Neutron confinement cell for investigating complex fluidsTonya L. Kuhl, Gregory S. Smith, Jacob N. Israelachvili, Jaroslaw Majewski, and William Hamilton Citation: Review of Scientific Instruments 72, 1715 (2001); doi: 10.1063/1.1347981 View online: http://dx.doi.org/10.1063/1.1347981 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/72/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A molecular beam epitaxy facility for in situ neutron scattering Rev. Sci. Instrum. 80, 073906 (2009); 10.1063/1.3169506 Surface force confinement cell for neutron reflectometry studies of complex fluids under nanoconfinement Rev. Sci. Instrum. 79, 103908 (2008); 10.1063/1.3005483 Understanding the complex rheological behavior of PEO–PPO–PEO copolymers in aqueous solution J. Rheol. 48, 1 (2004); 10.1122/1.1634988 Selfassembling of C60imidazole and C60pyridine adducts in the Langmuir and LangmuirBlodgett films viacomplex formation with watersoluble zinc porphyrins AIP Conf. Proc. 685, 3 (2003); 10.1063/1.1627974 Polarized reflectometer for the investigation of surface magnetism, the new polarized neutron reflectometer withpolarization analysis at the Laboratoire Léon Brillouin Rev. Sci. Instrum. 71, 3797 (2000); 10.1063/1.1310342

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Neutron confinement cell for investigating complex fluidsTonya L. Kuhla)

Department of Chemical Engineering and Materials Science, University of California, Davis, Davis,California 95616

Gregory S. SmithManuel Lujan, Jr. Neutron Scattering Center, LANSCE-12, MS H805, Los Alamos National Laboratory,Los Alamos, New Mexico 87545

Jacob N. IsraelachviliMaterials Research Laboratory and Department of Chemical Engineering, University of California,Santa Barbara, Santa Barbara, California 93106

Jaroslaw MajewskiManuel Lujan, Jr. Neutron Scattering Center, LANSCE-12, MS H805, Los Alamos National Laboratory,Los Alamos, New Mexico 87545

William HamiltonNeutron Scattering Section, Solid State Division, Oak Ridge National Laboratory, Oak Ridge,Tennessee 37831

~Received 18 September 2000; accepted for publication 20 November 2000!

We describe an apparatus for measuring the molecular density and orientation ofconfined, ultrathincomplex fluids under static and dynamic flow conditions. The device essentially couples the utilityof the surface forces apparatus—ability to control surface separation and alignment under appliedloads—with in situ structural characterization of the intervening material utilizing neutronreflectivity measurements. The apparatus is designed such that single crystal substrates of quartz orsapphire with areas up to tens of square centimeters can be kept parallel at controlled andwell-defined separations from millimeters to less than 100 nm. The large substrate surface areaenables direct structural measurements of the density profile of ‘‘soft’’ material placed between thealigned substrates. In addition, the cell is also designed to enable steady shear rates from 0.001 to20 Hz to be applied in order to follow the dynamic structural response of the confined material,especially at the solid-solution interface. Faster shear rates of order 104 can be obtained usingoscillatory motion. Current design specifications focus on the use of neutron reflectivity tocharacterize the structure of end-grafted polymer brush layers, but the device can be employed toprobe the structure of any complex fluid of interest and is amenable to other characterizationtechniques. ©2001 American Institute of Physics.@DOI: 10.1063/1.1347981#

I. INTRODUCTION

Polymer molecules at solid or fluid interfaces have anenormous spectrum of applications in a wide variety of tech-nologies. They provide a mechanism to impart colloid stabi-lization, they are used as protective coatings~including me-chanical protection of solids against friction and wear!, theygovern the interactions of biological cell surfaces, andthrough judicious design they are used to modulate disper-sion properties~such as rheology! under a variety of process-ing conditions. Knowledge of the conformations that ad-sorbed or terminally anchored chain molecules adopt whensubjected to confinement and/or solvent flow is essential forpredicting the interaction forces and rheological properties ofthe polymer layers involved in all of the above-mentionedapplications.

