Langmuir 1991, 7, 3076-3080
3076
Neutron Reflectivity and Ellipsometry Studies of a Polymer Molecular Layer Spread on the Water Surface L. T. Lee,? E. K. Mann,t D. Langevin,*vt and B. Farnouxt Laboratoire L&on Brillouin, CEN Saclay, GiflYvette, France, and Laboratoire de Physique Statistique de I'ENS, Paris, France Received March 11, 1991 We have studied poly(dimethylsi1oxane) (PDMS)layers spread at the free surface of water by using neutron reflectivity techniques. The results together with previous data from ellipsometry experiments confirmed that PDMS is able to form different kinds of stable molecular layers. The density of the layers is about 0.85 times the bulk polymer density for the first layer and evolves toward the bulk density when the surface concentration C is increased. The thickness evolves via discrete jumps when C increases and remains independent of the molecular weight. Thick layers spread with difficulty and the corresponding kinetics has also been studied.
Introduction Certain water-insoluble polymers can, like fatty acid substances, be spread a t the free surface of water.' They are interesting systems to check the theoretical models for macromolecules in two dimensions.2 The behavior of polymers a t interfaces controls a large variety of applications such as the stabilization of foams and emulsions by polymers, depletion stabilization of colloidal particles, and the coating, lubrication, and adhesion of solid surfaces. The investigation of the behavior of polymers a t liquid surfaces complements studied a t solid surfaces. This behavior depends mostly on the molecular interactions between the polymer segments and the substrate and, in particular, on the degree of hydrophilicity of this substrate. In the case of liquid surfaces, it is very easy to change these interactions by using liquid mixtures, or adsorbing a surfactant a t the surface (in the case of water, for instance, the surface changes from hydrophilic to hydrophobic). Spreading is faster on liquid surfaces, and dynamical behavior is more readily accessible. Poly(dimethylsi1oxane) (PDMS) is very insoluble in water. However, due to the oxygen of the siloxane groups, the polymer is amphiphilic and spreads a t the surface of water. The behavior has been investigated with surface tension measurement^^-^ and surface rheology."1° At low surface concentration, the molecule is thought to unfold from a bulk a-helix configuration to expose the polar groups to the water surface. At close-packing, a collapse takes place, after which the macromoleculesreturn progressively to their bulk configuration. In a recent study done using ellipsometry, we have shown that the evolution toward the bulk structure is not continuous, but proceeds via discrete steps.ll This behavior is very similar to that of + Laboratoire Leon Brillouin.
* Laboratoire d e Physique Statistique d e PENS.
(1)Gaines, G. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2)de Gennes, P. G. Adu. Colloid Interface Sci. 1987,27, 189. (3)Fox, W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947,39, 1401. (4)Newing, M. J. Tram. Faraday SOC.1950,46, 755. (5)Noll, W.; Steinbach, H.; Sucker, C. Ber. Bunsen-Ges. Phys. Chem. 1963, 67, 407. (6)Bernett, M. K.; Zisman, W. A. Macromolecules 1971,4,47. (7) Granick, S. Macromolecules 1985,18, 1597. Granick, S.; Clarson, S. J.; Formoy, T. R.; Semlyen, J. A. Polymer 1985, 26, 925. (8)Jarvis, N. L. J. Colloid Interface Sci. 1969,29, 647. (9)Garrett, W. D.; Zisman, W. A. J. Phys. Chem. 1970, 74, 1796. (10)Hard, G.; Neuman, R. D. J. Colloid Interface Sci. 1987,120,15. (11)Mann, E. K.; Langevin, D. Langmuir 1991, 7, 1112.
0743-7463/91/2407-3076$02.50/0
PDMS on silicon solid surfaces.12 In ellipsometry, the measured layer thicknesses are "optical" thicknesses, Le., the product of the real thickness times a function of the refractive index in the layer (or an integral in the case of avertical density profile). In the experiments of ref 11we assumed that the refractive index was that of the pure polymer. In order to better characterize the structure of the polymer layer, we have undertaken neutron reflectivity measurements, where both the thickness and the refractive index of the layer can be determined. Neutron reflectivity is a powerful technique to investigate the structure near surfaces.I3 It has been applied successfully to surfactant14J5 and polymer (including PDMS)leZ3 systems to study the polymer density profile near the interface. There are no previous published results on polymers spread on water.
