Effects of Molecular Weight and Molecular Weight Distribution on Mechanical Properties of Undrawn Filaments of Isotactic Polypropylene

Abstract

Fractions, blends and whole polymers from three kinds of isotactic polypropylene were melt spun at spinning draft 50 (diameter and length of straight die used, 0.45 and 0.5 mm,respectively, the rate of output 0.765 cc/min., winding velocity 250 m/min), at the optimum spinning temperature. The mechanical properties were measured on the undrawnfilaments having the same crystal form (monoclinic) and the same degrees of crystallinity and orientation. The results obtained are summerized as follows :(1) The optimum spinning temperature is a function of viscosity-average molecular weight only.(2) Since molecular weight degradation is avoidable during melt spinning process, the filament melt spun from fraction has rather wide distribution (M,,,/Mn=2--3, where, M." weightaverage molecular weight, Mn, number-average molecular weight).(3) Stress-strain relations vary widely with molecular weight of filaments. (4) Yield stress is remarkably influenced by the molecular weight retension during melt spinning process. Compared with the retention of the same degree yield stress increases with the increase of molecular weight.(5) Yield extension attains maximum at where, M" is viscosity-average molecular weight. (6) Both breaking strength and breaking energy increase with increasing molecular weight, depending on a polymer type.(7) Breaking extension reaches maximum at M"=45X104 independent of a polymer type and molecular weight distribution. This behabior is similar to molecular weight dependence of stretchability of filaments. (8) Elastic recovery at 10% extension reaches maximum at M"=1.8x106. (9) Relaxation modulus decreases in inverse proportion to log t (where, t time) and attains maximum at R,=8-13x104, compared with the same t.(10) Abrasion strength increases with molecular weight.