Temperature and time effects on the structural properties of a non-aqueous ethyl cellulose topical drug delivery system

The structural properties of ethyl cellulose and propylene glycol dicaprylate mixtures were investigated with a view to facilitating use of the system as excepient for topical drug delivery. The working protocol included small-deformation dynamic oscillation in combination with the principle of time–temperature superposition, micro and modulated differential scanning calorimetry, wide-angle X-ray diffraction patterns, infrared spectroscopy, and optical profile analysis in the form of gel particle roughness. In contrast to thermoreversible gelation upon heating of aqueous ethyl cellulose solutions reported widely in the literature, replacing water with propylene glycol dicaprylate and mixing with the polymer yields gels that revert to the solution state with increasing temperature. Time effects were also probed; the continuous increase in viscoelasticity of preparations as a function of time of observation at ambient temperature was accompanied by structural disintegration of the polymeric particles. This was rationalized by proposing that specific polymer–solvent interactions result with aging in particle erosion and the release of polymeric strands that are able to form a three-dimensional structure. It was thus documented that the time–temperature equivalence was active in the system producing a rubbery state in the master curve of viscoelasticity, which extends from ambient to subzero temperatures and should facilitate pharmaceutical applications.