First-principle simulations of electronic structure in semicrystalline polyethylene.

In order to increase our fundamental knowledge about high-voltage cable insulation materials, realistic polyethylene (PE) structures, generated with a novel molecular modeling strategy, have been analyzed using first principle electronic structure simulations. The PE structures were constructed by first generating atomistic PE configurations with an off-lattice Monte Carlo method and then equilibrating the structures at the desired temperature and pressure using molecular dynamics simulations. Semicrystalline, fully crystalline and fully amorphous PE, in some cases including crosslinks and short-chain branches, were analyzed. The modeled PE had a structure in agreement with established experimental data. Linear-scaling density functional theory (LS-DFT) was used to examine the electronic structure (e.g., spatial distribution of molecular orbitals, bandgaps and mobility edges) on all the materials, whereas conventional DFT was used to validate the LS-DFT results on small systems. When hybrid functionals were used, the simulated bandgaps were close to the experimental values. The localization of valence and conduction band states was demonstrated. The localized states in the conduction band were primarily found in the free volume (result of gauche conformations) present in the amorphous regions. For branched and crosslinked structures, the localized electronic states closest to the valence band edge were positioned at branches and crosslinks, respectively. At 0 K, the activation energy for transport was lower for holes than for electrons. However, at room temperature, the effective activation energy was very low (∼0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even below the mobility edges and suggests that charge carriers can be hot carriers above the mobility edges in the presence of a high electrical field.

Liquid-core nanocellulose-shell capsules with tunable oxygen permeability.

Encapsulation of oxygen sensitive components is important in several areas, including those in the food and pharmaceutical sectors, in order to improve shelf-life (oxidation resistance). Neat nanocellulose films demonstrate outstanding oxygen barrier properties, and thus nanocellulose-based capsules are interesting from the perspective of enhanced protection from oxygen. Herein, two types of nanocellulose-based capsules with liquid hexadecane cores were successfully prepared; a primary nanocellulose polyurea-urethane capsule (diameter: 1.66 µm) and a bigger aggregate capsule (diameter: 8.3 µm) containing several primary capsules in a nanocellulose matrix. To quantify oxygen permeation through the capsule walls, an oxygen-sensitive spin probe was dissolved within the liquid hexadecane core, allowing non-invasive measurements (spin-probe oximetry, electron spin resonance, ESR) of the oxygen concentration within the core. It was observed that the oxygen uptake rate was significantly reduced for both capsule types compared to a neat hexadecane solution containing the spin-probe, i.e. the slope of the non-steady state part of the ESR-curve was approximately one-third and one-ninth for the primary nanocellulose capsule and aggregated capsule, respectively, compared to that for the hexadecane sample. The transport of oxygen was modeled mathematically and by fitting to the experimental data, the oxygen diffusion coefficients of the capsule wall was determined. These values were, however, lower than expected and one plausible reason for this was that the ESR-technique underestimate the true oxygen uptake rate in the present systems at non-steady conditions, when the overall diffusion of oxygen was very slow.

Injection-molded nanocomposites and materials based on wheat gluten.

This is, to our knowledge, the first study of the injection molding of materials where wheat gluten (WG) is the main component. In addition to a plasticizer (glycerol), 5 wt.% natural montmorillonite clay was added. X-ray indicated intercalated clay and transmission electron microscopy indicated locally good clay platelet dispersion. Prior to feeding into the injection molder, the material was first compression molded into plates and pelletized. The filling of the circular mold via the central gate was characterized by a divergent flow yielding, in general, a stronger and stiffer material in the circumferential direction. It was observed that 20-30 wt.% glycerol yielded the best combination of processability and mechanical properties. The clay yielded improved processability, plate homogeneity and tensile stiffness. IR spectroscopy and protein solubility indicated that the injection molding process yielded a highly aggregated structure. The overall conclusion was that injection molding is a very promising method for producing WG objects.

Addendum to Food Additives and Contaminants 2002, 19, 492-501 migration of monomers from liquid crystalline poly(p-hydroxybenzoic acid-co-2-hydroxy-6-naphthoic acid).

Liquid-crystalline co-polyesters (e.g. a random copolyester based on p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) known as Vectra A950) offer good barrier properties, but for food-contact use they require overall and specific migration testing. For Vectra A950 films, the highest overall migration level obtained was 2.3 mg x dm(-2) (13.8 mg x kg(-1)) in olive oil (10 days at 40 degrees C), well below the EC limit of 10 mg dm(-2) (60 mg x kg(-1)). The highest specific migration of HBA was 15.2 microg x dm(-2) (91.2 microg x kg(-1)) in olive oil (2 h at 175 degrees C). In this case, the migration level was well below the EC limit of 10 mg dm(-2) (60 mg x kg(-1)). For HNA, the highest value obtained was 4.3 microg x dm(-2) (26 microg x kg(-1)) in 10% ethanol (4 h at 100 degrees C), well below the specific migration limit (SML = 50 microg x kg(-1)). The results obtained shows that even at these severe conditions, the migration values comply with the new European Union Directive 2002/72/EC, which regulates plastic materials and articles for food contact use. In addition, the polymer Vectra A950 complies with Food Contact Notification (FCN) No. 103 of the United States Food and Drug Administration (FDA). Vectra A950 is therefore permitted for food-contact use both in the European Union and the USA.

Migration of monomers from liquid crystalline poly(p-hydroxybenzoic acid-co-2-hydroxy-6-naphthoic acid).

Liquid-crystalline co-polyesters (e.g. a random copolyester based on p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) known as Vectra A950) offer good barrier properties, but for food-contact use require overall and specific migration testing. For Vectra A950 films, the highest overall migration level obtained was 2.3 mg kg(-1) in olive oil (10 days at 40 degrees C) well below the EC limit of 60 mg kg(-1). The highest specific migration for p-hydroxybenzoic acid was 15.2 microg dm(-2) in olive oil (2h at 175 degrees C). For 2-hydroxy-6-naphthoic acid, the highest value obtained was 4.3 microg dm(-2) in 10% ethanol (4h at 100 degrees C), although it was not on the EC positive and cannot yet be used for food-contact materials. At conditions considered as severe, the estimated daily intake for p-hydroxybenzoic acid was calculated as 11.9 microg/person day(-1) and for 2-hydroxy-6-naphthoic acid it was 5.3 microg/person day(-1). The results exceed the threshold of regulation of 1.5 microg/person day(-1).