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Evidence of Multimolecular Cooperative Charge Regulation In Weak Polyelectrolyte Brushes

Kevin N. Witte1, Jaehyun Hur2, Sangtae Kim2, and You-Yeon Won1. (1) School of Chemical Engineering, Purdue University, 480 Stadium Mall Dr, West Lafayette, IN 47907, (2) Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering (FRNY), 480 Stadium Mall Drive, West Lafayette, IN 47907

A novel system for the study of polymer brushes has been developed and utilized to study the thermodynamic stability of weak polyelectrolyte brushes. The brush system was created by at the air-water interface by spreading a unique diblock copolymer composed of poly(n-butyl acrylate) and poly(dimethylaminoethyl methacrylate) (PnBA-b-PDMAEMA). This polymer, synthesized by atom transfer radical polymerization (ATRP), has several characteristics that make it ideal for the study true polymer brushes at the air water interface. Namely, the hydrophobic PnBA anchoring block is non-glassy at ambient conditions, and has a positive spreading coefficient on water. Thus, the diblock copolymer will form a uniform monolayer at the interface that can be readily compressed to vary the available area per chain.

Isotherms for the copolymer were measured and their defining features explored in greater detail. At high area per chain, the behavior is governed by the anchoring block as it was compressed to form a uniform monolayer. The formation of this monolayer is evidenced by a sharp transition in the slope of the isotherm. This effect was further explored and verified by studying PnBA homopolymers of varying molecular length. The spread polymer was compressed to various values of the area per chain and deposited on a graphite substrate via a submersion Langmuir-Blodgett (LB) technique. The formation of the monolayer was then confirmed by fluid atomic force microscopy (AFM). At low area per chain, the behavior is governed by the interactions of the positively charged brush. We here show for the first time experimental evidence for thermodynamic instability in the single component brush system. Using the LB and fluid AFM techniques we show the existence of regions of high and low polymer density consistent with “phase” separation of the single component brush which can be related to the multimolecular cooperative charge regulation inherent in weak polyelectrolyte systems. The system also shows localized “microscopic” separation due to the build up of electrostatic repulsive energy and small ion translational entropy. This effect has been termed “frustrated” phase separation. The electrostatic nature of the behavior was confirmed by addition of 100 mM NaCl to the aqueous subphase. In this case the system remains uniformly dispersed across the interface through out the compression.