Langmuir 1993, 9, 646448
646
Polymer Surfactant Structure at the Air-Water Interface I, R. Gentle, P. M. Saville, and J. W. White* Research School of Chemistry, Australian National University, P.O.Box 4, Canberra, ACT, Australia 2601
J. Penfold Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, U.K. Received September 28,1992. In Final Form: January 11, 1993 We report the first study of the structure of a monofunctionalpolymeric surfadant, hydroxylterminated 1,4-polyisoprene,M W 3200, spread at the air-water interface from hexane solution. The system shows a highly reversible P A isotherm and remarkable constancy in the film thickness determined by neutron reflectivity at surface pressures above and below the classical "collapse transition". The thickness is approximately 30 A with head group penetration into the water layer of about 10 A and is consistent with a rigid chain conformation for the polymer. As the film is compressed,it thickens slightly indicating that the tilt of the chains increases from ca. 10" to ca. 1 5 O , but even at relatively low pressures up to 50% of the polymer present does not contribute to the reflectivity and is probably present as multilayers. Ellipsometry measurementsindicate that the same film can be transferredto a silicon substrate by Langmuk Bldgett methods.
Introduction The structure of polymeric films at the air-water interface is currently of interest1 and new information is accessible through neutron reflectivity measurementsn2 Studies of macromolecules such as poly(methy1 methacrylate) (PMMA) where the repeat unit contains polar groups from which the surface activity is derived represent one important class of systems3 and have reported that though the isotherms are well-behaved, some of the polymer may be "unobservable" due to film texture. This paper follows a different line and reports reflectivity measurements for a relatively rigid polymer with only one weak surface active terminal group per molecule. This is a first step toward elucidating the behavior of such systems as the length of the polymeric moiety is increased to the point where the polymex-polymer interactions are of the same order as the energy of interaction of the hydrophilic groups with the aqueous subphase. Unusual results akin to those for PMMA have been found and are here reported. Highly monodisperse, 1,4-polyisoprenes terminated by a single hydroxyl group (PIP-OH) have been used since aliphatic hydrocarbons of an appropriate length cannot be readily made. Anionic polymerization of isoprene, with care taken about cleanlinessand residual oxygen impurities to allowprecise termination with ethylene oxide, was used to make the polymers. The substances and their films are also interesting because of their electronic4 and possible optoelectronic properties. When spread from hexane a t the air-water interface (PIP-OH) shows adsorption isotherms like that in Figure 1. This isotherm is characterized by a steep rise in the surface pressure followed by a collapse, which leads to a visually observable film at high compressions. The polyisoprene films undergo reversible compression over the whole P A range. The purpose of the neutron reflectivity measurements reported here is to understand ~
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* To whom comments are to be addressed.
(1) Lee, L. T.; Mann, E. K.; Langevin, D.;Farnoux,B. Langmuir 1991, 7, 3076. (2) Penfold, J.; Thomas, R. K. J . Phys.: Condens. Matter 1990, 2, 1 RGQ.
(3) Henderson, J. A.; Richards, R. W.; Penfold, J.; Thomas, R. K. Macromolecules 1993,26, 65. (4)See for example Dai, L.; White, J. W. Polymer 1991,32,2120, and references therein..
10
I
0.15
0.20
0.25 0.30 Specific Area I m2 mg'
0.35
0 IO
Figure 1. Isotherms of hydrophilic-terminatedpolyisoprene at 20 "C showing neutron reflectivity experimental points.
the behavior of the film thickness along the isotherm and into the transition region beyond the collapse point. Measurements have been made on air contrast matched water (ACMW) and on heavy water for a fully deuterated polyisoprene (MW 3200 by stoichiometry, MW 2900 by GPC). Changing the contrast in this way puts constraints on the models fitted to the reflectivity data. These data are compared with ellipsometric measurements of Langmuir-Blodgett films of the same polymer deposited on a silicon substrate.
