Synthetic Methodologies to Tune Mixed Polymer Brush Morphology

📖 The Scoop

Polymer brushes are used for surface modification to tune interfacial properties. Mixed A/B polymer brushes, where two distinct types of polymers are tethered to the surface, are especially useful for creating nanoscale morphologies as well as switchable or responsive surfaces. Although simulations predict an array of unique phase-separated morphologies for mixed brushes, their phase behavior has not been extensively explored in experiments mainly due to challenges with synthesis. It is difficult to precisely tune molecular weight, grafting density, grafting ratio, and hence the conformation of the brushes, especially when A and B are highly incompatible with each other. This thesis addresses these synthetic challenges by a) development of a RAFT inimer for brush growth of previously difficult monomers along with efficient chain-end functionalization through click chemistry, b) design of a Y-shaped inimer with two orthogonal initiators attached at a common junction point for versatile growth of mixed A/B polymer brushes with a uniform 1:1 grafting ratio, and c) design of mixed inimer coating for non 1:1 stoichiometry of A:B. Using this chemistry, we perform the following morphological studies: a) probe the effect of a low-surface energy fluorinated chain end on the conformation of brushes grown from the RAFT inimer coating, b) demonstrate mixed PMMA/PS polymer brushes with 1:1 grafting ratio using the Y-inimer coating with a full range of microphase separated morphologies, and c) demonstrate a mixed PMAA/P2VP polyelectrolyte brushes with opposite charges that show a pH-dependent nanoscale porous morphology based on complex coacervation enabled by 1:1 grafting ratio. The main contribution of this thesis is in improving the synthesis methods for mixed polymer brushes to precisely control brush parameters and expand the monomer scope, hence paving the way for detailed experimental studies of morphological properties, especially in the high grafting density regime.

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