Reprinted from LANGMUIR, 1991, Z.Copyright @ 1991 by the American Chemical Society and reprinted by permission of the copyright owner.

Direct Measurement of Interfacial Interactions betweenSemispherical Lenses and Flat Sheets of

Poly(dimethylsiloxane) and Their Chemical Derivatives

Manoj K. Chaudhury*'t and George M. Whitesides-'t

Dow Corning Corporation, Midland, Michigan 48686, and Departrnent of Chemistry,Haruard Uniuersity, Carnbridge, Massachusetts 02138

Receiued May 5, 1990. In Final Form: Septernber 20, 1990

The deformations resulting on contacting small (1-2 mm) semispherical lenses of elastomeric poly-,(dimethylsiloxane) (PDMS) *ith the flat sheets of this material were measured in air and in mixtures ofwater and methanol. The measurements in air were carried out in two ways: as a function of externalloads, and under zero load but with variations in the sizes of the lenses. The measurements in liquids werecarried out under zero load and varied the composition of the liquid mixtures. These experimental datawere analyzed by using a theory of Johnson, Kendall and Roberts to obtain the works of adhesion betweenPDMS suifaces in the air and liquid media. The strength of interaction between PDMS surfaces decreasedin mixtures of water and methanol as the concentration of methanol increased. A small interaction persistedeven in pure methanol. The interfacial free energies (t.r) of the PDMS-liquid interfaces obtained fromthese measurements, together with the contact angles of these liquids on PDMS, were analyzed by usingYoung's equation. This analysis provided an estimate of the surface free energ-v of the polymer (y.") thatwas consistent both with the value obtained from measurements made in air and with the value estimatedfrom the analysis of the contact angles of nonpolar liquids on PDMS using the Good-Girifalco-Fowkesequation. This research also developed ways to modify the surface of PDMS chemically and thus tocontrol its properties. The chemically derivatized poly(dimethylsiloxanes), in the form of lenses and flatsheets, were subjected to load-deformation measurements similar to those used for unmodified PDMS.These functionalized PDMS surf'aces exhibited hysteresis in contact deformations, whereas no hysteresiswas detected for unmodified PDMS. The origin of this hysteresis is not clear at present. The observationof hysteresis at solid-solid interfaces is relevant to understanding adhesion to these surfaces.

1013

Introduction

A fundamental issue in surface science is to correlatemacroscop ic p rocesses-wet t ing , adhes ion , f r i c -tion-occurring at surfaces with their molecular-level finestructures. Although the basic concepts of surface ener-getics were worked out by the physicistsr'2 of the 19thcentury, attempts to correlate energetics with the con-stitutive properties of surfaces gained major impetus onlyafter Fox and Zisman.3 Their attempts to correlate wettingwith surface constitution stimulated Good and Girifalcoaand Fowkesb to develop general semiempirical models forinterfacial structure and energy. These models have beenconceptually important in understanding liquid-liquid,liquid-solid, and solid-solid interfaces, but their quan-titative aspects have gone largely unverified, mainlybecause thermodynamic parameters needed to test themodels were not always accessible experimentally.

Liquid-liquid interfaces are the simplest to analyzethermodynamically, because the surface free energies ofthe pure components and their interfacial free energiescan all be measured independently. The interfacial modelsproposed by Good, Girifalco, and Fowkes were thus testedwith such systems. Solid-solid and solid-liquid interfacesare more difficult to characterize, because neither theirinterfacial free energies nor the surface free energies ofthe solids are readily measurable. Studies of wetting of

t Dow Corning Corp.I Harvard University.(1) Young, T . Miscellaneow Worhs;Peacock, G., Ed.; Murray: London,

1866; 1.(2) Gibbs, J.W . Collected Worhs; Longmans, Green: New York, 1906;

Dover: New York, 1961; Vol. 1.(3) For, H. W.; Zisman, W. A. J. Colloid So. 1950,5, 514.(4) Girifalco, L. A.; Good, R. J. J. Phys. Chem. 1967, 61, 904. Good,

R. J.; Girifalco, L. A. J. Phys. Chem.1960, 64, 561.(5) Fowkes, F. M. Ind. Eng. Chem.196{, 56, 40.

solids try liquids provide estimates of the works of adhesionbetween the solids and liquids, from which predictionsabout the surface free energies of the solids can, at t imes,be made.a's For direct estimation of the surface freeenergies of solids, one would, however, hope to examinesolid-solid interfaces directly. If a convenient experi-mental protocol to examine solid-solid interfaces couldbe developed, systematic studies into relations betweenenergetics and surface constitution could be carried outat the level employed by Zisman et al.3'6 in their studiesof liquid-solid interfaces. We show here how this goalcan be achieved for elastomeric solids and demonstratehow these studies can complement the results obtainedfrom conventional contact angle measurements. Anexperimental system applicable to studies of interactionsbetween two elastomeric solids is not presently applicableto the broader problem of studying nonelastomeric ma-terials. Nonetheless, these techniques provide a significantextension of classical surface chemistry to solids andshould, with modifications, be applicable to many problemsinvolving one elastomeric and one nonelastomeric com-ponent.

When a convex elastic solid comes into contact withanother solid substrate, the adhesion forces, acting acrossthe interface, tend to deform the solids and thus to increasethe area of their contact. Since this deformation is opposedby the elastic restoring forces, its magnitude is small forsolids of high moduli butmeasurable for solidsof lowelasticmoduli, such as organic elastomers. Hertz first proposedthe theory of contact between two elastic solids.? Hecalculated both the profile of the region of deformationand the distribution of stresses around the contact zonewhen an external load was applied. In this treatment, no

(6) Zisman, W. A. Adu. Chem. Ser. 1964, No. 43, 1.(7) Hertz, H. Gesammelte Werhe, Leipzig, L895.

@ 1991 American Chemical Society07 43-7 463 I 9L I 2407 -101 3$02.50/ 0

1014 Langrnuir, Vol. 7, No. 5, 1991

consideration was given to the effects of surface forcesoperating across the interfaces. Solid-solid deformationinduced by the action of surface forces was successfullymodeled first by Johnson, Kendall, and Roberts (JKR).8These authors assumed that the attractive forces wereconfined within the area of contact and used the principleof detailed energy balance to develop a general expressionfor the contact deformation as a function of the surfaceand elastic properties of the solid materials.

An alternate model was proposed by Derjaguin, Muller,and Toporov (DMT)e as an improvement of an earliermodel proposed by Derjaguin,r0 where the assrrmption wasmade that all the attractive forces lay outside the area ofcontact and thatthe contact region was under compressiondescribed by the Hertzian strain profile. A full analysisof the problem of contact deformation was carried outmore recently by Muller, Yushchenko, and Derjaguin,llwho showed that both the JKR and DMT models arelimiting cases of a more general situation. Horn et al.have discussed the differences betweenthe various theoriesof deformation.l2 The analysis of Muller et al. showedthat the DMT model applies to solids of high elastic moduli,whereas the JKR model applies to solids of low elasticmoduli. Since our current studies are of a solid of lowelastic modulus, the JKR model is more relevant for ourpurpose.

For the contact between two spherical solids, the JKRmodel predicts the radius (o) of contact deformationresulting from the joint influences of surface and externalforces to be given by eqs 1-3

aB = (R lnP + grWR + [ tuWRp + (BrWE) t ]o t l (1 )

where

LIK= (3/4) l (1 - r r \1Er+(1 - r r ' \18r1 (2)

L ln = $ lR) + ( r lR2) (s )

P is an external load; 17 is the work of adhesion; .R1 andRzarc the radii of curvature of the two spheres. vL, v2andEu Ezare the Poisson ratios and elastic moduli of the twobodies. [Note: In this paper we use cgs units: P isexqressed in dynes; o and B in cm; E and K in dyn/cm2,and W in ergs/cm2.] For contact between a sphere anda flat_plate, the radius of curvature of the latter is infinityand R becomes the radius of curvature of the sphere. Ifthe elastic modulus of one of the components is muchlarger than the modulus of the other component, thedeformation will only be within the softer component.Thus, depending upon the values of Rr, Rz, Er, and 82,combination of a wide variety of materials can be describedby eq 1.

In order to test eq 1, Johnson et al.8 pressed togethertwo optically smooth, rubber hemispheres and measuredthe area of contact as a function of applied load. Theseexperiments were carried out in air, under water, and under

Chaudhury and Whitesides

a dilute detergent solution. The data obtained from thesestudies obeyed eq 1, from which the work of adhesionbetween the spheres could be determined. This studycreated an active interest among physicists,l2-tz *noapplied the concept to study the mechanistic aspects ofsuch phenomena as adhesion, friction, and fracture and,in turn, provided further evidence in favor of eq 1. Thistechnqiue has not, however, been properly exploited forsurface chemical investigations, despite its potential foryielding surface thermodlmam i c p ara mete rs-surface freeenergies of solids (?,u), works of adhesion (H4 at solid-solid interfaces, and interfacial free energies (y.i) at solid-liquid interfaces--complementary to those obtained fromother conventional studies such as contact angle. Ourcurrent major aim is to correlate the surface thermody-namic parameters obtained from these types of measure-ments with the constitutive properties of the interfaces inorder to develop an understanding of the chemistry ofsolid surfaces in general and of adhesion in particular.

A meaningful and systematic execution of such a studyrequires fulfilling the following stringent conditions. First,the surfaces of the deformable test materials must be verysmooth and homogeneous. It must be possible to castthem into spherical or semispherical shapes, and-in orderto vary their constitrrt ive properties --it should be possibleto modifv their surfaces chemically without affecting theirothe r p hvsical p ropert i es. Fort u natell ' , carefully preparedelastomeric pol l td imethy' ls i loxane) (PDMS) meets thesespecifications. The surface of this elastomer is verysmooth; no features could be found by electron microscopicexamination even at a resolution of 200-300 A. The contactangles of nonswelling l iquids (e.g. water and methanol) onPDMS exhibit negligible hysteresis (2o-3o), implying thatthe surface of PDMS is homogeneous. Stress-free poly-mers (qualitatively judged by examining them with a croiss-polarized microscope) can be prepared in the form ofconvex lenses and flat sheets suitable for the load-deformation studies. We also developed convenientmethods to modify the surfaces of these lenses and sheetschemically using the technology of self-assembled organicmonolayersls'le as a means to vary and control theirconstitutive properties.

