Microphase separation in bioerodible copolymers for drug delivery

This research examines the microstructure of bioerodible polyanhydrides with an eye towards precise design of drug delivery devices. Our main hypothesis is that the bioerodible copolymer poly(1,6-bis-p-carboxyphenoxyhexane-co-sebacic anhydride) (CPH : SA) undergoes micro-phase separation at certain copolymer compositions due to differences in relative hydrophobicity of the co-monomers, resulting in thermodynamic partitioning of drugs incorporated into these copolymers. We investigate the thermal properties, degree of crystallinity, and surface microstructure of several compositions of CPH : SA using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and atomic force microscopy (AFM). We observe that the degree of crystallinity decreases, while the crystal lamellar thickness increases with CPH content. Phase-imaging using AFM indicates the presence of micro-domains in 20 : 80 and 80 : 20 CPH : SA, while poly(SA) and 50 : 50 CPH : SA show no micro-phase separation. Finally, drug–polymer interactions are studied by loading the polymers with different amounts of brilliant blue (hydrophilic) and p-nitroaniline (hydrophobic). DSC and WAXD analysis shows that loading hydrophobic drugs into relatively hydrophobic polymers (poly(SA)) lowers melting point that becomes more pronounced with increased drug loading.