A highly active and chemoselective ionic organocatalyst is developed for room-temperature living/ controlled ring-opening polymerization of lactide. The catalysts are prepared by a simple dehydration reaction between tetra-n-butyl ammonium hydroxide and an N,N′diarylurea and used in cooperation with hydroxy initiators. Typically, poly(L-lactide) with near perfect isotacticity and widely tunable molar mass (4−130 kg mol −1 ) can be produced in <2 min (turnover frequency up to 120 000 h −1 ). Low molar mass distribution is observed in both short and substantially extended reaction times, clearly demonstrating the selectivity of catalyst for monomer enchainment over macromolecular transesterification. Versatile design and construction of diverse polylactide-based macromolecular structures are allowed thanks to the livingness of the polymerization and independence of initiator and catalyst. In addition to the hydrogen bond donor−acceptor type bifunctional activation mechanism, direct nucleophilic attack of the urea anion on the monomer and polymer is also shown which can be suppressed by the added hydroxy initiator.

2010)Development of a vehicle-track model assembly and numerical method for simulation of wheel-rail dynamic interaction due to unsupported sleepers, Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility, 48:12, 1535-1552 To link to this article: http://dx.In practice, it is not very uncommon to find railway track systems with unsupported sleepers due to the uneven settlement of a ballasted track system. These unsupported sleepers are among the major vibration excitations for a train and track system when a train moves forwards on a track. The vibration induced by unsupported sleepers can cause a large dynamic contact force between wheels and rails. For heavily loaded high-speed trains, the deteriorated sleeper support may lead to accelerated degradation of the railway track and vehicle components, and may thus impose safety risk to the operation. This paper presents analyses of a coupled vehicle-track assembly consisting of a roll plane vehicle model, a continuous track system model and an adaptive wheel-rail contact model. In order to improve the simulation efficiency, a numerical approach based on the central finite difference method is proposed in this investigation. The developed model assembly and proposed simulation method are utilised to simulate the vehicle-track dynamic interaction in the presence of unsupported sleepers. The dynamic response in terms of the dynamic wheel-rail interaction force due to one or multiple unsupported sleepers is studied. Important factors influencing the dynamic wheel-rail interaction force in the presence of sleeper voids are also investigated. The results show that the vehicle speed, the gap size and the number of unsupported sleepers primarily dictate the magnitude of impact load which can be significant.

This paper describes a simple and low-pollution surface modification strategy for fabricating flexible, semi-transparent, and robust bacterial cellulose (BC)/Kombucha tea-based BC films that are effectively resistant to water, water vapor, oxygen, ultraviolet light, and foodborne pathogenic bacteria. The strategy uses dimethyl sulfoxide as the pre-swelling solvent and long-chain alkenyl succinic anhydrides with different chain lengths as the esterifying agents. The proof of esterification, crystallinity index, film microstructure, and degree of surface substitution were systematically investigated. Our result showed that acylation with 2-octenylsuccinic anhydride for 8 h conferred the BC films with the highest water-vapor/oxygen resistance and optimal mechanical/thermal properties. In addition, the modified BC film has good antimicrobial properties and is capable of wrapping strawberries, increasing their longevity. Biodegradation and alkaline hydrolysis tests demonstrated that compostability of the film and recycling of the BC pulp are possible, revealing that such high-performance biofilm-based food packaging could have a green life cycle.

The application of cellulose in the food packaging field has gained increasing attention in recent years, driven by the desire for sustainable products. Cellulose can replace petroleum-based plastics because it can be converted to biodegradable and nontoxic polymers from sustainable natural resources. These products have increasingly been used as coatings, self-standing films, and paperboards in food packaging, owing to their promising mechanical and barrier properties. However, their utilization is limited because of the high hydrophilicity of cellulose. With the presence of a large quantity of functionalities within pristine cellulose and its derivatives, these building blocks provide a unique platform for chemical modification via covalent functionalization to introduce stable and permanent functionalities to cellulose. A primary aim of chemical attachment is to reduce the probability of component leaching in wet and softened conditions and to improve the aqueous, oil, water vapor, and oxygen barriers, thereby extending its specific use in the food packaging field. However, chemical modification may affect the desirable mechanical, thermal stabilities and biodegradability exhibited by pristine cellulose. This review exhaustively reports the research progress on cellulose chemical modification techniques and prospective applications of chemically modified cellulose for use in food packaging, including active packaging.

No abstract