The basic experiment was to bring a semispherical lensand a flat sheet of PDMS into contact (Figure 1) and thento measure the resulting contact deformation undercontrolled loads. In what follows next, we first presentresults obtained for unmodified PDMS. The surface freeenergy of the unmodified PDMS, as obtained from thedeformation studies, will be compared with that obtainedfrom detailed contact angle measurements. We will thendescribe a methodof modifyingthesurface of PDMS usingthe technology of self-assembled organic monolayers andreport results obtained by application of similar load-deformation experiments to these surfaces.

(13) Kendall, K. J. Phys., D: Appl. Phys. 1973, 5,1782; t9?6, 8, 1449,1722, 5L2; Proc. R. Soc. London, A 1975, A341, 409; lgZE, A544, 287; J,Adhesion 1975, 7, 137 ; J. Mater. Scl. 1976, II, 638, 1263, 126?; lg7 E, 10,101 1._ (tl) Maugis, D.;Barquine,M. Adhesionand Adsorptionof Polymers:Polyrner Science and Technology;Lee, L. [I., Ed.; P]enum Press: NewYork, 1980; 12A, 203; Barquins, M.; Courtel, R. Wear lgZE, 32, 1gg.

(15) Roberts, A. D.; Othman, A. B. Wear L977, 42, LLg.(16) Tabor, D. J. Colloid Interface Sci. 1977, 58, 2.(17) Lee, A. E. "/. Colloid Interface So. lg?8, 64,577.

_ L1q)Qqglu, J. :J. Ary. Chem. Soc. 1960, 102,92. Maoz, R.; Sagiv, J.J. Colloid Interfaee Sci. l9t{, 100,465.

(19) Waseerman, S. R.;Tao, Y.-T.; Whitesides, G.M. Lanemuir l9tg,5_, l0? 4. Wn1rerman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B.;Pershan, P. S.; Are, J. D. ./. Am. Chem, Soc. lgtg, 111,6ffi2.

(8) Johnson, K.L.; Kendall, K.; Roberts, A. D. hoc. R. Soc. London.A lg?t, 4324,301.^ .(9) P:rjgqui!, B. V.; Muller, V. M.; Toporov, Yu. p. J. C oiloid I nterfaceSci . 1975,53,314.

(10) Derjaguin, B. V. Kolliod Z.1931,69, 15b.- (11] M.$9r,, V. M.; Yushchenko, V. S.; Derjaguin, B. V. J. ColloidI,nterface_Sci. 1980, 77,91. (See dso Pashley, M. D. Colto;ds Sur/. lgt4,12,69.) Muller et al. dietinguiehed the behavior of contact based on adim eneionle-8E parn m eter p =_ (SZ I Sr)t (9n nz) / (8 r IQ Ztly t t, Z being th-intermolecular-separation. It wai ehown that'the DMT'nreihod ap"pliesylqn-l ie.smdl ((1), and the JKR method applies when p is large.'ForPDI-vIp, the value of p was found to be on the order of 1d{, so tfie JKRmodel was the clear choice for PDMS.^ .(t?J^Eoq,_R,_9.; Ieraelachvili, J. N.;Pribac, F. J. Colloid InterfaceSci. 1987, 115,480.

Interactions between Lenses and Flat Sheets

Figure l. Contact between a deformable semispherical solidwith radius of curvature E and a deformable flat plate results inthe formation of a circular region of radius o. The externalmedium might be either air or a liquid. The deformation at thezone of contact results from the simultaneous effects of the surfaceand external (P) forces. For clarity, the area of contact isexaggerated. In our experiments involving elastomeric poly(dim-ethylsiloxane), Pranged from 0 to 200 dyn;the radius of curvatureof the semisphere ranged from about 1 to 2 mm; the radius ofthe contact deformation ranged from about 100 to 250 rrm. Thethickness of the flat sheet was about 1.5 mm.

WHITE LIGHT

iI

: - - POLARIZER {A)

z ------1---wAvE PI-ATE (B)

-GLASS PLATE (E)-.LEAF SPRING (R

PDMS LENS (Hl- -__=----GL-ASS PI-ATE (G)

PDMS SHEET {r) -; LR'ER ARM (J)

NOMARSKI-- - PRTSM (O)

Langmuir , \ to l .7, ,Vo 5. 1991 i0 l ; r

Figure 3. Photornicrograph showing the contact area (radiits '-154 rrm) restrlt ing from the contact (in air) between a lens (Ii =1.44 mm) and a flat sheet of PDM-S. The edge of the lens is;out.side the field of view. There was llo external load on the lens

i,l$I"... the deformation was solelv dtte tc' the ef fect of sttrface

and the defrirrrrati( )I) was ni€it^stlr€J as a frtt ictit ln oi externalload.

'f he load was first In( reased frorrr zero to abriut 200

dyn and then decreased back to zero. In fhe secondexperiment, the deformations resulting frt lrn the i:ontactof lenses of various sizes with a flat sheet. of I'I)MS weremeasured by using the same apparatus, but at zero load.

Figure 4 summarizes the results obtained from tlie firstexperiment, where o3 is plotted as a functton of P fc"rr alens of radius 1.44 mrn. The data obtained from both theloading and unloading experiments fall on the same curve,indicating no hvsteresis in thesu cont*cl rlefcirmations:tha t i s . I h t ' r l t ' t i r t ' f i l i ,1 io t ; . a re f i j \ e rs r i ) iF . " l ' l tese exper imenta l da t t r t loar l r t r r : and r tn l t ra , . l t l t y . i l i i ta i t t ke t i tuge ther , twere analvzer l n ' i th eq I t ts ing a t t t tn ier ical regressionmethod with l t ' anr i K as inprt t i 'ar iables. The best f i tbetrveen the exper imental data and eq i y ie lded values ofWand A as 44 .1 ( * i .0 ) e rgs /cm2 and 4"83 ( *0 .06) x 106dl'n/ cm2, respectively.20

The contact deformations obtained for the lenses ofvarious sizes under zero load were analyzed as follows. Ifthere is no external load (P = 0), eq I redtrces to

a 3 = 6 r w V 2 1 x ( 4 )

According to eq 4, a plot of o3 versus R2 should be ri straightline passing through the origin. Figure 5 shou.s such aplot. The linear relation between o3 and R2 is as predictedby eq 4. From the slope of this straight line, and using theabove value of K (4.83 (+0.06) x 106 dyn/cm2), the valueof W is 42.5 (*0.5) ergs/cm2. The close agreement of thevalues <tf W obtairred from the above two experimentsdemonstrates the self-consistency of these two procedures.The surface free energy (y.u) of the polymer is given ashalf of the work of adhesion.a The average value of T.uestimated from the data in Figures 4 and 5 is thus 21.8(+0.8) ergs/cm2.

Surface Energy of PDMS from Contact Angles.This section briefly reviews the theory of contact anglesneeded to estimate the ygu of PDMS and for laterdiscussion.

(20) The components used to prepare cross-linked PDMS were obtainedcommercially, and its elastic modulus occasionally differed from batchto batch. This variation might be due to a number of factore, whichinclude (but are not restricted to) the differences in the concentration ofthe cross-linking agent and slight poisoning of the hydrosilation catalystby trace contaminations (e.g. mercaptans, amines, and phosphines) duringthe preparation of the polymer" Although the elastic modulus of PDMSvaried occasionally, its surface properties were not affected significantly.In order to compare the results from different batches of PDMS, it isnecessary to normalize the data by determining the value of K. Valueof K during this course of work clustered around 4.83 x 100 dyn/cmz.

i CAMEM (R)

Figure 2. Apparatus used to measure contact deformationbetween a PDMS lens and a PDMS sheet in air is shownschematically. The flat sheet (I) was placed on one end of thelever arm (J) whose other end was connected to an electrobal-ance (M). The leaf-spring (F) was a semicircular strip oftransparent adhesive tape. The glass plate (G) was mountedwith the leaf-spring simply by pressing it against the adhesivelayer. The lens (H) adhered sufficiently with the glass platewithout any adhesive. The lens (H) could be translated up, down,or sideways. When the lens came into contactwith the flat sheet,(I), any extra load was registered on the electrobalance. Thecorresponding contact deformation was recorded in the videomonitor (S).

Results and Discussion

Interaction between Semispherical Lenses and FlatSheets of PDMS in Air. Interactions between twoPDMS surfaces in air were studied by using the apparatusshown in Figure 2. The flat sheet rested on one end of alever arm: the other end of the lever arm was connectedto an electrobalance. The lens was brought into contactwith the sheet. Any positive or negative load applied onthe lens, while it was in contact with the sheet, wasdetermined by using the electrobalance and the corre-sponding contact deformation was recorded in the videomonitor. This apparatus was also equipped for stillphotography (not shown in the diagram). Figure 3 showsa typical photomicrograph of the contact zone resultingfrom the contact of a lens and a flat sheet of PDMS in air.

Two types of experiments were carried out in air. Inthe first experiment, the radius of the lens was kept fixed

l- "rr;1 ---1 MON|TOR

1016 Langrnuir, Vol. 7, No. 5, 1991 Chaudhury and Whitesides

R2(cm2 x 10O)

Figure 5. Plot of a3 against B2 following eq 4. The data wereobtained from the contact deformations of lenses of various sizesand a flat sheet of PDIvIS in air. There was no external load onthese ienses. and thus the deformations were purely due to surfaceforces.

the work of adhesion at the sol id-l iquid interface can beexpressed as a geometric mean of the two surface freeenergies as

14/ = 2(^y"ry,n,)o u

According to Dupr6,22 the work of adhesion Wexpressed in terms of 7.", ylv, and 7el BS

W = l e r * 7 t " - T . t

Combination of eqs 7 and 8 yields

?.r = l(7r,)o'u - (lr")o'ul' (g)

This revision23 shows that yt becomes zero as Tlv ap-proaches ̂yru. Consequently, 7. is equal to ?r" only whenthe predominant forces across an interface are purelydispersive. Equation 9, in conjunction with eq 5, becomeseq 10, known as the Good-Girifalco-Fowkes equation

cos d = -1 + 2(^r"rl tw)o'u

oo

Xo

Eo

oG

oo

Xo

Eo

oG

120 140 160 180 2@p (dynes)

Figure 4. Plot of os against P following the form expected fromeq 1. The data were obtained from the load-deformationexperiments using unmodified PDMS. The radius of the lensused in these measurements was 1.44 mm. The open circles (o)represent the data obtained from increasing loads and the closedcircles.(o) represent the data obtained from decreasing loads:there is no hysteresis. The solid line was obtained fiom theanalysis of these data by using eq 1.

