MOLECULAR-DYNAMICS OF CYCLIC AND LINEAR POLY(DIMETHYLSILOXANES)

Dielectric spectroscopy (10(-1) Hz to 10(7) Hz) has been employed to s tudy the molecular dynamics of a series of cyclic and linear polydimet hylsiloxanes (PDMS) of various molecular weights ranging from 300 to 1 0000 g/mol in the temperature range above the glass transition (from 1 30 K to 190 K). The observed alpha-relaxation depends strongly on both molecular weight and structure of the samples. For linear PDMS oligom ers, the alpha-relaxation shifts towards lower temperatures with decre asing molecular weight in good accordance with the Fox-Flory-model, Cy clic PDMS reveals a qualitatively different molecular weight dependenc e: for a given temperature the alpha-relaxation time increases with de creasing ring length, but has a maximum for small oligomers (degree of polymerization n approximate to 6). The shape of relaxation curves an d, with it, the relaxation time distribution is independent from lengt h and architecture of the chains. The observed experimental findings a re in qualitative agreement with dynamic Monte-Carlo simulations.