unit content. There was not so much difference between these two polysiloxanes in the behavior of the δC_p⋅Tg^-1 in spite of the variation of the content of siloxane unit with phenyl groups. So that, it was assumed that the effect on the statistical conditions was almost the same between these two polysiloxanes partially substituted with phenyl groups, PDMDPS and PDMMPS.