X-ray diffraction of polymer under load at cryogenic temperature

An x-ray diffraction apparatus, consisting of a load cell, a stretching device, and a cryogenic cell, has been constructed to observe the mechanical deformation of the crystal lattice of polymers at cryogenic temperature. The polymer specimen could be cooled down to 18 K using this apparatus. The equatorial diffraction profiles of polymer specimens were observed under various temperatures and loads with 18 K minimum temperature. The equatorial lattice spacings of polyethylene (PE) decreased at 18 K compared with those at 300 K; however, they were elongated by a constant applied tensile stress. The elastic modulus $E_t$ of the crystalline regions of PE in the direction perpendicular to the chain axis was evaluated. The $E_t$ value for the (200) plane of PE increased to 11 GPa at 18 K from 3.2 GPa at 300 K. This is considered to be due to the increase of the van der Waals interactions between the chains in the crystal lattice at very low temperature, where the incoherent thermal vibrations of the atoms can be ignored.