The molecular weight distribution of natural rubber

Repeatability and reproducibility of thermal field-flow fractionation in molecular weight determination of processed natural rubber

The purpose of this study is (1) to determine the repeatability and reproducibility of thermal field-flow fractionation (ThFFF) in measuring the molecular weight of compounded natural rubber, and (2) to examine the correlation between the molecular weights obtained from ThFFF and the rheological data. 8 batches of compounded natural rubber were obtained from a thermo-mechanical mastication process, and were analyzed by ThFFF in a designed testing sequence. ThFFF analysis showed the compounded natural rubbers range in weight-average molecular weight (M(w)) from 143,000 to 360,000. By taking into account both the short term variability (repeatability) as well as the long term variability (reproducibility) of the instrument, ThFFF was shown to be able to distinguish between samples differing by as little as 21,000 in M(w) and 15,600 in number-average molecular weight, M(n) (based on cis-polyisoprene calibration); and thus is a useful tool for the molecular weight analysis of natural rubber-related materials. It was also found that the rheological data (G' and tan delta) measured on both the virgin and the compounded natural rubber correlated well with the molecular weights obtained from ThFFF when normalized.

Structural Characterization of α-Terminal Group of Natural Rubber. 2. Decomposition of Branch-Points by Phospholipase and Chemical Treatments

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.

Structural Characterization of α-Terminal Group of Natural Rubber. 1. Decomposition of Branch-Points by Lipase and Phosphatase Treatments

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.