Yi-Ming Sun

Curing kinetics of the synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA) with ethylene glycol dimethacrylate (EGDMA) as a crosslinking agent

An experimental study was carried out to investigate the effect of ethylene glycol dimethacrylate (EGDMA, as a crosslinking agent) content on the curing kinetics of the polymerization of 2-hydroxyethyl methacrylate (HEMA), using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). EGDMA may cause a crosslinking-facilitated gel effect which reduces the termination rate of living free radicals and enhances the overall reaction rate, but it may also induce a diffusional resistance for the reactants so that some free monomers are trapped and pendant vinyl groups are prohibited from reaction by the crosslinked structure. At higher content of EGDMA, the later effect becomes predominant, and the reaction rate and the final conversion are limited. The exothermic peak of the curing reaction tends to carry a shoulder and then split into two peaks as the amount of EGDMA is increased, possibly due to a later reaction of the trapped monomers and pendant vinyls. The heat of reaction measured by DSC in the scanning mode is 61.2 kJ/mol CC. The activation energy (E) of the curing reaction ranges from 56.5 to 78.3 kJ/mol CC depending on the EGDMA content and the type of operation. The diffusion-limited reaction rate and the different thermal history experienced in the nonisothermal and isothermal curing can result in variations of the results in the activation energy measurement.

Spray-coated and solution-cast ethylcellulose pseudolatex membranes

Abstract Membranes of dibutyl sebacate (DBS) plasticized ethylcellulose pseudolatex were prepared by spray-coating and solution-casting methods for the studies of their transport and mechanical properties. The structure of sprayed membranes was more porous than that of cast membranes, so the sprayed membranes were more permeable than the cast membranes. The permeabilities of drug or water vapor through sprayed membranes decreased with increasing plasticizer concentration; on the contrary, those through cast membranes increased with increasing plasticizer concentration. The diffusion through pores accounts for the transport through sprayed membranes, while the solution–diffusion through plasticized polymer phase may become an important role for that through the cast membranes. Mechanically, the sprayed membranes were harder and more brittle than the cast membranes of similar formulation, but membranes prepared from both methods possessed similar breaking tensile strength. The tensile strength and Youngs modulus were reduced and the elongation was improved with increasing plasticizer concentration for both kinds of membranes. The sprayed membranes would become less permeable and tougher after the post thermal treatment as the degree of coalescence between the latex particles in the membranes was enhanced.

Fluidized-bed spray coated porous hydrogel beads for sustained release of diclofenac sodium

Abstract Swellable porous hydrogel beads were loaded with diclofenac sodium and then coated with pseudo-latex ethylcellulose (EC) in a fluidized-bed spray coater. The drug release profile in-vitro can be modified by adding 25% or more triethyl citrate (TEC) or dibutyl sebacate (DBS) as a plasticizer. The plasticized coating film can overcome the swelling stress of the core and function as a barrier for sustained drug release. Adding TEC resulted in a near-zero order kinetics after an initial burst in the release profile, and adding DBS showed a first initial burst, a second stage of linear release, a third stage of slow release. The initial burst is due to the flaws or cracks on the surface of the coated-beads and it can be reduced by increasing the amount of plasticizer or film thickness and also by adjusting the thermal treatment time. A 4- to 6-h treatment at 60°C was found to be the optimal condition to give the smallest initial burst for coating film containing TEC as a plasticizer. Adding hydrophilic hydroxylpropyl methylcellulose (HPMC) could smooth the initial burst and change the release profile to a Fickian-type release, and in this case increasing the thermal treatment time could reduce the release rate. Two kinds of EC-coated beads were tested in-vivo by using rabbits as animal models and showed good sustained release behaviors as the drug concentration in plasma could be maintained above 0.4 μg/ml and lower than 1.5 μg/ml for 24 h.

Structure formation and characterization of EVAL membranes with cosolvent of isopropanol and water

Abstract Microporous poly (ethylene-co-vinyl alcohol) (EVAL) membranes can be prepared by the solution casting method in combination of thermally induced phase separation (TIPS) with solvent evaporation. A cosolvent of isopropanol and water (3:2 w/w) is used to prepare the casting solution. EVAL membranes of different bulk or true densities, porosity, flexibility, crystallinity, and structure can be obtained by adjusting the casting temperature. A pseudo-binary temperature–concentration phase diagram of the EVAL–cosolvent system has been proposed, based on the information obtained from SEM, DSC, and light transmittance studies, to account for the membrane formation mechanism. The dense membranes were obtained when the casting temperature was higher than the upper critical solution temperature (UCST) of the system, but they might contain some nonSEM-observable micropores if the casting temperature was below the glass transition temperature (Tg) of EVAL. The porous and the least crystalline membranes with a honeycomb-like morphology were obtained through liquid–liquid demixing and vitrification at a casting temperature between the UCST and the dynamic crystallization temperature (Tc). Highly porous and crystalline membranes were obtained when they were prepared at temperatures near Tc. In this case, the particulate membranes were resulted from solid–liquid demixing mainly via TIPS, and membranes with leafy morphology were created through liquid–liquid demixing and then followed with immediately crystallization.