Influence of the temperature factor on the deformation properties of polymer filaments and films

The paper presents a study of the deformation properties of polymer materials. The authors consider the effect of the temperature factor, unaccounted for previously in modeling and forecasting, on the deformation properties of polymer filaments and the influence of changes in material temperature during deformation. The derivations of the main thermodynamic functions for polymer materials are given. The work examines the energy diagram typical for polymers that shrink. An explanation is given for the “dormancy” of the recovery process, as well as for the identical values of the highly elastic deformation when the load is removed during the recovery process in polymer threads and films. Based on the equation of condition for polymer filament and the well-known thermodynamic identities, the following basic thermodynamic functions were determined: internal energy, enthalpy and entropy depending on temperature and dimensionless stress. The cases of application of the first law of thermodynamics to deformation processes are examined. From the standpoint of thermodynamics, the main models of deformation are considered, i.e. creep, stress relaxation and active stretching of polymer filaments and films. Explanations are given for some of the phenomena observed experimentally in course of these processes. It was concluded that it is necessary to take into account the change in local temperature. Based on the model on the equation of condition for polymer filament and the analogue of the Clapeyron- Clausius equation, an expression for the temperature coefficient of material pressure is obtained. The work enabled to explain the “dormancy” of the recovery process of polymer and to determine the basic thermodynamic functions for a polymer material which take into account the material temperature change during deformation. The thermodynamic coefficient for the polytropic process is obtained. The described temperature coefficient makes it possible to analyze the tensile diagrams depending on the strain rate and the temperature change rate of the sample. The highly elastic part of deformation in thermodynamic function is expressed through the elastic deformation determined by the mechanical stress, which can be measured directly during the experiment.