Stress−strain relations and strain-induced crystallization (SIC) of unvulcanized and vulcanized states of natural rubber (NR) and synthetic polyisoprene (IR) were studied using synchrotron X-ray at various temperatures from −50 to +75 °C. Unvulcanized IR is a polymer melt that shows a viscous response with yield stress that is related to entanglement and no SIC at 25 °C. However, unvulcanized IR shows SIC at 0, −25, and −50 °C. Entanglements in unvulcanized IR become pivots to align chains and induce crystals at low temperatures. On the other hand, unvulcanized NR shows SIC and stress upturns in stress−strain relations at 25 °C. Since a permanent set is observed after large extension and retraction, unvulcanized NR has a pseudo end-linked network. The pseudo end-linked networks make entanglements as permanent entanglements and show stress upturn and SIC. Vulcanization makes IR to a rubber which shows a stress upturn and SIC by chemical bond network. The stress of vulcanized NR and IR appear almost the same at strains less than 3.0, however the stress of vulcanized NR is much higher than vulcanized IR beyond strain 3.0. The onset strain of SIC of vulcanized NR is much smaller than vulcanized IR. This different behavior is caused by the pseudo end-linked network. The stress in stress−strain relation at higher temperatures is significantly lower than the stress at lower temperatures. This tendency does not seem to follow the theory of rubber elasticity. The onset of SIC delays the upturn of stress as a shoulder or plateau in the stress−strain relation. SIC contributes to the stress, even though the stress smoothly increases with strain. At higher strain, SIC become big network points to bind many chains and reduce the limit of extensibility. The effect of SIC and the limited extensibility to the stress is not distinguishable.

Deproteinized natural rubber latex (DPNR-latex) was treated with lipase and phosphatase in order to analyze the structure of the chain-end group (alpha-terminal). The enzymatic treatment decreased the content of long-chain fatty acid ester groups in DPNR from about 6 to 2 mol per rubber molecule. The molecular weight and intrinsic viscosity were reduced to about one-third after treatment with lipase and phosphatase. The Huggins' k' constant of the enzyme-treated DPNR showed the formation of linear rubber molecules. The molecular weight distribution of DPNR changed apparently after treatment with lipase and phosphatase. (1)H NMR spectrum of rubber obtained from DPNR-latex showed small signals due to monophosphate, di-phosphate and phospholipids at the alpha-terminus. Treatment of DPNR-latex with lipase and phosphatase decreased the relative intensity of the (1)H NMR signals corresponding to phospholipids, whereas no change was observed for the signals due to mono- and diphosphates. The residual mono- and diphosphate signals as well as some phospholipid signals after lipase and phosphatase treatments indicate that mono- and diphosphate groups are directly linked at the alpha-terminus with the modified structure, expected by aggregation or linking with phospholipid molecules.

The treatment of deproteinized natural rubber (DPNR) latex with phospholipases A(2), B, C, and D decreased significantly the long-chain fatty acid ester contents in DPNR and also the molecular weight and Higgins' k' constant, except for phospholipase D treatment. This indicates the presence of phospholipid molecules in NR, which combine rubber molecules together. Transesterification of DPNR resulted in the decomposition of the functional group at the terminal chain-end (alpha-terminal), including phospholipids and formed linear rubber molecules. The addition of small amounts of ethanol into the DPNR solution reduced the molecular weight and shifted the molecular weight distribution (MWD) comparable to that of transesterified DPNR (TE-DPNR). The addition of diammonium hydrogen phosphate into DPNR-latex in order to remove Mg2+ ions yielded a slight decrease in molecular weight and a slight shift in MWD. The phospholipids are expected to link with mono- and diphosphate groups at the alpha-terminal by hydrogen bonding and/or ionic linkages. The decrease in the molecular weight and Huggins' k' constant of DPNR demonstrates the formation of linear molecules after decomposition of branch-points by this treatment, showing that phospholipids participate in the branching formation of NR. The branch-points formed at the alpha-terminus are postulated to originate predominantly by the association of phospholipids via micelle formation of long-chain fatty acid esters and hydrogen bonding between polar headgroups of phospholipids.