Low temperature crystallization behavior of natural rubber by dynamic mechanical analysis

Abstract

A fundamental study of low temperature crystallization of natural rubber (NR) gum polymer (raw elastomer) was conducted using dynamic mechanical analysis (DMA) in oscillatory shear rheology mode. Isothermal crystallization was followed using DMA for crystallization temperatures ranging from-15 to-35 degrees C, with the maximum rate of crystallization noted at-25 degrees C. After the isothermal crystallization (annealing) for times from 6 to 9 h, DMA heating scans revealed two melting transitions (a and b ) with locations that depended on the prior annealing temperature. The locations of these melting transitions were comparable with literature results for melting peaks by differential scanning calorimetry. At temperatures above these melting transitions, we identified two additional relaxations in the DMA heating trace that did not depend on the prior crystallization history. We also found evidence of the melt memory effect in polymer crystallization, which is discussed. During annealing at-25 degrees C, high cis-1,4 isoprene rubber (IR) showed considerably slower and lower extent of crystallization than NR, and crosslinked NR did not show noticeable crystallization within the 12-h experiment.