Dramatically Tuning Friction Using Responsive Polyelectrolyte Brushes

We present a paradigm that dramatically tunes friction from superior lubrication (μ ∼ 10^–3) to ultrahigh friction (μ > 1) using responsive polyelectrolyte brushes. The tunable friction is based on counterion-driven interactions in polyelectrolyte brushes that can be simply achieved by exchanging the counterions. We systematically investigated the effects of opposite counterions of different types on the friction properties of polyanionic, polycationic, and polyzwitterionic brushes. For cationic brushes with quaternary ammonium groups, the friction coefficient was progressively tuned from ∼10^–3 to ∼10⁰ according to the counterions series Cl^– < ClO₄^– < PF₆^– < TFSI^–. The friction of anionic brushes can be tuned by oppositely charged surfactants (tetraalkylammonium) with different length of hydrophobic tails, multivalent metal ions, and protons. The friction increase of cationic brushes is due to the dehydration and the collapse of polyelectrolyte chains induced by ion-pairing interactions. For anionic brushes, the friction increased with the length of hydrophobic tails of surfactants, which resulted from hydrophobicity induced electrostatic interaction among surfactants and polymer chains. The anionic brushes with the carboxylate and the sulfonate side groups revealed different friction responses, which is owing to the carboxylate groups getting stronger specific interaction with the quaternary ammonium and thus with the multivalent metal ions as well. The mechanism of tuning friction was finally concluded; that is, highly hydrated and swelling polymer brushes show superior lubrication, partially collapsed polymer chains have moderate lubrication, and completely dehydrated and collapsed conformation loses lubricating capability

The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior

We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brush-covered surfaces. For cationic (PMETAC) and anionic (PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh friction was observed upon adding oppositely charged surfactant as a result of the electrostatic and hydrophobic interactions between surfactant and polymer. The surfactant exchange led to a strong dehydration of the brush and a concomitant increase in friction. Upon adding surfactant above the CMC, we find a reduction in friction for the anionic brushes, while the cationic brushes maintain a high friction. This difference between the two lubrication systems suggests a different interaction mechanism between the polymers and the surfactants. For zwitterionic (PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic and hydrophobic interactions could be negligible, there is nearly no surfactant uptake and also no effect of surfactant on lubrication

Additional file 1 of Early brass from the Foyemiaowan-Xindiantai cemetery, 265–439 ce: the origin and diffusion of brass in ancient China

Additional file 1: Table S1. Analysis results of brass inclusions

Additional file 2 of Early brass from the Foyemiaowan-Xindiantai cemetery, 265–439 ce: the origin and diffusion of brass in ancient China

Additional file 2: Table S2

The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior

We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brush-covered surfaces. For cationic (PMETAC) and anionic (PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh friction was observed upon adding oppositely charged surfactant as a result of the electrostatic and hydrophobic interactions between surfactant and polymer. The surfactant exchange led to a strong dehydration of the brush and a concomitant increase in friction. Upon adding surfactant above the CMC, we find a reduction in friction for the anionic brushes, while the cationic brushes maintain a high friction. This difference between the two lubrication systems suggests a different interaction mechanism between the polymers and the surfactants. For zwitterionic (PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic and hydrophobic interactions could be negligible, there is nearly no surfactant uptake and also no effect of surfactant on lubrication

Various Tetraphenylethene-Based AIEgens with Four Functional Polymer Arms: Versatile Synthetic Approach and Photophysical Properties

There are a lot of demands to develop abundant superior properties of aggregation-induced emission (AIE) polymers used in the aggregation state. Here we report a practical and versatile approach for the preparation of AIE polymers by conventional free radical polymerization. As an initiator, TPE-AZO has an excellent ability to initiate polymerization of various kinds of vinyl monomers to obtain AIE functional materials. It was found that TPE polymers emit multiple colors with wavelengths ranging from 370 to 482 nm, and they also exhibit the advantages of combining the AIE characteristic with unique properties of polymers. The fluorescence properties of temperature-sensitive TPE-PNIPAM, pH-sensitive TPE-PMAA, and counterion-sensitive TPE-PMETAC were investigated, and the results indicated that all of the responsive behaviors of the three TPE polymers were related to the change in fluorescence. Our versatile approach would provide a platform to fabricate AIE polymers with various superior properties using the azo-based AIE molecular initiator under mild conditions

Mussel-Inspired Thermoresponsive Polypeptide–Pluronic Copolymers for Versatile Surgical Adhesives and Hemostasis

Inspired by marine mussel adhesive proteins, polymers with catechol side groups have been extensively explored in industrial and academic research. Here, Pluronic L-31 alcoholate ions were used as the initiator to prepare a series of polypeptide–Pluronic–polypeptide triblock copolymers via ring-opening polymerization of l-DOPA-<i>N</i>-carboxyanhydride (DOPA-NCA), l-arginine-NCA (Arg-NCA), l-cysteine-NCA (Cys-NCA), and ε-<i>N</i>-acryloyl lysine-NCA (Ac-Lys-NCA). These copolymers demonstrated good biodegradability, biocompatibility, and thermoresponsive properties. Adhesion tests using porcine skin and bone as adherends demonstrated lap-shear adhesion strengths up to 106 kPa and tensile adhesion strengths up to 675 kPa. The antibleeding activity and tissue adhesive ability were evaluated using a rat model. These polypeptide–Pluronic copolymer glues showed superior hemostatic properties and superior effects in wound healing and osteotomy gaps. Complete healing of skin incisions and remodeling of osteotomy gaps were observed in all rats after 14 and 60 days, respectively. These copolymers have potential uses as tissue adhesives, antibleeding, and tissue engineering materials