| Controlled release systems have been the subject of increasing interest in many research fields. The study of the diffusion process in those systems is very useful. We have studied diffusion into aqueous solutions and hydrogels by pulsed field gradient NMR spectroscopy and water diffusion into tablets by NMR imaging.; We have determined the self-diffusion coefficient (D) of a series of polymeric diffusants such as poly(propylene imine) dendrimers with triethyleneoxy methyl ether terminal groups (PPI(TEO)x with x = 8, 32 and 64), hyperbranched polyglycidols (HBPG) and linear poly(ethylene glycol)s (PEG). Results obtained show that D decreases with the molecular weight of the diffusant and with the PVA concentration. Furthermore, increasing values of D are obtained with increasing the temperature. However, for the effect of the shape of the diffusant, comparison between diffusants with similar molecular weight but different in the shape leads to a decrease of D from PPI(TEO)x to HBPGs and then to PEGs. Activation energy of the diffusion process obtained is less important for PPI(TEO)x than for the two other diffusants (E PPI(TEO)x < EPGHB < EPEG). From these results, it is clear that the shape of the diffusant is an important parameter of the diffusion process. The effect of the diffusant/polymer interactions has been also investigated. In this case, a series of small molecules with different functional groups have been studied. For the diffusants with similar molecular weight the value of D obtained for each of them is different. That difference in the D value is associated to the diffusant/polymer interactions. A difference in the D value is also obtained when polymer matrices with different functional groups are used.; All these studies were used to test the applicability of a new physical model developed by our research group (the model of Petit et al.). This model has been successfully used to describe some studied effects. However, the model in its present form does not provide an adequate description of the effects of both shape of the diffusants and the diffusant/polymer interactions on the diffusion process.; Classic spin echo sequence which allows a slice selection has been used to obtain pseudo-3D NMR images of the diffusion process of water into high amylose starch tablets as a function of time and temperature. Quantitative data from these images show that the water uptake as a function of time is a diffusion Case II type at 25°C and the diffusion process comes anomalous at 37°C. (Abstract shortened by UMI.)... |