Experimental Section Neutron Reflectivity. Neutron reflectivity is governed by the same principles as those underlining electromagnetic radiation. The reflectivity function can be calculated by starting from the Schr6dinger equation for coherent elastic scattering given byz4
where Q is the neutron wave function, E the incident neutron (12)Heslot, F.; Fraysse, N.; Cazabat, A. M. Nature 1989, 338, 640. (13)Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990,2, 1369. (14)Bradley, J. E.;Lee, E. M.; Thomas, R. K.; Willatt, A. J.; Penfold, J.; Ward, R. C.; Gregory, D. P.; Waschkowski, W.Langmuir 1988,4,821. (15)Lee,E. M.;Thomas,R. K.;Penfold, J.;Ward,R. C. J.Phys. Chem. 1989, 93, 381. (16)Fernandez, M. L.; Higgins, J. S.; Penfold, J.; Ward, R. C.; Shackleton, C.; Walsh, D. J. Polymer 1988,29, 1923. (17)Russell, T.P.; Karim, A.; Mansour, A.; Felcher, G. P. Macromolecules 1988, 21, 1890. (18)Anashiadis, S.H.;Russell, T. P.; Satija, S. K.; Marjkrzak, C. F. Phys. Reu. Lett. 1989, 62, 1852. (19)Sun, X.;Bouchaud, E.; Lapp, A.; Farnoux, B.; Daoud, M.; Jannink, G. Europhys. Lett. 1988, 6, 207. (20)Rennie, A. R.; Crawford, R. J.; Lee, E. M.; Thomas, R. K.; Crowley, T. L.; Roberta, s.;Qureshi, M. s.;Richards, R. W. Macromolecules 1989, 22, 3466. (21)Lee, L. T.; Guiselin, 0.; Farnoux, B.; Lapp, A. Macromolecules 1991, 24, 2518. (22)Cosgrove, T.;Heath, T. G.; Phillips, J. S.; Ridchardson, R. M. Macromolecules 1991, 24, 94. T. Europhys. (23)Sta", M.; Huttenbach, S.; Reiter, G.; Springer, . _ _ Lett. 1991, 14, 451. (24)Werner, S. A.; Klein, A. G. In Neutron Scattering; Skold, K., Price, D. L.,Eds.; Academic Press: New York, 1986.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 12, 1991 3077
Polymer Layer Spread on Water Surface energy, m the neutron mass, and V the potential that represents the effective interaction of neutron with the medium. At an interface, the wave propagation id9 O”(z)
+ k2(nZ(z)- cos2Bo)O(z)= 0
(2)
r is the Fresnel reflectivity coefficient and
ko = 2r/X is the incident wave vector, 00 the incident angle, and n(z) the refractive index at distance z from the interface. Neglecting absorption, the neutron refractive index n is defined asz4 (3)
where X is the neutron wavelength, N the number density, and b the average coherent scattering length. From Snell’s law, one obtains approximatelyz4 A, = Bo(r/Nb)’/2
(4)
where X, is the critical wavelength. This relationship permits the use of the time-of-flight methodz5where the incident angle is fixed and neutrons with a distribution of wavelengths are employed. Total reflection is obtained for X > A,. This method, which avoids changes in incident angles and movement of the sample, facilitates measurements of liquid surfaces. Reflectivity is defined as the ratio of the intensity of the specularly reflected beam to the intensity of the incident beam. The reflectivity profile is thus a function of the wave vector transfer, q, perpendicular to the reflecting surface where q = 4 r sin #/A. From eq 4, the critical wave vector transfer is qe = 4 ( ~ N b ) ’ For /~. z > 0, eq 2 is derived to be Q”(2)
+ q2O(z) = V(z)Q(z)
(5)
where qo = 412 and V(z) = 4 r ( N b , - N b , ) ( d ( z )- &),. Nb, and Nb, are scattering length densities of pure monomer and pure solvent molecule, respectively, and &) and &, are volume fractions of polymer at distance z from the interface and in the bulk solution, respectively. The reflectivity, R can be expressed as26
O is the physical solution of eq 6. Detailed derivations and discussions of these equations can be found in refs 26 and 27. For a homogeneous and structureless interface where the refractive index varies as a step function from one medium to the other (Fresnel), the reflectivity, RF,is simplified tozs
(7)
where x = q / q c . For a nonhomogeneoussurface, the deviation of the reflectivity from the Fresnel reflectivity provides information on the variations of the refractive index n(z) perpendicular to the interface as a function of the distance, 2, from the interface. Since the refractive index is a function of the scattering length density of the atom, one can therefore deduce the composition of the interface. A method commonly used to calculate a reflectivity function consists of replacing the continuous function V ( z )by a series of discrete homogeneouslayers. The standard optical methods are then applied to determine the Fresnel reflection and transmission coefficients at each interface.@-29 In the case of a discrete layer between two bulk substrates, the reflectivity is reduced toz8 (25)Farnoux, B.Proceedings of Neutron Scattering in the Nineties; IAEA Vienna, 1985.
