Showing papers by "Si-Xue Cheng published in 2003"

Temperature‐Sensitive Poly(N‐isopropylacrylamide) Hydrogels with Macroporous Structure and Fast Response Rate

Abstract: Macroporous temperature-sensitive poly(N-isopropylacrylamide) (PNIPA) hydrogels were prepared by a novel phase-separation technique to improve the response properties. In comparison with a conventional PNIPA hydrogel prepared in water, these macroporous hydrogels, prepared by polymerization in aqueous sucrose solutions, have higher swelling ratios at temperatures below the lower critical solution temperature and exhibit much faster response rates to temperature changes. Scanning electron microscopy image of the surface of a PNIPA hydrogel, prepared in 1.50 M aqueous sucrose solution.

Macroporous poly(N-isopropylacrylamide) hydrogels with fast response rates and improved protein release properties.

Abstract: A series of temperature-sensitive poly(N-isopropylacrylamide) (PNIPA) hydrogels with large pore size and fast response were prepared by carrying out polymerizations in aqueous sodium chloride solutions with different concentrations. In comparison with conventional PNIPA hydrogels, the PNIPA gels thus prepared have remarkably larger swelling ratios below their lower critical solution temperature (LCST), and exhibit much faster response rates as the temperature is raised above their LCST. The improved properties are due to the presence of inorganic salt, NaCl, which leads the phase separation and formation of a heterogeneous porous structure during the polymerizations. The study on the drug release properties shows the macroporous hydrogels exhibit modulated release in response to temperature. The model protein, bovine serum albumin (BSA), can be released completely from the porous hydrogels at the temperature lower than the LCST because the pore size of the hydrogels is larger than the protein molecules. However, the release of BSA from the gels almost stops after a "burst" release in the initial stage at the temperature higher than the LCST because the pores are closed at the high temperature.

Poly(vinyl alcohol)/poly(N-isopropylacrylamide) semi-interpenetrating polymer network hydrogels with rapid response to temperature changes

Abstract: A series of thermosensitive and fast-response poly(vinyl alcohol) (PVA)/poly(N-isopropylacrylamide) (PNIPA) hydrogels were prepared by incorporating PVA into cross-linked PNIPA to form a semi-interpenetrating polymeric network (semi-IPN). Compared to the conventional PNIPA hydrogel, the semi-IPN hydrogels thus prepared exhibit significantly faster response rates and undergo full deswelling in 1 min (lose about 95% water within 1 min) when the temperature is raised above their lower critical solution temperature, and have larger equilibrium swelling ratios at room temperature. These improved properties are attributed to the incorporation of PVA, which forms water-releasing channels and results in increased hydrophilicity, into the PNIPA hydrogel networks.

Gas‐sorption properties of 6FDA–durene/1,4‐phenylenediamine (pPDA) and 6FDA–durene/1,3‐phenylenediamine (mPDA) copolyimides

Abstract: We investigated the sorption isotherms of O2, N2, CH4, and CO2 gases in 6FDA–durene, 6FDA–1,4-phenylenediamine (6FDA–pPDA), and 6FDA–1,3-phenylenediamine (6FDA–mPDA) homopolymers and 6FDA–durene/pPDA and 6FDA–durene/mPDA copolyimides. The solubilities decrease in the order of the inherent condensabilities of the penetrant gases, namely, CO2, CH4, O2, and N2. The chemical structures of the polymer, as well as the chain packing, determine the sorption properties of these homopolymers and copolymers. The FDA–durene homopolymer has the highest solubility for all gases because of its high specific free volume and fractional free volume. The solubilities of the copolymers increase with an increasing 6FDA–durene content, while the solubility selectivities of the copolymers only vary slightly. The values of KD (Henry's law constant) and CH′ (Langmuir site capacity) of these copolyimides decrease with a decreasing 6FDA–durene content. To our surprise, contradictory to the previous known fact that the meta-connected materials tend to have denser molecular packing than that of the para-linked materials for homopolymers, the 6FDA–durene/mPDA 80/20 copolymer has higher gas solubilities than those of the 6FDA–durene/pPDA 80/20 copolymer. The random moiety sequence within the copolymer may be the main cause for the abnormal phenomenon. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2187–2193, 2003

Synthesis and Characterization of Novel Biodegradable Copolymers of 5‐Benzyloxy‐1,3‐dioxan‐2‐one and Glycolide

Abstract: A series of novel biodegradable random copolymers of 5-benzyloxy-1,3-dioxan-2-one (5-benzyloxy-trimethylene carbonate, BTMC) and glycolide were synthesized by ring-opening polymerization. The copolymers were characterized by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The incorporation of BTMC units into the copolymer chains results in good solubility of the polymers in common solvents. The in vitro degradation rate can be tailored by adjusting the composition of the copolymers. The in vitro degradation of the homopolymers and poly(BTMC-co-GA) copolymers.

Liquid Crystalline Polymers, Main-Chain

Abstract: This review covers the most recent study on the nature of thermotropic liquid crystalline polymers (LCPs) and their progress. We summarize and review the newly developed thin-film polymerization technique for the investigation of liquid crystalline formation as a function of monomer moieties, and for the study of reaction kinetics, the evolution of liquid crystal texture, and surface energy during polycondensation reactions. We also highlight various modification approaches to yield main-chain LCPs suitable for conventional processing equipment, without compromising the unique liquid crystalline characteristics and the superior mechanical properties. The thermal stability, degradation behavior, and crystallization kinetics of commercial available main-chain LCPs are recapitulated. Attention is also given to the rheology, polymer blends, micro- and nanostructures, and processing of LCPs and their applications. Keywords: thermotropic polymers; liquid crystalline polymers; thin-film polymerization