Surface-grafted assemblies of cyclic polymers: Shifting between high friction and extreme lubricity

The Structural Dispersity of Oligoethylene Glycol-Containing Polymer Brushes Determines Their Interfacial Properties

Ought to their bioinert properties and facile synthesis, poly[(oligoethylene glycol)methacrylate]s (POEGMAs) have been raised as attractive alternatives to poly(ethylene glycols) (PEGs) in an array of (bio)material applications, especially when they are applied as polymer brush coatings. However, commercially available OEG-methacrylate (macro)monomers feature a broad distribution of OEG lengths, thus generating structurally polydisperse POEGMAs when polymerized through reversible deactivation radical polymerization. Here, we demonstrate that the interfacial physicochemical properties of POEGMA brushes are significantly affected by their structural dispersity, i.e., the degree of heterogeneity in the length of side OEG segments. POEGMA brushes synthesized from discrete (macro)monomers obtained through chromatographic purification of commercial mixtures show increased hydration and reduced adhesion when compared to their structurally polydisperse analogues. The observed alteration of interfacial properties is directly linked to the presence of monodisperse OEG side chains, which hamper intramolecular and intermolecular hydrophobic interactions while simultaneously promoting the association of water molecules. These phenomena provide structurally homogeneous POEGMA brushes with a more lubricious and protein repellent character with respect to their heterogeneous counterparts. More generally, in contrast to what has been assumed until now, the properties of POEGMA brushes cannot be anticipated while ruling out the effect of dispersity by (macro)monomer feeds. Simultaneously, side chain dispersity of POEGMAs emerges as a critical parameter for determining the interfacial characteristics of brushes.

Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

Metallacyclobuta-(2,3)-diene: A Bidentate Ligand for Stream-line Synthesis of First Row Transition Metal Catalysts for Cyclic Polymerization of Phenylacetylene

Described here is a direct entry to two examples of 3d transition metal catalysts that are active for the cyclic polymerization of phenylacetylene, namely, [(BDI)M{κ2 -C,C-(Me3 SiC3 SiMe3 )}] (2-M) (BDI=[ArNC(CH3 )]2 CH- , Ar=2,6-i Pr2 C6 H3 ; M=Ti, V). Catalysts are prepared in one step by the treatment of [(BDI)MCl2 ] (1-M, M=Ti, V) with 1,3-dilithioallene [Li2 (Me3 SiC3 SiMe3 )]. Complexes 2-M have been spectroscopically and structurally characterized and the polymers that are catalytically formed from phenylacetylene were verified to have a cyclic topology based on a combination of size-exclusion chromatography (SEC) and intrinsic viscosity studies. Two-electron oxidation of 2-V with nitrous oxide (N2 O) cleanly yields a [VV ] alkylidene-alkynyl oxo complex [(BDI)V(=O){κ1 -C-(=C(SiMe3 )CC(SiMe3 ))}] (3), which lends support for how this scaffold in 2-M might be operating in the polymerization of the terminal alkyne. This work demonstrates how alkylidynes can be circumvented using 1,3-dianionic allene as a segue into M-C multiple bonds.

Vanadium Alkylidyne Initiated Cyclic Polymer Synthesis: The Importance of a Deprotiovanadacyclobutadiene Moiety

