Thermo-responsive amphiphilic di- and triblock copolymers based on poly(N-isopropylacrylamide) and poly(methoxy diethylene glycol acrylate): Aggregation and hydrogel formation in bulk solution and in thin films

In this feature, we provide a comprehensive view and conclusions on recent investigations on the micellar aggregation of amphiphilic model polymers, the subsequent hydrogel formation, and the thermoresponsive behavior. The results obtained in bulk solution as well as in thin films are combined and compared, from the structural as well as kinetic point of view. The studies used two extensive series of diblock and symmetrical triblock copolymers, which were prepared by reversible addition-fragmentation chain transfer (RAFT). Derived from the thermo-responsive parent polymers poly(N-isopropylacrylamide) (PNIPAM) and poly(methoxy diethylene glycol acrylate) (PMDEGA), respectively, both series exhibit a lower critical solution type phase transition in aqueous media in the range of 30–40 °C. The model polymers consist of a long hydrophilic, thermo-responsive middle block, which is end-capped by two relatively small, but strongly hydrophobic blocks made from various vinyl polymers, preferentially from polystyrene. Their aggregation and hydrogel formation as well as their thermo-responsive behavior are systematically studied in dilute and concentrated aqueous solution as well as in thin films. For that, complementary methods were applied such as turbidimetry, fluorescence correlation spectroscopy (FCS), dynamic light scattering (DLS), small-angle X-ray (SAXS) and neutron scattering (SANS), rheology, white light interferometry, atomic force microscopy (AFM), optical probes, X-ray (XRR) and neutron reflectivity (XRR), grazing-incidence small-angle X-ray (GISAXS) and neutron scattering (GISANS) as well as attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). All amphiphilic block copolymers self-organize at several hierarchical levels in bulk solution as well as in thin films. First, the association of the hydrophobic building blocks results in micelle-like aggregates. Then, the micelles cluster and eventually form networks, that make the systems gel. At elevated temperatures, the hydrophilic blocks undergo a collapse transition, inducing major structural changes at the molecular as well as supramolecular levels. Characteristic differences between PNIPAM and PMDEGA based solutions and thin films are worked out, concerning the self-organization, the width and hysteresis of the transition and the switching kinetics. Thin films of PNIPAM and PMDEGA based polymers differ with respect to long ranged correlations and the stability against dewetting. When probing polymer collapse, aggregation behavior, segmental dynamics and mechanical properties of the micellar solutions and the hydrogels, both the chain architecture and the chemical nature of the thermo-responsive block are found to play an important role for the detailed phase behavior.