(26)Guiselin, 0. J . Phys. (Paris) 1989,50,3407. (27)Dietrich, S.;Schack, R. Phys. Reu. Lett. 1987,58, 140. Wolf, E. Principles of0ptics;Pergamon Press: Oxford, (28)Born, M.; 1975. (29)Lekner, J. Theory of Reflection; Martinus Nijhoff: Dordrecht, 1987.
r . .= ’J
nisin Bi - nj sin Bj ni sin Bi + nj sin 9,
2 rn z d zsin B2 0 =T
The subscripts 1 and 3 denote the bulk substrates above and below the interfacial layer 2, respectively, and dz is the thickness of the layer. In the presence of surface roughness, ( p),the reflectivity function can be modified with an introduction of a DebyeWaller fact0r.~O,3~For the present system, thermally induced roughness of the liquid surface, which is estimated to be about 3 A, does not affect the reflectivity data in the range of q studied. Another method is an integral form which relates the deviation of the specularly reflected intensity from that of Fresnel’s to the square of the Fourier transform of the derivative of the refractive index The neutron reflectivity experiments were conducted on a prototype time-of-flight reflectometer DESIR in the ORPHEE reactor. A detailed description of the reflectometer has iven el~ewhere.~~ The neutron wavelengths range from 3 to 15 and . angular resolution of the the incident angle is ~ 0 . 4 0 ~The spectrometer ( 5 % ) is determined by using high-punty solvents (such as deuterated water, toluene, or benzene), the scattering densities of which are known. The sample container (100 X 30 X 1 mm) is made of aluminum; it is enclosed in a first cell to minimize evaporation of the liquid. This first cell is equipped with a thermocouple to control the sample temperature. This ensemble is then enclosed in asecond cell which helps maintain constant the temperature of the air surrounding the first cell. Such careful control of temperature is necessary to prevent condensation of the liquid sample on the cell windows. Data acquisition time is between 10 and 15 h per spectrum. Deuterated PDMS ( M , = 19 OOO, 84 000, and 800 OOO with M,/M,, = 2.3, 1.3, and 2.0, respectively) is spread on the water surface by using protonated n-hexane as spreading agent. The surface concentration of the spread polymer is increased by successive addition of the spreading solution. For this system, it has been shown that there is no difference in surface pressure depending on whether the surface concentration is increased by successive addition of polymer to a constant surface area or by compression of surface area in the presence of a fixed amount of polymer? After each addition of polymeron the water surface, there is an interval of about 15 min before acquisition of the neutron reflectivity spectrum to ensure complete evaporation of the spreading agent. Ellipsometry. In this technique, one measures the ellipticity of the light reflected at the Brewster angle, where the phase shift between the electric fields reflected with a polarization respectively parallel (E,II)and perpendicular (Erl)to the plane of incidence is u/2. For an ideal surface, sharp and flat, Erll= 0, and the ellipticity p = Er,l/ErL is zero. Real surfaces are never sharp; the properties of the upper and lower media vary continuously across the surface over a distance d. They are also rough because of thermal fluctuations (for liquid surfaces) and can be birefringent if there is a degree of molecular ordering in the surface region. All these features contribute to the ellipticity, and assumptions must be made to disentangle them.34 In the case of spread monolayersthe followingis a possible set of assumptions: the roughness contribution pr is equal to that measured for pure water (thevariation of prdue to surface tension
-
l
(30)Beckmann, P.;Spizzichino,A. The ScatteringofElectromagnetic Waves from Rough Surfaces; Pergamon Press: New York, 1963. (31)Nevot, L.; Croce, P. Reu. Phys. Appl. 1980,15,761. (32)Ala-Nielsen, 2.Phys. B: Condens. Matter 1985,61,411. (33)Farnoux,B.Rapport d’ Actiuit&;LaboratoireLBonBrillouin,CEA: Gif Yvette, 1987-1988. (34) Bercegol, H.; Gallet, F.; Meunier, J.;Langevin, D. J . Phys. (Paris) 1989,50,2277.
