Unravelling a novel mechanism in polymer self-assemblies: An order-order transition based on molecular interactions between hydrophilic and hydrophobic polymer blocks

Thermoresponsive hydrogel formation upon cooling in aqueous media is rarely described for synthetic polymers in the literature. However, if the sol-gel transition occurs in the physiologically relevant range (0-40 °C), there are many possible applications in areas such as drug delivery and biofabrication. Here, we describe a new mechanism of a thermally induced order-order transition in polymer self-assembly of an ABA triblock consisting of hydrophilic A blocks and a hydrophobic aromatic B block. Small-angle X-ray scattering confirmed worm-to-sphere transition upon heating on the nanoscale level while wide-angle X-ray scattering indicated a more uniform ordering of the macromolecular chains on the scale of 4-7 A. NMR spectroscopy showed reduced mobility of various polymer segments in the hydrogel state, especially in the hydrophobic aromatic region. More importantly however, solution and solid-state NMR investigations also revealed close proximity of hydrophobic and hydrophilic repeat units in the gel state, which is less pronounced in the sol state. This interaction between the hydrophilic and hydrophobic block is responsible for the order-order transition and –ipso facto– inverse thermogelation. This unusual interaction is supported in silico by molecular dynamics modeling. Changes in the structure of the hydrophilic A blocks can be used to tune the gel strength, persistence, and gelation kinetics. This order-order transition based on unexpected and previously not described interactions between the hydrophilic and the hydrophobic repeat units opens new avenues to control and design macromolecular self-assembly.