Reported is the catalytic cyclic polymer synthesis by a 3d transition metal complex: a V(V) alkylidyne, [(dBDI)V≡CtBu(OEt2)] (1-OEt2), supported by the deprotonated β-diketiminate dBDI2- (dBDI2- = ArNC(CH3)CHC(CH2)NAr, Ar = 2,6-iPr2C6H3). Complex 1-OEt2 is a precatalyst for the polymerization of phenylacetylene (PhCCH) to give cyclic poly(phenylacetylene) (c-PPA), whereas its precursor, complex [(BDI)V≡CtBu(OTf)] (2-OTf; BDI- = [ArNC(CH3)]2CH, Ar = 2,6-iPr2C6H3, OTf = OSO2CF3), and the zwitterion [((C6F5)3B-dBDI)V≡CtBu(OEt2)] (3-OEt2) exhibit low catalytic activity despite having a neopentylidyne ligand. Cyclic polymer topologies were verified by size-exclusion chromatography (SEC) and intrinsic viscosity studies. A component of the mechanism of the cyclic polymerization reaction was probed by isolation and full characterization of 4- and 6-membered metallacycles as model intermediates. Metallacyclobutadiene (MCBD) and deprotiometallacyclobutadiene (dMCBD) complexes (dBDI)V[C(tBu)C(H)C(tBu)] (4-tBu) and (BDI)V[C(tBu)CC(Mes)] (5-Mes), respectively, were synthesized upon reaction with bulkier alkynes, tBu- (tBuCCH) and Mes-acetylene (MesCCH), with 1-OEt2. Furthermore, the reaction of the conjugate acid of 1-OEt2, [(BDI)V≡CtBu(OTf)] (2-OTf), with the conjugated base of phenylacetylene, lithium phenylacetylide (LiCCPh), yields the doubly deprotio-metallacycle complex, [Li(THF)4]{(BDI)V[C(Ph)CC(tBu)CC(Ph)]} (6). Protonation of the doubly deprotio-metallacycle complex 6 yields 6-H+, a catalytically active species toward the polymerization of PhCCH, for which the polymers were also confirmed to be cyclic by SEC studies. Computational mechanistic studies complement the experimental observations and provide insight into the mechanism of cyclic polymer growth. The noninnocence of the supporting dBDI2- ligand and its role in proton shuttling to generate deprotiometallacyclobutadiene (dMCBD) complexes that proposedly culminate in the formation of catalytically active V(III) species are also discussed. This work demonstrates how a dMCBD moiety can react with terminal alkynes to form cyclic polyalkynes.

Design, preparation, and characterization of lubricating polymer brushes for biomedical applications

The lubrication modification of biomedical devices significantly enhances the functionality of implanted interventional medical devices, thereby providing additional benefits for patients. Polymer brush coating provides a convenient and efficient method for surface modification while ensuring the preservation of the substrate's original properties. The current research has focused on a "trial and error" method to finding polymer brushes with superior lubricity qualities, which is time-consuming and expensive, as obtaining effective and long-lasting lubricity properties for polymer brushes is difficult. This review summarizes recent research advances in the biomedical field in the design, material selection, preparation, and characterization of lubricating and antifouling polymer brushes, which follow the polymer brush development process. This review begins by examining various approaches to polymer brush design, including molecular dynamics simulation and machine learning, from the fundamentals of polymer brush lubrication. Recent advancements in polymer brush design are then synthesized and potential avenues for future research are explored. Emphasis is placed on the burgeoning field of zwitterionic polymer brushes, and highlighting the broad prospects of supramolecular polymer brushes based on host-guest interactions in the field of self-repairing polymer brush applications. The review culminates by providing a summary of methodologies for characterizing the structural and functional attributes of polymer brushes. It is believed that a development approach for polymer brushes based on "design-material selection-preparation-characterization" can be created, easing the challenge of creating polymer brushes with high-performance lubricating qualities and enabling the on-demand creation of coatings. STATEMENT OF SIGNIFICANCE: Biomedical devices have severe lubrication modification needs, and surface lubrication modification by polymer brush coating is currently the most promising means. However, the design and preparation of polymer brushes often involves "iterative testing" to find polymer brushes with excellent lubrication properties, which is both time-consuming and expensive. This review proposes a polymer brush development process based on the "design-material selection-preparation-characterization" strategy and summarizes recent research advances and trends in the design, material selection, preparation, and characterization of polymer brushes. This review will help polymer brush researchers by alleviating the challenges of creating polymer brushes with high-performance lubricity and promises to enable the on-demand construction of polymer brush lubrication coatings.