There exists a substantial body of theoretical literatureand experimental data on thestatic morphologies of ad-sorbed polymers, grafted chains, block copolymers, etc., at a

single surface and under confinement. Here, we will focus onthe behavior of tethered diblock polymer chains in selectivesolvents, where one end of the polymer block anchors thechain to the surface and the other block extends away fromthe interface into solution. On the experimental side, a lot ofinformation regarding the structure of grafted polymer layershas been deduced from force measurements utilizing tech-niques like the surface forces apparatus~SFA!1 and variousscanning probe microscopies.2 Such measurements are verypowerful and sensitive surface probes, however, the informa-tion they provide is not molecular. As a result, in many casesour understanding and interpretation of experimental datahas evolved from theoretical studies based on scalingarguments,3 mean field theories,4 and/or computersimulations.5

At the molecular level, neutron reflectivity experimentshave been very successful in providing detailed density dis-tribution profiles of polymeric materials at single interfaces~depth profiling!, where the structure of adsorbed diblockpolymers in good, theta, and poor solvents has been investi-a!Electronic mail: tlkuhl@ucdavis.edu

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 72, NUMBER 3 MARCH 2001

17150034-6748/2001/72(3)/1715/6/$18.00 © 2001 American Institute of Physics

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gated as a function of grafting density.6,7 Probing the struc-ture the tethered polymer layer adopts under confinement hasproven more elusive. The difficulty here lies in the need ofultraflat surfaces of a suitably large surface area for neutronscattering measurements. Not only must these surfaces beclosely opposed~intersurface separations below a few hun-dred nanometers or less are required!, but they must also bekept aligned and parallel throughout the duration of the mea-surement in order to confine the film of interest uniformly.Early work by Cosgrove and co-workers paved the way forthe apparatus we describe here.8 In their design, large, opti-cally polished quartz flats were forced to closely approachusing a hydraulic ram. Intersurface separations of about 100nm were attained, however it proved difficult to maintain aconstant gap separation during the course of an experiment,which greatly complicated data interpretation.

Although the structure of tethered polymer layers atsingle interfaces or confined has received much attention, forthe most part thedynamicbehavior of polymer layers undershear has only been inferred. Techniques such as ellipsom-etry or the hydrodynamic flow method enable average poly-mer layer thicknesses to be evaluated. However, they do notprovide information on the segmental conformation or den-sity profile of the polymer chains away from the interface.Likewise, even less known is the effect of geometric confine-ment imposed by a second surface although this more closelymatches the conditions of colloidal processing and the manyother technological situations listed previously. Recently,there has been more focus on understanding these effects.For example, dynamic shear investigations utilizing the SFAindicate that polymer brush layers extend and generate a nor-mal force component during shearing motion,9 while a novelshear cell for neutron reflectivity measurements has beenused to investigate the conformations tethered polymerchains adopt when exposed to flowing solvent at a singlesurface.10 In addition, a new device developed by Granickand co-workers, micrometer-gap optorheometer, shouldprove to be a powerful tool for probing the rheology andstructure of complex fluids at the mesoscale, gaps of a fewmicrons and larger.11

In this paper, we describe an apparatus, the neutron con-finement cell, which can be used for determining~i! the mo-lecular orientations and density distributions of physicallyadsorbed and chemically grafted polymer chains on solidsupports,~ii ! how these are affected by shear at a singlesurface, and~iii ! how the proximity of a second surface~im-posed confinement! alters the conformation, intersurfaceforces, and rheology of polymer layers in thin films. Currentdesign specifications focus on the use of neutron reflectivityto characterize the structure of end-grafted polymer brushlayers, but the device can be employed to probe the structureof any complex fluid of interest and is amenable to othercharacterization techniques.

