Jürgen Springer

Determination of interfacial tension from the profile of a pendant drop using computer-aided image processing

The reliability of the method described in the preceding paper (B. Song and J. Springer, J. Colloid Interface Sci. 183, in press, 1996) for the determination of surface and interfacial tension (IFT) of liquid-fluid systems from the profile of a pendant drop has been studied experimentally. Influences on the resultant IFT values from factors such as the location of the drop profile and illumination conditions are considered. The reproducibility and accuracy of the measurement method were examined by measuring the time dependence of polyethylene glycol (PEG600)/air and water/air systems and by determining the temperature dependence of the polyethylene glycol (PEG6000)/N2 and LD (low-density) polyethylene/N2 systems. The effect of drop vibrations on the accuracy of the method is discussed.

Surface characterization of natural fibers; surface properties and the water up-take behavior of modified sisal and coir fibers

The influence of fiber surface modifications like dewaxing, alkali treatment and methyl methacrylate grafting on the thermal and electrokinetic properties of coir (coconut) and sisal fibers has been investigated. Additionally scanning electron microscopy was performed to follow changes in the fiber surface morphology. Electrokinetic properties were measured using the streaming potential method. The measured time dependence of the ζ-potential offers the possibility to characterize the water up-take, i.e. the swelling behavior of natural fibers. The investigated natural fibers, as expected, contain dissociable surface functional groups as verified by measuring the pH-dependence of the ζ-potential. The influence of fiber surface modifications on the ζ-potential was compared to the influence of fiber surface modifications on measured (tensile and flexural strength) mechanical biocomposite properties. The ζ-potential measurement is a very efficient technique to investigate the various changes effected by different surface modifications, which are necessary to improve the compatibility of such natural fibers and polymer matrices for making eco-friendly and low cost composite materials.

INFLUENCE OF FLUORINATION ON THE PROPERTIES OF CARBON FIBRES

Abstract Various fluorination methods were applied to modify the surfaces of carbon fibres. Mild fluorination techniques, such as low pressure fluorine at various temperatures and pressures, were compared with strong fluorination agents, such as HF-F 2 mixtures or ClF 3 . The relationship between the degree of fluorination and the physical and chemical properties of two polyacrylonitrile-based carbon fibres (Torayca, FT 300 6000-99; Sign, Sigrafil C320.00A) was studied using a combination of contact angle hysteresis, electron spectroscopy for chemical analysis (ESCA), resistivity measurements and X-ray diffraction (XRD).

Morphology and gas permselectivity of blend membranes of polyvinylpyridine with ethylcellulose

Binary blend membranes of poly(4-vinylpyridine) (PVP) with ethylcellulose (EC) have been prepared by a solution casting technique with chloroform as solvent. The membranes appear macroscopically miscible but microscopically immiscible based on wide-angle X-ray diffraction, differential scanning calorimetry and scanning electron microscope investigations. Permeating feature of five pure gases through the blend membranes has been described in detail. The permeability, diffusivity, solubility, and their selectivities of oxygen, nitrogen, carbon dioxide, methane and hydrogen through the blend membranes have been measured over an entire range of blend composition in a time-lag apparatus. A remarkably and continuously enhanced selectivity has been achieved for important gas pairs including oxygen/nitrogen, carbon dioxide/methane, and hydrogen/nitrogen with increasing PVP content from zero to 100 wt%. The highest oxygen over nitrogen, carbon dioxide over methane, and hydrogen over nitrogen selectivity coefficients are, respectively, equal to 6.8, 25, and 70. Experimental permeability was compared with that calculated on the basis of blend composition. The various transport parameters of the five gases through the blends seem normally higher than composition weighted mean computed from pure PVP and pure EC values using semilogarithmic coordinates. The temperature- and pressure-dependences of the permeability of the five gases through a PVP/EC (50/50) blend membrane are discussed.

Basic surface oxides on carbon fibers

To influence the material properties of carbon fiber reinforced thermoplastics, basic carbon fiber surfaces were produced by heating fibers at 905°C under nitrogen, cooling to 25°C and exposing them to oxygen. Pure oxygen as well as air were used at different temperatures. Zeta (ζ)-potential (ζ=f(c, pH)) measurements confirmed the existence of basic surface functionalities on most of the carbon fibers investigated (high tensile (HT) and high modulus (HM) with the exception of ultra high tensile strength (UTS) carbon fiber). The obtained basic HT-carbon fibers were compared to thermally oxidized fibers (acidic character) of the same type, neglecting other methods for the preparation of acidic surfaces. Scanning electron microscopy micrographs revealed that the thermo-treated carbon fiber surfaces became slightly rougher. The basic carbon fibers obtained exhibited hydrophobic surfaces and a positive ζ-potential whereas acidic carbon fibers are hydrophilic and show a negative ζ-potential. The contact angles of basic carbon fibers measured by the modified Wilhelmy-technique versus water increase whereas they decrease versus diiodomethane. Solid surface tensions were calculated from the obtained contact angle values.

