Towards hierarchically ordered functional porous polymeric surfaces prepared by the breath figures approach

This article reviews the approaches developed to prepare and characterize porous structured materials by using the breath figures (BF) methodology. In particular, we have analyzed the topographical modifications of the surface that can be tuned with this approach, such as the control of the pore characteristics, changes in the pore morphology or use of non-planar substrates to create the porous materials among others. We have also given special attention to the functionality inside of the pores and how this can be created by using different kinds of polymers, from homopolymers to hybrid materials, as well as by changing the pore functionally after chemical modification. The approaches followed to obtain hierarchical structures, for example, by combination of the BF approach and nanostructure formation within the pores or by using soft-lithography have also been examined. In addition, we discuss the feasibility of obtaining stimuli-responsive honeycomb structured surfaces. The potential applications in different areas such as biomedicine, optics and so on, are also pointed out. Finally, we comment on some future perspectives of breath figures approach.     

Chemical composition and physico-chemical properties of polymer surfaces

The analysis of polymer surfaces in terms of functional groups can be undertaken by X-ray photoelectron spectroscopy. Correlations between chemical composition and water contact angle indicate the dependence of polymer wettability on the surface concentration of oxygen carrying a high electron density, whatever the specific functional groups present. Electrokinetic methods, which can be applied to plates or films, provide tools for examining the surface electric properties and for detecting ionizable groups at polymer surfaces. Surface characterization provides basic information for understanding the adhesion process through molecular interactions. However the study of polypropylene has shown that adhesiveness is not only influenced by surface properties but also by the deformability of the polymer and, consequently, by its crystallinity and its thermal history.     

Study of nisin adsorption on plasma-treated polymer surfaces for setting up materials with antibacterial properties

Setting up antibacterial materials by nisin adsorption on surfaces depends mainly on the surface properties and the surface treatments allowing the modification of such properties. In order to investigate the factors affecting such adsorption, the native low density polyethylene (LDPE) was modified using Argon/Oxygen (Ar/O₂) plasma, nitrogen (N₂) plasma and plasma-induced grafting of acrylic acid (AA). The films were studied by various characterization techniques. The chemical surface modification was confirmed by X-ray photoelectron spectroscopy (XPS), the wettability of the surfaces was evaluated by contact angle measurements, the surface charge was determined by the zeta potential measurements, and the changes in surface topography and roughness were revealed by atomic force microscopy (AFM). Nisin was adsorbed on the native and the modified surfaces. The antibacterial activity, the nisin adsorbed amount, and the peptide distribution were compared for the four nisin-functionalized films. The highest antibacterial activity was recorded on the Ar/O₂ followed by AA then by N₂ treated films and the lowest activity was on the native film. The observed antibacterial activity was correlated to the type of the surface, hydrophobic and hydrophilic interactions, surface charge, surface topography, nisin adsorbed amount, and nisin distribution on the surfaces.     

Robust grafting of PEG-methacrylate brushes from polymeric coatings

A study is presented of the grafting of poly(ethylene glycol)methyl ether methacrylate (PEGMA) from polymeric macroinitiator films to form well-defined polymer brushes, using activators generated/regenerated by electron transfer (AGET/ARGET) atom transfer radical polymerization (ATRP). Polymer brush coatings can potentially be obtained on surfaces of virtually any shape and composition, because of the ease of conformal casting of the anchoring macroinitiator film. Polymer brush coatings are synthesized in a robust way, as ARGET and AGET ATRP require little to no deoxygenation and make use of stable catalysts. The monomer, catalyst, ligand and reducing agent concentrations, the amount and type of initiating moiety in the anchoring films, and the choice of solvents are optimized, resulting in control over the rate of reaction, and the molecular weight of poly(PEGMA). The best conditions are determined for the formation of a poly(PEGMA) brush with high grafting density, controlled thickness and “living” ends available for post-functionalization.     

Surface rearrangement of tailored polyurethane-based coatings

Polyurethane coatings with different network composition were prepared from an oligomeri diol, a diisocyanate, and a low molecular weight triol. The glass transition temperature of the network was tuned by the ratio of diol and triol and the composition (aromatic or aliphatic) of the diisocyanate. All coatings were studied for their bulk properties as well as their surface properties. It was found that by the addition of a fluorinated additive the surface free energy of the coating was lowered by approximately 15 mN·m⁻¹, leaving the bulk properties intact. It was also shown that these polyurethane coatings are able to adapt their surface free energy in a reversible manner when exposed to water. The magnitude and rate of surface rearrangement is strongly dependent on the network density of the coating.     

