The influence of repellent coatings on surface free energy of glass plate and cotton fabric

The aim of this research was to determine the influence of chemical finishes on the surface properties of glass plate, considered as a model homogeneous smooth surface and cotton fabric as a non-ideal heterogeneous rough surface. Microscopic slides and 100% cotton fabric in plain weave were coated with fluorocarbon polymers (FCP), paraffin waxes with zirconium salts (PWZ), methylolmelamine derivatives (MMD), polysiloxanes with side alkyldimethylammonium groups (PSAAC) and aminofunctional polysiloxanes (AFPS). From the goniometer contact angle measurements of different liquids, the surface free energy of the coated glass plates was calculated according to approaches by Owens-Wendt-Kaelble, Wu, van Oss-Chaudhury-Good, and Li-Neumann-Kwok. The results showed that all the coatings decreased the surface free energy of the substrate, which was also influenced by the liquid combination and the theoretical approach used. In spite of the fact that the liquid contact angles were much higher on the coated fabric samples than on glass plates and resulted in the lower values of work of adhesion, a very good correlation between the coatings deposited on both substrates was obtained. The presence of repellent coatings FCP, PWZ and MMD converted the solid surface from polar to highly apolar by masking the functional groups of glass and cellulose. PSAAC and AFPS coatings did not decrease the solid surface free energy to such an extent as the former three coatings due to their monopolar character.     

An XPS study of pulsed plasma polymerised allyl alcohol film growth on polyurethane

The growth of highly functionalised poly allyl alcohol films by pulsed plasma polymerisation of CH₂CHCH₂OH on biomedical grade polyurethane has been followed by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Film thickness is observed to increase approximately linearly with plasma modification time, suggesting a layer-by-layer growth mode of poly allyl alcohol. Water contact angle measurements reveal the change in the surface free energy of wetting decreases linearly with plasma modification up to the monolayer point after which a constant limiting value of −24 mJ m⁻² was attained. Films prepared at 20 W plasma power with a duty cycle of 10 μs:500 μs exhibit a high degree of hydroxyl (OH) retention with minimal fragmentation of the monomer observed. Increasing the plasma power up to 125 W is found to improve OH retention at the expense of ether formation generating films close to the monomer stoichiometry. Duty cycle plays an important role in controlling both film composition and thickness, with longer off times increasing OH retention, while longer on times enhance allyl alcohol film growth.     

The influence of UV irradiation on surface composition of collagen/PVP blended films

The surface chemical composition and surface properties of collagen/poly(vinyl pyrrolidone) (PVP) blended films before and after UV irradiation (λ = 254 nm) were investigated using X-ray Photoelectron Spectroscopy (XPS), FTIR-ATR spectroscopy and Atomic Force Microscopy (AFM).The XPS results showed that collagen is enriched on the surface of the collagen/PVP blend. The surface composition of the collagen film was changed more by UV irradiation than the surface composition of the collagen/PVP blend.FTIR-ATR spectra showed that the positions of the amide bands in collagen are more altered after UV irradiation than those for the collagen/PVP blends.AFM images showed that the collagen surface is ordered contrary to PVP. The blend surface was similar to the pure collagen surface and confirms that there is more collagen present at the surface (higher concentration of collagen at the surface compared to PVP). UV irradiation caused only the small changes in the surface morphology of the collagen/PVP films. All of the results confirm that the surface of the collagen/PVP blend is more photoresistant than collagen.     

The surface properties of ionomers based on styrene-co-acrylic acid copolymers

The surface properties of styrene-co-acrylic acid copolymers and of the obtained ionomers containing alkali metal ions, before and after UV-irradiation, were investigated by the measurements of water contact angles and ATR-FTIR spectroscopy.It was examined, how the content and nature of the alkali metal introduced into the initial copolymer influences the water contact angle, surface free energy and its components, polar and dispersive.It was ascertained that the water contact angle, surface free energy and its components depend on the content of introduced acid or salt. These values were practically independent of the nature of the alkali metal introduced into the copolymer. However, more considerable changes were observed for the ionomers containing sodium ions.The research has indicated that UV-irradiation causes the increase in surface hydrophilicity of the investigated ionomers.     

Regulation of the elastic modulus of polyurethane microarrays and its influence on gecko-inspired dry adhesion

We studied the influence of the elastic modulus on the gecko-inspired dry adhesion by regulating the elastic modulus of bulk polyurethane combined with changing the size of microarrays. Segmented polyurethane (PU) was utilized to fabricate micro arrays by the porous polydimethyl siloxane (PDMS) membrane molding method. The properties of the micro arrays, such as the elastic modulus and adhesion, were investigated by Triboindenter. The study demonstrates that bulk surfaces show the highest elastic modulus, with similar values at around 175 MPa and decreasing the arrays radius causes a significant decrease in E, down to 0.62 MPa. The corresponding adhesion experiments show that decrease of the elastic modulus can enhance the adhesion which is consistent with the recent theoretical models.     

