PROTON SPIN RELAXATION IN LIQUID POLYDIMETHYLSILOXANE - MOLECULAR-MOTION AND NETWORK FORMATION

Proton spin relaxation time measurements are reported for a series of liquid polydimethylsiloxanes covering the temperature range from minus 50 to 200 degree C. These include fractionated and whole polymer samples (M//w from 10**4 to 10**6), binary mixtures, and crosslinked material. This paper is mainly devoted to the behavior of the spin-spin relaxation (T//2) which is sensitive to the low-frequency long-range configurational motions occurring in the liquid phase. These motions determine the time scale over which final motional averaging of the nuclear dipolar interactions occur, prior to the onset of normal liquid-like behavior of T//2. A further aspect of this work concerns the observed sensitivity of T//2 to network formation which considerably restricts the long-range motions that determine T//2. It is possible to determine the fraction of material present in the form of a network from the relative proportions of network and non-network T//c2 components. This is not only true for a permanent crosslinked network but also for a network arising from transient entanglement couplings.