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.

Measuring Rolling Friction at the Nanoscale.

Colloidal probe microscopy, a technique whereby a microparticle is affixed at the end of an atomic force microscopy (AFM) cantilever, plays a pivotal role in enabling the measurement of friction at the nanoscale and is of high relevance for applications and fundamental studies alike. However, in conventional experiments, the probe particle is immobilized onto the cantilever, thereby restricting its relative motion against a countersurface to pure sliding. Nonetheless, under many conditions of interest, such as during the processing of particle-based materials, particles are free to roll and slide past each other, calling for the development of techniques capable of measuring rolling friction alongside sliding friction. Here, we present a new methodology to measure lateral forces during rolling contacts based on the adaptation of colloidal probe microscopy. Using two-photon polymerization direct laser writing, we microfabricate holders that can capture microparticles, but allow for their free rotation. Once attached to an AFM cantilever, upon lateral scanning, the holders enable both sliding and rolling contacts between the captured particles and the substrate, depending on the interactions, while simultaneously giving access to normal and lateral force signals. Crucially, by producing particles with optically heterogeneous surfaces, we can accurately detect the presence of rotation during scanning. After introducing the workflow for the fabrication and use of the probes, we provide details on their calibration, investigate the effect of the materials used to fabricate them, and report data on rolling friction as a function of the surface roughness of the probe particles. We firmly believe that our methodology opens up new avenues for the characterization of rolling contacts at the nanoscale, aimed, for instance, at engineering particle surface properties and characterizing functional coatings in terms of their rolling friction.

Dispersity within Brushes Plays a Major Role in Determining Their Interfacial Properties: The Case of Oligoxazoline-Based Graft Polymers.

Many synthetic polymers used to form polymer-brush films feature a main backbone with functional, oligomeric side chains. While the structure of such graft polymers mimics biomacromolecules to an extent, it lacks the monodispersity and structural purity present in nature. Here we demonstrate that side-chain heterogeneity within graft polymers significantly influences hydration and the occurrence of hydrophobic interactions in the subsequently formed brushes and consequently impacts fundamental interfacial properties. This is demonstrated for the case of poly(methacrylate)s (PMAs) presenting oligomeric side chains of different length (n) and dispersity. A precise tuning of brush structure was achieved by first synthesizing oligo(2-ethyl-2-oxazoline) methacrylates (OEOXMAs) by cationic ring-opening polymerization (CROP), subsequently purifying them into discrete macromonomers with distinct values of n by column chromatography, and finally obtaining poly[oligo(2-ethyl-2-oxazoline) methacrylate]s (POEOXMAs) by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Assembly of POEOXMA on Au surfaces yielded graft polymer brushes with different side-chain dispersities and lengths, whose properties were thoroughly investigated by a combination of variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCMD), and atomic force microscopy (AFM) methods. Side-chain dispersity, or dispersity within brushes, leads to assemblies that are more hydrated, less adhesive, and more lubricious and biopassive compared to analogous films obtained from graft polymers characterized by a homogeneous structure.

Magnetic propulsion of colloidal microrollers controlled by electrically modulated friction.

Precise control over the motion of magnetically responsive particles in fluidic chambers is important for probing and manipulating tasks in prospective microrobotic and bio-analytical platforms. We have previously exploited such colloids as shuttles for the microscale manipulation of objects. Here, we study the rolling motion of magnetically driven Janus colloids on solid substrates under the influence of an orthogonal external electric field. Electrically induced attractive interactions were used to tune the load on the Janus colloid and thereby the friction with the underlying substrate, leading to control over the forward velocity of the particle. Our experimental data suggest that the frictional coupling required to achieve translation, transitions from a hydrodynamic regime to one of mixed contact coupling with increasing load force. Based on this insight, we show that our colloidal microrobots can probe the local friction coefficient of various solid surfaces, which makes them potentially useful as tribological microsensors. Lastly, we precisely manipulate porous cargos using our colloidal rollers, a feat that holds promise for bio-analytical applications.

Roughness-dependent tribology effects on discontinuous shear thickening.

