PLASMA FLUORINATION FOR IMPROVING OF THE PERMEABILITY, WETTING, BIOCOMPATIBILITY AND OPTICAL ABSORPTION OF DIFFERENT POLYMERS Fabienne Poncin-Epaillard*, Dominique Debarnot PCI, UMR 6120 CNRS-Universite du Maine, Le Mans, France Keywords: plasma, tetrafluormethane, flourine, hydrogen, substitution, polymer, epoxy Abstract: Polymer materials are of great Interest for many applications due to ttieir easy processing, mass production and low cost compared to ottier materials. However, some of their surface properties are poor and must be improved for the selected application. For example, polymers such as unsaturated polyesters, (co)poiyolefins are sensitive to water sorption. Reversible phenomena as water diffusion, irreversible phenomena such as chain scissions and chemical degradations can be both at the origin. CF4 plasma-treated layer improves drastically the barrier effects. Superhydrophobic surfaces, as lotus leaves correspond to hydrophobic surfaces whose water contact angle is higher than 150°. Such surfaces present weak physical properties like optical and mechanical ones but good blocompatlblllty. Two new different approaches of CF4 plasma treatment are proposed leading to stable and transparent polymeric superhydrophobic surfaces. Many integrated photonic devices for micro-electro-optlcal-mechanical system based on epoxy polymer present high optical losses at near-Infrared (IR) wavelengths mainly assigned to absorption from vibrational overtones of the carbon-hydrogen bonds. These losses can be reduced by the substitution of hydrogen atoms from C-H bonds with heavier atoms such as fluorine obtained through a plasma fluorination. Modifikacija polimerov s plazemsko flourinacijo za izboljšanje prepustnosti, omočljivosti, biokompatibilnosti in optičnih lastnosti Kjučne besede: plazma, tetrafiuormetan, fluor, vodik, substitucija, polimer, epoksi Izvleček: Pollmerni materiali so zanimivi za uporabo zaradi preproste obdelave In nizke cene. Nekatere njihove lastnosti pa niso najboljše, zato jih moramo pred uporabo spremeniti. Nekateri polimeri, kamor sodijo tudi nenasičeni poliestri In pollolefini, so občutljivi na absorpcijo vode. V tovrstnih materialih se pojavljajo reverzibilni procesi (na primer difuzija vode) in nereverzibilni procesi (na primer trganje poiimernih verig In degradacija polimerov). Obdelava s plinsko plazmo, ki vsebuje plin CF4, lahko drastično spremeni tovrstne procese. Po obdelavi lahko opazimo izredno povečanje hidrofobnosti materiala, saj lahko kontaktni kot vodne kapljice naraste preko 150°. Takšno stanje površine, ki ga včasih imenujemo pojav lotusovega lista, vodi k zelo dobri biokompatibilnosti materialov. V prispevku prikazujemo dva originalna pristopa k funkcionalizacijl površine poiimernih materialov s CF4 plazmo, ki vodita k stabilni funkcionallzaciji in s tem superhidrofobnosti obdelovancev. Mnoge Integrirane optične naprave, ki temeljijo na uporabi epoksi polimerov, imajo visoke optične Izgube v bližnjem infrardečem področju, kar razlagamo predvsem z absorpcijo na vezeh ogljika z vodikom. Tovrstne izgube lahko zmanjšamo s substitucijo vodikovih atomov s težjimi atomi, kot so fluorovl. Tovrstno substitucijo najlažje dosežemo s plazemsko flourinacijo. 1 Introduction Many advances have been made in developing new surface treatments, such as plasma treatment, to modify physical and chemical properties of polymer surface without altering bulk properties /1-7/. One of the applications is dealing with the superhydrophobic surfaces. Natural superhydrophobic surfaces /8-9/ as observed with leaves of plants (lotus) or feathers of some birds, correspond to hydrophobic surface whose water contact angle (Q) is higher than 150° /10/. Such surfaces are sometimes called self-cleaning surfaces since a water flux will push away the particles of dirtiness /9/. Different methods to prepare such surfaces were proposed, some of them devoted to inorganic materials /11/, the other to organic materials / 12-16/. The fabrication of inorganic model superhydrophobic surfaces /17-19/ allows studying the influence of the roughness on the water contact angle. Therefore, Si wafers were etched by lithography techniques to produce different topographic structures whose shape depends on the mask used. Then a hydrophobic layer of dimethyldichlo-rosilane, N-octyldimethylchlorosilane or heptadecafluoro-1,1,2,2- tetrahydrodimethylchlorosilane was deposited from liquid, vapour or plasma phase /20/. The contact angle increase with the hydrophobic character of the layer depends also on the number of plots but not on their respective heights. Other superhydrophobic model surfaces are synthesized from successive alkyi ketene dimmer (AKD) and dialkyi ketone (DAK) recrystallisations /21,23/. The controlled re-crystallization of such compounds allows producing surfaces with a fractal dimension varying from 2.18 to 2.29. Sol-gel techniques /24-27/ have also been developed through the copolymerization of fluoroalkylsilane with different organometallic