Langmuir 1991, 7, 3099-3109
3099
Characterization of Thin Films of Poly(dimethylsi1oxane) Formed from Surface Diffusion across Defined Polymer Substrates H. F. Webster and J. P. Wightman* Chemistry Department, Center for Adhesive and Sealant Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received January 24, 1991. In Final Form: July 10, 1991
Poly(dimethylsi1oxane)spreading on polymer surfaces was studied in the spreading regime where gravitational and inertial forcesare neglibible by use of the techniques of X-ray photoelectronspectroscopy (XPS) and contact angle analysis. XPS analysis indicated that the surface spreading or “creeping”was very slow, moving only several centimeters per several hundred hours in some cases. The spreading film also reached a “steady-state”concentration when covering an area. Similar results were observed for both horizontal spreading from a thin siloxane source and creep on an inclined surface from a silicone oil reservoir. There also seemed to be a dependence of the rate of film formation and final film structure on the type of polymer substrate. From angular-dependent XPS analysis, the siloxane film formed on polysulfone was found to be of monolayer proportions and covered less than 50% of the total substrate surface area. Contact angle analysis was used to monitor the siloxane movement and good agreement with the XPS data was found. There was a contact angle dependence on X-ray exposure time, indicating cross-linking of the surface film. Contact angle variations were also used to show that the film thickness and distribution may be dependent on the nature of the polymer substrate. 1. Introduction The spreading or wetting of liquids on solid surfaces is a common phenomenon that is desirable is some cases while detrimental in others. Practical applications requiring good wetting include adhesion, lubrication, and paint or ink application, while uncontrolled spreading can lead to loss of liquid or contamination of surrounding surfaces. The special problem of contamination from silicone polymer sources has been examined by several authors,1’2and studies have dealt with the prevention of this p r ~ b l e m . ~ The investigations ofthe spreading of liquids on solid surfaces can generally be divided into two categories. The first deals with theoretical treatments that try to establish the basic laws governing motion a t the contact line. The second involves experimental observations of the spreading liquid on a solid in which investigators are interested in the development of empirical models to predict spreading rates. While theoretical approaches can be found in abundance in the literature, relatively few experimental studies have been performed to validate proposed models. An excellent review of work done in the area has been presented by de G e n n e ~ . ~ An important aspect of the spreading process first observed in some of the earliest work is the presence of a very thin film that extends much farther than the bulk of the spreading liquid. Hardy first observed a film estimated to be 1 pm in thickness extending in front of spreading drops of various polar liquids on glass and steel and attributed this ”primary film” to adsorption of liquid from the vapor phase.5 Bangham and Saweris observed a similar phenomenon with the spreading of various liquids
* To whom corremondence should be addressed.
(1)Kitchen, N.M.; Russell, C. A. IEEE Trans.Parts,Hybrids, Packag. 1976, PHP-12, 24. (2) Clark, D. T. In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D. W., Fabish, T. J., Thomas, H. R., Eds.; ACS Symposium Series 162;American Chemical Society: Washington DC, 1981;p 247. (3)Haque, C. A. Appl. Surf. Sci. 1983, 15, 293. (4)de Gennes, P.G. Reu. Mod. Phys. 1985,57, 827. ( 5 ) Hardy, W. B. Philos. Mag. 1919, 38, 49.
0743-7463f 91f 24Q7-3Q99$02.50 f0
on mica and assumed the film formed by both adsorption from the vapor phase and surface diffusion.‘j Bascom et al. examined the primary film formed by nonpolar liquids spreading on steel by ellipsometry and found that ita thickness dropped sharply to values below 100 A.7 More recently, Marmur and Lelah observed that the rate of water spreading on glass slides was dependent on the size of substrate.* They reasoned that this dependence was due to the existence of a primary film that reached the edge faster than the bulk spreading liquid and thus affected the spreading rate. Chang et al. used ellipsometry to study the spreading of motor oil on silicon and concluded that a primary film existed that was formed by surface diffusion and not adsorption from the vapor phase.9 A recent detailed investigation of the thickness of the precursor film by Daillant et al. using X-ray reflectivity showed a nonhomogeneous final film for poly(dimethy1siloxane) spreading on silicon.1° In this study, the rate of the primary film formation and the structure of the resulting film for poly(dimethylsiloxane) (PDMS) on model polymer substrates was studied by X-ray photoelectron spectroscopy (XPS) and contact angle analysis. Both horizontal spreading and inclined creep were examined, and this represents one of the few studies to investigate the nature of the primary film first formed from a spreading liquid. 2. Experimental Section Materials. Substrate polymers used were nitrocellulose lacquer (Fixallspray enamel),polysulfone (UDELP1700, Union Carbide),andpoly(methy1methacrylate) (PMMA)(Elvasite2041, Du Pont). Nyebar, a fluorinated methacrylate (William Nye, (6) Bangham, D. H.; Saweris, Z. Trans. Faraday SOC.1938,34, 554.
(7)Bascom, W. D.; Cottington, R. L.; Singleterry, C. R. In Contact Angle, Wettability, and Adhesion; Gould, R. F., Ed.; Advances in Chemistry 43; American Chemical Society: Washington, DC, 1964; p 355. (8)Marmur, A.; Lelah, M. D. J. Colloid Interface Sci. 1980, 78, 262. (9)Chang, W. V.; Chang, Y. M.; Wang, L. J.; Wang, Z. C. Organic Coatings and Applied PoEymerProceedings;AmericanChemical Society: Washington, DC, 1982;Vol. 47. (10)Daillant, J.; Benattar, J. J.; Bosio, L.; Leger, L. Europhys. Lett. 1988, 6, 431.
0 1991 American Chemical Society
3100 Langmuir, Vol. 7, No. 12, 1991
loool-5 1 0 0 0 1 - 2
loool-1 t t t A
B
C
COLUMN Figure 1. Schematic of the base plate used for siloxanemigration with designations for plate sampling positions.
