Tunable nanolubrication between dual-responsive polyionic grafts

Weak polyelectrolyte brushes

I review experimental developments in the growth and application of surface-grafted weak polyelectrolytes (brushes), concentrating on their surface, tribological, and adhesive and bioadhesive properties, and their role as actuators.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

Enrichment of Charged Monomers Explains Non-monotonic Polymer Volume Fraction Profiles of Multi-stimulus Responsive Copolymer Brushes

Multi-stimulus responsive poly(2-(2-methoxyethoxy)ethyl methacrylate-co-2-(diethylamino)ethyl methacrylate) [P(MEO₂MA-co-DEA)] 80:20 mol % copolymer brushes were synthesized on planar silica substrates via surface-initiated activators continuously regenerated via electron transfer atom transfer radical polymerization. Brush thickness was sensitive to changes in pH and temperature as monitored with ellipsometry. At low pH, the brush is charged and swollen, while at high pH, the brush is uncharged and more collapsed. Clear thermoresponsive behavior is also observed with the brush more swollen at low temperatures compared to high temperatures at both high and low pH. Neutron reflectometry was used to determine the polymer volume fraction profiles (VFPs) at various pH values and temperatures. A region of lower polymer content, or a depletion region, near the substrate is present in all of the experimental polymer VFPs, and it is more pronounced at low pH (high charge) and less so at high pH (low charge). Polymer VFPs calculated through numerical self-consistent field theory suggest that enrichment of DEA monomers near the substrate results in the experimentally observed non-monotonic VFPs. Adsorption of DEA monomers to the substrate prior to initiation of polymerization could give rise to DEA segment-enriched region proximal to the substrate.

Interplay of Composition, pH, and Temperature on the Conformation of Multi-stimulus-responsive Copolymer Brushes: Comparison of Experiment and Theory

Poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO₂MA), a thermoresponsive polymer with a lower critical solution temperature of ∼28 °C, and poly(2-(diethylamino)ethyl methacrylate) (PDEA), a weak polybase with an apparent pK_a of ∼7.5, have been statistically copolymerized using activators continuously regenerated via electron transfer atom transfer radical polymerization to form multi-stimulus-responsive polymer brushes. The stimulus-responsive behavior of these brushes has been investigated with ellipsometry and numerical self-consistent field (nSCF) theory. The pH- and thermoresponsive behaviors of a PDEA homopolymer brush were investigated experimentally in order to benchmark the nSCF theory calculations. nSCF theory was able to reproduce the responsive behavior of PDEA and PMEO₂MA homopolymer brushes. Three copolymer compositions (90:10, 70:30, and 50:50 mol % MEO₂MA:DEA) were investigated experimentally with pH ramps performed at low and high temperatures and temperature ramps performed at low and high pH. A broader range of compositions were investigated with nSCF theory and compared to the experimental results, with the nSCF calculations able to capture the general behavior of the homopolymer and copolymer brushes. The responsive behavior of each brush to a given stimulus (temperature or pH) was dependent on both the polymer composition and environment (temperature or pH). The influence of pH on the brush increased with higher DEA mol % with a copolymer brush response transitioning from temperature-dominant to pH-dominant. The temperature response of PMEO₂MA was completely masked at low and high pH values by the presence of at least 30 mol % polybase in the copolymer.

Nanotribology of hydrogels with similar stiffness but different polymer and crosslinker concentrations

HYPOTHESIS

The stiffness has been found to regulate hydrogel performances and applications. However, the key interfacial properties of hydrogels, like friction and adhesion are not controlled by the stiffness, but are altered by the structure and composition of hydrogels, like polymer volume fraction and crosslinking degree.

EXPERIMENTS

Colloidal probe atomic force microscopy has been use to investigate the relationship between tribological properties (friction and adhesion) and composition of hydrogels with similar stiffness, but different polymer volume fractions and crosslinking degrees.

FINDINGS

The interfacial normal and lateral (friction) forces of hydrogels are not directly correlated to the stiffness, but altered by the hydrogel structure and composition. For normal force measurements, the adhesion increases with polymer volume fraction but decreases with crosslinking degree. For lateral force measurements, friction increases with polymer volume fraction, but decreases with crosslinking degree. In the low normal force regime, friction is mainly adhesion-controlled and increases significantly with the adhesion and polymer volume fraction. In the high normal force regime, friction is predominantly load-controlled and shows slow increase with normal force.

