Plastic deformation of polypropylene

Abstract

The heats of fusion of drawn polypropylene at different draw temperatures (ranging from 22°C–140°C) and draw rates (ranging from 0.05 cm min −1 to 50 cm min −1) were determined to establish the plastic deformation mechanism of polypropylene. At draw ratio λ = 6 and a draw rate R d = 0.5 cm min −1 the heat of fusion of drawn polypropylene is a unique function of the draw temperature and is completely independent of the crystallinity of the starting material. The heat of fusion does not change with increased draw rate on drawing at 22°C, but decreases with draw rate on drawing at 60°C or higher temperatures. These results may be interpreted by the phase-transition model of plastic deformation of a semicrystalline polymer. During drawing of polypropylene ‘mechanical melting’ and strained recrystallization take place at the draw temperature and must be very fast. The new structure of drawn polypropylene is solely determined by the draw temperature, with a slight modification. At a T d of 60°C or higher, and at higher draw rates, as a consequence of the shorter exposure of drawn polypropylene to the draw temperature at these higher rates, crystallization proceeds for a shorter time at T d and hence results in a smaller increase in crystallinity.