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.

Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates.

Nanostructured biological materials inspire the creation of materials with tunable mechanical properties. Strong cellulose nanofibrils derived from bacteria or wood can form ductile or tough networks that are suitable as functional materials. Here, we show that freeze-dried bacterial cellulose nanofibril aerogels can be used as templates for making lightweight porous magnetic aerogels, which can be compacted into a stiff magnetic nanopaper. The 20-70-nm-thick cellulose nanofibrils act as templates for the non-agglomerated growth of ferromagnetic cobalt ferrite nanoparticles (diameter, 40-120 nm). Unlike solvent-swollen gels and ferrogels, our magnetic aerogel is dry, lightweight, porous (98%), flexible, and can be actuated by a small household magnet. Moreover, it can absorb water and release it upon compression. Owing to their flexibility, high porosity and surface area, these aerogels are expected to be useful in microfluidics devices and as electronic actuators.

Effect of head surface roughness and sterilization on wear of UHMWPE acetabular cups.

The impact of femoral head surface roughness on wear of gamma-irradiation sterilized (3 MRad in nitrogen, crosslinked) and nonsterilized (not crosslinked) UHMWPE acetabular cups has been evaluated. Gravimetric wear testing was performed in a hip joint simulator for 2 x 10(6) cycles. CoCrMo heads were used with different surface roughness (R(a) = 15 nm and R(a) = 400 nm). The surface roughness after wear test was unchanged for the roughened heads, whereas the initially smooth heads showed a few scratches. The roughened heads increased the wear of the acetabular cups 2-fold. The gamma-irradiated cups tested against rough heads underwent the highest wear. The absorption of water was highest for the gamma-irradiated cups (0.0204% compared to 0.0031% after 85 days). Raman spectroscopy showed small but significant crystallinity changes in the wear zone, where the gamma-irradiated cups with the most extensive abrasion increased in crystallinity, whereas the nonsterilized cups underwent a crystallinity decrease.

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).