Organic coatings for marginally prepared steel surfaces

The present study, carried out in the frame of a common research programme between the Institute of Chemistry, Faculty of Engineering at the University of Genoa and AGIP CORM Department about alternative surface preparation methods, is a preliminary contribution to the development of a series of rust-prevention coatings that can be applied directly in order to protect steel structures exposed to aggressive environments. The use of ‘rust converters’ may be considered a tool for alternative surface preparation but it is necessary to control the amount of loose rust layers and to ensure their complete removal. The poor adhesion level of converted layers is a problem not yet solved and it does not allow for a subsequent coating and/or finishing step. The application of the ‘surface tolerant’ coating systems seems to be a more useful way to protect steel structures without sandblasting and all its related economic and environmental disadvantages. The application of these systems on marginally prepared steel surfaces shows promising reliability both for the intrinsic protective power and for the adhesion to the metallic substrata. Their use during repainting maintenance is a reliable proposition and a profitable way to overcome not only technical problems but also subsequent environmental pollution and related constraints.     

Polymer structure and properties of heterophase and self-stratifying coatings

Phase separations in bulk and during the film forming process on a substrate, accompanied with the evaporation of solvents and chemical reactions of curing, have been studied in incompatible polymer blends (combinations of solid epoxy resin with selected thermoplastic resins, priorly dissolved in organic solvents) in order to be able to control the heterophase polymer structure of coatings. The resultant polymer/ polymer heterogeneous and sometimes non-homogeneous-in-layer (ultimately double-layer) polymer structures of coatings were evaluated by scanning electron microscopy (SEM). A few driving forces for self-stratification and the formation of double-layer coating structures, where each layer is enriched with a certain polymer component, are discussed as possibly associated with selective wetting of a substrate, phase contraction, interfacial tension gradients, etc. Examples are given to illustrate non-additive changes of structure-dependent coating properties with the composition of the epoxy/thermoplastic resin blends, which is characteristic for polymer/polymer heterophase materials. Depending on the phase state of the binder in the paint (formulated with the use of a selected incompatible polymer blend, combination of solvents and sometimes with an additive, controlling the phase separation process) and conditions of application to a substrate and film formation, various polymer/polymer heterophase or self-stratifying one-coat coatings can be obtained. Some of them, particularly those that combine partial stratification with polymer/polymer heterogeneity, are capable of meeting the demands for advanced performance due to improved combinations of bulk and surface properties.     

Synthesis and characterization of functionalized carbon nanotubes with different wetting behaviors and their influence on the wetting properties of carbon nanotubes/polymethylmethacrylate coatings

The synthesis and characterization of hydrophobic functionalized multi-wall carbon nanotubes (MWCNTs) were investigated and their influence on the wetting properties of organic coatings and composites was studied. Functionalization was performed using oxidation, 1,3-dipolar cycloaddition, and silanization. Silanization was conducted using three hydrophobic silane precursors: 1H,1H,2H,2H-perfluorodecyltrimethoxysilane, 1H,1H,2H,2H-perfluorooctyltrimethoxysilane, and triethoxyoctylsilane. Functionalization was directly confirmed and characterized by Fourier transform infrared spectroscopy, thermal gravimetric analysis, and scanning electron microscopy. The water contact angle of the functionalized MWCNTs was 40–142° for different surface functionalities and the functionalized MWCNTs were incorporated into an acidic solution of polymethylmethacrylate. The effect of surface functionality and the concentration of the functionalized MWCNTs on the wetting properties of these composites were studied by measuring the water contact angle. Under optimum conditions, composite surfaces with water contact angles greater than 110° were obtained. Atomic force microscopy was used to determine the topography of the surface and energy dispersion spectroscopy was used to determine the distribution of the functionalized MWCNTs in the composite film. It was shown that the hydrophobic functionalized MWCNTs migrated to the surface; this was more pronounced for the more hydrophobic MWCNTs.     

Effect of a moving flame on the temperature of polymer coatings and substrates

A computational model is developed to predict the temperature profile over an organic coating on a metal surface as a result of the action of a moving flame. The deflection of the flame as it impinges on the surface is simulated and its consequent heat transfer to the polymer is determined. The scanning action of the flame across the substrate is quantified and the temperature profiles within the polymer are calculated. The results show a substantial build up of temperature at the surface and large temperature gradients throughout the thickness, which are due to the low thermal conductivity of polymers. This can be particularly detrimental for polymers owing to their low softening and decomposition temperatures. The model can be applied to flame impingement on a bulk polymer or on an organic coating on a metal substrate. The research shows the risks of a moving flame overheating a polymer surface and indicates remedial measures.     

A critical appraisal of the potential of self healing polymeric coatings

Several approaches and concepts to self healing materials have appeared in the literature over the last years, all presenting advantages and disadvantages, but definitely showing the great potential of this new class of materials to increase the lifetime of structures and to decrease maintenance costs. Also the field of coatings technology can benefit from this new concept. In this paper the authors critically discuss the potential of existing self-healing approaches applied or likely to be applied to the field of organic coatings, analyzing the pros and cons of each healing mechanism and highlighting the potential of development of non-explored areas of coatings technology.     