II. APPARATUS

A. Overall features of the apparatus

The apparatus and a schematic are shown in Fig. 1. Theframe and interior components of the neutron confinement

cell ~NCC! are constructed from 304 and 316 stainless steel.All inlets, outlets, and openings are sealed with Teflon™gaskets or o-rings, enabling liquid vapor pressure to be main-tained and preventing contamination from entering the cham-ber. Single crystal quartz windows~1 mm thick! act as beamports for the incident and reflected neutron beam.

The heart of the device is the substrates used to confinethe complex fluid of interest. Theoretically, the minimumgap obtainable with the device is solely a function of thesmoothness~waviness! of the substrates used. In otherwords, the gap separation is equivalent to the separation be-tween the two substrates. In the current design, the NCC usessingle crystal quartz and single crystal sapphire substrates ofnominal surface waviness less thanl/25 andl/20, respec-tively. Thus, the peak-to-valley height difference on each

FIG. 1. ~A! Photograph of the neutron confinement cell~NCC! with theouter steel housing removed. A 12 in. ruler on the left-hand side gives anidea of the size of the apparatus.~B! Cross section of the NCC. The neutronbeam passes perpendicular to the view shown. The apparatus is constructedwith 304 and 316 stainless steel and all inlets and outlets are sealed byTeflon o-rings or gaskets. A hydraulic ram can be used to apply high loads,which are calibrated by measuring the compression of Belleville washers ofvariable spring constant. The upper quartz substrate mounts into the top ofthe outer housing. The lower sapphire~quartz! substrate mounts on a me-chanical slider and can be translated~sheared! relative to the upper surfaceusing a mechanical motor drive assembly.

1716 Rev. Sci. Instrum., Vol. 72, No. 3, March 2001 Kuhl et al.

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substrate across the inner 90% of the surface is about 300 Å.The outer 10% of each substrate is rounded off to ensure thatthere are no edge asperities.

Another critical concern is that these substrates must bekept in a parallel alignment at a constant gap separation forthe duration of the reflectivity measurement~2–12 h!. Oth-erwise, contributions from different substrate separationswill tend to smear out the reflectivity profile.8 In our appa-ratus, the substrates are mounted into recesses within thesteel housing~upper surface! and in the shearing mount~lower surface!. The steel surfaces upon which the substratesrest are machined to a lapped tolerance of less than 10mm. ATeflon gasket material of Durlon 9000 is placed along thebottom and sides of the substrate recesses to distribute theapplied load evenly across the substrate surface. Moreover,the flexibility of the gasket material and the compliance ofthe substrate materials themselves, quartz against sapphire,enable the surfaces to conform and self align. As we shallshow in Sec. III, very small gap separations were obtainedusing this construction. The assembled device fits on top of apiston ram, which can be used to apply large normal loads tothe substrates through a series of variable spring constantBelleville washers. By measuring the deflection of theBelleville washers, the applied load can be accurately deter-mined. A locking collar enables the applied load to bemaintained if the device is removed from the piston ramassembly.

B. Substrate chemistry

We found that contamination on the surfaces frequentlydetermined the minimum separation as opposed to the physi-cal smoothness of the substrate surface. Both bare quartz andsapphire have high surface energies. As a result, it is difficultto remove particles or contamination from these surfaces.One important aspect of our design was hydrophobizing thesingle crystal substrates with a molecular layer of eitherchemically grafted octadecyltrichlorosile~OTS! or physi-cally adsorbed octadecylphosphonic acid~OPA! on thequartz and sapphire, respectively. The purpose of hydropho-bizing the substrates was twofold. First, the hydrophobic lay-ers provided a protective coating on the two ultrasmoothsurfaces. Second, hydrophobic surfaces were much easier toclean and to keep clean of contaminants and particles, inparticular dust. Indeed, we found that by effectively remov-ing dust and other particles from the substrate surfaces, ex-tremely small intersurface separations of less than 70 nmwere obtainable.

