Processing and strain induced crystallization and reinforcement under strain of poly(1,4-cis-isoprene) from Ziegler–Natta catalysis, hevea brasiliensis, taraxacum kok-saghyz and partenium argentatum

Strain induced crystallization and reinforcement were studied for poly(isoprene)s from the following sources: Ziegler–Natta catalysis, hevea brasiliensis (HNR), taraxacum kok-saghyz (TKS), known as the russian dandelion, partenium argentatum (GR), known as guayule. Two HNR samples were studied, with high (HNR-H) and low (HNR-L) molar mass. Investigated guayule samples were: as isolated from the latex (GR-R) and after extraction with acetone (GR-P). All of the samples had weight average molar mass higher than 1.5 × 106 Da. TKS was found to be the most stereoregular sample, with undetectable amounts of stereoerrors. Wide angle X-Ray diffraction patterns were collected on samples, unstretched after processing and during stretching, up to 5 as the strain ratio. Quasi-static tensile measurements were performed. HNR and GR-P exhibited rubber crystallinity already in the undeformed state and the orientation of their crystalline phase remained low also for the highest strain ratios. GR-R and TKS were amorphous at low strain and developed highly oriented crystalline phases under stretching. TKS developed extraordinary mechanical reinforcement under stretching: stresses at large elongations were much higher than those obtained with HNR. It is thus shown that the formation of highly oriented crystalline phases brings large mechanical reinforcement. In conclusion, an amorphous NR sample from a natural source, such as TKS, which has high molar mass and does not contain non rubber components which could act as plastifiers, is able, under stretching, to develop crystallinity and a high degree of axial orientation. Crystallization occurs at high strain ratio, when the chains are prevailingly aligned. The biosynthesis of TKS is likely to play a strategic role, as it promotes the chain end crosslinking of the polymer chains.