Release mechanisms of semipolar solutes from poly(dimethylsiloxane) elastomers: Effect of a hydrophilic additive

The release profiles of three xanthine derivatives from matrices of pure poly(dimethylsiloxane) (PDMS), and the effect of incorporating 10% w/w polyethylene glycol (PEG)−3000 in the matrix are presented. Theobromine (TBR), theophylline (TPL), and caffeine (CAF), although structurally very similar, are characterized by different physicochemical properties. In addition, differential scanning calorimetry and scanning electron microscopy measurements indicate different physical interactions with the polymeric matrices. The observed rates of release from pure PDMS matrices loaded at 0.065 g/g with each one of the drugs increase in the order TBR < TPL < CAF. The same order holds for the corresponding permeabilities derived from the release kinetic data. The slopes of the release curves versus the square root of time were linearly correlated to the square root of calculated solubilities in the polymer, as expected by the Higuchi equation for diffusive transport of solutes of the same diffusivity. The incorporation of 10% w/w PEG in the PDMS matrix accelerates the release rate of each drug, because of the concurrent water uptake induced by the hydrophilic additive. The extent of the corresponding permeability enhancement for theophylline was close to that predicted by the Maxwell equation for a composite two-phase system, consisting of a PDMS continuous phase characterized by a much lower permeability than that of the fully swollen PEG dispersed phase. The corresponding enhancement of permeability was higher for TBR and lower for CAF. Possible reasons for these differences are discussed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40782.