Lee e t al.
3078 Langmuir, Vol. 7,No. 12, 1991
!
!
c
[m9. m-‘]
Figure 1. Surface pressure (T)isotherm of PDMS on waber as a function of surface concentration C for silicone oil 47V100 ( O ) , silicone oil 47V100 without low mass fraction ( O ) ,and silicone oil 47V1000 (+).
\e.
variations are less than experimental error); the anisotropy contribution pa is not taken into account; the thickness contribution pt is taken from the Drude formula Pt
=-
(n:
+ 1)’’’
(rz? - l)(nf2-):n
n2-1
where d is the layer thickness, X the light wavelength, n, the refractive index of water, and nf the refractive index of the film. nf was taken equal to the refractive index of the pure polymer unless stated otherwise. The experiments were performed on an instrument, built inhouse, using the phase modulation technique. We have observed that pr can vary significantly between series of experiments, probably because of residual surface contamination; since p = pI + pt, this leads to an uncertainty on d of about 1 A. Surface Pressure Measurements. The surface pressure T of the monolayer can be easily obtained by measuring the difference between the surface tension of pure water yo and the surface tension of water covered by the monolayer y : =~yo - y. The measurements were done with an open frame version of the Wilhelmy plate. These experiments were done on silicone oils 47V100 and 47V1000 from Rhhe-Poulenc including a sample from which the lower mass compounds had been removed. The PDMS molecular weights, M,, were respectively lo4 and 3.3 X 104and the polydispersity indices M,/M, 1.8 and 2.4. This large polydispersity does not have an important influence, since the properties of PDMS monolayers were shown to be rather independent of molecular m eight.^ The ellipsometry measurements were done on the silicone oil 47V100.
Results and Discussion Figure 1 shows the surface pressure as a function of surface concentration of PDMS on water. The data agree well with those in the l i t e r a t ~ r e . ~ -At ~ low surface concentration C, the surface pressure is zero within instrumental accuracy. s increases abruptly within a narrow region around C1 = 0.75 mg/m2 to a plateau value 9.1 mN/m, interpreted as a collapse pressure. Figure 2 shows the ellipsometry results for the PDMS thickness d , as a function of C. Below C1, we observe alternatively ellipticities corresponding either to pure 4.5 A. This water (d = 0) or to a nonzero d value: d suggests that the surface is partially covered with polymer domains of size greater than the illuminated area (ca. millimeters). This was confirmed by direct imaging of the ~urface.3~ Above C1, the thickness is constant and equal to 6.5 A up to CZ= 1.6 mg/m2. At CZthe thickness increases again but steeply and remains constant up to C3 3C1: d = 14 A. Above CS,the thickness fluctuates from place to place between apparently discrete values. This was also confirmed by microscopic observations in which isolated
-
-
1 1
om
owa
0010
0012
0014
0016
oota
0020
qiA1
Figure 3. Neutron reflectivity curve (log R vs q ) of deuterated
PDMS on air contrast matched water. The solid continuousline is the calculated curve obtained by using &, = 0.85.
