On the friction and adhesion hysteresis between polymer brushes attached to curved surfaces: Rate and solvation effects

Computer simulations of friction between polymer brushes are usually simplified compared to real systems in terms of solvents and geometry.In most simulations,the solvent is only implicit with infinite compressibility and zero inertia.In addition,the model geometries are parallel walls rather than curved or rough as in reality.In this work,we study the effects of these approximations and more generally the relevance of solvation on dissipation in polymer-brush systems by comparing simulations based on different solvation schemes.We find that the rate dependence of the energy loss during the collision of brush-bearing asperities can be different for explicit and implicit solvent.Moreover,the non-Newtonian rate dependences differ noticeably between normal and transverse motion,i.e.,between head-on and off-center asperity collisions.Lastly,when the two opposing brushes are made immiscible,the friction is dramatically reduced compared to an undersaturated miscible polymer-brush system,irrespective of the sliding direction.

Heavy Metal Ions Removal by Nano-sized Spherical Polymer Brushes

Nano-sized spherical polymer brushes （SPBs） consisting of both a polystyrene （PS） core and a brush shell of poly（acrylic acid） （PAA）, poly（N-acrylcysteamine） （PSH）, or poly（N-acrylcysteamine-co-acrylic acid） （P（SH-co-AA））, were prepared by photo-emulsion polymerization. The core-shell structure was observed by dynamic light scattering and transmission electron microscopy. Due to the strengthened Donnan effect, the PAA brush can adsorb heavy metal ions. Effects of the contact time, thickness of PAA brush and pH value on the adsorption results were investigated. Due to the coordination between the mercapto groups and heavy metal ions as well as the electrostatic interactions, SPBs with mercapto groups are capable to remove heavy metal ions selectively from aqueous solutions. The order of adsorption capacity of the heavy metal ions by SPBs with mercapto groups is： Hg2＋ ≈ Au3＋ 〉 Pb2＋ 〉 CH2＋ 〉 Ni2＋. The adsorbed heavy metal ions can be eluted from SPB by aqueous HCI solution, and the SPBs can be recovered. After three regenerations the recovered SPBs still maintain their adsorption capacity.

Cylindrical polymer brushes-anisotropic unimolecular micelle drug delivery system for enhancing the effectiveness of chemotherapy

Polymer systems can be designed into different structures and morphologies according to their physical and chemical performance requirements,and are considered as one of the most promising controlled delivery systems that can effectively improve the cancer therapeutic index.However,the majority of the polymer delivery systems are designed to be simple spherical nanostructures.To explore morphology/size-oriented delivery performance optimization,here,we synthesized three novel cylindrical polymer brushes(CPBs)by atom transfer radical polymerization(ATRP),which were cellulose-g-(CPT-b-OEGMA)(CCO)with different lengths(~86,~40,and~21 nm).The CPBs are composed of bio-degradable cellulose as the carrier,poly(ethylene glycol)methyl ether methacrylate(OEGMA)as hydrophily block,and glutathione(GSH)-responsive hydrophobic camptothecin(CPT)monomer as loaded anticancer drug.By controlling the chain length of the initiator,three kinds of polymeric prodrugs with different lengths(CCO-1,CCO-2,and CCO-3)could be self-organized into unimolecular micelles in water.We carried out comparative studies of three polymers,whose results verified that the shorter CPBs exhibited higher drug release efficiency,more cellular uptake,and enhanced tumor permeability,accompanied by shortened blood circulation time and lower tumor accumulation.As evidenced by in vivo experiments,the shorter CPBs exhibited higher anti-tumor efficiency,revealing that the size advantage has a higher priority than the anisotropic structure advantage.This provided vital information as to design an anisotropic polymer-based drug delivery system for cancer therapy.