There are several methods3{ to estimate the surfacefree energy of a solid from the contact angles (0) of non-wetting liquids, all involving Young's equationl

71, cos 0 = "y""- ̂ f ,t (5)

Here, Tii (ij stands for lv, sv, and sl) represents the surfacetension (or surface free energy) values of the i-j interface.2lSin99 Jd c&nnot in general be determined a piiori, eq 5 byitself is not useful for estimating y.r, and hence approx-imate methods are needed. The earliest method is due toFox and Zisman.s According to this method, tlie cosinesof the contact angles of a number of liquids on a solidsurface are plotted against the surface tensions of the testliquids. The line obtained from such a plot is extrapolatedto cos 0 = L;the corresponding surface tension of the liquid,which deinarcates those liquids that would spread ott thesolid as a thin continuous film from those that would not,is termed the critical surface tension of the solid (^y.). Thedefinition o{ t. and a simple application of young'sequation yieldss

7c = 7s' -

7gl (6)

Equation 6 shows that l. is a measure of the surface freeenergy of the solid but is not necessarily equal to it, becauseTrl need not be zero even when 0 is zero. Fox and Zisman'sappoach was later revised by Good and Girifalcoa andFowkes,6 who recognized the importance of separating thevarious forces that constitute the surface and interfacialfree energies. If the forces operating across an interfaceare purely dispersive in nature and if the 7ru and 71" valuesrepresent the true surface free energies of the solid andliquid (i.e. when the adsorptions of vapor are negligible),

(7)

can be

(8)

(10)According to eq 10, surface free energy of a nonpolar solidwould be found more accurately by plotting cos d against(tru){'5 rather than 71u. Equation 10 also allows theestimation of 7r, of a nonpolar solid from the contact angleof a single nonpolar liquid.

For PDMS, 7." is expected to result mainly fromdispersion forces. Hence 7. for PDMS should, as a firstapproximation, be equal to its 7r". The values of ?c ofPDMS reported in the literaturez4,z6 are in the range of22-24 ergs/cm2. We also made an independent estimateof its 7r" from contact angle measurements. An experi-mental difficulty was encountered in measuring the contactangles of organic liquids on PDMS, since most organic

(21) The term'surface ercesE free energy'is more appropriate for aeolid eurface than eurface tengion (appropriatc for liquidii. ttre iaenlitybetween the surface free energy andluriace tension is eiricilv valid forliquid. eurfaces. For liquids itis poesible to change the surfac;ar"" qu"-sistatically without doilr.wor.k agailst the elaitic forceg. r,o, soiias,however, the prese-nce of the elastic forces complicates the derrnition ofsurface teneion. The definition of eurface teneion when applied to solidsc_an.be juetified only if the composition of the surface rei-ains conetantduring an erperiment.

(22) Dupr6, A. Th1orie Micanique de la Chaleur. Parie, 1869.(23) Raleigh [Raleigh, Philos. Mog. 1883, 16, g09l wag the firet to uE€

a geometric mean approach to problems of interfacial tensione. He arrivedat eq 9, tested it for water-oil interface and rejected it. He did, however,explain the discre-pancy by remarking that "the action of one fluid upongnollr_er might follow an altogether different law from its action uponitself."_ Qq Zlqman, W. A. In Symposium on Adhesion and Cohesion; WeisE,P., Ed.; Elsevier: New York, 1962; p 1?6.

(25) Owen, M. J. J. Coatings Teihnol.lgSl, 93, 49.

Interoctions between Lenses and Flat Sheets

liquids swell PDMS to a greater or lesser degree. Meth-ylene iodide was one organic liquid that did not swellPDMS. The contact angle of methylene iodide on PDMSwas 70o with no visible hysteresis. With this value, ?a, ofPDMS was found to be 22.L ergslcm2 from eq 10. Contactangles of two other probe liquids, nnrp€ly, hexadecane andparaffin oil, were also measured. Both of these liquidsswelled PDMS. In order to minimize the effects due toswelling, we measured the contact angles within a fewseconds after the application of the drops on the surface.The advancing and receding contact angles of hexadecaneof PDMS were 40o and 260, respectively. With thesevalues, ?rv of PDMS was calculated to be 21.6 and 24.9ergs/cm2, respectively. By use of the advancing (51') andreceding (40") contact angles of paraffin oil on PDMS, Tr"of PDMS was found to be 21.5 and 25.3 ergs lcmz,respectively. Note that both the liquids yield similar valuesof ?., for PDMS. Because of swelling, however, we believethe values of ̂ y." from receding contact angles to be lessaccurate than those from advancing contact angles. Valuesof y." of PDMS obtained from the advancing contact anglesare similar to that (22.1 ergs/cm2) obtained from thecontact angle of methylene iodide and are also close to thevalues (2L-22.5 ergs/cm2) obtained from contact defor-mation experiments.

Surfact tension of liquid PDMS,26 in the limit of infinitemolecular weight, is about 2L ergslcm2, well within thelimit predicted for solid PDMS. The agreement betweenthe surface free energies of solid and liquid PDMS indicatesa similar orientation of the surface groups (i.e. methylgroups) in both the liquid and cross-linked solid state.Owenb first suggested this possibility on the basis of thehigh flexibility of PDMS backbone.

Interaction between a PDMS Lens and Flat Sheetin WaterMethanol Mixtures. The interactions be-tween PDMS surfacee were measured by bringing a lensof radius 1.52 mm into contact with a flat sheet in water-methanol mixtures of different compositions2? using theapparatus shown in Figure 6. Figure 7 summarizes thedata obtained by calculating the work of adhesion betweenthe PDMS surfaces in these liquid mixtures using eq 4.The adhesion between PDMS surfaces was strongest inpure water and deceased as the hydrophobicity of theliquid increased; a weak but measurable adhesion persistedbetween PDMS surfaces even in pure methanol. Thisexperiment shows the influence of the medium on thework of adhesion between two hydrophobic surfaces ofconstant chemical composition. Since the interfacial freeenergy ?sl at a solid-liquid interface is given by half of thevalue of l,[/ru, its magnitude is easily calculated from thedata shown in Figure 7. Figure 8 compares these valuesof ?l with the surface tensions (lr") of water-methanolmixtures measured at airliquid intcrfaces. The two curvesare almost parallel (except at very low concentration ofmethanol), indicating that the surface activity of methanolat air-olution interfaces is roughly the same as that atPDMS-solution interfaces.

Direct Eetimation of the ?av of PDMS UsingYoung's Equation. We discussed above a method toestimate ?sv of PDMS from the contact angle of hexade-

Langrnuir, Vol. 7, No. 5, 1991 1017

Figure 6. Contact deformations under liquids measured byplacing a lens (D) in contact with a flat sheet (E) of PDMS ina liquid medium. The experimental cell is made of Sylgard 170(C), which has a rectangular hole at the center. The cell issandwiched between two glass slides (B and F). This assemblyis placed in between the condenser (A) and objective (G) of anoptical microscope.

Volume Percent of Methanol

Figure 7. Works of adhesion ( W.rJ between two PDMS surfacesin mixtures of water and methanol plotted against the volumepercent of methanol in the mixtures. To generate this plot, thedata obtained from the deformation experiments under themixtures of water and methanol were analyzed according to eq4. The radius of the lens used in these experiments was 1.52 mm.

cane and paraffin oil using the Good-Girifalco-Fowkes(GGF) equation (eq 10). This equation assumes that theinteraction is entirely due to dispersion forces and that ageometric mean combining rule for the interfacial inter-action could be employed.

Here, we describe an alternate method to determine 7."of PDMS from Young's original equation. This methodis neutral with respect to the nature of forces constituting?ev or 71". Since Tlv, ?sl, and 0 were all measured for water-methanol mixtures on PDMS, we could construct a plotof 71, cos d vs 74. This plot, according to Young's equation,should be a straight line, whose intercept in the Trv cos 0axis is the surface free energy of the polymer. Figure 9summarizes the results and indicates that the expectedlinear relation is observed. To generate such a plot, boththe advancing and receding contact angles of water-methanol mixtures were used. The average value of ?.,

N

Eooc',o

6

c9o

J

o=

(26) Wu, S, Polymer Interface and Adhesion; Marcel Dekker: NewYork, 1982.

(27) We estimated the influence of buoyancy as followe: The densityof PDMS is about 0.99 g/cms-a value roughly the eame ae watcr (1g/cm8). Hence there should not be any buoyancy effect in water. Inmethqlol, the weight of the lens, after correcting for buoyancy, was 0.54mg. This weight correeponds to a value of P o10.5 dyn, which is muchlower than the effective weight l6rWR = 21 dynl of the gurface forces(so€ ref 8). Hencc, the effect of buoyancy could be safely neglected inmethanol as well.

,ID

t -=-+

1991

Volume Percent of MethanolFigure 8. Surface tensions (rr") of water-methanol mixtures atthe air*olution interfaces compared with the interfacial freeenergies ("yrJ of water*"methanol mixtures at PDMS-liquidinterfaces. 'Ihe

closed symbois (O) repr:esent -yr" and the opensvrnL'ols (61) represert )sl . In the inset. l l is plotted against -y,, . .

