Costas Panayiotou

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.

Processing and characterization of starch/polycaprolactone products

Poly(e-caprolactone)/plastisized starch blends varying in starch content were processed by conventional extrusion, injection molding, and film blowing techniques. X-ray diffraction was used to investigate starch destructurization carried out by extrusion. The mechanical energy input was also recorded. The effect of starch content on the blends was evaluated by differential scanning calorimetry, mechanical property measurements, and scanning electron microscopy. A finer starch phase dispersion was achieved in injection molded products than in films, probably due to the development of higher shear rates at injection molding. Starch incorporation in polycaprolactone resulted in a material with decreased strength and elongation at, both, yield and break, while the modulus increased. At high starch content, particle coalescence was associated with a further mechanical property decrease. Polycaprolactone melting temperature was only slightly depressed by starch addition.

Properties of Fatty-Acid Esters of Starch and Their Blends with LDPE

In the present study, starch octanoates OCST1.8 and OCST2.7 with degrees of substitution (d.s.) of 1.8 and 2.7, respectively, and dodecanoate DODST2.7 (d.s. A 2.7), were prepared by esterification of native starch with fatty acid chlorides. Our analyses, including elemental analysis, FTIR, contact angle, DSC, and TGA measure- ments confirmed the esterification reaction of starch and the degree of substitution. The ester group was found to act like an internal plasticizer, with an increase in the number and the size of fatty acyl chains grafted onto starch. These starch esters were mixed with low density polyethylene (LDPE) at various proportions in a Haake Rheo- mixer. Water and moisture absorption, thermal and mechanical properties, and biodeg- radation were investigated as a function of blend composition. The DODST2.7/LDPE blends showed, in general, better thermal stability and higher elongation, but lower tensile strength and water absorption, than did corresponding OCST/LDPE blends. The addition of starch esters to LDPE led to a very slow rate of biodegradation of these blends. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 705-721, 1997

Properties of octanoated starch and its blends with polyethylene

Octanoated starch (OCST) was prepared by esterification of native starch with octanoyl chloride. The new material was characterised by 1H NMR, FTIR, and elemental analysis. This showed that the esterification had proceeded to a degree of substitution of 2.7. The octanoated starch was subsequently mixed with low density polyethylene (LDPE) at various proportions in a Haake Rheomixer. Similar blends of LDPE were also prepared with plasticised starch (PLST) for comparison purposes. The thermomechanical properties were determined as a function of blend composition. The prepared LDPE/OCST blends show better mechanical properties compared to their respective LDPE/PLST blends. In particular, the elongation at breaking is significantly higher in the LDPE/OCST blends. The latter exhibit also a higher thermal stability and a low water absorption.

Mechanical properties and viscoelastic behavior of basalt fiber-reinforced polypropylene

In the present article, a series of commercial-grade polypropylenes (PP) filled with different contents of short basalt fibers were studied. This composite material presented deterioration of both mechanical characteristics, for example, stress and strain at yield with increasing of the fiber content. On the other hand, the impact strength was fourfold higher than that of unfilled PP. A poor adhesion between the PP matrix and the basalt fibers was detected. This is why interfacial interactions were promoted by the adding of poly(propylene-g-maleic anhydride) (PP-g-MA). It was observed that the tensile properties of the obtained materials and their impact strengths increased significantly with increasing of the amount of PP-g-MA in the blend. The adhesion improvement was confirmed by scanning electron microscopy as well. Fourier transform infrared spectroscopy was applied to assess if any chemical interactions in the system PP/PP-g-MA/basalt fibers exist. Dynamic mechanical thermal analysis data showed an increase of the storage modulus with increasing fiber content. The conclusion was made that the modification of the PP matrix led to a higher stiffness but its value remained constant, irrespective of the PP-g-MA content. With increasing fiber content, damping in the β-region decreased, but increase of the coupling agent content restored its value back to that of PP. The loss modulus spectra presented a strong influence of fiber content on the α-relaxation process of PP. The position of the peaks of the above-mentioned relaxation processes are discussed as well.

