Water diffusion and free volume in hydrophilic polymers

Abstract: During moisture uptake in polymers, the diffusion of the water molecules is believed to occur via unoccupied voids. These voids are called free volume cavities and their sizes are continuously altered when the moisture content in the polymer is enhanced. Not only the free volume cavity sizes, but also other polymer properties such as the degree of crystallinity, are changed as a consequence of moisture uptake. By modifying the polymer properties, the conditions for water diffusion are altered, which is the issue at stake in this thesis. A study of the coupling between the self-diffusion coefficients of water and the free volume hole sizes in polymer-water systems based on poly(ethylene oxide) (PEO) and hydroxypropyl methylcellulose (HPMC) has been performed by means of nuclear magnetic resonance (NMR) and positron annihilation lifetime spectroscopy (PALS). Moreover, the change in the tensile storage modulus upon adding water and a model drug to HPMC as well as the change in the degree of crystallinity and the glass transition temperature upon adding water and a nano-sized clay filler to PEO was studied by means of dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). In the HPMC-water system, an increase in the free volume hole size seemed to be associated with an increase in the self-diffusion coefficient. In contrast, the water self-diffusion in PEO seemed not to be controlled by the free volume hole size, at least not in the water weight fraction range where more than 50 % of the sample still was crystalline. The reduction of the crystalline phase in the PEO as a function of water weight fraction seemed to result in a relaxation of a strained amorphous phase. Strong indications of water clustering in the measured water weight fraction region were observed in the HPMC, but not in the PEO. Addition of triacetin to the HPMC resulted in a reduction of the amount of water absorbed by the HPMC. On a molar basis, the triacetin decreased the tensile storage modulus to a higher extent than water did. At constant water weight fractions, the self-diffusion coefficient of water and the free volume hole size increased upon addition of triacetin. The self-diffusion coefficient of water in the HPMC-water system was more influenced by an increasing amount of water than by an increasing amount of triacetin, whereas the opposite effect was observed for the free volume hole size. The presence of a nano-sized clay filler in PEO resulted in less water absorbed by the PEO and seemed to slightly increase the influence of water on the glass transition temperature and the free volume hole size.