Probing the frictional properties of soft materials at the nanoscale

The understanding of friction in soft materials is of increasing importance due to the demands of industries such as healthcare, biomedical, food and personal care, the incorporation of soft materials into technology, and in the study of interacting biological interfaces. Many of these processes occur at the nanoscale, but even at micrometer length scales there are fundamental aspects of tribology that remain poorly understood. With the advent of Friction Force Microscopy (FFM), there have been many fundamental insights into tribological phenomena at the atomic scale, such as 'stick-slip' and 'super-lubricity'. This review examines the growing field of soft tribology, the experimental aspects of FFM and its underlying theory. Moving to the nanoscale changes the contact mechanics which govern adhesive forces, which in turn play a pivotal role in friction, along with the deformation of the soft interface and dissipative phenomena. We examine recent progress and future prospects in soft nanotribology.

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.

Surface Density Variation within Cyclic Polymer Brushes Reveals Topology Effects on Their Nanotribological and Biopassive Properties

While topology effects by cyclic polymers in solution and melts are well-known, their translation into the interfacial properties of polymer "brushes" provides new opportunities to impart enhanced surface lubricity and biopassivity to inorganic surfaces, above and beyond that expected for linear analogues of identical composition. The impact of polymer topology on the nanotribological and protein-resistance properties of polymer brushes is revealed by studying linear and cyclic poly(2-ethyl-2-oxazoline) (PEOXA) grafts presenting a broad range of surface densities and while shearing them alternatively against an identical brush or a bare inorganic surface. The intramolecular constraints introduced by the cyclization provide a valuable increment in both steric stabilization and load-bearing capacity for cyclic brushes. Moreover, the intrinsic absence of chain ends within cyclic adsorbates hinders interpenetration between opposing brushes, as they are slid over each other, leading to a reduction in the friction coefficient (μ) at higher pressures, a phenomenon not observed for linear grafts. The application of cyclic polymers for the modification of inorganic surfaces generates films that outperform both the nanotribological and biopassive properties of linear brushes, significantly expanding the design possibilities for synthetic biointerfaces.

Friction and adhesion control between adsorbed layers of polyelectrolyte brush-grafted nanoparticles via pH-triggered bridging interactions

HYPOTHESIS

Adsorption of polyelectrolyte brush-grafted nanoparticles (BGNPs) produces a heterogeneous interface with sub-monolayer surface coverage resulting from lateral electrostatic repulsions that limit packing. As a result, the interaction forces between opposing BGNP layers include an adhesive cross-surface BGNP-substrate bridging force that depends on the interparticle spacing, particle size, and strength of electrostatic interactions. We hypothesize that BGNPs with pH-responsive, annealed polyelectrolyte brushes can undergo controlled changes in surface area coverage through post-adsorption swelling or de-swelling into non-equilibrium layer conformations and that such changes in surface coverage can switch off or switch on particle intercalation, bridging attractions, and enhanced energy dissipation upon sliding. This work aims to characterize the nature of surface forces in heterogeneous BGNP adsorbed layers and to utilize pH-sensitive bridging forces as a mechanism to tune friction and adhesion.

EXPERIMENTS

Colloidal probe atomic force microscopy (CP-AFM) is used to measure normal and lateral forces between negatively charged silica surfaces with adsorbed pH-responsive cationic BGNPs. The BGNPs are poly(2-dimethylaminoethyl methacrylate) brush-grafted silica nanoparticles. Adhesion force and friction analysis is complemented by simultaneous quartz-crystal microbalance and ellipsometry measurements under conditions that render the particles strongly charged and swollen (acidic) or weakly charged and de-swollen (basic).

FINDINGS

Adsorbed BGNPs can be swollen or de-swollen via pH rinses, enabling direct control of surface coverage and bridging interactions. Transitions from adhesive bridging contacts with high friction to non-adhesive contacts with low friction forces occur when adsorbed BGNP layers are switched from a de-swollen/weakly charged state to a swollen/highly charged state. The ability to controllably shift the character of normal and lateral forces via coverage-mediated bridging interactions is a unique feature of adsorbed nanoparticulate brush constructs and highlights their potential to condition surfaces with additional functionality compared to dense, planar homopolymer brushes.

