Chain orientation in natural rubber, Part II: 2H-NMR study

Abstract

Stress-induced crystallisation (SIC) and stress-induced melting (SIM) in natural rubbers (NR), unfilled and filled with carbon black (CB) have been studied by (2)H-NMR measurements. Various materials have been swollen with small amount (< 2%) of deuterated alkane chains. The orientation of the amorphous chains, then the local deformation of the amorphous chains during deformation cycles and during stress relaxation, permits to clarify the SIC and SIM processes during hardening and recovery. By mechanical, WAXS and NMR measurements one determines the same critical draw ratio for appearance lambda(A) and disappearance lambda(E) of the crystallites. It is demonstrated that the hysteresis observed by the different techniques (stress sigma, crystallinity chi, NMR splitting Deltanu) are due to the supercooling effect ( lambda(A) > lambda(E), at constant temperature). During hardening at constant strain rate it is found that the local draw ratio remains constant and equal to lambda(A), whereas the crystallinity increases linearly with the macroscopic draw ratio lambda. The hardening sigma approximately (lambda - lambda(A))(2) is then interpreted as a reinforcement effect due to the crystallites, which act as new crosslinks. This confirms the prediction of Flory. In filled rubber the same effects are observed, and the stress amplification factor is determined as a function of the CB content. It is found that the fillers act as nucleation centres for the NR crystallites. The reinforcement of such materials is due principally to this nucleation effect and to the presence of a super network formed by both the NR crystallites and the CB fillers.