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C. Jama, J-D. Quensierre, L. Gengembre+, V. Moineau+, J. Grimblot+, O. Dessaux and P. Goudmand
 Abstract
Polyethylene ultra high molecular weight (PEUHMW) plates were treated using radio frequency capacitive CF4 plasma discharge. In this paper, we present results on the heating effects of PEUHMW surfaces after CF4 capacitive plasma fluorination. For characterizing the surface chemical modifications, XPS and surface free energy gs analyses have been carried out for fluorinated samples before and after heating at different temperatures : ambient, 100 and 240°C. The presented results show incorporation of CFx groups leading to the observed decrease of gs values after CF4 plasma treatment. The evolution of the chemical surface composition and the repartition of the fluorinated chemical groups induced by such heating are also discussed. The most important changes are due to an increase of the dispersive component and are a consequence of surface reorganisation, producing phenomena such as molecular chain scission, leading to CF2 and CF3 groups departure and are accompanied by a progressive oxidation phenomenon by heating the treated samples.
Introduction
Polyethylene (PE) polymers have been applied successfully in many industrial fields such as
automotive, packaging, biomaterials, microelectronic devices, thin film
technology, adhesion, friction, wear and protective coatings. Indeed PE
polymers are inexpensive, easy to process and have good resistance to
weathering and excellent bulk physical and chemical properties. However,
special surface properties such as controlled roughness, crosslinking density,
inertness and low friction coefficients are in general required to success in
these applications. As normally, PE polymers do not possess the surface
properties needed for the required applications, the materials have to be
treated to modify its superficial behaviour. One of the main used process for
surface modification is plasma fluorination [1-3]. Indeed, this process is
used to decrease the surface free energy and increase hydrophoby, surface
lubricity and the surface inertness.
Such plasma treatments permit also to increase the control and the diffusion process through porous materials with a better control. In the literature, only very few works deal with the increase of the hydrophobic character to control the diffusion through porous materials using plasma fluorination [4]. A very recent patent describes the fluorination process of powdered or porous materials [5]. Porous materials submitted to fluorination show an important gradient of the concentration of grafted chemical groups concentration from the periphery to the bulk. A high hydrophobic character is observed at the polymer periphery with a gradual decrease by progressing into the bulk. To achieve bulk homogeneous fluorination, one possible solution can be a surface fluorination of powdered material followed by a sintering process to agglomerate the final solid. In this process, the powder is heated to form the required porous material. The present work is aimed to answer to two important questions relative to the process just described : does the hydrophobic character disappear after heating ? Does the heating process affect on the concentration of the grafted functions ? In this paper, we present results on the effects of thermal treatments of PE surfaces fluorinated by a CF4 capacitive plasma. For characterizing the surface chemical modifications, both angle-resolved XPS and surface free energy μs analyses have been carried out. The fluorinated samples have been examined without heating or after heating at 100 and 240°C. The evolution of the chemical surface composition and the repartition of the fluorinated chemical groups induced by such heating are also discussed. Introduction
Samples: The samples were plates of PE with a ultra high molecular weight (PEUHMW) (30mm'30mm'1.5 mm) plates from Goodfellow. The as received
samples were subjected to ultrasonic degreasing in acetone during 5 min. The CF4
plasma treated samples were heated in air for 5 min in a microwave oven (Panasonic)
equipped by a forced convection system for a homogeneous heating. The oven was
pre-heated at the chosen temperatures before samples introduction.
Plasma reactor: The plasma reactor has already been described [6]. It consists of a glass reactor (0.045 m3) containing two parallel electrodes (290 mm diameter with a separation distance 70 mm) powered by a radio frequency generator (13.56 MHz). The substrate was placed on the lowest electrode. The reactor was initially purged with argon and then pumped to 0.5 Pa (Alcatel, 33 m3.h-1). A flow rate of CF4 ( from Air Liquide, purity : 99%) equal to 30 sccm was introduced into the reactor and was controlled with a mass flow controller (MKS). The working pressure of 180 Pa was measured by a Pirani sensor (MKS). The fluorination treatment duration was 5 min with an incident plasma power of 25 W. Surface analyses - Angle resolved XPS: The XPS analyses were performed with a Thermo Electron ESCALAB 220XL spectrometer with the natural MgKa radiation (1253.6 eV). The X-ray gun operated at a low power (10 kV with a current emission of 20 mA) with a position kept 10 mm backward of the standard working position to avoid X-ray induced damages on the samples. The lens were working in the electrostatic mode (SA150). The spectrometer was initially calibrated using the Cu 2p3/2 (932.7 eV) and Ag 3d5/2 (368.26 eV) binding energy peaks positions. The C1s, F1s and O1s spectra were recorded in the binding energy range of 25 eV with 0.1 eV steps and 100 ms counting time. Each region was scan four times. The angle resolved experiments were performed at three takeoff angles F (90°, 30° and 10°) related to the sample surface. The sampling depth D for each takeoff angle is related to the sampling depth d obtained at F=90° by the relationship : Possible radiation damages were checked by back-recording the spectra at 90°, they were considered as being negligible. The atomic ratios were determined by the ratio nA/nB = (IAi/IBj)´(KBj/KAi), where IAi(Bj) is the i(j) photopeak intensity of the element A(B) and KAi(Bj) is the result term between the cross section of the i(j) core level orbital, the inelastic mean free path and the transmission factor of the analyser, both of the latter being energy dependent. The efficiency of the electron detector was considered to be constant. The curve resolving procedure involved the fitting of the experimental spectra with gaussiang-lorentzian mix components using the Thermo Electron Eclipse software. Accuracy of the fits was appreciated from c2values. The sample binding energy calibration was based on the F1s peak at 689.2 eV. Contact angle measurements: The dynamic contact angles measurements were obtained with a processor tensiometer (KRÜSS; K12) according to the Wilhelmy gravimetric method [7]. The chosen liquids were water, formamide, ethanediol, dimethylformamide, and 1,4-dioxan. The solid surface free energy and dispersive components were used to evaluate the effect of fluorination on the PE plates and their evolution upon further thermal treatments. |