The Role of Nanomechanics in Healthcare

Nanomechanics has played a vital role in pushing our capability to detect, probe, and manipulate the biological species, such as proteins, cells, and tissues, paving way to a deeper knowledge and superior strategies for healthcare. Nanomechanical characterization techniques, such as atomic force microscopy, nanoindentation, nanotribology, optical tweezers, and other hybrid techniques have been utilized to understand the mechanics and kinetics of biospecies. Investigation of the mechanics of cells and tissues has provided critical information about mechanical characteristics of host body environments. This information has been utilized for developing biomimetic materials and structures for tissue engineering and artificial implants. This review summarizes nanomechanical characterization techniques and their potential applications in healthcare research. The principles and examples of label-free detection of cancers and myocardial infarction by nanomechanical cantilevers are discussed. The vital importance of nanomechanics in regenerative medicine is highlighted from the perspective of material selection and design for developing biocompatible scaffolds. This review interconnects the advancements made in fundamental materials science research and biomedical technology, and therefore provides scientific insight that is of common interest to the researchers working in different disciplines of healthcare science and technology.

Behavior of Weak Polyelectrolyte Brushes in Mixed Salt Solutions

Hydrophilic and hydrophobic weak polybasic brushes immersed in aqueous solutions of mixed salt counterions are considered using a mean-field numerical self-consistent field approach. On top of the solvent quality of the polymer, the counterion–solvent interactions are accounted for by implementing Flory–Huggins interaction parameters. We show that ion specificity within the brush can bring about large changes in conformation. It is found that the collapse transition of hydrophobic, weak polyelectrolyte brushes features an intermediate two-phase state wherein a subset of chains are collapsed in a dense layer at the substrate, while the remainder of chains are well-solvated and strongly stretched away from the it. Besides pH and ionic strength, solvent quality of counterions and the composition of ions in the solvent are important control parameters for the behavior of polyelectrolyte brushes. Increasingly hydrophobic counterions penetrate deeper within the brush and stabilize the collapsed region, while hydrophilic counterions do the opposite.

Mutable polyelectrolyte tube arrays: mesoscale modeling and lateral force microscopy

In this study, the pH-dependent friction of layer-by-layer assemblies of poly(allylamine hydrochloride) and poly(acrylic acid) (PAH/PAA) are quantified for microtube array structures via experimental and simulated lateral force microscopy (LFM). A novel coarse-grain tube model is developed, utilizing a molecular dynamics (MD) framework with a Hertzian soft contact potential (such that F ∼ δ^3/2) to allow the efficient dynamic simulation of 3D arrays consisting of hundreds of tubes at micrometer length scales. By quantitatively comparing experimental LFM and computational results, the coupling between geometry (tube spacing and swelling) and material properties (intrinsic stiffness) results in a transition from bending dominated deformation to bending combined with inter-tube contact, independent of material adhesion assumptions. Variation of tube spacing (and thus control of contact) can be used to exploit the normal and lateral resistance of the tube arrays as a function of pH (2.0/5.5), beyond the effect of areal tube density, with increased resistances (potential mutability) up to a factor of ∼60. This study provides a novel modeling platform to assess and design dynamic polyelectrolyte-based substrates/coatings with tailorable stimulus-responsive surface friction. Our results show that micro-geometry can be used alongside stimulus-responsive material changes to amplify and systematically tune mutability.

Chemical physics of electroactive materials: concluding remarks

It is an honour to be charged with providing the concluding remarks for a Faraday Discussion. As many have remarked before, it is nonetheless a prodigious task, and what follows is necessarily a personal, and probably perverse, view of a watershed event in the Chemical Physics of Electroactive materials. The spirit of the conference was captured in a single sentence during the meeting itself."It is the nexus between rheology, electrochemistry, colloid science and energy storage". The current scientific climate is increasingly dominated by a limited number of global challenges, and there is thus a tendency for research to resemble a football match played by 6 year olds, where everyone on the field chases the (funding) ball instead of playing to their "discipline". It is thus reassuring to see how the application of rigorous chemical physics is leading to ingenious new solutions for both energy storage and harvesting, via, for example, nanoactuation, electrowetting, ionic materials and nanoplasmonics. In fact, the same language of chemical physics allows seamless transition between applications as diverse as mechano-electric energy generation, active moisture transport and plasmonic shutters - even the origins of life were addressed in the context of electro-autocatalysis!