1. Quartz

By virtue of its hydrolyzable functional groups, OTSmolecules were covalently bound to the quartz substrate hy-droxyl groups as well as cross-linked within the organosilox-ane layer. As a result, a robust, chemically inert, protectivemonolayer was formed on the quartz substrate. The proce-dure was as follows. The quartz surface was first cleaned andthe surface hydroxylated by immersion in a strong basic so-lution of 10 wt% NaOH for 10 min at 50 °C. The quartz wasthen rinsed in Millipore water and dried in a clean stream ofnitrogen gas. Next, following the procedure of Moav and

Sagiv,12 the quartz was immersed in a solution of 1 mM OTSin dried bicyclohexyl. The OTS monolayer was allowed toself-assemble for 1 h.13 Upon removal from solution, themonolayer coated substrate was autophobic and the qualityof the formed monolayer was initially judged from the de-wetting of the solution from the substrate surface. The sub-strate was then rinsed in clean chloroform and baked in anoven at 102 °C for 3–6 h. Baking the monolayer increasedthe cross-link density of the organosiloxane layer to the sub-strate as well as intermolecularly. The advancing contactangle of water on the now hydrophobized quartz substratewas 110° and the receding contact angle was 100°65°.

2. Sapphire

Similarly, the sapphire substrate was hydrophobizedwith a self-assembled monolayer of OPA. OPA forms astrong physisorbed monolayer on sapphire through ionicbonds between the phosphonate group and the two negativeoxygens above the aluminum ions. The neutral oxygen~P5O! is above the vacancy site and the P–C bond is normalto the surface.14 Adapting the procedure followed by Ber-mann et al.,15 the sapphire substrate was first thoroughlycleaned in Helmenex solution, rinsed with copious Milliporewater, and dried thoroughly in a stream of clean nitrogen.The substrate was then immersed in a solution of 0.5 mMOPA in bicyclohexyl at 60 °C for 1 h. Afterwards, the sub-strate was rinsed in clean chloroform to remove residual sol-vent and unbound surfactant. The resulting coating was ho-mogeneous as probed by contact angle measurements withan advancing contact angle of 80° and a residing contactangle of 60°65° with water.

C. Shear capabilities

Lateral motion~shear! is accomplished using a variable-speed motor-driven screw, which presses or pulls the lowersubstrate holder relative to the stationary upper substrate.The lower substrate holder is mounted on a low frictionslider rail, which ensures smooth motion even when the sub-strates are under a high applied load. To maintain parallelmotion, the motor driven screw is aligned with the interfacebetween the two substrates. Thus, the substrates are neitherdriven apart nor toward each other during the shearing mo-tion. Depending on the gap separation between the two sub-strates and the time of the neutron reflectivity measurement,shear rates spanning over 4 orders of magnitude are attain-able, 0.001–20 Hz. Faster shear rates can be used, but thelimitation of data collection time requires that anoscillating—back-and-forth—motion must be used.

Higher shear rates would be desirable for some experi-ments involving polymer brush layers. Indeed, previouswork by Baker et al. has shown little change in polymerbrush structure in a good solvent at shear rates above 10 000Hz.10 In those experiments, shear was accomplished by sol-vent flow, whereby the solvent was pumped at relativelyhigh rates but still under laminar flow conditions past anadsorbed polymer layer. In our case, the substrates slide lat-erally past one another. As a result, the shear is at the sub-strate interface as opposed to the outermost portion of the

1717Rev. Sci. Instrum., Vol. 72, No. 3, March 2001 Confinement cell

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layer exposed to the solvent flow. It will be interesting tocompare our results with those reported previously withmore conventional solvent flow shear cells as well as to in-vestigate the effects of oscillatory shear motion.10,16

D. Neutron reflectivity measurements

The NCC was tested on the MIRROR17 neutron reflec-tometer on the HB-3A beam line of the High Flux IsotopeReactor at Oak Ridge National Laboratory. This reflectome-ter is set up for vertical sample geometry. Although the cellwas originally designed for horizontal sample geometry, wefound it operated well in vertical geometry.

