Neutron confinement cell for investigating complex fluids

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 72, NUMBER 3 MARCH 2001

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#

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I. INTRODUCTION

Polymer molecules at solid or fluid interfaces haveenormous spectrum of applications in a wide variety of tenologies. They provide a mechanism to impart colloid stalization, they are used as protective coatings~including me-chanical protection of solids against friction and wear!, theygovern the interactions of biological cell surfaces, athrough judicious design they are used to modulate dission properties~such as rheology! under a variety of processing conditions. Knowledge of the conformations that asorbed or terminally anchored chain molecules adopt wsubjected to confinement and/or solvent flow is essentialpredicting the interaction forces and rheological propertiesthe polymer layers involved in all of the above-mentionapplications.

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

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single surface and under confinement. Here, we will focusthe behavior of tethered diblock polymer chains in selectsolvents, where one end of the polymer block anchorschain to the surface and the other block extends away fthe interface into solution. On the experimental side, a lotinformation regarding the structure of grafted polymer layhas been deduced from force measurements utilizing teniques like the surface forces apparatus~SFA!1 and variousscanning probe microscopies.2 Such measurements are vepowerful and sensitive surface probes, however, the infortion they provide is not molecular. As a result, in many caour understanding and interpretation of experimental dhas evolved from theoretical studies based on scaarguments,3 mean field theories,4 and/or computersimulations.5

At the molecular level, neutron reflectivity experimenhave been very successful in providing detailed densitytribution profiles of polymeric materials at single interfac~depth profiling!, where the structure of adsorbed diblocpolymers in good, theta, and poor solvents has been inv

5 © 2001 American Institute of Physics

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

gated as a function of grafting density.6,7 Probing the struc-ture the tethered polymer layer adopts under confinementproven more elusive. The difficulty here lies in the needultraflat surfaces of a suitably large surface area for neuscattering measurements. Not only must these surfaceclosely opposed~intersurface separations below a few hudred nanometers or less are required!, but they must also bekept aligned and parallel throughout the duration of the msurement in order to confine the film of interest uniformEarly work by Cosgrove and co-workers paved the waythe apparatus we describe here.8 In their design, large, opti-cally polished quartz flats were forced to closely approausing a hydraulic ram. Intersurface separations of aboutnm were attained, however it proved difficult to maintainconstant gap separation during the course of an experimwhich greatly complicated data interpretation.

Although the structure of tethered polymer layerssingle interfaces or confined has received much attentionthe most part thedynamicbehavior of polymer layers undeshear has only been inferred. Techniques such as ellipsetry or the hydrodynamic flow method enable average pomer layer thicknesses to be evaluated. However, they doprovide information on the segmental conformation or dsity profile of the polymer chains away from the interfacLikewise, even less known is the effect of geometric confiment imposed by a second surface although this more clomatches the conditions of colloidal processing and the mother technological situations listed previously. Recenthere has been more focus on understanding these effFor example, dynamic shear investigations utilizing the Sindicate that polymer brush layers extend and generate amal force component during shearing motion,9 while a novelshear cell for neutron reflectivity measurements has bused to investigate the conformations tethered polymchains adopt when exposed to flowing solvent at a sinsurface.10 In addition, a new device developed by Graniand co-workers, micrometer-gap optorheometer, shoprove to be a powerful tool for probing the rheology astructure of complex fluids at the mesoscale, gaps of amicrons and larger.11

In this paper, we describe an apparatus, the neutronfinement cell, which can be used for determining~i! the mo-lecular orientations and density distributions of physicaadsorbed and chemically grafted polymer chains on ssupports,~ii ! how these are affected by shear at a sinsurface, and~iii ! how the proximity of a second surface~im-posed confinement! alters the conformation, intersurfacforces, and rheology of polymer layers in thin films. Curredesign specifications focus on the use of neutron reflectito characterize the structure of end-grafted polymer brlayers, but the device can be employed to probe the strucof any complex fluid of interest and is amenable to othcharacterization techniques.

