Changing the Rate of Crystallization of Trans-1, 4-Polysoprene By Sulfur Crosslinks

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Abstract
The effect of vulcanization on unstretched natural rubber in retarding crystallization is well known. The present study has shown the crystallization rate of balata and a synthetic trans-1, 4-polyisoprene can be likewise decreased by crosslinking. Two types of cure systems with sulfur were studied using (1) N, N, oxydiethylene-2-benzothiazyl sulfenamide (OBTS) and (2) a combination of mercaptobenzothiazole (MBT) and tetra methylthiuram disulfide (TMTD). Included in this study was the effect of increasing cure times for each compound at temperatures of 140°C and 160°C. The results have shown the feasibility of varying the amount of crystallinity as well as delaying its formation for periods of one to two weeks for compounds cured with MBT-TMTD and sulfur. An increase in the percent monosulfide linkages has been shown to occur in the case of the compounds with OBTS cured at 160°C compared to those cured at 140°C. In contrast, a high percentage of monosulfide linkages was present in the compounds with MBT-TMTD cured at both 140°C and 160°C. Our studies have shown that the higher the crosslink density the slower the rate of crystallization. We speculate that the monosulfide linkages in the polymer matrix may play an important role in changing the crystallization rate. These observations have been substantiated by (1) hardness measure ments with time, (2) X-ray diffraction scans of the crystallization, and (3) crosslink density studies. The results obtained for both balata and the synthetic trans-1, 4-polyisoprene have shown good agreement.

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