In the measurements presented here, a perpendicularscattering vectorQz ~54p sinu/l! range from 0.004 to about0.05 Å21 was covered at MIRROR’s operating neutronwavelength l52.59 Å, which corresponds to reflectionangles,u, between 0.05° and 0.6°. Reflected neutrons werecounted using an Ordela model 1150N linear position sensi-tive 3He detector with;1 mm ~rms! resolution at a sampleto detector distance of 3.37 m, giving the instrument a limitresolution ofsQ;0.0005 Å21. To avoid reflections from therounded surfaces at the edges of the substrates, a slit justupstream of the sample was varied according to the angle ofincidence to maintain a constant 20 mm illumination length~footprint! near the center of the sample interface. Dependingon the instrument configuration, the reflectometer resolutionvaried from 0.0005 to 0.001 Å21 ~;2p/7000 Å! during thecourse of a measurement. Assuming a minimum sampling ofabout four points per period, the instrument could thereforeresolve interference fringes between the reflections from suf-ficiently parallel quartz and sapphire surfaces for separationsless than about 3000 Å. Thus, the visibility of interferencefringes in our data is a sensitive indication of well-controlledintersubstrate separations below this limit, as shown inSec. III.

Initial tests were performed without material betweenthe two substrates~an air gap! to determine the intersubstrateseparation obtainable with the device. Subsequently, the re-flectivity profile was measured when a mixture of hydroge-nated and deuterated toluene was placed between the sub-strates at a ratio chosen for good neutron contrast relative toboth the quartz and sapphire substrates. In our initial mea-surements we restricted the beam height to 1.5 mm to betterensure sample uniformity over the illuminated region. Withthis restriction, runs obtaining good statistics and signal tobackground ratios for reflectivities down to 1023 requiredabout 12 h@Fig. 2~a!#. Larger area illumination with a beamheight of 5 mm ~interfacial area of 20 mm35 mm5100mm2! required significantly shorter data collection times ofabout 4 hours@Figs. 2~b! and 3#.

The reduced data are plotted asR* Qz4 versus the perpen-

dicular scattering vector,Qz ~this compensates for the;1/Qz

4 decrease of the reflectivity due to the Fresnel’s law!.The vertical error bars on the data represent the statisticalerrors in the measurements after background subtraction~standard deviation,sR!. The horizontal error bars representthe rms instrumentQz resolution calculated on a point-by-point basis~with no allowance made for sample irregularity

or curvature!. Reflectivities were normalized to illuminatedsample area and scaled to unity below the critical edge fortotal reflection. The fits to the reflectivity~calculated usingthe iterative, dynamical method6 as described in the follow-ing! included an additional parameter to normalize the cal-culated reflectivity to the data. However, this parameter didnot vary more than 5% from unity—a value comparable tothe normalization error due to the scatter of data points in theR51 region below the critical angle.

III. EXPERIMENTAL RESULTS

Results from two reflectivity measurements with an airgap separating the quartz and sapphire in the NCC are shownin Fig. 2. A clear series of interference fringes is visible forboth, indicative of the small intersurface separations attain-able with the NCC. The coherent neutron scattering lengthdensities~b! of the materials used in this work and the mod-eling are shown in Table I.18 To calculate the reflectivityprofile R(Qz ,t), the key structural components of the inves-tigated system@the octadecyltrichlorosiloxane~OTS!, the oc-tadecylphosphonic acid~OPA!, and the gap between thequartz and sapphire crystals# were described by boxes of aconstant thickness and scattering length densityb(z). Thereflectivity R(Qz ,t) was calculated using the optical matrix~or dynamical! method.6,19

To obtain good model fits to the experimental reflectivitydata, it was necessary to account for sample irregularitiesacross the illuminated beam footprint due to, for example,substrate waviness. One possible way to account for thesample spacing irregularities would be to add a ‘‘sample cur-vature resolution’’ term in quadrature to the instrument reso-

FIG. 2. Neutron reflectivity data for air gaps of~a! 1925 Å and~b! 874 Åbetween a single crystal quartz and sapphire substrate mounted in the NCC.The solid curves are fits to the data using the model described in the text.Parameters are tabulated in Table I.