Co.) was used as a reference polymer and spreading inhibitor. The spreading liquid used was 50- and 100-centistoke(cSt)-grade DC200 silicone fluid from Dow Corning. The viscosities were chosen to maximize the spreading rate while minimizing vapor pressures for ultrahigh vacuum work. Substrate Preparation. Substrates were prepared as thin polymer films cast from solution on smooth mirror substrates. Polysulfone and poly(methy1 methacrylate) were prepared as weight per volume solutions in chloroform and the commercial lacquer was prepared as weight per weight solutions based on the solids content of Tru Test lacquer thinner. Nyebar solutions were prepared from the commercial solution (2% w/v) by using 1,1,2-trichloro-1,1,2 trifluoroethane. The metal substrates used were ferrotype plates obtained from Thompson Photo Products (Bedford Park, IL) and consisted of a 0.51-mm steel base plated with a layer of nickel and a surface layer of chrome. Thin polymer base films were then deposited on these substrates in one of two ways: Method 1. A volume of polymer solution (0.5-1.0% w/v), sufficient to cover the entire surface, was deposited on a small ferrotype plate of specified size. A small rectangular bar (1.9 cm x 10.3 cm) of mild steel, polished to a fine edge, was drawn across the surface to leave a thin film. Method 2. Ferrotype plate substrates were cleaned in an oxygen plasma for 15min by use of a Plasmod dry asher followed by rinsing with deionized water and then acetone. The cycle was then repeated, the plates were air-dried, and the optical constants (Ns, K,) for each plate were determined at a 70° incidence angle by ellipsometry. The substrate was then flooded with polymer solution (0.1-1.0%) and spin-coated at 1500-1600 rpm to yield a uniform film. Poly(dimethylsi1oxane)Spreading. Experimental methods were devised to examine both horizontal spreading and creep on inclined substrates. Horizontal Spreading. For these spreading studies, ferrotype plates were machined precisely to either a 3.16 cm X 5.08 cm rectangle or a 3.16 cm X 3.16 cm square. Polymer films were cast on these plates from known concentrations by either coating method. The plates were tilted to approximately 65" with respect to horizontal, and a volume of siloxane solution in either isooctane or cyclohexane was deposited by microliter syringe to the bottom edge. The volume used ranged from 5 to 10 wL, and the concentration was adjusted so that if the siloxane uniformly spread over the entire substrate surface area, a 25-nm film would result. The solvent evaporated quickly,resulting in avery narrow (2-3 mm) visible siloxane film at the edge of the plate. The plates were then placed horizontally on a covered aluminum rack in a thermostated oven. Periodically, individual plates were removed for analysis. To monitor siloxane spreading, a punch and die were designed to remove 6.35" circular disks from defined locations on the larger plate. Since the die was designed for 3.175 cm X 5.08 cm ferrotype plates of 0.51-mm thickness, and the width of the machined plates was 3.16 h O.Ol.cm, a specificdisk position could be monitored with consistency for both 5.08- and 3.16-cm-length plates. The design and designation for each available position
Webster and Wightman is shown in Figure 1. In all cases, the second sampling position was the first monitored, and the plate center (B position) was exclusively examined for siloxane spreading. Inclined Creep. In this case, polymer-coatedsubstrates (3.16 cm X 3.16 cm) were tilted to an angle of approximately 60" to the horizontal and the edge was immersed in a pure siloxane reservoir such that only a 2-3-mm portion of the plate bottom was in contact with the liquid. To prevent siloxane migration on the uncoated side of the plate, it was coated along the edges with Nyebar to serve as a barrier film. This polymer is very similar in structure to those studied by Haque in the prevention of silicone oil creep.3 To analyze for spreading, plates were removed at given intervals and excess oil was removed by placing the plate edge against absorbent paper. To completelyeliminate contamination problems, the portion of the plate edge still showing bulk silicone oil was sheared off approximately 3-7 mm from the edge. Sample disks were then removed from the plate as described for substrates used in horizontal spreading. To examine the effectiveness of Nyebar as a barrier film for silicone spreading, a small narrow band of the polymer (2% w/v) was deposited from solution across the width of the coated ferrotype substrate approximately 5-7 mm from the siloxane source. Sample disks were then analyzed for siloxane spreading as before. Analysis. A. XPS. XPS analysis of surface spreading was performed using a Kratos XSAM 800 electron spectrometer with a magnesium X-ray source (1253.6 eV) operated at 13 kV and 20 mA. The operating pressure was typically 5 X 10-8Torr,and the analyzer was operated in the fixed retarding ratio FRR mode. To measure the extent of siloxane migration, techniques were needed to allow measurement of the absolute Si 2p photopeak intensity indicative of the siloxane layer present within the substrate disk area. Ratios to other elements, as is usually done in this type of analysis, would produce ambiguous results as all elemental concentrations may vary with film coverage. To measure the absolute intensity of silicone, a disk holder was designed that allowed reproducible positioning of the sample in the spectrometer. Two designs were used, and in the first, a 9.5" circular aluminum holder was constructed in such a way that the punched 6.35-mm disk was seated flush within the larger holder. This assembly could be mounted to the sample probe and reproducibly positioned in the spectrometer and the Si 2p photopeak monitored as a function of time. The second and improved design consisted of a small ferrotype rectangle (10 mm X 21.6 mm) that was coated with a 2% (w/v) solution of Nyebar and then consistently dried for 30 min at -80 "C. Plates were used immediately upon preparation. The disk to be analyzed was centered reproducibly on the end of the rectangle and the entire assembly mounted on the sample probe and positioned in the spectrometer. The second design was an improvement over the first in that it provided a reference fluorine signal to normalize Si 2p photopeak intensities from the spreading siloxane and alleviate the obvious problems inherent in absolute signal measurement. It also removed the need to substract background contributions to the Si 2p photopeak intensity from the aluminum holder itself since the Nyebar film showed no silicon signal. Due to the extremely low surface energy of the Nyebar film, an added advantage was that the fluorine reference signal did not change substantially over short periods of time as a result of atmospheric contamination. The use of consistent width rectangleswas crucial to the reproducibility of this experiment, and the error in the width was less than 5%. A requirement for either holder was that the entire area of the circular sample disk be analyzed by XPS, and that the signal intensity did not depend on sample positioning. To examine this criterion for the aluminum holder, a small amout of Nyebar was placed on the edge of a 6.35-mm chrome ferrotype disk, and the ratio of F 1s to Cr 2p signals was monitored as a function of disk rotation. Deviation in the F/Cr ratio in this case was approximately 20%. Similarly with the second holder, a ferrotype disk with a small amount of polysulfone deposited on the edge was used and the ratio of S 2p to F 1s monitored. Deviation in the S/Fratio here was approximately 10% as a function of disk rotation. Experiments were also performed to determine the reproducibility of sample probe positioning. With the first holder, chromium disks were used, and the ratio of the Cr 2p to A1 2p was monitored with repeated
Langmuir, Vol. 7, No. 12, 1991 3101
Poly(dimethylsi1oxane) Thin Films removal and repositioning of the sample; reproducibility here was within 10%. XPS angular-dependent studies of the film formed after siloxane spreading over a defined area were done using a PHI 5300 spectrometer operated with a Mg K q 2 X-ray source at 14 kV and approximately 18 mA. Typical operating pressures were 5-7 X 10-8 Torr and the instrument was operated in the fixed analyzer transmission (FAT) mode. Circular disks (9.53 mm) were taken from an area known to be covered by spreading film. Only samples from the study of inclined creep were analyzed, and takeoff angles of 10, 20, 30, and 90’ (with respect to the sample plane) were used in this analysis. B. Contact Angle Analysis. A Rame-Hart contact angle goniometer (Model 100-00) was used in all measurements. To ensure that contact angles were measured in equilibrium with their vapor, each measurement was taken in an environmental chamber (Rame-Hart Model 100-07)fitted with small reservoirs filled with the test liquid. Two methods of contact angle analysis were used. The first was a sessile drop technique and involved depositing a 20-pL drop of test liquid onto the substrate and subsequently measuring the contact angle. Both left and right angles were measured for each drop and the average was taken. The second method used for the determination of the surface energy characteristics of the polymer films involved taking the advancing and receding contact angles for each liquid. Twomicroliter volumes of the test liquid were added to the drop with a microliter syringe until a stable advancing contact angle was obtained. Receding contact angles were measured in a similar manner by withdrawing 2-pL volumes. Measurements were always made with the syringe needle in the drop. For all contact angle measurements, 5-10 repetitions were performed to monitor the reproducibility of the measurement. To analyze the effect of the migrating siloxane film on contact angle, small 6.35-mm disks were taken at various times and analyzed either directly from the plate or after XPS analysis. Water was used as the test liquid, and analysis was made by the sessile drop technique. To test the effect of X-rays on contact angle, 6.35-mm disks were exposed to a magnesium X-ray source (PHI XPS system) operated at a power of 250 W for varying lengths of time. C. Ellipsometry. Ellipsometry was used to determine the film thickness of spin-coated substrate samples. A Gaertner L116A dual-mode ellipsometer was used. Both linear and circular polarization were used in the analysis with automatic data sampling at 72 points for each state of polarization. A 1-mW He-Ne (632.8 nm) source was used with a spot size of approximately 1mm at 90°. Optical constanta for the ferrotype plates were determined followed by spin-coating and thickness determination. Various thicknesses were deposited and several incidence angles used to determine the refractive index and absolute thickness of the deposited film. Refractive index (N,) and attenuation coefficient (K,) values of 3.51 0.03 and 4.26 0.02 were found for the ferrotype substrate and compared well with literature values of approximately 3.63 and 4.57 found for chromium.” Refractive index values of 1.59 and 1.47 for freshly prepared polysulfone and poly(methylmethacry1ate)films agreed well with literature values of 1.633 and 1.489, respectively.12 Slightlybetter agreement could be obtained by thermal annealing at temperatures just below the polymer glass transition temperature.
