Crystallization-correlated phase separation of polypropylene random copolymers

Abstract

Ethylene segments are incorporated into stereoregular propylene-propylene-propylene (PPP) homo-sequences to create locally disordered ethylene-propylene (EP) segments in polypropylene random copolymer (PPR) through a phase separation process, resulting in large quantities of spherical EP rubbery particles for improving impact toughness. However, the manipulation on size and distribution of these spherical EP rubbery phases remains insufficiently understood. In this study, the phase separation of PPR and the influence of crystallization on phase separation were investigated using rheological measurements and small-angle x-ray scattering (SAXS) methods. Combined with the isothermal crystallization method, phase separation was differentiated by the crystallization rate. The molecular chain states corresponding to different crystallization conditions were studied to interpret the varying rubbery particle sizes resulting from phase separation. The results showed that phase separation of EP segments from PPP is highly related to crystallization. Isothermal crystallization at temperatures ranging from 110 to 130°C leads to a high degree of phase separation due to the fast crystallization rate, as evidenced by the presence of small quantities of large pores in microscopic observations. This work provides insights into the phase separation of random copolymers and establishes correlations for manipulating the low-temperature toughness of PPR.