Inverse gas chromatographic characterization of polystyrene and organo-montmorillonite/polystyrene nanocomposites

Polymer nanocomposites of a polystyrene (PS) matrix containing 2% and 5% organo-montmorillonite (OMt) by mass were prepared using the solution blending method with sonication. Hexadecyl trimethyl ammonium bromide was used to modify the montmorillonite after its surface was saturated with Na⁺ ions. X-ray diffraction and transmission electron microscopy revealed the mixed nanomorphology of nanocomposites. The majority of OMt is dispersed in the polymer matrix in the form of an ordered tactoid (multilayer particles) structure consisting of few silicate layers and a small amount of exfoliation was achieved. The synthesized nanocomposites showed a higher decomposition temperature in comparison with pure PS according to the thermogravimetric analysis. Inverse gas chromatography under infinite dilution conditions was applied to evaluate the surface properties of the PS and OMt/PS nanocomposites. The IGC results were in agreement with conceptual and theoretical expectations. The dispersive component of the free energy of adsorption and specific interactions (acid–base properties) were determined using polar and non-polar adsorbates (probes) of known properties in the temperature range 40–70 °C. The IGC data showed that the introduction of a very small amount of OMt into the polymer matrix significantly changed the surface characteristics of the final material.     

Peptide-Based Surface Plasmon Resonance Biosensor for Detection of Staphylococcal Enterotoxin B

A label-free biosensor based on surface plasmon resonance (SPR) has been developed to evaluate the potential of peptide ligands as biorecognition molecules for the detection of bacterial toxins. A 24-mer peptide, previously identified from a phage display library and modified for better binding, was used for the detection of staphylococcal enterotoxin B (SEB). The peptide was immobilized covalently onto the gold-coated surface modified with self-assembly monolayer. The limit of detection (LOD) for SEB was found to be 20 μg/mL. The selectivity of the peptide-based sensor for SEB was verified by injecting bovine serum albumin (BSA), and the sensor response for the BSA was negligible. Further, to probe the effectiveness of the sensor in complex food environments, it was tested with milk samples spiked with SEB, showing that it is applicable to complex food matrices. The results of this study demonstrate the potential of peptides, selected from phage display libraries, as recognition agents for detecting bacterial toxins using biosensors.     

Statistical Modeling of β-galactosidase Inhibition During Lactose Hydrolysis

A statistical model approach called response surface methodology was used to describe the product and substrate inhibition effect on β-galactosidase enzyme during lactose hydrolysis. The effect of independent variables, namely the initial concentrations of lactose (73 – 146 mM), galactose (44 – 122 mM) and glucose (83 – 167 mM) on the reaction rate of β-galactosidase was evaluated. The enzymatic reaction rate was influenced by both combined and individual effects of all the substrate and products. Although, glucose acted as an activator at low lactose and low galactose concentrations, glucose caused the inhibition of β-galactosidase at higher concentrations of lactose and galactose. The effect of galactose concentration on β-galactosidase enzyme was in the direction of inhibition. At low lactose concentrations and high glucose concentrations, galactose concentration became more effective on the reaction rate.     

Direct pyrolysis mass spectrometry analyses of polyamide‐6 containing melamine and boron compounds

In this work, the thermal degradation characteristics of polyamide 6 (PA6) containing melamine (Me) and a boron‐containing additive, borophosphate (BPO₄), zinc borate (ZnB) or a boron and silicon‐containing oligomer (BSi) are investigated systematically via direct pyrolysis mass spectrometry. In the presence of boron compounds, not only the release of melamine and its degradation products but also thermal characteristics of PA6 are affected significantly and the interactions between melamine and PA6 are enhanced. For the composite involving BPO₄, sublimation of melamine and degradation of PA6 are slightly shifted to high temperatures. Whereas, for the composites involving ZnB and BSi, sublimation of melamine is depressed due to the glassy surface formed by ZnB and BSi and release of thermal degradation products of PA6 are shifted to lower temperatures noticeably. The glassy surface generated by ZnB allows the release of degradation products of melamine to a certain extent. Conversely, for the composite involving BSi, reduction in the yields of degradation products of melamine is also noted. The trap of degradation products of melamine in the polymer matrix causes significant decrease in the the temperature region where thermal degradation products of PA6 are recorded. POLYM. COMPOS. 34:1389–1395, 2013. © 2013 Society of Plastics Engineers     

A variable neighborhood search for minimizing total weighted tardiness with sequence dependent setup times on a single machine

This paper deals with the single machine scheduling problem to minimize the total weighted tardiness in the presence of sequence dependent setup. Firstly, a mathematical model is given to describe the problem formally. Since the problem is NP-hard, a general variable neighborhood search (GVNS) heuristic is proposed to solve it. Initial solution for the GVNS algorithm is obtained by using a constructive heuristic that is widely used in the literature for the problem. The proposed algorithm is tested on 120 benchmark instances. The results show that 37 out of 120 best known solutions in the literature are improved while 64 instances are solved equally. Next, the GVNS algorithm is applied to single machine scheduling problem with sequence dependent setup times to minimize the total tardiness problem without changing any implementation issues and the parameters of the GVNS algorithm. For this problem, 64 test instances are solved varying from small to large sizes. Among these 64 instances, 35 instances are solved to the optimality, 16 instances' best-known results are improved, and 6 instances are solved equally compared to the best-known results. Hence, it can be concluded that the GVNS algorithm is an effective, efficient and a robust algorithm for minimizing tardiness on a single machine in the presence of setup times.     

Performance of local search heuristics on scheduling a class of pipelined multiprocessor tasks

This paper presents the evaluation of the solution quality of heuristic algorithms developed for scheduling multiprocessor tasks for a class of multiprocessor architectures designed to exploit temporal and spatial parallelism simultaneously. More specifically, we deal with multi-level or partitionable architectures where MIMD parallelism and multiprogramming support are the two main characteristics of the system. We investigate scheduling a number of pipelined multiprocessor tasks with arbitrary processing times and arbitrary processor requirements in this system. The scheduling problem consists of two interrelated sub-problems, which are finding a sequence of pipelined multiprocessor tasks on a processor and finding a proper mapping of tasks to the processors that are already being sequenced. For the solution of the second problem, various techniques are available. However, the problem remains of generating a feasible sequence for the pipelined operations. We employed three well-known local search heuristic algorithms that are known to be robust methods applicable to various optimization problems. These are Simulated Annealing, Tabu Search, and Genetic Algorithms. We then conduct computational experiments and evaluate the reduction achieved in completion time by each heuristic. We have also compared the results with well-known simple list-based heuristics.