*
*
3. Results and Discussion 3.1. Substrate Surface Energy Analysis. Due to the importance of both substrate and spreading liquid surface energy in determining whether spreading will occur (i.e., a positive spreading coefficient (S)),characterizing the surface energy of the polymer substrates could be of critical importance in both the spreading rate and the nature of the spreading precursor film. The term S is defined as ysv- (rsl+ y d , where ysv,ysl,and ylvrepresent (11)Ganin, G. V.; Kirillova, M. M.; Nomerovannaya, L. V.; Shirokovskii, V. P. Fiz. Met. Metalloued. 1977, 43, 907. (12) Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1975; p 111-241.
Table I. Contact Angle Analysis. liquid water glycerol formamide methyleneiodide ethylene glycol bromonaphthalene
advancin& sessile drops PSULF PMMA LACQ PSULF PMMA LACQ 9Of1 70f6 63f1
25f2 56 f 1 9f2
7111 66f4 53f4 38f2 51 f 1 16
75i2 75f3 45f4 46f1 51 f 1 22f 1
82f2 66i1 60i1
70f3
24fl
34f2
52 f 1 7f2
16
75f2 72f 1 42 i 2 38f2 44 f 2 22il
Values in degrees. PSULF,polysulfone;PMMA,poly(methy1methacrylate); LACQ, nitrocellulose lacquer. a
the solid/vapor, solid/liquid, and liquid/vapor free energies. Contact angle analysis can provide a convenient means to estimate the surface energy of the films used in this work. Advancing contact angles of five liquids on the three polymer substrates (spin-coated) are listed in Table I. From these data, the values for the lacquer were found to be similar to those for poly(methy1 methacrylate) although some deviations can be seen for glycerol and formamide. Contact angles made by the sessile drop technique are also included in Table I. The values of contact angles measured in this way are consistently lower than those measured by the advancing angle technique and may be a contributing factor to many discrepancies in the values of contact angles reported in the literature. A consistent method of analysis is needed for accurate correlation of data between laboratories. Although not shown, several liquids were tested on polymer films cast by method 1and no significant difference was seen when compared to the spin-coated samples. Contact angle analysis by the sessile drop technique for various thicknesses of substrate polymers (5-90 nm) was done and showed no change in contact angle within this range. Both water and methylene iodide were used as a polar and nonpolar test liquid, respectively. 3.2. Determination of Surface Energy. Zisman provided a great contribution to the estimation of surface energies through his concept of “critical surface tension”.13 From measurement of the contact angles of various liquids on a solid, a plot of cos 8 vs ylvyields a rectilinear line that can be expressed by the following empirical equation: cos e = 1 + b(rc - rlv) (1) where b is the slope of the line and yc is termed the critical surface tension. This term represents that value of the surface energy of a liquid that just completely wets the solid. A problem with the analysis is that although it provides a convenient “indexn of energies for solids, the value of y c is dependent on the nature of the liquids used in its determination.14J5 To further understand the dependence of contact angle on the nature of the liquid used, an understanding of the specific interfacial interactions between solid and liquid was needed. Girifalco and Good suggested that the energy of interaction could be expressed by the geometric mean of the energy of cohesion of each phase.16 The interfacial energy (712) could now be defined by the following relationship: where
was defined as the interaction parameter that
(13) Zisman, W. A. In Contact Angle, Wettability, and Adhesion; Gould, R. F., Ed.; Advances in Chemistry 4%AmericanChemical Society Washington, DC, 1964; p 1. (14) Siochi, E. M. Masters Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1984. (15) D a n , J. R. J . Colloid Interface Sci. 1970, 32, 302. (16) Girifalco, L. A.; Good, R. J. J . Phys. Chem. 1967,61, 904.
Webster and Wightman
3102 Langmuir, Vol. 7, No. 12,1991 Table 11. Literature Values. for the Surface Tension of Various Liauids and Their DisRersion Force Contributions
liauid
Yl"
Yd
72.2 58.3 51.2 64.0 44.8 48.3
22.1 32.0 50.8 32.4 47.0b 28.6
I C
water formamide methylene iodide glycerol bromonaphthalene ethylene glycol
e 0.5 0.c
I
A
0 M
~
I
PMMA
a Values taken from ref 14. * Since the dispersion component is greater than the surface tension, the value will be taken as equal to the surface tension.
corrected for disparities in molecular volumes of the two phases.16 Fowkes proposed that the total surface free energy term could be divided into various components that could be grouped into dispersion and nondispersion for~es.'~J*He assumed that the dispersion forces also interacted by the geometric mean relationship to give the following equation: d d 1/2 (3) = Y 1 + Yz - 2(Y, Yz ) where +yld and -yzd represent the dispersion force contributions to the total energy of phase 1 and phase 2, re~pective1y.l~ Starting with the Young-Dupre' equation, ylVcos 0 = ys-ysl- re, substitution of eq 3 for the interfacial free energy yields an expression referred as the GoodGirifalco-Fowkes-Young equation (GGFY).15 712
cos 0 = -1
+ 2[YsdYlvd11/2/ n v - T , / Y l V
(4)
This equation only predicts the contact angle in cases where dispersion forces are at work at the interface. Tamai et al. introduced a modification of the equation to allow for nondispersion interactions at the interface and this is given by the following equation:19 = -1 + ~ [ Yd~S ld 1v1 / 2/ ~ l v+ Isl/Ylv - *e/Ylv (5) where Isl denotes the combined nondispersion interactions forces at the interface. Both KaelbleZ0and Owens and Wendt21 have treated these interactions as a term also defined by the geometric mean relationship. Thus, the Isl parameter is defined as 2 ( ~ , p y l , p ) ~ /where ~ ysP and 714' represent the polar contributions to the total energy of the solid and liquid phase, respectively. The inclusion of this expression in eq 5 with knowledge of the polar and dispersion components of any two liquids allows solution for the polar and dispersion components of a solid surface by solving simultaneous equations.z0,21 However, this relationship has been criticized by Fowkes as a "forced fit" of all nondispersion terms into one polar expression.22 To more carefully analyze the contact angle data in this work, both Zisman's critical surface tension analysis and the analysis by the Good-Girfalco-Fowkes-Young equation were used to give an estimation of the surface free energy of the polymer films. For the critical surface tension determination, values of the liquid surface tension are required, and for the analysis by the GGFY equation, both this value and the dispersion force contribution are needed. Table I1 lists values for these two quantities that will be used in later calculations. The critical surface tension analysis is shown in Figure 2 for each polymer studied and the critical surface tension
Surface Tension (dyne/"
Figure 2. Critical surface tension analysis of thin polymeric films. Table 111. Analysis of Dispersive and Nondispersive Contributions to the Surface Free Energy of Polymer Films A. r , a n d ~ d ~ ~ _ _ _ _ ~
YC
Dolvmer polysulfone poly(methy1methacrylate) lacquer
4176. (20) Kaelble, D. H. J.Adhes. 1970, 2, 66. (21) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969,13,1741. (22) Fowkes, F. M. J. Adhes. Sci. Technol. 1987, I, 7.