Embedded metal nanoparticles as localized heat sources: An alternative processing approach for complex polymeric materials

Metal nanoparticles were utilized as heating elements within nanofibers to demonstrate an alternative approach to thermally process nanostructured polymeric materials. In the photothermal process, resonant light excites the surface plasmon of the nanoparticle and the absorbed energy is converted into heat due to electron-phonon collisions. This heating is efficient and strongly localized, generated from the nanometer-sized metal particles embedded within the polymer. Composite polyethylene oxide (PEO) nanofibers, containing differing concentrations and types of nanoparticles, were fabricated by electrospinning and irradiated by a low intensity laser tuned specifically to the metal nanoparticle surface plasmon absorbance; aggregation of fibers, loss of fibrous structure, and ultimately, complete melting were observed. The photothermal response to irradiation increased with nanoparticle concentration as long as particle aggregation was avoided. Pure PEO nanofibers, or those containing metal nanoparticles possessing a non-resonant surface plasmon, were also irradiated but no melting occurred, demonstrating the controllable specificity of this approach.     

Non-destructive 3-dimensional mapping of microcapsules in polymeric coatings by confocal Raman spectroscopy

Nowadays, the properties of polymeric coatings are enhanced by various additives mixed into the resin. Recently, embedding of polymeric microcapsules into the coating matrix has been investigated to provide special on-demand features to the coating. The detection and characterization of such microcapsules in a polymeric coating are of major importance but difficult, because both are built up by similar molecules with similar densities. Current analysis methods require complex sample preparation to allow reliable measurements.In contrast, confocal Raman spectroscopy allows fast and non-destructive differentiation between characteristic molecular bonds at a spatial resolution below one micrometer. Hence, the objective of this research was to apply this technique on microcapsules embedded in a coating and provide answers to the following questions: Can one detect microcapsules embedded in a coating and clearly identify them? Can one differentiate between full and empty microcapsules and the coating matrix? Can one determine the exact location of the capsules and their distribution in the coating?Therefore, several two-dimensional confocal Raman spectroscopy mappings recorded at different depths allowed a three-dimensional reconstruction of the polymeric coating with the polymeric microcapsules in it. Thereby, the distribution of the capsules within the coating could be determined with micrometer resolution. As a result Raman tomography provides a more detailed insight into the distribution of microcapsules through the possibility of three-dimensional reconstruction.     

Functional coatings for anti-biofouling applications by surface segregation of block copolymer additives

Temperature responsive or bactericidal coatings with poly(n-butyl methacrylate) (PBMA) as bulk material and surface segregated poly(n-butyl acrylate)-block-poly-(N-isopropylacrylamide) (PBA-b-PNIPAAm) or poly(n-butyl acrylate)-block-quaternized poly(2-(dimethylamino)ethyl methacrylate) (PBA-b-PDMAEMAq) as additive were prepared via sequential solvent evaporation of polymer solutions in a solvent mixture. The degree of enrichment at the air surface of the coating and the functionality were examined for different molecular weight additives with different block ratios obtained via Atom Transfer Radical Polymerization (ATRP). The design of the block copolymers with an anchor block (PBA) which is compatible with the bulk polymer (PBMA) and water-compatible functional blocks (PNIPAAm and PDMAEMAq) along with the selection of suited solvent mixtures based on pre-estimation of the selective solubility and sequential evaporation via the Hansen solubility parameters and vapor pressures, respectively, were found to work very well. A small fraction of water in the solvent mixture had been crucial to obtain surface segregation of the functional block, e.g., a PNIPAAm surface with temperature-switchable wettability. Reversible temperature dependent wettability and long term stability of the functionalization, based on contact angle data, were obtained for an optimized PBA-b-PNIPAAm additive. Surface charge density, estimated from dye binding and zeta potential measurements, and killing efficiency against Staphylococcus aureus were investigated for PBA-b-PDMAEMAq as additive. Both block copolymer additives were found to dominate the surface properties and the functionality of the PBMA coating.     

