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Some insight into hydrolytic scission mechanisms in bioerodible polyesters Abstract PLLA, PDLLA, and PLGA copolymers have been studied to understand the details of their degradation behavior. All polymers exhibited a homogeneous mode of degradation, with uniform rates of degradation throughout the film. Crystallinity inhibited water absorption and hence retarded degradation. The degradation rate was increased by the presence of glycolic acid units in the PLGA copolymer; this effect overwhelms any decrease in degradation rate because of increased crystallinity due to the additional GA units. This effect is demonstrated quantitatively in this study. In PLGA polymers, there is evidence of unusual recrystallization behavior as degradation proceeds, due mainly to the higher rate of hydrolytic scission of the glycolide linkage compared with the lactide, as verified quantitatively with the use of 1 H NMR studies. Application of a Monte Carlo model to the degradation results, however, appears to show a random scission process. The details of the mechanistic study of different factors influencing the process of degradation, as reported here, may have important implications in terms of selecting the right material for specific biomedical applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3111–3117, 2006
Wiley - Tập 102 Số 4 - Trang 3111-3117 - 2006
Immobilization of α‐amylase onto chitosan and its amino acid condensation adducts Abstract α‐Amylase from Bacillus subtilis was immobilized on insoluble chitosan and its amino acid (L ‐glutamic acid and 4‐aminobutyric acid) condensation adducts with the direct covalent attachment method and with glutaric dialdehyde (GDA) as a crosslinking agent. The immobilization process was carried out at 25°C and pH 6.9, and the maximum retained activity was obtained with 3 mg of α‐amylase. The properties of the immobilized α‐amylase were investigated and compared with those of the free α‐amylase. For the assays carried out via the crosslinking method at 25°C and pH 6.9, the retained activities were found to be 68.59, 97.36, and 79.50% for chitosan, chitosan–L ‐glutamic acid, and chitosan–4‐aminobutyric acid crosslinked with 1% GDA, respectively. The immobilized α‐amylase had better stability and higher retained activities with respect to the pH, temperature, and storage stability than the free α‐amylase. In the repeated‐use experiments, the α‐amylase immobilized with chitosan–GDA (1%) retained about 46.45% of its original activity after 25 uses. In contrast, the activities of α‐amylase immobilized on chitosan–L ‐glutamic acid–GDA (1%) and chitosan–4‐aminobutyric acid–GDA (1%) did not change after 11 and 8 uses, respectively. The retained activities after 25 uses were 79 and 71% with respect to the original activity for the aforementioned carriers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009
Wiley - Tập 112 Số 2 - Trang 805-814 - 2009
Quantitative explanation of the mechanism of corrosion of poly(tetrafluoroethylene) caused by active alkali metals Abstract The known method of surface treatment of poly(tetrafluoroethylene) (Teflon) by alkali metals was studied quantitatively. It was shown that the reaction proceeds by an electrochemical mechanism in solid phase. The alkali metal acts as anode and Teflon as a cathodic active material of a typical galvanic corrosion cell. A relation describing the rate of the penetration of this reaction into the depth of Teflon was derived.
Wiley - Tập 19 Số 12 - Trang 3201-3210 - 1975
Attachment of amino groups to polymer surfaces by radiofrequency plasmas Abstract Low temperature gaseous plasmas of ammonia or nitrogen–hydrogen mixtures contain NH2 groups, or precursors thereof, formed in the plasma, which experimental evidence strongly suggests, can add to various polymer surfaces. The plasmas were established in the 0.3–1.5 torr range by radiofrequency (13.56 MHz) electrodeless excitation at powers ranging from 50 to 500 W. Samples of polypropylene, poly(vinyl chloride), polytetrafluorethylene, polycarbonate, polyurethane, and poly(methl methacrylate) were investigated. All these polymers added amino groups to varying degrees of amino site densities depending on the choice of plasma parameters and the reactivity of the polymer itself. In every instance the polymer was rendered more wettable, although no quantitative wettability measurements were made. Following the plasma treatment, degrees of amino attachment to the polymer were followed radiometrically and reported in terms of “heparin thicknesses” resulting from ionic heparin 35 S attachment to quaternary sites produced from the amino groups. Two implications of such a surface modification are to adhesion and blood compatible materials preparation.