Polymer brushes for friction control: Contributions of molecular simulations

When polymer chains are grafted to solid surfaces at sufficiently high density, they form brushes that can modify the surface properties. In particular, polymer brushes are increasingly being used to reduce friction in water-lubricated systems close to the very low levels found in natural systems, such as synovial joints. New types of polymer brush are continually being developed to improve with lower friction and adhesion, as well as higher load-bearing capacities. To complement experimental studies, molecular simulations are increasingly being used to help to understand how polymer brushes reduce friction. In this paper, we review how molecular simulations of polymer brush friction have progressed from very simple coarse-grained models toward more detailed models that can capture the effects of brush topology and chemistry as well as electrostatic interactions for polyelectrolyte brushes. We pay particular attention to studies that have attempted to match experimental friction data of polymer brush bilayers to results obtained using molecular simulations. We also critically look at the remaining challenges and key limitations to overcome and propose future modifications that could potentially improve agreement with experimental studies, thus enabling molecular simulations to be used predictively to modify the brush structure for optimal friction reduction.

Nanoarchitectonics beyond perfect order - not quite perfect but quite useful

Nanoarchitectonics, like architectonics, allows the design and building of structures, but at the nanoscale. Unlike those in architectonics, and even macro-, micro-, and atomic-scale architectonics, the assembled structures at the nanoscale do not always follow the projected design. In fact, they do follow the projected design but only for self-assembly processes producing structures with perfect order. Here, we look at nanoarchitectonics allowing the building of nanostructures without a perfect arrangement of building blocks. Here, fabrication of structures from molecules, polymers, nanoparticles, and nanosheets to polymer brushes, layer-by-layer assembly structures, and hydrogels through self-assembly processes is discussed, where perfect order is not necessarily the aim to be achieved. Both planar substrate and spherical template-based assemblies are discussed, showing the challenging nature of research in this field and the usefulness of such structures for numerous applications, which are also discussed here.

Conformation-dominated surface antifouling and aqueous lubrication

Antifouling and aqueous lubrication are important properties for biomaterials, especially for those with implantation purposes. In order to better understand the polymer conformation dependence of the surface antifouling and lubrication properties, poly(ethylene glycol) (PEG) polymers with mono-functional and difunctional catechol anchors were designed and anchored on surface to adopt tail and loop conformations. Diblock and triblock copolymers with poly(dopamine methacrylamide) (PDMA) block as anchors and PEG block as the main body were synthesized and anchored on silicon surfaces by a "grafting to" strategy. The chemical composition, film thickness, and surface roughness of both coatings were controlled to be similar to give a direct comparison of looped brushes and tailed analogues. Then, the antifouling and surface friction behaviors were detected to verify the topological conformation effect of PEG polymer brushes. Results showed that PEG triblock copolymer modified surface exhibited an obviously better antifouling property and a lower friction coefficient of ∼0.011 than that of PEG diblock copolymer modified surface. Additionally, calculation and simulation results demonstrated that triblock copolymer had higher adsorption energy and anchored on surface with looped conformation. It is indicated that the strongly anchored PEG loops are effective for excellent antifouling and lubricating properties due to its strong hydration and steric hindrance. The conformation-dominated enhanced antifouling and reduced interfacial friction is an effective method for the development of excellent antifouling surfaces.

Cyclic Polyethylene Glycol as Nanoparticle Surface Ligand

Cyclic polymers behave different than linear polymers due to the lack of end groups and smaller coil dimensions. Herein, we demonstrate that cyclic polyethylene glycol (PEG) can be used as an alternative of classical linear PEG ligands for gold nanoparticle (AuNP) stabilization. We observed that the brush height of cyclic PEG on AuNPs of diameter 4.4 and 13.2 nm increases identically as that of linear brushes with (Nσ^1/2)^0.7 (N, number of monomers in a chain and σ, grafting density) and that cyclic brushes are more stretched than their linear analogues when compared to their unperturbed dimensions. Such structural effect and the reduced footprint diameter in cyclic brushes with the entire chain in a concentrated polymer brush regime explains the distinct response of NPs to ionic strength and temperature, respectively, compared to linear analogues. These experiments are an important step in understanding the effect of polymer brush topology on colloidal properties.

