New Method to Probe the Surface Properties of Polymer Thin Films by Two-Dimensional (2D) Inverse Gas Chromatography (iGC)

Polymer-based functional surface coatings are extensively used in advanced technologies, including optics, energy, and environmental applications. Surface thermodynamic properties profoundly impact the molecular interactions that control interfacial behaviors, such as adhesion and wettability, which in turn dictate coating processes and performance. Conventionally, contact angle measurements are used to assess the surface energy of polymer films and coatings, where the wettability of a surface is assessed using probe fluids (liquid drops). However, contact angle measurement oftentimes can be nontrivial due to the roughness or chemical heterogeneity of the solid surface, as well as the potential for the liquid drop to swell or even dissolve the material being measured. Alternatively, inverse gas chromatography (iGC) is a versatile technique to measure surface thermodynamics and Lewis acid-base properties while also providing environmental control such as temperature and humidity. Despite these benefits, the application of iGC has been limited to powders or fibers, while the direct measurement of supported thin films or coatings is still a nascent area of research. This creates a challenge when using iGC as a comprehensive platform for measuring the physicochemical properties of solid surfaces. Here, we demonstrate how to effectively use iGC to characterize the surface energy of supported polymer thin films by using a two-dimensional (2D) film holder and modifying operational controls, such as the concentration range of the injected gas probe molecules. This enables the precise control of surface coverage required for analyzing samples having minimal surface area, such as thin films. Poly(methyl methacrylate) (PMMA) was employed as a benchmark to determine suitable iGC parameters and to validate our approach on polymer thin films. The seminal work presented here expands the capability of state-of-the-art iGC to embrace supported thin films (2D iGC) that could either be smooth or display texture/roughness (patterned films) as well as coatings with heterogeneous chemical/structural composition.

Corrosion Resistance and Cytocompatibility of Magnesium-Calcium Alloys Modified with Zinc- or Gallium-Doped Calcium Phosphate Coatings

In orthopedic surgery, metals are preferred to support or treat damaged bones due to their high mechanical strength. However, the necessity for a second surgery for implant removal after healing creates problems. Therefore, biodegradable metals, especially magnesium (Mg), gained importance, although their extreme susceptibility to galvanic corrosion limits their applications. The focus of this study was to control the corrosion of Mg and enhance its biocompatibility. For this purpose, surfaces of magnesium-calcium (MgCa1) alloys were modified with calcium phosphate (CaP) or CaP doped with zinc (Zn) or gallium (Ga) via microarc oxidation. The effects of surface modifications on physical, chemical, and mechanical properties and corrosion resistance of the alloys were studied using surface profilometry, goniometry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), nanoindentation, and electrochemical impedance spectroscopy (EIS). The coating thickness was about 5-8 μm, with grain sizes of 43.1 nm for CaP coating and 28.2 and 58.1 nm for Zn- and Ga-doped coatings, respectively. According to EIS measurements, the capacitive response (Y_c) decreased from 11.29 to 8.72 and 0.15 Ω^-1 cm^-2 sⁿ upon doping with Zn and Ga, respectively. The E_corr value, which was -1933 mV for CaP-coated samples, was found significantly electropositive at -275 mV for Ga-doped ones. All samples were cytocompatible according to indirect tests. In vitro culture with Saos-2 cells led to changes in the surface compositions of the alloys. The numbers of cells attached to the Zn-doped (2.6 × 10⁴ cells/cm²) and Ga-doped (6.3 × 10⁴ cells/cm²) coatings were higher than that on the surface of the undoped coating (1.0 × 10³ cells/cm²). Decreased corrosivity and enhanced cell affinity of the modified MgCa alloys (CaP coated and Zn and Ga doped, with Ga-doped ones having the greatest positive effect) make them novel and promising candidates as biodegradable metallic implant materials for the treatment of bone damages and other orthopedic applications.

