Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Materials Science and Engineering

Committee Chair/Advisor

Dr. O. Thompson Mefford

Committee Member

Dr. Feng Chen

Committee Member

Dr. Igor Luzinov

Committee Member

Dr. Marek Urban


Release of small molecules from polymeric materials has wide applications in the delivery systems of active molecules such as drug, fragrance, and semiochemical. Rubber materials are good candidates for the excipients of those systems due to properties such as good flexibility, permeability, and biocompatibility. Factors like material properties, mechanical loading, and molecular interactions may affect the release of small molecules in those systems. Therefore, understanding how those factors affect the release is key to the formulation, design, and evaluation of those systems.

To study the effects of materials properties, vulcanized natural rubber sheets with different crosslink densities, loaded with small molecules with different boiling points, molecular weights, and chemical moieties were prepared. Release experiments were performed to determine the mass transfer and diffusion coefficients of those molecules and corresponding numerical models were built. Good agreements between experiments and numerical simulation were observed. It was found that the mass transfer coefficients of those small molecules decreased with increasing boiling points but remained practically constant among rubber sheets with different crosslink densities. The diffusion coefficients did not show evident correlation with molecular weights but their relationships with crosslink densities indicated that there might be an optimal crosslink density where a small molecule reached its maximum diffusion coefficient.

To study the effects of uniaxial tensile strain, silicone rubber sheets loaded with triacetin were stretched and held at different lengths up to 125% engineering strain. The mass transfer and diffusion coefficients of triacetin were determined experimentally. It was found that there was no significant change of diffusion coefficient as the applied strain increased, which might result from two microstructure changes that had conflicting effects on diffusion: chain orientation and free volume deformation.

To study the effects of molecular interactions, the mass transfer and diffusion coefficients of three model molecules, octanol, octyl acetate, and octyl butyrate were determined from silicone rubber sheets loaded with only one or two of the three molecules. Differential scanning calorimetry and Fourier transform infrared spectroscopy were used to characterize molecular interactions. It was found that the release of octanol conformed to the Fickian diffusion pattern at low initial concentration but deviated as the concentration increased due to hydrogen bonding between octanol and the silanol group of the silica filler. When two small molecules were released simultaneously, different effects of one model molecule on the diffusion coefficient of another were observed and explained by the competing effects of plasticizing and hydrogen bonding/dipole-dipole interaction.

In summary, the outcome of this work promoted the understanding on important influencing factors of the release of small molecules from rubber and other polymeric materials and can serve as support and reference for the formulation, design, and evaluation of such delivery systems.



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