Biocompatible hierarchical zwitterionic polymer brushes with bacterial phosphatase activated antibacterial activity

Hierarchical polymer brushes have been considered as an effective and promising method for preventing implant-associated infections via multiple antibacterial mechanisms.Herein,a bacterial phosphatase re-sponsive surface with hierarchical zwitterionic structures was developed for timely dealing with the poly-meric implant-associated bacterial infection.The hierarchical polymeric architecture was subtly realized on model polypropylene(PP)substrate by sequential photo living grafting of poly(2-(dimethylamino)ethyl methacrylate(PDMAEMA)bottom layer and zwitterionic poly(sulfobetaine methacrylate)(PSBMA)upper layer,followed by the conversion of the PDMAEMA into the zwitterionic structure via succes-sive quaternization and phosphorylation reactions.Owing to shielding the bottom polycations,the hi-erarchical zwitterionic polymer brushes guaranteed the surface with the optimal biocompatibility under the normal physiological environment.Once bacteria are invaded,the surface bactericidal activity of the bottom layer can be rapidly and automatically activated owing to the transition triggered by bacterial phosphatase from zwitterion to polycation.Additionally,ameliorated by the upper layer,the hierarchical surface showed obvious adhesion resistance to dead bacterial cells and notably migrated the cytotoxic-ity of exposed polycation after completion of the bactericidal task.As a proof-of-principle demonstration,this self-adaptive hierarchical surface with sensitive bacterial responsiveness and biocompatibility showed great potential in combating hernia mesh-related infection.This work provides a promising and universal strategy for the on-demand prevention of medical device-associated infections.

Structures and Surface States of Polymer Brushes in Good Solvents:Effects of Surface Interactions

The influence of the surface interaction on the mesoscopic structure of grafted polymers in good solvents has been examined. At high surface coverage, tethered polymers are in the brush state and the parabolic segment density profile is confirmed by self-consistent field theory （SCFT） calculations. It is found that this is a universal behavior for a whole range of surface interactions from complete repulsion to strong attraction. More interestingly, finite surface repulsion may lead to the maximum in the proximal layer of its segment density profile, which is significantly different from both the depletion layer of pure repulsion and the adsorbing layer of attraction. In addition to the brush state on both repulsive and attractive surfaces, three additional surface states were identified by analyzing the scaling behavior of the layer thickness of polymer brushes： the mushroom state on repulsive substrates, the dilute and the semidilute surface states on attractive substrates.

Tuning Polymer-Grafted Particle Monolayer Structure at the Air-Water Interface by Introducing Anisotropic Features

Fabrication of anisotropic material is one of the important topics and we examined to introduce “anisotropic” nature by spreading polymer-grafted particle on the medium with polymer-reactive potential. Poly (tert-butyl methacrylate) (PtBMA) was polymerized from polystyrene latex (PSL) surface by ATRP to give PtBMA-grafted PSL (PSL-PtBMA). Particle monolayer was formed at air-water and air-acidic water interfaces and the monolayer characteristics were compared by π-A isotherm measurements, SEM observations, and contact angle measurements. π-A isotherms, in particular, indicates that the interaction between polymer chains become stronger by changing the subphase condition, which suggests that anisotropicparticle monolayer formation.

Simultaneous Bulk-and Surface-initiated Living Polymerization Studied with a Heterogeneous Stochastic Reaction Model

To better characterize the properties of surface-initiated polymers, simultaneous bulk-and surface-initiated polymerizations are usually carried out by assuming that the properties of the surface-initiated polymers resemble those of the bulk-initiated polymers. Through a Monte Carlo simulation using a heterogeneous stochastic reaction model, it was discovered that the bulk-initiated polymers exhibit a higher molecular weight and a lower dispersity than the corresponding surface-initiated polymers, which indicates that the equivalent assumption is invalid. Furthermore, the molecular weight distributions of the two types of polymers are also different, suggesting different polymerization mechanisms. The results can be simply explained by the heterogeneous distributions of reactants in the system. This study is helpful to better understand surface-initiated polymerization.

Ordered quasi-two-dimensional structure of nanoparticles in semiflexible ring polymer brushes under compression

Molecular dynamics simulations of a coarse-grained bead-spring model of ring polymer brushes un- der compression are presented. Flexible polymer brushes are always disordered during compression, whereas semiflexible polymer brushes tend to be ordered under sufficiently strong compression. Fur- ther, the polymer monomer density of the semiflexible polymer brush is very high near the brush surface, inducing a peak value of the free energy near the surface. Therefore~ when nanoparticles are compressed in semifiexible ring polymer brushes, they tend to exhibit a closely packed single-layer structure between the brush surface and the impenetrable wall, and a quasi-two-dimensional ordered structure near the brush surface is formed under strong compression. These findings provide a new approach to designing responsive applications.