a
lit.
a
lit.
42 38 38
4514 3915
46 42 39
4114 41'5
~
B. Zsl Determination
COS 0
(17)Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40. (18) Fowkes, F. M. J. Phys. Chem. 1962, 66, 382. (19) Tamai, Y.; Makuuchi, K.; Suzuki, M. J. Phys. Chem. 1967, 71,
Yd
liquid water glycerol formamide ethylene glycol This work.
polysulfone a lit. 8.3 7.4 6.4 0.02
16.614 7.214 7.614
a
PMMA lit.
35.0 7.6 18.5 7.3
32.315 14.914 16.514
lacquer a 32.0 6.6 26.8 9.3
(I
was determined by an extrapolation of the cos e vs ylvplot to a cos 0 value of 1using a variety of test liquids. Linear regression was applied to determine the value of ye The scatter in the data is significant, and whether the functions are linear or slightly curving is difficult to determine. Dan+ has shown that the value of the critical surface tension can strongly depend on the liquids used, and that curvilinear functions of cos 0 vs ylv are common. Even with these difficulties, the critical surface tensions were determined to give a relative estimation of the surface energy between substrates. The values are shown in Table 111, and comparison with literature values shows good agreement in general. Analysis by the GGFY equation can be done graphically by plotting the cos e as a function of 2(y1vd)1/2/y1v and assuming the spreading pressure term is negligible. This plot contains no terms for polar interactions with the test liquid and requires an intercept of -1. The dispersion component of the surface free energy can be derived from the slope of this plot, and the deviation gives an indication of the polar interaction between the test liquid and the solid. The values of methylene iodide, bromonaphthalene, and the intercept were used to calculate the slope of the line. Deviations from this line were converted to the
_
_
Langmuir, Vol. 7, No. 12, 1991 3103
Poly(dimethylsi1oxane) Thin Films
Table IV. Analysis of Thin Polymer Films before and after Thermal Treatment A. Ellipsometric Analysis thickness, A
0
-I
0.0
0.1
- 0.2
0.3
0.0
0.1
-
0.2
0.3
polymep PMMA PSULF LACQ
before 46 f 3 51 f 3 39f2
after 51 f 2 61 f 3 40f2
B. Contact Angle Analysis 8 (sessile drop), deg before after
0.0
0.1
0.2
2 R h I "
0.3
6.0
0.1
0.2
0.3
24QYIV
polymer" PMMA PSULF LACQ
CHzIz 34*2 24f 1 38f2
H2O 68 f 1 81 f 3 72f1
CHzIz 36 f 3 20* 1 39* 1
C. XPSAnalysis atomic ratio
Figure 3. Surface energy analysis of substrate polymer films using the GGFY equation. polar force contributions by inclusion of the Isl term using eq 5. This method is similar to that of Dann, where liquids with small polar force contributions were used to calculated Yd of the solid, and Isl values were calculated from the difference between the calculated and measured spreading coefficients.15 Figure 3 shows the result of the graphical analysis, and values of the dispersion force contribution to the solid's surface energy and Isl are given in Table 111. From the graphical representation, the amount of deviation from the ideal line increases with both the polarity of the solid and the test liquid. Polystyrene is shown as an example of a relatively nonpolar polymer and shows almost no deviation from the ideal line. Deviation increases with polysulfone as polar sulfone and ether bonds are added to the aromatic structure. The greatest deviation is seen with poly(methy1methacrylate) and the lacquer substrates, indicating that these two polar substrates are also similar in surface polarity. Tabulated values for the dispersion component of the surface free energy indicated that all polymers studies are similar, with Isl values that vary in accordance with the graphical representation of Figure 3. An important point in the analysis by this method is that the value of Yd is dependent on the square of the slope of the calculated line. This can lead to large changes in calculated values with relatively small changes in measured contact angle. Another source of error is the interaction of methylene iodide and bromonaphthalene with some surfaces. 3.3. Thin-Film Analysis. The integrity of the substrate films is essential for accurate analysis of siloxane spreading across polymer films. Spin-coated control samples were held a t 39 f 2 "C for approximately 600 h and then analyzed for film damage. Table IV shows the results for ellipsometry, contact angle, and XPS analysis. From ellipsometry of very thin films, a film thickness increase of 3-20 5% is indicated, and this can most probably be attributed to deposition of atmospheric contaminants. The thickness increase is of questionable meaning, however, since the contamination is most likely nonhomogeneous, and the equations of ellipsometry are based on homogeneous isotropic layers. Contact angle analysis for 20-30-nm-thickness films shows surprising agreementwith the unexposed polymer samples. Since both a polar (water) and nonpolar (methylene iodide) liquid were tested, changes in both the hydrophilic and hydrophobic natures of the surface could be monitored. Since contact angle analysis provides one of the most sensitive techniques to surface modification, l i t t l e change m u s t have
HzO 70 f 3 82 f 2 75 f 2
polymeP O/C
before S/C N/C
Si/C O/C
after S/C N/C Si/C
0 0 0 a PMMA, poly(methy1methacrylate);PSULF, polysulfone;LACQ, nitrocellulose lacquer. PMMA PSULF LACQ
0.36 0.15 0.022 0.29 0.035
0 0 0
0.32 0.16 0.037 0.40 0.030
occurred in the surface energy of the substrate over the course of the experiment. XPS analysis of 20-40-nm-thickness films shows no chromium signal from the metal substrate, indicating no major film destruction had occurred. Except for the polysulfone S 2p/C 1s ratio, the atomic ratios are also in fairly good agreementwith unexposed samples, considering the extreme sensitivity of the technique to small amounts of contamination. Atomic ratios were calculated by a first principles methodZ3tz4 utilizing the following equation for elastically scattered electrons:
N , = C,(E,iE)p(a+w, (6) where C, is an instrument-dependent constant involving specific spectrometergeometries, X-ray flux,and efficiency of detector, E,/E is a term to account for the fact that most spectrometers in use retard the energy of the exiting electrons to improve both resolution and sensitivity, p is the atomic density of the element, au/dQ defines the differential cross section or probability of ionization of a particular atomic level, and A, accounts for the photoelectron attenuation as it travels in the solid. This attenuation can be described in terms of the umean free path" and is the distance for which l / e of photoelectrons have been elastically scattered and escape. The particular instrument geometry employed in all cases here used a fiied angle of 5 4 O between the X-ray propagation direction and photoelectron emission direction, and this allowed the use of the total cross-section values, with no effect due to the asymmetry parameter often needed for other ~onfigurations.2~ Atomic concentrations were calculated by use of Scofield's cross-section values,25mean free path values calculated from X = aKE5 where CY is a constant and B is (23) Fadley, C. S.;Baird, R. J.; Siekhaus,W.; Novakov, T.;Bergstr6m, S. A. L. J. Electron Spectrosc. Relat. Phenom. 1974, 4, 93.
