Simultaneous X-ray diffraction/DSC of the reversible orthorhombic-hexaghonal transition in gel-drawn polyethylene

Thermal analysis (DSC) and small-angle X-ray diffraction have been combined in a single experiment using synchrotron radiation. We have performed both small- and wide-angle diffraction studies simultaneously with DSC (XDDSC) on a variety of systems involving polymers and molecular and liquid crystals [2,3]. We also introduced improvements to the method, such as highly X-ray transparent and thermally conductive ceramic materials for specimen containers. The block scheme of the experimental set-up is shown. One particularly useful application of this technique is in studies of metastable short-lived phases. A good example is the hexagonal phase of polyethylene which has been observed, when a highly oriented high molecular weight polyethylene fibre or tape is constrained to inhibit retraction and then heated to above the normal melting point of orthorhombic polyethylene, i.e. above ca. 140°C. Rather than melt, the polymer will undergo a transition into the hexagonal phase.