Langmuir 1995,11,905-911
905
Formation of Poly(tetrafluoroethy1ene) Thin Films on Alumina by Admicellar Polymerization Chung-Li Lai, Jeffrey H. Harwell, and Edgar A. O'Rear* School of Chemical Engineering a n d Materials Science, The University of Oklahoma, Norman, Oklahoma 73019
Shigeki Komatsuzaki, Juichi Arai, Takayuki Nakakawaji, and Yutaka Ito Hitachi Research Laboratory, Hitachi, Ltd., Hitachi-shi, Ibaraki-ken, J a p a n Received July 15, 1994. I n Final Form: November 9,1994@ Partitioning of tetrafluoroethylene gas into admicelles of sodium perfluoroheptanoate on alumina was measured in a high-pressurereactor. Polymerizationoftetrafluoroethylenewithin the adsorbed surfactant bilayer took place after thermal initiation with the formation of fluorocarbonthin films on aluminum oxide powder. In the formation of poly(tetrafluoroethy1ene) (PTFE),the effects of surfactant concentration and of initiator concentration were studied. Modified powder was characterized by a number of techniques including Fourier transform infrared (FTIR) spectroscopy, wavelength-dispersivespectroscopy (WDS), atomic force microscopy (AFM),and wettability testing. Measurements of friction coefficients and contact angles verified that a thin poly(tetrafluoroethy1ene) film was formed on alumina plates using the same procedure.
Introduction For high-speed magnetic storage devices used in the electronics industry, it is necessary to have small gap spacings between the reading head and the fast moving magnetic medium. Solid lubricants play a n important role, helping to protect the system from vibrations or collisions; in general, they are materials having a characteristic crystalline structure which causes them to shear into thin, flat plates, which easily slide over one another and thus generate a low friction effect.l High chemical stability and resistance to oxidation a t high temperature are important factors for choosing a solid lubricant. Fluorocarbon polymer coatings are prospective solid lubricants. Although their cost is higher than that of hydrocarbons, poly(tetrafluoroethy1ene)coatings can be used over a wide range of temperatures and are stable to heat and oxidation. A flexible method for the formation of thin fluorocarbon coatings might make them economically viable. In bulk solution, the polymerization of tetrafluoroethylene can be accomplished in the presence of water and water-soluble free-radical initiators by using a method similar to emulsion polymerization.2 Since most fluorinated monomers are sparingly water-soluble, perfluorocarbon surfactants, used as emulsifiers, offer a good environment for this technique. Because of the magnetic media application^^-^ in the areas of protecting surfaces which come in contact during sliding over a solid polymer surface, perfluoropolyethers and other lubricants have been prepared with far-UV irradiation6,' of a thin film ofthese lubricants on inorganic @Abstract published in Advance A C S Abstracts, February 1 ,
1995. (1)Braithwaite, E. R. In Solid Lubricants and Surfaces; Pergamon: New York, 1964; Chapter 1. (2) Piirma, I. In Emulsion Polymerization; Academic Press: New York, 1982; Chapter 2. (3) Moulder, J. F.; Holland, L.; Smith, K. L. Appl. Surf. Sci. 1986, 25,446-454. (4) Miyamoto, T.; Sato, I.; Ando, Y. Tribol. Mech. Magnetic Storage Systems 1988,5,SP-25,55-61. ( 5 ) Scarati, A. M.; Caporiccio, G. IEEE Trans. Mag. 1987, Mag23, 106-108. (6) Kellock, A. J.; Nyberg, G. I.; Williams, J. S. Vacuum 1985, 35, 625-628. (7) Saperstein, D. D.; Lin, L.J. Langmuir 1990, 6, 1522-1524.
