Comparative study of the crystalline morphology present in isotropic and uniaxially stretched “conventional” and metallocene polyethylenes
Introduction
The understanding of the structure–properties relationship of ethylene-based polymers has been one of the main topics of fundamental research over the last decades. During this time span, the development of these commodity materials has been continuous in order to overcome the successive challenges imposed by the market and the advent of new competitive products, as well as to improve and diversify overall performance. New technologies in polymerisation processes and catalysts have broadened considerably the range of materials available. The discovery and posterior development of low density (LDPE) and high-density (HDPE) polyethylenes was followed by lineal (LLDPE) and bimodal polyethylenes. More recently (in the 1990s), a new range of polyethylene grades with a highly regular molecular design has been attained by means of single site metallocene-based catalysts [1] and vanadium-based catalysts [2]. These materials are usually called homogeneous polyethylenes because they display a highly homogeneous molecular structure with narrow molecular weight distribution and a random incorporation of the comonomer inter- and intra-molecularly. Conversely, heterogeneous polyethylenes, i.e. LLDPEs and bimodal materials, display a broader molecular weight distribution and heterogeneous incorporation (inter- and/or intra-molecularly) of the comonomer over the molecular weight [3]. The improvement in the control over the molecular design achieved by this new generation of catalysts provides the researchers with a new range of materials to extend the characterisation of this family of polymers.
The microstructure of the homogeneous copolymers obtained by the Dow's INSITE™ constrained geometry catalyst technology (based on metallocene chemistry) has been recently classified as a function of sample density using transmission electron microscopy (TEM) and others techniques [1]. The suggested morphologies go from lamellaes superstructured into spherulites for the highest densities (HDPE-like) to bundle-like fringed micellar crystals for the lowest densities (<890 kg/m3) with elastomeric-like behaviour. Intermediate densities show a mixture of both morphologies. In a paper by Mathod et al. [2] it is shown that vanadium-based catalysts can yield similar homogeneous polyethylene's.
A previous study [4] using Raman spectroscopy and other techniques, and dealing with the characterisation of uniaxially stretched (cold drawn) HDPE materials, showed that a decrease in the Raman crystallinity (ca. 30%) and a shift in position (+1 cm−1) of the crystallinity band occurs upon cold drawing. The Raman crystallinity band is thought to arise from interchain interaction within the orthorhombic unit cell called factor group splitting. The decrease in the Raman orthorhombic crystallinity was attributed to the creation of a highly disrupted and defective orthorhombic crystalline morphology with some phase transformation to monoclinic phase (characterised by wide angle X-ray scattering (WAXS)) and possibly, some chain unfolding. The disruption of some of the orthorhombic crystals occurred to such an extent that no longer results in effective interchain interaction, i.e. in factor group splitting, is thought to cause this drop in the Raman crystallinity. The shift in position towards a higher wavenumber observed for the crystallinity band was associated with a decrease in the interchain interaction efficiency within the lattice, and therefore to lateral disorder. Crystallinity by DSC though, measured no difference between isotropic and cold drawn materials. The annealing of the stretched structure restored the Raman orthorhombic crystalline phase towards the perfectness of the isotropic undeformed crystals and left virtually unchanged the high molecular orientation of the stretched specimens. The study concluded that uniaxial deformation at room temperature and below of HDPE fractures and subsequently extends the crystalline lamellae due to molecules being pulled through the crystals. The latter effect results at the molecular scale in a dramatic loss of the chain lateral order in the orthorhombic lattice, to which the Raman crystallinity band is highly sensitive. A recent study [5] carried out on a varied range of polyethylenes, reported a significant increase in all-trans chain segments with decreasing density upon uniaxial stretching. The increase of the all-trans segments within the chains was attributed to orientation in the amorphous regions and in particular to stress-induced orientation of tie-molecules and other topological constrains trapped between crystals.
In the work reported here, we carry out a comparative characterisation of the crystalline structure of a group of PEs covering a broad range of density and molecular designs. The polymers were subjected to uniaxial deformation and annealing and were studied by Raman spectroscopy, DSC and WAXS. We aim to assess all the structural features cited above and get more insight into the Raman analysis of the polyethylene crystallinity as a function of density by comparison with DSC and WAXS data.
Section snippets
Samples
Five different polyethylene grades (kindly supplied by Dow Iberica, Spain) were used in this study. Some sample characteristics are summarised in Table 1. All materials except LDPE are ethylene-1-octene copolymers. The content of the comonomer increases with decreasing density although the actual numbers were not supplied. Samples were selected to match, as close as possible, molecular weight. Consequently, differences in behaviour can be mainly attributed to differences in molecular
Isotropic materials
The melting endotherms and Raman spectra of the isotropic samples are shown in Fig. 2. From Fig. 2(A) it is clear that as sample density decreases (from top to bottom in Fig. 2) the end of melting occurs at lower temperatures. The latter observation does not apply for the LLDPE when compared to LDPE. This is attributed to the LLDPE showing a multiple melting peak endotherm. Multiple melting features are characteristic of LLDPE materials and are caused by the presence of a broad distribution of
Conclusions
A comparative study of the crystalline structure of a number of polyethylenes covering a broad range in density and molecular designs has been carried out using Raman, DSC and WAXS. The materials were compared in the isotropic state, after cold drawing, and after annealing of the cold drawn material. Crystallinity for the isotropic samples, as determined by DSC and WAXS, agreed reasonably well and decreased with decreasing density. Crystallinity by Raman was systematically lower and decreased
Acknowledgements
This research was funded by the MINER/Spain (programme ATICA) and the Agency of Economic Development (ADE) of Castilla y León/ Spain.
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