Micrometre and nanometre scale patterning of binary polymer brushes, supported lipid bilayers and proteins

Binary polymer brush patterns were fabricated via photodeprotection of an aminosilane with a photo-cleavable nitrophenyl protecting group. UV exposure of the silane film through a mask yields micrometre-scale amine-terminated regions that can be derivatised to incorporate a bromine initiator to facilitate polymer brush growth via atom transfer radical polymerisation (ATRP). Atomic force microscopy (AFM) and imaging secondary ion mass spectrometry (SIMS) confirm that relatively thick brushes can be grown with high spatial confinement. Nanometre-scale patterns were formed by using a Lloyd's mirror interferometer to expose the nitrophenyl-protected aminosilane film. In exposed regions, protein-resistant poly(oligo(ethylene glycol)methyl ether methacrylate) (POEGMEMA) brushes were grown by ATRP and used to define channels as narrow as 141 nm into which proteins could be adsorbed. The contrast in the pattern can be inverted by (i) a simple blocking reaction after UV exposure, (ii) a second deprotection step to expose previously intact protecting groups, and (iii) subsequent brush growth via surface ATRP. Alternatively, two-component brush patterns can be formed. Exposure of a nitrophenyl-protected aminosilane layer either through a mask or to an interferogram, enables growth of an initial POEGMEMA brush. Subsequent UV exposure of the previously intact regions allows attachment of ATRP initiator sites and growth of a second poly(cysteine methacrylate) (PCysMA) brush within photolithographically-defined micrometre or nanometre scale regions. POEGMEMA brushes resist deposition of liposomes, but fluorescence recovery after photobleaching (FRAP) studies confirm that liposomes readily rupture on PCysMA "corrals" defined within POEGMEMA "walls". This leads to the formation of highly mobile supported lipid bilayers that exhibit similar diffusion coefficients to lipid bilayers formed on surfaces such as glass.

A facile route towards PDMAEMA homopolymer amphiphiles

Well-defined poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) has been modified at low degrees of quaternization of the tertiary amine groups using alkyl halides with long alkyl chains as the quaternization agents. The resulting PDMAEMA-co-PQDMAEMA copolymers were studied in aqueous solution using potentiometric titrations, turbidimetry, surface tensiometry, dynamic light scattering and zeta potential measurements. An increase of the hydrophilicity of the precursor polymer, leading to an increase or even elimination of the lower critical solution temperature (LCST) for the quaternized copolymers was found; this extended the temperature range of the stable polymer solution. At the same time, it was shown that the hydrophobic character of the polymer increases upon quaternization, leading to higher surface activity compared to the precursor PDMAEMA homopolymer, and, thus, to more effective polymeric surfactants. This contradiction in the copolymer behavior was attributed to the interplay between the polymer self-assembly in the aqueous medium and the polymer adsorption at the air/water interface, which dominate the cloud point and the surface properties, respectively.

Lubrication with Soft and Hard Two-Dimensional Colloidal Arrays

Normal and friction forces between immobilized two-dimensional (2D) homogeneous non-close-packed colloidal arrays made of different particles are compared in aqueous media. Soft pH-responsive (microgels) and nonresponsive hard (silica) particles of different sizes were used to create the 2D arrays. The results show that the friction of soft responsive structured layers can be successfully modulated by varying the pH, with a friction coefficient varying by nearly 3 orders of magnitude (10^-2 to 1). This important change in lubricating properties is mainly correlated with the particle swelling behavior, i.e., the friction coefficient decreasing exponentially with an increase in the swelling ratio regardless of the size, surface coverage, and degree of ionization of the particles. In addition, the robustly attached microgel particles were able to sustain high pressure (up to 200 atm) without significant surface damage. The 2D arrays of nonresponsive hard particles also gave rise to a very low friction coefficient (μ ≈ 10^-3) under similar experimental conditions and could sustain a larger pressure without damage (≤600 atm). The low friction dissipation observed between the hard arrays resulted from a rolling mechanism. Even though rolling requires nonimmobilized particles on the substrates, the results show the importance of attaching a certain proportion of particles on the surfaces to reduce friction.

