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Increasing Molecular Weight (increasing + molecular_weight)
Selected AbstractsLocalization of deposited polycyclic aromatic hydrocarbons in leaves of PlantagoENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2001Martine I. Bakker Abstract After deposition to foliage, polycyclic aromatic hydrocarbons (PAHs) may remain on the leaf surface, accumulate in the cuticular wax, or diffuse into the remaining interior of the plant. In a field study, the location of deposited PAHs in the leaves of two Plantago species was determined. To this aim, leaves of Plantago major and Plantago media were divided into three fractions. First, the leaves were washed (wash-off fraction), then cuticular wax was extracted (wax fraction). Finally, the remaining leaf material was extracted (interior fraction). The presence of PAHs could be demonstrated in all three fractions. For both plants, the distribution of PAHs over the three fractions changed with molecular weight (mol wt) of the PAHs. The wash-off fraction increased with increasing molecular weight, likely because high molecular-weight PAHs occur predominantly bound to particles, which can be readily washed off from the leaves. In contrast, the amount of PAHs detected in the interior of the leaves decreased with increasing molecular weight. This can be explained by a slow desorption of the PAHs from the particles and a low diffusion rate of the larger molecules. This study shows that washing reduces the amount of high molecular-weight PAHs on plant surfaces. Therefore, washing of leafy vegetables is important to minimize human dietary intake of PAHs. [source] Some biophysical properties of castor oil esterified with some acid anhydridesEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 10 2008Mona A. Saied Abstract A series of castor oil adducts were synthesized by esterification of castor oil with acid anhydrides: phthalic, maleic and succinic anhydrides. The chemical structure of castor oil and the prepared adducts were characterized by means of IR and 1H,NMR spectroscopy. The number-average and weight-average molecular weights were measured by gel permeation chromatography. The electrical properties were studied through the permittivity, dielectric loss and conductivity measurements, which are considered to be in the range of electrical insulation. The electrical conductivity, which describes the ionic mobility of the systems, was found to be in the range of 10,9 to 10,12,S/cm. This indicates that castor oil and its esters could be used for antistatic applications. The viscosity,, and the activation energy,E,, obtained from the dependency of viscosity on temperature using the Arrhenius equation, were found to increase with increasing molecular weight of the system. The effect of different concentrations of castor oil and its esters on the growth activities of the sugar beet pathogens R.,solani and S.,rolfsii was studied through the determination of percent germination, average length of hyphal extensions, dry mass yield and the production of sclerotia. The obtained data indicate that the esterification of castor oil with anhydrides improves the antifungal activity. [source] Control of thermo reversible gelation of methylcellulose using polyethylene glycol and sodium chloride for sustained delivery of ophthalmic drugJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2010Mrinal Kanti Bain Abstract The effect of molecular weight of polyethyleneglycol (PEG) and sodium chloride (NaCl) on the gelation temperature of methylcellulose (MC) was studied with the objective to develop a MC based formulation for sustained delivery of ophthalmic drug. The gelation temperature of 1% MC was 60 ± 0.40°C. It was found that the gelation temperature of MC was reduced with the addition of 10% PEG and extent of reduction of gelation temperature was depended on the molecular weight of PEG at same PEG concentration of 10%. The gelation temperature of MC was reduced by 10.4 to 5.9°C with the increasing molecular weight of PEG starting from 400 to 20,000 (Mn) depending on the method of determination of gelation temperature. To reduce the gelation temperature of MC close to physiological temperature (37°C), 6% NaCl was added in the different MC-PEG combinations containing different molecular weight of PEG. It was observed that the drug release time increased from 5 to 8 h with the increase in molecular weight of PEG from 400 to 20,000 (Mn) and this was due to the maximum viscosity and gel strength of MC-PEG20000-NaCl ternary combination. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Structure and gas permeation properties of asymmetric polyimide membranes made by dry,wet phase inversion: Influence of the polyimide molecular weightJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2010Naoko Seki Abstract In this article, we report the influence of the polyimide molecular weight (1.2 × 105, 2.6 × 105, and 4.1 × 105) on the structure and the gas permeation properties of asymmetric polyimide membranes made by the dry,wet phase-inversion process. The apparent skin layer thickness of the asymmetric membrane increased with increasing molecular weight, and the thicknesses of the membranes prepared from the three polyimides with a casting polymer solution containing 8.0 wt % butanol were 132, 350, and 739 nm, respectively. That is, the gas permeance in the asymmetric membranes increased with decreasing molecular weight. In contrast, the gas selectivity of the asymmetric membranes did not depend on the skin layer thickness. The solvent evaporation in the dry phase-inversion process and the nonsolvent diffusion in the dry process were important factors that determined the formation of the asymmetric membrane. