Synthesis and Behavior of Amphiphilic pH-Responsive Binary Heterografted Molecular Bottlebrush Polymers at Interfaces

Molecular bottlebrush (MBB) polymers are a class of architecturally complex polymers characterized by a backbone polymer densely grafted with relatively shorter polymeric side chains. Due to the dense grafting nature and the large molecular size, MBBs exhibit many unique characteristics and have attracted considerable interest. This dissertation research primarily explored the synthesis of novel amphiphilic pH-responsive heterografted bicomponent MBBs with tertiary amine-containing side chains and their behavior at interfaces.

A series of linear MBBs composed of heterografted poly(n-butyl acrylate) (PnBA) and pH-responsive poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA) side chains with various molar ratios was synthesized by a click “grafting to” method, along with two homografted MBBs. At pH 4.0, below the PDEAEMA’s pK_a of 7.4, MBBs with appropriate side chain compositions were effective and efficient emulsion stabilizers, producing water-in-toluene emulsions with excellent long-term stability after vortexing. Upon adjusting the pH to 10.0, the emulsions were broken. The formation and breaking of emulsions can be repeated 10 times, and atomic force microscopy showed that the MBBs remained intact. Pendant drop tensiometry revealed that the lowest toluene-water interfacial tension decreased with increasing molar content of PDEAEMA in the bottlebrushes, but no clear correlation was found between the interfacial tension reduction and the MBB’s emulsion stabilization ability. Interfacial jamming of MBBs with certain side chain compositions, manifested in wrinkled interfaces, was observed at pH 4.0 upon reducing the interfacial area of droplets, which correlated well with their abilities to stabilize emulsions. This was attributed to the strong adsorption of MBBs to the interface with PnBA and protonated PDEAEMA side chains extending only into the toluene and aqueous phase, respectively. Using a Langmuir-Blodgett trough, heterografted PnBA/PDEAEMA MBBs exhibited wormlike-to-globular shape transitions upon lateral compression on the acidic water surface when PnBA was the major component of side chains. Additionally, stepwise conformational changes of linear MBBs from wormlike to partially collapsed, and stable globular were achieved in aqueous solutions using two distinct stimuli-responsive polymers and poly(ethylene oxide) as the side chains. The insights obtained from this dissertation research will provide guidelines for design of stimuli-responsive MBBs for potential applications, e.g., in emulsion and substance delivery.