Light-triggered unfolding of single linear molecular bottlebrushes from compact globular to wormlike nano-objects in water
This article reports on the light-triggered unfolding of single stimuli-responsive linear molecular bottlebrushes, composed of either homografted poly(methoxydi(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate) (P(DEGMA-co-NBA)) or heterografted poly(ethylene oxide) (PEO) and poly(o-nitrobenzyl acrylate) (PNBA) side chains, from compact globular to wormlike shapes in an effort to mimic the triggered unravelling of proteins such as the von Willebrand factor and talin. PDEGMA is a thermosensitive polymer with a lower critical solution temperature (LCST) of 38 °C in water. The hydrophobic o-nitrobenzyl group can be cleaved by UV irradiation, yielding a hydrophilic carboxylic acid group. This increased the LCST or solubility of the corresponding side chains in water, triggering the shape transitions of brush molecules from globular to wormlike. The brushes were synthesized by a “grafting to” method using the copper-catalyzed cycloaddition of alkyne end-functionalized side chain polymers and an azide-bearing backbone polymer. For 0.2 mg g−1 P(DEGMA-co-NBA) molecular brushes in water, a dynamic light scattering study showed that the apparent hydrodynamic size decreased with increasing temperature, with the LCST occurring at 23 °C. Atomic force microscopy revealed that the brush molecules changed from an extended wormlike morphology at 4 °C to compact globular nano-objects upon heating to 30 °C. UV irradiation of the solution at 30 °C for 70 min unfolded the brushes to an extended wormlike conformation. For heterografted PEO/PNBA molecular brushes, the collapsed compact nano-objects in water, stabilized by PEO side chains, were prepared by gradually adding a DMF solution of brushes into water, followed by dialysis to remove DMF. Similarly, upon UV irradiation, the brush molecules changed to wormlike conformations.