PIEZOELECTRIC PROPERTIES OF SINGLE-STRAND DNA MOLECULAR BRUSH BIOLAYERS

The paper is devoted to investigations on nanomechanical behaviors of biochips in label-free biodetections. The chip consists of Si-layer, Ti-layer, Au-layer and single-strand DNA （ssDNA） molecular brush biolayer immobilized by self-assembly technology of thiol group. Unlike previous viewpoints, such as force-bending, entropy-bending and curvature electricity effect, etc., the piezoelectric effect of the biopolymer brush layer is viewed as the main factor that induces nanomechanical bending of biochips, and a classical macroscopic piezoelectric constitutive relation is used to describe the piezoelectric effect. A new laminated cantilever beam model with a piezoelectric biolayer in continuum mechanics, the linearized Poisson-Boltzmann equation in statistical mechanics and the scaling method in polyelectrolyte brush theory are combined to es- tablish a relationship between the nanomechanical deflection of DNA chips and the factors such as nanoscopic structural features of ssDNA molecules, buffer salt concentration, macroscopic mechanical/piezoelectric parameters of DNA chips etc. Curve fitting of experimental data shows that the sign of the piezoelectric constant of the biolayer may control the deflection direction of DNA chips during the packaging process.

Adsorption of Poly(L-lysine) on Surface-attached Poly(methacrylic acid) Monolayers

Adsorption of poly（L-lysine） on surface-attached poly（methacrylic acid） monolayers formed through in situ free radical polymerization was investigated. A strong ＂template effect＂ was observed for the adsorption of poly（L-lysine） on poly（methacrylic acid） layers, which were perpendicularly grown from the surface of substrates. The adsorbed amount of poly（amino acid） increases linearly with the increase in initial layer thickness of poly（methacrylic acid） monolayers. In addition, the adsorbed poly（amino acid） is relatively stable in medium concentration of salt solution but can be completely released from the brush in high salt concentration.

Preparation of Poly(o-toluidine)/Nano ZrO2/Epoxy Composite Coating and Its Application for Corrosion Protection of Steel

The main objective was to study the anticorrosion performance of poly（o-toluidine）/nano ZrO2/epoxy composite coating.Poly（o-toluidine）/nano ZrO2 composite was prepared by in situ polymerization of o-toluidine monomer in the presence of nano ZrO2 particles.Fourier transformation infrared spectroscopy（FT-IR）,UV-visible spectroscopy（UV-vis）,X-ray diffraction（XRD）,Scanning electron microscopy（SEM）,and Thermogravimetric analysis（TGA） were used to characterize the composition and structure of the composite.Poly（o-toluidine）/nano ZrO2 composite was mixed with epoxy resin through a solution blending method and the three components poly（o-toluidine）/nano ZrO2/epoxy composite coating was coated onto the surface of steel sample by the brush coating method.The anticorrosion performance of poly（o-toluidine）/nano ZrO2/epoxy composite coating on steel sample was studied by polarization curve and electrochemical impendence spectroscopy in 3.5% Na Cl solution as corrosion environment and also compared with that of poly（o-toluidine）/epoxy composite coating and pure epoxy coating.It was observed that the composite coating containing poly（otoluidine）/nano ZrO2 composite has got higher corrosion protection ability than that of poly（o-toluidine）.The electrochemical measurement results demonstrated that poly（o-toluidine） fillers improve the electrochemical anticorrosion performance of epoxy coating and the addition of nano ZrO2 particles increases the tortuosity of the diffusion pathway of corrosive substances.

Polypeptide Brushes Grown via Surface-initiated Ring-opening Polymerization of α-Amino Acid N-Carboxyanhydrides

Polypeptide brushes are attractive platforms to generate functional and responsive interfaces that are of potential interest due to their possible biodegradability, biocompatibility and tunable secondary structures. Surface-initiated ringopening polymerization（SI-ROP） of α-amino acid N-carboxyanhydrides represents a powerful and versatile strategy to generate polypeptide brushes. This review is an attempt to capture the state-of-the-art in this field and highlights the latest developments in several selected areas. In addition to presenting an overview of the synthetic methods that have been used to generate polypeptide brushes via SI-ROP, this article will discuss the preparation of patterned polypeptide brushes, the conformational properties of surface-tethered polypeptides, ways to control chain orientation at surfaces as well as properties and applications of these thin polymer films.