(24)Powell,C.J.In QuantitatiueSurfaceAnalysis ofMateriaLP;McIntyre, N. S.,Ed.; ASTM Special Technical Publication 643;American Society for Testing Materials: Philidelphia, PA, 1978. (25) Scofield,J. H. J. Elect. Spectrosc. Relat. Phenom. 1976,8, 129.
3104 Langmuir, Vol. 7, No. 12, 1991
I
I
OU4-J0
100
Webster and Wightman
I
I
I
200
300
400
1
500
Time (hours) Figure 4. XPS analysis of 100-cSt poly(dimethylsi1oxane) migration on nitrocellulose lacquer for the 2B (0) and 3B ( 0 ) plate-sampling positions using a solution source.
taken as 0.75,26and a transmission function of E-' for the PHI spectrometer in the FAT mode and E+' for the Kratos in the FRR mode. While Seah and Anthony have noted the problems in using these simple relationships for the transmission function, particularly for the FAT mode,27 analysis of nine polymer standards containing the elements in this study indicated that the experimentally determined sensitivity factors compared well with those calculated from first principles methods using the E-' relationship for the transmission function with the PHI system. The N 1s signal for the nitrocellulose lacquer was not reproducible due to the degradation by X-rays and loss of NO2 functionality. Similar loss of this functionality in pure nitrocellulose has been reported by Clark and Stephenson.28 3.4. XPS Analysis of Poly(dimethylsi1oxane)Surface Diffusion. A. Horizontal Spreading. Nitrocellulose Laquer/ 100-cSt Poly(dimethy1siloxane).Figure 4 shows the results for speading of siloxane across this polymer substrate. The intensity of the Si 2p photopeak rises from zero, indicating the migration of the siloxane into the sample area. This plot indicates that the time of migration is very slow, taking almost 150 h to reach the next sampling position. An interesting point, however, is the short time the silicone takes to reach the 2B position compared to the time of crossing it. Since the 2-3-mm line of silicone deposited initially is approximately 10mm away from the second sampling position, this seems to indicate that the siloxane moves very rapidly initially and then slows to the later observed rate. The explanation is that since the solvent used to deposit the siloxane was a hydrocarbon of low surface tension and viscosity, the dilute solution would have a positive spreading coefficient (S) on the polymer substratesand spread rapidly up the tilted plates upon application. Since the evaporating solvent used has a lower surface tension than the poly(dimethylsiloxane), Maragoni effects would also play a role as the solvent evaporated more quickly in the spreading film leading to surface tension gradients. These effects would change the profile of the siloxane source, but would quickly diminish due to the fast evaporation of solvent. The rate of spreading across the third sampling position is similar to that for the second, and although there is scatter in the data, this rise in signal for the third position comes after a large change in slope for the second position plot. This would indicate a siloxane diffusion from the initial source and rule out the possibility of silicone contamination through vapor transfer. The secondposition plot levels after 150-200 h, and the third-position (26) Siqjman, J.;Lieaegang, J.; Jenkin, J.G.;Leckey,R.C. G .J.EZectron Spectrosc. Relat. Phenom. 1981,23, 97. (27) Seah, M. P.; Anthony, M. T. Surf. Interface Anal. 1984,6,230. (28)Clark,D. T.; Stephenson, P. J. PoZym. Degrad. Stab. 1982,4,185.
plot also seems to approach this level. If this region of the plot is indeed flat, this would indicate a "steady-state" diffusion across the disk. This is an interesting phenomenon since its postulates that molecules are continually moving into the second position after coverage, with no net change in concentration per unit area or unit thickness. This assumes that the signal is not saturated due to a film where the thickness is on the order of the mean free path length of the detected electron. An important parameter of such a plot would be the level of the "steady-state" portion of the migration curve. The area of the photopeak in this region would then be directly related to the amount of siloxane on the surface. If the surface layer were thicker than the XPS sampling depth (approximately 3XsizP),this signal would represent the maximum attainable signal for poly(dimethylsi1oxane) and should mask any signal from the underlying polymer. To investigate this, the N 1s photopeak from the lacquer was monitored as a function of time. In all cases, due to X-ray degradation discussed previously, nitrogen was scanned immediately to approximate the true signal. In some random cases, two N 1s photopeaks were observed, and in these, only the higher binding energy peak assigned to the nitrocellulose was used (-407 eV). Although there was substantial scatter in the data, the analysis indicated that the N 1s photopeak did not disappear upon coverage with the migrating film, even in the region where the Si 2p photopeak intensity is relatively constant. Both the second and third positions were analyzed, and although the third had a slightly higher N 1s signal, the overall results for all disks were not statistically different when compared to control samples. In the plateau region (see Figure 4)) the Si 2p to N 1s atomic ratio provides some indication of the amount of siloxane at the surface. The general problem of a thin surface layer on a bulk material can be examined by XPS through modification of eq 623as follows:
~ ( $ 1 ,= ~ , 0 [ 1 - e-d/'+in01
(7)
N($)b= NbOe-d/k+d
(8)
where d is the overlayer thickness, 4 is the exit angle, N(d)*and N($)bare the measured photopeak signals from the surface and bulk, respectively, and N(4),O and N(r$)bo represent the maximum signals from the surface and bulk layers and incorporate the various constant terms in eq 6. These values are assumed to be approximately independent of takeoff angle. Through use of these equations, the thickness of the overlayer required to give the experimentally determined surface/ bulk ratios can be calculated. The maximum bulk intensities were determined by analyzing control samples of pure lacquer and 100-cSt poly(dimethylsiloxane), using the Nyebar-coated holder with a reference fluorine signal. The mean free paths for the N 1s (2.3 nm) and the Si 2p (2.9 nm) electrons were scaled from the work of Roberts et al., assuming a Eo.75d e p e n d e n ~ e Analysis .~~ of points in the plateau region included second-position samples greater than 200 h, and third-position samples taken after 500 h. The average atomic ratio of Si 2p to N 1s in this region was 1.6 f 0.4, and the maximum atomic fraction ratio for these elements was experimentally determined to be approximately 9. To obtain this level of signal, a homogeneous thickness of less than 0.4-0.5 nm would be required. This thickness is somewhat smaller than the lateral dimensions of the (29) Roberta, R. F.; Allara, D. L.; Pryde, C. A,; Buchanan, D. N. E.; Hobbins, N. D. Surf. Interface Anal. 1980, 2, 5.
