The interfacial behaviour of single poly(N,N-dimethylacrylamide) chains as a function of pH

Abstract

We have studied the pH-dependent conformational behaviour of poly(N,N-dimethylacrylamide) (PDMAc) at silicon and gold surfaces using single-molecule force spectroscopy and a quartz crystal microbalance (QCM). Despite the pH dependence, nuclear magnetic resonance and titration experiments demonstrate that PDMAc is not a weak polybase. The interaction between single chains and a silicon surface (with native oxide layer intact) in aqueous solution was investigated using force spectroscopy. Single-molecule force measurements were performed using thiol-functionalized PDMAc and gold-coated AFM cantilevers. The forces of interaction between PDMAc and the native oxide-coated silicon surface vary with the pH. The shape of the retraction curve for low pH solution includes a greater number of 'train' conformations, which suggests a stronger interaction with the surface relative to the surrounding media at low pH. The adsorption behaviour of PDMAc, from liquid onto silicon surfaces, was monitored using a QCM, which shows greater PDMAc adsorption onto silicon at low pH. The force spectroscopy and QCM investigations confirm that the PDMAc chain is more extended and stiffer in low pH solution. We attribute the pH-dependent behaviour to an increased number of hydrogen bonding sites on the silicon surface at low pH.

Determination of the molar mass of surface-grafted weak polyelectrolyte brushes using force spectroscopy

“…Experiments were conducted in deionized water (15 MΩ resistivity, Elga PURELab option water purifier) at room temperature with a z-piezo velocity of 400 nm/s. Details of the MFP and the force-distance curve analysis can be found elsewhere [34,39]. The spring constant of each cantilever was characterized using the built-in thermal calibration method [40].…”

“…For example, a single peak shows there is only one contact point between polymer and surface, indicating that the chain is mostly immersed in the solution, with only a small train of monomers on the surface, whereas a plateau suggests the chain takes a pancake conformation. More details can be found elsewhere [10,34,43].…”

“…By stretching single molecules between an atomic force microscope (AFM) tip and a supporting substrate, not only will intermolecular interactions be revealed, but also the contour length of the chain, and the conformation of the molecules on the surface or interface [10,[32][33][34][35][36]. The number of repeat units of each individual chain can then be estimated by dividing the calculated contour length with the length of monomer units.…”

Carbon nanotube size effect on the mechanical properties and toughness of nanocomposites

“…Greenfels and Wanger [29] built an analytical model to investigate the effect caused by the filler geometry (diameter and length) on the toughness, strength and stiffness of nanocomposites, and they found that theses material properties can be improved simultaneously by reducing the nanotube crosssectional aspect ratio (cross-sectional aspect ratio 5 diameter/length). Koratkar et al [30,31] reported that the addition of carbon nanotubes with an average length of 15 mm to an epoxy polymer matrix reduced the rate of crack growth when the nanocomposites were subjected to cyclic-fatigue loading. The increase in fatigue resistance was also demonstrated in a methylmethacrylate polymer modified with carbon nanotubes [32].…”

“…In comparison, the addition of milled carbon fibers to a similar polymer gave no improvement in fatigue performance [33], indicating that the effect of using carbon nanotubes on the fatigue performance of polymer-based nanocomposites is substantially more efficient compared with the use of conventional carbon fibers. The main mechanism through which the addition of carbon nanotubes increases the observed toughness of polymer-based composites was identified in examinations on the fractography of nanocomposites [15,[28][29][30] as pullout of the nanotubes from the polymeric matrix when the crack faces open during crack propagation. The nanotube pullout mechanism involves interface debonding between nanotubes and the polymer matrix, followed by friction as the nanotubes are extracted from the matrix.…”