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

Heterografted molecular bottlebrushes (MBBs) with side chains composed of poly( n -butyl acrylate) (P n BA) and pH-responsive poly(2-( N , N -diethylamino)ethyl methacrylate) (PDEAEMA, p K 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.