15 20 25

Yo {ergs/cm2)

Chaudhury and Whitesides

vapor or solid-l iquid interface cannot in general bemeasured independently. There are, however, two re-ported cases where all four parameters of Young's equationcould be estimated independently. Johnson et al.8 studiedthe interaction of two smooth rubber spheres in air andin water. They estimated the values of y." and 74 as 35and 3.4 ergs/cm2, respectively. With these values, thepredicted contact angle (64") of water on rubber agreedwell with the experimental value of 66o. This demon-stration of Johnson et al. was the first experimental proofof Young's equation. The other report was by Pashleyand Israelachvili,zs'2e who estimated the ?r" (2i + 2 eryslcm2) of mica coated with an organic monolayer and the 7r1(11 + 2 ergsf cm2) at the interface between this surfaceand a dilute surfactant solution (7r" = 40 ergs/cm2) usingforce balance experiments. The experimental contactangle (64") of the surfactant solution on the monolayercoated mica agreed with the prediction (66") based onYoung's equation. Neither of these two reports discussedthe possible influence of nonidealities of their surfaces ofthe type expected to give rise to contact angle hysteresis.Our experiments with PDMS provide a third exampiewhere all the parameters of Young's equation wereindependently measured for 12 different conditions, usingboth the advancing and receding contact angles. We takethe agreement between the ̂ y.u values obtained from Figure9 (using both advancing and receding contact angles) andthe values estimated from the measurements in air asanother direct proof of Young's equat ion.

There is, however, a detail about these data that deservescomment. Even though PDMS exhibits a low hysteresisin contact angles (2o-3o), in terms of energy, the hysteresisis significant for water-methanol mixtures, especially inthe range of higher water concentrations (Figure 9). Sinceanalysis of advancing and receding angles gives differentvalues of T.r, one may ask which value is the more accurate(or, perhaps, the more appropriate for a given type ofexperiment). For this discussion, let us consider the caseof pure water where the hysteresis is most pronounced.The term ^y1y cos 0 for water for PDMS obtained fromadvancing and receding contact angles is -18.8 and -15.9ergs/cmz, respect ively. Using the value of 7ru as 21.8 ergs/cm2 (an average vaiue obtained fr<-rm the measurementsin air), we estimate the interfacial free energy (trr) at thewater-PDMS interface to be 40.6 ergs/cm2 from d" and37.i ergslcm2 from 0r, respectively.30 The latter valueagrees with the tt (37.2 ergs/cm2) obtained from contactdeformations. This agreement is consistent with thepicture that the lens, while coming to contact with the flatsheet, expels the liquid from between the two solids. Sincethe liquid being expelled is in the receding mode, the workof adhesion between the two PDMS surfaces is related tothe 7.1value obtained from the receding contact angle ofthis liquid in an air-liquid-solid system.

Effect of Surface Pressure. The surface free energyof a solid (yr) is half of the reversible work needed toseparate two semiinfinite slabs of the solid under vacuum(or in an atmosphere of a gas that does not interact with

(28) Pashley, R. M.; Israelachvili, J. N. Colloids Sur/. lg8l, 2, 169.Pashley et al. (Pashley, R. M.;McGuiggan, P. M.; Horn, R. G.; Ninham,B. W..f. Colloid Intert'ace Sci. 1988, 126,569) Iater improved the qualityof monolayers adsorbed on mica by using higher purity eurfactant. Thevalue of "!,d obtained with this improved system was higher (28-36 ergs/cmr) than the value reported in their earlier work.

(29) Ieraelachvili, J. N. Adu. Colloid Interface Sci. 1982, 16, 31.(30) The interfacial tension at the liquid PDMS-water interface wae

found tnbe 42-44 ergef cmz [Kanellopouloe, A. G.; Owen, M. J. ?rans.Faraday Soc. 1971, 67,3L271. These values were closer to the valueobtained from advancing contact angles of water of PDMS than the valueobtained from receding contact angles.

RrT - . .

lII.ll o

601- l tE {O Ia soJ9 ls le l

E Yq I r,.'f eoJ 'l

i1I

201I

J

I1 0 1

I1

I

o J--'---0

f gtrooc D 4g

\ r nooO> - a

Figure 9. Plot of ^),r" cos d against ̂ y.1 for the mixtures of waterand methanol on PDMS obeying Young's equation (eq b). Closed(O) and open (O) circles correspond to the data obtained fromthe advancing and receding contact angles, respectively. Thevalues of "y1w€r€ obtained from Figure 8. The linear correlationsbgtween llv cos 0 and .f .r are in accordance with Young's equation.The intercepts in the 71" cos 0 axis yield the value of i," of pOtvtsas 20.9ergs/cm2 (from 0,\ and2l.2ergs/cm2 (from 0J, respectively.

for PDMS was estimated as 21.1 (+0.2) ergs/cm2, inagteementwith the value (2L-22.5 ergs/cm2) obtained fromthe measurements made in air. Since the hysteresis incontact angles is small over the entire range of compo-sitions, the slopes of the two lines shown in Figure g arealso very close (1.08 from 0" and 0.99 from dr)-

We conclude this section with a special note on Young'sequation" We m.entioned before that ?r" or ?el at the solid-

Interoctions between Lenses and FIat Sheets

the surface). Adsorption of vapor onto the surface reducesthe magnitude of 7, by a term (r = "y"- ?r"), known as thesurface pressure.sl'32 There is substantial discussion inthe literature concerning the importance of surface pres-sure (r) to contact angles. Adamsonss suggested that thesurface pressure resulting from the adsorption of vaporon solid surface can be significant, even for low-energysurfaces. Zisman,6 Good,il and Fowkesss suggested that?r may be significant for high energy surfaces and when dis close to zero, but its effect can be safely neglected forlow energy surfaces and when d ig much greater than zero.

For phenomenological treatments of contact angles, theimportance and interpretaton of r are irrelevant;for properinterpretation of the value of ?ru (as is the case with themeasurements with PDMS described in this paper), themagnitude of r is important.

The best method to estimate the value of r is gasadsorption.s3 Since we have no data on gas adsorption,our arguments are only qualitative. Had the term n beensignificant, we feel that the 7ru obtained from analysis ofthe data in Figure 9 would have differed from the valueobtained from the measurements made in air. In addition,since r depends on the composition of the solvents, itshould have caused the plots of 7r" cos 0 vs 7sl (Figure g)

to deviate from linearity. Since this plot is linear, webelieve that the value of r is small. This inference suggeststhat ?., obtained from these measurements is the truesurface free energy (rJ of the polymer.

Effect of Surface Roughnest. Up to this point, ouranalysis of contact deformations has assumed that thePDMS lens and PDMS sheet make molecular contact andthat any liquid between the two is completely excludedwhen the two are brought into contact. The assumptionthat the polymer surfaces make uniform van der Waalscontact has no direct experimental support. The valuesfor the surface free energy obtained by the JKR anaiysisand from the contact angle measurements are, however,in satisfactory agreement is indirect support for the validityof the assumption of contact. If intimate molecular con-tact is established between two surfaces, it must be becausethe surfaces are smooth or that the elastomeric polymerconforms to the other surface by depressing or extendingits surface asperities. Verification of this assumptionthrough direct measurements is important and is thesubject of future investigations. Roughness can complicatethe analysis of contact deformations and contact anglesin other ways. Contact angle is influenced roughness. Thecontact angle on a rough surface is not strictly describedby Young's equation, and the effect of hysteresis must beconsidered. Rough surfaces and those showing othernonidealities may exhibit metastable states.36 Poly(di-methylsiloxane) surfaces prepared from Sylgard 170 (DowCorning) however, exhibit negligible hysteresis in contactangles for water-methanol mixtures (Figure 9). The closeagreement of 7." of PDMS (i.e. 2L.2 and 20.9 ergs/cm2)obtained by using the advancing and receding contactangles of water-methanol mixtures on PDMS suggeststhat the hysteresis in contact angles has no profound effect.We also believe, based on the following arguments, thatwhen the two surfaces are brought into contact in liquids,the liquids are displaced from between them.

(31) Bangham, D. H.; Razouk, R. I. Trcns. Faraday Soc. 1937, 33,1469.

(32) Harkins, W. D.; Livingston, H. K. "L Chem, Phys.1912,10,342.(33) Adameon, A. W. Physical Chemiatry of Surfaces, 3rd ed.; John

Wiley and Sona: New York, 1976.(34) Good, R. J. .I. Colloid Scl. 1976, 52, 308.(35) Fowkee, F. M.; McCarthy, D. C.; Moetafa, M. A../. Colloid Interface

Sci. 19t0, 78, 200.(36) Dettre, R. H.; Johnson, R. E. Adu. Chem. Ser. 1964, No. 43, Ll2.

Langmuir, VoL 7, No.5, 1991 1019

A B C

Figure 10. Three possible ways a lens might come into contactwith a flat sheet under a liquid: (A) lens and flat sheet make truemolecular contact with each other with no liquid between them;(B) what might happen if the surfaces were rough; (C) a film ofliquid present between the two surfaces.

Figure 10 shows the various possibilities for a lensopposing a flat sheet through a liquid. The liquid may becompletely excluded from between the two surfaces andintimate contactmaybe made betweenthem (Figure 10A);the lens may make partial contact with the sheet and poolsof liquid may remain between them (Figure 108); surfacesmay repel one another (Figure 10C). This last possibilityis unlikel-v- for our system. We wish to determine whetherthe situation shown in Figure 10A or Figure 108 is mostrepresentative of our case. A detailed analysis of the effectof roughness on adhesion is complex; here we present asimplified but plausible description of the problem.Assume that Figure 108 is correct; the apparent interfacialwork of adhesion ( W'.rJ is then proportional to the fractionof the total area in molecular contact, i.e.