Surface roughness affects many properties of colloids, from depletion and capillary interactions to their dispersibility and use as emulsion stabilizers. It also impacts particle-particle frictional contacts, which have recently emerged as being responsible for the discontinuous shear thickening (DST) of dense suspensions. Tribological properties of these contacts have been rarely experimentally accessed, especially for nonspherical particles. Here, we systematically tackle the effect of nanoscale surface roughness by producing a library of all-silica, raspberry-like colloids and linking their rheology to their tribology. Rougher surfaces lead to a significant anticipation of DST onset, in terms of both shear rate and solid loading. Strikingly, they also eliminate continuous thickening. DST is here due to the interlocking of asperities, which we have identified as "stick-slip" frictional contacts by measuring the sliding of the same particles via lateral force microscopy (LFM). Direct measurements of particle-particle friction therefore highlight the value of an engineering-tribology approach to tuning the thickening of suspensions.

Load and Velocity Dependence of Friction Mediated by Dynamics of Interfacial Contacts.

Studying the frictional properties of interfaces with dynamic chemical bonds advances understanding of the mechanism underlying rate and state laws, and offers new pathways for the rational control of frictional response. In this work, we revisit the load dependence of interfacial chemical-bond-induced (ICBI) friction experimentally and find that the velocity dependence of friction can be reversed by changing the normal load. We propose a theoretical model, whose analytical solution allows us to interpret the experimental data on timescales and length scales that are relevant to experimental conditions. Our work provides a promising avenue for exploring the dynamics of ICBI friction.

Brushes, Graft Copolymers, or Bottlebrushes? The Effect of Polymer Architecture on the Nanotribological Properties of Grafted-from Assemblies.

Surface-grafted polyzwitterions (PZW) have gained a foothold in the design of synthetic materials that closely mimic the lubricious properties of articular joints in mammals. Besides their chemical composition, the architecture of PZW brushes strongly determines their morphological, nanomechanical, and nanotribological characteristics. This emerges while comparing the properties of linear poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes with those displayed by graft copolymer and bottlebrush brushes, either featuring a low or a high content of PMPC side chains. Surface-initiated atom transfer radical polymerization (SI-ATRP) enabled the synthesis of different branched-brush architectures from multifunctional macroinitiators via multiple grafting steps, and allowed us to modulate their structure by tuning the polymerization conditions. At relatively low grafting densities (σ), long PMPC side segments extend at the interface of bottlebrush and graft copolymer brushes, providing both morphology and lubrication properties comparable to those shown by loosely grafted, linear PMPC brushes. When σ > 0.1 chains nm-2 the effect of the branched-brush architecture on the nanotribological properties of the films became evident. Linear PMPC brushes showed the lowest friction among the studied brush structures, with a coefficient of friction (µ) that reached 1 × 10-4, as measured by atomic force microscopy (AFM). Bottlebrush brushes showed comparatively higher friction, although the high content of hydrophilic PMPC side chains along their backbone substantially improved lubrication compared to that displayed by the more sparsely substituted graft copolymer brushes.

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.

Understanding Complex Tribofilms by Means of H3BO3-B2O3 Model Glasses.

The discovery of the spontaneous reaction of boric oxides with moisture in the air to form lubricious H3BO3 films has led to great interest in the tribology of boron compounds in general. Despite this, a study of the growth kinetics of H3BO3 on a B2O3 substrate under controlled relative humidity (RH) has not yet been reported in the literature. Here, we describe the tribological properties of H3BO3-B2O3 glass systems after aging under controlled RH over different lengths of time. A series of tribological tests has been performed applying a normal load of 15 N, at both room temperature and 100 °C in YUBASE 4 oil. In addition, the cause of H3BO3 film failure under high-pressure and high-temperature conditions has been studied to find out whether the temperature, the tribostress, or both influence the removal of the lubricious film from the contact points. The following techniques were exploited: confocal Raman spectroscopy to characterize the structure and chemical nature of the glass systems, environmental scanning electron microscopy to examine the morphology of the H3BO3 films developed, atomic force microscopy to monitor changes in roughness as a consequence of the air exposure, focused-ion-beam scanning electron microscopy to measure the average thickness of the H3BO3 films grown over various times on B2O3 glass substrates and to reveal the morphology of the sample in the vertical section, tribological tests to shed light on the system's lubricating properties, and finally small-area X-ray photoelectron spectroscopy to investigate the composition of the transfer film formed on the steel ball while tribotesting.