substrates. Far-UV irradiation has been shown to lower surface free energy and to fix fluorinated lubricants to most inorganic surfaces. In this paper a process for the formation of ultrathin films from fluorocarbon surfactants, fluorocarbon monomers, and metal oxide substrates is presented. The filmforming process includes three major steps: (1) the adsorption of perfluorocarbon surfactants on aluminum oxide substrates to form admicelles; (2) the solubilization of tetrafluoroethylene gas monomer into the admicelles; (3)the initiation of polymerization within the fluorocarbon bilayers. These procedures are similar to those where surfactant bilayers have been used as two-dimensional solvents for the formation of polystyrene ultrathin 61ms.8-" This study, however, is the first to use perfluorocarbon surfactants in the surfactant bilayers and fluorocarbon materials as monomers. Moreover, the three-step film forming process studied by Wu et al.s-10 is carried out here for the first time with a gas phase monomer. The schematic diagram for the process of forming fluorocarbon thin films is presented in Figure 1. In this process, the transport of gas monomers through the aqueous phase into the admicelles on the solid substrates plays an important role. It was expected that, with suitable modifications to the reactor, this technique of forming polymeric thin films could be adapted to gas monomers as well. For this study, two types of alumina samples were used. An aluminum oxide powder (y-alumina, particle size 0.02 mm, and average pore size 80 A with surface area 100120 mVg, obtained from Degussa corporation) provided sufficiently high surface area to determine solubilization of the gaseous monomer. The planar geometry needed for friction testing and contact angle measurements came from A 1 2 0 3 chips. The alumina plates were provided by Hitachi Research Laboratory to the group a t the University of Oklahoma and were bought from Hitachi Chemical Ltd. These plates are alumina fired under pressure. The (8) Wu, J.;Harwell,J. H.; O'Rear,E.A.Langmuir 1987,3,531-537. (9) Wu, J.; Harwell, J. H.; O'Rear, E. A.; Christian, S. D. AIChE J. 1988,34 (9), 1511. (10)Wu, J.; Harwell, J. H.; ORear, E. A. Colloids Surf. 1987, 26, 155. (11) U.S. Patent 4770906, 1988.
0743-746319512411-0905$09.00/0 0 1995 American Chemical Society
Lai et al.
906 Langmuir, Vol. 11, No. 3, 1995 FLUOROCARBONSURFACTANT BILAYER
iiiTli
GF13COONa
FLUOROCARBONSURFACTANT BILAYER
ADSOLUBILIZATIONOF TETRAFLUOROETHYLENE MONOMERS
in the following systems: (1)100 mL of 0.06 M CsF13COONa solution with 0.1 M NaCl adjusted to pH 4; (2) system (1)plus 5 g ofAl203 powder. The samples were equilibrated for 15 days. The equilibrium adsolubilization constant, Kads, is a partition coefficient analogous to that used in solubilization studied2
Kad8= (ratio of adsolubilized TFE molecules to adsorbed surfactant molecules)/(concentration of TFE in the supernatant) Additional experiments were carried out to find the solubility and solubilization of TFE in distilled deionized water, a t the critical micelle concentration (cmc) (0.1 M SPHN) and above the cmc (0.4 M SPHN and 0.4 M SPHN with 0.1 M NaCl).
Polymerization of TFE Gas Monomer within Sodium Pertluoroheptanoate Admicelles. After an adsolubilization
ADSOLUBILIZATIONOF TETRAFLUOROETHYLENE MONOMERS
I - fluorocarbon surfactant
POLYMERIC THIN FILM
M - C2F4 monomer
Figure 1. Schematic of the process of in situ fluorocarbon thin film formation. purity of A 1 2 0 3 plates is 96%, and the surface roughness of these chips is less than 5 p m .
Experimental Section Adsolubilization of Tetrafluoroethylene (TFE) Gas Monomer. Five grams of aluminum oxide powder was put into a stainless steel Parr Model 4561 mini high pressure reactor with a size of 300 mL. A 100-mL aqueous solution of surfactant with a known initial concentrationof sodium perfluoroheptanoate CsF13COONa (SPHN)and containing 0.1 M NaCl at pH 4 (before adsorption, for an equilibrium supernatant concentration in - l ~fed ) into the reactor region 3 of the adsorption i s o t h e ~ - m ~ ~was and the reactor sealed. The reactor was purged with nitrogen. After this, TFE gas monomer, obtained from PCR Inc., was introduced into the reactor a t pressures of 100-140 psig. Temperature and stirring speed were controlled a t 30 "C and 20 rpm, respectively. The pressure decrease with time was recorded to determine the adsolubilizationof TFE monomer. The solubility/solubilization ofTFE in the supernatant was checked by using the gas trap method as well as pressure loss. Blank tests for the gas trap a t certain pressures over the adsolubilization range were carried out by introducing TFE gas into the reactor, equilibrating, and then venting the reactor to the atmosphere immediately. The gas was trapped into two 1000-mLgraduated cylinders filled with water a t 1atm and the gas volume recorded. After the recorded volume was converted to moles of gas (by using the ideal gas law) a mass balance was performed to determine the solubilizatiordadsolubilization of TFE monomer (12) Rosen, M. J. In Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1978; Chapter 4. (13)Scamehom, J. F.; Schechter, R. S.; Wade, W. H. J . Colloid Interface Sci. 1982,85,463. (14) Scamehom, J. F.; Schechter, R. S.; Wade, W. H. J . Colloid Interface Sci. 1982,85,479. (15) Scamehorn, J . F.; Schechter, R. S.: Wade, W. H. J . Colloid Interface Sci. 1982,85, 494. (16) Bitting, D.; Hanvell, J. H. Langmuir 1987, 3, 500. (17) Scamehorn, J.F.;Schechter, R. S.;Wade, W. H. J.Am. Oil Chem. Soc. 1983,60 (71, 1345.