W'"b= rWrt" ( 11 )

Here, W*"(=2^y.1) is the true work of adhesion and r is thefraction of the total area that is in intimate molecularcontact,3? Tising eq 11, Young's equation becomes

11u cos 0 = 1a,- (L l r ) (7 '"r1

Here, ] 'er (= W'rul2) is the apparent interfacial free energy

obtained from contact deformations using rough surfaces.According to eq 12, a plot of ylv cos d vs ?'rr will be astraight line, whose slope isL I r. Note thatthis plotshouldstill yield the correct value of T.u, although the slope ofthis line would differ from unity. The values of the slopesobtained from the two plots, shown in Figure 9, are 1.08from d" and 0.99 from d.; both of these values are close tounity. For this reason, we feel that ?'.r and 7r1 areexperimentally indistinguishable, that is, the liquids are,in fact, displaced and contact is made between PDMSsurfaces (Figure 10A).

Forces Rcquired to Pull a PDMS Lens from a FlatSheet of PDMS. Equation 1 indicates that the contactarea will be reduced to zero only if P is negative. If P isnegative, a real solution of eq 1 exists when 6IWRP 3(3rWR)2, where the equality sign represents the limitingcase of the two solids just touching each other. By use ofthis equality, the force necessary to separate the two solidsis given by

P = L. \ tRW

(12)

(13)We tested this result of Johnson et al.8 by measuring theforce necessary to pull PDMS lenses of various sizes offa flat sheet using the apparatus shown in Figure 2. Theresults are summarized in Figure 11. The linear depen-dence of the adhesion force on I is in accordance with eq13. From the slope of this line (1.5zrW = 213 ergs/cm2),we calculated the value of Was 45.2eryslcm2. This valueof W yields a value for 7r" of 22.6 ergs/cmz, in agreementwith the values obtained from the equilibrium contactdeformations. This result is another indication that the

(37) Wenzel, R. N. Ind. Eng. Chem.1936, 2t, 988.

0,0,

Goc

0ruoob 2 0

IL

c'=, 15o.cE

1020 Longmuir, Vol. 7, No. 5, 1991

0 0.025 0.0s 0.075 0.1 0.125 0.15 0.175 0.2

R (cm)

Figure ll. Adhesion (pull-off) forces between semisphericallenses and a flat sheet of PDMS varing linearly with the radius(fi) of curvature of the lenees. These data are in accordance witheq 13. The surface free energy of PDMS obtained from this plotis 22.6 ergs/cm2.

deformations on contact between the lenses and flat sheetsof PDMS are mostly elastic in nature: had viscousdissipative effects in the PDMS been important, theexperimental pull-off force would have been higher thanthat predicted by eq 13.

Experiments with Synthetic Model Surfaces. Thecontact deformation experiments carried out with unmod-ified PDMS established four important properties of thissystem: (i) the surface of PDMS was smooth and homo-geneous; (ii) the contact between PDMS surfaces waspurely elastic; (iii) conta.ct deformations were reversible;(iv) intimate contact was established spontaneously be'tween two PDMS surfaces both in air and in liquids. Thispattern of characteristics was ideal for experiments of thetype described here and stimulated us to investigatewhether similar contact deformation experiments couldbe conducted with surface-modified PDMS. Experimentswith modified surfaces could be useful for studying therelationship between adhesion and surface constitution.In this section, we first describe methods to modify thesurface of PDMS. We then discuss the results obtainedfrom the load-deformation studies using these modifiedsurfaces. We conclude by comparing the surface freeenergies of these modified surfaces with values obtainedfrom contact angles.

The surface of PDMS is converted to silica on exposureto an oxygen plasma.38 These oxidized surfaces can befurther functionalized by reaction with alkyltrichlorosi-lanes (Scheme I). Reaction with silanes is a methodcommonly used to modify glass, silica, or other oxidesurfaces.se Recent studiesls'le have indicated that long-chain alkyl trichlorosilanes on reaction with silica formwell-ordered monolayer films. We hoped that long-chainalkyltrichlorosilanes would also form well-ordered mono-layers on reaction with oxidized PDMS (PDMS-).

We carried out these reactions by exposing PDMS''tothe vapors of these silanes under reduced pressure rather

(38) Fakes, D. W.; Daviee, M. C.; Browng, A.; Newton, J. M. Sur/.Interface Anal. 1988, 13,233.

(39) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: NewYork, 1982.

ffiA

Chaudhury and Whitesides

Scheme I. PDMS Functionalized by Oxidation in anOxygen Plasma To Generate a Silica Surface'

n n R nl l l l

oH oH oH oH ,ai'",j'"'j'"'i*

OrygenPlasma

o This superficial silica layer is further functionalized by reactionwith functional alkyltrichlorosilanes.

Table I. Surface Free Energies of Silane-Modified PDMSSurfacee'

0^, 0,'(deg) (deg)

7rr, ergs/cm2, from

system

PDMSPDMSOTPDMSo{gSi(CHz)gCHsPDMS'- OsSi(CHtz(CFz)zCFg

l0 (s) 26 (s) 2L6 24.90 0

42 40 2L.0 21.683 69 8.7 12.8

o d" and 0, are the advancing and receding contact angles of hexa-decane in degrees. Surface free energies ("rr") are calculatpd for eachsample by using both the advancing (dJ and receding (0') contactangles and using the Good-Girifalco-Fowkes equation (eq 10, seetext). (s) indicates that PDMS swells in hexadecane; the swelling ofPDMS was, however, prevented on the monolayer-coated PDMSsurfaces. ( -) ind icates that the value of ?r' was not estimated becauseof inadequate contact angle values.

than to solution of silanes, because PDMS swells in mostorganic Iiquids. Studies of the reaction of PDMSoT witha number of organofunctional silanes wil l be reportedseparately. Here we present the results concerning surfaceth-ermodynamics obtained by using Cl3Si(CHz)gCHs andCl3Si(CHz)z(CFz) rCFg.

The surface free energies (t.") of PDMS.'-OBSi(CH2)e-CHs (the product of reaction of PDMS.'and CIBSi(CH2)e-CHs) and PDMSo-OsSi(CH2)2(CFz)zCFe (the analogousmaterial from reaction of PDMS and ClsSi(CHilz(CFdz-CFs) were obtained from the contact angles of hexade-cane and by using eq 10; these energies are summarizedin Table I. Although we have no direct estimate of thestructural order exhibited in these modified surfaces' webelieve, by analogy with the previous work, that theoutermost layer of PDMSo-OgSi(CHz)gCHg is populatedwith CHs groups and has order similar to that of otherself-assembled mo.tolayers comprising n-alkyl units. Theorder in the fluorocarbon containing surface is less certain.

Zisman and his collaboratorss'6 first suggested values of

7. for solid surfaces that were characteristic of certainfunctional groups: -CHs, 22 ergslcm2; -CHz-, 31 ergs/cm2;-CFz-, 18 ergs/cm2;-CFs, 6 ergs/cm2. These ?cvdueswere obtained by plotting the cos 0 of several liquids asa function of their surface tensions and extrapolating tocos 0 = 1. As mentioned before, a better way to obtain 7"(and thus ^ys, of a nonpolar solid) is to plot cos 0 against(1/(yr,)0'5). This procedure reduces the errors of longextrapolations inherent in Fox and Zisman's procedure.Fowkis,s'ao using the latter method, found values of 7r' ofthese surfaces that followed the same sequence as Foxand Zisman's 7. values: ̂ yru values of surfaces composedof -CHs, -CHz-, -CFz-, and -CFs groups were 21, 35, 19.5,and 10.4 ergs/cm2, respectivelY.

The comparison between ys, of PDMSo-OsSi(CHts-CHe with Fowkes' values indicates that its surface iscomposed mainly of -CHs groups. This value dso agteeswittr-two othervalues of ?svreported for surfaces composed

(40) Fowkes, F. M. Adu. Chem. Ser. 1964, No.43,99. Fow-k-es, F. M.lnSurfaces and Interfaces; Burke, Reed, Weieg, Eds.; Syracuse UnivenityPrese: Syracuee, NY, 196?; p 197.

0

Interactions between Lenses and Flat Sheets

of close-packed -CHs groups: one (7r, = 19.3 ergs/cm2)was prepared by adsorbing long-chain alkanethiols(HS(CH2)zrCHe) onto goldal and the other (?,, = 20 eryslcm2) by reacting long-chain alkyltrichlorosilanes (ClsSi-(CHdtzCHs) on silica.a2 These values of surface freeenergy, coupled with the fact that PDMS.-OBSi(CHte-CHs exhibits low hysteresis in contact angle, stronglysuggest that its outer surface is ordered and composedmainly of -CHs groups.

We cannot reach a clear conclusion concerningthe orderof PDMSo-OgSi(CHz)z(CFdzCFs, other than to note thatthe low value of 7." clearly indicates that the surface iscomposed mainly of fluorinated groups. Values of 7r" forthis surface estimated from the advancing and recedingcontact angles of hexadecane range from 8.7 to L2.8 ergslcm2, values close (10.4 ergs/cm2) to that expected of asurface populated mainly with -CF3 groups. The highhysteresis in these contact angles suggests significant (butdifficult to quantify) disorder for this surface.a3

Interaction between Oxidized PDMS Surfaces.The surface free energy of oxidized PDMS is much higherthan that of unoxidized PDMS. This assertion is basedon two major observations: zero contact angle of water,and high adhesion between two oxidized surfaces of PDMS.We found that two oxidized surfaces of PDMS, whenbrought into contact, adhered so strongly that they couldnot be separated without causing cohesive failure in thepolymer samples. Strong adhesion between these surfaceswas also reflected in the load-deformation experiments asdiscussed below.

The deformation resulting on contacting a lens (B =1.21 mm) of PDMS.T with a flat sheet of this material wasmeasured as a function of external load. Because thecontact area did not decrease during the unloadingexperiments and the joint fractured only cohesively, nouseful information could be obtained from the unloadingexperiments other than to note that the force required tofracture such a joint was nearly 2 orders of magnitudehigher than that required for unmodified PDMS. Thedata obtained from the loading experiments are summa-rized in Figure 12. The analysis of these data in light ofeq 1 yielded values of W and K as 117 (+2) ergslcm2 and5.11 (+0.06) x tOo dyn/cm2, respectively. Note that thevalue of K obtained from this measurement is similar tothe value of K (4.8 X 106 dyn/cm2) for unmodified PDMS.The value of 7ru of PDMSoT was found to be 58.5 ergs/cmz,which is nearly 3 times the value of ?ru for unmodifiedPDMS.

