C. Riccardi

Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment

The surface chemical and physical modifications of poly(ethylene terephthalate) (PET) fibers induced by radiofrequency air plasma treatments were correlated with the characteristics of the discharge parameters and the chemical composition of the plasma itself, to identify the plasma-induced surface processes prevailing under different operating conditions. Treated polymer surfaces were characterized by water droplet absorption time measurements and XPS analysis, as a function of the aging time in different media, and by AFM analysis. They exhibited a remarkable increase in hydrophilicity, accompanied by extensive etching and by the implantation of both oxygen- and nitrogen-containing polar groups. Etching was mainly a consequence of ion bombardment, yielding low molecular weight, water soluble oxidation products, while surface chemical modifications were mainly due to the action of neutral species on the plasma-activated polymer surface.

Self-similarity of the plasma edge fluctuations

The rescaled range analysis techniques are used to investigate long-range dependence in plasma edge fluctuations [Mandelbrot and Wallis, Water Resources Res. 4, 909 (1969)]. This technology has been applied to data from several confinement devices such as tokamaks, stellarators, and reversed-field pinch. The results reveal the self-similar character of the electrostatic fluctuations at the plasma edge with self-similarity parameters ranging from 0.62 to 0.72. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times of the order of the confinement time. In cold plasma devices (Te<1 eV at the core), there is no evidence of algebraic tails in the autocorrelation function. Some other characteristic features of the autocorrelation function and power spectrum have been investigated. All of these features are consistent with plasma transport as characterized by self-organized criticality.

Cold plasma treatment of PET fabrics: AFM surface morphology characterisation

Atomic force microscopy (AFM) has been used to investigate the morphology changes in the surface of poly(ethyleneterephthalate) (PET) fabrics due to cold plasma treatments. This has resulted in the possibility to measure quantitatively the root-mean-square (rms) surface roughness and the surface area of the samples developed after the treatment. The morphology changes, mainly rms surface roughness and surface area, on the PET fabrics surface due to air cold plasma have been measured as a function of treatment time and as a function of gas pressure. The same quantities as a function of pressure were measured also for He, Ar, SF6 and CF4 gases. The changes in morphology in the cases of air, He and Ar gases seems to be due mainly to etching effects. The situation is different for SF6 and CF4 gases where reorganisation of the surface, possibly due to fluorine atoms grafting, seems to be effective.

Characterisation of poly(ethylene terephthalate) and cotton fibres after cold SF6 plasma treatment

Poly(ethylene terephthalate) (PET) and cotton fibres, treated in an appositely set up RF SF6 plasma reactor under different operating conditions, were characterised by XPS, EPR, DSC, XRD, ATR analyses, water contact angle and water droplet roll-off angle measurements. The ageing of plasma-treated samples was also investigated under different post-treatment conditions. Plasma treatment led to efficient implantation of fluorine atoms on the surface of both polymers; this resulted in water repellence without altering the bulk properties of the polymers. The radical species formed in the plasma-activated polymer surface were involved in its fluorination and in the subsequent uptake of atmospheric oxygen. Surface reorganisation of polymer segments, tending to reduce the interfacial energy between the polymer and the phase in contact with it, induced the surface modifications observed under ageing and in samples plasma-treated several times. An increase in the depth of the fluorinated layer, leading to outstanding stable hydrorepellence, was achieved by repeated SF6 plasma treatments, followed by surface rearrangements favoured by swelling.

Statistical characterization of fluctuation wave forms in the boundary region of fusion and nonfusion plasmas

The statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion (tokamaks and stellarators) and nonfusion plasmas. Fluctuations in ion saturation current and floating potential have a near-Gaussian character in the proximity of the velocity shear layer (rsh). However, fluctuations deviate from a Gaussian distribution when moving inside of the plasma edge (r rsh). Furthermore, fluctuations show sporadic pulses that are asymmetric in time. The present analysis shows a coupling of those pulses and the averaged flow in the shear layer region.

