1,720,976 research outputs found
Controlled Crosslinking Is a Tool To Precisely Modulate the Nanomechanical and Nanotribological Properties of Polymer Brushes
No full textCovalent crosslinking of weak polyelectrolyte brushes widens the tuning potential for their swelling, nanomechanical, and nanotribological properties, which can be simultaneously adjusted by varying the crosslinker content and the pH of the surroundings. We demonstrate that this is especially valid for poly(hydroxyethyl methacrylate) (PHEMA) brushes and brush hydrogels, and their ionizable, succinate-modified derivatives (PHEMA-SA), covalently crosslinked with different amounts of di(ethylene glycol) dimethacrylate (DEGDMA) during surface-initiated atom transfer radical polymerization (SI-ATRP). Atomic force microscopy (AFM) methods highlight how pristine PHEMA films are stiff and display high coefficients of friction in water. Their succinate derivatives swell profusely in aqueous, media. Under acidic conditions they are neutral, compliant, and lubricious, with apparent Young's moduli (E*) lying between 10 and 30 kPa. Their contact mechanical behavior can be described by either the Johnson-Kendall-Roberts (JKR) or the Derjaguin-Muller-Toporov (DMT) model, depending on crosslinker content. In contrast, under basic conditions, brushes and brush hydrogels become charged, expand, and present a rigid, electrostatic barrier toward the AFM probe. Friction is extremely low at relatively low applied loads, whereas it increases at higher loads, to an extent that is regulated by the number of crosslinks within the films
Surface-grafted assemblies of cyclic polymers: Shifting between high friction and extreme lubricity
No full textThe interfacial physicochemical properties of “brushes” constituted by cyclic polymers strongly depend on their surface density, and they can be modulated within a broader range with respect to those displayed by linear brush analogues of identical composition. This is especially valid for the nanotribological characteristics of poly(2-ethyl-2-oxazoline) (PEOXA) brushes, assembled on titanium oxide surfaces by grafting-to technique. At low surface coverage, cyclic PEOXA (C-PEOXA) grafts flatten down towards the grafting surface and provide high friction when sheared against an identical countersurface. In contrast, densely grafted C-PEOXA assemblies stretch vertically forming a molecularly smooth surface that hinders interpenetration with the opposing brush, and dramatically reduces friction, greatly surpassing the lubricious properties typically attained by applying linear grafts.
Differently from their nanotribological properties, C-PEOXA brushes always show improved resistance towards the adsorption of proteins if compared to their linear counterparts, irrespective of their grafting density. The enhancement in biopassivity achieved by applying cyclic PEOXAs is ascribed to their intrinsic tendency to hinder protein penetration within a brush layer
Topological Polymer Chemistry Enters Surface Science: The Interfacial, Physico-Chemical Properties of Linear, Cyclic and Loops Brushes
No full textThe application of distinctive polymer topologies, beyond linearity, to yield cyclic and “loops”-forming surfacegrafted
polymer assemblies, enables a broad modulation of highly relevant, interfacial, physicochemical properties
that are difficult to be addressed by linear polymer brushes. On flat surfaces, the ultra-dense and highly compact
character of cyclic poly-2-ethyl-2-oxazoline (PEOXA) brushes provide enhanced steric stabilization of the interface,
improved biopassivity and highly lubricious behavior, if compared to linear brushes with similar molar mass. The
application of random PEOXA-based copolymers with a variable concentration of surface-reactive co-monomers
generates mixtures of grafted loops and “tails”. The precise tuning of the relative surface concentration of these two
polymer topologies allows shifting of the interfacial properties either towards the ones showed by linear brushes, or
near the characteristics of cyclic analogues. An increase in the loops fraction generates an increment of steric
repulsion, a concomitant decrease of friction, and an improvement of the antifouling character of the polymer films. All
these findings highlight how polymer topological effects, typically observed in bulk or in solution are amplified by
adding an additional boundary such as a grafting surface. Their precise tuning translates into materials with
unprecedented characteristics and extremely high applicability
Engineering Lubricious, Biopassive Polymer Brushes by Surface-Initiated, Controlled Radical Polymerization