Hairy and Slippery Polyoxazoline-Based Copolymers on Model and Cartilage Surfaces.

Comb-like polymers presenting a hydroxybenzaldehyde (HBA)-functionalized poly(glutamic acid) (PGA) backbone and poly(2-methyl-2-oxazoline) (PMOXA) side chains chemisorb on aminolized substrates, including cartilage surfaces, forming layers that reduce protein contamination and provide lubrication. The structure, physicochemical, biopassive, and tribological properties of PGA-PMOXA-HBA films are finely determined by the copolymer architecture, its reactivity toward the surface, i.e. PMOXA side-chain crowding and HBA density, and by the copolymer solution concentration during assembly. Highly reactive species with low PMOXA content form inhomogeneous layers due to the limited possibility of surface rearrangements by strongly anchored copolymers, just partially protecting the functionalized surface from protein contamination and providing a relatively weak lubrication on cartilage. Biopassivity and lubrication can be improved by increasing copolymer concentration during assembly, leading to a progressive saturation of surface defects across the films. In a different way, less reactive copolymers presenting high PMOXA side-chain densities form uniform, biopassive, and lubricious films, both on model aminolized silicon oxide surfaces, as well as on cartilage substrates. When assembled at low concentrations these copolymers adopt a "lying down" conformation, i.e. adhering via their backbones onto the substrates, while at high concentrations they undergo a conformational transition, assuming a more densely packed, "standing up" structure, where they stretch perpendicularly from the substrate. This specific arrangement reduces protein contamination and improves lubrication both on model as well as on cartilage surfaces.

Chemical Design of Non-Ionic Polymer Brushes as Biointerfaces: Poly(2-oxazine)s Outperform Both Poly(2-oxazoline)s and PEG.

The era of poly(ethylene glycol) (PEG) brushes as a universal panacea for preventing non-specific protein adsorption and providing lubrication to surfaces is coming to an end. In the functionalization of medical devices and implants, in addition to preventing non-specific protein adsorption and cell adhesion, polymer-brush formulations are often required to generate highly lubricious films. Poly(2-alkyl-2-oxazoline) (PAOXA) brushes meet these requirements, and depending on their side-group composition, they can form films that match, and in some cases surpass, the bioinert and lubricious properties of PEG analogues. Poly(2-methyl-2-oxazine) (PMOZI) provides an additional enhancement of brush hydration and main-chain flexibility, leading to complete bioinertness and a further reduction in friction. These data redefine the combination of structural parameters necessary to design polymer-brush-based biointerfaces, identifying a novel, superior polymer formulation.

Loops and Cycles at Surfaces: The Unique Properties of Topological Polymer Brushes.

Grafting synthetic polymers to inorganic and organic surfaces to yield polymer "brushes" has represented a revolution in many fields of materials science. Polymer brushes provide colloidal stabilization to nanoparticles (NPs), prevent and/or regulate the adsorption of proteins on biomaterials, and significantly reduce friction when applied to two surfaces sheared against each other. Can the performance of polymer brushes as steric stabilizers and boundary lubricants be improved? The answer to this question encompasses the application of polymer grafts presenting different chain topologies, beyond linearity. In particular, grafted polymers forming loops and cycles at the surface have been recently demonstrated to enable the modulation of interfacial physicochemical properties, including nanomechanical and nanotribological, to an extent that is difficultly addressed by using their linear counterparts. Loop and cyclic polymer brushes provide enhanced steric stabilization to surfaces, increase their biopassivity and show superlubricious behavior. Their distinctive structure, the methods applied to fabricate them and their application in several technologically relevant fields of materials science are reviewed in this contribution.

Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces.

