In vitro biostability evaluation of polyurethane composites in acidic, basic, oxidative, and neutral solutions.

New and improved properties can often be achieved by compounding two or more different but compatible materials. But, can failure possibility also be increased by such a compounding strategy? In this article, we compared the in vitro biostability of composites with that of the pure polymer. We tested three model composites in oxidative, acidic, basic, and neutral solutions. We found that oxidation degradation was much more profound in the composites than in the corresponding pure polymer. This degradation seemed to be an intrinsic property of composite materials. We also observed the well documented interfacial debonding between filler and matrix and its effects on the mechanical reinforcement of the hydrated composites. The improvements in acid and base resistance were also observed.

Kinetics and time-temperature equivalence of polymer degradation.

We studied the hydrolysis kinetics of amorphous polylactide. It was found the hydrolysis rate had a slow-to-fast transition at a certain molecular weight (Mn). This transition was not correlated with the mass loss and water uptake of samples, nor the pH values of testing media. We speculated that this transition was due to the slow diffusion of polymer chain ends. The chain ends did not significantly promote the hydrolysis of samples until their concentrations (approximately 1/Mn) reached a critical value. The degradation tests were also conducted over a temperature range from 37 to 90 degrees C. A time-temperature equivalent relationship of degradation processes was established and a master curve spanning a time range equivalent to 3-5 years at 37 degrees C was constructed. This master curve can be used to predict polymer degradation processes based on accelerated tests. The functional time and disappearance time of degradable polymers were also discussed.

Adjusting drug diffusivity using miscible polymer blends.

Tuning the release rates of drugs was accomplished with miscible polymer blends. Dexamethasone (DX) was formulated in a miscible polymer blend composed of two poly(ether urethane)s, one in which DX diffuses relatively quickly and the other in which DX diffuses relatively slowly. Matrices that provide drug diffusion coefficients from 7 x 10(-13) to 3 x 10(-19) cm2/s were obtained by adjusting the blend ratio. Tunable diffusion coefficients were also achieved with partially miscible blends, such as poly(vinyl acetate) (PVAC)/cellulose acetate butyrate (CAB). However, similar tunable diffusion was not observed with an immiscible polymer blend composed of poly(carbonate urethane) (PCU) and polysulfone (PSF).