Surface microstructural modification and fracture behavior of tensile deformed polypropylene with different percentage crystallinity

Microstructural evolution during tensile deformation of injection molded polypropylene (PP) at the micro- and nano-scale level was studied using atomic force and scanning electron microscopy techniques. Atomic force microscopy (AFM) enabled microstructural changes of tensile deformed PPs with different percentage crystallinity to be captured. AFM of undeformed slow-cooled (SC-PP: high crystallinity) and water-quenched (WQ-PP: low crystallinity) PPs suggested that the fibrils are relatively more closely packed in the SC-PP with higher average surface height of ∼7.5 nm as compared to ∼5.4 nm in the case of WQ-PP. Tensile deformed SC-PP and WQ-PP at displacement rates of ∼125–500 mm/min (strain rates of ∼0.04 s−1 to 0.16 s−1) indicated that the fibrils/microfibrils are aligned along the tensile axis, with WQ-PP exhibiting enhanced stretching of fibrils/microfibrils/chain-folded lamellae in comparison to SC-PP. Three fracture morphologies were identified at different strain rates, and include crazing/tearing (C), brittle fracture in association with crazing/tearing (B1), and brittle fracture together with ductile pulling of ligaments (B2). The fracture morphology exhibited by both SC-PP and WQ-PP was similar, but the percent area fraction of the three identified morphologies varied. WQ-PP with lower crystallinity was characterized by a decrease in percent of crazing/tearing (C) and brittle+crazing/tearing (B1), and increase in brittle+ductile pulling of ligaments (B2). The fracture characteristics of PPs with differences in crystallinity was consistent with AFM observations.