The influence of the scattering anisotropy parameter on diffuse reflection of light

We discuss the impact of the scattering anisotropy parameter on the path length distribution of multiply scattered light. A quantitative treatment based on the radiative transfer equation is compared to an extended photon diffusion formalism. We compare both models with diffusing-wave spectroscopy measurements using randomly polarized light.     

RA-IR Studies on the Structure of Mercapto-Ended Azobenzene Derivatives in Self-Assembled Monolayers

Self-Assembled Monolayers (SAMs) were prepared from mercapto-ended azobenzene derivatives with the structure of n-C_nH_2n+1, AzoO(CH₂)_mSH (n=4,6,8,10,12; m=3,5). the structure of these SAMs was thoroughly studied with grazing-angle incident reflection absorption FT1R technique and wettability measurement. the results suggested that the plane of Azobenzene system of the assembling molecules in the SAMs lies on its back with an approximate angle of 22° included between the substrate surface normal and the Azobenzene plane. Tail alkyl groups (n-C_nH_2n+1) in these assembling molecules were considered to be in an all-trans conformation, as if they were in a crystalline-like environment. and the C-C-C plane of these all-trans tail alkyl groups, while n≤8, lies also on its back with an angle about 70° between its plane and the substrate surface normal. the conformation of these head groups (-O(CH₂)_mSH) in SAMs are disturbed by many structural factors. While m=3 or 5, the head group chain was proposed to remain in a gauche conformation to fit the upright orientation of the Azobenzene plane. the packing density was investigated by measuring the contact angle of water on these SAMs. the results showed that the self-assembled monolayer films are perfectly packed and the coverage density might be improved with increasing the length of both tail and head alkyl chains.     

Surface properties of ionomers based on styrene-b-acrylic acid copolymers obtained by copolymerization in emulsion

Surface properties of styrene-b-acrylic acid copolymers obtained in emulsion and suitable ionomers before and after UV-irradiation were studied by measurements of contact angles and FTIR-ATR spectroscopy.The research focused on the influence of different content of carboxylic acid groups in copolymers, of various types and contents of alkali metal salts in ionomers and of cesium acrylate or methacrylate in ionomers on hydrophilicity of the surfaces of these samples and the course of photodegradation in them.Hydrophilicity of initial copolymer surfaces was higher than this of polystyrene as a result of presence of carboxylic acid groups, which also made the surfaces of these copolymers more sensitive to UV-irradiation.Hydrophilicity of the surfaces of ionomers containing cesium acrylates depended on the content of cesium salt in the samples. The course of ionomer photooxidation was also dependent on the content of this salt.The surface of ionomer containing cesium methacrylate was more polar than this of ionomer containing cesium acrylate.Styrene-based ionomers containing 3.7 mol% of various alkali metal acrylates had less polar surfaces than initial copolymer and they were also more resistant to UV-irradiation in comparison to the initial copolymer.Copolymers obtained in emulsion and suitable ionomers had more polar surfaces and they were more sensitive to UV-light compared to copolymers obtained in bulk and their ionomers.     

Wetting mode transition of nanoliter scale water droplets during evaporation on superhydrophobic surfaces with random roughness structure

During evaporation, shape changes of nanoliter-scale (80-100 nL) water droplets were evaluated on two superhydrophobic surfaces with different random roughness (nm-coating, μm-coating). The square of the contact radius and the square of the droplet height decreased linearly with evaporation time. However, trend changes were observed at around 170 s (nm-coating) and around 150 s (μm-coating) suggesting a wetting mode transition. The calculated droplet radii for the wetting mode transition from the average roughness distance and the average roughness height of these surface structures were approximately equal to the experimental values at these trend changes. A certain level of correlation between the roughness size and droplet radius at the wetting mode transition was confirmed on surfaces with random roughness.     

Influence of tensile deformation on the crystalline organization of ethylene copolymers

The influence of tensile deformation on the crystalline properties of ethylene copolymers (ethylene-vinyl acetate [EVA] copolymers) was investigated by differential scanning calorimetry (DSC). The consequence of drawing on the mobility of the amorphous phase also was investigated through the study of the glass transition temperature. The results indicate that more disorganized crystals, melting at a lower temperature, are present after the tensile deformation, reducing the mobility of the amorphous chains, as shown by an increase of the glass transition temperature. For the lowest molecular weight copolymer. less crystalline changes are observed after the tensile test, probably due to the fact that no stiffening appears during the drawing.     

