Quantum effects in molecular dynamics and fracture kinetics of polymers

Molecular dynamics in crystallites of a number of polymers, such as polyethylene, nylon-6, poly-(vinyl alcohol), was studied at temperatures ranging from 4.2 to 400 K using the X-ray diffraction technique. Thermal expansion of the lattice was measured by the temperature-induced displacements of diffraction maxima. The thermal expansion coefficient was found to grow from 0 to ∼10⁻⁴ K⁻¹, this fact being the evidence of the quantum statistics in the vibrational dynamics of straightened molecules over the whole temperature range. Characteristic temperatures of vibrations were estimated. The kinetics of fracture was studied using highly - oriented nylon-6 and KEVLAR-49 fibers. Tensile strength (σ_r) was measured as a function of loading rate σ and temperature (T) in the range from 4.2 to 400–600 K. The observed specific features of the σ_r(σ) and σ_r(T) curves at low temperatures correspond to tunnel scission of stressed chain molecules.