Biomedical subjects

Dongshun Bai

Publications and source records attributed to Dongshun Bai.

3 recordsLinked to original sources

PubMed2005

pH-responsive copolymer films by surface-catalyzed growth.

We have engineered a new class of pH-responsive polymer films on gold surfaces by first developing a controlled, surface-catalyzed polymerization to prepare a copolymer film consistent with poly(methylene-co-ethyl acetate) and subsequently hydrolyzing the ester side chains to varying extents to yield carboxylic acids (denoted as PM-CO2H). When pH is increased, the acid groups become deprotonated or charged, dramatically increasing their water solubility and greatly altering the film properties. The carboxylic acid content within the copolymer film can be adjusted by changing the monomer concentration ratio used in the polymerization process or the length of time for the hydrolysis. We have designed PM-CO2H films to consist predominately (>95%) of polymethylene (PM) so that the film is hydrophobic in the uncharged state and, thereby, exhibits an extremely large pH-induced response in barrier properties once ionized. The effect of polymer composition on pH response was investigated by electrochemical impedance spectroscopy (EIS), reflectance-absorption infrared spectroscopy (RAIRS), and contact angle measurements. At a 1%-4% molar acid content, the copolymer film exhibits a 5 orders of magnitude change in its resistance to ion transport over 2-3 pH units. The pH at which this response begins can be tailored from pH 5 to pH 10 by decreasing the acid content in the film from 4% to 1%.

Journal Article↗

PubMed2005

Interaction between a nonionic surfactant and a hydrophobically modified 2-hydroxyethyl cellulose.

Interaction between the nonionic surfactant Tergitol 15-S-7 and hydrophobically modified 2-hydroxyethyl cellulose (HMHEC) was studied rheologically in a semidilute regime of HMHEC. The low-shear viscosity of HMHEC was increased with addition of surfactant from 25 to 250 ppm, in which the critical micelle concentration of surfactant was near 39 ppm, and then decreased to a value smaller than that of pure HMHEC with further addition of surfactant to 1000 ppm. An interesting shear-induced phenomenon was observed. The steady-state shear measurements show that there exist crossovers between viscosity-shear rate curves of HMHEC solutions with and without surfactant added, whereas it was not observed in the HEC-surfactant systems. Moreover, added Tergitol 15-S-7 reversed the temperature effect on the viscosity of the HMHEC solution. That is, increasing temperature to or near the cloud point raises the viscosity of the HMHEC-surfactant aggregates, in contrast to the viscosity decrease in the pure HMHEC solutions. A possible mechanism based on the necklace model and the clouding phenomenon is conjecturally introduced to explain such phenomena.

Cellulose↗

PubMed2005

Surface-catalyzed growth of polymethylene-rich copolymer films on gold.

We report the preparation of a new class of copolymer films that consist of polymethylene (PM) with a low percentage ( approximately 1-4%) of randomly distributed ethyl ester side groups, consistent with poly(methylene-co-ethyl acetate). The films are prepared through a surface-catalyzed polymerization on gold surfaces upon exposure to a dilute solution of diazomethane (DM) and ethyl diazoacetate (EDA) in ether at 0 degrees C. While EDA alone does not polymerize at gold surfaces but DM does decompose on gold to grow PM films, the combined presence of EDA and DM results in dramatic enhancements in film growth and promotes an alternative mechanism for propagation as compared with that for the PM homopolymerization. The rate of copolymer film growth is constant over a approximately 24 h period, consistent with a controlled polymerization in which chain terminations are minimized. Carefully controlled experiments indicate that chain propagation does not occur at the outer film-solvent interface, but more likely, at the film-metal interface, suggesting a catalyzed insertion mechanism that extends the chain and pushes the outer chain termini further away from the metal-polymer interface. The results also suggest that adsorbed intermediates of EDA function as co-catalysts to promote the propagation reaction. Of particular importance for materials modification is that the ester side chains of these copolymer films can be hydrolyzed to carboxylate groups that exhibit pH-dependent wettability.

Journal Article↗