The nanotribological properties of hydrophilic polymer brushes are conveniently analyzed by lateral force microscopy (LFM). However, the measurement of friction for highly swollen and relatively thick polymer brushes can be strongly affected by the tendency of the compliant brush to be laterally deformed by the shearing probe. This phenomenon induces a "tilting" in the recorded friction loops, which is generated by the lateral bending and stretching of the grafts. In this study we highlight how the brush lateral deformation mainly affects the friction measurements of swollen PNIPAM brushes (below LCST) when relatively short scanning distances are applied. Under these conditions, the energy dissipation recorded by LFM is almost uniquely determined by stretching and bending of the compliant brush back and forth along the scanning direction, and it is not correlated to dynamic friction between two sliding surfaces. In contrast, when the scanning distance applied during LFM is relevantly longer than the brush lateral deformation, sliding of the probe on the brush interface becomes dominant, and a correct measurement of dynamic friction can be accomplished. By increasing the temperature above the LCST, the PNIPAM brushes undergo dehydration and assume a collapsed morphology, thereby hindering their lateral deformation by scanning probe. Hence, at 40 °C in water the recorded friction loops do not show any tilting and LFM accurately describes the dynamic friction between the probe and the polymer surface.

Next-Generation Polymer Shells for Inorganic Nanoparticles are Highly Compact, Ultra-Dense, and Long-Lasting Cyclic Brushes.

Cyclic poly-2-ethyl-2-oxazoline (PEOXA) ligands for superparamagnetic Fe3 O4 nanoparticles (NPs) generate ultra-dense and highly compact shells, providing enhanced colloidal stability and bio-inertness in physiological media. When linear brush shells fail in providing colloidal stabilization to NPs, the cyclic ones assure long lasting dispersions. While the thermally induced dehydration of linear PEOXA shells cause irreversible aggregation of the NPs, the collapse and subsequent rehydration of similarly grafted cyclic brushes allow the full recovery of individually dispersed NPs. Although linear ligands are densely grafted onto Fe3 O4 cores, a small plasma protein such as bovine serum albumin (BSA) still physisorbs within their shells. In contrast, the impenetrable entropic shield provided by cyclic brushes efficiently prevents nonspecific interaction with proteins.

Crosslinking Polymer Brushes with Ethylene Glycol-Containing Segments: Influence on Physicochemical and Antifouling Properties.

The introduction of different types and concentrations of crosslinks within poly(hydroxyethyl methacrylate) (PHEMA) brushes influences their interfacial, physicochemical properties, ultimately governing their adsorption of proteins. PHEMA brushes and brush-hydrogels were synthesized by surface-initiated, atom-transfer radical polymerization (SI-ATRP) from HEMA, with and without the addition of di(ethylene glycol) dimethacrylate (DEGDMA) or tetra(ethylene glycol) dimethacrylate (TEGDMA) as crosslinkers. Linear (pure PHEMA) brushes show high hydration and low modulus and additionally provide an efficient barrier against nonspecific protein adsorption. In contrast, brush-hydrogels are stiffer and less hydrated, and the presence of crosslinks affects the entropy-driven, conformational barrier that hinders the surface interaction of biomolecules with brushes. This leads to the physisorption of proteins at low concentrations of short crosslinks. At higher contents of DEGDMA or in the presence of longer TEGDMA-based crosslinks, brush-hydrogels recover their antifouling properties due to the increase in interfacial water association by the higher concentration of ethylene glycol (EG) units.

Topological Polymer Chemistry Enters Surface Science: Linear versus Cyclic Polymer Brushes.

The cyclic polymer topology strongly alters the interfacial, physico-chemical properties of polymer brushes, when compared to the linear counterparts. In this study, we especially concentrated on poly-2-ethyl-2-oxazoline (PEOXA) cyclic and linear grafts assembled on titanium oxide surfaces by the "grafting-to" technique. The smaller hydrodynamic radius of ring PEOXAs favors the formation of denser brushes with respect to linear analogs. Denser and more compact cyclic brushes generate a steric barrier that surpasses the typical entropic shield by a linear brush. This phenomenon, translates into an improved resistance towards biological contamination from different protein mixtures. Moreover, the enhancement of steric stabilization coupled to the intrinsic absence of chain ends by cyclic brushes, produce surfaces displaying a super-lubricating character when they are sheared against each other. All these topological effects pave the way for the application of cyclic brushes for surface functionalization, enabling the modulation of physico-chemical properties that could be just marginally tuned by applying linear grafts.