Nanotribological properties of nanostructured poly(cysteine methacrylate) brushes

The nanomechanical properties of zwitterionic poly(cysteine methacrylate) (PCysMA) brushes grown from planar surfaces by atom transfer radical polymerisation have been characterised by friction force microscopy (FFM). FFM provides quantitative insights into polymer structure-property relationships and in particular illuminates the dependence of brush swelling on chain packing in nanostructured materials. In ethanol, which is a poor solvent for PCysMA, a linear friction-load relationship is observed, indicating that energy dissipation occurs primarily through ploughing. In contrast, in a good solvent for PCysMA such as water, a non-linear friction-load relationship is observed that can be fitted by Derjaguin-Muller-Toporov (DMT) mechanics, suggesting that the relatively small modulus of the swollen polymer leads to a large contact area and consequently a significant shear contribution to energy dissipation. The brush grafting density was varied by using UV photolysis of C-Br bonds at 244 nm to dehalogenate the surface in a controlled fashion. The surface shear strength increases initially as the brush grafting density is reduced, but then decreases for UV doses greater than 0.5 J cm^-2, reaching a limiting value when the brush thickness is ca. 50% that of a brush monolayer. Below this critical grafting density, a collapsed brush layer is obtained. For nm-scale gradient brush structures formed via interferometric lithography, the mean width increases as the period is increased, and the lateral mobility of brushes in these regions is reduced, leading to an increase in brush height as the grafted chains become progressively more extended. For a width of 260 nm, the mean brush height in water and ethanol is close to the thickness of a dense unpatterned brush monolayer synthesised under identical conditions. Both the surface shear stress measured for PCysMA brushes under water and the coefficient of friction measured in ethanol are closely correlated to the feature height, and hence to the chain conformation.

Nanotribological Investigation of Polymer Brushes with Lithographically Defined and Systematically Varying Grafting Densities

Following controlled photodeprotection of a 2-nitrophenylpropyloxycarbonyl-protected (aminopropyl)triethoxysilane (NPPOC-APTES) film and subsequent derivatization with a bromoester-based initiator, poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes with various grafting densities were grown from planar silicon substrates using atom transfer radical polymerization (ATRP). The grafting density correlated closely with the extent of deprotection of the NPPOC-APTES. The coefficient of friction for such PMPC brushes was measured by friction force microscopy in water and found to be inversely proportional to the grafting density due to the osmotic pressure that resists deformation. Deprotection of NPPOC-APTES via near-field photolithography using a range of writing rates enabled the fabrication of neighboring nanoscopic polymeric structures with dimensions ranging from 100 to 1000 nm. Slow writing rates enable complete deprotection to occur; hence, polymer brushes are formed with comparable thicknesses to macroscopic brushes grown under the same conditions. However, the extent of deprotection is reduced at higher writing rates, resulting in the concomitant reduction of the brush thickness. The coefficient of friction for such polymer brushes varied smoothly with brush height, with lower coefficients being obtained at slower writing rate (increasing initiator density) because the solvated brush layer confers greater lubricity. However, when ultrasharp probes were used for nanotribological measurements, the coefficient of friction increased with brush thickness. Under such conditions, the radius of curvature of the tip is comparable to the mean spacing between brush chains, allowing the probe to penetrate the brush layer leading to a relatively large contact area.

A temperature, pH and sugar triple-stimuli-responsive nanofluidic diode

In this article, we have demonstrated for the first time a triple stimuli-responsive nanofluidic diode that can rectify ionic current under multiple external stimuli including temperature, pH, and sugar. This diode was fabricated by immobilizing poly[2-(dimethylamino)ethyl methacrylate]-co-[4-vinyl phenylboronic acid] (P(DMAEMA-co-VPBA)) onto the wall of a single glass conical nanopore channel via surface-initiator atom transfer radical polymerization (SI-ATRP). The copolymer brushes contain functional groups sensitive to pH, temperature and sugar that can induce charge and configuration change to affect the status of the pore wall. The experimental results confirmed that the P(DMAEMA-co-VPBA) brush modified nanochannel regulated the ionic current rectification successfully under three different external stimuli. This biomimetically inspired research simulates the complex biological multi-functions of ion channels and promotes the development of "smart" biomimetic nanochannel systems for actuating and sensing applications.