Novel poly(arylene ether) as membranes for gas separation

Fluorine containing polymers are of special interest because of their possible use as gas selective membranes, their remarkably low water absorption and their low dielectric constant and enhanced flame resistance. In this work, the gas transport and separation properties of H2, CO2, O2, N2 and CH4 were studied for four novel poly(arylene ether)s (PAEs) membranes, two containing the 2,6-bis(trifluoromethylphenylene)pyridine unit (6FPPy series) and the others containing 2,5-bis(3-trifluoromethylphenylene)thiophene unit (6FPT series) in the polymer backbone. The preparation of this new class of semifluorinated polymers relied on the aromatic substitution polymerization of trifluoromethyl activated bishalo monomers with different bis(phenol)s. It was found that the introduction of CF3 groups into the bisphenol unit increases the gas permeabilities and decreases slightly the selectivities for both series of membranes. The gas permeability coefficients of 6FPPy membranes are higher than those of 6FPT membranes while their selectivities are comparable. These results were discussed based on the gas diffusion and solubility in the membranes and could be ascribed to the polymer structure. The effect of temperature on the gas transport properties of these membranes was measured and the corresponding activation energies for gas permeation and diffusion were calculated.

Surface properties of PAN-based carbon fibers tuned by anodic oxidation in different alkaline electrolyte systems

High modulus carbon fibers were anodic oxidized in various alkaline electrolyte systems. The success of the modification was proved by contact angle measurements. The fiber surface tension was calculated from the measured contact angles using the harmonic mean method. With the exception of one electrolyte system, all in alkaline electrolytes modified carbon fibers showed an increased fiber surface tension. The increased fiber surface tension is due to the increased number of oxygen containing surface groups detected by energy dispersive X-ray microanalysis (EDX). Anodic oxidation causes a roughening of the fiber surfaces observed by scanning electron microscopy (SEM). Cyclo-voltammetric investigations give an possible mechanism of the anodic oxidation in alkaline electrolytes.

Electrografting of thiophene, carbazole, pyrrole and their copolymers onto carbon fibers : electrokinetic measurements, surface composition and morphology

Abstract In order to improve the surface properties of carbon fiber without affecting the mechanical properties of the reinforcing fibers, various monomers (pyrrole (Py), carbazole (Cz) and thiophene (Th)) were used to electrograft conjugated copolymers. The electrocoated carbon fibers were characterized with respect to their surface morphology and surface composition, using scanning electron microscopy (SEM) and FTIR-reflectance spectroscopy, respectively. In general very homogeneous and continuous coated carbon fiber surfaces were obtained. The pH-dependent zeta ( ζ )-potential measurements were applied to characterize the surface properties of all electrocoated fibers. The performed ζ -potential measurements reflect that the surface state is strongly dependent not only on the composition of the grafted polymers (different homopolymers, copolymers and a single terpolymer), but also on the extent of doping by the counter ion from the supporting electrolyte. Polymers or copolymers formed in solution during the electrografting process were also analyzed using FTIR-reflectance, 1 H NMR and UV–VIS spectroscopy, as well by gel permeation chromatography (GPC) and elemental analysis. Such a detailed investigation of well known electrografted copolymers opens the possibility to use such ‘superior’ materials in the field of carbon fiber reinforced polymeric composite materials. The main advantages of such homogeneous and dense electrocoatings are their thermal stability and the fact that one can design the surface composition and morphology.

Basic and acidic surface oxides on carbon fiber and their influence on the expected adhesion to polyamide

This study presents contact angle measurements versus test liquids and directly between carbon fibers and a polyamide melt in combination with zeta (ζ)-potential measurements to predict the adhesion in carbon fiber reinforced polyamide composites. In addition and to compare the results of the physico–chemical measurements model, composites were prepared to determine the interfacial shear strength as a measure for the adhesion between acidic or basic modified carbon fibers and a polyamide matrix.

Gas transport properties of highly fluorinated polyamideimides

Abstract Gas transport properties of a series highly fluorinated polyamideimides based on 2,2-bis(3,4-decarboxyphenyl)hexafluoropropane dianhydride and N,N -bis(amino-phenyl)-perfluoroalkane-α,ω-dicarboxamide diamines have been measured as a function of upstream pressure and temperature. The results show that the permeability coefficients P for the penetrant gases in any of the four polyamideimides decrease in the gas order: P (CO 2 ) > P (O 2 ) > P (N 2 ) > P (CH 4 ), while the permeabilities for a given gas in these polymers increase with the increase of oligo(tetrafluoroethene) segment length. The effects of the substitution pattern of the phenyl ring and the length of the oligo(tetrafluoroethene) segments in the diamine moieties on gas permeabilities, diffusivities and solubilities are discussed. It seems that, comparing with the effect of the flexible oligo(tetrafluoroethene) segment, the substitution patterns of phenyl group in the diamine moieties have no notable effect of the diffusivities in the polyamideimides as the length of the oligo(tetrafluoroethene) segment increasing from 8 to 12 carbons. The activation energies of permeation and diffusion were obtained for the gas/polymer pairs studied and correlated with the size of penetrant gas.