measurement, the desired concentrationof ammonium persulfate (NH4)2SzOs, used as an initiator and obtained from Aldrich Chemical Co., sodium bisulfate NaHS04 (one-fifth of quantity of ammonium persulfate by weight), and 2 ppm ferrous sulfate FeS04,18usedas an initiator regulator and acquired from Aldrich Chemical Co. and J.T. Baker Chemical Co., respectively, were added. These agents were charged into the reactor by purging the reactor with nitrogen, venting, and then opening the reactor. Desired amounts of ammonium persulfate used in these polymerization experiments were 0.025%,0.25%,and 2.5% by weight, respectively. The initiator regulator was included to improve the initiator effectiveness at low temperatures. After being repressurized with TFE at 140 psia, the reactor was heated with stirring a t 100 rpm to 80 "C. A decrease in pressure indicated adsolubilization and the beginning of polymerization. In order to maintain the pressure, it was necessary to introduce more TFE periodically. The reaction mixture was heated and continuously stirred a t 100 rpm for 36 h. The stirring speed was then reduced to zero, the reactor contents were cooled, and the pressure was decreased to atmospheric by venting the reactor. The supernatant above the aluminum oxide powder was poured off carefully. The remainingslurry was thoroughly washed once with 65% methanol (volume percent) and then several times with distilled and deionized water in order to make sure that sufficient surfactant had been washed off t o expose the PTFE film. The slurry was then filtered and air-dried. After air-drying, the sample like a filter cake was placed in an oven at 60 "C overnight in order to remove bulk water in the sample. After drying, the material is easily broken up by rubbing between the thumb and forefinger. A mortar and pestle was used with a light force to produce a uniform powder for the additional tests. The pressure decrease with time a t the reaction temperature for polymerization was recorded. Values for polymerization (g/g of alumina) were obtained from the weight differences in the alumina sample after drying. The conversion was determined by the calculation of a mass balance between the weight differences in the alumina sample before and afler polymerization and total TFE charged into the reactor. Alumina chips prepared under similar conditions were carefully packaged and sent by express mail to Japan for contact angle and friction measurements. Fourier Transform Infrared (FTIR)Spectroscopy. The FTIR spectra of PTFE-coated aluminum oxide powder and surfactant adsorbed on the aluminum oxide powder were obtained by diffuse reflectance measurements on a BIO-RAD FTS-40 spectrometer. A small amount of the PTFE-coated sample was thoroughly ground in a clean mortar. A mixture of the finely ground sample (5 wt %) and potassium bromide powder (FTIR grade from Aldrich Chemical Co.) was pressed in a FTIR sample cell for testing a t room temperature. Scanning Electron Microscopy (SEMI. An electron microprobe SX50 (Cameca Instruments, Inc.) was used to analyze fluorine content in the sample of PTFE-coated aluminum oxide powder and to check fluorine levels in a control sample with surfactant first adsorbed and then washed off. In SEM, wavelength-dispersive spectroscopy(WDS) is a method of X-ray analysis that employs a crystal spectrometer to discriminate characteristicX-raywavelengths. In order to have some electrical (18)Coker, J. N. J . Polym. Sci.: Polym. Chem. Ed. 1975,13,24732489.
PTFE Thin Films
Langmuir, Vol. 11,No. 3, 1995 907 Load 4
Load Cell
I9I "'le
apphire Hemisphere p4
adropofwater
-
Surface Treated Sample
samplestage
I
I _
I-
(b)
LoadWeight
hortzontdtop Ine water drop
/
Figure 3. Schematic diagram of the test system of friction and contact wearing. 4.0
T = 3OoC,W = 20 rpm T (contactangle) = T1+72
Figure 2. Contact angle measurements: (a) putting a small pure water drop on substrate; (b) measuring water drop edge through an equipped microscope; (c) view through the microscope.
.
0 0 0 0 0
6n 0
0
B
2.4
0 0
E
1.6
c.
