Investigation of the Aqueous Dissolution of Semicrystalline Poly(Ethylene Oxide) Using Infrared Chemical Imaging: The Effects of Molecular Weight and Crystallinity

Fourier transform infrared (FT-IR) imaging was used to successfully explore several factors influencing the dissolution of poly(ethylene oxide). The effect of the degree of crystallinity on the rate of dissolution of mid-range molecular weight PEO was negligible over the temperature ranges studied. The influence of molecular weight on polymer dissolution was found to be much greater than the changes in morphology. An examination of the polymer and solvent images and absorbance profiles, compared with the results of the bulk polymer/solvent boundary movement, confirmed this relationship. An investigation of the bulk polymer/solvent boundary using a crystalline-sensitive polymer band showed the crystalline to amorphous phase change occurred over a short distance. Moreover, solvent diffusion ahead of the bulk polymer/solvent front was minimal, most likely a result of the required phase change, which in turn regulated the degree of solvent ingress. Modeling of the dissolution was performed using the Peppas (power law) model. Physical parameters of the dissolution process were obtained from fitting the release profiles to the power law (fraction released = k × tn, where k is the dissolution rate constant and n is the release exponent). Results indicated the model worked well to describe dissolution at all molecular weights. By varying the number of data points input to the model and then comparing the generated graphs, it becomes clear that not only does the dissolution slow down over the course of the experiment, but an increase in molecular weight enhances this effect. The effect of different types of drug on the rate of polymer dissolution was also studied. The dissolution of neat polymer was compared to the dissolution of polymer containing 10% (by weight) of theophylline, etophylline, or testosterone. The general trend of all the dissolution curves was the same, with the addition of etophylline and testosterone tracing almost the same route in terms of movement of the bulk polymer/solvent front.