Experimental Section The anionic polymerization of isoprene was initiated with n-butyllithium and the products were characterized by gel permeation chromatography and NMR spectroscopy. The polydispersitywas about 2 % . Solutionswere made with distilled HPLC grade hexane. For all Langmuir fiis,high-puritywater, with conductivity 118Mil cm, was used to make up the subphase and, where possible, maintained at 20 O C . Film compression rates were of the order of 0.4 cm2 s-l. Neutron reflectivity measurements were performed with the instrument CRISP at the ISIS facility at Rutherford-AppletonLaboratory, and data collection times were typically 2 h. The ellipsometry measurementa were performed on films picked up on Si wafers which were pulled vertically from the water subphaseafter the f i ihad reached an equilibrium under constant pressure. Constant surface pressure was maintained while these films were being
0743-7463/93/2409-0646$04.00/00 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No. 3, 1993 647
2.5
T4 2.0
’’. 2
1.5
1.0
0.5
0.0 0.05
0.10
0.15
0.20
0.25
0.30
Momentum Transfer Q / A-’
Figure 2. Reflectivityprofiles for a polyisoprene film spread on
ACMW (filed circles) and DzO (open circles) at an area of 125
Azper molecule (point C on Figure 1). Solid lines are best fits using parameters described in the text and Table I.
deposited. These films were measured within 18 h of deposition with a single wavelength SOFIE STEP0 ellipsometer calibrated on a S i 0 wafer of known thickness and refractive index.
Results and Discussion In order to c o n ~ t r a i nmodels ~ * ~ for the scattering length density profile of the surface, measurements were made with air contrast matched water (ACMW) and D2O as subphases. Direct methods using the kinematic approximation5J18 with many different contrasts appear to offer good confidence about model uniqueness; however, the necessary variations were not possible in this experiment, and so standard reflectivity modeling methodss were used to interpret the data. While a single-layer model of the monolayer was adequate to fit the profiles obtained from the deuterated polymer on ACMW, where the deuterated portion of the polymer is the major contributor to the reflectivity, it was not possible to obtain a good fit using a single layer model for the (very different) profiles of the polymer on D20. Figure 2 shows how different the reflection profiles are and also how a two-layer model involvinga polymer layer and a head group/subphaselayer fits well for both subphases. All subsequent fits used this two-layer model. Thicknesses and scattering length densities obtained in this way for the data on ACMW and DzO are shown in Table I. The scattering length density profiles for the ACMW and D2O subphases are shown in Figure 3. For the deuterated polymer on ACMW, the two-layer fit gave a first layer thickness (dl) of about 30 “A with a head group subphase layer (d2) of about 8 A. This compares well with the thicknesses obtained for the same polymer on DzO before the collapse point, and this agreement is considered to show the validity of this model for representing the interface in this region. The thicknesses and scattering length densities shown in Table I were interpreted as indicating a first layer of deuterated polymer chains only, with a similar density to the bulk polymer, while the second layer is mainly composed of subphase molecules with the head groups of the polymers (-CH2CH20H)extendingintoit. The low scatteringlength density of the -CH2CH20H group means that it is in low B -
(5) Crowley, T. L.; h e , E. M.; Simister, E. A.; Thomas, R. K. Physica
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1991. 173. 143. - ~ (6) Penfold, J. In Neutron, X-Ray and Light Scattering, Lindner, P., Zemb, T., Eds.; Elsevier: Amsterdam: 1991; p 223. (7) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J . Phys. Chem. 1992,96, 1373. (8) Lu, J. R.; Simister, E. A.; Lee, E. M.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1992,8, 1837.
contrast when on a subphase of ACMW, but on D2O this group significantly lowers the mean scattering length density of the second layer. Beyond the collapse, the ACMW results have a dl of about 26 A while for D20 the layer models to 34 A. This difference may be due to different collapse mechanisms, dependent on compression rates and spreadinghistory, producing different structures in the multilayer. This point needs to be further explored in terms of the sensitivityof the reflectivityto the scattering length density profiles created by the multilayer. The variation in the film thickness along the isotherm for ACMW is displayed in Figure 1. There is a general trend of increasingfilm thickness up to and includingpoint E; point F may be in a region where the film is unstable and has collapsed to some extent. An interesting aspect of the data is that while the area per molecule as measured macroscopically has decreased by nearly a factor of 2 from A to I, the film thickness has stayed much more nearly constant than would have been expected from this. The area per head group (from the fitting of the neutron reflectivity) and the area per molecule (calculated from the isotherm) are also compared in Table I and seen to divergeat higher compression. This somewhat paradoxical behavior is similar to that