Solid-state NMR characterization of biodegradable shape-memory polymer networks

Copolymer networks synthesized from dilactide and diglycolide were characterized by solid-state C-13 CPMAS NMR in terms of composition, cross-link density, and rate of cross-linking by UV irradiation. The latter is directly evident by a signal at 44 ppm in the C-13 NMR spectrum. Comparison of solid-state NMR data with the determination of the gel content revealed that this NMR method is sensitive to the chemical cross-link density whereas the gel content is also influenced by physical constraints such as entanglement. Furthermore, these copolymer networks show a shape-memory effect, i.e. a temporary macroscopic shape can be programmed by heating the network above its glass transition temperature together with fixation during cooling. Reheating without fixation recovers the permanent shape. The recovery of the permanent shape could be followed by H-1 DQ NMR buildup curves for a sample that was stretched by 80%.