Urea‐induced conformational changes of chitosan molecules and the shift of break point of Mark–Houwink equation by increasing urea concentration

Chitosan solutions of the same 83% degree of deacetylation (DD) but different weight average molecular weights (M_ws) (78–914 kDa) in 0.01M HCl containing different concentrations of urea (0–6M) were prepared. Intrinsic viscosity ([η]) and weight average molecular weight (M_w) of chitosan were measured with a capillary viscometer and light scattering, respectively. Mark–Houwink exponent a was used as the parameter of conformational index. The Mark–Houwink exponent a increased with increasing concentrations of urea. When solutions contained 0, 2, 3, 4, and 6M urea, the value of a increased from 0.715 to 0.839, 0.894, 1.000, and 1.060, respectively. This indicates the occurrence of urea‐induced conformational transitions of chitosans. The break point shifted from 223 kDa in solutions containing no urea to 280 kDa in 2M urea solutions, to 362 kDa in 4M urea solutions and further to 481 kDa in 6M urea solutions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 452–457, 2000     

Syntheses of regular copolyamides using triphenyl phosphite and pyridine

In this article, a method of preparing of regular copolyamide (co‐PA) in a reactor using triphenyl phosphite/pyridine as the activating agent was investigated. In two‐step reactions of copolymers prepared from two kinds of dicarboxylic acids and one kind of diamine, oligomer PAs containing amine end groups were first synthesized; then, the second dicarboxylic acid was added to the same reactor to polymerize high molecular weight and regular co‐PAs. However, when oligomers having carboxylic acid end groups were first prepared and another kind of diamine was added, the degree of polymerization was low. The reason proposed was that if the oligomer has an amine end group, it would coexist with the activating agent without deactivating; if the end group was the carboxylic acid end group, deactivation would occur. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 116–122, 2000     

Natural antioxidant extract from galangal (Alpinia galanga) for minced beef

Raw and cooked minced beef samples containing commercial antioxidants or galangal (Alpinia galanga) extract were evaluated for storage stability at 4 ± 1 °C. Thiobarbituric acid (TBA) values of raw and cooked samples containing galangal extract at 5 and 10% (w/w) were significantly (p < 0.05) lower than those of the controls. In raw beef, galangal extract at 10% (w/w) was as effective as α‐tocopherol at 0.10% (w/w) and butylated hydroxytoluene (BHT) at 0.02% (w/w) in inhibiting/minimising lipid oxidation. Galangal extract also delayed the induction period of lipid oxidation in cooked beef but was again observed to be effective at a higher concentration than the commercial antioxidants studied. No difference in total plate count was observed in raw beef, but samples with galangal extract had higher Hunter a (redness) and chroma (colourfulness relative to the brightness of the surroundings) values. However, cooked beef treated with galangal extract or antioxidants had lower Hunter a values and total microbial counts than the control. Thus galangal extract may be a possible natural antioxidant source for meat and meat products. © 2000 Society of Chemical Industry     

Image‐guided robotic navigation system for neurosurgery

This paper presents an image‐guided robotic navigation system for neurosurgery, which can be applied to the electro‐stimulation treatment of Parkinson's leisure, the biopsy of deep tumors, and haematoma evacuation. The system integrates a computer containing CT images for surgical planning, a magnetic tracking device for measuring the coordinates of the markers and surgical instruments, and a robot manipulator for guiding surgical instruments to the preplanned position and orientation. The computer display of brain anatomy offers a convenient tool for surgeons to diagnose brain diseases and to plan safe surgical paths, while the tracking device guides the robot manipulator to automatically move surgical instruments to the preplanned position and orientation. An experiment of using a skull model for simulating a robotic biopsy of brain tumor has been done to verify the performance of the robotic navigation system. The results show that the positioning accuracy of the robot relative to the tracker frame is only related to the positioning resolution of the robot manipulator and the positioning accuracy of the tracking device. In other words, the positioning accuracy of the robot manipulator does not affect the final positioning accuracy of the surgical instruments. Therefore, using a robot manipulator for precise surgical navigation is feasible and reliable. © 2000 John Wiley & Sons, Inc.     

Effects of polymer matrix and processing on the conductivity of polymer blends

The influence of different hosts, processing conditions and conducting fillers on the percolation threshold (Φ_c) of the resulting conducting blends was investigated. Results indicate that microscopic properties, such as the dipole moments of the side‐groups, and distribution of these groups on the host polymer backbone govern the strength of host–filler interactions, and to a large extent, the value of Φ_c, rather than macroscopic properties such as surface tension. The grade of carbon black used in this experiment was found to be polar in nature and it resulted in lower values of Φ_c with the polar hosts, contrary to published literature. In general, melt blending has been shown to result in higher values of Φ_c when compared to hot pressing alone. In the latter method the conductive filler was found to be isolated at the grain boundaries of the polymer host, resulting in the formation of continuous conducting pathways at low filler concentration. © 2001 Society of Chemical Industry     

Simultaneous full‐interpenetrating polymer networks of blocked polyurethane and vinyl ester. II. Static and dynamic mechanical properties

