Effect of the protein on the strain induced crystallization in natural rubber

Abstract

This study systematically investigated 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 revealed an increase in the effective overall value of T₂ ($$) from 1560 μs for NR to 1648 μs for CNR-SP and further to 1942 μs for DPNR, demonstrating protein-induced restriction of the amorphous network mobility. Analysis of stress-strain curves indicated that endogenous proteins exhibit superior capacity in enhancing tensile stress and shear modulus compared to exogenous soy protein. The average molecular weights between adjacent crosslinking points ($$) derived from stress-strain relations increased sequentially from 2164 g/mol (NR) to 3209 g/mol (CNR-SP) and 4652 g/mol (DPNR). Correspondingly, the crosslinking densities ($$) determined by equilibrium swelling measurements decreased successively in the order of 4.04 x 10^-4 mol/cm³ (NR), 3.76 x 10^-4 mol/cm³ (CNR-SP) and 2.79 x 10^-4 mol/cm³ (DPNR). These results indicated the participation of the protein in the formation of the so-called naturally occurring network, resulting in an increase in the network density. Furthermore, in situ wide angle X-ray diffraction (WAXD) experiments unraveled that the protein promotes the degree of SIC as evidenced by an earlier initiation of SIC and a higher ultimate crystallinity. This improvement in SIC directly contributes to the superior mechanical properties of NR. From a microstructural standpoint, the enhanced SIC arises from an increased network density facilitated by the involvement of protein in the naturally occurring network. Moreover, an increase in molecular orientation is observed from DPNR to CNR-SP to NR, which is indicative of a tighter amorphous network with the presence of protein. The observed tighter amorphous network is accompanied by a shorter T₂ relaxation time and a smaller $$, 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.