0.8
Y
s
0 0
0
3
w cr
o
A A A A A &
4
a
12
16
20
Time (days)
n
9
QfA
0.0
b
u
0
0
I
a
conductivity, the PTFE-modified A 1 2 0 3 WDS sample was coated with carbon and then mounted on the sample cell. Atomic Force Microscopy (AFM). An atomic force microscope (Digital Instruments, Inc. NanoScope 11)was used to study the surface characteristics of PTFE thin films on thin A 1 2 0 3 chips. The atomic force microscope (AFM) can image both conductors and nonconductors to atomic resolution. With forces usually on the order of 10-lo-lO-*l N, an AFM reads a surface by touching it with the tip on a cantilever and then tracing the surface profile. Verified displacement of the tip is sensed by laser light reflected from the cantilever mount. Because the force applied by the tip is so small, an AFM can probe a wide range of substrates without damaging the surfaces. Contact Angle Measurements. Contact angles were measured on planar alumina chips by placing a drop of pure water on the substrate. After 30 s from this process, angles of the edge of a water drop were measured by use of a microscope equipped with a goniometer eyepiece. The schematic diagram for the procedure for contact angle measurements is shown in Figure 2. The contact angle was obtained from the sum of angles T1 and T2 in Figure 2c due to geometry. The reason for not using direct measurement is that measuring a high contact angle is more difficult than it appears. Evaluationof Friction Propertiesand Wearing. Figure 3a is a schematic diagram of a n experimental apparatus for evaluation of friction properties. The friction test was carried out between the upper sapphire hemisphere and the surfacetreated sample; that is, the surface was covered by the above described three-step film-forming process with a thin PTFE film. For reference, 0.1 mm thick PTFE sheets were spread for testing on alumina plates by mending tape; the original alumina plates before surface treatment were also tested. Reference materials were cleaned supersonically prior to the tests, first by using acetone for 5 min twice and then by using CFC-113 for 5 min once. Experimental conditions were as follows: (1) applied vertical load, 5-300 $, (2) slidingvelocity, 30 m d m i n ; (3)radius of sapphire, 30 mm; (4) sliding distance, 10 mm; (5) sliding movement, one way. The method used in the wearing evaluation was a contact type wearing durability test. A schematic diagram of the wear testing system is drawn in Figure 3b. Experimental conditions were as follows: (1)load weight, 5 &, (2) rotation
3.2
1
1
4.0 1
3.2
T = 3OoC,W = 100 rpm
L
W
b
o
4
a
12
16
20
Time (days)
Figure 4. Adsolubilization and solubility as a function of time: (1)adsolubilization of TFE for the initial pressure of (a) 150 psi and (c) 121 psi; (2) blank test of the solubility of TFE for the initial pressure of (b) 145 psi and (d) 120 psi. rate, 3 d s ; and (3)maximum number ofrotation pass, 20Kpasses. The size of alumina disks used in this wearing evaluation was 3.75 in. diameter.
Results and Discussion Effects on Adsolubilizationof TFE Gas Monomer. Figure 4a shows the total uptake of TFE versus time obtained from a mini high pressure reactor a t 30 "C and a stirring speed of 20 rpm. The data indicate that the uptake of TFE is 1.54 g by an aqueous slurry (108 g) of 100 mL of 0.06 M CeF&OONa and 5 g of A1203with an initial pressure of 150 psi. Figure 4c displays the results at 30 "C and a stirring speed of 100 rpm with an initial pressure of 121 psi. Figure 4b illustrates a blank (surfactant-free) test for the solubility of TFE monomer; it should be noted that although a slight solubility of TFE
908 Langmuir, Vol. 11, No. 3, 1995
Lai et al. Table 1. Adsolubilization Dataa
initial pressure of TFE (psi) 121 130 140
final pressure of TFE (psi) 116 121 120
final TFE concn in supernatant(M) 1.47 x 1.47 x 1.47 x
TFE adsolubilization
TFE/C6F13COONa ratio
Kads
OlmoVg) 253 691 1895
in admicelles 0.59:l 1.54:l 3.99:l
(M-1) 40 104 271
a The conditions for the adsolubilization of TFE were as follows: (1)100mL feed solution of 0.06M CsF13COONa with 0.1M NaCl adjusted to pH 4 with 0.1 M HC1; (2)5 g of A1203 powder; (3)the C6F13COONa adsorptions bmoVg) were 429 for the initial pressure of 121 psi, 450 for the initial pressure of 130 psi, and 475 for the initial pressure of 140 psi. The amount of surfactant adsorbed per gram of solid substrate was determined by measuring the concentrations of surfactants before and after adsorption using a high-performance liquid
chromatograph. 0.14 L
I
15
L
0
20
40
60 80 100 . Pressure (pslg)
120
I40
160
Figure 5. Solubilizationof TFE in sodium perfluoroheptanoate solution at 30 "C: (a)only DI water (Le. no surfactant), (b) 0.1 M, (c) 0.4 M, (d) 0.4 M with 0.1 M NaCl. The cmc of sodium perfluoroheptanoateis about 0.1 M.
