Connecting nanoscale morphology to rheology in self-assembled poly (propylene glycol), poly (ethylene glycol) and poly(ε-caprolactone) thermogels: a SANS investigation

Thermogels are hydrogels that undergo a sol-gel transition in response to temperature. They are a leading class of biomaterials that are highly promising for numerous biomedical applications such as sustained drug delivery and as vitreous substitutes for treatment of retinal detachment. Amongst various thermogelling polymer constructs, a class of multi-block copolymers comprising poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG) and poly(caprolactone) (PCL) linked together by urethane bonds (EPCs) show tremendous promise. Despite their potential, the nanostructures formed by the self-assembly of these polymers when dissolved in water remain largely unknown. Herein, we elucidate the nanostructures formed in solution by seven of these constructs with varying structure and molecular weights (24-74 kDa), using small-angle neutron scattering (SANS), which we relate to their rheological properties: thermogelation, and viscoelasticity. Our findings reveal that EPCs self-assemble into elliptical structures, with a polar radius ranging from 24 to 43 Å and an equatorial radius ranging from 118-190 Å at 25°C. These structures become less anisotropic with temperature as the polar radius increases to 47-57 Å at 50°C. EPCs with higher molecular weights show a higher aggregation propensity, leading to larger self-assembled structures and stronger gels, with a lower onset of gelation. Altogether, these structural insights provide a molecular-level understanding of the self-assembly behaviour of these polymers in solution, with potential implications for biological applications.