Transformations of Thermo-Sensitive Hyperbranched Poly(ionic liquid)s Monolayers.

We synthesized amphiphilic hyperbranched poly(ionic liquid)s (HBPILs) with asymmetrical peripheral composition consisting of hydrophobic n-octadecylurethane arms and hydrophilic, ionically linked poly(N-isopropylacrylamide) (PNIPAM) macrocations and studied low critical solution temperature (LCST) induced reorganizations at the air-water interface. We observed that the morphology of HBPIL Langmuir monolayers is controlled by the surface pressure with uniform well-defined disk-like domains formed in a liquid phase. These domains are merged and transformed to uniform monolayers with elevated ridge-like network structures representing coalesced interdomain boundaries in a solid phase since the branched architecture and asymmetrical chemical composition stabilize the disk-like morphology under a high compression. Above LCST, elevated individual islands are formed due to the aggregation of the collapsed hydrophobized PNIPAM terminal macrocations in a solid phase. The presence of thermo-responsive PNIPAM macrocations initiates the monolayer reorganization at LCST with transformation of surface mechanical contrast distribution. The heterogeneity of elastic response and adhesion distributions for HBPIL monolayers in a wet state changed from highly contrasted two-phase distribution below LCST to near uniform mechanical response above LCST due to the hydrophilic to hydrophobic transformation of the PNIPAM phase.