Effect of polymer molecular weight on the structural properties of non aqueous ethyl cellulose gels intended for topical drug delivery

The structural properties of five ethyl cellulose polymers (EC) in mixture with the non aqueous solvent of propylene glycol dicaprylate (PGD) have been investigated with a view to facilitating topical drug delivery. Total concentration of the polymer varied from 12 to 20% (w/w) and its weight average molecular weight was in the range of 64-223 kDa. Experimental techniques utilised were small-deformation dynamic oscillation in shear, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) and wide-angle X-ray diffraction. The nature of the time and temperature dependence on viscoelastic functions of all samples could be treated with the Chambon and Winter criterion allowing determination of the critical gelation temperature in relation to polymer concentration and molecular weight. A further refinement of this school of thought allowed correlation between the viscoelastic relaxation exponent and fractal dimensions in thermally treated EC/PGD matrices. Tangible evidence on the packing arrangements of the ethyl cellulose network and the molecular nature of its interaction with the non aqueous solvent as a function of polymer concentration and molecular weight was provided by X-ray diffraction and infrared studies.