The influence of molecular weight distribution on melt viscosity,melt elasticity,processing behavior and properties of polystyrene

The influence of molecular weight and molecular weight distribution on the melt rheological behavior of two polystrenes of approximately the same weight average molecular weight, but of widely different molecular weight distribution, was determined. Then, using a series of capillaries with different length-to-diameter ratios in an Instron Capillary Rheometer, the entrance correction methods of E. B. Bagley and the relationships of W. Philippoff and F. H. Gaskins, the recoverable shear strain (S_R) in the melt at the capillary wall for these mono- and polydisperse polystyrenes was determined. Shear modulus (G) and normal stress (P_N) were calculated using the relationships: G = τ_RC/S_R and P_N = 2τ_RC S_R, where τ_RC is the corrected shear stress at the capillary wall. These are compared to values obtained using a Weissenberg Rheogonimeter. These two polystyrenes were also injection molded into an ASTM specimen mold over a wide range of stock temperature, using a 12 OZ . in-line reciprocating screw injection press, and evaluated for mechanical property values. The effects of the elasticity parameters (S_R G & P_N) and their magnitude on the rheology, processability and mechanical properties of these polystyrenes are discussed.