Plasma-induced graft-polymerization of polyethylene glycol acrylate on polypropylene films: Chemical characterization and evaluation of the protein adsorption

This work deals with the optimization of argon plasma-induced graft-polymerization of polyethylene glycol acrylate (PEGA) on polypropylene (PP) films in order to obtain surfaces with a reduced protein adsorption for possible biomedical applications. To this end, we examined the protein adsorption on the treated and untreated surfaces. The graft-polymerization process consisted of four steps: (a) plasma pre-activation of the PP substrates; (b) immersion in a PEGA solution; (c) argon plasma-induced graft-polymerization; (d) washing and drying of the samples. The efficiency of these processes was evaluated in terms of the amount of grafted polymer, coverage uniformity and substrates wettability. The process was monitored by contact angle measurements, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS) and atomic force microscopy (AFM) analyses. The stability of the obtained thin films was evaluated in water and in Phosphate Buffer Saline (PBS) at 37 degrees C. The adsorption of fibrinogen and green fluorescent protein (GFP)--taken as model proteins--on the differently prepared surfaces was evaluated through a fluorescence approach using laser scanning confocal microscopy with photon counting detection. After plasma treatments of short duration, the protein adsorption decreases by about 60-70% with respect to that of the untreated film, while long plasma exposure resulted in a higher protein adsorption, due to damaging of the grafted polymer.

A SF6 RF plasma reactor for research on textile treatment

This research concerns the development of a SF6 RF discharge at low pressure in a small reactor for industrial applications. The plasma is produced in the pressure range 0.05-1 mbar by a RF supply. The pumping system sustains a flowrate of about 50 cm s-1, with residence time in the discharge of about 0.2 s at a pressure of 0.1 mbar. The discharge parameters were measured at a low operation power. Measurements were performed by means of movable electrostatic probes and a photodiode. Particular care in the analysis of the data proved to be necessary due to the presence of a substantial amount of negative ions. The reactor has been employed for textile treatment in order to modify the surface properties of the fibres. Favourable operating conditions leading to an improved hydrophobicity of the textiles were achieved.

Modeling and diagnostic of an SF/sub 6/ RF plasma at low pressure

This research concerns the experimental and theoretical analysis of an SF/sub 6/ radio-frequency (RF) discharge at low pressure in a small reactor for industrial applications. The plasma is produced in the pressure range of 0.05-1 mbar by a 13.56-MHz RF supplier at moderate power, up to 25 W. The pumping system sustains a flowrate of about 50 cm/s, with residence time in the discharge of about 0.2 s. The discharge parameters have been measured and lie in the range n/sub e//spl sime/10/sup 7/-10/sup 9/ cm/sup -3/, n/sub i//spl sime/10/sup 10/-10/sup 12/ cm/sup -3/, T/sub e//spl sime/5-10 eV. Measurements were performed by means of movable electrostatic probes and of a photodiode. Particular care In the analysis of the data proved necessary due to the presence of a substantial amount of negative ions. On the other hand, we have performed simulations of the discharge composition through the implementation of a suitable numerical model of the chemical kinetics in the device. On the basis of the experiments a comparison of charged and neutral species production with those predicted by the theory was performed, and a more realistic description of the involved phenomena was obtained. In particular, several assumptions concerning the ion diffusion coefficient were tested, allowing us to pin down the relative importance of transport and bulk processes in the discharge state.

Surface Modifications of Silk by Cold SF6 Plasma Treatment

Silk samples, treated in an appositely set-up radio frequency SF6 plasma reactor under different operation conditions, were characterized by XPS, EPR, DSC, XRD, ATR analyses and water contact angle measurements, also as a function of storage time after the treatment. Efficient attachment of fluorine atoms on the polymer surface occurred during the plasma treatment, imparting water repellence to it. Also the percentage of surface oxygen increased after the treatment, partially as a consequence of radical scavenging by molecular oxygen and the formation of peroxides. An increase in silk crystallinity during the treatment seems to contribute to ensuring durable water repellence properties.

Experimental analysis of drift waves destabilization in a toroidal plasma

This paper concerns the study of the development of turbulence in a toroidal magnetoplasma [C. Riccardi et al., Plasma Phys. 36, 1791 (1994)]. This analysis has been performed by evaluating wave number, frequency spectra, and bicoherence coefficients of density fluctuations associated to drift wave propagation. Plasma parameters have been changed over a wide range, in order to identify and characterize density fluctuations both in absence and in presence of nonlinear phenomena.