Determination of degree of crosslinking in natural rubber vulcanizates. Part II

Abstract

Abstract Natural rubber vulcanizates have been prepared by a method permitting determinations by chemical analysis of the number of crosslinking introduced. The crosslinking agent was di‐ tert ‐butyl peroxide, and the peroxide and its decomposition products– tert ‐butanol, acetone, methane, and ethane—were determined by infrared spectroscopic and manometric methods. tert ‐Butanol and methane result from the abstraction of α‐methylenic hydrogen atoms from the rubber hydrocarbon by means of tert ‐butoxy and methyl radicals, respectively. On the assumption that the resultant rubber hydrocarbon radicals undergo exclusive mutual combination a chemical evaluation of degree of crosslinking, 1/2 M c , is given by ½(moles of tert ‐butanol + moles of methane), where M c is the average molecular weight between crosslinks. Physical values of 1/2 M c for rubber of infinite initial molecular weight have been obtained by determining the volume fraction of swollen rubber, v r ‐, in n ‐decane, and by using the method described in Part I for correcting for the finite initial molecular weight of the rubber. A comparison of the physical and chemical values of 1/2 M c indicates that the former is greater than the latter for all degrees of crosslinking, the major divergence being accounted for by an additive term which is pictorially visualized as due to entanglements.