Interactions between Surface-FunctionalizedPDMS Surfaces: Cohesive Interactions. This sectiondescribes the results of the load-deformation experimentsinvolving surface-functionalized PDMS lenses and PDMSsheets. Both the lens and sheet had the same chemicalgroups. PDMS sheets and PDMS lenses, which had beenfunctionalized according to Scheme I, were subjected tothe load-deformation studies (the lens and flat sheet hadthe same chemical groups) using the apparatus shown inFigure 2. The results of these experiments are shown inFigures 13 and 14. In contrast to the unmodified PDMS(Figure 4), hysteresis was observed in these experiments.Horn, Israelachvili, and Pribacl2 also noted a hvsteresis

^ (4_1) Eain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J. E.; Whiteeides,G.M.; Nuzzo, R. G. J. Am. Chem. Soc. 1g89, 111,32L., (42) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. C. J. Am.Chem. Soc. 1988, tI0, 6136.

(43) H. Biebuyck (Harvard Univereity, unpublished observations)fo-und- that_a^mon_olayer su_rface composed of :CFg gtoups prepared byadeorbing HS(CH)z(CFz)zCFs onto gold erhibitcd [igner hysteresis incontact-angles than the surface prepared by adsorbing HS(CHz)eCHgonto gold.

Langmuir, Vol. 7, No. 5, 1991 1021

P (dynes)

Figure 12. PIot of o3 against P for PDMS.'following the formexpected from eq 1. The data were obtained from the loadingexperiments only. The radius of the lens was 1.21 mm.

P (dynes)

Figure 13. Plots of cs against Pshowing weak hysteresis for thesurfaces of PDMSo-OsSi(CHrsCHs. The radius of the lens was1.4 mm. The open circles (O) represent the data obtained fromthe increasing loads and the closed circles (O) represent the dataobtained from the decreasing loads. The solid lines in both plotsare predicted from eq 1.

in the deformation resulting from the contact betweentwo curved mica surfaces. The authors suggested plasticdeformation in the glue supporting the mica surface to bea possible cause of this hysteresis. In our case, the lackof hysteresis in contactdeformation for unmodified PDMSimplies that its occurrence in experiments using func-tionalized PDMS must originate from surface effects andnot from any bulk viscoelastic effects. The hysteresis wasmuch smaller on the surface of PDMSo-OeSi(CHtsCHsthan on that of PDMSo-OsSi(CHdz(CFdzCFs surfaces.These results follow the same general trend observed inthe hysteresis of contact angles. The loading and un-loading data obtained for each surface were analyzedseparately by using eq 1. For PDMSo-OsSi(CHrgCHe,the values of K obtained from the loading and unloadingexperiments are 4.91 (*0.07) x t00 and 5.02 (+0.14) x 106dyn/cmz, respectively. For PDMSo-OaSi(CHdz(CFdrCF3, these values are 5.08 (+0.38) x 106 and 4.89 (+0.28)x 106 dyn/cm2, respectively. All of these values are veryclose to the value of K (4.83 (*0.06) X 106 dyn/cmz)

€o

. X

Eo

oG

oo

Xo

Eo

1022 Langrnuir, VoL7, No. 5, 1991 Chaudhury and Whitesides

P (dynes)Figure 15. Results of the load-deformation experiments usinga lens of PDMS"'-OsSi(CHz)gCHs and a flat sheet of PDMS.-OBSi(CHtz(CFtzCFg. The open (o) symbols indicate the dataobtained from increasing loads and the closed symbols (O)represent the data obtained from decreasing loads.

of W obtained from the decreasing load-deformationexper iments.

Adhesive Interact ion htween Alkyls i loxane andFluoroalkyls i loxane Monolayers. This sect ion dis-cusses the interaction between PDMSqTSi ( C Hd z( CFr;r-CF3 and PDMSo'-OaSi(CHz)gCHe. Deformations result-ing from the contact between a lens (R = 1.28 mm) ofPDMSo'-OsSi(CHtgCHr and a sheet of PDMS.{3Si-(CH;rlgptzCFs were measured as a function of increasingand decreasing loads. Significant hysteresis in contactdeformations was observed in these experiments (Figure15). Although this hysteresis is qualitatively similar tothat observed for surfaces containing similar functionalgroups (Figures 13 and 14), there is an important differencebetween them in terms of kinetics. While the areas ofcontact between the surfaces composed of identicalfunctional groups did not change with time during eitherthe loading and unloading experiments, the area of contactbetween the fluorocarbon and hydrocarbon surfaceschanged with time during unloading experiments. Nonoticeable time-dependent response was observed duringthe loading experiments, however. Figure 16 exemplifiesthe relaxation kinetics of the area of contact betweenfluorocarbon and hydrocarbon surfaces. In this experi-ment a load of 200 dyn was applied to a lens of PDMS.-OaSi(CH2)gCHe while it was in contact with a sheet ofPDMS.'-OBSi (CHtz(CFz)zCF3; the load was subsequentlyreduced to zero. The contact area continued to decreasewith time and reached a plateau value after about 800 s(Figure 16). This relaxation process is qualitatively similarto what is known as 'creep" in the fracture of polymers.ft

The data corresponding to the unloading experiments(Figure 15) were taken within 15-20 s of varying the loads;hence these values were highly nonequilibrium. Theeffective work of adhesion calculated from the unloadingdata is 58.0 (+3.0) ergs/cm2, which is higher than the valuesobtained for the surfaces containing similar functionalgroups (see above). The value of K obtained from thisexperiment is 5.58 (+0.18) x 106 dyn/cm2, which is alsoslightly higher than the value of K (4.8 x 106 dyn/cm2) forunmodified PDMS.

The data obtained from the loading experiments werebetter behaved and did not exhibit dissipative charac-

o

P 4X

F6 3

oG

ooF 5

x

E 4o

oG 3

0o 20 & 60 80 100 120 1& 100 180 2@ 20

P (dynes)

Figure 14. Plots of os against P for the surfaces of PDMS.-OsSi(CH2)z(CFtzCFs exhibiting large hysteresis. The radius ofthe lens wag 1 mm. The open circles (O) represent the dataobtained from the increasing loads and the closed circles (o)represent the data obtained from the decreasing loads. The solidlines are obtained from the analysis of these data using eq 1.

obtained for unmodified PDMS. This similarity suggeststhat the elastic modulus of PDMS is not affected by thesteps used in surface modifications.

The values of W for PDMSoLOsSi(CHz)gCHr obtainedfrom the increasing loads (W = 41.1 (+1.0) ergs/cm2) aresimilar to those obtained from decreasing loads (W = 48.1(+2.1) ergs/cm2). The corresponding values of I4l forPDMS'' OaSi(CHz)z(CFtzCFs (14.2(+2.8) and 42.0 (+4.1)ergsf cmz from loading and unloading experiments, re-spectively) differ to a much greater extent. The surfacefree energies of PDMS.LOBSi(CH)gCH3 and PDMSo-O ssi (C Ht z ( C Fz) zC Fg obtained from the advancing contactanglesof hexadecane are 21.0 and 8.7 ergs/cm2, respectively(Table I), predicting values of Wfor these surfaces of 42.0and 17.4 ergs/cm2. These values are comparable to thoseobtained from the loading experiments but are lower thanthe values obtained from unloading experiments (seeabove). Using receding contact angles of hexadecane, thesevalues of W are predicted as 43.2 and 25.6 eryslcm2 forthese-CHg and-CFs surfaces, respectively. Although theagreement between this value of Wand that obtained fromunloading experiments is satisfactory for PDMSo-OsSi-(CHdgCH3, the prediction is poor for PDMSo-OsSi-(CH;rlgPrzCFe.

In summary, the works of adhesion between thesesurfaces obtained from increasing load deformations arecomparable to the predictions based on advancing contactangles. The work of adhesion obtained from the decreasingload deformations has little or no correlation with thevalues predicted from either advancing or receding contactangles. PDMSo-OeSi(CHdsCHa is better behaved thanPDMS.LO3Si(CH2)2(CF2}CF3. The origin of the hys-teresis at the solid-solid interfaces is unclear.

We also measured the pull-off forces between these func-tionalieed surfaces using the apparatus shown in Figure2. For PDMSo'-OsSi(CHz)eCHs, the force needed to pulloff a lens of B = 1.4 mm from a flat sheet was 32 (+1) dyn.For PDMSo-OsSi(CHz)z(CFz)zCFe, the force needed topull off a lens of R = 1 mm from a flat sheet was 19 (+1)dyn. These pull-off forces, in view of eq 13, predict valuesf,or W of 48.5 and 40.3 ergs/cm2 for PDMSo-OeSi(CHrs-CHs and PDMSo-OsSi(CH2)z(CFz)zCFs, respectively.These values are in excellent agreement with the values

Interactions between Lenses and Flat Sheets

rime tHcl 8oo 3600

Figure 16. Area of contact between a lens of PDMS"-O3Si-(CHdgCHs and a flat eheet of PDMS"-OgSi(CHz)z(CFz)zCFs asa function of time when a load is applied on the lens and thenremoved. The data are presented in the dimensionless form,a3(r)/o3(0), where a(t) is the contact radius at time t and o(0) isthe initial contact radius.

teristics; consequently, the value of K (4.80 (+0.09) X 106dyn/cm2) obtained from this experiment is close to that(4.83 x 106 dyn/cm2) of unmodified PDMS. The work ofadhesion (23.3 (+0.1) ergs/cm2) between the fluorocarbonand hydrocarbon surfaces as obtained from these loadingexperiments is intermediate to the values obtained forsurfaces# containing similar functional groups (41.1 ergs/cm2 for PDMSo-OgSi(CHz)sCHs and 14.2 ergs/cm2 forPDMSoLOaSi (CH2) z(CFz)zCFa.

