Effect of the Protein on the Strain Induced Crystallization in Natural Rubber

Abstract

This study systematically investigates the effects of endogenous and exogenous proteins on chain dynamics and strain-induced crystallization (SIC) in natural rubber (NR) systems through comparative analysis of NR, deproteinized natural rubber (DPNR), and centrifugal natural rubber with addition of soy protein (CNR-SP). The proton NMR transverse magnetization relaxation (T2 relaxation) measurements demonstrate protein-induced restriction of amorphous network mobility. Analysis of stress-strain curves reveals that endogenous proteins exhibit superior capacity in enhancing tensile stress and shear modulus compared to exogenous soy protein, though both protein types show qualitatively similar effects. The presence of the protein reduces the average molecular weight between adjacent crosslinking points ( ) due to its participation in the formation of the so-called naturally occurring network, resulting in an increase in the network density. The endogenous protein was found to reduce  more effectively than soy protein. Furthermore, in situ WAXD experiments unravel that the protein promotes the degree of SIC as evidenced by an earlier initiation of SIC and a higher ultimate crystallinity. Moreover, an increase in molecular orientation is observed from DPNR to CNR-SP to NR, which is indicative of a tighter amorphous network with the presence of protein. The observed tighter amorphous network is accompanied by a shorter T2 relaxation time and a smaller , which correlates directly with enhanced mechanical performance. Evidently, simple addition of soy protein cannot fully reconstruct the naturally occurring network in NR. These findings provide mechanistic insights into the structural basis of NR's superior mechanical properties, highlighting the irreplaceable role of endogenous protein arrangements in naturally occurring network formation.

Keywords: Natural rubber, protein, strain-induced crystallization