Alternative radiopacifiers for polymethyl methacrylate bone cements: Silane-treated anatase titanium dioxide and yttria-stabilised zirconium dioxide

Poly (methyl methacrylate) (PMMA) bone cement is widely used for anchoring joint arthroplasties. In cement brands approved for these procedures, micron-sized particles (usually barium sulphate, BaSO₄) act as the radiopacifier. It has been postulated that these particles act as sites for crack initiation and subsequently cement fatigue. This study investigated whether alternative radiopacifiers, anatase titanium dioxide (TiO₂) and yttria-stabilised zirconium dioxide (ZrO₂), could improve the in vitro mechanical, fatigue crack propagation and biological properties of polymethyl methacrylate (PMMA) bone cement and whether their coating with a silane could further enhance cement performance. Cement samples containing 0, 5, 10, 15, 20 and 25%w/w TiO₂ or ZrO₂ and 10%w/w silane-treated TiO₂ or ZrO₂ were prepared and characterised in vitro in terms of radiopacity, compressive and bending strength, bending modulus, fatigue crack propagation, hydroxyapatite forming ability and MC3T3-E1 cell attachment and viability. Cement samples with greater than 10%w/w TiO₂ and ZrO₂ had a similar radiopacity to the control 10%w/w BaSO₄ cement and commercial products. The addition of TiO₂ and ZrO₂ to bone cement reduced the bending strength and fracture toughness and increased fatigue crack propagation due to the formation of agglomerations and voids. Silane treating TiO₂ reversed this effect, enhancing the dispersion and adhesion of particles to the PMMA matrix and resulted in improved mechanical properties and fatigue crack propagation resistance. Silane-treated TiO₂ cements had increased nucleation of hydroxyapatite and MC3T3-E1 cell attachment in vitro, without significantly compromising cell viability. This research has demonstrated that 10%w/w silane-treated anatase TiO₂ is a promising alternative radiopacifier for PMMA bone cement offering additional benefits over conventional BaSO₄ radiopacifiers.

Stimuli-Responsive Rifampicin-Based Macromolecules

This paper presents the modification of the antibiotic rifampicin by an anionic polyelectrolyte using a simplified electrochemically mediated atom transfer radical polymerization (seATRP) technique to receive stimuli-responsive polymer materials. Initially, a supramolecular ATRP initiator was prepared by an esterification reaction of rifampicin hydroxyl groups with α-bromoisobutyryl bromide (BriBBr). The structure of the initiator was successfully proved by nuclear magnetic resonance (1H and 13C NMR), Fourier-transform infrared (FT-IR) and ultraviolet-visible (UV-vis) spectroscopy. The prepared rifampicin-based macroinitiator was electrochemically investigated among various ATRP catalytic complexes, by a series of cyclic voltammetry (CV) measurements, determining the rate constants of electrochemical catalytic (EC') process. Macromolecules with rifampicin core and hydrophobic poly (n-butyl acrylate) (PnBA) and poly(tert-butyl acrylate) (PtBA) side chains were synthesized in a controlled manner, receiving polymers with narrow molecular weight distribution (Mw/Mn = 1.29 and 1.58, respectively). "Smart" polymer materials sensitive to pH changes were provided by transformation of tBA into acrylic acid (AA) moieties in a facile route by acidic hydrolysis. The pH-dependent behavior of prepared macromolecules was investigated by dynamic light scattering (DLS) determining a hydrodynamic radius of polymers upon pH changes, followed by a control release of quercetin as a model active substance upon pH changes.

Highly Transparent Polyethylcyanoacrylates from Approved Eco-Friendly Fragrance Materials Demonstrating Excellent Fog-Harvesting and Anti-Wear Properties

Superglue monomers belong to a family of cyanoacrylates that are known for their very rapid polymerization upon contact with moist surfaces. Their biodegradation and low toxicity make them attractive as medical and veterinary adhesives. Although the fast-acting polymerization characteristics have been successfully utilized to design nanoscale polymeric particles that can carry drugs or other inorganic nanoparticles, it constitutes a significant drawback if one desires to produce other forms of functional biodegradable acrylics, such as coatings, sheets, or nanocomposites. This is because rapid polymerization in air creates highly porous and brittle structures. Here, we address this drawback by reporting a simple and inexpensive method of fabricating highly transparent (>92%) polyethylcyanoacrylate (PECA) coatings by dispersing the monomer in a fragrance-classified green liquid, cyclopentanone. The resulting transparent coatings were hydrophilic but with slippery wetting characteristics, suitable as efficient fog-harvesting templates. Furthermore, another fragrance liquid, benzyl alcohol, is introduced as a plasticizer and co-solvent to overcome its brittleness while retaining its transparency. The same plasticized monomer solutions, dispersing low concentrations of graphene (<0.5 wt %), were allowed to self-assemble on stainless steel surfaces, forming low-friction and anti-wear dry lubricants by decreasing the steel friction coefficient and wear rate by 6- and 10-fold, respectively.

