Melting and Glass Transitions in Polyisobutylene

Abstract

Abstract The purpose of this work was to investigate the crystallization behavior of uncompounded, unstressed polyisobutylene and butyl rubber. Glass transition data obtained during the work are also reported. The data were determined dilatometrically. The work was prompted by a literature survey which collected transition data on elastomers. It was found that crystallization information for polyisobutylene and butyl rubber was based largely on x-ray studies of these polymers under high elongation. In fact, it has been assumed generally that these polymers would crystallize only when highly oriented, as by stretching. Because of the resistance of polyisobutylene to ordering when in the unstressed state, no measurements of optimum crystallization temperature or melting temperature, Tm, have previously been reported. Information of this type is important because of its bearing on the mechanical behavior of a polymer. Many types of elastomers will crystallize readily if oriented by a large stress. However, only those having relatively simple structures crystallize readily under zero stress. Such behavior is, of course, a limiting factor in low-temperature serviceability. Polyisobutylene might seem at first to have the required simplicity of structure; however, steric hindrance of the side methyl group forces the chain to crystallize in a helical configuration having a molecular repeat distance of 18.6 A, as shown by Fuller, Frosch, and Pape. This is the greatest repeat distance of any of the elastomers, suggesting that a lengthy exposure period would be required if ordering were to occur at all. A clue to ordering in an isobutylene copolymer was furnished by Gehman, Woodford, and Wilkinson, who reported that a butyl tread stock showed a greatly increased torsional stiffness after storage for about 40 days at −30° C. The aged butyl sample exhibited an x-ray diffraction pattern suggestive of crystallization. These results may be the first showing crystallization in a butyl polymer essentially free of stress. In other work, Radi and Britt, using equilibrium temperature-retraction techniques with samples elongated 300 per cent initially, found that two types of butyl polymers showed optimum crystallization rates at −30° and −41° C. The aggregate of these results suggests that crystallization might occur in unstressed polyisobutylene if sufficient time were allowed, and indicates an optimum rate in the range of −30° to −40° C. This is reasonable by comparison with natural rubber, which has a glass transition temperature very close to that of butyl rubber, and an optimum crystallization rate at about −25°C.