Ioannis Zuburtikudis

Superhydrophobic Composite Films Produced on Various Substrates

Hydrophilic silica (SiO2) nanoparticles were dispersed in solutions of poly(methyl methacrylate) (PMMA) and in solutions of a commercial poly(alkyl siloxane) (Rhodorsil 224), and the suspensions were sprayed on glass surfaces. The effect of the particle concentration on the hydrophobic character of PMMA-SiO2 and Rhodorsil-SiO2 films was investigated and showed the following: (i) Static contact angles (theta s), measured on surfaces that were prepared from dilute dispersions (particle concentration <1% w/v), increase rapidly with particle concentration and reach maximum values (154 and 164 degrees for PMMA-SiO2 and siloxane-SiO2, respectively). Further increases in particle concentration do not have any effect on theta s. (ii) The effect of particle concentration on the contact angle hysteresis (thetaAlpha - thetaR) is more complicated: as the particle concentration increases, we first notice an increase in hysteresis, which then decreases and finally becomes constant at elevated particle concentrations. The lowest thetaAlpha - thetaR values were 5 degrees for PMMA-SiO2 and 3 degrees for siloxane-SiO2, respectively. (iii) SEM and AFM images show that a two-length-scale hierarchical structure is formed on the surface of the superhydrophobic films. It is demonstrated that superhydrophobicity can be achieved using various hydrophilic nanoparticles (alumina and tin oxide nanoparticles were successfully tested) and that the substrate has almost no effect on the hydrophobic character of the applied coatings, which were produced on silicon, concrete, aluminum, silk, wood, marble, and of course glass. The results are discussed in light of Wenzel and Cassie-Baxter models.

Biodegradable polymer nanocomposites: The role of nanoclays on the thermomechanical characteristics and the electrospun fibrous structure

Polymer nanocomposites, based on poly(e-caprolactone) (PCL) and organically modified montmorillonite, were prepared by the solution intercalation technique. The thermal stability of the prepared materials was analyzed by thermogravimetric analysis. Investigation of their mechanical properties revealed that incorporation of the high aspect ratio montmorillonite sheets into the matrix significantly enhanced the polymer stiffness without sacrificing its ductility. Fibrous membranes of neat and nanocomposite PCL were fabricated by electrospinning. The effect of the applied voltage, the solution concentration and the clay content of the nanocomposite materials on the final fibrous structure was investigated. The results showed that the introduction of the inorganic filler and the increase in the applied voltage from 7.5 to 15 kV facilitated the formation of fine fibers with fewer bead defects. The presence of nanoclay resulted in narrower fiber size distributions, although the mean fiber diameter was not significantly altered. The increase in the solution concentration led to the formation of more uniform fiber structures and to a slight increase in the mean fiber diameter. Furthermore, the electrospinning process affected significantly the structure of the nanocomposite material by increasing the interlayer spacing of the inorganic mineral.

Polymer-Silica nanoparticles composite films as protective coatings for stone-based monuments

The decrease of surface energy of mineral substrates similar to those used in many stone monuments of cultural heritage by the application of protective polymer coatings along with the simultaneous increase of their surface roughness can increase their ability to repel water substantially. In this work, the effect of artificially induced roughness on the water repellency of mineral substrates coated with protective polymer films was investigated. Natural marble samples or home made calcium carbonate blocks were tried as the mineral substrates. The roughness increase was achieved by mineral chemical etching or by creation of nanoscale binary composition film on the substrate surface. PMMA and PFPE were the polymers used, while different-sized silica nanoparticles were employed for the production of the nanocomposite films. Examination of the coated and uncoated surfaces with profilometry and AFM and measurements of water contact angles reveal a pronounced effect of the surface roughness on water repellency. Especially in the case of nanocomposite coatings, the surfaces become super-hydrophobic. This result indicates that the nanoscale binary composition film scheme, which is characterized by its simplicity and low cost, is a suitable candidate for the water protection of stone-based monuments on large scale.