Tailoring Ink-Substrate Interactions via Thin Polymeric Layers for High-Resolution Printing

The surface properties of a substrate are among the most important parameters in the printing technology of functional materials, determining not only the printing resolution but also the stability of the printed features. This paper addresses the wetting difficulties encountered during inkjet printing on homogeneous substrates as a result of improper surface properties. We show that the wetting of a substrate and, consequently, the quality of the printed pattern, can be mediated through the deposition of polymeric layers that are a few nanometers thick. The chemical nature of the polymers determines the surface energy and polarity of the thin layer. Some applications, however, require a rigorous adjustment of the surface properties. We propose a simple and precise method of surface-energy tailoring based on the thermal decomposition of poly(methyl methacrylate) (PMMA) layers. A smooth transition in the wetting occurs when the thickness of the PMMA layer approaches zero, probably due to percolation of the underlying surface of the substrate, which enables the inkjet printing of complex structures with a high resolution. In particular, the wetting of three substrate-ink systems was successfully adjusted using the thin polymeric layer: (i) a tantalum-oxide-based ink on indium-tin-oxide-coated glass, (ii) a ferroelectric lead zirconate titanate ink on a platinized silicon substrate, and (iii) a silver nanoparticle ink on an alumina substrate.

Selective Mechanical Transfer Deposition of Langmuir Graphene Films for High-Performance Silver Nanowire Hybrid Electrodes

In this work, we present silver nanowire hybrid electrodes prepared through the addition of small quantities of pristine graphene by mechanical transfer deposition from surface-assembled Langmuir films. This technique is a fast, efficient, and facile method for modifying the optoelectronic performance of AgNW films. We demonstrate that it is possible to use this technique to perform two-step device production by selective patterning of the stamp used, leading to controlled variation in the local sheet resistance across a device. This is particularly attractive for producing extremely low cost sensors on arbitrarily large scales. Our aim is to address some of the concerns surrounding the use of AgNW films as replacements for indium tin oxide (ITO), namely, the use of scarce materials and poor stability of AgNWs against flexural and environmental degradation.

Apparent Contact Angle Calculated from a Water Repellent Model with Pinning Effect

A set of new theoretical equations for apparent contact angles is proposed. The equations are derived from an equilibrium of interfacial tensions of a three-phase contact line pinned at the edges of a fine structure. These equations are validated by comparison with contact-angle measurement results for 2 μL water droplets on poly(methyl methacrylate) microstructured samples with square pillars or holes. The equilibrium contact angles predicted by the new equations reasonably agree with the experimental results. In contrast, the values predicted by the Cassie-Baxter equation or the Wenzel equation do not qualitatively agree with the experimental results in pillar pattern cases because the Cassie-Baxter equation and the Wenzel equation do not account for the pinning effect.

Quantification of Type, Timing, and Extent of Cell Body and Nucleus Deformations Caused by the Dimensions and Hydrophilicity of Square Prism Micropillars

Novel digital analysis strategies are developed for the quantification of changes in the cytoskeletal and nuclear morphologies of mesenchymal stem cells cultured on micropillars. Severe deformations of nucleus and distinct conformational changes of cell body ranging from extensive elongation to branching are visualized and quantified. These deformations are caused mainly by the dimensions and hydrophilicity of the micropillars.

Zwitterionic Nanofibers of Super-Glue for Transparent and Biocompatible Multi-Purpose Coatings

Here we show that macrozwitterions of poly(ethyl 2-cyanoacrylate), commonly called Super Glue, can easily assemble into long and well defined fibers by electrospinning. The resulting fibrous networks are thermally treated on glass in order to create transparent coatings whose superficial morphology recalls the organization of the initial electrospun mats. These textured coatings are characterized by low liquid adhesion and anti-staining performance. Furthermore, the low friction coefficient and excellent scratch resistance make them attractive as solid lubricants. The inherent texture of the coatings positively affects their biocompatibility. In fact, they are able to promote the proliferation and differentiation of myoblast stem cells. Optically-transparent and biocompatible coatings that simultaneously possess characteristics of low water contact angle hysteresis, low friction and mechanical robustness can find application in a wide range of technological sectors, from the construction and automotive industries to electronic and biomedical devices.