II. APPARATUS

A. Overall features of the apparatus

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


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cell ~NCC! are constructed from 304 and 316 stainless stAll inlets, outlets, and openings are sealed with Teflongaskets or o-rings, enabling liquid vapor pressure to be mtained and preventing contamination from entering the chaber. 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 conthe complex fluid of interest. Theoretically, the minimugap obtainable with the device is solely a function of tsmoothness~waviness! of the substrates used. In othewords, the gap separation is equivalent to the separationtween the two substrates. In the current design, the NCCsingle crystal quartz and single crystal sapphire substratenominal surface waviness less thanl/25 andl/20, respec-tively. Thus, the peak-to-valley height difference on ea

FIG. 1. ~A! Photograph of the neutron confinement cell~NCC! with theouter steel housing removed. A 12 in. ruler on the left-hand side givesidea of the size of the apparatus.~B! Cross section of the NCC. The neutrobeam passes perpendicular to the view shown. The apparatus is constwith 304 and 316 stainless steel and all inlets and outlets are sealeTeflon o-rings or gaskets. A hydraulic ram can be used to apply high lowhich are calibrated by measuring the compression of Belleville washervariable spring constant. The upper quartz substrate mounts into the tothe outer housing. The lower sapphire~quartz! substrate mounts on a mechanical slider and can be translated~sheared! relative to the upper surfaceusing a mechanical motor drive assembly.


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

substrate across the inner 90% of the surface is about 30The outer 10% of each substrate is rounded off to ensurethere are no edge asperities.

Another critical concern is that these substrates muskept in a parallel alignment at a constant gap separationthe duration of the reflectivity measurement~2–12 h!. Oth-erwise, contributions from different substrate separatiwill tend to smear out the reflectivity profile.8 In our appa-ratus, the substrates are mounted into recesses withinsteel housing~upper surface! and in the shearing moun~lower surface!. The steel surfaces upon which the substrarest are machined to a lapped tolerance of less than 10mm. ATeflon gasket material of Durlon 9000 is placed alongbottom and sides of the substrate recesses to distributeapplied load evenly across the substrate surface. Moreothe flexibility of the gasket material and the compliancethe substrate materials themselves, quartz against sappenable the surfaces to conform and self align. As we sshow in Sec. III, very small gap separations were obtaiusing this construction. The assembled device fits on toppiston ram, which can be used to apply large normal loadthe substrates through a series of variable spring consBelleville washers. By measuring the deflection of tBelleville washers, the applied load can be accurately demined. A locking collar enables the applied load tomaintained if the device is removed from the piston raassembly.

B. Substrate chemistry

We found that contamination on the surfaces frequendetermined the minimum separation as opposed to the phcal smoothness of the substrate surface. Both bare quartzsapphire have high surface energies. As a result, it is diffito remove particles or contamination from these surfacOne important aspect of our design was hydrophobizingsingle crystal substrates with a molecular layer of eitchemically grafted octadecyltrichlorosile~OTS! or physi-cally adsorbed octadecylphosphonic acid~OPA! on thequartz and sapphire, respectively. The purpose of hydropbizing the substrates was twofold. First, the hydrophobic lers provided a protective coating on the two ultrasmosurfaces. Second, hydrophobic surfaces were much easclean and to keep clean of contaminants and particlesparticular dust. Indeed, we found that by effectively remoing dust and other particles from the substrate surfaces,tremely small intersurface separations of less than 70were obtainable.