Patterning with Organized Molecules

Decades of progress in the semiconductor industry has led to lithographically printed dimensions that are small enough that the positions of individual molecules and the stochastic variation in the number of photons have a significant effect on the quality of photoresist patterns.These effects scale badly and will be more important as feature sizes continue to shrink.Selforganizing materials can provide regular patterns of molecules that have the potential to minimize stochastic effects.Some such reported materials are block copolymers,bottle brush polymers and DNA,all of which have been used as part of lithographic patterning.A key challenge for selforganizing materials is defect levels.The energy to rearrange has to be high enough that random defects aren’t created thermally but low enough that rearrangement into preferred domains can occur.All of the methods can generate accurate CDs based on the chemical composition of the material,but they all need some way to control the positions of the feature edges.There are methods for guiding the self-organization,but the final position is the sum of the guide pattern misalignment and the intrinsic alignment error of the self-organizing materials.Thus it can be worse than the positioning of the guide structures.Alignment and defect levels are thus two big challenges for manufacturing introduction of self-organizing materials.

应用于高效雾气收集的润滑微沟槽锥

液滴在不对称一维表面的定向输送具有广泛应用,包括雾气收集、油水分离、海水淡化及传感等.液滴所受的驱动力和阻力共同作用决定液滴的运动性能,为提高液滴的运动性能,常见的策略是通过在运动方向引入各种不对称结构以增加驱动力,或者构建光滑表面以减少阻力.然而,到目前为止将增强驱动力和降低阻力同时整合到一个体系中的策略仍未见报道,这主要是由于传统的液体填充润滑表面所需的润滑油很难在物理或化学不对称的表面上稳定存在.本研究中,我们提出在具有微沟槽结构的锥表面接枝柔性聚合物分子刷,即通过协同表面结构和化学修饰来增大液滴在锥结构表面拉普拉斯驱动力并减小粘滞阻力.结果表明,与仅具有微沟槽结构的锥和仅具有聚合物分子刷的光滑锥相比,润滑微沟槽锥可显著提高液滴的运动性能,本研究为开发高效液滴运输智能表面提供了新思路.

A Facile and Environment-friendly Method for Fabrication of Polymer Brush

A novel environment-friendly system is proposed to fabricate polymer brush, which has the advantages including non-toxic and inexpensive initiator （eosin Y）, visible-light exposure （λ = 515 nm）, water medium and ambient environment. The experimental results from UV-Vis spectroscopy, AFM-based single molecule force spectroscopy （SMFS） and other measurements indicate that a polymer brush with a living nature is fabricated via free radical polymerization. This polymer brush may find applications in coatings, bio-interfaces and so forth.

Ceiling Degree of Polymerization for Brush Polymers Prepared via ROMP of Poly（tert-Butyl Acrylate） Macromonomers

Controlled preparation of brush polymers is important in the design of functional materials. In this study, poly（tert-butyl acrylate） macromonomers functionalized with norbornenyl end group（NB-PtBA） were synthesized via atom transfer radical polymerization in three different molecular weights, 2000（NB-PtBA-2k）, 3000（NB-PtBA-3k）, and 8000（NB-PtBA-8k）. Additionally, brush polymers with PtBA as side chains were synthesized via ring-opening metathesis polymerization（ROMP）. Kinetic studies on ROMP of NB-PtBA showed that there was a ceiling degree of polymerization（CDP） for the brush polymers, beyond which the polymerization of NB-PtBA was out of control. For brush polymers of P[NB-PtBA-2k] and P[NB-PtBA-3k], CDPs were estimated to be ca. 400, but the value of P[NB-PtBA-8k] was ca. 100. Therefore, the controlled ROMP of brush polymers was critical at the CDP limit with increased macromonomer molecular weight.

How Does a Polymer Brush Repel Proteins?

The captioned question has been addressed by the steric effect; namely, the adsorption of proteins on a surface grafted with linear polymer chains decreases monotonically as the grafting density increases. However, there is no quantitative and satisfactory explanation why the adsorption starts to increase when the grafting density is sufficiently high and why polyethylene glycol（PEG） still remains as one of the best polymers to repel proteins. After considering each grafted chain as a molecular spring confined inside a ＂tube＂ made of its surrounding grafted chains, we estimated how its free energy depends on the grafting density and chain length, and calculated its thermal energy-agitated chain conformation fluctuation, enabling us to predict an adsorption minimum at a proper grafting density, which agrees well with previous experimental results. We propose that it is such a chain fluctuation that slows down the adsorption kinetically.

