SWELLING-INDUCED ZERO-ORDER RELEASE FROM RUBBERY POLYDIMETHYLSILOXANE BEADS

The release of cinnamyl alcohol from spherical polydimethylsiloxane beads of about 1 mm diameter exhibits a considerable zero-order region in pentane. The corresponding release in water is, however, completely Fickian. The observed penetrating front movement and transient dimensional changes in pentane are Fickian-like and, therefore, do not provide sufficient clues to the underlying causes of this anomaly. Deborah number analyses in conjunction with the time-dependent diffusion coefficient approach indicates that both the polymer relaxation in response to solvent swelling and the solute diffusion occur on a comparable time scale, which would suggest a non-Fickian diffusion mechanism. However, the spherical geometry would have rendered the fractional release less linear even under conditions which would normally lead to zero-order release in sheet samples. One possible explanation attributes the apparent zero-order release region to the unusually large swelling of the polymer in pentane ( approximately 90% by volume), which causes time-dependent increases in both the surface area as well as diffusion coefficient.