1718 Rev. Sci. Instrum., Vol. 72, No. 3, March 2001 Kuhl et al.

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lution. Adding a term of this sort did reduce the calculatedfringe visibility to values near those observed in our data.However, this treatment gives a uniform reduction overQz ,while in our data a definite weakening of the fringe visibilityis evident asQz increases. A surface roughness parameterhas a similar effect, but also dramatically reduces the overallreflectivity. Using these descriptions, we were unable to pro-duce statistically satisfactory fits without using either un-physical neutron scattering length densities or modifying theresolution beyond limits consistent with the experimentallyobserved spread of the specularly reflected beam.

Instead, we accounted for deviations in the average sub-strate separationt0 by assuming that there was a Gaussiandistribution of thicknesses~gaps! between the two crystals ofstandard deviationsw . The reflectivity of each constantthickness regiont was described by a functionRt(Qz ,t)5ur t(Qz ,t)u2, where r t(Qz) is a complex function whichcharacterizes the reflectance of a domain of thicknesst. Forthe case of macroscopically large domains of differing thick-nesses, one would expect that the domains would reflect theneutron beam independently of one another and thus themeasured reflectivity is the~incoherent! average of the inten-sities:

R~Qz ,t0!51

swA2pE

2`

`

expS 2~ t02t !2

2sw2 DR~Qz ,t !dt,

where t0 is the average thickness of the gap between twocrystals andt is the thickness of each individual domain. Thestandard deviationsw then describes the distribution ofthickness around the average valuet0 due to waviness orcurvature of the crystals and resulting variation in the gapseparation. Averaging incoherently assumes that the lateralsize of the reflecting regions with different separations ex-ceeds the coherence length of the neutron beam projected onthe reflecting surface~;10–100mm! and that their angles toone another are less than the instrument’s angular resolution.Reflecting facets of a smaller length scale or at larger angleswould produce beam scattering or broadening effects that

were not observed. Moreover, this calculation method satis-factorily reproduced the measured weakening of fringe vis-ibility with increasingQz and no correction to the instrumentresolution was necessary. An additional ‘‘Gaussian’’ rough-ness at the surface~by modifying the interfacial reflectanceby the so-called Ne´vot–Croce factor, roughness parameters!was not used here since in all cases the values were small~rms ,5 Å! and their inclusion had negligible effect on thefit quality over theQz range covered in these measurements.

The solid curves are fits to the data based on the modelparameters listed in Table I. Fits to these measurements gaveaverage intersubstrate separationst0 of 1925 Å~a! and 874 Å~b!, with respective values forsw of 66 and 47 Å. As can beseen by the quality of the fits, this model using only thescattering length densities of the quartz and sapphire sub-strates, air thickness~intersubstrate separation!, and the inco-herent average of the reflectivity over a relative flatness orparallelism parameter accurately models the system.20 Thesmaller value forsw for smaller substrate separations@Fig.2~b!# is consistent with a greater parallelism of the substratesunder higher applied load. For consistency the contributionof the hydrocarbon monolayers to the measured reflectivityprofiles, OTS and phosphonic acid on the quartz and sap-phire, of thickness 24 and 15 Å, respectively, was includedwhen calculating these fits. However, because of the weakscattering contrast (b'20.431026 Å22) with air addingthese layers to the model did not significantly improve thequality of the fits in comparison with a simple air-gap model.In all cases, similar results were obtained when the modelwas refined using either nonlinear versions of Marquardt–Levenburg methods or Nelder and Mead’s downhill simplexalgorithm.