Simultaneous full‐interpenetrating polymer networks (full‐IPNs) based on blocked polyurethane (PU) and vinyl ester (VE) have been prepared. The static and dynamic properties of these IPNs have been examined. Results show that the tensile strength and flexural strength of IPNs increased with blocked PU content to a maximum value at 7.5 wt % PU content and then decreased. The tensile modulus, flexural modulus, and hardness of IPNs decreased with increasing blocked PU content. The impact strength of IPNs increased with increasing blocked PU content. The tensile strength, flexural strength, tensile modulus, and flexural modulus of IPNs increased with filler (kaolin) content to a maximum value at 20 to 25 phr filler content and then decreased. The higher the filler content, the greater the hardness, and the lower the impact strength of IPNs. The tensile strength, flexural strength, tensile modulus, flexural modulus, and hardness of IPNs increased with increasing VE initiator content. The dynamic technique was used to determined the damping behavior across a temperature range. Results show that the glass transition temperature (T_g) of IPNs are shifted inwardly compared with pure PU and VE, which indicated that the blocked PU–VE IPNs showed excellent compatible. Meanwhile, the glass transition temperature was shifted to a higher temperature with increased filler content. The dynamic storage modulus (E′) of IPNs increased with increasing VE and filler content. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1977–1985, 1999     

Effects of ionic strength and pH on the diffusion coefficients and conformation of chitosans molecule in solution

The effects of ionic strength and pH on the diffusion coefficients and gross conformation of chitosan molecules in solution were studied. Chitosan with 83% degree of deacetylation (DD) was prepared from red shrimp (Solemocera prominenitis) processing waste. Ten different molecular weight chitosans were prepared by ultrasonic degradation, and their molecular weights were determined by static light scattering. The weight-average molecular weight (M_w) were between 78 to 914 kilo dalton (KDa). Solution of different ionic strengths (I = 0.01, 0.10, and 0.20) but the same pH (2.18) and different pHs (2.37, 3.10, and 4.14) but the same ionic strength (I = 0.05) were prepared to measure their mutual diffusion coefficient (D_m). The diffusion coefficients for standard condition (D_20,w) were derived from D_m. Intrinsic viscosities ([η]) were determined by a capillary viscometer in different pH solutions. The Mark–Houwink exponents a and ε were obtained from plots of Log [η] and Log D_20,w versus Log M_w, respectively. The results show that diffusion coefficients increased with increasing ionic strength or with increasing pH or with decreasing M_w. Value of ε and a were between 0.503 to 0.571 and ranged from 0.543 to 0.632, respectively. The results indicates that chitosans conformation were in random coil in solutions in the ranges of ionic strength and pH studied. The values of a*, ε* and a**, ε**, Mark–Houwink exponents of smaller and higher than 223 KDa chitosans, respectively, were between 0.752 to 0.988 and 0.585 to 0.777 for smaller M_w chitosans and 0.406 to 0.428 and 0.430 to 0.518 for larger M_w chitosans, respectively. Molecular-weight-induced conformational transition occurred because smaller M_w chitosans was more extended than higher M_w chitosans. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2041–2050, 1999     

Immobilization of lipase from Candida rugosa on synthetic polymer beads for use in the synthesis of fatty esters

Lipase from Candida rugosa was immobilized on three different supports, i.e. Amberlite XAD7, poly(methylmethacrylate) (PMMA) and celite. With the conditions tested, maximum adsorption can be achieved after 30 min. The activities of the immobilized lipases were determined by the esterification reaction of oleic acid and butanol. The immobilized lipases were found to be very effective in the esterification reaction. The immobilized activities generally were high in apolar organic solvents with log P values from 2·0 to 4·0. The preference for fatty acids as acyl donors differed in all cases of immobilized lipases. Lipase immobilized on XAD7 and PMMA exhibited high preference of acyl donors (fatty acids) with chain lengths 12–18 and 8–18, respectively. Lipase immobilized on celite, however, showed high activity in all cases of fatty acids. The nucleophile (alcohol) selectivity studies showed that lipase immobilized on XAD7 and celite was more accessible to alcohols of chain lengths 3–12. However, lipase immobilized on PMMA showed a significant preference towards alcohols of chain lengths from 3 to 10.     

Proportionally fair load balancing with statistical quality of service provisioning for aerial base stations

Aerial base stations (ABSs) seem promising to enhance the coverage and capacity of fifth-generation and upcoming networks. With the flexible mobility of ABSs, they can be positioned in air to maximize the number of users served with a guaranteed quality of service (QoS). However, ABSs may be overloaded or underutilized given inefficient placement, and user association has not been well addressed. Hence, we propose a three-dimensional ABS placement scheme with a delay-QoS-driven user association to balance loading among ABSs. First, a load balancing utility function is designed based on proportional fairness. Then, an optimization problem for joint ABS placement and user association is formulated to maximize the utility function subject to statistical delay QoS requirements and ABS collision avoidance constraints. To solve this problem, we introduce an efficient modified gray wolf optimizer for ABS placement with a greedy user association strategy. Simulation results demonstrate that the proposed scheme outperforms baselines in terms of load balancing and delay QoS provisioning.