Controlled Crosslinking Is a Tool To Precisely Modulate the Nanomechanical and Nanotribological Properties of Polymer Brushes

Abstract

Covalent crosslinking of weak polyelectrolyte brushes widens the tuning potential for their swelling, nanomechanical, and nanotribological properties, which can be simultaneously adjusted by varying the crosslinker content and the pH of the surroundings. We demonstrate that this is especially valid for poly­(hydroxyethyl methacrylate) (PHEMA) brushes and brush hydrogels, and their ionizable, succinate-modified derivatives (PHEMA-SA), covalently crosslinked with different amounts of di­(ethylene glycol) dimethacrylate (DEGDMA) during surface-initiated atom transfer radical polymerization (SI-ATRP). Atomic force microscopy (AFM) methods highlight how pristine PHEMA films are stiff and display high coefficients of friction in water. Their succinate derivatives swell profusely in aqueous media. Under acidic conditions they are neutral, compliant, and lubricious, with apparent Young’s moduli (E*) lying between 10 and 30 kPa. Their contact mechanical behavior can be described by either the Johnson–Kendall–Roberts (JKR) or the Derjaguin–Müller–Toporov (DMT) model, depending on crosslinker content. In contrast, under basic conditions, brushes and brush hydrogels become charged, expand, and present a rigid, electrostatic barrier toward the AFM probe. Friction is extremely low at relatively low applied loads, whereas it increases at higher loads, to an extent that is regulated by the number of crosslinks within the films.

Using Polymers to Impart Lubricity and Biopassivity to Surfaces: Are These Properties Linked?

“…In the absence of crosslinking, PHEMA brushes display a friction coefficient μ = 0.49, while crosslinked PHEMA containing 1 or 2 mol-% DEGDMA were measured as μ = 0.67 or 0.76, respectively. [75] Thus, in the case of PHEMA brushes and brushhydrogels, biopassivity and lubrication appear to be properties that can be conveniently decoupled by varying the molecular composition of PHEMA films. This is due to the modest swelling properties of this polymer in aqueous media, and the concomitantly high stiffness of the films, which was further enhanced by covalent crosslinking.…”

The Role of Nanomechanics in Healthcare

“…Polymer brushes have been emerging as highly versatile coatings for a variety of applications in surface and biomaterials science such as antifouling, bioactive, and lubricating surfaces . The lubricating properties of polymer brushes have attracted the attention of materials scientists and researchers .…”

“…The quantification of friction at nanoscale can provide information about the behaviors of coating at swollen and shrunken conditions. Dehghani et al investigated the nanotribological performance of polymer brushes hydrogels, and their ionizable, succinate‐modified derivatives during surface‐initiated atom transfer radical polymerization . While changing the pH, the conformation of the polymer brushes changes resulting in transformation of interaction mechanism from “sliding of shrunken coating” at the pH value of 5 to “deformation of the swollen coating” at the pH value of 9.…”

Effect of Architecture of Thermoresponsive Copolymer Brushes on Switching of Their Adsorption Properties

“…They are also able to demonstrate switchable properties due to the stimuli‐responsive behavior of the polymer chains that were grafted onto the surface . The responsiveness of these brushes was utilized to design surfaces with tunable wetting properties with regard to adhesion, adsorption, friction, and other properties . In one approach for the design of stimuli‐responsive brushes, the switching is provided by changing the properties (charge and hydrophobicity) of the polymer themselves .…”