Worm-globule transition of amphiphilic pH-responsive heterografted bottlebrushes at air–water interface

Abstract

Heterografted molecular bottlebrushes (MBBs) with side chains composed of poly(n-butyl acrylate) (PnBA) and pH-responsive poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA, pK_a = 7.4) have been shown to be efficient, robust, and responsive emulsifiers. However, it remains unknown how they respond to external stimuli at interfaces. In this work, the shape-changing behavior of six hetero- and homografted MBBs at air–water interfaces in response to pH changes and lateral compression was investigated using a Langmuir–Blodgett trough and atomic force microscopy. At a surface pressure of 0.5 mN m⁻¹, PDEAEMA-containing MBBs showed no worm-globule transitions when the pH was increased from 4.0 to 10.0, at which PDEAEMA becomes insoluble in water. Upon lateral compression at pH 4.0, MBBs with a mole fraction of PDEAEMA side chains (x_PDEAEMA) < 0.50 underwent pronounced worm-globule shape transitions; there was an increasing tendency for bottlebrushes to become connected with increasing x_PDEAEMA. At x_PDEAEMA = 0.76, the molecules remained wormlike even at high compression. These observations were presumably caused by the increased electrostatic repulsion between protonated PDEAEMA side chains in the subphase with increasing x_PDEAEMA, hindering the shape change. At pH 10.0, MBBs with x_PDEAEMA < 0.50 showed a lower tendency to change their wormlike morphologies upon compression than at pH 4.0. No shape transition was observed when x_PDEAEMA > 0.50, attributed to the relatively high affinity toward water and the rigidity of PDEAEMA. This study revealed the shape-changing behavior of amphiphilic pH-responsive MBBs at air–water interfaces, which could be useful for future design of multicomponent MBBs for potential applications.