STUDIES OF CYCLIC AND LINEAR POLYDIMETHYLSILOXANES .32. DIELECTRIC-RELAXATION INVESTIGATIONS

The dielectric cu-relaxation of a series of narrow-molecular-weight fr actions of cyclic and linear polydimethysiloxanes (PDMSs) has been inv estigated as a function of frequency in the temperature range 140.5-15 7.5 K. The temperature of the alpha-transition decreases with increasi ng chain length for cyclic PDMS and increases with increasing chain le ngth for linear PDMS, in agreement with the previously reported differ ential scanning calorimetry (d.s.c.) glass transition. The cooperativi ty of the relaxation process, according to the Kohlrausch-Williams-Wat ts analysis, is greater in linear PDMS. The activation energy and mole cular dipole moment increase with increasing chain length for both cyc lic and linear molecular types. Experimental values of the dipole mome nt ratio are in good agreement with those measured by static dielectri c techniques and with the theoretical long-chain value. Copyright (C) 1996 Elsevier Science Ltd.