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

Isotactic polypropylene (iPP) was doped with pimelic acid/calcium stearate to promote the formation of the hexagonal (beta) crystals during melt crystallisation. The conversion of growth pattern from the beta- to the alpha-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 degrees C to produce almost pure beta-phase and then the crystallising polymer was cooled to different temperatures between 115 and 25 degrees C for a second-step crystallisation. It was found that the amount of alpha-phase was negligible after the second-step crystallisation process at 85 degrees C or above. When the second-step crystallisation process was below 85 degrees C, however, the amount of a-PP increased abruptly with a corresponding drop in the P-phase. This indicated that a major conversion of the beta-growth to alpha-growth occurred near 85 degrees C. Recrystallisation of the beta-phase during heating was dependent upon the heating rate as well as the crystallisation condition of the samples. (C) 1999 Elsevier Science Ltd. All rights reserved.