Elucidating Interfacial Chain Conformation of Superhydrophilic Polymer Brushes by Vibrational Sum Frequency Generation Spectroscopy

Abstract

Surface-tethered macromolecules (polymer brushes) are a potent means to modify surfaces with stimuli-responsive properties while avoiding delamination problems. This vibrational sum frequency generation spectroscopy study describes how the conformation of hydrophilic polymer brushes changes in response to environmental conditions, that is, changes in humidity (in air) and upon exposure to liquid water. Three hydrophilic brushes were prepared on silicon oxide surfaces by surface-initiated reversible deactivation radical polymerization of cationic (quaternary ammonium), anionic (sulfonate), and zwitterionic (containing both) monomers. The average tilt angle of methyl groups was analyzed and used to deduce the chain conformations of the polymer brushes. In air, the brush films absorb water and swell with increasing humidity. This is accompanied by the rotation of interfacial polymer chains. The degree of water uptake and chain conformation vary with the nature of the charged hydrophilic moieties. The hydrophilic polymer brush surfaces appear to remain relatively dry except in near-condensation conditions. In water, the quaternary ammonium groups of cationic and zwitterionic brushes are aligned nearly parallel to the surface. The anionic brush chains appear to assume nearly random conformations in water.

Thermoresponsive polymer brush photocatalytic substrates for wastewater remediation

“…This implies that CH 3 groups at the brush end are tilted about 35° from the surface normal when the brush is extended below LCST and reoriented nearly parallel to the surface when the brush is collapsed above LCST. 67 Also, the –N(H)– and –OH groups appear to be nearly normal to the surface below LCST (strong in the ssp spectrum) and become more parallel to the surface above LCST (stronger in the ppp spectrum).…”

Recent Advances in Antifouling Surface Polymer Brushes

“…Sum frequency generation (SFG) vibrational spectroscopy is a sensitive analytic technology to investigate surfaces and interfaces. 17,18 By use of SFG, different water structures at the polymer−water interface can be distinguished to assist in understanding of the hydration behavior of surface polymer brushes (Figure 1b). Chen et al revealed in a series of studies that water molecules can bond tightly or weakly to the surface of polymers, which is critical to unraveling the molecular origin of antifouling properties of polymers.…”

“…However, this characterization only generates information on the hydration of surface polymer brushes at a macroscopic scale. Sum frequency generation (SFG) vibrational spectroscopy is a sensitive analytic technology to investigate surfaces and interfaces. , By use of SFG, different water structures at the polymer–water interface can be distinguished to assist in understanding of the hydration behavior of surface polymer brushes (Figure b). Chen et al revealed in a series of studies that water molecules can bond tightly or weakly to the surface of polymers, which is critical to unraveling the molecular origin of antifouling properties of polymers. − For hydrophilic polymers, water molecules are tightly associated through hydrogen bonding, leading to strong surface hydration.…”

Switchable Slippery Surfaces Controlled by a Humidity‐Induced Glass Transition of Polyelectrolyte‐Grafted Brushes

“…[23][24][25] Previous measurements of water uptake as a function of relative humidity for such brushes typically show absorption curves with a convex shape displaying accelerating water uptake at higher levels of relative humidity. [26][27][28][29] Additionally, others have noted strong connections between the swelling state of a brush (swollen/collapsed) [11,12,30] or the amount of unbound water available [31] with a switching friction coefficient for a brush in the presence of liquid water. It may therefore be expected that friction forces should decrease in a similar manner as the water uptake or the swelling of the brush.…”

“…Although, prior studies considering polyelectrolyte brush water uptake in humid vapor generally have not considered the possibility of such a transition, possibly due to the continuous nature of the observed water absorption. [28,29,38,26,39] Yet, a glass transition could itself have implications on the properties of a brush beyond just the water uptake due to the mechanical properties and chain relaxation dynamics that can change rapidly. [40,41] Despite this outstanding knowledge gap and potential applications, no research work to date has examined in detail how the water absorption behavior of a polyelectrolyte brush with humidity affects properties such as friction.…”