Effect of particle stiffness and surface properties on the non-linear viscoelasticity of dense microgel suspensions.

HYPOTHESIS: Particle surface chemistry and internal softness are two fundamental parameters in governing the mechanical properties of dense colloidal suspensions, dictating structure and flow, therefore of interest from materials fabrication to processing. EXPERIMENTS: Here, we modulate softness by tuning the crosslinker content of poly(N-isopropylacrylamide) microgels, and we adjust their surface properties by co-polymerization with polyethylene glycol chains, controlling adhesion, friction and fuzziness. We investigate the distinct effects of these parameters on the entire mechanical response from restructuring to complete fluidization of jammed samples at varying packing fractions under large-amplitude oscillatory shear experiments, and we complement rheological data with colloidal-probe atomic force microscopy to unravel variations in the particles' surface properties. FINDINGS: Our results indicate that surface properties play a fundamental role at smaller packings; decreasing adhesion and friction at contact causes the samples to yield and fluidify in a lower deformation range. Instead, increasing softness or fuzziness has a similar effect at ultra-high densities, making suspensions able to better adapt to the applied shear and reach complete fluidization over a larger deformation range. These findings shed new light on the single-particle parameters governing the mechanical response of dense suspensions subjected to deformation, offering synthetic approaches to design materials with tailored mechanical properties.

Modulating the conformation of microgels by complexation with inorganic nanoparticles.

HYPOTHESIS: The complexation of microgels with rigid nanoparticles is an effective way to impart novel properties and functions to the resulting hybrid particles for applications such as in optics, catalysis, or for the stabilization of foams/emulsions. The nanoparticles affect the conformation of the polymer network, both in bulk aqueous environments and when the microgels are adsorbed at a fluid interface, in a non-trivial manner by modulating the microgel size, stiffness and apparent contact angle. EXPERIMENTS: Here, we provide a detailed investigation, using light scattering, in-situ atomic force microscopy and nano-indentation experiments, of the interaction between poly(N-isopropylacrylamide) microgels and hydrophobized silica nanoparticles after mixing in aqueous suspension to shed light on the network reorganization upon nanoparticle incorporation. FINDINGS: The addition of nanoparticles decreases the microgels' bulk swelling and thermal response. When adsorbed at an oil-water interface, a higher ratio of nanoparticles influences the microgel's stiffness as well as their hydrophobic/hydrophilic character by increasing their effective contact angle, consequently modulating the monolayer response upon interfacial compression. Overall, these results provide fundamental understanding on the complex conformation of hybrid microgels in different environments and give inspiration to design new materials where the combination of a soft polymer network and nanoparticles might result in additional functionalities.

Study of adhesion and friction properties on a nanoparticle gradient surface: transition from JKR to DMT contact mechanics.

We have previously investigated the dependence of adhesion on nanometer-scale surface roughness by employing a roughness gradient. In this study, we correlate the obtained adhesion forces on nanometer-scale rough surfaces to their frictional properties. A roughness gradient with varying silica particle (diameter ≈ 12 nm) density was prepared, and adhesion and frictional forces were measured across the gradient surface in perfluorodecalin by means of atomic force microscopy with a polyethylene colloidal probe. Similarly to the pull-off measurements, the frictional forces initially showed a reduction with decreasing particle density and later an abrupt increase as the colloidal sphere began to touch the flat substrate beneath, at very low particle densities. The friction-load relation is found to depend on the real contact area (A(real)) between the colloid probe and the underlying particles. At high particle density, the colloidal sphere undergoes large deformations over several nanoparticles, and the contact adhesion (JKR type) dominates the frictional response. However, at low particle density (before the colloidal probe is in contact with the underlying surface), the colloidal sphere is suspended by a few particles only, resulting in local deformations of the colloid sphere, with the frictional response to the applied load being dominated by long-range, noncontact (DMT-type) interactions with the substrate beneath.