Polymerisation and structure–property relationships of Ziegler–Natta catalysed isotactic polypropylenes

Abstract

Polypropylene homopolymer samples were prepared with a Ziegler–Natta catalyst using two different external donors, namely diphenyldimethoxysilane (DPDMS), and methylphenyldimethoxysilane (MPDMS). Each donor was used in varying molar ratios to the catalyst in order to prepare samples for physical testing. The polymers were fully characterised and also fractionated by preparative TREF with characterisation of the fractions. In terms of the polymerisation reactions the DPDMS external donor exerts greater influence at the active sites than the MPDMS and produces polymer of higher molar mass and lower polydispersity. The physical properties of the polymers were investigated using microhardness measurements. It is revealed that the microhardness is strongly dependent on the stereoregularity of the samples.

Introduction

The correlation of physical properties with molecular characteristics is vital in order to further the development of a polymer and improve on the properties attainable by the polymer. Polypropylene is one of the materials which has experienced a huge growth in consumption over the last few decades [1]. This has not materialised without significant contributions from the development of new catalyst systems and the increased understanding of why the material behaves as it does under a variety of conditions and testing methods.

In recent years there have been a number of studies which have contributed to the overall understanding of the effect of molecular characteristics on polymer properties. A series of papers by De Rosa et al. [2], [3], [4], [5] have revealed much in terms of the effect of copolymerisation, and stereo- and regio-defects on the crystallisation and properties of polypropylene. Busico et al. [6], [7] have made considerable contributions to the understanding of the polymerisation mechanism of Ziegler–Natta catalysis with the proposal of the 3-sites model. These developments have provided much assistance to the investigation of the structure–property relationships of these materials since one can better understand the manner in which the chains are produced. Therefore, one can tailor-make polymers with certain properties, especially if one also understands the effect of the microstructure on the crystallisation, and ultimately the mechanical properties of the materials.

The microhardness technique has developed into an important method to characterise the visco-plastic deformation in polymer samples [8]. The technique has proven to be able to detect very small transitions in polymer materials such as the glass transition temperature (T_g) [9], differences in the crystal phase [10], [11], [12], [13], [14], material anisotropy [8], [15], thermal history [16], [17], as well as polymer composition [18], [19]. It is generally accepted that the principal factor affecting the microhardness of semi-crystalline polymers is the crystallinity of the material, although the rate of hardness increase with crystallinity is different for different materials [9]. Factors such as the crystal thickness have also been found to influence the hardness [19]. It is therefore apparent that the molecular characteristics affecting polymer crystallisation also affect the final properties of the material.

Whereas it is quite simple to evaluate the changes in molecular composition as a function of polymerisation conditions, the link between the polymerisation conditions and the final properties of the polymer is not as simple to explain and predict. The aim of the current study is to tailor the polymerisation conditions such that polymers can be produced with controlled differences in microstructure for the investigation of structure–property relationships of polypropylene. Polymerisations were conducted with and without external donors, in order to produce samples with larger differences, and also with varying Si/Ti ratios to invoke small changes in the microstructure. Microhardness measurements were used as a tool to probe the physical properties of the polymers.