Elsevier

Polymer

Volume 47, Issue 15, 12 July 2006, Pages 5643-5656
Polymer

Influence of molecular composition on the development of microstructure from sheared polyethylene melts: Molecular and lamellar templating

Dedicated to Professor David Bassett.
https://doi.org/10.1016/j.polymer.2006.01.097Get rights and content

Abstract

A combination of in situ and ex situ X-ray scattering techniques and transmission electron microscopy has been used to study the crystallization behaviour of polyethylene, following the imposition of melt shear. In the case of a branched material, the imposition of shear flow up to a rate of 30 s−1 was found to induce no anisotropy. Although shearing the linear material only ever induced a very small degree of anisotropy in the melt, for shear rates >0.15 s−1, subsequent crystallization resulted in increasing anisotropy. Blends of the above two polyethylenes were produced, in which the linear material constituted the minority fraction (∼10%). Isothermal crystallization at temperatures where extensive crystallization of the branched material does not occur demonstrated that the behaviour of the linear component of the sheared blend mirrored that of the linear polyethylene alone. However, in addition, it was found that when crystallized in the presence of an oriented morphology, the branched polymer also formed anisotropic structures. We have termed the process templating, in which the crystallization behaviour of the bulk of the system (∼90% branched material) is completely altered (spherulitic to oriented lamellar) by mapping it onto a pre-existing minority structure (∼10% linear polymer).

Introduction

The development of a comprehensive understanding of the crystallization behaviour of polydisperse polymer melts following the imposition of melt flow, represents a major challenge and, consequently, is a topic that has attracted considerable recent attention [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Whilst it is well appreciated that a degree of molecular variability is required to ensure processability and adequate final properties, the precise role of each molecular fraction in technologically relevant materials is not well understood. Following early studies of extensional flow, which revealed many important general concepts [11], [12], [13], [14], it was soon realised that similar structures could also develop from a sheared melt; the characteristic morphology was termed a shish-kebab. At the simplistic level, extension of the longest chains occurs first, in response to the imposed flow field, to give oriented ‘shish’ structures, which then act as contiguous nucleation sites for the lateral growth of lamellar crystals (‘kebabs’) [4], [15], [16] containing shorter molecules. In this process, shish formation is critical, and a range of different interpretations stressing the role of highly oriented molecular segments [17], long-chain/long-chain overlap [5], [18], fringed micellelar conformations within the melt [19] and particulate impurities [6] have all been proposed. Indeed, ordered structures have been observed in amorphous sheared isotactic polystyrene, when quenched from the melt [9], and it has even been demonstrated that the addition of high molar mass atactic polypropylene can enhance the nucleation of the isotactic stereo-isomer [20]. Both these observations have led to the suggestion that shish structures are associated with aligned mesomorphic conformations within the melt.

Realistic technological applications of crystallizable polymers exploit materials that contain a broad and continuous spectrum of molecular lengths. Therefore, while fundamental theory is best derived from the study of monodisperse systems, these are not so relevant from a practical perspective. In the case of polyethylene, even when crystallization occurs from a quiescent melt, molecular segregation and fractionation processes often result in complex microstructures, which are both hierarchical and compact [21], [22], [23]. To enhance understanding of such complex lamellar textures, blending has often been used to induce morphological simplicity. In this respect, the exploitation of isotactic/atactic blends represents a long-standing strategy [24], [25]. In polyethylene, the equivalent involves crystallizing a linear material in the presence of less crystallizable branched polymer [26], where the level of branching limits the thickness of lamellar crystals that is attainable and, therefore, the temperature range within which the polymer can be crystallized. Typically, low density polyethylenes do not crystallize above ∼110 °C, whereas linear grades can be crystallized isothermally up to ∼130 °C. This, therefore, gives a temperature window of about 20 °C within which the branched polymer effectively acts as a non-crystallizable diluent, such that crystallization of the linear polymer occurs relatively slowly to give simple lamellar textures [27], [28], [29]. Although co-crystallization occurs to a degree that is dependent upon the crystallization temperature, it is nevertheless, often convenient to consider the final morphologies in terms of the initial nucleation and growth of lamellae of linear polyethylene (LPE), followed by the separate crystallization of the branched polyethylene (BPE). Our usage of these terms acknowledges that the former can include the more linear segments from the BPE, whilst the latter may include the shortest LPE fraction.

Previously, we have explored the crystallization behaviour of sheared linear polyethylene melts through the strategy of combining real-time, in situ X-ray scattering techniques, with the ex situ examination of larger scale morphologies using transmission electron microscopy (TEM) [30], [31]. For the study reported here, we chose, in addition, to exploit the structural simplifications that result from blending a linear polyethylene with a less crystallizable branched material. In such a system, the majority BPE matrix would be expected to behave both as a stress transfer medium and a diluent and, thereby, more clearly reveal the effect of shear flow on crystal nucleation and growth in the minority linear component.

Section snippets

Materials

Samples ranging in composition from 0 to 20% LPE were examined during the course of this investigation but, here, we will concentrate on data derived from systems containing 10 or 20% linear plus, respectively, 90 or 80% branched polyethylene. The LPE was supplied by BP Chemicals and exhibits a broad molecular mass range, with M¯w=312,000 and M¯n=33,000. The BPE used here is a conventional low density grade produced by Borealis Polymers using a high pressure synthetic route. It is characterised

DSC behaviour

Fig. 2 illustrates the crystallization and melting behaviour of the LPE and BPE materials, together with that of a blend containing 10% LPE. Fig. 2(a) contains three non-isothermal crystallization exotherms, from which it is apparent that both the LPE and the BPE, in isolation, exhibit singular crystallization peaks (LPE: peak temperature 119 °C; BPE: peak temperature 98 °C). Conversely, the blend system is characterized by two transitions; the LPE crystallizes first at 110 °C, the BPE

Discussion

By combining the X-ray scattering and TEM results described above, it is possible to describe the sequence of processes that occurs during the crystallization of our blend systems and to relate each stage to the molecular composition of the system.

Shearing the melt results in the orientation of a small fraction of the material (orientation parameter 〈P2〉 ∼0.01 [30]), which corresponds to the very longest molecules within the system [5], [31], [41]. On cooling, these structures form ‘thread-like

Conclusions

The above study has applied a combination of in situ and ex situ techniques to the study of the crystallization behaviour of sheared polyethylene blends. This strategy was adopted to reduce overall crystallization rates, to aid real time scattering experiments, and to produce open morphologies that are more amenable to interpretation. Both scattering techniques and TEM indicate that the BPE used in this investigation does not form anisotropic structures when exposed to melt shear rates up to 30 s

Acknowledgements

This work was performed as part of the Royal Society, Academica Sinica funded link between The University of Reading and the Institute for Applied Chemistry, Changchun. The synchrotron radiation based experiments were performed at the CCLRC Daresbury Laboratory and we thank Anthony Gleeson for his assistance on the beam-line. JJH acknowledges the support of EPSRC and BP Chemicals snc through a CASE award. We thank BP Chemicals and Borealis Polymers for the supply of the linear and branched

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