Crystallinity development in cellular poly(lactic acid) in the presence of supercritical carbon dioxide

This article investigates the crystallinity development in cellular poly(lactic acid) (PLA) and the effect of the achieved crystalline content on its properties and microstructure. Carbon dioxide (CO₂) in its supercritical state was used as the expansion agent for three different grades of PLA that differed in terms of L‐lactic acid content. Cellular PLA was produced on a twin‐screw extrusion line using capillary dies of various diameters. The obtained crystalline contents were measured by differential scanning calorimetry and X‐ray diffraction techniques. The morphology of the cellular structures was examined using scanning electron microscopy. The crystallinity developed on expansion depended on L‐lactic acid content, on supercritical CO₂ concentration, polymer flow rate, and die diameter. Cellular PLA, with densities as low as 30 kg/m³, was obtained under the most favorable conditions. It was shown that the crystallinity development in PLA enhances its cellular structure formation and enables the fabrication of quality cellular materials at lower CO₂ concentration. The presence of PLA crystallites within expanded cell walls leads to a peculiar 2D‐cavitation phenomena observed only in the cell walls of semicrystalline foams. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Interface/morphology relationships in polymer blends with thermoplastic starch

In this paper, the interface/morphology relationship in polyethylene/TPS blends prepared by a one-step extrusion process is examined in detail. Emulsification curves tracking the change in phase size with added quantity of PE-g-MA copolymer are used to identify the critical concentration required for saturation of the interface as well as to estimate the areal density of grafted copolymer chains at the interface. The level of glycerol content in the TPS is shown to lead to different emulsification behaviors. Dynamic mechanical analysis clearly shows a partial miscibility between glycerol and starch in the TPS with glycerol-rich and starch-rich peaks being clearly identified. This phase separation is more evident in the case of high glycerol levels in the TPS (>24% glycerol). Furthermore, the glycerol-rich peak decreases in intensity with added PE-g-MA graft copolymer. At high glycerol contents (>24% glycerol) in the TPS, a 20% thermoplastic starch-based binary blend with polyethylene can reach an elongation at break value as high as 200%. When also modified at the appropriate level with a PE-g-MA copolymer, this elongation at break further increases to 600%. However, at lower glycerol contents, the elongation at break is comparatively low at 20-50% even after the addition of PE-g-MA copolymer. We explain these results through a proposed double mechanism of interfacial modification between the HDPE matrix and the TPS dispersed phase. Under dynamic melt-mixing conditions, it is suggested that a small portion of the low molecular weight glycerol-rich phase tends to migrate to the HDPE-TPS interface as predicted by Harkins spreading theory. Once at the interface, this glycerol-rich outer layer is readily deformed by an applied stress and this stress is then transferred to the starch-rich phase due to their mutual partial miscibility. Added PE-g-MA copolymer initially reacts with the glycerol-rich outer layer but if the level of copolymer is high enough, it then reacts with the starch-rich phase via a classic interfacial modification protocol. Also, both the elongation at break and impact properties dramatically increase at a copolymer level associated with interfacial saturation. The above mechanism effectively explains all the emulsification and mechanical property observations.     

Optimization of Large Transport Networks Using the Ant Colony Heuristic

Abstract:   Long-term transportation planning for larger regions must assess synergies and interference among sets of projects. The objective is the maximization of the overall benefit within specific budget restrictions by finding the most favorable bundle of projects, that is, solving the network design problem. For large numbers of projects, complete enumeration of all combinations is not feasible for detailed networks. The ant colony heuristic is suitable for this kind of problem. According to our knowledge, this article presents the above-mentioned heuristic's first application to a realistically sized network. A detailed multimodal network assignment of a substantial Swiss city provides the basis for calculations. First, each infrastructure project is assessed using a cost–benefit analysis. The ant colony heuristic is then successfully executed and the bundles are evaluated. The article provides new insights into applications of the heuristic in large networks and focuses on problematic calibration details as well as the choice of objective function. Suggestions are made for general applications and further research .     

Sulphite binding in ciders

Summary The extent of sulphite binding was measured in commercial ciders, and experimentally observed binding curves were compared with theoretically derived curves based on assessment of the levels of individual sulphite binding compounds determined in the ciders. Subsequently, experimental ciders were fermented under various controlled conditions using nine different strains of cider yeasts. The results indicated considerable differences both in the levels of sulphite binding compounds produced, and in the ability of different yeast strains to produce SO₂ in the cider. Juice concentration and the presence of cloud in juice had little or no effect on sulphite binding. Other factors which affected sulphite binding included the type and condition of juice (especially the effect of pectinase treatment) and, in some instances, the use of added yeast nutrients. Significant sulphite binding was also attributable to unaccountable components, probably derived from poor quality fruit, which were present in the apple juice prior to fermentation.     