1. Quartz

By virtue of its hydrolyzable functional groups, OTmolecules were covalently bound to the quartz substratedroxyl groups as well as cross-linked within the organosiloane layer. As a result, a robust, chemically inert, protecmonolayer was formed on the quartz substrate. The produre was as follows. The quartz surface was first cleanedthe surface hydroxylated by immersion in a strong basiclution of 10 wt% NaOH for 10 min at 50 °C. The quartz wathen rinsed in Millipore water and dried in a clean streamnitrogen gas. Next, following the procedure of Moav a


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Sagiv,12 the quartz was immersed in a solution of 1 mM OTin dried bicyclohexyl. The OTS monolayer was allowedself-assemble for 1 h.13 Upon removal from solution, themonolayer coated substrate was autophobic and the quof the formed monolayer was initially judged from the dwetting of the solution from the substrate surface. The sstrate was then rinsed in clean chloroform and baked inoven at 102 °C for 3–6 h. Baking the monolayer increasthe cross-link density of the organosiloxane layer to the sstrate as well as intermolecularly. The advancing contangle of water on the now hydrophobized quartz substwas 110° and the receding contact angle was 100°65°.

2. Sapphire

Similarly, the sapphire substrate was hydrophobizwith a self-assembled monolayer of OPA. OPA formsstrong physisorbed monolayer on sapphire through iobonds between the phosphonate group and the two negoxygens above the aluminum ions. The neutral oxyg~P5O! is above the vacancy site and the P–C bond is norto the surface.14 Adapting the procedure followed by Bermann et al.,15 the sapphire substrate was first thoroughcleaned in Helmenex solution, rinsed with copious Millipowater, and dried thoroughly in a stream of clean nitrogThe substrate was then immersed in a solution of 0.5 mOPA in bicyclohexyl at 60 °C for 1 h. Afterwards, the sustrate was rinsed in clean chloroform to remove residualvent and unbound surfactant. The resulting coating wasmogeneous as probed by contact angle measurementsan advancing contact angle of 80° and a residing conangle of 60°65° with water.

C. Shear capabilities

Lateral motion~shear! is accomplished using a variablespeed motor-driven screw, which presses or pulls the losubstrate holder relative to the stationary upper substrThe lower substrate holder is mounted on a low frictislider rail, which ensures smooth motion even when the sstrates are under a high applied load. To maintain paramotion, the motor driven screw is aligned with the interfabetween the two substrates. Thus, the substrates are nedriven apart nor toward each other during the shearing mtion. Depending on the gap separation between the two sstrates and the time of the neutron reflectivity measuremshear rates spanning over 4 orders of magnitude are atable, 0.001–20 Hz. Faster shear rates can be used, bulimitation of data collection time requires that aoscillating—back-and-forth—motion must be used.

Higher shear rates would be desirable for some expments involving polymer brush layers. Indeed, previowork by Baker et al. has shown little change in polymebrush structure in a good solvent at shear rates above 10Hz.10 In those experiments, shear was accomplished byvent flow, whereby the solvent was pumped at relativhigh rates but still under laminar flow conditions pastadsorbed polymer layer. In our case, the substrates slideerally past one another. As a result, the shear is at thestrate interface as opposed to the outermost portion of


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layer exposed to the solvent flow. It will be interestingcompare our results with those reported previously wmore conventional solvent flow shear cells as well as tovestigate 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 IsotoReactor at Oak Ridge National Laboratory. This reflectomter is set up for vertical sample geometry. Although the cwas originally designed for horizontal sample geometry,found it operated well in vertical geometry.

In the measurements presented here, a perpendicscattering vectorQz ~54p sinu/l! range from 0.004 to abou0.05 Å21 was covered at MIRROR’s operating neutrowavelength l52.59 Å, which corresponds to reflectioangles,u, between 0.05° and 0.6°. Reflected neutrons wcounted using an Ordela model 1150N linear position setive 3He detector with;1 mm ~rms! resolution at a sampleto detector distance of 3.37 m, giving the instrument a limresolution ofsQ;0.0005 Å21. To avoid reflections from therounded surfaces at the edges of the substrates, a slitupstream of the sample was varied according to the anglincidence to maintain a constant 20 mm illumination leng~footprint! near the center of the sample interface. Dependon the instrument configuration, the reflectometer resoluvaried from 0.0005 to 0.001 Å21 ~;2p/7000 Å! during thecourse of a measurement. Assuming a minimum samplinabout four points per period, the instrument could therefresolve interference fringes between the reflections fromficiently parallel quartz and sapphire surfaces for separatless than about 3000 Å. Thus, the visibility of interferenfringes in our data is a sensitive indication of well-controllintersubstrate separations below this limit, as shownSec. III.