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Poly(,-caprolactone)- b -poly(ethylene glycol)- b -poly(,-caprolactone) triblock copolymers: Synthesis and self-assembly in aqueous solutionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2007Yaqiong Zhang Abstract Nontoxic and biodegradable poly(,-caprolactone)- b -poly(ethylene glycol)- b -poly(,-caprolactone) triblock copolymers were synthesized by the solution polymerization of ,-caprolactone in the presence of poly(ethylene glycol). The chemical structure of the resulting triblock copolymer was characterized with 1H NMR and gel permeation chromatography. In aqueous solutions of the triblock copolymers, the micellization and sol,gel-transition behaviors were investigated. The experimental results showed that the unimer-to-micelle transition did occur. In a sol,gel-transition phase diagram obtained by the vial-tilting method, the boundary curve shifted to the left, and the gel regions expanded with the increasing molecular weight of the poly(,-caprolactone) block. In addition, the hydrodynamic diameters of the micelles were almost independent of the investigated temperature (25,55 °C). The atomic force microscopy results showed that spherical micelles formed at the copolymer concentration of 2.5 × 10,4 g/mL, whereas necklace-like and worm-like shapes were adopted when the concentration was 0.25 g/mL, which was high enough to form a gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 605,613, 2007 [source] Synthesis, crystallization, and morphology of star-shaped poly(,-caprolactone)JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2005Jing-Liang Wang Abstract Six-arm star-shaped poly(,-caprolactone) (sPCL) was successfully synthesized via the ring-opening polymerization of ,-caprolactone with a commercial dipentaerythritol as the initiator and stannous octoate (SnOct2) as the catalyst in bulk at 120 °C. The effects of the molar ratios of both the monomer to the initiator and the monomer to the catalyst on the molecular weight of the polymer were investigated in detail. The molecular weight of the polymer linearly increased with the molar ratio of the monomer to the initiator, and the molecular weight distribution was very low (weight-average molecular weight/number-average molecular weight = 1.05,1.24). However, the molar ratio of the monomer to the catalyst had no apparent influence on the molecular weight of the polymer. Differential scanning calorimetry analysis indicated that the maximal melting point, cold crystallization temperature, and degree of crystallinity of the sPCL polymers increased with increasing molecular weight, and crystallinities of different sizes and imperfect crystallization possibly did not exist in the sPCL polymers. Furthermore, polarized optical microscopy analysis indicated that the crystallization rate of the polymers was in the order of linear poly(,-caprolactone) (LPCL) > sPCL5 > sPCL1 (sPCL5 had a higher molecular weight than both sPCL1 and LPCL, which had similar molecular weights). Both LPCL and sPCL5 exhibited a good spherulitic morphology with apparent Maltese cross patterns, whereas sPCL1 showed a poor spherulitic morphology. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5449,5457, 2005 [source] Synthesis and Rheological Properties of Polylactide/Poly(ethylene glycol) Multiblock CopolymersMACROMOLECULAR BIOSCIENCE, Issue 11 2005Feng Li Abstract Summary: Ring-opening polymerization of D,L -lactide was carried out in the presence of poly(ethylene glycol), using Zn powder as catalyst. The hydroxyl-capped PLA-PEG-PLA triblock copolymers were coupled with adipoyl chloride at different molar ratios under mild conditions. N -Dimethylaminopyridine (DMAP) was used as catalyst of the coupling reaction. The resulting PLA/PEG multiblock copolymers were characterized by various analytical techniques such as IR, 1H NMR, SEC, and DSC. Sol-gel transition properties of the multiblock copolymers were investigated by mechanical rheology. The data showed that the sol-gel transition temperature and the transition modulus increased with increasing molecular weight and the solution concentration of the multiblock copolymers. Variation of storage modulus (G,) and loss modulus (G,) as a function of temperature for a 20% sample of MB3. [source] Atom Transfer Radical Polymerization and Third-Order Nonlinear Optical Properties of New Azobenzene-Containing Side-Chain PolymersMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 4 2007Najun Li Abstract The atom transfer radical polymerization (ATRP) technique has been successfully applied to synthesize a series of nonlinear optically (NLO) active homopolymers, 4-(4-nitrophenyl-diazenyl) phenyl acrylate (P - NPAPA) and 4-(4-methoxyphenyl-diazenyl) phenyl acrylate (P - MPAPA), containing azobenzene groups on the side chain. The third-order NLO properties of the polymer films were measured by the degenerated four-wave mixing (DFWM) technique. A dependence of the ,(3) values and response times of polymers on their number-average molecular weight and the electronic effect of the substituent (nitro- or methoxy-) on the azobenzene group have been evidenced. The increasing ,(3) value of the polymer films at the magnitude of about 10,10 was displayed with increasing molecular weight and the presence of the push-pull electronic system contributes much in enhancing the third-order NLO susceptibility of polymers. [source] Self-reinforced polypropylene/LCP prepregs and laminatesPOLYMER COMPOSITES, Issue 5 2002Y. C. Liang Polypropylenes (PPs) of various molecular weights were mixed with a thermotropic liquid crystal polymer (LCP) to prepare unidirectional sheets (prepregs), quasi-isotropic and unidirectional laminates. The mechanical properties and the morphology of the prepregs and the laminates at 0° and 90° with respect to the machine direction were investigated as a function of draw ratio, LCP concentration and molecular weight of the PP. The results for prepregs and laminates showed that both drawing and LCP concentration generally enhanced modulus and tensile strength in machine direction. The morphology of LCP changed from spherical or ellipsoidal droplets to elongated fibrils as the draw ratio increased. The diameter of LCP fibrils decreased with increasing molecular weight of the PP matrix, indicating more effective droplet breakup and better mixing in the case of high molecular weight PP. [source] | |||||||||||||