Formation of hydrated layers in PMMA thin films in aqueous solution

Applied Surface Science 353 (2015) 829–834

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Formation of hydrated layers in PMMA thin films in aqueous solution Peter W. Akers a , Andrew R.J. Nelson b , David E. Williams a,c , Duncan J. McGillivray a,c,∗ a

School of Chemical Sciences, University of Auckland, Auckland, New Zealand The Bragg Institute, Australian Nuclear Science and Technology Organisation, Menai, NSW, Australia c MacDiarmid Institute of Advanced Materials and Nanotechnology, Wellington, New Zealand b

a r t i c l e

i n f o

Article history: Received 16 April 2015 Received in revised form 29 June 2015 Accepted 30 June 2015 Available online 11 July 2015 Keywords: Protective coatings PMMA thin films Neutron reflectometry Surface and bulk properties PMMA hydration Delamination

a b s t r a c t ˚ poly(methylmethaNeutron reflectometry (NR) measurements have been made on thin (70–150 A) crylate) (PMMA) films on Si/SiOx substrates in aqueous conditions, and compared with parameters measured using ellipsometry and X-Ray reflectometry (XRR) on dry films. All techniques show that the thin films prepared using spin-coating techniques were uniform and had low roughness at both the silicon and subphase interfaces, and similar surface energetics to thicker PMMA films. In aqueous solution, NR measurements at 25 ◦ C showed that PMMA forms a partially hydrated layer at the SiOx interface 10 A˚ under the film, while the bulk film remains intact and contains around 4% water. Both the PMMA film layer and the sublayer showed minimal swelling over a period of 24 h. At 50 ◦ C, PMMA films in aqueous solution roughen and swell, without loss of PMMA material at the surface. After cooling back to 25 ◦ C, swelling and roughening increases further, with loss of material from the PMMA layer. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ultrathin PMMA coatings are in use in a wide range of applications such as medicine [1], microelectronics [2] and nanotechnology [3] with the number of different uses projected to increase even further [4]. Because of this, information about the response of these films to a wide range of external stimuli is becoming more and more important. Although most applications are in dry environments, analysis of PMMA films in aqueous solutions is of relevance because of their potential as coatings for various types of medical implants [5–7] and general use in other applications where protective surface coatings in aqueous solution are required. Studying PMMA films in situ in aqueous solutions is challenging, but neutron reflectometry has been previously used in various studies which characterise thin films of hydrated polymers prepared via spin-coating, in particular with studies of glass transition temperature (TG ) [11,12] and solvent retention [13,14]. Formation of blisters in PMMA [8] films in aqueous solution have been observed with AFM and optical microscopy, where it was suggested that deformation depends on film thickness and Young’s modulus of PMMA. It has also been shown that liquids in which the polymer

∗ Corresponding author at: School of Chemical Sciences, University of Auckland, Auckland, New Zealand. Tel.: +64 9 9238255; fax: +64 9 373 7422. E-mail address: [email protected] (D.J. McGillivray). http://dx.doi.org/10.1016/j.apsusc.2015.06.199 0169-4332/© 2015 Elsevier B.V. All rights reserved.

is not soluble, such as water, cause swelling of the polymer film at the solvent-film interface [9]. Similar but more pronounced effects were observed by Berkelaar et al. [10] in polystyrene (PS) films deposited on Si wafers, where blisters formed at the Si–PS interface in aqueous solution, which expanded to 12 ␮m in diameter after 30 min. In applications where devices must be used in both dry and aqueous environments, changes in the polymer film due to sorption of water may have downstream effects that change the mechanical properties and overall performance of the film. To explore this topic further we characterised dry ultrathin PMMA films using contact angle, ellipsometry and X-Ray reflectometry (XRR) measurements and re-measured in aqueous solution using neutron reflectometry (NR) to test swelling and changes at PMMA/SiO2 and PMMA/air interfaces. Further, we heated a film to 50 ◦ C in aqueous solution to test whether increased temperature has an effect on interfacial structure. 2. Materials and methods 2.1. Materials PMMA (average Mw ∼ 120,000) was obtained from Sigma– Aldrich and used without further purification. Analytical grade toluene was obtained from Scharlau S.L (Spain) and stored in 3 A˚ molecular sieves before use. Polished Silicon wafers, 1 1 0, with

