Flow-induced epitaxial growth of high density polyethylene in its blends with low crystallizable polypropylene copolymer

Abstract

In this work, we investigated the epitaxial crystallization and its determinant factor in the blends of high density polyethylene (HDPE) and a low crystallizable propylene-ethylene random copolymer (PPR) during injection molding. For PPR dominated blend (blended with 30 wt.% HDPE), epitaxial growth of HDPE on the PPR crystals can be achieved, as demonstrated by two-dimensional wide-angle X-ray diffraction and small-angle X-ray scattering measurements, though the crystallization of PPR was far behind that of HDPE component under quiescent conditions. In-situ shear infrared experiments indicated that PPR crystallized in advance mainly due to larger crystallization window and flow-induced crystallization acceleration. As for the blend with 70 wt.% HDPE, however, one could not observe epitaxy. Shear-induced crystallization of HDPE hindered its epitaxial growth. Once the formation of shish-kebab of HDPE was activated, the epitaxial growth would be suppressed to a large extent.

Introduction

Heterogeneous epitaxy, a specific interfacial crystallization due to lattice matching, between different polymer pairs has attracted much attention in the past decades [1], [2], [3], [4], [5], [6], [7]. The importance of these studies lies not only in academic values, expanding our insights into interfacial crystallization, but in that heterogeneous epitaxy at interfaces may be an active way for improving the mechanical properties of polymer blends [8], [9], [10], [11], [12], especially for incompatible systems [8], [9]. As a model system, high density polyethylene (HDPE)/isotactic polypropylene (iPP) blend has been investigated intensively in terms of the mechanism and the influence factors for epitaxial crystallization under ideal experimental conditions [4], [5], [6], [7]. When HDPE epitaxially crystallizes on iPP substrate, the zigzag chains of PE are at 50° apart from the c-axial direction of iPP crystal lattice, which has been explained in terms of the interaction of HDPE chains with rows of methyl groups that populate the (010) plane of iPP a-crystals, since PE chains can fit well into the valleys formed by the methyl groups of iPP [13]. On the other hand, iPP can also epitaxially grow on the lamellae of HDPE via controlling the crystallization of iPP from solution on the oriented HDPE film, with their long axes inclined at an angle of 40° from the HDPE axis [13], [14]. The lattice coincidence is: (010)_iPP//(100)_PE, [101]_iPP//[001]_PE. That is to say, the epitaxial growth between HDPE and iPP depends to a large extent on that which one crystallizes first and serves as substrate to induce the interfacial crystallization of the other.

To obtain epitaxy during industrial processing and achieve super polyolefin blends, the epitaxial growth of HDPE as well as linear low density polyethylene (LLDPE) on the oriented iPP lamellae have been clearly demonstrated in the injection-molded bars in our previous works [7], [15], [16], in which chain orientation induced by shearing has been proved to greatly facilitate epitaxy. As for the epitaxy of iPP on the HDPE substrate, however, it has not been realized by shear during injection molding. HDPE cannot act as the template for the epitaxy of iPP, for iPP always crystallizes prior to HDPE under shear field. Since the sequence of crystallization between both components is critical for their epitaxial relationship, one may wonder that the epitaxial growth of polypropylene on the HDPE can be obtained if a polypropylene copolymer with low crystallizability and crystallization temperature is used.

In this article, HDPE was blended with a low crystallizable polypropylene-ethylene random copolymer (PPR), which had the same crystalline structure as that of isotactic polypropylene [17], [18]. The injection-molded bars of HDPE/PPR blends were prepared with the aid of dynamic packing injection molding (DPIM), the main feature of which was to introduce the oscillatory shear field on the melt/solid interface of the cooling melt during the packing stage by two hydraulically actuated pistons that moved reversibly with a constant rate. As a part of the long-term project aimed at super polyolefin blends, we are seeking to establish a fundamental understanding of the relationship between epitaxial crystallization and properties of polyolefin blends. The goals of this work are twofold: (1) to further explore the importance of the sequence of crystallization between polypropylene and HDPE by using a low crystallizable polypropylene-ethylene random copolymer (PPR); and (2) to explore the competition between the formation of shish-kebab structure and the epitaxial structure under shear field.