Abstract
Gaining new insight and better understanding of the residual catalyst-cum-UV-degraded polyolefins, and their melting behaviors are important. Hence, such oxidative degradation of an unstabilized Z-N HomoPE film was studied by measuring the carbonyl index, Tpm (peak melting temperature), and % crystallinity as a function of the exposure time. The carbonyl index increased whereas Tpm decreased as the exposure time increased. The variation of % crystallinity showed to be irregular and disturbed. The melting behavior of the degraded films as a function of UV exposure time was modeled from a nanoscopic perspective. A degradation mechanism, based on a unified molecular level concept of surface chemistry and structure of the residual catalyst, has been proposed. The model predictions provide an entirely new insight into what the UV dosage causes to the exposed films on a nanoscopic scale. The UV radiation introduced consistent lamellar (chain fold/period) and repeat unit shrinkages into the matrix of the exposed films; and decreased the most probable lamellar thickness L0 and chain fold CH2 repeat unit n0 in a similar fashion. However, the ratio of lamellar to repeat unit shrinkages remained unaffected. The model successfully explained why Tmp gradually decreased with the UV exposure time. This is attributed to the analogous variation of L0 and n0. The model-predicted Tmps well matched the corresponding DSC values. The decrease in L0 and n0 with the increase in exposure time has been explained. These findings are likely to help develop better catalysts, design nanostructured polyolefin blends, and advance the polyolefin degradation and recycling areas.
Keywords: Residual catalyst surface chemistry and structure, UV-induced oxidative degradation, ethylene homopolymer, thermal properties, modeling of melting behavior, Polyethylene Film, polymer, transition metal, Lewis acidity, crystallite lamellae, autooxidation, ketone carbonyl, hydroperoxide, lactone, Synthesis of Catalysts
39