Initial tests were performed without material betwethe two substrates~an air gap! to determine the intersubstraseparation obtainable with the device. Subsequently, theflectivity profile was measured when a mixture of hydrognated and deuterated toluene was placed between thestrates at a ratio chosen for good neutron contrast relativboth the quartz and sapphire substrates. In our initial msurements we restricted the beam height to 1.5 mm to beensure sample uniformity over the illuminated region. Wthis restriction, runs obtaining good statistics and signabackground 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 timesabout 4 hours@Figs. 2~b! and 3#.

The reduced data are plotted asR* Qz4 versus the perpen

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

4 decrease of the reflectivity due to the Fresnel’s la!.The vertical error bars on the data represent the statiserrors in the measurements after background subtrac~standard deviation,sR!. The horizontal error bars represethe rms instrumentQz resolution calculated on a point-bypoint basis~with no allowance made for sample irregulari


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or curvature!. Reflectivities were normalized to illuminatesample area and scaled to unity below the critical edgetotal reflection. The fits to the reflectivity~calculated usingthe iterative, dynamical method6 as described in the follow-ing! included an additional parameter to normalize the cculated reflectivity to the data. However, this parameternot vary more than 5% from unity—a value comparablethe normalization error due to the scatter of data points inR51 region below the critical angle.

III. EXPERIMENTAL RESULTS

Results from two reflectivity measurements with angap separating the quartz and sapphire in the NCC are shin Fig. 2. A clear series of interference fringes is visible fboth, indicative of the small intersurface separations attaable with the NCC. The coherent neutron scattering lendensities~b! of the materials used in this work and the moeling are shown in Table I.18 To calculate the reflectivityprofile R(Qz ,t), the key structural components of the invetigated system@the octadecyltrichlorosiloxane~OTS!, the oc-tadecylphosphonic acid~OPA!, and the gap between thquartz and sapphire crystals# were described by boxes ofconstant thickness and scattering length densityb(z). Thereflectivity R(Qz ,t) was calculated using the optical matr~or dynamical! method.6,19

To obtain good model fits to the experimental reflectivdata, it was necessary to account for sample irregulariacross the illuminated beam footprint due to, for exampsubstrate waviness. One possible way to account forsample spacing irregularities would be to add a ‘‘sample cvature resolution’’ term in quadrature to the instrument re

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 theThe solid curves are fits to the data using the model described in theParameters are tabulated in Table I.


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

lution. Adding a term of this sort did reduce the calculatfringe visibility to values near those observed in our daHowever, this treatment gives a uniform reduction overQz ,while in our data a definite weakening of the fringe visibiliis evident asQz increases. A surface roughness paramehas a similar effect, but also dramatically reduces the overeflectivity. Using these descriptions, we were unable to pduce statistically satisfactory fits without using either uphysical neutron scattering length densities or modifyingresolution beyond limits consistent with the experimentaobserved spread of the specularly reflected beam.

Instead, we accounted for deviations in the average sstrate separationt0 by assuming that there was a Gaussdistribution of thicknesses~gaps! between the two crystals ostandard deviationsw . The reflectivity of each constanthickness 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 thinesses, one would expect that the domains would reflecneutron beam independently of one another and thusmeasured reflectivity is the~incoherent! average of the intensities:

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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 chax2 by unity multiplied by ax2/(n2p) correction factor, wheren5numberof data points andp5number of fit parameters, i.e., square root of treduced chi squared.

