A SPIN-PROBE STUDY OF HETEROGENEITY IN THE NATURAL-RUBBER MATRIX - THE EFFECT OF MOLECULAR-WEIGHT, MOLECULAR-WEIGHT DISTRIBUTION AND GEL PHASE

The electron spin resonance (e.s.r.) spin-probe method has been applie d to investigate the matrix heterogeneity of natural rubbers differing in molecular weight, molecular weight distribution and gel content. T he composite temperature-dependent e.s.r. spectra above the glass tran sition temperature (T-g) were evaluated in order to obtain the populat ion of spin probes contributing to restricted and fast motion. From th e ratio of spin probes distributed in the two motionally different env ironments, microstructural heterogeneities of the rubber matrix are ob served. It is shown that the restricted spin-probe motion above T-g is predominantly determined by the amount of gel phase and structure of the gel. Chemical treatment and mastication of natural rubber, which h ave a complex influence on gel fraction, molecular weight and intermol ecular interactions, have been considered in analysis of the temperatu re dependence of the e.s.r. spectra. A comparison between natural rubb er and a synthetic polyisoprene confirms the contribution of the gel p hase and its structure to the microstructural characteristics of rubbe r matrices. (C) 1998 Elsevier Science Ltd. All rights reserved.