Dynamic light scattering study on sulfur-cured natural rubber

Abstract

Dynamic light scattering (DLS) was measured for sulfur-cured natural rubber (NR) as a function of sulfur content and curing time; the cooperative diffusion constant, Dcoop, and/or the normalized variance μ2 Λ ⁻² were determined by the cumulant method. Dcoop was proportional to polymer weight concentration at the equilibrium state of swelling Ce to a larger power than the theoretical prediction, 0.75, and μ2 Λ ⁻² decreased with increasing Ce. As easily estimated from these facts, the dependence of Dcoop on average molecular weight between crosslink sites determined from the application of the C* theorem, which method has been experimentally confirmed to be valid by employing unimodal end-linked polydimethylsiloxane (PDMS) model networks, was also not in agreement with the theoretical prediction. In addition, the correlation length calculated from Dcoop was progressively greater than the calculated average distance of network chain between crosslink sites with decreasing C _e. On the other hand, the autocorrelation function measured with DLS was well reproduced by using double-exponential terms, in which the fast and slow modes were, respectively, assigned to the cooperative gel-like diffusion mode of the network chains and to the cooperative self-diffusion mode based upon the motions of a certain cluster (region) in the network structure. Such behavior is naturally dependent on sulfur content and is rather similar to those in bimodal end-linked PDMS networks. Heterogeneous network structure is suggested from these experimental results; its formation is discussed in detail.