Tuning Liquid Crystal Ordering Through Precise Positioning of Photoswitchable Units Within Amphiphilic Polymers

Azochromophore-bound hyperbranched polymers have substantial scientific interest in modern technological and sensing applications. Our study explores the self-assembly and photoresponsive behavior of two compositionally similar polymeric amphiphiles (HBP-Azo-TDP and HBP-OEG-Azo) in aqueous solution and at liquid-crystal (LC)-aqueous interfaces. These two amphiphiles consisted of a common hyperbranched core with hydrophobic and hydrophilic segments randomly grafted from it. We installed photoresponsive azobenzene units either in hydrophobic or in hydrophilic segments, so that their location could differ substantially within the self-assembled structure. The amphiphiles self-assemble into spherical micellar aggregates in an aqueous medium and tune the anchoring transition of LC molecules at the LC-aqueous interfaces. However, the optimum doping concentration required to attain homeotropic alignment of LC at the LC-aqueous interfaces was lower for the HBP-Azo-TDP amphiphiles (≥0.3 wt.%) than in HBP-OEG-Azo amphiphiles (≥1 wt.%). Also, despite similar compositions, HBP-OEG-Azo exhibits a faster optical response upon photoillumination, which can be attributed to its location at the periphery of the nanoaggregates and to the intrinsic topology of the polymeric amphiphiles. In contrast, the photoresponsive behavior of the HBP-Azo-TDP amphiphile at the LC-aqueous interface was hindered by the azobenzene's location within the micellar nanoaggregates core. Therefore, the location of photoresponsive units plays a vital role in the design of a new class of LC-aqueous interfaces for the reversible photoisomerization of azo-bound polymeric amphiphiles in a label-free manner.