Temperature-Induced Changes in the Nanostructure of Hydrogels Based on Reversibly Cross-Linked Hyperbranched Polyglycidol with B(OH)4⊖ Ions

Solid-state boron nuclear magnetic resonance NMR) and positron annihilation lifetime spectroscopies (PALS) were used to study the molecular structure of self healing hydrogels based on cross-linked hyperbranched polyglycidol (HbPGL) with borax at basic pH. The lifetime and intensity of orthopositronium allowed characterizing the micro- and nanostucture of hydrogels at various thermal conditions. Stepwise changes in the free volume parameters were found in pure HbPGL as well as in hydrogels based on this polymer. However, the shift in the phase transition temperature suggests that the important properties of the hydrogel arise from the water building these systems. Rheological measurements demonstrated the subsequent reduction of the average cross-link lifetime within the polymer network under heating. Composition of boronic species within hydrogel systems also diverged upon change in temperature range from -10 degrees C to +70 degrees C. The reduced fraction of boronic diester upon heating was quantitatively rebuilt after cooling to ambient temperature. Heating the hydrogel at 70 degrees C launched the irreversible release of a small fraction of HbPGL macromolecules from the polymer network, generating its defects, still present after cooling. The structural studies carried out in a nanoscale facilitated the distinction in cross-linking density of two analyzed hydrogel systems. The PAL spectroscopy turned out to be a valuable tool to exclude entanglements between individual macromolecules of pristine HbPGL.