Effect of Matrix Molecular Mass on the Crystallization of β-Form Isotactic Polypropylene around an Oriented Polypropylene Fiber

The β-form of isotactic polypropylene (iPP) is a technologically useful material. In this context, pathways to produce this material have been intensively studied for at least two decades. One method, developed by Varga, is to selectively nucleate β-iPP on preexisting α-iPP row nuclei. 1-5 This method utilizes the different temperature dependences of crystal growth rates of α- and β-iPP, there being a narrow temperature window, 100-140 °C, in which the growth of the β-phase is faster than that of the α-phase. 1.6 It is required then only to provide the β-form nuclei in this regime, for β-form growth to outstrip that of α and become the predominant phase. A simple procedure for growing β material is to embed existing, oriented α-iPP fibers into an iPP melt and then lower the temperature into the temperature region which favors β-form growth. 7-9 With this method, it has been shown that the oriented fibers must be partially melted, by insertion into the existing melt at temperatures above 170 °C, in order to successfully nucleate β-crystallization at a lower temperature. 8 Nothing has been reported on the β-nucleation process for PP β-crystallization induced by oriented fibers. In the work reported here, we explore the role of matrix iPP on this process and, on the basis of these results, suggest a role of the matrix polymer in the nucleation process.