of 0.192 g in a slurry of 100 g of water and of 5 g Ai203 a t the initial pressure of 145 psi could be measured, our technique was unable to detect TFE uptake with a n initial pressure of 120 psi (Figure 4d). Table 1 presents the adsolubilization calculated from the pressure drop and the solubility results and the Kaasat three specified initial pressures of TFE, viz. 121,130, and 140 psi, respectively. The value ofKadsincreases with increasing initial pressure of TFE and is in the range of 40-270 M-l. Ratios of TFE monomer to perfluorosurfactant in admicelles were found to vary between 0.6 monomer per surfactant molecule to as high as 4 TFE monomers per surfactant. The change in adsolubilization from 120 to 140 psi may be due to pore condensation and Laplace effect. Any factor that reduces gas solubilization, for instance, the permeation of water into the admicelle, will be reflected as an increase in Laplace pressure. It is apparent from this that the great hydrophobicity of the TFE monomer makes the adsolubilization process in perfluorosurfactant bilayers highly efficient. Solubilization of TFE in Sodium Perfluoroheptanoate (SPHN) micelle^.'^ In the solubilization experiment, the pressure differences combined with system temperature, and gas phase volume were translated into solubilities through the ideal gas law. Figure 5 shows the solubilities ofTFE in deionized water, and in 0.1 M SPHN (Le. cmc), and solubilization in 0.4 M SPHN, and in 0.4 M SPHN with 0.1 M NaC1. These results are obtained for a system temperature of 30 "C. In the 0.1 M SPHN solution of perfluorinated surfactant, in which almost no micelles were formed, the solubility of TFE appeared to be very close to that in water, indicating almost no solubilization occurred. Though saturated at low pressure, the solubility of TFE in water increased nonlinearly from 70 to 105 psia before reaching a second saturation point. At 3 times the saturation concentration of that a t low pressure, aggregation of TFE without surfactant may be (19)Chen, H.Y .M.S. Thesis,The University ofOklahoma, Norman,
OK,1993.
o ~ ~ ~ ~ " ~ ~ ~ " ' " ' ~ " " ' " " ' ' ' " ' ' " ~ 0
5
10
15 20 25 Reaction Time (hour)
30
35
Figure 6. Polymerization versus reaction time for different total concentrations of surfactant C6F13COONa: (A) 0.2 M, (B) 0.06 M. Conditions for the polymerization: (1)initiator, 0.25 wt % (NH~)zSZOS; (2) initial pressure; 135 psi; (3)temperature, 80 "C.
occurring. At a surfactant level 4 times cmc, however, the solubility of TFE was greatly enhanced due to the solubilization of TFE in SPHN micelles in the aqueous phase. The addition of electrolyte tends to decrease the repulsion between the similarly charged head groups of surfactants in micelles. This reduction in mutual repulsions of surfactant head group typically leads to the increase in aggregation number and causes closer packing of surfactant molecules in the palisade layer. The increase in aggregation number results in an increase of solubilization in the inner core ofthe micelle;however, the closer packing in the palisade layer reduces the space available there for solubilization. The lowered solubility of TFE in 0.4 M SPHN with 0.1 M NaCl, compared with that ofTFE without NaC1, could be the result of the balance of these two effects. When the data of Figure 4a, and the data of Figure 5 are compared, the uptake of TFE in the SPHN admicelles appeared to be 3 times that of SPHN micelles (4 times cmc)for similar amount of surfactant. This shows that introducing alumina to allow formation of the perfluorosurfactant bilayers makes the uptake of TFE greatly enhanced. Polymerization of TFE Gas Monomer. As shown in Figure 6 , polymerization (g of PTFE/g of Al2O3) versus reaction time is almost independent of surfactant concentrations in the period of the first 10 h. Because there are more surfactant molecules adsorbed onto the aluminum oxide powder for a higher initial concentration of C ~ F I ~ C O Othe N ~polymerization , for 0.2 M C6F13COONa is slightly higher than that for 0.06 M of C6F13COONa in the period of final reaction time. Figure 7 shows the relation between polymerization and reaction time a t various initiator concentrations. Polymerization increases remarkably with increasing initiator concentrations from 0.25 to 2.5 wt %. Three examples of conversion versus reaction time curves are also shown in Figure 7. For all experiments, the conversion data are based on total TFE introduced into the reactor. The conversion reaches 35% for an initiator concentration of 2.5 wt %, while it reaches
PTFE Thin Films
Langmuir, Vol. 21, No. 3, 1995 909 35
28
n
s
w
21 0.375
14
0.250
..6E ;
0.125
0
10 20 30 40 50 Reaction Tme(banr)
Figure 7. Polymerization and conversion as a function of reaction time for different initiator concentrations: (a) 2.5 wt % (NH4)zSzOa; (b) 0.25 wt % (NH4)2SzOa; (c) 0.025 wt % (m4)2sZo8.
