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

Here we report the formation of stimulus-responsive chemically end-grafted ‘‘brush-brushes’’ by synthesizing, mono thiol(end)-functionalized poly(methacrylic acid-g-ethylene glycol) or poly(MAA-g-EG) comb-type graft copolymers via 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 via chemisorption, resulting in molecular separation distances of y3–4 nm. pHDependent swelling was confirmed to take place gradually above pH 4–5 and quantified by heights measured by contact mode AFM imaging of microcontact printed (mCP) samples. Swelling factors (maximum height/minimum height) were fairly large (3.6–7.3) and a decrease in molecular weight by y26 and side chain graft density by y46 resulted in a decrease in swelling factor by y26. Layer height versus normal force for all three polymers measured by contact mode atomic force microscope imaging on mCP samples at pH 9 showed a nonlinearly decreasing relationship and complete compression y,2 nm for forces >10 nN. At pH 4, all polymer layers were largely collapsed (heights y,4 nm) and incompressible (i.e., heights were independent of normal force).