Time-Resolved SAXS/WAXD under Tensile Deformation: Role of Segmental Ethylene–Propylene Copolymers in Impact-Resistant Polypropylene Copolymers

• Published in: ACS applied polymer materials 2021-12, Vol.3 (12), p.6394-6406
• Main Authors: Rungswang, Wonchalerm, Jarumaneeroj, Chatchai, Jirasukho, Prapasinee, Juabrum, Sawitree, Pakawanit, Phakkhananan, Soontaranon, Siriwat, Rugmai, Supagorn
• Format: Article
• Language: English
• ISSN: 2637-6105; 2637-6105
• DOI: 10.1021/acsapm.1c01159

Structural evolution of impact-resistant polypropylene copolymers (IPCs), having different compositions of segmental ethylene–propylene (sEP) copolymers, under tensile deformation was investigated by time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) at elevated temperatures. At 40 °C, IPCs with a higher P content in sEP copolymers showed much longer elongation at break ratios than IPCs with a lower P content, which suggested the role of interfacial adhesion between the ethylene–propylene random (EPR) copolymer and homo polypropylene (hPP) phases. Polarized optical microscopy revealed the existence of the interfacial crystallite grown from the hPP spherulitic interface into the EPR phase. This indicated that the P segment might have promoted the interfacial adhesion between the EPR/hPP phases. To further verify this hypothesis, tensile experiments were carried out at 130 °C, a temperature higher than the melting temperature of the copolymer crystal. As expected, the tensile property of the IPCs became comparable with that of hPP, which also confirmed the role of the interfacial crystallite. SAXS and WAXD results revealed that for IPCs with higher interfacial adhesion, the crystal fracture was delayed at the strain near the yield point. However, its fibrillation process became faster than that of IPCs with poorer interfacial adhesion. The nano- and micro-void formation was investigated through SAXS and X-ray tomography techniques, respectively. Interestingly, both nano- and micro-voids, which first elongated along the transverse direction and then sequentially reoriented along the stretching direction, were observed for the first time, and it was termed the “double cavitation” process. The results indicated that the nano- and micro-voids of IPCs with high interfacial adhesion were smaller than those of hPP. This study shows the role of interfacial adhesion in sEP copolymers in improving the failure resistance during tensile deformation.