precursors /28-29/, on aluminium /30-31 /, or on titanium dioxide /32/, Organic superhydrophobic surfaces are obtained when poly(tetrafluoroethylene) (PTFE) /33/ or polystyrene (PS) /18-19,34/ spherical microparticles are dispersed in a hy- drophobic polymer matrix. Hozumi and coworkers /25,35/ describe the fabrication of superhydrophobic surfaces based on plasma enhanced chemical vapor deposition of organosilanes in presence of oxygen onto silicon wafer. Then in a second step, the fluoroalkylsilane replaces O2 in the plasma phase and the deposited layer becomes hydrophobic and superhydrophobic. Pulsed plasma of 1H,1H,2H,2H-heptadecafluorodecylacrylate /13/ also leads to the deposition of a fluorinated layer whose chemical structure is close to the monomer vapour and its roughness varies from 0 to 100 em depending on the initial roughness of the PTFE substrate /14/. Favia et al. /15/ proposed the pulsed RF plasma deposition of tetrafluoroeth-ylene (TFE). Superhydrophobic surfaces composed of juxtaposition of very thin ribbons whose crystallinity is close to PTFE lead toa 150°-165° contact angle. However, this surface structure depends strongly on the plasma parameters (value of the duty cycle in pulsed RF glow discharges fed with TFE). CF4 plasma surface modification of spin coated polybutadiene onto various substrates leads also to superhydrophobic surfaces (Q = 172%) /16/. Another illustration is given with the sputtering of PTFE onto polypropylene (PP) film /36/; the roughness is due to the PTFE sputtering by Ar+ bombardment but also by the PP degradation during the sputtering. The dissolution of polypropylene in ap-xylene/methyl ethyl ketone solution followed by its deposition onto the substrate and the solvents evaporation also leads to a rough hydrophobic surface /37/. Here, the synthesis in dry medium (plasma) is proposed leading to stable and transparent polymeric superhydrophobic polyethylene surfaces. Barrier properties for packaging materials have been improved over the years to protect packaged foodstuffs from the oxidation and the moisture. Owing to their favourable performances as efficient barrier materials, ethylene-vinyl alcohol copolymers (EVOH) are used in many applications and therefore are widely used in packaging and protective coating. Among all the new technologies involved in improving the barrier properties of EVOH films, one in particular is often used: the creation of multilayer systems mostly through the plasma technique /38/. The glycidyl ether of bisphenol A (epoxy-based) polymer whose common name is SU-8, widely used as negative photoresist, has also been studied for micro-electro-optl-cal mechanical system (MEOMS) applications and optical waveguides /39-43/. However, it presents high optical losses at near-infrared (IR) wavelengths mainly assigned to absorption from vibrational overtones of the carbon-hydrogen (C-H) bonds. These losses can be reduced by the substitution of hydrogen atoms of C-H bonds with heavier atoms such as fluorine (F), deuterium (D) or chlorine (CI) ones /11 -14/ thanks to the plasma technique allowing the substitution of hydrogen atoms by fluorine ones. The chemical and topographic modifications of the polymeric surface induced by plasma treatment were found to be strongly influenced by the type of feed gas and param- eters employed. Studied plasma parameters are the discharge power, treatment time and nature of plasma (CF4 or mixtures of CF4/H2), in order to optimize plasma treatment for different applications. 2 Plasma fluorination of polymers Poly(ethylene-co-vinyl alcohol) and polyethylene films have been modified by CF4 microwave plasma. The influence of vinyl alcohol comonomer contents on the properties of these materials under CF4 microwave plasma has been studied by means of surface energies, atomic force microscopy (AFM), secondary ion mass spectroscopy (SIMS) and calorimetric measurements (DSC). The CF4 plasma treatment improved the hydrophobicity and the roughness of the film surfaces was hardly changed. Lower the vinyl alcohol comonomer content, higher is the increase of the water contact angle value (Table 1). Table 1: Wettability of CF4 plasma-treated polymers Sample eHjO T" r" / LLDPE [OH] = 0 mol% 92 26.3 1.2 27.5 LLDPE 50W 15 min 95 21.0 1.1 22.1 EVOH(E) [OH] = 56 mol% 76 32.8 3.8 36.6 EVOH(E) 50W 15 min 98 12.0 3.2 15.2 EVOH(F) [OH] = 68 mol% 72 30.5 7.3 37.8 EVOH(F) 50W 15 min 99 15.9 2.7 18.6 EVOH(S)[OHl = 71mol% 52 29.3 17.2 46.6 EVOH(S) 50W 15 min 105 12.2 1.5 13.7 The decrease of surface energy of polymeric material, corresponding to an increase of the water contact angle, was associated to a chemical surface modification. XPS analyses of EVOH, polyethylene (Fig. 1) or SU8 plasma-treated surfaces give evidence of fluorinated groups attachment. Depending on plasma parameters, the chemical surface structure varies from CHF group to the Teflon-like structure. The atomic F/C ratio can reach the value of 0.5 with pure CF4 plasma treatment. Addition of hydrogen to the CF4 plasma phase allows to control the fluorine atom density in the plasma phase and therefore to modify the surface fluorination rate. Addition of 10% of hydrogen leads to the lowest polar surface energy of plasma-treated SUB surface (Fig. 2). 