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.

The quasi-chemical approach for non-randomness in liquid mixtures. Expressions for local surfaces and local compositions with an application to polymer solutions

Abstract Explicit expressions for local surfaces and local compositions in mixtures obtained from the quasi-chemical approach of Guggenheim are presented and their relations with empirical expressions discussed. The treatment is greatly simplified by introducing well-defined, non-random factors. A method is proposed for the solution of the set of non-linear equations arising from the quasi-chemical approach. Detailed equations are given for the case of binary and ternary systems. The extension of the general approach in terms of groups is discussed. The quasi-chemical expressions for the local surface fractions are combined with the formalism of the New Flory Theory in order to account for non-randomness in polymer solutions. The resulting expressions are applied to experimental data of the systems carbon tetrachloride—polypropylene oxide, chloroform—polypropylene oxide and benzene—polyisobutylene.

Development of micro- and nano-porous composite materials by processing cellulose with ionic liquids and supercritical CO2

Three lines of green chemistry were combined in this study, in order to produce porous materials with pore size distributions in the micro- and nano-scales. These lines are: (i) the renewable and biodegradable sources (cellulose), (ii) ionic liquids, and (iii) supercritical fluids. By dissolving cellulose in a room temperature ionic liquid and regenerating with water or methanol we obtained cellulose hydrogels and methanogels. The liquid mixtures were separated by vacuum distillation with high yield of recovery. The obtained gels were processed by supercritical carbon dioxide to give porous materials. A novel foaming procedure was applied to hydrogels in order to obtain microporous structures of cellulose and cellulose composites, while in alcogels the supercritical point drying method resulted in nanoporous aerogels. For elucidating physicochemical aspects involved in the two processes and for characterization of the produced materials, X-ray diffraction, sorption measurements (by a modified mass loss analysis and the BET method) and scanning electron microscopy were used. The role of various process parameters on the final porous structure was investigated.

Electrospun fiber mats containing shikonin and derivatives with potential biomedical applications.

Alkannin, shikonin (A/S) and their derivatives are naturally occurring hydroxynaphthoquinones with a well-established spectrum of wound healing, antimicrobial, anti-inflammatory, antioxidant and antitumor activity. Clinical studies over the years revealed that A/S derivatives-based wound healing preparations (such as HELIXDERM(®)) are among a very small group of therapeutics that modulate both the inflammatory and proliferative phases of wound healing and present significant tissue regenerative activity. The purpose of the present work was to combine the biological properties of A/S and the advantages of electrospun meshes to prepare a potent topical/transdermal biomaterial for A/S. Four biocompatible polymers (cellulose acetate, poly(L-lactide), poly(lactide-co-glycolide) LA/GA:50/50 and 75/25) were used for the first time, to produce electrospun fiber mats containing either shikonin or A/S mixture in various amounts. Both drugs were effectively loaded into the above biomaterials. The incorporation of drugs did not considerably affect fibers morphology and their mean diameter size varied from 315 to 670 nm. High drug entrapment efficiencies (ranged from 74% to 95%) and appropriate release profiles were achieved, that render these fibers as potential A/S topical/transdermal wound healing dressings. Given the multifunctional activity of the natural products alkannins and shikonins, their consideration as bioactive constituents for tissue engineering scaffolds seems a promising strategy for repairing and regenerating tissues and mainly skin.

Processing and characterization of LDPE/starch/PCL blends

Low-density polyethylene/plasticized starch/polycaprolactone blends were processed by conventional extrusion, injection molding, and film blowing techniques. The glass transition temperatures of plasticized starch were determined using differential scanning calorimetry. The blends were characterized by mechanical property measurements and scanning electron microscopy. The blend properties were found to depend not only on composition but also on the generated morphology. In films the fine dispersion of polycaprolactone phase in the polyethylene/starch matrix resulted in mechanical property increase, while in injection specimens there was property decrease due to phase coalescence. It appears that the different conditions existing at the two different shaping processes i.e. film blowing and injection molding could account for the final obtained morphology.