Sureerut Amnuaypornsri

Strain-induced Crystallization of Natural Rubber: Effect of Proteins and Phospholipids

Abstract The effects of proteins and phospholipids in natural rubber (NR) on the strain-induced crystallization behavior during uniaxial deformation were studied by in-situ synchrotron wide-angle X-ray diffraction (WAXD) technique and simultaneous measurements of stress-strain relation. The influences of proteins and phospholipids in NR were evaluated separately by decomposition methods using deproteinization and lipase treatment, respectively. It was found that both components form a naturally occurring network, which is responsible for the strain-induced crystallizability of unvulcanized NR and the corresponding high mechanical property. This network also plays a significant role in strain-induced crystallization of vulcanized natural rubber.

Molecular dynamics of natural rubber as revealed by dielectric spectroscopy: The role of natural cross–linking

In order to understand the molecular dynamics of natural rubber, the dielectric relaxation behavior of its different components were investigated. These components included: (1) the linear polyisoprene fraction, obtained after deproteinization and transesterification of natural rubber (TE–DPNR), (2) the gel (GEL) fraction, corresponding to pure natural chain–end cross–linked natural rubber, (3) deproteinized natural rubber (DPNR), in which the protein cross–links at the ω–end have been removed, and (4) natural rubber (CNR) purified (through centrifugation) but still containing proteins, phospholipids and the sol phases. The dielectric relaxation behaviour of natural rubber revealed a segmental mode (SM) which is not affected by natural chain-end cross-linking (so-called naturally occurring network) and a normal mode (NM) which depends on a naturally occurring network. The dynamics of the NM, which is associated to chain mobility, seems to be strongly affected by natural chain-end cross-linking. We propose a model based on a hybrid star polymer in which the low mobility core (phospholipids) controls the mobility of the polyisoprene arms.

Role of Gel Content on the Structural Changes of Masticated Natural Rubber

In this study, natural rubber (NR) containing various amounts of gel was prepared by accelerated storage hardening to investigate the role of gel content on the structural changes of masticated NR. The NR samples containing various amounts of gel were subjected to mastication at various times, and subsequently characterized for the change of gel content, molecular weight, and Mooney viscosity to evaluate the role of gel content on these parameters. Furthermore, an oscillatory shear experiment using a strain sweep test was applied in this study to elucidate the structural changes of rubber samples after mastication. The results revealed that the Mooney viscosity was related to the percentage of gel fraction that has been proven to be the result of the interactions of proteins and phospholipids at the chain ends. The gel fraction of NR can be decomposed into a sol fraction by shear force during mastication and the mastication time for decomposition of gel relates to the initial gel content of the rubber. After mastication for 15 min, although the gel fraction of NR can be decomposed to ~0% w/w, the interactions of proteins and phospholipids at the chain ends still existed, and their quantities is corresponded to the gel content of raw rubber.