Modification of the surface state of rough substrates by two different varnishes and influence on the reflected light

Modification of the visual appearance when a rough surface is covered by a varnish is mostly attributed to the levelling of the substrate surface, which depends on the molecular weight of the varnish. The topography of varnished surfaces, however, has never been measured directly. Surfaces of varnishes applied over glass substrates of varying roughness were studied, therefore, using mechanical profilometry. Two different varnishes made with a low and a high molecular weight resin were studied. Both varnishes lower the r.m.s. roughness of the substrates and filter the high spatial frequencies. These results are amplified for the varnish containing the low molecular weight resin. The light reflected by the varnished samples is modelled from these topographical data. Its angular distribution, calculated from the probability density of slopes is presented, taking into account separately the air/varnish and the varnish/substrate interfaces. These analyses are presented in a back-scattering configuration. They show that varnishing significantly reduces the angular width of the reflected light and that this effect is magnified for the low molecular weight resin. Modelling furthermore shows that the influence of the roughness of the varnish/substrate interface is negligible in the total reflected light.     

The effect of working pressure on the chemical bond structure and hydrophobic properties of PET surface treated by N ion beams bombardment

Polyethylene terephthalate (PET) surface was bombarded by N ion beams at room temperature. Varying the working pressure of the ion beams, PET surfaces with different composition and properties were obtained. Characterization by X-ray photoelectron spectrometry showed that only on film surface, ester bonds, especially C-O bonds, were broken and N element chemical bonded with C. The influence depth was less than 5 nm because of the lower ion energy (about 10³ eV). Contact angle results revealed that with increasing the working pressure of ion beams, the contact angle of PET surface to pure water increased from 51° to 130°. With these results, one conclusion could be deduced that the hydrophilic and hydrophobic properties of PET surface could be influenced by N atom chemical bond with C, which in turn is controlled by the working pressure of N ion beams.     

Deformation and motion by gravity and magnetic field of a droplet of water-based magnetic fluid on a hydrophobic surface

Motion and deformation of a water-based magnetic fluid on a hydrophobic surface were investigated under gravity and a magnetic field. Surface energy and the resultant contact angle of the magnetic fluid depend on the surfactant concentration. The fluid viscosity is governed mainly by magnetite concentration. The front edge of the droplet moved under a weak external field. The rear edge required a higher external field for movement. The forces of gravity and the magnetic field for moving of the front edge are almost equal. However, those of the rear edge are different. The motion of magnetic fluids by an external field depends on concentrations of surfactants and magnetic particles, the external field, and experimental assembly.     

Annealing effect on surface segregation behavior of hydroxypropylcellulose/polyethylenimine blend films

The surface properties of hydroxypropylcellulose (HPC) and polyethylenimine (PEI) blend films prepared by solution casting method before and after annealing were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and contact angle measurements. SEM and AFM analysis of the blends revealed that the PEI component segregated on the surface of the air-surface side of the blend films with increasing annealing temperature. The band intensity of PEI component at 1630 cm⁻¹ also increased depending on annealing temperature. The water contact angle decreased abruptly with increasing annealing temperature and reached almost 31° on the surface of the air-surface side of the blends at 150 °C. The results of these studies showed that the PEI chains with low surface energy segregated or enriched mainly on the surface of the air-surface side, and that, on the other hand, HPC chains with high surface energy oriented to the surface of the glass-surface side and inside of the films with increasing annealing temperature.     

Formation process of a strong water-repellent alumina surface by the sol-gel method

A novel strong water-repellent alumina thin film is fabricated by chemically adsorbing stearic acid (STA) layer onto the porous and roughened aluminum film coated with polyethyleneimine (PEI). The formation process and the structure of the strong water-repellent alumina film are investigated by means of contact angle measurement and atomic force microscope (AFM). Results show that the water contact angles for the alumina films increase with the increase of the immersion time in the boiling water, and meanwhile, the roughness of the alumina films increases with the dissolution of the boehmite in the boiling water. Finally, the strong water-repellent film with a high water contact angle of 139.1° is obtained when the alumina films have distinct roughened morphology with some papillary peaks and porous structure. Moreover, both the roughened structure and the hydrophobic materials of the STA endow the alumina films with the strong water-repellence.     