Immobilization of heparin on chitosan-grafted polyurethane films to enhance anti-adhesive and antibacterial properties

Infections caused by bacteria adhering to implant surfaces are one of the main reasons for the failure of the implants. In this study, polyurethane (PU), which is the most commonly used polymer in the production of medical devices, was synthesized and surfaces of polyurethane films were modified by chitosan (CH) grafting and heparin (Hep) immobilization in order to enhance anti-adhesiveness and antibacterial properties. Functional groups present on the surface, topographical shapes, and free energies of the polyurethane films were determined. Pristine polyurethane, chitosan-grafted polyurethane (PU–CH), and heparin immobilized polyurethane (PU–CH–Hep) films demonstrated high anti-adhesive efficacy against bacteria in the given order, where PU–CH–Hep was the most effective one. When PU–CH–Hep samples were incubated with different bacteria, complete death was observed for Pseudomonas aeruginosa (Gram negative), Staphylococcus aureus (Gram positive), and Staphylococcus epidermidis (Gram positive). Some living Escherichia coli (Gram negative) were observed after 24 h of incubation. Pristine and modified polyurethane samples demonstrated no adverse effect on proliferation of L929 fibroblast cells and were found to be biocompatible according to MTT cytotoxicity tests.

Mussel-inspired dopamine- and plant-based cardanol-containing polymer coatings for multifunctional filtration membranes

A series of copolymers [PCD#s, where # is the weight percentage of dopamine methacrylamide (DMA) in polymers] containing mussel-inspired hydrophilic dopamine and plant-based hydrophobic cardanol moieties was prepared via radical polymerization using DMA and 2-hydroxy-3-cardanylpropyl methacrylate (HCPM) as the monomers. PCD#s were used as coating materials to prevent flux decline of the membranes caused by the adhesion of biofoulants and oil-foulants. Polysulfone (PSf) ultrafiltration membranes coated with PCD#s showed higher biofouling resistance than the bare PSf membrane, and the bactericidal properties of the membranes increased upon increasing the content of HCPM units in the PCD#s. Serendipitously, the PSf membranes coated with the more or less amphiphilic PCD54 and PCD74, having the optimum amount of both hydrophilic DMA and hydrophobic HCPM moieties, showed noticeably higher oil-fouling resistance than the more hydrophilic PCD91-coated membrane, the more hydrophobic PCD0-coated membrane, and the bare PSf membrane. Therefore, multifunctional coating materials having biofouling- and oil-fouling-resistant and bactericidal properties could be prepared from the monomers containing mussel-inspired dopamine and plant-based cardanol groups.

Surface modification and adhesion improvement of polyester films

Facile surface modification of polyester films was performed via chemical solutions treatment. Surface hydrolysis was carried out by means of sodium hydroxide solutions, leading to the formation of carboxylate groups. Three commercial polyester films of 100 μm in thickness were used in this work: AryLite™, Mylar™, and Teonex™, hydrolysis time being the main modification parameter. FTIR-ATR analysis, topography and contact angle (CA) measurements, surface free energy (SFE) and T-Peel adhesion tests were carried out to characterize the modified films. A quantitative estimate of the carboxylates surface coverage as a function of treatment time was obtained through a supramolecular approach, i.e. the ionic self-assembly of a tetracationic porphyrin chromophore onto the film surface. The surface free energy and critical surface tension of the hydrolyzed polyesters was evaluated by means of Zisman, Saito, Berthelot and Owens-Wendt methods. It was shown that NaOH solution treatment increases roughness, polarity and surface free energy of polymers. As a result, T-Peel strengths for modified Mylar™ and Teonex™ films were respectively 2.2 and 1.8 times higher than that for the unmodified films, whereas AryLite™ adhesion test failed.

Wettability and antifouling behavior on the surfaces of superhydrophilic polymer brushes

The surface wettabilities of polymer brushes with hydrophobic and hydrophilic functional groups were discussed on the basis of conventional static and dynamic contact angle measurements of water and hexadecane in air and captive bubble measurements in water. Various types of high-density polymer brushes with nonionic and ionic functional groups were prepared on a silicon wafer by surface-initiated atom-transfer radical polymerization. The surface free energies of the brushes were estimated by Owens-Wendt equation using the contact angles of various probe liquids with different polarities. The decrease in the water contact angle corresponded to the polarity of fluoroalkyl, hydroxy, ethylene oxide, amino, carboxylic acid, ammonium salt, sulfonate, carboxybetaine, sulfobetaine, and phosphobetaine functional groups. The poly(2-perfluorooctylethyl acrylate) brush had a low surface free energy of approximately 8.7 mN/m, but the polyelectrolyte brushes revealed much higher surface free energies of 70-74 mN/m, close to the value for water. Polyelectrolyte brushes repelled both air bubbles and hexadecane in water. Even when the silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolled up and detached from the brush surface. The oil detachment behavior observed on the superhydrophilic polyelectrolyte brush in water was explained by the low adhesion force between the brush and the oil, which could contribute to its excellent antifouling and self-cleaning properties.

