Molecular dynamics of cyclic and linear poly(dimethylsiloxanes)

Abstract

Dielectric spectroscopy (10⁻¹ Hz to 10⁷ Hz) has been employed to study the molecular dynamics of a series of cyclic and linear polydimethylsiloxanes (PDMS) of various molecular weights ranging from 300 to 10 000 g/mol in the temperature range above the glass transition (from 130 K to 190 K). The observed α-relaxation depends strongly on both molecular weight and structure of the samples. For linear PDMS oligomers, the α-relaxation shifts towards lower temperatures with decreasing molecular weight in good accordance with the Fox-Flory-model. Cyclic PDMS reveals a qualitatively different molecular weight dependence: for a given temperature the α-relaxation time increases with decreasing ring length, but has a maximum for small oligomers (degree of polymerizationn≈6). The shape of relaxation curves and, with it, the relaxation time distribution is independent from length and architecture of the chains The observed experimental findings are in qualitative agreement with dynamic Monte-Carlo simulations.