Topological Polymer Chemistry Enters Materials Science: Expanding the Applicability of Cyclic Polymers

Polymer-topology effects can alter technologically relevant properties when cyclic macromolecules are applied within diverse materials formulations. These include coatings, polymer networks, or nanostructures for delivering therapeutics. While substituting linear building blocks with cyclic analogues in commonly studied materials is itself of fundamental interest, an even more fascinating observation has been that the introduction of physical or chemical boundaries (e.g., a grafting surface or cross-links) can amplify the topology-related effects observed when employing cyclic polymer-based precursors for assembling multidimensional objects. Hence, the application of cyclic polymers has enabled the fabrication of coatings with enhanced biorepellency and superior lubricity, broadened the tuning potential for mechanical properties of polymer networks, increased the thermodynamic stability, and altered the capability of loading and releasing drugs within polymeric micelles.

Water-Stable Plasma-Polymerized N,N-Dimethylacrylamide Coatings to Control Cellular Adhesion

The plasma polymerization of amide-based precursors is a nearly unexplored research area, which is in contrast with the abundance of reports focusing on amide-based surface modification using wet chemistry. Therefore, this study aims to profoundly investigate the near-atmospheric pressure plasma polymerization of N,N-dimethylacrylamide (DMAM) to obtain stable coatings. In contrast to the unstable coatings obtained at lower discharge powers, the stable coatings that were obtained at higher powers showed a lower hydrophilicity as assessed by water contact angle (WCA). This decrease in hydrophilicity with increasing plasma power was found to be related to a reduced preservation of the monomer structure, as observed by Fourier transform infrared (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and XPS C₆₀ depth profiling, a rarely used but effective combination of techniques. Furthermore, the chemical composition of the coating was found to be in good agreement with the plasma active species observed by optical emission spectroscopy. Additionally, XPS C₆₀ depth profiling indicated a difference between the top layer and bulk of the plasma polymer due to spontaneous oxidation and/or postplasma coating deposition. Finally, the stable coatings were also found to have cell-interactive behavior toward MC3T3 as studied by in vitro live/dead fluorescence imaging and (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) (MTS) assays. With the latter technique, a cell viability of up to 89% as compared with tissue culture plates after 1 day of cell culture was observed, indicating the potential of these coatings for tissue engineering purposes.

Unraveling the nanomechanical properties of surface-grafted conjugated polymer brushes with ladder-like architecture

Surface-grafted polymer brushes with ladder-like architecture enforce extended conformation of the chains affecting their mechanical and tribological properties.

A mussel-inspired catecholic ABA triblock copolymer exhibits better antifouling properties compared to a diblock copolymer

The scheme of the chemical architecture, aggregation, assembly and antifouling properties of two copolymers.

Bioinert and Lubricious Surfaces by Macromolecular Design

The modification of a variety of biomaterials and medical devices often encompasses the generation of biopassive and lubricious layers on their exposed surfaces. This is valid when the synthetic supports are required to integrate within physiological media without altering their interfacial composition and when the minimization of shear stress prevents or reduces damage to the surrounding environment. In many of these cases, hydrophilic polymer brushes assembled from surface-interacting polymer adsorbates or directly grown by surface-initiated polymerizations (SIP) are chosen. Although growing efforts by polymer chemists have been focusing on varying the composition of polymer brushes in order to attain increasingly bioinert and lubricious surfaces, the precise modulation of polymer architecture has simultaneously enabled us to substantially broaden the tuning potential for the above-mentioned properties. This feature article concentrates on reviewing this latter strategy, comparatively analyzing how polymer brush parameters such as molecular weight and grafting density, the application of block copolymers, the introduction of branching and cross-links, or the variation of polymer topology beyond the simple, linear chains determine highly technologically relevant properties, such as biopassivity and lubrication.

Synthesis of macrocyclic poly(ethylene oxide)s containing a protected thiol group: a strategy for decorating gold surfaces with ring polymers

Macrocyclic poly(ethylene oxide)s containing a protected thiol group are able to attach to gold substrates without thiol deprotection enabling surface modification.