Templated synthesis of patterned gold nanoparticle assemblies for highly sensitive and reliable SERS substrates

Formation of plasmonic structure in closely packed assemblies of metallic nanoparticles(NPs)is essential for various applications in sensing,renewable energy,authentication,catalysis,and metamaterials.Herein,a surface-enhanced Raman scattering(SERS)substrate is fabricated for trace detection with ultrahigh sensitivity and stability.The SERS substrate is constructed from a simple yet robust strategy through in situ growth patterned assemblies of Au NPs based on a polymer brush templated synthesis strategy.Benefiting from the dense and uniform distribution of Au NPs,the resulting Au plasmonic nanostructure demonstrates a very strong SERS effect,while the outer polymer brush could restrict the excessive growth of Au NPs and the patterned design could achieve uniform distribution of Au NPs.As results,an ultra-low limit of detection(LOD)of 10^(−15)M,which has never been successfully detected in other work,is determined for 4-acetamidothiophenol(4-AMTP)molecules and the Raman signals in the random region show good signal homogeneity with a low relative standard deviation(RSD)of 7.2%,indicating great sensitivity and reliability as a SERS substrate.The LOD values of such Au plasmonic nanostructures for methylene blue,thiram,and R6G molecules can also reach as low as 10^(−10)M,further indicating that the substrate has a wide range of applicability for SERS detection.With the help of finite difference time domain simulations(FDTD)calculation,the electric field distribution of the Au plasmonic nanostructures is simulated,which quantitatively matches the experimental observations.Moreover,the Au plasmonic nanostructures show good shelf stability for at least 10 months of storage in an ambient environment,indicating potentials for practical applications.

Artificial cilia for soft and stable surface covalent immobilization of bone morphogenetic protein-2

Preservation of growth factor sensitivity and bioactivity(e.g.,bone morphogenetic protein-2(BMP-2))post-immobilization to tissue engineering scaffolds remains a great challenge.Here,we develop a stable and soft surface modification strategy to address this issue.BMP-2(a model growth factor)is covalently immobilized onto homogeneous poly(glycidyl methacrylate)(PGMA)polymer brushes which are grafted onto substrate surfaces(Au,quartz glass,silica wafer,or common biomaterials)via surface-initiated atom transfer radical polymerization.This surface modification method multiplies the functionalized interfacial area;it is simple,fast,gentle,and has little effect on the loaded protein owing to the cilia motility.The immobilized BMP-2(i-BMP-2)on the surface of homogeneous PGMA polymer brushes exhibits excellent bioactivity(-87%bioactivity of free BMP-2 in vitro and 20%-50%higher than scaffolds with free BMP-2 in vivo),with conformation and secondary structure well-preserved after covalent immobilization and ethanol sterilization.Moreover,the osteogenic activity of i-BMP-2 on the nanoline pattern(PGMA-poly(N-isopropylacrylamide))shows-110%bioactivity of free BMP-2.This is superior compared to conventional protein covalent immobilization strategies in terms of both bioactivity preservation and therapeutic efficacy.PGMA polymer brushes can be used to modify surfaces of different tissue-engineered scaffolds,which facilitates in situ immobilization of growth factors,and accelerates repair of a wide range of tissue types.

Hydration lubrication

The hydration lubrication paradigm,whereby hydration layers are both strongly held by the charges they surround,and so can support large pressures without being squeezed out,and at the same time remain very rapidly relaxing and so have a fluid response to shear,provides a framework for understanding,controlling,and designing very efficient boundary lubrication systems in aqueous and biological media.This review discusses the properties of confined water,which-unlike organic solvents-retains its fluidity down to molecularly thin films.It then describes lubrication by hydrated ions trapped between charged surfaces,and by other hydrated boundary species including charged and zwitterionic polymer brushes,surfactant monolayers,liposomes,and biological macromolecules implicated in synovial joint lubrication.Finally,challenges and prospects for future development of this new boundary lubrication approach are considered.