Homothetic tube model predictive control

The robust model predictive control for constrained linear discrete time systems is solved through the development of a homothetic tube model predictive control synthesis method. The method employs several novel features including a more general parameterization of the state and control tubes based on homothety and invariance, a more flexible form of the terminal constraint set and a relaxation of the controlled dynamics of the sets that define the state and control tubes. Under natural assumptions, the proposed method is computationally efficient and it induces strong system theoretic properties.     

Simple computations involving two-component symmetric trilayers

The equivalent properties of a symmetric three-layer structure are derived by a nontraditional method that provides useful insights and a simplified application to some thin-film design problems. The equations derived from this method may be used to design a three-layer replacement for a single layer in an interference coating. The equations are especially helpful for cases that involve complex numbers, such as metal layers or above-critical angle propagation in dielectric layers. Several multilayer design problems are solved to demonstrate the application of this approach.     

Modulation transfer function measurement of sparse-array sensors using a self-calibrating fringe pattern

A simple method for the measurement of the pixel modulation transfer function (MTF) of sparse-array (extended MTF) sensors has been developed. We use a phase-shifting Twyman-Green interferometer to generate a series of single spatial-frequency fringe patterns incident on the sensor The resulting signal modulation is measured. We achieve self-calibration by restricting the measured spatial frequencies to multiples of the Nyquist frequency. The aliased patterns at these frequencies are unique and easily identifiable. Spatial frequencies of 480 cycles/mm are generated and measured. This frequency value is more than ten times that of the sensor sampling frequency. The expected MTF shape is obtained at multiples of the sampling frequency. At odd multiples of the Nyquist frequency, the MTF's are affected by the electronic bandwidth and cross talk in the charge-injection device sensor.     

Maintaining large time steps in explicit finite element simulations using shape matching

We present a novel hybrid method to allow large time steps in explicit integrations for the simulation of deformable objects. In explicit integration schemes, the time step is typically limited by the size and the shape of the discretization elements as well as by the material parameters. We propose a two-step strategy to enable large time steps for meshes with elements potentially destabilizing the integration. First, the necessary time step for a stable computation is identified per element using modal analysis. This allows determining which elements have to be handled specially given a desired simulation time step. The identified critical elements are treated by a geometric deformation model, while the remaining ones are simulated with a standard deformation model (in our case, a corotational linear Finite Element Method). In order to achieve a valid deformation behavior, we propose a strategy to determine appropriate parameters for the geometric model. Our hybrid method allows taking much larger time steps than using an explicit Finite Element Method alone. The total computational costs per second are significantly lowered. The proposed scheme is especially useful for simulations requiring interactive mesh updates, such as for instance cutting in surgical simulations.     

In vivo studies of the biosynthesis of vernolic acid in the seed of Vernonia galamensis

In vivo radiotracer experiments using [1-¹⁴C]acetate as the precursor were conducted to investigate the biosynthesis of vernolic acid (12, 13-epoxy-cis-9-octadecenoic acid) in the seeds of Vernonia galamensis. The acetate precursor radioactively labeled vernolate in phosphatidylcholine (PC), diacylglycerol, and triacylglycerol. Time-course kinetics of the incorporation of the radioactive tracer indicated that vernolate is synthesized while the acyl moiety is esterified to PC. Pulsechase experiments provided additional supporting evidence that vernolate is synthesized while esterified to PC. These results are consistent with the hypothesis that linoleoyl PC is the precursor of vernoleoyl-PC. Subsequently, vernolate is quickly moved from the PC pool to the triacylglycerol pool, where it accumulates.     

The influence of thermal history on the properties of poly(3-hydroxybutyrate-co-12%-3-hydroxyvalerate)

The influence of thermal history on the properties of commercial P(3HB-co-12%-3HV) was studied. Thermogravimetric analysis and differential scanning calorimetry revealed that the plasticizer evaporated at 140°C or higher. The loss of plasticizer during thermal treatment at 170 and 180°C resulted in a slight increase of the melting temperature of the polymer. The processing time and temperature, as well as the cooling procedure influenced the thermal behaviour of the material. A decrease in molecular weight with time was found at the temperatures investigated and this significantly affected the mechanical properties of the polymer prepared at 180 and 200°C. The rate constant k_d of thermal degradation was slightly higher for samples during a shape-forming process in a Minimax apparatus than during a quiescent heating process (DSC) and its value increased with temperature. Limiting the processing at 170°C to 2 min gave a material with useful properties but increasing the residence time resulted in a decrease in strength, elongation at break, molecular weight and viscosity although it did not significantly influence the modulus of elasticity. Materials prepared at 180 or 200°C were more brittle and longer residence times resulted in a deterioration of the mechanical properties. © 1996 John Wiley & Sons, Inc.