Vitamin E can be used to hinder scissioning in radiation cross‐linked UHMWPE during high‐temperature melting

High‐temperature melting (HTM) of ultrahigh molecular weight polyethylene (UHMWPE) was shown to improve its elongation and toughness. This was believed to be due to increased scissioning and increased diffusion of polymer chains. It was hypothesized here that the toughness of previously radiation cross‐linked UHMWPEs could also be improved by HTM. To test this hypothesis, the wear resistance, tensile mechanical properties, and Izod impact strength of radiation cross‐linked virgin (no additive) and antioxidant‐blended (with vitamin E) UHMWPEs were tested. The results suggested that although the impact strength of cross‐linked UHMWPEs could be improved significantly by HTM, the wear resistance was decreased. Thus, this procedure can be optimized to be especially suited in high‐stress applications, such as total knee replacements with lessened wear concerns. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42735.     

Degradation and biodegradation of polyethylene with pro‐oxidant aditives under compost conditions establishing relationships between physicochemical and rheological parameters

In the present work, an analysis is carried out to provide a relationship between the Molecular Weight (M_w) of degraded LDPE films (containing Mn stearate as pro oxidant (MnSt‐LDPE) and changes in viscosity, elongation at break (EB %) and carbonyl index (CI) occurring during thermal degradation in the thermophilic phase of the compost process. The thermal treatment comprised various temperatures (50°C, 60°C, and 70°C) and exposure times, and was characterized through a so‐called Energy‐Time Factor (the product of thermal energy and exposure time). Changes in viscosity, EB %, and CI were correlated to this factor. A modified Mark‐Houwink equation was used to relate the zero shear‐rate viscosity and M_w of the degraded LDPE films. Results indicate that the EB %, M_w and viscosity decrease simultaneously with an increase in the CI as the Energy‐Time Factor augments, allowing the assessment of the variation of these properties with M_w. Calculations of the percentage abiotic degradation (%D) of LDPE films indicate that a M_w of 6 kg mol^?1 corresponds to a maximum abiotic degradation degree of 91.85%, which is henceforth susceptible to biodegradation. The film treated with Energy‐Time Factor of 2.79E+09 J s mol^?1 reached a 74% of biodegradation in 90 days (average time of the composting process). Results exhibit clearly the correlation between abiotic and biotic degradation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42721.     

A unified polymer reaction engineering methodology for catalytic olefin polymerization: From reaction conditions and catalyst reaction performance to molecular and rheological properties for forward,reverse engineering and deconvolution applications

This work presents a unified polymer reaction engineering methodology for the catalytic olefin polymerization process. The proposed modelling approach offers a modelling pathway from the polymerization recipe to production rate and polymer microstructure, and finally to rheological properties. Furthermore, this work introduces for the first time the constraint of the actual reaction performance of the polymerization catalyst in the inverse rheology and microstructural deconvolution problem, limiting the solution only to the most realistic potential molecular weight distributions (MWDs) that a specific catalyst can produce. This approach can be applied for both single- and multi-site catalysts, providing not a potential MWD but the unique one that the selected catalyst can offer under given polymerization conditions. Depending on the available catalyst reaction performance insight, the constraint can vary and include from the number of active sites in use to the exact kinetic parameters of each site type. The potential of the proposed methodology is highlighted within a series of indicative examples, including forward, reverse engineering and deconvolution applications.     

Polyolefin microstructural deconvolution methods: The good,the bad,and the ugly

The deconvolution of the molecular weight distribution (MWD) of polyolefins into Schultz–Flory most probable distributions has become the standard method to identify the number of site types on multiple-site-type olefin polymerization catalysts such as Ziegler–Natta, Phillips, and some supported metallocenes. This method has been used to quantify the effect of polymerization conditions and catalyst formulations on polyolefin MWD and olefin polymerization kinetics. Related methods have also been developed to deconvolute other polyolefin microstructure features, such as the chemical composition and comonomer sequence length distributions. In this paper, I explain the premises behind these deconvolution models and review the publications in this area, highlighting the advantages, disadvantages, and misuses of these methods. I also propose a revised formulation on how to model the MWD of polyolefins made with multiple-site-type catalysts using ratio distributions for propagation and chain transfer frequencies. The main objective of this overview article is to highlight the strengths, but also show the pitfalls, of polyolefin microstructure deconvolution methods.