Conformational studies of poly(methacrylic acid). I. Conformational energy calculations on lsotactic and syndiotactic poly(methacrylic acid)

The conformational energies of isotactic and syndiotactic poly (methacrytic acid) were calculated for isolated chains in order to determine the most probable helical conformations. It was concluded that the most probable backbone conformation of the isotactic polymer is either a 5₂ or 8₃ helix, where the internal rotations of the two helices are nearly equal. The left-handed and right-handed conformations were also essentially equienergy conformations. Two lowenergy helical conformations were found for the syndiotactic polymer. One of these conformations had backbone internal rotations nearly identical to those of the 5₂ and 8₃ helices of isotactic poly(methacry1ic acid). The second conformation was a nearly planar structure containing two monomer units in the axial repeat unit, with backbone rotation angles nearly identical to those of the 5₂ and 8₃ helices or their negatives.

Deformation of the backbone C─C─C bond angles was considered in the conformational energy calculations. The optimum bond angles for both isotactic and syndiotactic polymer chains were considerably larger than the normal tetrahedral bond angle. The relative energies calculated for helical conformations were influenced by the backbone bond angles. It was demonstrated that the failure to consider backbone bond-angle deformation may lead to erroneous conformational assignments in disubstituted vinyl polymers.     

Wettability and surface energy of parylene F deposited on PDMS

For the first time, the wettability and surface energy of parylene F were investigated. The results showed that parylene F had a hydrophobic surface with an eigen water contact angle of 104.7 ± 0.6°. We found that 3.5 μl probe liquid was an optimal value for the contact angle measurement of parylene F. Moreover, we found that the Lifshitz–van der Waals/acid–base approach was unsuitable for determining the surface energy of parylene F, whereas the Owens–Wendt–Kaelble approach and the limitless liquid–solid interface wetting system were compatible. The surface energy of parylene F was estimated to be 27.06 mJ/m² (Owens–Wendt–Kaelble) and 40.41 mJ/m² (Limitless liquid-solid interface wetting system). Furthermore, this investigation also provided a reference for the applications of empirical and physics-based semi-empirical approaches for the estimation of surface energy.     

Plasma-chemical model, describing the surface treatment of polymers in RF-discharge

The surface modification of the polymers has been studied in RF-discharge. A plasma-chemical model is presented, describing the alteration of the adhesion work and the wetting contact angle of the treated materials. It is presumed that the adhesion work can be expressed by its volume and surface components. In the first approximation, only the surface adhesion work is changed in the course of the plasma treatment and it is due to creating of two common functional groups. We suggest a basic system of kinetic equations for quantitatively determining these groups. The system is solved analytically. This gives possibilities to obtain the analytical solutions of the adhesion work and the contact angle as a function of the time of treatment.     

The influence of light intensity on surface recombination in GaS single crystals

Gallium sulphide (GaS) is a layer structure semiconductor with relatively wide energy gap (E_g (295 K) = 2.5 eV and E_g (80 K) = 2.62 eV). It has potential applications in some areas of optoelectronics. This paper presents the investigations of the influence of light intensity on surface recombination velocity of charge carriers in GaS single crystals. To attain this purpose spectral dependences (between 420 and 550 nm) of absorption coefficients, reflectivity coefficients and photoconductivity were measured in vacuum. The investigations were performed for various light intensities in several temperatures from 80 to 333 K. The least square method was applied to fit the theoretical dependences of photoconductivity on wavelength and intensity of illumination at these temperatures. From the fittings the temperature and light intensity dependences of surface recombination velocity and bulk lifetime of charge carriers were obtained.     

Sliding behavior of oil droplets on nanosphere stacking layers with different surface textures

Two facile coating techniques, gravitational sediment and spin coating, were applied for the creation of silica sphere stacking layers with different textures onto glass substrates that display various sliding abilities toward liquid drops with different surface tensions, ranged from 25.6 to 72.3 mN/m. The resulting silica surface exhibits oil repellency, long-period durability > 30 days, and oil sliding capability. The two-tier texture offers a better roll-off ability toward liquid drops with a wide range of γ_L, ranged from 30.2 to 72.3 mN/m, i.e., when the sliding angle (SA) < 15°, the oil droplet start to roll off the surface. This improvement of sliding ability can be ascribed to the fact that the two-tier texture allows for air pockets (i.e., referred to as the Cassie state), thus favoring the self-cleaning ability. Taking Young-Duprè equation into account, a linearity relationship between sine SA and work of adhesion (W_ad) appears to describe the sliding behavior within the W_ad region: 2.20-3.03 mN/m. The smaller W_ad, the easier drop sliding (i.e., the smaller SA value) takes place on the surfaces. The W_ad value ∼3.03 mN/m shows a critical kinetic barrier for drop sliding on the silica surfaces from stationary to movement states. This work proposes a mathematical model to simulate the sliding behavior of oil drops on a nanosphere stacking layer, confirming the anti-oil contamination capability.