Wiley - Tập 13 Số 4 - Trang 807-816 - 1969
The effect of surface treatments on wear of polytetrafluoroethylene Abstract This paper reports the effect of surface chlorination, oxidation, and phosphatization of carbon steel, copper, and aluminum, and surface treatment with sodium naphthalenide of PTFE on wear of PTFE. It was found that the wear behavior of untreated PTFE is not affected by surface treatments of metals but treatment of PTFE can reduce its wear by a factor of at least 100. The wear mechanism of PTFE is discussed.
Wiley - Tập 40 Số 7-8 - Trang 1141-1148 - 1990
Kinetics of the solid‐state polymerization of nylon‐6 Abstract The kinetics of the thermally induced solid‐state polymerization (SSP) of nylon‐6 were examined in both a fixed‐bed reactor and a rotary reactor. Factors such as the regulator content, the reaction temperature and time, the particle size, the type and geometry of the nylon‐6 prepolymer, the nitrogen gas flow rate, the water content of the nitrogen gas flow, and the polymerization process were studied. The results showed that the regulator content, the reaction temperature and time, and the particle size were the primary factors, and that the others were negligible. Moreover, the SSP rate and number‐average molecular weight (M n M n
Wiley - Tập 84 Số 3 - Trang 616-621 - 2002
Effect of dissolved carbon dioxide on the glass transition and crystallization of poly(lactic acid) as probed by ultrasonic measurements Abstract The effects of dissolved carbon dioxide (CO2 ) molecules on the glass‐transition temperature as well as the crystallization kinetics of poly(lactic acid) have been investigated with a novel device that combines ultrasonic and volumetric measurements. Ultrasonic parameters such as the sound velocity are very sensitive to crystallization and can be used to monitor the crystallization kinetics. The glass‐transition temperature has been found to decrease nonlinearly as the CO2 concentration increases. The maximum in the crystallization rate has been found to increase with the addition of CO2 . © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009
Wiley - Tập 112 Số 3 - Trang 1345-1355 - 2009
Chain extenders for polyesters. IV. Properties of the polyesters chain-extended by 2,2′-bis(2-oxazoline)
Wiley - Tập 33 Số 8 - Trang 3069-3079 - 1987
Multifunctional coupling agents. II. Chain extension and terminal group modification of polyamides Abstract New bifunctional coupling agents possessing one 2‐oxazoline group and one 2‐oxazinone group were converted in a Haake melt mixer or extruder with PA6 and PA12. It was shown by means of NMR spectroscopic investigations that the 2‐oxazoline group reacted mainly with carboxylic groups whereas the oxazinone group reacted preferably with the amino groups. Both reactions proceeded with high selectivity and independent from each other. In the case of carboxy/amino group terminated polyamides, the conversions resulted in increased molecular weights since both reactive terminal groups of the polyamides were addressed simultaneously by the coupling agent. In the case of amino group terminated polyamides, the conversion with the bifunctional coupling agent resulted in oxazoline terminated polymer chains. Unlike PA6, it was possible to convert PA12 in two steps. At lower temperatures (210°C) and short reaction times (2 min), the reaction of the oxazinone group with the amino groups was predominant, whereas the reaction of the oxazoline group with the carboxylic groups proceeded to a sufficient extent only after longer reaction times or at higher temperatures. In the case of PA6, processing temperatures of about 250°C were necessary. Here, a side reaction was observed that resulted in the formation of cyclic quinoxaline structures under evolution of water. This side reaction did not disturb the chain extension significantly. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2170–2177, 2004
Wiley - Tập 94 Số 5 - Trang 2170-2177 - 2004
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