7%for a n initiator concentration of 0.25 wt %. Although the conversion is higher for an initiator concentration at or above 2.5 wt %, it should be pointed out that a t this concentration there is considerable polymerization in the bulk solution even though the concentration of surfactant is below cmc (a murky solution being observed in the supernatant);however, this never seems to happen for an initiator concentration a t or below 0.25 wt % (a clear solution in the supernatant). In studies by Takeshi Suwa et al.,zOa n interesting and possibly relevant observation was reported in that PTFE, a hydrophobic polymer produced by radiation-induced emulsion polymerization, formed as a stable latex in the absence of surfactant. A few workers had investigated surfactant-free polymerization of styrene,21122 methyl m e t h a ~ r y l a t eand , ~ ~styrenebutadienez4 with K2SZOs as initiator in the absence of surfactants. According to this literature, a small degree of polymerization in the bulk solution is not unreasonable. The temperatures for adsolubilization and polymerization are intentionally different. I t was our experience that, on occasion with similar systems, the polymerization of TFE would self-initiate prior to or concurrent with adsolubilization. The end result was a polymer coating on the inside of the reactor vessel that required cleaning by blasting with glass beads. This coating was not observed when carried out as described. Unfortunately, we cannot provide a direct estimate of the degree of polymerization in the admicelles. The inertness and insolubility of PTFE make its quantification very difficult. We know from work with other polymers that the films remain hydrophilic if not washed aRer the polymerization. However, we also know from assays in similar systems that about halfthe surfactant generally remains with the film. FTIR and WDS Studies. A typical FTIR spectrum of the PTFE sample prepared by using the above method a t a n initial C,$'&OONa concentration of 0.06 M and a n initiator concentration of 0.25 wt % is shown in Figure 8B; for comparison, the FTIR spectrum of a PTFE standard, obtained from Aldrich Chemical Co., is also shown (C). One characteristic F'TFE band appears between 1100 and 1300 cm-l; the other characteristic peak (20)Suwa, T.;Watanabe, T.; Okamoto, J,; Machi, S. J.Polym. Sci.: Polym. Chem. Ed. 1978,16,2931-2944. (21)Juang, M. S.D.; Krieger, I. M. J.Polym. Sci.: Polym. Chem. Ed. 1976,14,2089. (22)Konno, M.; Terunuma,Y.; Saito, S. J. Chem. Eng. 1991, - Jpn. . 24, 4. (23)Ono,H.; Saeki, H. Colloid Polym. Sci. 1976,253, 744. (24)Ceska, G.W. J.Appl. Polym. Sci. 1974,18,427.
1
,
,
,
,
,
,
*
,
2400 2200 2000 1800 1600 1400 1200 loOD
Wavenumbers(cm")
Figure 8. A comparison of FTIR spectra of in situ growth of PTFE films with a PTFE standard (A) 5 h reaction time; (B) 35 h reaction time; (C)PTFE standard; (D) adsorbed surfactant only.