Surface and Cell Affinity Properties of Chitosan-Gelatin-Hydroxyapatite Composite Films

Addition of hydroxyapatite (HAp) into polymeric matrices and use them as supporting material for tissue engineering has gained attraction since hydroxyapatite (HAp) type crystals are the main inorganic components of the bone structure. In this study, composite films were prepared from chitosan (C) and gelatin (G) with addition of HAp, and the surface characterization and cell affinity properties were examined. The hydrophilicities and morphologies of the films were investigated by contact angle measurements and by scanning electron microscopy (SEM), respectively. Cell affinities were studied by using human osteosarcoma (SaOs-2) cell line. Crosslinked and HAp containing films were found to be more hydrophilic than that of the others, while surface free energy (SFE) values were almost similar for all films. Cell proliferation and spreading of SaOs-2 cells were higher for the films with high gelatin content and no HAp; whereas for HAp containing composites higher cell affinities were obtained for the samples which have high chitosan content.

Perfluorocarbon thin films and polymer brushes on stainless steel 316 L for the control of interfacial properties

Perfluorocarbon thin films and polymer brushes were formed on stainless steel 316 L (SS316L) to control the surface properties of the metal oxide. Substrates modified with the films were characterized using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), contact angle analysis, atomic force microscopy (AFM), and cyclic voltammetry (CV). Perfluorooctadecanoic acid (PFOA) was used to form thin films by self-assembly on the surface of SS316L. Polypentafluorostyrene (PFS) polymer brushes were formed by surface-initiated polymerization using SAMs of 16-phosphonohexadecanoic acid (COOH-PA) as the base. PFOA and PFS were effective in significantly reducing the surface energy and thus the interfacial wetting properties of SS316L. The SS316L control exhibited a surface energy of 38 mN/m compared to PFOA and PFS modifications, which had surface energies of 22 and 24 mN/m, respectively. PFOA thin films were more effective in reducing the surface energy of the SS316L compared to PFS polymer brushes. This is attributed to the ordered PFOA film presenting aligned CF(3) terminal groups. However, PFS polymer brushes were more effective in providing corrosion protection. These low-energy surfaces could be used to provide a hydrophobic barrier that inhibits the corrosion of the SS316L metal oxide surface.

Wettability and surface free energy of graphene films

Graphene sheets were produced through chemical exfoliation of natural graphite flake and hydrazine conversion. Subsequently, graphene sheets were assembled into a thin film, and microscale liquid droplets were placed onto the film surface for measurement of wettability and contact angle. It is found that a graphene oxide sheet is hydrophilic and a graphene sheet is hydrophobic. Isolated graphene layers seem more difficult to wet in comparison to graphite, and low adhesion work was found in the graphene-liquid interface. Approximation of solid-liquid interfacial energy with the equation of state theory was applied to determine the graphene surface energy. The results indicate that surface energy of graphene and graphene oxide is 46.7 and 62.1 mJ/m2, respectively, while natural graphite flake shows a surface free energy of 54.8 mJ/m2 at room temperature. These results will provide valuable guidance for the design and manufacturing of graphene-based biomaterials, medical instruments, structural composites, electronics, and renewable energy devices.

Correlation of effective dispersive and polar surface energies in heterogeneous self-assembled monolayer coatings

We show, theoretically, that the measured effective dispersive and polar surface energies of a heterogeneous surface are correlated; the correlation, however, differs whether a Cassie or an Israelachvili and Gee model is assumed. Fluorocarbon self-assembled monolayers with varying coverage were grown on oxidized (100) silicon surfaces in a vapor phase process using five different precursors. Experimentally, effective surface energy components of the fluorocarbon self-assembled monolayers were determined from measured contact angles using the Owens-Wendt-Rabel-Kaelble method. We show that the correlation between the effective surface energy components of the heterogeneous surfaces coated with fluorocarbon self-assembled monolayers is in agreement with the Cassie model.