Surface-Grafted Stimuli-Responsive Block Copolymer Brushes for the Thermo-, Photo- and pH-Sensitive Release of Dye Molecules

We present a general approach for using surface-grafted stimuli-responsive diblock copolymer brushes as stimuli-sensitive and controllable release systems. Surface-initiated ATRP was used to grow sequentially a first block serving as an inner reservoir for loading and a second block that acts as a stimuli responsive outer layer controlling the closure or opening of the brush in water. We show that the release kinetics of loaded model dyes (hydrophobic or hydrophilic) could be controlled by the second block switchable between the collapsed and extended brush chain states in response to temperature or pH change or exposure to light. On the one hand, diblock copolymer brushes of polystyrene-b-poly(N-isopropylacrylainide) (PS-b-PNIPAM) and poly(N,N'-dimethylacrylamide)-b-poly(N-isopropylacrylamide) (PDMA-b-PNIPAM) were synthesized to demonstrate the thermosensitive release of dyes based on the LCST-determined solubility switching between swollen and collapsed PNIPAM chains. On the other hand, a diblock copolymer brush of polystyrene-b-poly(4,5-dimethoxy-2-nitrobenzyl methacrylate) (PS-b-PNBA) was designed to investigate the possibility of tuning the dye release kinetics with light. The photocontrol was achieved by controlling the photocleavage degree of photolabile o-nitrobenzyl groups, which determines the number of hydrophilic methacrylic acid (MA) groups in the outer layer. Moreover, complete photocleavage of o-nitrobenzyl groups converted the photosensitive PS-b-PNBA brush into a pH-sensitive PS-b-PMA brush with which pH-dependent dye release was observed due to the water solubility switching of PMA chains with protonated or ionized carboxylic acid groups. The interest, the versatility, and the generality of the approach were demonstrated in this study with three different stimuli, namely, temperature, pH, and light.