Worm-globule transition of amphiphilic pH-responsive heterografted bottlebrushes at air-water interface.
basic_science · Level V
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- Record sourced from PubMed, PMID 38230501.
- Also identified by DOI 10.1039/d3sm01635h.
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Abstract
Heterografted molecular bottlebrushes (MBBs) with side chains composed of poly(<i>n</i>-butyl acrylate) (P<i>n</i>BA) and pH-responsive poly(2-(<i>N</i>,<i>N</i>-diethylamino)ethyl methacrylate) (PDEAEMA, p<i>K</i><sub>a</sub> = 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<sup>-1</sup>, 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 (<i>x</i><sub>PDEAEMA</sub>) < 0.50 underwent pronounced worm-globule shape transitions; there was an increasing tendency for bottlebrushes to become connected with increasing <i>x</i><sub>PDEAEMA</sub>. At <i>x</i><sub>PDEAEMA</sub> = 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 <i>x</i><sub>PDEAEMA</sub>, hindering the shape change. At pH 10.0, MBBs with <i>x</i><sub>PDEAEMA</sub> < 0.50 showed a lower tendency to change their wormlike morphologies upon compression than at pH 4.0. No shape transition was observed when <i>x</i><sub>PDEAEMA</sub> > 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.