Langmuir, Vol. 7, No. 12, 1991 3105
Poly(dimethylsi1oxane) Thin Films I
n
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Time (hours) Figure 5. XPS analysis of 100-cSt poly(dimethylsi1oxane) migration on poly(methy1methacrylate)(A)and polysulfone (A) from a solution source. Inset shows the polysulfone sulfur signal as a function of time.
20
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48
20 I00
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700
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Figure 6. XPS analysis of 50-cSt poly(dimethylsi1oxane) migration on polysulfone from a solution source monitoring the (a) Si 2p/F Is and the (b) S 2p/F 1s atomic ratios for plate positions 2B-5B. siloxane molecule (0.5-0.6 nm)30and, combined with the fact that the Si 2p photopeak intensity increased with no substantial decrease in N 1s signal, probably indicates a somewhat patchy film or bulk diffusion. Polysulfone, Poly(methy1 methacrylate)/lOO-cSt Poly(dimethylsi1oxane). Figure 5 shows the spreading analysis with both polysulfone and poly(methy1 methacrylate) as substrates. A similar rise in photopeak signal to a constant value is seen as was shown in Figure 4, although in this case, the "steady-state" regions for the two polymers clearly represent two concentrations of surface siloxane. The analysis of the S 2p photopeak area for polysulfone is also shown in Figure 5 (see inset) and again shows only a slight decrease with film coverage. The rate of siloxane migration in these experiments is difficult to compare because of the inital rapid period that may depend on the spreading coefficient ( S ) of the spreading solvent on a given polymer. Polysulfone/50-cSt Poly(dimethylsi1oxane). Figure 6a shows a more extensive analysis of siloxane migration with 50-cSt poly(dimethylsi1oxane) on polysulfone. In all cases the second holder was used and results are reported as atomic ratios of silicon to fluorine. In this figure, there is significant scatter in the data as the siloxane initially enters a disk area. For this experiment, since the siloxane ~~
(30) Fox, H.W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947, 39, 1401.
Time (hours) Figure 7. XPS analysis of 100-cSt poly(dimethylsi1oxane) migration for poly(methy1methacrylate)2B ( 0 )and polysulfone 2B (A) and 3B (A)positions from inclined creep. Also shown are points for poly(methy1 methacrylate) (B)and polysulfone (0) substrates with a Nyebar barrier film. used was of lower viscosity, the initial spreading rate for the solution would be faster, causing a less reproducible starting point of diffusion. The result would be data scatter as a disk is slowly covered by the migrating film, with reproducibility returning to normal once the plate was fully covered. In this plot, the Si 2p signal increases to a characteristic "steady-state" level, and more defiiite proof of this unchanging level is given by the fact that the Si 2p signal from the second position remains virtually unchanged even after the film has reached the fifth sampling position. This is strong evidence that a relatively uniform density film is migrating from the source to eventually cover the entire plate. Figure 6b shows the variation in the S 2p/F 1s photopeak intensity ratio from the underlying polysulfone as a function of time. As the siloxane migrates over each disk, the signal from the underlying substrate remains essentially unchanged. This is identical with the results found for the nitrocellulose lacquer. 3.5. Inclined Creep. The inclined creep experimental design eliminated the effect of the solvent, and the results are shown in Figure 7. From this plot, a much longer time is required to reach the second sampling position than was seen in the previous analysis using solvent. This is in agreement with the previous conclusion that the solvent used was the primary cause of the initial rapid spreading. The movement is very slow and reproducible, in agreement with horizontal spreading results, and there also seems to be a characteristic plateau region in the plot. In this case, however, whether the slope is completely flat or still increasing slightly is difficult to determine. Also shown in Figure 7 is the identical experiment with a substrate of PMMA, and the results indicate that the rate of migrating siloxane film is slower than found with polysulfone. Due to the experimental difficulty in reproducing the exact starting position of each plate, there is a degree of error in the comparison, although several repetitions indicated the same trend. With the PMMA substrate, the Si 2p photopeak appears to rise to almost the same level as in polysulfone. This is in contrast to the results shown Figure 5, which indicate the amount of siloxane was different on the two substrates. Included in Figure 7 for both substrates is the effect of placing a barrier film in the path of migration. A small strip of Nyebar (4mm) was placed in the path of migration before the second disk position, and since the critical surface tension of this material has been shown to be approximately 11dyn/cm,14no siloxane (rc= 24 dyn/cm)S1 should spread on this substrate. The results show that no siloxane migration is observed for (31) Owen,
M.J. Ind. Eng. Chem. R o d . Res. Dew
1980, 19, 97.
Webster and Wightman
3106 Langmuir, Vol. 7, No. 12, 1991 6
27 0
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Figure 8. XPS analysis of the relative rates of poly(dimethylsiloxane) migration from both inclined creep (0) and horizontal ( 0 )spreading on polysulfone substrates.
I
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any samples with the barrier film. This is also evidence against bulk diffusion, since if the Nyebar remains on the surface, bulk diffusion should allow siloxane to move under the film. Also, due to the very low surface free energy of poly(dimethylsiloxane),the migration into the much higher surface energy bulk polymer would probably be thermodynamically unlikely at the temperatures used. Clark found bulk diffusion of low molecular weight poly(dimethylsiloxane) fractions migrating on polyethylene from adhesive tape a t 75 "C, but not a t room temperature.2 Considering that the surface energy of polyethylene = 31 dyn/cm)'3 is much lower than that of the polymers studied here, and that the migrating polymer in Clark's study was probably of lower molecular weight than that used here, the possibility of bulk diffusion at the temperatures used here seems unlikely. The results in this section have been given for a variety of experimental conditions. While it is difficult to correlate the migration rates across a single disk due to experimental difficulties, a comparison of the migration rate on polysulfone substrates for horizontal spreading and inclined creep experiments may be useful. To do this, because of the variable starting positions, one needs to reference the migration to some common point. The point chosen here was the initial time that the film begins to enter the second disk area, and this time can be estimated by extrapolating to zero Si 2p intensity. The results, shown in Figure 8, are preliminary evidence that the migration rates are relatively independent of the differing sources of siloxane. Another useful analysis of all experiments using polysulfone as the substrate is the comparison of the "steadystate" region for a number of experimental conditions. The atomic ratio of Si 2p to S 2p was averaged in the plateau region for all experiments and resulted in an average atomic ratio of 1.07 with a standard deviation of approximately 20%. Within experimental error, there was no difference due to viscosity of the PDMS used or whether solvent was usedin the initial deposition. To provide more quantitative information, pure 100-cStDC200 silconefluid and a polysulfone control sample (with Nyebar reference) were analyzed under identical operating conditions to give the maximum bulk atomic ratio of approximately 9. Comparison of this value to the experimentally determined value of 1.07 for the migrating film again gives some indication of the surface siloxane concentrations. Equations 2 and 3 can be applied to calculate the required thickness to reduce the maximum signal to that measured for the migrating film. As an approximation, the mean free path values for the Si 2p and S 2p electrons were assumed to be equal. The relative difference between the two values would be less than 10% if calculated from the equation CYEBwhere 8 ranges from 0.5 to 0.8. By use of
0
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1