Surface Properties of Superhydrophobic Coatings for Stone Protection

Superhydrophobic films are produced by a simple and low cost method. Silica (SiO2) nanoparticles are dispersed in solutions of Rhodorsil 224, a commercial poly(alkyl siloxane) which is used for the protection of outdoor cultural heritage objects, and the suspensions are sprayed on glass surfaces. It is shown that the siloxane-nanoparticle composite films prepared from dispersions of high particle concentrations (≥ 0.5% w/v) exhibit superydrophobic properties (high static contact angle and small hysteresis) which can be rationalized by the Cassie-Baxter model, according to quantitative measurements obtained by SEM images. Siloxane-nanoparticle films are then deposited (sprayed) on “Opuka”, a fine-grained argillite which was used for the restoration of the castle of Prague. It is shown that the treated stone surfaces exhibit superydrophobic properties, similar to the treated glass surfaces. The efficacy of the superhydrophobic films to protect Opuka is evaluated by performing water contact angle, water capillary absorption, water vapor permeability and colorimetric measurements. It is shown that the use of nanoparticles in the protective coating has a positive effect on the results of the aforementioned tests, except for the colorimetric measurements.

A Simple Route for Purifying Extracellular Poly(3-hydroxybutyrate)-depolymerase from Penicillium pinophilum

This work proposes the purification of an active and efficient enzyme, extracellular poly(3-hydroxybutyrate) (PHB)-depolymerase, suitable for industrial applications. This is achieved by the application of an easy, fast, and cheap route, skipping the chromatography step. Chromatography with one or two columns is a common step in the purification procedure, which however renders the isolation of the enzyme a time consuming and an expensive process. A strain of the fungus Penicillium pinophilum (ATCC 9644) is used for the isolation of extracellular PHB-depolymerase. The molecular weight of the purified enzyme is about 35 kDa and is estimated by gel electrophoresis (SDS-PAGE, 12% polyacrylamide). The enzymatic activity of the isolated enzyme is determined to be 3.56-fold similar to that found by other researchers that have used chromatography for the isolation. The as-isolated enzyme disintegrates the poly(3-hydroxybutyrate) (PHB) films successfully, as it is demonstrated by the biodegradation test results provided here.

Surfactant-induced morphology and thermal behavior of polymer layered silicate nanocomposites

Poly(L-lactic acid) nanocomposites were prepared by the addition of montmorillonite modified with various loadings of hexadecylammonium cation. The influence of alkylammonium on the morphology and surface charge of the clay was investigated by Xray diffraction (XRD) analysis and electrokinetic measurements, respectively. The structural characteristics of the inorganic-organic hybrids were studied by XRD, transmission electron microscopy (TEM) and atomic force microscopy (AFM). Thermal analysis was carried out by thermogravimetric analysis (TGA) under constant nitrogen flow and under air. The results showed that high concentration of surfactant present in the clay greatly increases clays dispersibility into the matrix and this substantially improves the thermal stability of the pristine polymer.

Nanofibrous morphology of electrospun chitosan nanocomposites reinforced with WS2 nanotubes: A design-of-experiments study:

Chitosan nanofibers reinforced with tungsten disulfide inorganic nanotubes (INT-WS2) were fabricated in this study. The aim was to investigate the effect of the material parameters and the electrospinning process parameters on the obtained nanofibrous morphology of the mats. The INT-WS2 content, the polymer solution concentration, the electric field strength, and the solutions flow rate were the investigated factors within the framework of response surface methodology. Scanning electron microscopic and image analysis were used for the dimensional characterization of the nanofibrous morphology and the estimation of three selected responses. Two responses were related to the quality of the nanofibrous morphology: the number surface density of the beads (Nbead) and the average bead-to-fiber diameter (Dbead/Dfiber). The third response was indicative of the fiber thickness (Dfiber). The developed models as well as the coupling and the individual effects of the four investigated factors are given. The results indicate that the electrospun nanofibrous morphology is mostly affected by the polymer solution concentration, the electric field strength and the INT-WS2 loading. Furthermore, the response-surface results reveal possible experimental pathways that may be followed in order to obtain specified nanofibrous chitosan/INT-WS2 morphologies.

PBSA/layered silicate nanocomposites and the role of nanoclay on their electrospun fibrous structure

Polymer nanocomposites based on poly (butylene succinate-co-butylene adipate) (PBSA) with various concentrations of organically modified montmorillonite were fabricated by the solvent casting technique. The morphology of the prepared materials was investigated by X-ray diffraction (XRD) and their thermal behaviour was analysed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Fibrous mats of pure and nanocomposite PBSA were subsequently prepared by electrospinning. Solution concentration, flow rate and applied voltage were kept constant in order to reveal the effect of the clay content alone on the final fibrous structure using scanning electron microscopy (SEM).