3 Superhydrophobic plasma-fluorinated polymers Whatever the plasma phase composition is, the elemental composition of the CF4 plasma-treated polyethylene is associated to the grafting of CHF-CH2, CF CF2 and CF3functional groups whose proportions depend on plasma parameters. But another side effect of the plasma interaction is observed and corresponds to the surface degradation and roughness. Drastic powers, long durations or oxygen plasma pre-treatment emphasize this phenomenon. The s >1 i800a 16000 14000 12000 10000 800a 6000" 4000- 200a 0--2000 a) ffl^-CHF 278 280 282 284 286 288 290 292 294 Binding energy (eV) 20000- 18000- 16000- 14000- 12000- S 10000- g 8000- 6000- 4000- 2000- 1 ' 1 284 286 288 290 292 294 296 Binding energy (eV) Fig. 1: Plasma-fluorinated HOPE in different conditions. combination of surface fluorination and roughness induces the superhydrophobic character appearance (i.e. water contact angle higher than 150°). Beside this evolution, if the discharge power and/or duration are important, the hysteresis of the contact angle becomes smaller. Figure 3 illustrates the domain of roughness leading to polyethylene superhydrophobic surface. 4 Applications of plasma-fluorinated polymers These plasma-fluorinated surfaces may be used in different domains. The first one is dealing with the biomaterials field. The bioadhesion or the non adhesion of proteins, cells onto the biomaterials is strongly dependent on the nature of the interactions between these two entities. Among them, hydrophobic-hydrophobic and electrostatic ones are the predominant ones. However, in the case of nonadhesion processes, biosurfaces bearing a superhydrophobic character may act as self-cleaning surface towards the biomolecules adhesion. The self-cleaning property implies a fast dynamic de-wetting of a water thin film as shown in Fig. 4. Another application is dealing with barrier properties and permeability of plasma-treated polymers. If the water per- b) ^ 5- r-i I-; 4" 3- (U s U L 0 2 4 6 8 10 12 14 16 18 [H,] (%) Fig. 2: Dependence of gaseous fluorinated species on discharge power (a), of surface energy of piasma-treated SU8 on hydrogen proportions (b). 2 steps □ Istep ■ 50 100 150 200 250 300 roughness RMS (nm) Fig. 3: Dependence of the contact angle versus roughness of piasma-fiuorinated poiyethyiene. meability coefficient is almost constant after the plasma fluorination of linear low density polyethylene, this coefficient is strongly altered by the surface fluorination of EVOH, especially when the vinyl alcohol content is increasing (Table 2). This barrier effect is also noticed with plasma fluorination of Polyamide 12. Complete dewetting PTFE PEBD UH A-^^qi^PEBD UH 00 0,05 0,10 0,15 0,20 0,25 0,95 1,00 1,05 1,10 time (s) Fig. 4: Dynamic de-wetting of thin water layer onto poiymer surfaces. Table 2: Water permeability of plasma-fluorinated polymers. Sample [OH] Thickness P(Ban) yS (mol %) (um) LLDPE 0 55 68 27.5 rOHl = 0 mol% LLDPE 0 55 64 22.1 50W 15 min EVOH(E) [OH] = 56 mol% 56 15 241 36.6 EVOH(E) 56 14 86 15.2 50W 15 mm EVOH(F) rOHl = 68 mol% 68 15 222 37.8 EVOH(F) 50W 15 min 68 15 1865 18.6 EVOH(S) 76 14 31350 46.6 [OH] = 71 mol% EVOH(S) 50W 15 min 76 101 17754 13.7 Fluorination, obtained with the CF4 plasma treatment allows reducing water permeability of the films. The higher hydrophilicity of the base materials, the more water barrier effect of CF4 plasma treatment is obtained compared to the non-treated materials. However, the very significant enhancement of the water barrier properties of EVOH(S) after plasma treatment was explained not only by the introduction of fluorine groups but an increase of the crystallin-ity too, so that the diffusivity and solubility of water through the CF4 plasma treated film were more reduced. The effect of surface modification on water permeability decreased the water uptake at the film surface at the feed side so that the plasticization effect of the hydrophilic films by the sorbed water molecules was decreased during the water vapour permeation. The last illustration will be given with the decrease of optical losses thanks to the plasma fluorination. The measured propagation loss values of untreated and plasma-treated SU8 single-mode planar waveguides for both transverse electric (TE) and transverse magnetic (TM) polarizations at 632.8, 1304 and 1540 nm are shown in Fig. 5. ^ SU-8 blank ^ SU-8 CF^ plasma-treated xEq Mode SU-8 CFyH2 plasma-treated 632,8 nm 1304 nm Wavelength (nm) 1540 nm b) 632,8 nm 1304 nm Wavelength (nm) 1540 nm Fig. 5: Opticai iosses of plasma-fluorinated SU8 waveguide 5 Conclusions Plasma fluorination of polymers leads to a chemical surface modification through the grafting of fluorinated groups. 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