Temperature-/CO2-dual-responsiveness of a zwitterionic “schizophrenic” copolymer

A zwitterionic “schizophrenic” copolymer with dual-responsiveness to temperature and carbon dioxide self-assembles to undergo a reversible phase transition in a weakly alkaline borate buffer solution.

Synergies in lubrication

In living organisms the aqueous medium is used for providing low friction forces. This is achieved by synergistic actions of different biomolecules that together accomplish a high load bearing capacity and sustain an easily sheared water layer.

Ionic Surfactant Binding to pH-Responsive Polyelectrolyte Brush-Grafted Nanoparticles in Suspension and on Charged Surfaces

The interactions between silica nanoparticles grafted with a brush of cationic poly(2-(dimethylamino) ethyl methacrylate) (SiO2-g-PDMAEMA) and anionic surfactant sodium dodecyl sulfate (SDS) is investigated by dynamic light scattering, electrophoretic mobility, quartz crystal microbalance with dissipation, ellipsometry, and atomic force microscopy. SiO2-g-PDMAEMA exhibits pH-dependent charge and size properties which enable the SDS binding to be probed over a range of electrostatic conditions and brush conformations. SDS monomers bind irreversibly to SiO2-g-PDMAEMA at low surfactant concentrations (∼10(-4) M) while exhibiting a pH-dependent threshold above which cooperative, partially reversible SDS binding occurs. At pH 5, SDS binding induces collapse of the highly charged and swollen brush as observed in the bulk by DLS and on surfaces by QCM-D. Similar experiments at pH 9 suggest that SDS binds to the periphery of the weakly charged and deswollen brush and produces SiO2-g-PDMAEMA/SDS complexes with a net negative charge. SiO2-g-PDMAEMA brush collapse and charge neutralization is further confirmed by colloidal probe AFM measurements, where reduced electrosteric repulsions and bridging adhesion are attributed to effects of the bound SDS. Additionally, sequential adsorption schemes with SDS and SiO2-g-PDMAEMA are used to enhance deposition relative to SiO2-g-PDMAEMA direct adsorption on silica. This work shows that the polyelectrolyte brush configuration responds in a more dramatic fashion to SDS than to pH-induced changes in ionization, and this can be exploited to manipulate the structure of adsorbed layers and the corresponding forces of compression and friction between opposing surfaces.

Stretching of collapsed polymers causes an enhanced dissipative response of PNIPAM brushes near their LCST

Poly(N-isopropyl acrylamide) (PNIPAM) is a stimulus-responsive polymer that can switch in water from an expanded state below the lower critical solution temperature (LCST) of 32 °C to a globular state above the LCST. It was recently shown that, as a consequence of this conformational transition, the interfacial and (tribo-)mechanical properties of polymeric systems composed of PNIPAM can be switched between two states. Here we show that the tribo-mechanical properties of a particular type of PNIPAM system, which is the PNIPAM brush, do not just change between two states, but instead evolve continuously and non-monotonically upon increasing/decreasing temperature. To do so, we present atomic force microscopy experiments in which we measure the adhesion hysteresis and the friction upon bringing a gold colloid in relative motion with PNIPAM brushes at temperatures around the LCST. Both the friction and the adhesion hysteresis display a pronounced maximum exactly at the LCST. The force vs. distance data captured at these temperatures show a long-ranged adhesive interaction upon moving the colloid away from the original point of contact, which indicates that during this retraction the partly collapsed polymers in the brush become strongly stretched.

Weighing the surface charge of an ionic liquid

Electrochemical quartz crystal microbalance has been used to measure changes in the composition of the capacitive electrical double layer for 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate, an ionic liquid, in contact with a gold electrode surface as a function of potential. The mass difference between the cation and anion means that the technique can effectively "weigh" the surface charge accurately with high temporal resolution. This reveals quantitatively how changing the potential alters the ratio of cations and anions associated with the electrode surface, and thus the charge per unit area, as well as the kinetics associated with these interfacial processes. The measurements reveal that it is diffusion of co-ions into the interfacial region rather than expulsion of counterions that controls the relaxation. The measured potential dependent double layer capacitance experimentally validates recent theoretical predictions for counterion overscreening (low potentials) and crowding (high potentials) at electrode surfaces. This new capacity to quantitatively measure ion composition is critical for ionic liquid applications ranging from batteries, capacitors and electrodeposition through to boundary layer structure in tribology, and more broadly provides new insight into interfacial processes in concentrated electrolyte solutions.

