| Abstract: | Crosslinking and stretching (2.5 times along the circumferential direction) of the molten polymer during extrusion produced pipes with dominantly circumferential orientation and a lower degree of axial chain orientation. Differential scanning calorimetry (crystallinity and crystal thickness), density measurements (crystallinity), X-ray diffraction (c-axis orientation), infrared dichroism measurements (crystalline and amorphous chain orientation) and contraction measurements (molecular draw ratio) assessed the microstructure of the pipe material. The mechanical properties of the oriented material were assessed by uniaxial tensile tests. The orientation was biaxial with the main orientation in the circumferential direction and a lesser orientation in the axial direction. The maximum degree of circumferential orientation was obtained at the inner wall of the pipe. The lower degree of crosslinking of the core material allowed slippage of chains during the stretching of the molten polymer and it is suggested that this is the cause of the lower degree of orientation of the core material. The oriented pipe material exhibited a 5-10% higher degree of crystallinity and higher crystal thickness than conventionally crosslinked material. The tensile modulus and the tensile strength of the oriented, cross-linked material was greater along the axial direction than along the circumferential direction. The circumferential and axial moduli for the oriented, crosslinked pipe were greater than the corresponding moduli of the non-oriented cross-linked pipe material. Another pipe based on crosslinked PE that were first circumferentially stretched 2.5 times and later axially stretched 10 times (in the molten state) showed, despite the fact that it exhibited pronounced axial orientation almost a balanced tensile modulus (4.3±0.2 GPa) in the axial-circumferential plane. Atomistic modelling showed that the orientational dependence of the density of the amorphous phase is small. |
