Effect of Matrix Molecular Mass on the Crystallization of $\beta $-Form Isotactic Polypropylene Around an Oriented Polypropylene Fiber

The role of molecular mass in an existing method for inducing the $\beta $-phase of isotactic polypropylene (iPP) has been investigated. The method consists of introducing oriented $\langle $-phase fibers into a molten iPP matrix, followed by crystallization of $\alpha $- or $\beta $-form iPP in the matrix around the fiber at a lower temperature. Comparing the effects of iPPs of two different molecular masses, it is found that the lower molecular mass polymer is more efficient in inducing the $\beta $ form. This result, coupled with results on the effects of fiber introduction temperature and crystallization temperature, leads to the hypothesis that the matrix chains are involved in the $\beta $-nucleation event and that partial relaxation of the surface of the oriented fiber is essential to the penetration of matrix chains into the oriented material prior to nucleation.