of PTFE shows a t 2350 cm-l. Figure 8 also compares FTIR spectra of PTFE samples at 5 h reaction (A) and at 35 h reaction (B). The FTIR spectra are consistent with PTFE being present on the surfaces of the aluminum oxide powder. For comparison, a n FTIR spectrum of adsorbed surfactant only is shown in Figure 8D. A spectrum for checking fluorine levels in the PFTEcoated A1203 sample (prepared at a n initial C&'1&OONa concentration of 0.06 M and a n initiator concentration of 0.25 wt % for 35 h reaction time) by using WDS is shown in Figure 9A. Presence of fluoropolymer is supported by a W D S study (Figure 9B) which reveals that a sample of alumina exposed to the surfactant solution under experimental conditions without polymerization and with subsequent washing seems to be essentially fluorine-free while a polymerized sample with subsequent washing has high fluorine content. The counting level for fluorine in Figure 9B is 5 and, even a t this higher sensitivity, the figure does not show a specific peak for fluorine in the characteristic range of X-ray wavelengths between 17.86 and 18.77 A; however, the counting value is 90 in Figure 9A and the figure shows a n intense, specific peak for fluorine. This shows that the fluorine in the PTFE-coated A1203 sample is not simply adsorbed surfactant. Part of the signal in Figure 9A could be from surfactant, but the surfactant spectra are much weaker. AFM Study. Figure 10A presents an AF'M line plot of a PTFE thin film on a n A1203 chip. This picture suggests how the fluoropolymer layer extends over a several hundreds ofnanometers. Shown in Figure 10B is an AFM line plot of a n original aluminum oxide chip, showing the roughness of the original chip on this scale. The AFM indicated that the polymer film formed by the in situ polymerization conformed closely to the original contours ofthe chip surface; however, while the chip surface showed variations in height in this sampling on the order of 70
910 Langmuir, Vol. 11,No. 3, 1995
h i et al. Table 2. Conditions of Samples Prepared for the Test of Friction Coefficient and Contact Angle
Sp: 5 Scale:90 cts Xtal: TAP 15 Kev
conditions contact sample C6F13coONa (NH4)2S~08NaHS04 FeS04 angle no. (MI (wt%) (wt%) (ppm) (deg) ~
1 2 3
4
I
II
1
PTFE alumina
1
256
Channel Sinus theta Lambda (A)
0.69499 17.8535
0.73060 18.7700
Figure 9. Spectra for checking fluorine levels: (A) sample
after in situ polymerization on alumina powder; (B)the sample with no polymerization and with the surfactant washed off (note differences in scale).
0
250
sod N(
Figure 10. AFM line plots: (A) FTFE formed in situ on an
alumina chip; (B)original alumina chip; (C)reference PTFE sheet. nm, the coated surface showed variations on the order of only 30 nm. Pictures of alumina chips at higher magnification exhibited substantial roughness of a fine scale with many spikes. An AFM image of a reference PTFE sheet is given in Figure 1OC showing variations on the order of 50 nm, between the orders of an original alumina
0.02 0.03 0.04 0.04
~~
~~
0.3 0.3
0.5 1.5
0.06 0.06 0.1 0.3
2 2 2 2
113.0 113.0 111.2 109.5 116.0 52
plate and a PTFE-treated alumina plate. The objective of the AFM measurements really was to get some idea of the uniformity of the film. In the end, we felt that, with the data available, we could not make a conclusive statement about uniformity of the film. Contact Angles. The PTFE-treated aluminum oxide powder was extremely hydrophobic and floated on the surface ofwater, unlike the untreated powder. All powder samples prepared by the above technique floated on water even a h r washing them with acetone, methanol, and tetrahydrohan. Every surface-treated sample, as shown in Table 2, had a much higher contact angle than the original alumina plate, approaching the same level as the reference PTFE sheet. The reproducibility of the contact angle measurements is f2". Friction and Wear Evaluationof Surface-Treated Alumina Plates. Friction coefficientsare plotted against loads in Figure 11. Every sample had a lower friction coefficient than the original alumina plate, though they did not approach that of the reference FTFE sheet. This difference may arise from the surface roughness on the alumina plates, which was apparent with the AFM, and fewer points of contact. PartsA and B of Figure 12 show that the friction coefficients change through the test as a function of feed pH values for surfactant concentrations of 0.02 and 0.1 M,respectively. There were two modes in the friction change of wearing test. In one mode (samples B4 and B6),their friction coefficients showed a minimum value at the beginning of the test and then increased monotonically with the number of rotation passes. In the other mode for the remaining test samples, the friction coefficients showed the highest value at the beginning, then decreased to a minimum value until around a thousand passes, and then increased sharply with the number of passes. For evaluation of friction properties, no large differences of friction coefficients were found among the surface-treated samples; the 0.1 mm thick reference PTFE sheet had the lowest value among the test samples. This indicates that film thickness itself may have an effect on the friction coefficientfor ultrathin PTFE films. For the wearing evaluation, as far as the initial friction coefficientwas concerned,it clearly depended upon the pH value of the deposition solution. It had a minimum value at pH 4 for both surfactant concentrations of 0.02 and 0.1 M. Moreover, the values for both pH 4 and pH 6 were almost the same for both concentrations of 0.02 and 0.1 M, respectively. However, at pH 2, we could detect a difference. The value of the initial friction coefficient formed at the concentration of 0.1 M was lower than that formed at 0.02 M. In addition, in the wear tests the sample formed at pH 4 was the only one which showed a lower friction coefficientthan that ofbare A1203 substrate(0.225, initial). As for the friction coefficient after the wearing test, the trend of pH dependency differed for each surfactant concentration. In the case of 0.02 M,there was a minimum point at pH 4. On the other hand, coefficients increased monotonically with pH values in the case of 0.1 M surfactant. All values of friction
Langmuir, Vol. 11, No. 3, 1995 911
PTFE Thin Films 0.2
0.15
.$ -A-
8
Samph 1
--bSample 4
g 0.05
0 50
100
150
250
200
300
Load (00
Figure 11. Relation between applied load and friction coefficient.