Summary and Conclusiong

The objective of our research was to explore determi-nation of contact deformation at solid-solid interfaces asa technique for determining surface thermodynamicproperties. Poly(dimethylsiloxane), with its excellentsurface and bulk properties, is a good model system forthese studies. The deformations resulting on contactingsemispherical lenses and flat sheets of PDMS are spon-taneous and reversible (i.e. free of hysteresis). Thesecontact deformations conform well with the JKR modeland yield values of Tr" in agreement with the predictionsbased on contact angles. The agreement between the ?r"values obtained from these measurements and the adhe-sion (pull-offl forces indicates that the deformation at thecontact between a lens and flat sheet of PDMS is purelyelastic in nature; i.e. there are no concomitant dissipativeprocesses. Comparisons of the experiments carried out inair and under mixtures of water and methanol establishthat intimate contact between PDMS surfaces takes placein both cases.

An important part of these studies was to develop waysto modify the surface of PDMS and to subject thesemodified surfaces to the deformation experiments similarto those used for unmodified PDMS. The experimentalprocedure based on alkylsilylation of the SiOz layer on thesurface of plasma oxidized PDMS seems to yield surfacemonolayers having order similar to that achieved on

(44) Further ernmination of the energetica at the fluorocarbon-hydrocarbon interface ie not possible at present because of the highhyeteresie in contact angles and contact deformatione. We are currentlydeveloping ways to prepare monolayer eurfaceg of low hysteresie. Anaccount of theee studiee will b€ published in due course.

Langmuir, Vol. 7, No. 5, 1991 1023

silicon/Sio2 wafers. Experiments with these function-alized PDMS surfaces exhibit hysteresis in contact de-formations, an observation that is qualitatively consistentwith the trend observed in contact angle hysteresis. Valuesof ?ru for these modified surfaces obtained from increasingload deformations are similar to the predictions fromcontact angles; values obtained from decreasing loaddeformation are higher than the predictions based oncontact angles. The important conclusions from thesestudies are as follows:

(1) Studies of contact deformations can provide surfacethermodynamic param eters for solid-olid and solid-liquidinterfaces that are not available from conventional studiesof contact angles. Contact deformations in conjunctionwith contact angles provide a more complete analysis ofsolid-solid and solid-liquid interfaces than either tech-nique alone.

(2) This method is limited by the constraints that bothsolids must be smooth and at least one must be elasto-meric and deformable. The breadth of applicability canbe increased by modifying the surfaces of the components.

(3) A special reason for studying the chemistry of solidsurfaces is to elucidate the joint roles of surface chemistryand rheology in adhesive fracture processes. Our currentstudies are limited to pure elastic responses of the twocomponents. The adhesion forces are thus primarilydetermined by the surface properties and by the geometryof the interacting components. The adhesion forcesobserved for unmodified PDMS are consistent with thesurface free energies obtained from contactangles, becauseunmodified PDMS exhibits negligible hysteresis in contactdeformations. For functionalized PDMS, which exhibitedhysteresis both in contact angles and in contact defor-mations, the measured adhesion forces have little corre-lation with the predictions based on contact angles. Theobservation of hysteresis complies with the existence ofmetastable surface states,s which might even give rise totime-dependent responses. The origin of these metastablestates needs to be examined in detail to understand theirinfluence on adhesion and fracture. It should be possibleto extend these measurements to viscoelastic systems.Bycarefully controlling the viscoelastic and surface properties,we hope to develop a more complete picture of solid-olidadhesion.

Experimental Section

General Information. Poly(dimethylsiloxane) (Sylgard 170)was supplied by Dow Corning Corp., Midland, MI. The liquidsused for the various measurements were water, methanol, me-thylene iodide, paraffin oil, and hexadecane. Water was purifiedwith a Nanopure water purifier (Barnstead) and had a surfacetension of about 72.8 eryslcm2. When the surface of this waterwas compressed in a Langmuir trough to one-tenth of its originalarea, the surface tension of the water decreased by only 0.3 ergs/cm2, indicatingthatthe surfaceof the water was notcontaminated.Methanol (Fisher) was high-purity HPLC grade and had a surfacetension of 22.8 ergs/cm2. Methylene iodide (Aldrich) was ueedwithout further purification; the surface tension of methyleneiodide was found to be 49.4 ergs/cm2. Paraffin oil (Fisher) waspurified by equilibriating it with neutral grade alumina (Fisher)and had a surface tension of 32.4 ergs/cm2. Hexadecane waEpurified by passing it through a column of neutral grade alumina.The measured surface tension of hexadecane was 27.6 erysf cmz.The trichlorosilanes CIBSi(CH2)gCHg and ClgSi(CHdz(CFdzCFswere obtained from Petrarch and PCR, respectively, and distilledprior to use. The microsyringes (Gilmont) used to measurecontact angles and to prepare PDMS lenses were obtained fromVWR. Plasma oxidation was carried out in a Harrick PlasmaCleaner (Model PDC-23G, 100 W). Contact angle measurementswere carried outwith a Ram6 Hartgoniometer (Model 100). Themicroscope used to measure contact deformation was purchased

ooG

oo 0.4 %-o {t-

1024 Langmuir, Vol. 7, No. S, lggl

from Nikon (Nikon Diaphot Inverted Microscope), equipped witha video camera, a video monitor, and a still camera.

'

Preparation of semispherical Lenees and Flat sheete ofPDMS. The components for preparing the elastomers werelupplied in two parts, Sylgard 1?0A and Sylgard 1?0B (DowCorning Co., Midland, MI). These two parts piimarily comprisedthe components of a reaction mixture having vinyl lnd-cappedoligom_eric dimethylsiloxane (H2C:911 (Si @ grlrO),Si lciir; r_ql{:CHl_(gv_erqge value of n is about 2b0)), a -etttyt t ydrojensiloxane ( (Hsc)ssi (osiHcHJ6(osi(cHs)2)Bosi (cHr)r)

"r cross-

linking agent, and a platinum complex as a catalyrt rb. the hy-drosilation reaction. As obtained, these co-pottents boihcontained reinforcing fillers. upon storage the fillers sedimenteddown. The clear components were poured out of the containers.A 50:50 mixture (w/w) of the two clear components was stirredin a plaslig weighing cup using a glass rod. Trapped air bubblesresulting from the agitation of the mixture *ere ,emoved byapplying gentle vacuum (6fT0 mmHg for about 30 min). Smalldrops (L-2 pL) of the transparent mixture were applied with amicrosyringe onto the surface of asmooth glass micioscope slide,whjch ha{ previously been treated with C[Si(CHd2(CF2)7CF3 toreduce adhesion to it. The hydrosilation polymerization reactionwas canied out at 65 oc for t h. The fluoroalkylsilane-treatedglass slide served two purposes: first, it provided a flat substratefrom which the lenses could be removed easily after they had9g9g completely; second, the silicone drops formed a finite angle(66o) of contact on the surface. By controtting the volume of thedrops, lenses of various radii of curvature .ourd be formed. Theradii ofcurvature ofthe cured drops at the central regions weremeasured directly from their photographs. The prepied lenseshad radii of curvatures less ttran 2 m-. ttte .ur.d lens could beeasily removed from the glass slides and manipulated by holdingit at its edge by a fine-pointed tweezer. The flat sheets of rili.onlelastomer (thickness 1.5 mm) were prepared from the snmematerials used in preparing the lens. ThesL sheets were obtainedbycuringthe two-component reaction mixture in a flat-bottomedpolystyrene petri dish. The surfaces of the sheets exposed to theair during curing were the ones contacted to the lens during thedeformation experiments. Electron microscopy of these PDMSsurfaces revealed no surface features even wlien examined at aresolution of 200-300 A. Long ripples (about 1 mm) wereoccasionally visible by eye on portions of these surfaces; theseregions were avoided during the deformation measurements.

ApparatueUeed To MeasureContact Deformation in Air.Figure 2 is the diagram of the apparatus used to measure contactdeformation as a function of external load. It is related to theapparatus used by Barquins and Courtcl to study rub ber fri ction. laThelens (H) could be broughtslowly in contaciwith, or detachedslowly from, the flat sheet (I) of PDMS using a manipulator (not

*gy" j.n the-figure) attached to the clamp (D). Th; leaf spring(F) indicated in Figure 2 was a semicircular strip of adhesivetape. since the load transferred to the assembly oi th. lens andflat sheet was controlled by the vertical displacement of the clamp(D), much of the vertical displacement resurted in deformationof the leaf-spring, and thus the relative displacement of the lenswas small. The leaf-spring acted as a displacement buffer andthus provided finer control of the load transmitted to the lens.The flat sheet of PDMS rested on one end of a lever arm (J)whose other end was connected to an electrobalance (M). Thecenter of the arm was freely suspended on a sharp pivot (K). Anye-xcess load applied on the lens during the couise of a load-deformation experiment was registered on the electrobalance,which had a sensitivity of 1 dyn. The lens could be brought intocontact with the flat sheet at zeto load by careful operition ofthe manipulator. The area of contact was measured by aninverted.microscope (Nikon, Diaphot) equipped with a videocamera (R) and a video monitor (S). Difierential inte.ferencecont-rast (DIC) optics was used to view the contact area, whichyielded a sharply defined edge of the contact circle. The DICwas particularly useful when measurements were made in liquids.All the measurements were made directly from the screen ortnevideo monitor.

Apparatue Ueed To Meaeure Contact Deformationsunder Liquids. Measurements in a liquid were carried out ina rectangular cell, prepared from Sylgard 1?0 (Figure 6). Arectangular hole (3.0 cm X 1.5 cm x 0.? cm) *". cteat"d at the

Chaudhury and Whitesides

center of a rectangular slab (5.0 cm x 4.8 cm x 0.? cm) of a curedsilicone elastomer. This slab was placed flat on a grass slide (b.0x 7.5 cm) previously treated with ChSi(CHdz(CFdzCFg, whichrendered it hydrophobic. The hydrophobic polymer and thehydrophobic glass slide created a good hydrophobic seal, throughwhich neither methanol nor water leaked. A strip of pDMs sheet(1.0 cm x 2.0 cm) was placed inside this cell and then the cellwas filled with the desired liquid. The PDMS rene was held atits edge by a fine-pointed tweezer and brought slowly throughthe liquid and placed onto the flat sheet. The top of the cell wassealed by covering it with another glass slide alio treated withf_luoroalkylsilane. All the components of this cell were periodicallydismantled for rigorous cleaning. The cleaning was done byeth-anol, methanol, and distilled water. (Note: This cell could- havebeen designed with all glass components. silicone elastomer wagused primarily because it was readily available, the cells wereeasy to prepare, and several cells could be designed at once.) Thecontact area was viewed through a 4X microscope objective thathad a working distance of lG.2 mm from the-specimen. Theworking distance from the stage to the condensei was 20.5 mm,which permitted enough room to manipulate the specimens.