Nanomechanical properties of human skin and introduction of a novel hair indenter

The mechanical resistance of the stratum corneum, the outermost layer of skin, to deformation has been evaluated at different length scales using Atomic Force Microscopy. Nanomechanical surface mapping was first conducted using a sharp silicon tip and revealed that Young׳s modulus of the stratum corneum varied over the surface with a mean value of about 0.4GPa. Force indentation measurements showed permanent deformation of the skin surface only at high applied loads (above 4µN). The latter effect was further demonstrated using nanomechanical imaging in which the obtained depth profiles clearly illustrate the effects of increased normal force on the elastic/plastic surface deformation. Force measurements utilizing the single hair fiber probe supported the nanoindentation results of the stratum corneum being highly elastic at the nanoscale, but revealed that the lateral scale of the deformation determines the effective elastic modulus.This result resolves the fact that the reported values in the literature vary greatly and will help to understand the biophysics of the interaction of razor cut hairs that curl back during growth and interact with the skin.

Anion-specific effects on the behavior of pH-sensitive polybasic brushes

The anion-specific solvation and conformational behavior of weakly basic poly(2-dimethylamino)ethyl methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (poly(DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly(DPA)) brushes, with correspondingly increasing inherent hydrophobicity, have been investigated using in situ ellipsometric and quartz crystal microbalance with dissipation (QCM-D) measurements. In the osmotic brush regime, as the initial low concentration of salt is increased, the brushes osmotically swell by the uptake of solvent as they become charged and the attractive hydrophobic inter- and intrachain interactions are overcome. With increased ionic strength, the brushes move into the salted brush regime where they desolvate and collapse as their electrostatic charge is screened. Here, as the brushes collapse, they transition to more uniform and rigid conformations, which dissipate less energy, than similarly solvated brushes at lower ionic strength. Significantly, in these distinct regimes brush behavior is not only ionic strength dependent but is also influenced by the nature of the added salt based on its position in the well-known Hofmeister or lyotropic series, with potassium acetate, nitrate, and thiocyanate investigated. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Increasing thiocyanate concentration causes the brushes to collapse at lower ionic strength than for the other two anions. This study of weak polybasic brushes demonstrates the importance of all ion, solvent, and polymer interactions.

Tethered poly(2-isopropyl-2-oxazoline) chains: temperature effects on layer structure and interactions probed by AFM experiments and modeling

Thermoresponsive polymer layers on silica surfaces have been obtained by utilizing electrostatically driven adsorption of a cationic-nonionic diblock copolymer. The cationic block provides strong anchoring to the surface for the nonionic block of poly(2-isopropyl-2-oxazoline), referred to as PIPOZ. The PIPOZ chain interacts favorably with water at low temperatures, but above 46 °C aqueous solutions of PIPOZ phase separate as water becomes a poor solvent for the polymer. We explore how a change in solvent condition affects interactions between such adsorbed layers and report temperature effects on both normal forces and friction forces. To gain further insight, we utilize self-consistent lattice mean-field theory to follow how changes in temperature affect the polymer segment density distributions and to calculate surface force curves. We find that with worsening of the solvent condition an attraction develops between the adsorbed PIPOZ layers, and this observation is in good agreement with predictions of the mean-field theory. The modeling also demonstrates that the segment density profile and the degree of chain interpenetration under a given load between two PIPOZ-coated surfaces rise significantly with increasing temperature.

Hydrophobic effects within the dynamic pH-response of polybasic tertiary amine methacrylate brushes

Monomer hydrophobicity dominates the kinetics of the pH-response of tertiary amine methacrylate brushes as determined by in situ ellipsometry and QCM-D kinetic and equilibrium measurements.

