Biodegradable shape-memory polymer networks:: characterization with solid-state NMR

Polymer networks made from oligo[(L-lactide-ran-glycolide)]dimethacrylates by UV curing are characterized by solid-state NMR. These polymer networks show a shape-memory effect and could be used as temporary implant materials for medical applications. The C-13 spectra enable the direct determination of the cross-link density by a signal at 44 ppm. This is used to correlate its intensity with the chain segment length as well as to study the kinetics of photo-cross-linking. The latter is compared with the gel content, and it is found that the NMR method detects the real amount of covalent cross-links whereas the gel content also depends on influences from constraints such as physical entanglements. The shape-memory effect of the polymer networks can be followed as well by H-1 double quantum buildup curves of samples that are programmed by stretching. Results indicate the reversibility of the shape-memory process.