polymer domains of different uniform thicknesses were seen to float on the surface.35 In ellipsometry, one assumes an average index of refraction for the monolayer; information on the composition of the layer is lost. In order to obtain such compositional information as the amount of water and air incorporated in the polymer layer, we have used the method of contrast variation to alter the scattering density of the water substrate in the neutron reflectivity experiments. Figure 3 shows the reflectivity curve of deuterated PDMS (C= 1.2 mg/m2) spread on air contrast matched water. This mixture of water, containing 8.1 7% D20 and 91.9 % H2O by volume has an average zero scattering length density and is therefore “invisible” to neutrons. In this case, neutron reflection from the surface is only due to the deuterated polymer layer. Thus the ratio of the scattering length density deduced for the polymer layer to that of the pure bulk polymer gives us directly the volume fraction of polymer &, in the layer. In this figure, the continuous line passing through the experimental points is a calculated curve obtained by using a polymer layer thickness d = 12 8, and Nb = 4.26 X lo4 A-z (& = 0.85). The volume fraction of water .#, incorporated in the polymer layer can then be obtained from a different isotopic system where the water substrate contributes to (35) M v n , E. K.; HBnon, S.; Meunier, J.; Langevin, D. Manuscript in preparation.
Langmuir, Vol. 7, No. 12,1991 3079
Polymer Layer Spread on Water Surface .1
w
30
&+
+JJ
20 X
d
A
[AI
j 0010
OW8
0012
0016
0014
0018
0 .
.
0
0020
0
4(11
T I c1
Figure 4. Neutron reflectivity curve of deuterated PDMS on H20. The solid continuous line is the calculated curve obtained by using &, = 0.85.
T 2
3
4
5
c2
c
[mg *m"]
Figure 6. Thickness of PDMS layer as a function of C measured by neutron reflectivity for polymers of M, = 19 OOO (o),84 OOO (X),
3"
A 0
A
TI
O K 6
OW@ !)
A
ox 10
-3 w
0
X
and 800 OOO (A).
i
,
~
OW6
,
,
0008
. 0010
,
, 0012
, 0014
0016
0018
0020
0022
q (A '1
Figure 5. Neutron reflectivity curve of deuterated PDMS on a mixture of 22 % DzO and 78% H2O. The solid continuous line is the calculated curve obtained by using d,, = 0.85. Table I. Composition of Spread PDMS Layer as a Function of Surface Concentration C. me/m2
ab
0.8
0.85 0.85
1.2 1.6
0.90
the reflectivity. Figures 4 and 5 show the reflectivity curves for deuterated PDMS spread on pure HzO and on mixtures of 22% DzO and 78% HzO, respectively, for the same surface concentrations as the previous case. The continuous lines are calculated curves obtained by using d = 12 A, = 0.85, and +w = 0.10, which give reasonably good fits. Unfortunately, we are not able to determine precisely the values of +w and +a. Equally good fits are obtained for 0 I 50.15 and 0.15 2 +a 2 0. Table I shows the values of +,and &for different surface concentrations of the polymer. As C increases, +p also increases slightly, suggesting that beyond monolayer coverage, additional polymer that is introduced on the surface spreads onto the monolayer to form a denser multilayer. The evolution of the thickness of the polymer layer with surface concentration evaluated from neutron reflectivity is given in Figure 6. At low surface coverage, C < CI, we do not observe thickness variations as in ellipsometry experiments. But the illuminated area in neutron reflectivity experiments is larger than that in ellipsometry; therefore the results of the former are averaged over a larger surface, which may be larger than the size of the
+,
+
polymer domains. At monolayer coverage (0.75 mg/m2), d = 8 A, a value which is higher than that obtained by using ellipsometry (d = 6.5 A). T o deduce the thickness d from the ellipsometry data, i t was necessary to assume a uniform layer with a given index of refraction, which we chose, as a first trial, equal to that in the bulk polymer. This is clearly not necessarily the case; the polymer is not in the bulk configuration and the peripheries of the layer may mix with water or air. With the neutron reflectivity experiment, it was possible to measure both a thickness and an index of the layer. Under conditions of optimum