Capturing the sol and gel states of thermoresponsive polyoxazoline / -oxazine hydrogels by ambient and subambient solid-state NMR

Recently it was shown that ABA-type triblock copolymers with pPheOzi as central B-block undergo a cooling induced order-order transition from spherical to worm-like micelles accompanied by inverse thermogelation. Previous attempts to modulate the chemical structure of the pPheOzi block prevented worm-formation or even thermogelation. Here, two novel polymer variants were synthesized bearing -CH3 or -OCH3 at the para-position of the phenyl group of pMeOx-b-pPheOzi-b-pMeOx. Rheological, µDSC and AFM analyses proved thermogelation and formation of worm-like micelles of pMeOx-b-pMeOPheOzi-b-pMeOx, while pMeOx-b-pMePheOzi-b-pMeOx remains a sol of spherical micelles. In order to understand the macroscopic phenomena at the molecular level, a detailed NMR study has been carried out. NMR spectroscopy in solution was used to visualize the subset of mobile polymer moieties in the corona, whereas rigid moieties were analysed in-detail by solid-state NMR. Intermediate motions can interfere with classical solid-state NMR analyses, but freezing the samples successfully improved the visibility of moieties in this dynamic regime. Combining solid-state NMR of frozen samples with DSC revealed three types of water —non-freezable bound, freezable-bound, and free water— with indications for water also being present in the core of micelles and worms. Based on the complementary insights due to the additional frozen sol- and gel-state NMR experiments, the -OCH3 within the B-block could also be identified to stabilize the core-water interaction leading to prolonged thermal stability for the new pMeOx-b-pMeOPheOzi-b-pMeOx gels.