STUDIES ON NON LINEAR VISCOELASTIC BEHAVIOR RELATED TO POLYMER PROCESSING

The increase in diameter of a polymer extrudate or a polymer solution upon emerging from an extrusion tube or die is usually referred to as "die swell." The present study establishes that the die swell is mainly due to nonlinear viscoelastic behavior of the material and the entrance effects are unimportant. Novel experiments in "Tube Flow Apparatus" were carried out without attendant end effects. The extrusion tube is filled with polymer, and the polymer is allowed to relax completely. The polymer is then extruded at a low rate by pushing new material into the tube. The volume of material leaving the tube is less than 20 percent of the total volume of the tube. The material is immobilized by cooling it radially below its glass transition or crystalline melting temperature while maintaining the driving pressure to preserve the flow induced orientations prior to removal from the tube as a glassy rod. The glassy rod is sectioned and annealed (heating to above glass temperature or melting temperature.) The increase in diameter observed after annealing is reported as unconstrained transverse recovery, a ratio of the diameter after annealing to the diameter of glassy rod. A correction arising from contradiction shearing is obtained and applied to unconstrained transverse recovery results.; High molecular weight, broad distribution commercial Polystyrene (Dylene 8) and Polyvinyl Acetate samples were used in this study. In addition, a 3.85 weight percent blend of high molecular weight narrow distribution polystyrene, PC14A, with commercial polystyrene, Dylene 8, was prepared and used in this study.; The die swell values and flow curves at different temperatures obtained from a capillary rheometer are reported along with recoverable compliance and viscosity data obtained at different stress levels from torsional creep recovery measurements. The unconstrained transverse recovery results for Dylene 8 obtained under different conditions (different stress levels, temperature and tube dimensions) are also reported.; The corrected unconstrained transverse recovery results show quantitative agreement with die swell results. Also the maximum observed in the unconstrained transverse recovery with extrusion time compares well with the maximum observed in total recovery as a function of time of creep.