found3for isotactic PMMA and interpreted as part of the polymer folding away from the surface either into the water or the air above the film. We suppose again that even well before the collapse point there is formation of islands of PIP-OH, which contribute only weakly to the reflectivity. This formation requires that the chains may be readily extracted from the subphase to form multilayers, which would seem more likely than their being forced into the subphasedue to the hydrophobic nature of the isoprene repeat unit. The same relatively small variation of the observed film thickness and similar absolute values were also observed by ellipsometry for the same polymer films picked up on silicon (Table 11). This consistencywith the neutron measurements indicates that the structure of the Langmuir films is preserved during deposition on the silicon substrate and that the material unobserved by neutron reflectivity is either not transferred or again “hidden” from the ellipsometric measurement. One possible model to describe the thickness of the observable films at the aidwater and air/silicon interfaces is that the polymer chaiqs retain a solution phase conformation, i.e. are random coils above the interface. This is unlikely unless the polymer shrinks greatly since the solution radius of gyration (Q, measured by small angle X-ray scattering (SAXS)? yields an area which is too large compared to those measured on the Langmuir trough (e.g. the cross sectional area for a MW 2000 polymer extrapolated from SAXS measurements is of the order of 1400 A2 while the isotherm gives a maximum area at the “toe” of 125 A2 and about 80 %L2 at the collapse point). An alternative structure which fits both neutron and ellipsometry measurements is that the polymer films are composed of essentially rigid 1,4-polyisoprenechains lying at an angle of about 10” to the surface. The increase in thickness up to the collapse point is beyond the error of the measurement and is equivalent to an increase in the tilt angle to about 15”. The thicknesses and tilt angles, at the toe of the isotherm, yield an occupation of the subphase, by the head groups, of about 15%, which is comparable to the 12.5% calculated from the scattering length densities of the second layer. We conclude, therefore, that films of 1,4-polyisopreneOH have the form of closely packed chains lying at (9) Dai, L.; White, J. W.; Ken, J.; Thomas, R. K.;Penfold, J.; Aldissi, 1989,223, D69.
J. Synth. Metals
648 Langmuir, Vol. 9,No. 3, 1993
pointn ACMW subphase
A C D
E F G H I
Table I. PolvisoDrene Films at the A i d w a t e r Interface layer Ib/A layer zC/A NbldllW A-2 NbZe/10-6A-2 29 5 6.0 0.07 33 9 6.4 0.7 32 9 5.9 0.1 35 8 6.7 1.5 33 8 6.0 0.2 28 6 5.7 0.5 25 6 6.2 0.5 25 6 6.0 0.5
B
DzO subphase
Letters
31 33 35 35 34 35
C E
G H I
7 9 9 7 7 7
6.6 6.4 6.5 6.5 6.4 6.2
A.H.G.fIA2 180 150 170 135 160 200 210 210
A.H.G.gIA2 170 125 116 112 104 96 88 80
5.6 5.8 5.4 5.2 5.3 5.4
See Figure 1. Thickness of first layer, adjacent to air phase. Thickness of second layer, adjacent to subphase. Scattering length density of first layer. e Scattering length density of second layer. f Area per head group calculated from fit to neutron reflectivity, neglecting the contribution from layer 2.8 Area per head group from volume of solution spread, molecular weight, and trough area. Table 11. P o l y i s o p r e n d H Films on Silicon
Air
P
P
Air
H
H
ACMW
D20
-
H Head group
P Polymer
Figure 3. Scattering length density profiles for the two-layer model on each subphase.
approximately looto the water interface with penetration of about 8 A for all of the surface areas studied, but even at low surface pressures some multilayer formation occurs. At the collapse point and beyond the two-layer model no longer fits the reflectivity well as can be seen by comparing the layer 1thicknesses on both subphases for points G, H, and I. The transition to a three-dimensional phase is essentially unhindered, the.polyisoprene chains sliding over one another with the extraction of OH groups from the air/water interface. The multilayer islands so produced, like those before the collapse point, were not
II/mN m-1 1.25 2.12 3.25 5.12 5.37
area/m2 mg-1 0.248 0.238 0.227 0.171 0.125
f i i thicknewlA 26 28 30 38 50-63
detectable by the neutron reflectivity measurements, presumably due to their low density and the experimental conditions. There may be some slight increase in roughness at areas less than the collapse point but this is at the level of the statistics of the measurements. Further measurements over a wider range of momentum transfer (especially to lower Q) are necessary to verify these suppositions. Measurementsof the off-specular reflection have recently detected periodic structure parallel to the interface for polystyrene films at high compression,1° and if the f i i s studied here are forming multilayer islands, they may also be detectable in a similar way.
Acknowledgment. The authors gratefully acknowledge the assistance of Dr. R. K. Thomas for the loan of a trough for the reflectivity measurements,Joe McCarney and Dr. J. R. P. Webster for technical assistance, and the ISTAC grants scheme of the Australian Department of Industry, Technology and Commerce for financial aid. (10) Gentle, I. R.; Saville, P. M.; White, J. W.; Penfold, J. Manuscript in preparation.