Design and development of self-stratifying systems as sustainable coatings

Thermosetting polyurethane coating formulations which stratify into two distinct layers after application have been prepared. In a thermosetting system the polymer phase separation among other factors is seen to be kinetically controlled and have less dependence on parameters such as surface tension gradient and polymer solubility. Certain coating applications including those used in architectural, automotive, industrial maintenance and aerospace require a combination of primer/topcoat or basecoat/clearcoat layers. These multilayered systems necessitate complex application and curing procedures. Multiple product formulation, application and processing steps not only require significant amounts of processing time, and contributing to environmental waste generation and pollution they also consume excessive amount of energy and manpower until a solid film has been produced. It would be desirable to reduce the number of layers to a minimum while providing the equal or better overall performance. In this paper we report the design and development of thermosetting polyurethane coatings that self-stratify to two distinct phases upon application and cure. It was demonstrated that kinetically controlled reactions aided by incompatible polymers possessing gradient surface free energies can afford self-stratifying coatings. This investigation also revealed that solvent type, evaporation rate, and gravity did not contribute to stratification of thermosetting coatings. Prototype pigmented systems were applied and cured and characterized by FTIR, SEM/EDX and were evaluated using standard coating test methods. The results of this investigation provide a new understanding of stratification phenomenon and establish that preferentially reactive polymers can be a basis for formulating cross-linkable self-stratifying coatings.     

Study on reconstruction mechanism at the surface of a glassy polymer

The introduction of hydrophilic functional groups at various depths in the functionalized interfacial region of poly(4-methylstyrene) (P4MS) provided a system for studying the surface reconstruction mechanism of this glassy polymer. The degree of surface reconstruction (RD) and the rate of surface reconstruction (1/t_1/2) were employed to compare the surface reconstruction behavior for various samples. The results showed that 1/t_1/2 decreased with increase of the depth in the functionalized interface region at a temperature below T_g of poly(4-methylstyrene). By studying the relation of water contact angles and the surface free energy of the samples with temperature, it was found that surface reconstruction of surface-oxidized P4MS samples took place in two steps when the samples were heated in air. The first step took place below 80 °C, in which polar side groups turned into the bulk, leaving a relatively nonpolar backbone projecting out of the surface to form a “hydrophobic conformation”. The second step occurred above the P4MS T_g (110 °C), in which P4MS molecular chains migrated to the surface in order to minimize the interfacial free energy between surface-oxidized P4MS film and air, since oxidized P4MS molecules containing carbonyl groups have higher surface free energy than the unmodified P4MS molecules. As the depth of the functionalized interfacial region increases, a longer time is needed for the polar side groups to reorient (the first step) and for unoxidized P4MS molecules to migrate to the surface (the second step), which resulted in the sample with deeper functionalized region having lower reconstruction rate and RD using the same treatment condition.     

Formation of thin boron nitride coating on multiwall carbon nanotube surfaces

Thin boron nitride films were deposited onto outer surfaces of multiwall carbon nanotubes (MWCNTs) by dip coating, which involves infiltration by boric acid solutions and subsequent nitridation of the boron oxide in ammonia flow at 1050 °C. The overall composition of the samples was determined by electron energy loss (EELS) and X-ray photoelectron spectroscopy (XPS), the surface composition and chemical structure of the BN coatings by XPS, the morphology of the BN/MWCNT composites by scanning and transmission electron microscopy (SEM, TEM), and the resistance against oxidation at elevated temperatures by thermal analysis (TGA). It was proved that single and multilayer BN coverage were achieved at the applied experimental conditions, and the coated samples showed significantly increased oxidation resistance compared to the uncoated MWCNTs.     

Poly(m-phenylendiamine-co-o-aminophenol) coatings on mild steel: Effect of comonomers feed ratio on surface and corrosion protection aspects

Copolymer of m-phenylendiamine (mPD) and o-aminophenol (oAP) has been electrochemically deposited on mild steel from an aqueous solution of oxalic acid using cyclic voltammetric technique. The copolymer structure was confirmed by Proton Nuclear Magnetic Resonance (¹H NMR) and Attenuated Total Reflection Infrared (ATR-IR) spectroscopic techniques. Surface characterisation results indicated that the formed copolymer coatings posses uniform, smooth surface and strong adherence towards the metal surface. Corrosion performance of copolymer coatings was investigated in 0.5 M HCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) methods. Results showed that corrosion performance of copolymer coatings depends on the feed ratio of both the monomer units in copolymer chain. It was found that the copolymer feed ratio of mPD:oAP = 70/30 in coatings exhibited the highest performance due to its superior surface nature than other copolymer coatings.     

Effect of surface microstructure on the hydrophobicity of coatings

In this paper, coatings adopted SiO₂ nanoparticles ranging from 1 wt% to 7 wt% were in situ polymerized with low surface energy polymer, and different surface microstructures were built automatically during painting film. Copper microstructure surface was also achieved by chemical etching method and then was modified by low surface energy resin as contrast. The contact angle and the peel strength between the coating and bio-gooey simulacrum were studied. The results showed that the hydrophobicity of the coating was remarkably affected by the surface microstructure. The coating containing 5 wt% SiO₂ nanoparticles was covered with micro- and nano-scale salience and holes, and had the maximum contact angle and the minimum peel strength. Copper slices presented uniform micron and sub-micron structure surface when etching for 15 min and the contact angle reached a maximum of 125° after being modified.