PFS-b-PNIPAM: A first step towards polymeric nanofibrillar hydrogels based on uniform fiber-like micelles

Amphiphilic crystalline-coil diblock copolymers polyferrocenyldi-methylsilane-block-poly(N-isopropylacrylamide) of two different block ratios (PFS56-b-PNIPAM190 and PFS26-b-PNIPAM520) were synthesized by a copper-catalyzed azide−alkyne coupling reaction. They exhibited pronounced differences
in self-assembly in alcohol solvents. While PFS56-b-PNIPAM190 formed mixtures
of spherical and rod-like micelles in ethanol and 2-propanol, PFS26-b-PNIPAM520 formed long fibers of uniform width in these solvents. We used a seeded growth protocol to grow rod-like PFS26-b-PNIPAM520 micelles of uniform lengths. There were two surprising features of this experiment: First, micelle growth was unusually slow and required a long aging time (40 days) for them to reach their final length. Second, the micelles were characterized by a low number of polymer chains per unit length as determined by multiangle light scattering. This result suggests a loose packing of PFS chains in the micelle core. In an attempt to prepare thermoresponsive nanofibrillar hydrogels from these micelles, we explored approaches to transfer them from 2-propanol to water. These attempts were accompanied by extensive fragmentation of the micelles. We believe the fragility of these micelles is related to the loosely packed nature of the PFS chains in the micelle core. Fragmentation may also be affected by the cononsolvency effect of 2-propanol-water mixtures on the PNIPAM corona of the micelles. We could show, however, that the micelle fragments in water retained their anticipated thermoresponsive behavior.