Langmuir-Schaefer (LS) macroinitiator film control on the grafting of a thermosensitive polymer brush via surface initiated-ATRP

A novel surface modification based on the Langmuir-Schaefer (LS) monolayer technique was utilized for controlled surface-initiated atom transfer radical polymerization (SI-ATRP) of polymer brushes. The fabrication of these films was prepared by spreading an insoluble monolayer of an amphiphilic macroinitiator, poly(styrene-copolymer-hydroxyethyl methacrylate-2-bromoisobutyryl bromide) (p(St-co-HeBiB)), capable of initiating the ATRP polymerization of n-isopropylacrylamide (NIPAM). During the LS monolayer deposition, the surface pressure was varied in order to probe its effect on the initiator distribution and subsequent grafting density and thickness. The surface-bound macroinitiator ranged from a loosely (low surface pressure) to a densely (high surface pressure) crowded initiation sites on a monolayer. Increasing LS macroinitiator film thicknesses were observed with deposition at increasing surface pressures, i.e. 5, 15, 20, 30, and 50 mN/m using ellipsometry. However, based on the XPS data, there was a decrease in the bromine signal for the 50 mN/m LS macroinitiator film. Contrary to this result, there was a higher nitrogen signal and thicker pNIPAM brushes obtained for the 50 mN/m LS brush film. This was explained by a macroinitiator surface rearrangement during polymerization accounting for the difference. Moreover, wetting measurements by static water contact angle and an in situ atomic force microscopy (AFM) experiment were performed to investigate the temperature dependence of the pNIPAM brush surface wettability and morphology, respectively.