contrast, this yielded directly the volume fraction of polymer in the layer, 85%. Using this value for the volume fraction of polymer, we can reevaluate the results of ellipsometry: the polymer layer thickness and the sensitivity to the presence of water or air. Because the optical refractive index is close to that of the water substrate, the ellipticity is not significantly changed if 10%water is mixed (ideally) into the layer; the actual thickness of the layer, water included, would then be 10% greater than that calculated as above from the ellipticity. On the other hand, incorporation of air in the layer would considerably reduce the ellipticity; 5% air mixed in the layer would imply a true layer thickness 2.5 A greater than that calculated from the ellipticity. This seems to indicate that there is less than 5% air in the polymer layer. Thus neutron reflectivity data, complemented with ellipsometry data, suggest that a t monolayer coverage, +p =0.85,0.10 I+w I 0.15, and 0 5 +a 5 0.05. For a bilayer, we obtain d = 15 A from neutron reflectivity data and denser packing as observed from the increase in the value of 4,. Beyond bilayer coverage, +p 2 0.95. Note that a t high C as the polymer layer becomes denser, the thicknesses measured by neutron reflectivity and by ellipsometry converge. The results in Figure 6 also show that the thickness of the spread layer does not depend on the polymer chain length. This confirms the often stated concept of a stretched out two-dimensional conformation of the polymer molecule on a hydrophilic surfaces2 Such a conformation is favored by hydrogen bonding between the oxygen of the silicone and the hydroxyl group of the water molecule. One can expect that a modification of this condition, a change in polymer-substrate interfacial tension such as that induced by the presence of surfactant molecules would significantly perturb this conformation. This has in fact been observed by ellipsometry."
Lee
3080 Langmuir, Vol. 7, No. 12, 1991
et
al.
with a region of thick unspread polymer which gives rise to the broad peak in the reflectivity curve. This peak disappears only after 9-12 h, giving a smooth curve which results from a homogeneous polymer layer of about 2.5 A.
0
Conclusion
1
I
I
1
Ow6
0008
0010
0012
0011
0011
0018
0020
4 (A ‘I
Figure 7. Neutron reflectivity curves for PDMS on water after 3 h (o),6 h (+), and 12 h (0).
A change in the surface energy of the substrate alters not only the configuration of the polymer molecule but also the kinetics of spreading of the polymer.36 We have observed that a t high C, the polymer spreads more slowly. Figure 7 shows the reflectivity curves taken a t different times after the addition of PDMS onto the water surface already covered with a multilayer of polymer. It can be seen that after 3 h, the polymer layer is inhomogeneous (36) Daillant, J.; Benattar, J. J.; Uger, L. Phys. Reu. A Cen. Phys. Silberzan, P.; Leger, L. Phys. Rev. Lett. 1991, 66,185.
1990,41, 1963.
PDMS is able to form different kinds of stable molecular layers on water. The thickness of these layers is independent of the polymer molecular weight. Monolayers obtained between 0.75 and 1.6 mg/m2 are less dense than the bulk polymer and have a thickness of about 8 A. “Bilayers” obtained between 1.6 and 3 mg/m2 are denser than the monolayer, but their thickness is less than twice the monolayer thickness. Their density approaches that of the bulk polymer and the thicknesses measured with neutron reflectivity and ellipsometry converge. Above 3 mg/m2, the layers become very inhomogeneous, and the spreading is difficult. This is probably due to slow chain reorientation and the kinetics has also been characterized in the neutron experiments. Previous authors have proposed that above CI, the configuration of the polymer might evolve from flat to helical. Our results demonstrate that although the densities in the different layers states evolve toward the bulk polymer density a t large C, the configuration changes are less progressive than suggested. Acknowledgment. We are grateful to A. Lapp for supplying us with deuterated polymers. Registry No. H20,7732-18-5; neutron, 12586-31-1.
