Strain-induced crystallization behaviors of natural rubber with additional lipids

Natural rubber (NR) is an indispensable material in our daily lives due to its excellent properties derived from strain-induced crystallization (SIC). However, the mechanism and the promoting factor of SIC have not been clarified yet, and the influence of non-rubber components to SIC has long been discussed. This study aims to elucidate the mechanisms through which non-rubber components enhance SIC, by mixing NR with three different lipids—glycerol monostearate (GMS), glycerol distearate (GDS), and glycerol tristearate (GTS)—and examining their effects on SIC behavior and tensile properties. The results revealed that GTS microcrystals act as nucleating agents within the NR matrix, lowering the energy required for SIC nucleation, and promote the SIC onset, even though the SIC promotion did not affect significantly on the tensile properties. This nucleating effect of GTS is attributed to the combination of the strong affinity with NR ensured by high hydrophobicity and its highly crystalline due to efficient packing of its alkyl chains. In contrast, GMS and GDS, possessing hydroxyl groups, did not act as a nucleating agent, but rather delayed the SIC by lowering crosslink density. These findings suggest that the dominant SIC-promoting factor is nucleating effect in this solution mixing system, and hydroxyl groups of the lipids seems not to interact with the functional groups of NR chain ends to form end-linking structure. This perspective is expected to advance the understanding of SIC mechanisms and guide future research in this field.