The measured reflectivity profile when a mixture of hy-drogenated and deuterated toluene, scattering length densityb'231026 Å22, was confined between the substrates isshown in Fig. 3. A similar model was used to fit the reflec-tivity profile—an inset shows scattering length density pro-file over the interface for the average spacing (t0). Howeverin this case, the contribution of the OTS and OPA monolay-ers had to be included to obtain a good fit between the cal-

FIG. 3. Neutron reflectivity data for a deuterated-hydrogenated mixture oftolueneb'231026 Å22 confined between a single crystal quartz and sap-phire substrate. The solid curves are fits to the data using the model de-scribed in the text. The inset shows the mean scattering length density pro-file ~before incoherent averaging!. Parameters are tabulated in Table I.

TABLE I. Model fitting parameters.

ParameteraLarge air

gapSmall air

gapToluene

gap

Intrasubstrate separation~Å! 192564 87463 121166Quartzb(1026 Å22) 4.17b 4.17b 4.17b

Sapphireb(1026 Å22) 5.70b 5.70b 5.70b

Air gap b(1026 Å22) 0.00b 0.00b NATolueneb(1026 Å22) NA NA 1.9460.02OTS on quartzb(1026 Å22) 20.4b 20.4b 0.960.2c

OTS on quartz thickness~Å! 24b 24b 24b

OPA on sapphireb(1026 Å22) 20.4b 20.4b 0.960.2c

OPA on sapphire thickness~Å! 15b 15b 15b

Gap variation-sw(Å) 6663 4765 8964

aErrors are estimated conventionally as parameter variation which changesx2 by unity multiplied by ax2/(n2p) correction factor, wheren5numberof data points andp5number of fit parameters, i.e., square root of thereduced chi squared.

bThese parameters were kept fixed during the refinement of the calculatedreflectivity profiles.

cThese scattering length density parameters were held to equal values duringfitting.

1719Rev. Sci. Instrum., Vol. 72, No. 3, March 2001 Confinement cell

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culated and measured reflectivity profiles, since the scatter-ing length density of hydrocarbon layers was significantlydifferent from the toluene mixture. In order to minimize thenumber of fitting parameters, the thicknesses of both hydro-carbon monolayers were fixed to reasonable values as listedin Table I, but their scattering length densities were allowedto vary together. A best fit was obtained for monolayer scat-tering length densityb of 0.931026 Å22. The differencefrom the dry scattering length density is consistent with somedegree of toluene penetration within the hydrocarbon mono-layers.

In summary, our reflectivity results demonstrate the abil-ity to obtain ultrathin intersubstrate separations utilizing theneutron confinement cell. Based on the success of these ini-tial measurements, this device provides a new technology toprobe the structure of complex fluids or other materials ofinterest under confinement in a defined geometry. We are inthe process of continuing these studies to probe the staticstructure of adsorbed polymer diblocks of PS–PEO in tolu-ene and water as a function of confinement. The effects ofdynamic shear will also be investigated.

ACKNOWLEDGMENTS

This work was supported under the auspices of theUnited States Department of Energy through a collaborativeUC/Los Alamos Research~CULAR! Grant No. 9853 andDOE PECASE Award No. 05419-0099-2K. The Manuel Lu-jan Jr., Neutron Scattering Center is a national user facilityfunded by the United States Department of Energy, Office ofBasic Energy Sciences-Materials Science, under ContractNo. W-7405-ENG-36 with the University of California. Dur-ing this work Oak Ridge National Laboratory has been man-aged for DOE by Lockheed Martin Energy Research Corpo-ration under Contract No. DE-AC05-96OR22464 and byUT-Battelle LLC under Contract No. DE-AC05-00OR22725. We thank Professor Abraham Ulman, Depart-ment of Chemistry, Polytechnic University Brooklyn, NY,for recommending and providing OPA to protect and hydro-phobize the sapphire substrates.

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20Durlon 9000 is composed of pure PTFE combined with inorganic fillers.The unique formulation results in a dimensionally stable material undercompressive loads. Thus, this material does not exhibit the cold flow prob-lems associated with virgin PTFE.

1720 Rev. Sci. Instrum., Vol. 72, No. 3, March 2001 Kuhl et al.

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