Structural and morphological aspects of some polymorphs of syndiotactic poly(p-methylstyrene)

Abstract

Novel data about the crystal morphology and the electron diffraction of syndiotactic poly(p-methylstyrene) (s-PPMS) in form I and in the clathrate forms including o-dichlorobenzene and tetrahydrofuran are presented. Preparative routes of s-PPMS samples in the various polymorphs suitable for electron microscopy and diffraction have been devised. This investigation provides data, unavailable in the past literature, helpful to get direct structural information and to better understand the ways of obtaining and stabilizing the various polymorphs.

Introduction

Syndiotactic poly(p-methylstyrene) (s-PPMS) presents a complex polymorphism: four different crystalline forms (named I, II, III and V), a mesomorphic form (IV) and several clathrate forms, including solvent molecules, have been observed so far [1], [2], [3], [4]. As described in Ref. [1] s-PPMS does not crystallize by cooling from the melt or by annealing the amorphous phase; crystallization may be induced through casting procedures, precipitation from solution or solvent-induced crystallization from the amorphous state. The obtaining of crystalline and clathrate forms depends on solvent, crystallization techniques or other parameters (casting-temperature, nature of the non-solvent during precipitation, etc.).

Two classes of clathrates, named α and β, have been obtained [5]. The two classes differ by the nature of the included guest molecules, mostly related to their steric hindrance illustrated by the presence or not of ortho substituents on the ring structure. For instance, solvents such as o-dichlorobenzene, o-chlorophenol, o-xylene and N-methyl-2-pyrrolidone induce the crystallization of α class clathrates, whereas solvents such as tetrahydrofuran, benzene, 1,4-dioxane, cyclohexane and cyclohexanone induce crystallization of β class clathrates.

In this paper we report novel data (optical and electron microscopy and electron diffraction), on single crystals and spherulites of s-PPMS in the various crystalline forms. Preparative routes of single crystals of s-PPMS in the clathrate forms and in form I have been devised. The new structural information, unavailable in the past literature, will help to better understand the ways of obtaining and stabilizing the various polymorphs.