Stress-strain behavior and plastic deformation of polymer

The plastic deformation and yield of crystalline polymers have been evaluated with particular regard to thermodynamics. It is envisioned that there are two ways to convert a crystalline polymer to the molten state. Traditionally, the polymer is heated, and the thermal energy induces thermal melting; alternatively, one can input mechanical work to induce mechanical melting at the draw temperature. External forces can separate polymer molecules from each other, making them reach the molten state. Theoretical value for the energy to achieve mechanical melting is derived. The experimental results show that the mechanical work of external force during drawing some polymers is enough to induce the mechanical melting. A formula for the theoretical yield stress of crystalline polymer is derived. The agreement between experimental and calculated values is excellent for some polymers. It indicates the validity of the stress activated phase transition deformation model.;The isothermality of the plastic deformation of crystalline polymer is investigated by the heats of fusion of drawn polypropylene. The heat of fusion is a more sensitive parameter to draw rate/temperature near room temperature than the long period is. It is a unique function of the draw temperature and is independent of the crystallinity of the starting material.;The influence of draw rate on the stress-strain behavior of polymer is systematically investigated. A new phenomenon in the tensile deformation of polymers is detected. When the draw rate is larger than a critical value, both modulus and yield stress for some polymers decrease rapidly with increasing draw rate; the normal time-temperature superposition principle does not appear to hold in these cases. This decrease in modulus and yield stress is not caused by a temperature rise during drawing. The preyielding process is approximately isothermal. A change in the yield mechanism is suggested as a possible reason for this decrease.;The temperature rises during post-drawing of polypropylene and polyethylene are measured. A method for measuring the fraction of mechanical work converted into heat during drawing is proposed. This fraction for polypropylene is 0.55, and that for polyethylene is 0.48.