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Figure 9. Angular-dependent XPS analysis of the poly(dimethylsiloxane)film formed from the inclined creep on a polysulfone substrate from a DC200 silicone fluid source. Also shown are both the plot for a 45% film coverage using a Xld value of 2.5 and Nbo/N,Ovalue of 7.5 and the plot for 100%coverage. For full monolayer coverage,a thicknessvalue of 0.55 nm and X value of 2.8 nm were used. Results were calculated from eqs 9 and 10. an angle of 90°, the thickness calculated by this method was -0.3 nm. This is less than that calculated previously for the lacquer film and is not a realistic thickness. Fox et al. found that, for the extended configuration of poly(dimethylsiloxane) on a water surface, the thickness was 0.5-0.6nm in the flattest c~nfiguration.~~ The explanation again must lie in the fact that the results suggest only partial monolayer coverage. 3.6. Angular-Dependent XPS Analysis. To further study the nature of the film left after inclined creep across polysulfone, 9.53-mm circular disks were removed from areas covered by the siloxane film (near the second sampling position) and studied by XPS angular analysis. Figure 9 shows the results of this analysis for creep from both 50- and 100-cSt poly(dimethylsi1oxane). In this plot the atomic ratio of Si 2p to S 2p is plotted as a function of takeoff angle. As can be seen, although the reproducibility is much better for the 100 cSt than for the 50 cSt, in both cases, the Si 2p to S 2p atomic ratio as a function of angle is very reproducible, as can be seen by the relatively small error bars present. An interesting point to note is that the average Si 2p to S 2p atomic ratio of 1.18 at 90" is in excellent agreement with the "steady-state" ratio of 1.07 found for the average of previous migration experiments, even though the analysis there was done on the Kratos system using a different analyzer mode (FRR). T o analyze this plot more quantitatively, eqs 7 and 8 were used to determine the thickness of the resulting siloxane film. For the maximum Si 2p to S 2p ratio, pure DC200fluid and polysulfone control samples were analyzed as a function of angle under identical operating conditions using the PHI system and employing PHI Omni smallspot optics (approximately l-mm circular spot). For freshly prepared samples, a Si 2p to S 2p atomic ratio of 7-8 was obtained, in reasonable agreement with the value of 9 found for the Kratos analysis of heat-aged samples. Angular analysis of heat-aged polysulfone control samples using the PHI system showed an increase in the C 1s to S 2p ratio of approximately 20% in going from a 90" to 10" takeoff angle, indicating some degree of contamination is present. To a lesser degree, this type of response is also observed for freshly prepared samples.
Langmuir, Vol. 7, No. 12, 1991 3107
Poly(dimethylsi1orane) Thin Films
If the siloxane film is assumed to be a close-packedmonolayer, the thickness needed to give the experimentally measured value a t 90° is about 0.3-0.4 nm if an average mean free path of 2.8 nm is assumed for both the Si 2p and S 2p photopeaks. This is a thickness comparable to that calculated from the average results of the "steadystate" region for polysulfone. To further interpret these results, eqs 7 and 8 can be modified to account for surface layers with only partial surface c0verage:~3
100,
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0
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,
I
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,
,
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where f is the fraction of the total surface area covered by the surface film. Figure 9 also shows calculated plots using eqs 9 and 10. Since the mean free paths for the two elements are taken to be the same, the calculated plots are given in terms of Aid. A reasonable fit to the data can be made using a Xld value of 2.5 and 45% surface coverage. If an estimate of the mean free path of sulfur and silicone is taken as approximately 2-3 nm, the corresponding thickness of the surface film would be 0.8-1.2 nm, somewhat higher than the lateral dimensions of a siloxane monolayer. The analysis must have some degree of error due to the presence of unknown amounts of atmospheric contamination, which were not corrected for in eq 6. Also, the assumption of a constant maximum Si 2p to S 2p ratio as a function of angle and uncertainty about the absolute mean free path values could lead to errors in the analysis. Such errors, however, would not invalidate the conclusion of a surface film of near-monolayer dimensions with submonolayer coverage. Clark et al., using a similar model and estimating a 0.5-nm siloxane film, found surface coverages of 3-27 % for polyquinolines contaminated with silicone films from grease s0urces.3~ For comparison, the 100% coverage plot for a layer of 5-6-nm thickness is given in Figure 9, and the large discrepancy with the experimental values can be seen. Although not shown, the ?r to ?r* transiton peak due to the aromatic backbone of polysulfone was also monitored and determined to be of a form similar to that above. Several important observations were noted when different experimental techniques were used in the angular analysis. When a 3.18-cm-length rectangular strip was cut instead of the 9.53-mm disk, the reproducibility of the angular dependence curve was lost. In some cases a much stronger dependence was seen, while others exhibited a behavior similar to that in Figure 9. In these experiments, there was no attempt to reproducibly analyze a specific portion of the plate, so any part may have been analyzed. The lack of reproducibility may indicate that the thickness may vary laterally across the plate. Since the reverse side of the substrate plates was covered with Nyebar to prevent siloxane spreading in these experiments, there is a possibility of ridge formation at these low-energy edges, similar to that seen by Bascom et al. for squalene in contact with a barrier film of perfluorodecanoate on steel.' Analysis near these thicker areas of the film would result in much larger siloxane signals, and since the area analyzed increases with decreasing angle, an increasing fraction of this part of the plate may have been analyzed. This could lead to very large Si 2p signals a t small angles for the analysis of undefined sections of the substrate. 3.7. Contact Angle Analysis of Surface Diffusion. Figure 10 shows the analysis of siloxane spreading by
contract angle analysis and represents the same experimental conditions as shown in Figure 5. All data were taken after exposure to X-rays in the XPS analysis and results show a trend similar to the XPS data. The measured contact angle starts at a value indicative of the base polymer and then rises to a constant higher value a t approximately the same time that the XPS data indicated a constant siloxane level (see Figure 5). While the initial contact angle a t zero time and the constant contact angle a t the long times have meaning, the intermediate values are meaningless since the water drop is in contact with both film-covered and noncovered surfaces, and the average of the left and right sides was simply taken in this case. They show intermediate values between initial and final contact angles. This figure also showsthat the contact angle at the plateau regions for poly(methy1 methacrylate) is quite different than that for polysulfone, indicating that the film present on the PMMA substrate does not represent the bulk properties of poly(dimethylsiloxane), and this is in agreement with previous XPS data. If the two base polymers were completely covered by the same thick homogeneous siloxane film, the contact angle should have been the same on both. Water contact angles for heat-treated poly(dimethylsi1oxane) have been reported to range from 102to l10°.33 The lower water contact angle values measured for both polysulfone and poly(methy1 methacrylate) is consistent with a submonolayer surface film of siloxane. Figure 11shows the contact angle analysis after X-ray exposure corresponding to the XPS data shown in Figure 6. The data from this graph agree remarkably well with the XPS results. Again, the contact angle rises to a characteristic value after the siloxane film has crossed the analysis disk. The constant value for the third-position disk is in reasonable agreement with that of the second although slightly lower. An interesting point is the plotted values for the contact angles of sample disks not exposed to X-rays. As can be seen, there is no change in the contact angle with the crossing of the migrating film, and this requires explanation. Siloxanes possess the ability, due to the extreme flexibility of the siloxane backbone, for orientation in response to their environment. Poly(dimethylsi1oxanes) in contact with water will orient the backbone to expose as many Si-0 linkages as possible to the water interface, and this is the reason for the abnormally low interfacial
(32)Clark,D.T.; Munro, H. S.; Recca, A.; Stille, J.K.Macromolecules 1984,17, 1871.