Measurements of Contact Deformatione Obtained byusing unfunctionalized PDMS in Air. The lenses and sheetswere r insed in HPLC grade methanol before use and air-dried.onlv those lenses * 'ere used that pro. lected circular aspects whenr rer+'ed through the mrcroscope and * 'ere free from gross defectsrFigure , l r . In order tL, carrv out a load-deformation erperiment,a lens ,r? = 1.{{ mm, \r 'as brought into contsct * i th a f lat sheetof PD\{S in air. \ei ther the remperarure nor the humidity ofthe room was controlled. The average t€mp€rature and relaiivehumidity of the laboratory were 24-25 oC and 45-50%, respec-tively. After the lens was placed on the flat sheet very carefully,the contact deformation was measured. Additional load wasapplied by pressing the lens against the flat sheet. After the loadreached a steady value, the contact deformation was measuredagain. The load was increased from a zero value to about 200dyn and the deformations were measured at random intervals.At the end of this experiment, load was removed from the rensand the deformation was again measured as the load continuedto decrease from 200 dyn to zero. This experiment was repeatedby placing the lens on different locations of the flat shlet. As-econd set of experiments was performed by measuring thedeformation as a function of the radius of curvatures of the lenses.Lenses of radii ranging from - | to 2 mm were placed on a flatsheet of PDMS under the condition of zero load. In allexperiments involving unmodif ied PDMS, the contact defor-mations were spontaneous and reversible. Although the mea-surements were generalll 'made within minutes of contact betweenthe lens and sheet, no noticeable change ofthe contact area couldbe seen even after an additionai hour of contact. This observationimplied that measurements were made under equilibrium con-ditions.

Measurements of Pull-Off Forcee. The maximum forceneeded to pull a lens out of contact with the flat sheet of pDMSwas measured by slowly removing the lens from the sheet. Wedid not have precise control over the speed of detachment; it wasdone as slowly as possible by manual operation. precise controlover speeds was not necessary in these experiments, because thesystem behaved purely elastically. The adhesion (pull-ofO forceswere measured as a function of the radii of curvatures of severallenses, which were previously used for equilibrium deformationexperiments.

Measurements of Contact Deformations in Liquide. Aset of measurements was carried out under liquids rather thanin air. since the work of adhesion depends on the medium, usinga liquid provided a convenient way to alter the surface *or[term.as The interactions between PDMS lens and PDMS sheetwere examined under mixtures of water and methanol. Toestablish the reversibility of these systems, the lens was removedfrom the surface and replaced at different locations; the observed

(45) Gent and Schultz (Gent, A. N.; Schulrz, J. J. Adhesion 1g72,3,281) used a similar procedure to alter the surface work term in theirstudies on the correlation between fracture surface energy and thermo,dynamic work function.

Interactions between Lenses and Flat Sheets

values of the contact areas were reproducible.s Cavitation{? was

sometimes a problem when the measurements were made in pure

water. When it occurred, a ring of vapor was found to surroundthe contact circle. Prolonged degassing of water eliminated thisproblem. In all experiments, degassed water was used even

though it was found that the cavitation did not occur in mixtures

of water and methanol.Meaeurements of Contact Anglee. Water, paraffin oil, hexa-

decane, methylene iodide, methanol, and various mixtures ofwater and methanol were used for the contact angle measure-ments. Quasistatic advancing and receding contact angles were.ea.uted according to the method of Neumann and Good.€Following this technique, a small drop of about 1 pL was formedon the solid surface using a needle attached to a microsy'ringe.While the drop was still in contact with the needle, additionalliquid was added to the drop to advance the drop edge as slowly

as possible. After the three-phase contact line had stoppedmoving, the advancing contact angle was measured'{e Recedingcontact angles were measured following the snme procedure afterwithdrawingthe liquid from the drop' While most measurementsof contact angles were done under quasistatic conditions, thecontact angles of paraffin oil and hexadecane on unmodifiedPDMS were measured quickly (within a few seconds) afteradvancing or retreating the drops, because these liquids swellPDMS. Contact angles of methylene iodide, paraffin oil, and

alkane were measured under ambient conditions. Contact anglesof water-methanol mixtures were measured in an environmentalchamber by equilibriating the atmosphere of the chamber with

solutions having compositions that were the same as those usedfor the contact angle measurements.

Chemical Functionalization of Polv(dimethylsiloxane ).The details of the techniques involved in functionalizing PDMSsurfaces will be described in a separate publication. Briefly,PDMS sheets and lenses were oxidized in an oxygen plasma for45 s at 0.2 Torr 02 pressure in a Harrick plasma cleaner at thelowest power setting. This procedure yielded a very hydrophilicsurface (d" of water was zero), which reacted readily with Clg-Si(CHz)sCHs and Clssi(CHrz(CFz)zCFe' Traditionallv' modi-fications of inorganic oxides by silanes have been done from anorganic solvent. Most organic solvents swell PDMS to a greater

Longmuir, Vol. 7, No. 5, 1991 t025

and lesser degree. Because of this problem, the reactions betweenPDMS.'and-the silanes were carried out by exposing the PDM-

So'to the vapors ofvarious chlorosilanes under reduced pressure.

A second advantage ofcarrying outthe adsorption from the vaporphase was that the higher molecular weight silanes could not

interfere with the adsorption of the silane monomers because

they could not transfer to the vapor phase. A plastic weighing

cup containing 3 g of paraffin oil and 200 p,L of silane was firstplaced in a desiccator, which was then evacuated to about 0.15

Torr to remove volatiles (the advantage of dissolving the silanein the paraffin oil was that the solution could be reused severaltimes). The desiccator was back-filled with nitrogen and the

oxidized PDMS samples were placed at a distance of about 1.5

cm trom the oil level.m The desiccator was again evacuated to

0.15 Torr. The desiccator at this point was disconnected from

the vacuum pump and allowed to remain in that condition for

2 h. At the end of 2 h, the samples were removed from the

desiccator. AII the steps starting from the insertion to the removal

of samples were performed inside a glovebag, purged with nitrogen(or argon).

loaa-Deformation Experiments with Functional izedPDMS. The load-deformation experiments involving function-

alized PDMS were similar to those used for unmodified PDMS

surfaces. The measurements were made in air by bringing a

functionalized lens into contact with a functionalized flat sheet.

The lens and the flat sheet contained either similar or dissimilarfunctional groups. Measurements were generally made within

minutes after the applied loads were varied. For the contact

between hydrocarbon and fluorocarbon surface, the contact area

exhibited dynamic response during unloading experiments and,hence, the measurements were made within L5-20 s after the

loads were varied. The measurements were repeated on several

locations of the flat sheet using a single lens except for PDMS''.For two surfaces of PDMS"' measurements could not be made

on several locations, because failure took place within the polymer

upon separation. For surfaces containing similar functionalgioup., the adhesion tpul l 'off) forces were measured from

diff . i .nt locatigns of the functional ized sheets. For the hydro-

carbon-f luc,rocarbon interface, the pul l-off forces depend sig-

nif icantl l . on the rate of separation and are not reported here.

This subject * ' i l l be discussed in a separate publ ication.

Acknowledgpent. We wish to acknowledge H. Bie-

buyck, G. S. Ferguson, C. D. Bain, and S. Wasserman ofHarvard University and M. J. Owen (Dow Corning) formany valuable discussions. We thank D. Stephenson andW. Anderson of Nikon for their help in installing instru-ments. The initial stages of this work at HarvardUniversity were supported in part by the Office of NavalResearch and the Defense Advanced Research ProjectsAgency through the University Research Initiative and inpart by the National Science Foundation under thebngineering Research Center Initiative to the M.I.T. Bio-technology Process Engineering Center (Cooperative Agree'ment CDR-88-03014).

Registry No. ClgSi(CHz)sCHg, 13829-21-5; ClsSi(CHz)r(CFdz-CFs, 78560-44-8; water, 7732-L8-5; methanol, 67-56-1.

(50) we later found that monolayers of identical qualities can be formed

by placing the same anywhere within the desiccator.

(46) Although we have not directly esrablished whether lhere was

damage in thJ lene during theee operatione, we infer from the reproducibility of the contact deformations on several locations of the flateheet thit the lens was not damaged. If the lens was contaminated bydust or damaged accidentally, the defect could be easily discerned by the

deviation of the area of contact from circularity.(4?) Christenson and Claesson (Christeneon, H. K.; Claesson, Per M'

science 1988, 239, 390) reported cavitation when two hydrqphobic mica

eurfaces were brought inio contact in degassed ryatcr. Th-e degr-ee of

cavitation was found to be related to the hydrophobicity of the surfaces.(48) Neumann, A. W.; Good, R. J. Sur/oc e ond Colloid Science;Good,

R. i., Stromberg, R. A., Eds.; Plenum Press: New York, 19?9; Vol. 11,p 3 1 .'

(49) Watching for the three-phase contact line to come to a quasistatic

value is very important. psp srample, the ilqtantaneous advancing contactangle of *at"rbn PDMS is about 108o. However, if watched carefully,thi three-phase line is found to advance slowly even after-the cessationof the addiiion of any further liquid to the drop-and a quasistatic contactancle of 105-106o iiobtained. The contact angle of a nonswelling liquid

onlDMS obtained from captive bubble methodss should, however, bemore reliable than the seesile drop method. our preliminary measure-ments of d" and d, of watcr on PDMS using the captive bubble methods

are, howevir, in agreement with the valuee obtained from the sessile dropmethod.