Phase morphology of nascent impact polypropylene copolymer and its evolution in the melt

Abstract

In this study, the morphology of the nascent grains of a commercial impact polypropylene copolymer (IPC) and its evolution upon melting was investigated by atomic force microscopy (AFM) and atomic force microscopy-infrared (AFM-IR). The grains were found to be heterogeneous, about half translucent and the rest opaque, containing 12.2 and 2.0 wt% ethylene comonomer, respectively. The ethylene-propylene (EP) rubber in these two kinds of grains differed in content, but was of the same composition, filling the voids between isotactic polypropylene (iPP) subglobules in irregular long strips. Upon melting, the EP rubber strips were found to quickly break into smaller sections, and their shape became more regular, within which rigid cores started to appear. Then, disperse particles with clear core-shell structure were observed, and they grew slightly bigger at longer heating time. Both translucent and opaque grains went through the same morphology evolution process, resulting in core-shell particles of the same phase compositions as assessed by AFM-IR analysis. In addition, the translucent grains were mixed with 5 wt% of a deuterated PP in a twin-screw extruder, and the blend pellet was examined by AFM-IR, and the results showed that the deuterated species was abundant in the PP matrix and totally absent within the disperse particles, implying no encapsulation of the molecules from the matrix surrounding in the formation of the core-shell structure in the melt processing.