of the chips. In this case, the PTFE may dominate the contact angle measurements but the ahmindsapphire head contact points may dominate the friction and wear testing results. The second possibility is that the PTFE film is thin and only loosely bound to the surface of the chip. In this case, the contact angle measurements are being made on the PTFE layer, but when a load is placed on the film, the PTFE film may be pushed aside by the sapphire head to allow direct sapphirelalumina contact. (Either hypothesis is consistent with the very rough surface of the bare alumina observed at very high magnification with the AFM.) Using the surface area of 100m2/gand a perfluorocarbon mass density of2.2 g/cm3,27 one obtains an estimate of film thickness of 30 A from the alumina powder results. Whichever model is most appropriate, however, the route to the improvement of the performance of the film is probably the same: introduction of a cross-linking agent into the polymerization process and/or binding of the polymer to the substrate, perhaps by U V radiation. It is also possible that use of a polymerizable surfactant, such as that recently reported by Esumi et al.,28,29 could improve the performance of the film in reducing friction.
Conclusions
--(-
-b-
1
2
3
4
5
6
initiai 2OKpasses 15Kpasses
7
pH Vd8e
Figure 12. Wearing test of thin PTFE film: (A) prepared at 0.02 M C6F&OONa surfactant;(B)prepared at 0.1 M C6F13-
COONa surfactant.
coefficient for the treated chips were smaller than that of substrate (0.3) after 15K passes. In addition, an estimate of the pKa of perfluoroheptanoic acid is 2.25 On the basis of the above acidic condition, at pH 2, the system approaches the acidity of perfluoroheptanoicacid, so that interfacial structures may include adsorption and/or precipitation aggregates. Although SPHN is a carboxylate surfactant and SPHN could become an acid type a t acidic conditions, the acid type of SPHN is still a surfactant having aggregation properties.26 Different aggregate structures may help to explain the pH dependence in the friction coefficient measurements. The most obvious question which arises from this study concerns the discrepancy between the very large contact angles, which approach that of neat F'TFE, and the disappointinglylarge friction coefficients,which are little different from that of the bare alumina. Two possibilities would explain these results. First, it may be that the PTFE layer is not uniformly distributed over the surface
A1203
(25)Dean, J. A. In Lange's Handbook of Chemistry, 4th ed.; McGraw-Hill: New York, 1993; Section 8. (26) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991,95, 1829-1836.
Pressure variation promises to be a main factor in the control of TFE adsolubilization into surfactant bilayers. The concentration of the initiator has a marked effect on polymerization in the range from 0.25 to 2.5 wt %, indicatingthat this factor must be taken into consideration during the analysis of kinetic data. FTIR spectral data indicate the existence of a PTFE coating on the aluminum oxide powder. The use of an Electron Probe Microanalyzer determined fluorine levels in the samples of aluminum oxide powder. The clear evidence is that PTFE has been successfully coated on both aluminum oxide powder and chips when compared with the samples with surfactant washed off. The measurement of friction coefficients and contact angles also verified that thin PTFE films were formed on the alumina plates. Contact angles for the fluoropolymer-coated samples approached those of pure PTFE. Frictional behavior seems to be related with film thickness and continuity; measurements of film thickness will be required in the future. Use of noncontact AFM may also detect holes in the film. It is concluded that the use of perfluorosurfactants with in situ formation of fluoropolymerhas been successfully demonstrated on the alumina surface and that this technique has significant potential for surface modification but will require improvement for lubricating applications.
Acknowledgment. Financial support received from Hitachi Research Laboratory, Hitachi, Ltd., for work performed at The University of Oklahoma is gratefully acknowledged. Additional support for this research was provided by Oklahoma Center for the Advancement of Science and Technology (Award No. ARO-075) and the National Science Foundation Grant No. CTS-8912806. LA940559A (27) Rodriguez, F. In Principles of Polymer Systems, 2rd ed.; McGraw-Hill: New York, 1982,425. (28)Esumi, K ; Watanabe, N.; Meguro, K. Langmuir 1989,5,1420. (29) Esumi, K.; Nakao, T.; Ito, S.J.Colloid Interface Sci. 1993,156, 256.
