Macromolecular dynamics during polymer dissolution: Molecular modeling and experimental characterization
Abstract
The dissolution mechanism of rubbery polymers was analyzed by dividing the penetrant concentration field into three regimes which delineate three distinctly different transport processes. The mode of mobility of the polymer chains was shown to undergo a change at a critical penetrant concentration expressed as a change in the diffusion coefficient of the polymer. Beyond the critical penetrant concentration, reptation was the dominant mode of diffusion. Molecular arguments were invoked to derive expressions for the radius of gyration, the plateau modulus, and the reptation time, thus leading to an expression for the reptation diffusivity. Transport in the second penetrant concentration regime occurred in a diffusion boundary layer adjacent to the polymer-solvent interface, where a Smoluchowski type diffusion equation was obtained. To understand the dissolution of glassy polymers, chain reptation was incorporated into penetrant transport models. In this case, solvent penetration through the polymer was modeled as a consequence of a diffusional flux and an osmotic pressure contribution. Experiments were performed to measure the positions of the moving fronts in a poly(ethylene glycol) - water system with the aid of a photographic technique. In addition, magnetic resonance imaging (MRI) studies of non-crystalline poly(vinyl alcohol) in water were carried out. These studies suggested the existence of a change in the mode of mobility of the polymer chains as the dissolution proceeded. Satisfactory agreement was obtained between the model predictions and the experimental data. Measurements of self-diffusion coefficients using MRI verified scaling laws that were proposed in the model. In addition, models were developed to understand the mechanism of drug release from dissolving polymers. Appropriate mathematical conditions were established for zero-order release of the drug. The model predictions were compared with available experimental data on drug release from such systems and good agreement was obtained.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering
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