Probing the brush structure of end-tethered poly(oligo(ethylene glycol) methyl ether methacrylate) chains

Polymer coatings of poly(oligo(ethylene glycol) methyl ether methacrylate) (poly(OEGMA)) have been often synthesized utilizing surface-initiated atom transfer radical polymerization (SI-ATRP) from halogen initiating moieties bound on the surface. This heterogeneous nonlinear analogue of polyethylene glycol has been widely used to introduce biocompatibility in various sensing, drug delivery, tissue engineering and microfluidics applications. While the chemical structure of the coating plays an important role in the biofunctionality, the physical conformational state of polymer chains determines the coating's properties. However, the actual state of the poly(OEGMA) polymer chains constituting the coatings has been rarely accessed. In this work, we have synthesized poly(OEGMA) polymer chains at various grafting densities by tuning the surface concentration of initiating groups in the range of 0.1-4.8 Br-atoms/nm2. We thoroughly examined the chemical and physical structure of the poly(OEGMA) chains through various surface-sensitive techniques. To probe the viscoelastic response of the poly(MEOGMA) brushes of various grafting densities, we combined ex-and in-situ acoustic quartz crystal microbalance with dissipation monitoring (QCM-D) and variable angle spectroscopic ellipsometry (VASE) measurements. The detailed analysis in wet state points to similar relaxation times for the swollen poly (OEGMA) chains of various densities, i.e. similar stretched polymer brush chain conformation irrespectively of their density. The concomitant QCM-D-VASE analysis provided insights of the scaling behavior of the end-tethered poly(OEGMA) brushes with an exponent n = 0.54 throughout the whole studied density region, i.e. sigma ranging from 0.04 to 0.27 chains/nm2. The attained brush state of the polymer chains was further corroborated by the reduced grafting density parameter. The observed scaling behavior with an exponent specific to polymer brushes in the high-density region is governed by the strong perpendicular swelling of the chains from the tethering substrate, which is further assisted by the bulky MeOEG side chains, which sterically limit the coiling of the main polymer chain.