Relationship between particle size and molecular weight of rubber from hevea brasiliensis

Rubber particles with different mean diameters obtained from fresh natural rubber latex (FL-latex) were separated by high-speed centrifugation. The small rubber particles (SRP), having mean diameter less than 250 nm, were found to be composed of both low and high number-average molecular-weight (M̄_n) rubber molecules; while larger rubber particles (LRP), with mean diameter larger than 250 nm, mainly consisted of low M̄_n rubber molecules. The content of longchain fatty acid ester are mainly due to functional groups such as phospholipids were less in SRP than that of LRP. This indicates clearly that LRP contains mainly rubber molecules which were terminated to form a functional terminal group containing fatty acid ester groups. The high molecular-weight rubber in SRP, containing lower amounts of ester groups, is presumed to be mainly linear molecules having no chain-end group to form branch-points. On the other hand, the low molecular-weight rubber molecules with high ester content in the LRP are presumably terminated with the functional group containing fatty acid esters to form long-chain branching. The molecular-weight of rubber in LRP increased with increasing particle sizes. This suggests that the LRP was derived from the aggregation and/or association of functional groups at ω- and α-terminal end groups.