Synthesis, preparation, and conformation of stimulus-responsive end-grafted poly(methacrylic acid-g-ethylene glycol) layers.

Abstract

Here we report the formation of stimulus-responsive chemically end-grafted "brush-brushes" by synthesizing, mono thiol(end)-functionalized poly(methacrylic acid--ethylene glycol) or poly(MAA--EG) comb-type graft copolymers a combination of protecting group chemistry and atom transfer radical polymerization using the initiator 2-(2,4-dinitrophenylthio)ethyl 2-bromo-2-methyl propionate. The polymers were synthesized with three different molecular weights (15 k, 17 k and 27 k), PEG side chain graft densities (EG/MAA mole ratio = 2.2, 0.4 and 1.9, respectively), and a PEG molecular weight = 1100 and then chemically end-grafted to gold substrates chemisorption, resulting in molecular separation distances of ∼3-4 nm. pH-Dependent swelling was confirmed to take place gradually above pH 4-5 and quantified by heights measured by contact mode AFM imaging of microcontact printed (µCP) samples. Swelling factors (maximum height/minimum height) were fairly large (3.6-7.3) and a decrease in molecular weight by ∼2× and side chain graft density by ∼4× resulted in a decrease in swelling factor by ∼2×. Layer height normal force for all three polymers measured by contact mode atomic force microscope imaging on µCP samples at pH 9 showed a nonlinearly decreasing relationship and complete compression ∼<2 nm for forces >10 nN. At pH 4, all polymer layers were largely collapsed (heights ∼<4 nm) and incompressible (, heights were independent of normal force).