Switchable friction using contacts of stimulus-responsive and nonresponding swollen polymer brushes

Stimulus-responsive (SR), solvated polymers can switch between an expanded state and a collapsed state via external stimuli. Using molecular dynamics simulations, I show that such SR polymers can be employed to control the frictional response between two opposing polymer brushes in relative sliding motion. By using a brush composed of SR polymers in contact with a nonresponding solvated polymer brush, the presence of capillaries and the overlap between molecules of the opposing brushes can be switched. When both brushes are solvated, a capillary is formed and polymers of the opposing brushes interdigitate. Interdigitation dominates friction upon shearing flat brush-bearing surfaces, while the breaking and formation of capillaries dominate friction in the low-velocity limit between rough brush-bearing surfaces. Thus, when either rough or flat polymer-bearing surfaces are sheared, friction between two swollen brushes can be high. In contrast, when the SR brush is collapsed, the solvent absorbs only in the brush that does not respond to the external stimulus. The latter circumvents the presence of capillaries and interdigitation of the brushes, which results in a low friction force upon shearing.

Collapse from the top: brushes of poly(N-isopropylacrylamide) in co-nonsolvent mixtures

Using a combination of ellipsometry and friction force microscopy, we study the reversible swelling, collapse and variation in friction properties of covalently bound poly(N-isopropylacrylamide) (PNIPAM) layers on silicon with different grafting densities in response to exposure to good solvents and co-nonsolvent mixtures. Changes in the thickness and segment density distribution of grafted films are investigated by in situ ellipsometry. Based on quantitative modelling of the ellipsometry spectra, we postulate a structural model, which assumes that collapse takes place in the contacting layer between the brush and the co-nonsolvent and the top-collapsed brushes remain hydrated in the film interior. Using the structural model derived from ellipsometry spectra, we analyse the AFM based friction force microscopy data, which were obtained by silica colloidal probes. Results show a large increase of the friction coefficient of PNIPAM grafts when the grafts swollen by water are brought in contact with co-nonsolvents. For instance, the value of the friction coefficient for a medium density brush in water is four times lower than the value observed in a water-methanol (50% v/v) mixture. This increase of friction is accompanied by an increase in adherence between the PNIPAM chains and the silica colloidal probes, and is a result of chain collapse in the graft when contacted by a co-nonsolvent mixture in agreement with the model postulated on the basis of ellipsometric characterisation. The kinetic behaviour of the collapse is assessed by measuring the temporal variation of friction in situ as a function of elapsed time following contact with the co-nonsolvent as a function of graft density. In conclusion, the effect of co-nonsolvency influenced both the thickness of the PNIPAM brushes and the tribological behavior of the brush surfaces.

Critical salt effects in the swelling behavior of a weak polybasic brush

The swelling behavior of poly(2-(diethylamino)ethyl methacrylate) (PDEA) brushes in response to changes in solution pH and ionic strength has been investigated. The brushes were synthesized by ARGET ATRP methodology at the silica-aqueous solution interface via two different surface-bound initiator approaches: electrostatically adsorbed cationic macroinitiator and covalently anchored silane-based ATRP initiator moieties. The pH-response of these brushes is studied as a function of the solvated brush thickness in a constant flow regime that elucidates the intrinsic behavior of polymer brushes. In situ ellipsometry equilibrium measurements show the pH-induced brush swelling and collapse transitions are hysteretic in nature. Furthermore, high temporal resolution kinetic studies demonstrate that protonation and solvent ingress during swelling occur much faster than the brush charge neutralization and solvent expulsion during collapse. This hysteresis is attributed to the formation of a dense outer region or skin during collapse that retards solvent egress. Moreover, at a constant pH below its pKa, the PDEA brush exhibited a critical conformational change in the range 0.5-1 mM electrolyte, a range much narrower than predicted by the theory of the osmotic brush regime. This behavior is attributed to the hydrophobicity of the collapsed brush. The swelling and collapse kinetics for this salt-induced transition are nearly identical. This is in contrast to the asymmetry in the rate of the pH-induced response, suggesting an alternative mechanism for the two processes dependent on the nature of the environmental trigger.

Organophosphonic acids as viable linkers for the covalent attachment of polyelectrolyte brushes on silica and mica surfaces

We report the first successful preparation of polyelectrolyte brushes using an ATRP initiator that was covalently grafted to silica and mica substrates via an organophosphonic acid.

Thermo-sensitive zwitterionic block copolymers via ATRP

Novel thermo-sensitive zwitterionic block copolymers synthesized by ATRP exhibited distinct thermo-sensitivity and excellent antifouling property.