Crystallization in isotactic polypropylene melts during contraction flow: time-resolved synchrotron WAXD studies

Abstract

Time-resolved 2D WAXD studies have been performed on α-nucleated isotactic polypropylene (iPP) in order to study the development of crystal growth and crystallite orientation during and subsequent to contraction melt flow at elevated temperatures in the range from 159 to 172°C. Prior to flow, an annealing procedure is carried out at 230°C for 90 min to erase memory effects in terms of crystal aggregates and molecular conformations. The onset of crystallization is visible at temperatures as high as 170°C, which is far above the crystallization range under quiescent conditions. Depending on the applied flow rates mainly two different crystalline orientations are observed. A low flow rate γ ̇ =9.1 s −1 at 159 and 161°C causes intensive meridional (110) and equatorial (040) reflections during flow (95–140 s) which can be described by lamellar branching via homoepitaxy. Whereas with increasing flow rate up to γ ̇ =127.1 s −1 the equatorial hk0 reflections become dominant, indicating a preferential orientation of crystallites with their c-axis in flow direction.