ABSTRACT
A new method of preparing composite poly(vinyl alcohol) (PVA) beads with a double-layer structure was developed (58) and involved a stepwise saponification of suspension polymerized poly(vinyl acetate) (PVAc) beads and subsequent stepwise cross-linking of the PVA core and shell with glutaraldehyde. This process results in PVA beads with thin, highly cross-linked outer shells and lightly cross-linked inner cores ofdifferent degrees ofcross-linking. In addition to structural characterization of the polymer based on equilibrium swelling measurements, the kinetics of water swelling and drug release from these beads were studied at 37°C using acetaminophen and proxyphylline as model drugs. The results showed that the outer shell functions as a rate-controlling membrane on increasing its cross-linking ratio X above 0.47. This aspect is reflected in the observed diffusional time lags and constant-rate regions during swelling C:lnd drug release. From the observed time lags, the diffusion coefficient of water through the outer PVA shell with a high cross-linking ratio of X == 0.5 was estimated to be at least six times higher than that of acetaminophen and proxyphylline. In addition, drug diffusion coefficients in the lightly cross-linked PYA core appeared to be at least ten times larger than that in the highly cross-linked outer shell. At lower shell cross-linking ratios (X<0.4), the diffusional time lags appeared to be absent and the diffusion profiles are apparently first-order (Fickian) in nature (58).
