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Neutron Reflectometry on Interfacial Structures of the Thin Films Of Polymer Journal, Vol. 39, No. 12, pp. 1238–1246 (2007) #2007 The Society of Polymer Science, Japan SPECIAL ISSUES -PROGRESS IN STRUCTURE ANALYSES OF POLYMERIC MATERIALS BY SYNCHROTRON RADIATION AND NEUTRON BEAM- REVIEW ARTICLE Neutron Reflectometry on Interfacial Structures of the Thin Films of Polymer and Lipid y yy Naoya TORIKAI,1; Norifumi L. YAMADA,1 Atsushi NORO,2 Masashi HARADA,2; Daisuke KAWAGUCHI,2 Atsushi TAKANO,2 and Yushu MATSUSHITA2 1Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba 305-0801, Japan 2Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan (Received August 1, 2007; Accepted September 27, 2007; Published November 2, 2007) ABSTRACT: Neutron reflectometry is a very powerful and essential technique for the studies on material interfa- ces due to its high spatial resolution, a few tenths of nm, along the depth direction. The use of neutron as a probe is a big advantage for structural analysis on soft-materials, since the scattering contrast can be highly enhanced in hydro- genous materials such as polymers, surfactants, lipids, proteins, etc., without big changes in their physical and chemical properties by substituting all or part of the hydrogen atoms in the molecules with deuterium (a deuterium labeling meth- od). Furthermore, the neutron reflectometry can explore deeply-buried interfaces such as solid/liquid interfaces in a non-destructive way, and make in situ measurements combined with various sample environments due to its high trans- mission to the materials. In this article, the neutron reflectometry is reviewed from the standpoint of researches on in- terfacial structures of the thin films of polymer and lipid, and its future prospects at a high-intensity pulsed-neutron source are presented. [doi:10.1295/polymj.PJ2007113] KEY WORDS Neutron Reflectometry / High Depth Resolution / Soft-material / Deuterium Labeling / Non-destructive Exploration / Deeply-buried Interface / In situ Measurement / At interfaces, materials often exhibit different struc- interfaces because of its high spatial resolution, a tures or physical properties from in bulk because they few tenths of nm, along the depth direction. Here, interact directly with different materials or phases in a the neutron reflectometry is reviewed from the stand- very narrow space. So far many interesting phenom- point of soft-material researches by highlighting ex- ena relevant to interfaces have been reported, e.g., perimental examples on the thin films of polymer lower surface glass transition temperature, de-wetting, and lipid systems, and its future prospects at a high- surface segregation, etc., in the research field of soft- intensity pulsed-neutron source are presented. materials.1,2 Also, material interfaces contribute large- ly to many practical phenomena known as coating, NEUTRON REFLECTOMETRY painting, adhesion, lubrication, etc. In addition, the re- cent development of nano-technology makes the size Neutron has unique properties as a probe for struc- of practical devices smaller, so that interfaces occupy tural analysis of materials, compared with electromag- the larger proportion of space in the devices, and then netic waves such as light or X-ray. The neutron is play an important role in the device performance. scattered through atomic interaction with nuclei, not Therefore, strong demands to clarify the structures electrons, so that scattering length, b, of element is and physical properties of materials at interfaces have not simply proportional to the atomic number Z, and been rapidly grown. also isotopes have different b values for neutron.3 Es- However, in reality it is generally difficult to ob- pecially, there is a big difference in b between hydro- serve detailed interfacial structure, since the interfa- gen (H, b ¼À3:74  10À15 m) and its isotope, deute- cial region is very narrow and has considerably small rium (D, b ¼þ6:67  10À15 m), and this gives a big volume in materials. The material interfaces are not advantage for soft-material researches on polymers, always exposed to air surface, but are deeply buried surfactants, lipids, proteins, etc., that possess many in materials. Neutron reflectometry is a very powerful hydrogen atoms. The contrast for neutron can be intro- and indispensable technique to the studies on material duced in these hydrogenous molecules with no big yTo whom correspondence should be addressed (Tel: +81-29-864-5614, Fax: +81-29-864-3202, E-mail: [email protected]). yyPresent Address: Toyota Central R&D Laboratories, Incorporated, Nagakute-cho, Aichi-gun 480-1192, Japan 1238 Neutron Reflectometry on Interfacial Structures Table I. The values of ðÆbiÞ and the critical QZ for total reflection of typical materials ðÆb Þ Critical Q Materials i Z (Â10À4 nmÀ2) (nmÀ1) Silicon Si 2.07 0.10 Quartz SiO2 4.18 0.14 Nickel Ni 9.21 0.22 Water H2O À0:56 — Deuterated water D2O 6.35 0.18 Polyisoprene (C5H8)n 0.27 0.04 Poly(trimethylsilylstyrene-d ) 9 3.6 0.13 (C11SiH7D9)n Figure 1. A schematic illustration of the geometrical configu- Dimyristoylphosphatidylcholine 5.17½1 0.16 ration in a reflectivity measurement. DMPC Sodium Iodide NaI 1.31 0.081 Poly(styrene-h )(CH ) 1.41 0.084 changes in their physical and chemical properties by 8 8 8 n Poly(styrene-d )(CD ) 6.47 0.18 substituting the part or all of the hydrogen atoms with 8 8 8 n Poly(2-vinylpyridine) (C NH ) 1.95 0.099 deuterium in the molecules (a deuterium labeling 7 7 n method). The contrast matching is also a quite useful [1] The number density of DMPC was calculated from the 4 concept, in which the contrast of one component is molar volume evaluated in aqueous solution experimentally. matched with the others by blending the deuterated 1=2 and normal species at adequate ratio. Another impor- ð16ðÆbiÞÞ , when the neutron is incident on the tant feature is that neutron has very high transmission objective interface from the side of the material to materials, e.g., transmissivity is still kept about 0.5 with lower n. The values of ðÆbiÞ and critical Qz even though the neutron passes through a silicon block for total reflection are tabulated for several materials with 50 mm length, so that deeply buried interfaces in Table I. In most cases, specular reflectivity profile such as solid/liquid interfaces with a block of silicon is analyzed by a conventional model fitting method or quartz can be explored in a non-destructive way. Al- with the Parratt’s recursion algorithm5 or optical 6 so, it is easy to combine neutron experiment with var- matrix formula, providing the ðÆbi) distribution ious sample environments such as a vacuum, a vapor, a along the Z depth direction. The further details on data pressure, etc., requiring the tight seal for a sample cell. analysis should refer to the existing review papers7,8 In a reflectivity measurement, a well-collimated and the textbook.9 neutron beam is incident on optically flat surface of At a reactor source, most of neutron reflectome- 10,11 a sample with the small grazing incident angle, in, ters use a monochromatic beam with constant and generally specular reflection, in which the reflec- and make a so-called -2 scan, in which the angles tion angle, out, is equal to in, as schematically illus- of a sample and a detector relative to the incident trated in Figure 1, is observed as a function of neutron beam are simultaneously changed keeping the specu- momentum transfer, Qz (¼ð4=Þ sin in, where is lar condition, to observe specular reflection. On the the wavelength of neutron), along the direction per- other hand, the reflectometers at a spallation neutron pendicular to the sample surface to probe the depth source with a proton accelerator8,12–14 or the ones in- structure of material with high spatial resolution, stalling a mechanical chopper at a reactor15,16 utilize a few tenths of nm. At the angular position of white neutrons with a wide band as the incident in 6¼ out, very weak signal of off-specular reflection beam. The obtained data is analyzed by using a originating from the in-plane structure along the QX- Time-of-Flight (TOF) method. It should be noted that direction shown in Figure 1 can be observed. The re- the pulsed-neutron reflectometer is able to measure fractive index, n, of material for neutron is expressed specular reflection in a wide QZ-range at one time ac- À2 with its scattering length density, ðÆbi) (nm ), as cording to the bandwidth without moving the sam- ple. This means that the pulsed-neutron reflectometer n2 ¼ 1 À 4ðÆb Þ=k2 i is suitable for the studies on free interfaces if it adopts where is the number density, (Æbi) is a sum of b a horizontal-sample geometry, and also for a time- over the constituent elements of the material, and k resolved measurement of reflectivity if the neutron is the wave vector defined as 2=. Generally, specu- source has high intensity in the incident beam. More- lar reflectivity profile exhibits total reflection with over, a simultaneous measurement of off-specular as reflectivity of unity, up to the critical Qz defined as well as specular reflections is possible without any Polym. J., Vol. 39, No. 12, 2007 1239 N. TORIKAI et al. scans by using simply a one-dimensional position-sen- Bragg peaks originating from the structure ordered sitive detector. preferentially along the film depth direction, though the X-ray data shows only the Kiessig fringes reflect- SOME EXPERIMENTAL EXAMPLES ing total film thickness due to the small difference in electron density between the deuterated PS and P2VP. Interfacial Structure of a Block Copolymer with a The best ðÆbiÞ profile along the film depth direction, Lamellar Microdomain obtained from the model fitting analysis for the neu- A block and graft copolymers with incompatible tron data, is shown in the inset.
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