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

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


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were not observed. Moreover, this calculation method safactorily reproduced the measured weakening of fringe vibility with increasingQz and no correction to the instrumenresolution was necessary. An additional ‘‘Gaussian’’ rougness at the surface~by modifying the interfacial reflectancby the so-called Ne´vot–Croce factor, roughness paramete!was not used here since in all cases the values were s~rms ,5 Å! and their inclusion had negligible effect on thfit quality over theQz range covered in these measuremen

The solid curves are fits to the data based on the moparameters listed in Table I. Fits to these measurementsaverage 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 tscattering length densities of the quartz and sapphire sstrates, air thickness~intersubstrate separation!, and the inco-herent average of the reflectivity over a relative flatnessparallelism parameter accurately models the system.20 Thesmaller value forsw for smaller substrate [email protected]~b!# is consistent with a greater parallelism of the substraunder higher applied load. For consistency the contributof the hydrocarbon monolayers to the measured reflectiprofiles, OTS and phosphonic acid on the quartz and sphire, of thickness 24 and 15 Å, respectively, was includwhen calculating these fits. However, because of the wscattering contrast (b'20.431026 Å22) with air addingthese layers to the model did not significantly improve tquality of the fits in comparison with a simple air-gap modIn all cases, similar results were obtained when the mowas refined using either nonlinear versions of MarquardLevenburg methods or Nelder and Mead’s downhill simpalgorithm.

The measured reflectivity profile when a mixture of hdrogenated and deuterated toluene, scattering length deb'231026 Å22, was confined between the substratesshown in Fig. 3. A similar model was used to fit the refletivity profile—an inset shows scattering length density pfile over the interface for the average spacing (t0). Howeverin this case, the contribution of the OTS and OPA monolaers had to be included to obtain a good fit between the

FIG. 3. Neutron reflectivity data for a deuterated-hydrogenated mixturetolueneb'231026 Å22 confined between a single crystal quartz and saphire substrate. The solid curves are fits to the data using the modescribed in the text. The inset shows the mean scattering length densityfile ~before incoherent averaging!. Parameters are tabulated in Table I.

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culated and measured reflectivity profiles, since the scaing length density of hydrocarbon layers was significandifferent from the toluene mixture. In order to minimize thnumber of fitting parameters, the thicknesses of both hydcarbon monolayers were fixed to reasonable values as lin Table I, but their scattering length densities were allowto vary together. A best fit was obtained for monolayer sctering length densityb of 0.931026 Å22. The differencefrom the dry scattering length density is consistent with sodegree of toluene penetration within the hydrocarbon molayers.

In summary, our reflectivity results demonstrate the aity to obtain ultrathin intersubstrate separations utilizingneutron confinement cell. Based on the success of thesetial measurements, this device provides a new technologprobe the structure of complex fluids or other materialsinterest under confinement in a defined geometry. We arthe process of continuing these studies to probe the sstructure of adsorbed polymer diblocks of PS–PEO in toene and water as a function of confinement. The effectsdynamic shear will also be investigated.

ACKNOWLEDGMENTS

This work was supported under the auspices ofUnited States Department of Energy through a collaboraUC/Los Alamos Research~CULAR! Grant No. 9853 andDOE PECASE Award No. 05419-0099-2K. The Manuel Ljan Jr., Neutron Scattering Center is a national user facfunded by the United States Department of Energy, OfficeBasic Energy Sciences-Materials Science, under ContNo. W-7405-ENG-36 with the University of California. During this work Oak Ridge National Laboratory has been maged for DOE by Lockheed Martin Energy Research Corration under Contract No. DE-AC05-96OR22464 andUT-Battelle LLC under Contract No. DE-AC0500OR22725. We thank Professor Abraham Ulman, Depment of Chemistry, Polytechnic University Brooklyn, NYfor recommending and providing OPA to protect and hydphobize the sapphire substrates.

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Volume C: Mathematical. Physical and Chemical Tables, 2nd ed., editedby E. Prince and A. J. C. Wilson~Kluwer Academic, Dordrecht, 1999!,pp. 126–130.

20Durlon 9000 is composed of pure PTFE combined with inorganic filleThe unique formulation results in a dimensionally stable material uncompressive loads. Thus, this material does not exhibit the cold flow plems associated with virgin PTFE.


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