No full textSurface-initiated controlled radical polymerization enables the fabrication of biopassive polymer brushes with interfacial, physicochemical properties that can be independently varied across a single substrate. Poly[(oligoethylene glycol) methacrylate] (POEGMA) brushes were synthesized by surface initiated atom transfer radical polymerization (SI-ATRP), locally varying the exposure of initiator-functionalized surfaces to the polymerization solution to yield POEGMA brush thickness gradients. A combination of variable-angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) demonstrated that brush swelling, grafting density, nano mechanical properties, and biopassivity towards protein adsorption all remained constant within a thickness range between 20 and 90 nm. However, the nanotribological properties of POEGMA brushes, investigated by lateral force microscopy (LFM), were found to vary progressively along the gradient, thinner brushes showing significantly lower friction than thicker and more viscoelastic grafts. The independent variation of lubricity across a biopassive brush gradient shows how SI-ATRP can be used to tailor surfaces destined for applications involving both contact with biological media and exposure to shear stresses, as is the case for tissue-replacement implants and scaffolds for tissue engineering
Stratified Polymer Grafts: Synthesis and Characterization of Layered ‘Brush’ and ‘Gel’ Structures
Full text linkAmplified Responsiveness of Multilayered Polymer Grafts: Synergy between Brushes and Hydrogels
No full textThe responsive properties of surface-grafted polymer films in aqueous media can be amplified by covalently layering thermosensitive brushes and hydrogels. This was demonstrated by synthesizing layers of linear poly(N-isopropylacrylamide) (PNIPAM) brushes, alternating with cross-linked, poly(hydroxyethyl)methacrylate (PHEMA) hydrogels via sequential surface-initiated atom-transfer radical polymerization (SI-ATRP) steps. Below the lower critical solution temperature (LCST) of PNIPAM, brush/hydrogel multilayered films swell similarly to linear PNIPAM homopolymer brushes, as measured by liquid ellipsometry. In contrast, above the LCST, the PHEMA hydrogel interlayer acts as stiffening element within the collapsed multilayered film, as monitored by atomic force microscopy (AFM) nanoindentation and lateral force microscopy (LFM). This translates into a 10-fold increase in Young's modulus by the collapsed, layered films compared to 2 4 PNIPAM homopolymer analogues. The (macro)molecular continuity between the brush main chains and hydrogel constituents thus enables a chemically robust layering to form graded, quasi-3D grafted polymer architectures, which display a concerted and amplified temperature-triggered transition
Crosslinking Polymer Brushes with Ethylene Glycol-Containing Segments: Influence on Physicochemical and Antifouling Properties
No full textThe 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
Fabrication and Interfacial Properties of Polymer Brush Gradients by Surface-Initiated Cu(0)-Mediated Controlled Radical Polymerization
No full textSurface-initiated Cu(0)-mediated controlled radical polymerization (SiCuCRP) can be successfully applied to fabricate poly[(oligoethylene glycol)methyl ether methacrylate] (POEGMA) brushes in one pot, presenting a grafting-density gradient across the surface. This is achieved by continuously varying the distance (d) between a copper plate, used as a source of Cu species, and the initiator-functionalized substrate. X-ray photoelectron spectroscopy (XPS) analysis of monolayers of Culselective ligands demonstrates that a higher concentration of activator species diffuses to the initiating substrate in areas closer to the copper plate, a progressive decrease in activator concentration being observed upon increasing the distance between the two surfaces. As confirmed by the SI-CuCRP kinetics measured at different positions along the gradient, radical-termination reactions between propagating chains limit the grafting density of POEGMA grafts where the diffusion of activators is favored (i.e., at d 0). This effect decreases with increasing d, ultimately yielding a gradual variation of POEGMA grafting density across the substrate. We have investigated the influence of grafting-density variation across the gradient on the swelling of POEGMA brushes as well as on their nanomechanical and nanotribological properties, measured by a combination of variable angle spectroscopic ellipsometry (VASE), colloidal-probe atomic force microscopy (CP-AFM), and lateral force microscopy (LFM). The results of these tests highlight how loosely grafted POEGMA chains incorporating a substantial amount of water can be significantly deformed by a shearing AFM probe, exhibit relatively high friction, and generate friction-vs-load (F-f-L) profiles that follow a sublinear trend described by a Johnson Kendall-Roberts (JKR) model-typical of deformable films of high surface energy. In contrast, more densely packed POEGMA brushes incorporate less solvent and display very low friction, with F-f-L data following a linear progression according to Amontons' law
Layering of ionic liquids on rough surfaces
No full textUnderstanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. In this work we have performed an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL–solid interface. Our work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contac
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