(33)Hunter, M. 3.;Gordon, M. S.; Barry,A. J.; Hyde, J. F.; Heidenreich, R. D. Ind. Eng. Chem. 1947,39,1389.
Time (hours) Figure 10. Water contact angle analysis of 100-cSt poly(di-
methylsiloxane) migration on polysulfone (A)and poly(methy1 methacrylate) (A)from a solution source.
Webster and Wightman
3108 Langmuir, Vol. 7, No. 12, 1991
Ot
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Exposure Time (min.) Figure 12. Contact angle analysis of the poly(dimethylsi1oxme) film formed by migration on polysulfone as a function of 74
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Figure 11. Water contact angle analysis of 50-cSt poly(dimethylsiloxane) migration on polysulfone from a solution source.
andafter ( 0 )X-ray exposure Points showbothsamplesbefore (0) for plate positions 2B-4B.
tensions measured in such a system.31 This type of orientation effect may play a role in the inability to detect the surface siloxane film. In addition, several other responses of the surface film may be possible. The surface film may be pushed to the periphery of the drop, essentially sweeping the surface clean of a surface film. This may be the preferred response instead of reorientation of the siloxane backbone in a patchy film where it would be likely that a significant number of the low-energymethyl groups would be exposed to the water. The film movement in this case may be an attempt to minimize the water contact with these groups. A second response might be the spreading of the thin film onto the much higher energy water surface. This would also change the surface free energy of the water and contribute to the change in contact angle, but at the levels present, the magnitude of the effect is hard to predict. The fact that the surface film has no effect on the water contact angle may be further evidence against bulk diffusion, where the siloxane movement would be more restricted, and there would be a greater chance for some type of contact angle response. No matter what the explanation for the inability to detect the surface siloxane film with contact angles, the exposure of the surface film to X-ray irradiation drastically changes the response of the measured contact angle to the presence of the migrating siloxane film. One possible explanation is that the surface siloxane film is cross-linked by X-ray exposure, limiting the surface mobility and ability to reorient, thus giving a more accurate representation of the structure present in the surface film. In early work on the effects of radiation on poly(dimethylsiloxane), Warrick showed that exposure of silica-filled poly(dimethy1siloxane) to X-rays from several targets led to cross-linking in the sample.34 The increase in water contact angle with increasing amount of migrating film indicates the presence (34) Warrick, E.L.Znd. Eng. Chem. 1955,47, 2388.
X-ray exposure. of the low-energy methyl groups at the surface, which now must come into contact with the water. Cross-linking of siloxane by heat treatment has long been used to allow accurate contact angle analysis of the true siloxane surface, free of artifacts caused by the orientation of the siloxane backbone.31 To examine the kinetic aspect of the surface film crosslinking, preliminary data were compiled from several experiments in which the contact angle was measured for the "steady-state" region after X-ray exposure for various lengths of time. The results are given in Figure 12 and show that the rise in contact angle is fairly rapid, reaching a maximum value after only -20 min of X-ray exposure. Noting that the average exposure time shown in Figures 10 and 11is greater than this time, there would be little variation expected due to inadequate cross-linking of the surface film. 4. Summary
XPS analysis showed the siloxane did migrate as a thin film both from a source created by solution evaporation and from pure fluid. In the case where low surface energy solvents were used, the rate was seen to be greatly influenced initially by the solvent and this is probably due to surface energy effects. Analysis of the rate of spreading once the solvent was removed showed a rate independent of the method providing the source. Given the surface sensitivity of XPS, one remarkable characteristic of the siloxane migration was that the substrate signal did not decrease substantially, as would be expected if the film thickness was significant. The ratio of this substrate signal to that indicative of the siloxane film led to the conclusion of submonolayer surface coverage. Migration was also characterized by a "steady-state" level, where the Si 2p photopeak intensity became constant within a specified area analyzed, although the film continued to move ahead. Siloxane viscosity changes and varying the method of siloxane source preparation did not change this "steady-state" level. XPS analysis showed that for siloxane migration from a solution-deposited source, the 'steady-state" levels were different for PMMA and polysulfone. Migration from a pure source, however, did not show such large differences although clearly more experimental work is needed to extend the analysis times used. XPS angular-dependent studies were performed, and comparison of proposed models with the measured response of photopeak intensities with takeoff angle confirmed previous observations that the film formed from migration was of near-monolayer thickness with submono-
Poly(dimethylsi1orane) Thin Films
layer surface coverage. Bulk diffusion was ruled unlikely as spreading could be prevented through the use of a barrier film of Nyebar. Contact angle analysis of the migrating film showed that after XPS analysis the rate of diffusion of the surface film could be monitored by this rather simple technique. Further studies showed that the water contact angle of the film-covered surface was dependent on the time of X-ray exposure, and this was attributed to possible crosslinking of the siloxane film. The fact that the contact angles measured were lower than that expected for a pure siloxane layer is also consistent with partial monolayer coverage. The implication of monolayer film with low surface coverage migrating from a source is an interesting result combined with the fact that the thickness and degree of coverage seem constant in the regime of spreadingstudied. The rate of migration and surface coverage of the film formed may be dependent on the magnitude of the spreading coefficient for a given system. This has been shown to be the case for the analysisof the final film formed from the spreading of poly(dimethylsi1oxane) on both silicon and an octadecyltrichlorosilane surface.1° More probably, the specific interactions present, including both nonpolar (London dispersion forces) and acid/base interactions, play a role in the specificorientation and density of the final film formed. In this case, careful analysis of the substrate surface energetics is critical in the future
Langmuir, Vol. 7, No. 12, 1991 3109
correlation of migration parameters with substrate type. Although polymer substrates are less subject to the quick atmospheric contamination usually associated with highenergy oxide surfaces, contamination of the polymer substrates may lead to surfaceheterogeneities,which would play a critical role in determining the spreading rate and final surface coverage of the siloxane film. Both ellipsometry and XPS analysis indicated that some contamination was probably present for the polymer surfaces in this study. Also, since commercial silicone fluids were used in this work, the relatively wide molecular weight distribution plays an as yet unknown role in the surface diffusion of silicone fluids. In conclusion, we have shown that the initial stages of the migration of poly(dimethylsi1oxane) across defined polymer surfaces can be easily monitored by XPS and contact angle analysis, giving some of the first detailed information of initial movement of the precursor film during spreading.
Acknowledgment. We thank Dr. M. J. Owen and Dow Corning for the supply of silcone fluid used in this work, Financial support of this work by Johnson Wax through a student fellowship (H.F.W.) is appreciated. The technical advice of Dr. Noubar Tcheurekdjian is particularly acknowledged. Registry No. UDEL P1700,25135-51-7;PMMA, 9011-14-7;
nitrocellulose, 9004-70-0.
