Conversion of growth and recrystallisation of β-phase in doped iPP

Abstract

Isotactic polypropylene (iPP) was doped with pimelic acid/calcium stearate to promote the formation of the hexagonal (β) crystals during melt crystallisation. The conversion of growth pattern from the β- to the α- phase due to lowering of the crystallisation temperature was studied by a two-step crystallisation process in a DSC. The polymer melt was first allowed to crystallise isothermally at 130°C to produce almost pure β-phase and then the crystallising polymer was cooled to different temperatures between 115 and 25°C for a second-step crystallisation. It was found that the amount of α-phase was negligible after the second-step crystallisation process at 85°C or above. When the second-step crystallisation process was below 85°C, however, the amount of α-PP increased abruptly with a corresponding drop in the β-phase. This indicated that a major conversion of the β-growth to α- growth occurred near 85°C. Recrystallisation of the β-phase during heating was dependent upon the heating rate as well as the crystallisation condition of the samples.