Highly crosslinked ionizable acrylates: Polymerization kinetics and network structure

Novel ionizable polymer networks were prepared from oligo(ethylene glycol) (OEG) multiacrylates and acrylic acid (AA), employing bulk radical photopolymerization techniques. The properties of these materials exhibit a complex dependence on the network structure and composition. Dynamic mechanical analysis and penetrant sorption experiments demonstrated that the crosslinked structure of the materials depends very strongly on the AA content, although the network chain population is expected to be composed solely of ethylene glycol oligomers. The results indicate that inter-chain interactions are diminished as the AA content is increased, due to the increased spatial separation of OEG chains. The compositional dependence of the glass transition temperature is qualitatively described by a treatment consistent with that employed for polymer blends, and deviations from ideal blend behavior point to the importance of system-specific free volume changes during the radical polymerization process. Hence, the glass transition temperature and other network properties are closely coupled to the polymerization kinetics. The impact of varying the AA content and the OEG chain length on the polymer chain dynamics was examined using diffusion and $\sp{13}$C NMR relaxation studies. The shapes of the penetrant uptake curves suggest a coupling of Fickian and relaxation-driven contributions to the overall swelling behavior. The effect of increasing the AA content on the characteristic chain relaxation time is reversed as the OEG chain length is varied, indicating that chain relaxation is controlled by structural considerations for shorter OEG chains and by compositional considerations for longer OEG chains. Measured compositional effects on solid state $\sp{13}$C NMR relaxation times support our conclusions. The polymerization kinetics of multiacrylate copolymer systems exhibited an increasing dependence of the polymerization rate on the OEG chain length. AA-containing systems exhibit a slight rate enhancement at high conversions, relative to multiacrylate homopolymer systems. These effects were captured in a free volume-based model of the diffusion-controlled polymerization kinetics. Multiacrylate copolymers with AA were used in the design of controlled release devices with precisely controlled properties. These devices exhibit pH- and composition- sensitive release rates and allow for sustained solute delivery over long time intervals