(217ag) A Laterally-Mobile Immiscible Mixed Polyelectrolyte/Neutral Polymer Brush Undergoes a Macroscopic Phase Separation When the Two Polymer Types Are of Comparable Size

We studied mixed poly(ethylene oxide) (PEO) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. The question we attempted to answer was: When the chain grafting points are laterally mobile, how will this lateral mobility influence the structure and phase behavior of the mixed brush? Three different model mixed PEO/PDMAEMA brush systems were prepared: (1) a mobile height-matched mixed brush by spreading onto the air-water interface a mixture of two diblock copolymers, poly(ethylene oxide)-poly(n-butyl acrylate) (PEO-PnBA) and poly(2-(dimethylamino)ethyl methacrylate)-poly(n-butyl acrylate) (PDMAEMA-PnBA), having comparable molecular weights for the brush-forming blocks, (ii) a mobility-restricted (inseparable) version of this height-matched mixed brush prepared using a PEO-PnBA-PDMAEMA triblock copolymer having respective brush molecular weights matched to those of the diblock copolymers, and (iii) a height-mismatched laterally-mobile mixed PEO and PDMAEMA brush prepared from a molecular-weight-mismatched combination of PEO-PnBA and PDMAEMA-PnBA diblock copolymers. These three mixed brush systems were investigated by surface pressure-area isotherm and X-ray (XR) reflectivity measurements. These experimental data were analyzed within the theoretical framework of a continuum self-consistent field (SCF) polymer brush model. The combined experimental and theoretical results suggest that the mobile mixed brush undergoes a lateral macroscopic phase separation at high chain grafting densities when the two brush chain types are of comparable height, whereas the height-mismatched system is only microscopically phase separated under comparable brush density conditions even though the lateral mobility of the grafted chains is unrestricted. The macroscopic phase separation observed in the height-matched mobile mixed brush system is in contrast to the microphase separation behavior commonly observed in two-dimensional laterally-mobile charged small molecule mixtures. Further study is needed to determine the detailed morphologies of the macro and microphase-separated mixed PEO/PDMAEMA brushes.