Stress-induced crystallization and reinforcement in filled natural rubbers:: 2H NMR study

Stress-induced crystallization (SIC) and stress-induced melting (SIM) in pure and filled (carbon black, CB) natural rubbers are studied by mechanical analysis, X-ray scattering, and quadrupolar NMR as a function of temperature and filler content. These complementary techniques allow to measure the crystallinity and the local deformation of the amorphous chains during the mechanical cycle. The critical draw ratio for appearance (stretching) and disappearance (recovery) of the crystallinity are compared by these three techniques. It is shown by WAXS that CB particles act as nucleation centers of the SIC. By H-2 NMR one finds that during crystallization in both types of materials the remaining amorphous chains relax partially, as predicted by Flory. The amplification factors A(sigma), A(chi), and A(NMR) deduced from the stress crystallinity and NMR splitting measurements are compared. The amplification factors measured by WAXS (A(chi)) and NMR (A(NMR)) are of the same order and are not far from the Bueche prediction. The amplification factor A(sigma) deduced from the stress-strain curves verifies the classical empirical Guth and Gold relation. The relation A(sigma) = 2.5A(NMR) observed at low deformation (no SIC) indicates that the reinforcement in these filled rubbers has two causes of similar importance: the classical overstrain of the chains due to geometrical effect and the role of particles as new effective cross-links. When the samples (filled and unfilled) crystallize, a third reinforcement effect appears; the crystallites act also as giant cross-links, and the stress increases drastically with the macroscopic deformation (hardening).