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a diameter of 76.2 mm and thickness of 5 mm were obtained from El-Cat Inc. (USA). 2.2. Methods 2.2.1. Preparation of PMMA films PMMA films were prepared using a previously reported procedure [21]. Briefly, PMMA (∼120 kDa) was suspended in toluene to 2.5 g L−1 and heated to 45 ◦ C until no solid particles were visible. The solution was sonicated for 15 min using an Elmasonic S 30 Elmasonic ultrasonic bath (Total Lab Systems Ltd., NZ) and filtered through a 0.2 ␮M pore size PTFE filter (Raylab Ltd., NZ) to remove any remaining aggregates and dust particles. PMMA was then spin coated onto polished and cleaned Si wafers at 2000 rpm for 45 s using a Laurell WS-400B-6NPP Lite spin coater (Laurell Technologies, USA) in a clean room facility. A single concentration of PMMA (2.5 g L−1 ) and spin speed (2000 rpm) were selected to produce ˚ Residual toluene was removed by films of approximately 100 A. leaving at room temperature overnight followed by curing at 80 ◦ C for 2 h. One further substrate was prepared and annealed at 125 ◦ C under vacuum, which is higher than the glass transition temperature (TG ) of PMMA (105 ◦ C). 2.2.2. Contact angle measurements Contact angle measurements were collected using a CAM 100 instrument (KSV NIMA, Finland). Drops of ultrapure water of 15 ␮L were placed on clean, dry surfaces using a Hamilton syringe for 5 s. Real time images of drops were recorded and contact angles measured using the instrument software. At least three measurements in multiple regions of each surface were taken. 2.2.3. Ellipsometry Ellipsometric thickness of PMMA films was measured using a Beaglehole Imaging Ellipsometer (Beaglehole Instruments, NZ) equipped with a red LED (633 nm) using optical constants of n = 3.882 and k = 0.019 for Si [15], n = 1.457 and k = 0 for the SiOx layer [16] and n = 1.489 and k = 0 for PMMA [17]. To unambiguously resolve the thickness of the SiOx layer, it was either measured before PMMA spin coating or determined by NR. At least 3 spots on each substrate were measured to test film uniformity. 2.2.4. Neutron and X-ray reflectometry Air-solid XRR measurements were recorded on a Panalytical ˚ X’Pert Pro instrument using a Cu K␣ X-ray radiation ( = 1.54 A). The line focus X-ray beam was produced using a Göbel mirror with a 1/32◦ divergence slit and post-sample parallel plate collimator. Reflectivity was collected over an angular range 0.05◦ ≤  ≤ 7◦ , corresponding to a Q-range of 0.007 < Q/Å−1 < 1 with a counting time of 2 s per step. Background was accounted for in data modelling. Neutron reflectometry measurements were performed on the Platypus time of flight neutron reflectometer at ANSTO [18] using ˚ 24 Hz neutron an incident beam spectrum between 2.8 A˚ and 18 A. pulses are generated using a disc chopper system, collimated, and reflected from the sample at different angles of incidence sufficient to measure a QZ range of approximately 0.01–0.35 A˚ −1 before detection with a 2D position sensitive detector. The reflectivity of a sample is calculated as the ratio of the reflected and incident beam intensities, with non-specular scattering measured and subtracted as background from the data. Reflectivity is expressed as a function of the perpendicular wave vector transfer QZ : Qz =

4 sin  

where  is the angle of incidence of the radiation, and  is the wavelength of the radiation.

We performed least squares analysis of the reflectometry data using the Motofit [19] data analysis program, using an optical matrix method in which a model of the surface structure is created and divided into homogeneous layers. Each layer is described by a volume fraction of solvent, a thickness, an interfacial roughness and a scattering length density (xSLD or nSLD, for X-rays or neutrons respectively): SLD =

˙i bi Vm

where for neutron reflectometry bi is the bound coherent neutron scattering length of each atom i and Vm is the molecular volume. In the case of X-rays bi are replaced by the product of the atomic number Z and the Thomson scattering length re . For PMMA, the nSLD was calculated to be 1.07 × 10−6 A˚ −2 and the xSLD 10.9 × 10−6 A˚ −2 and these values were fixed during data fitting. Error values for NR data were obtained using Monte Carlo error analysis methods [20], while XRR error values were obtained using Genetic Algorithm + Levenberg Marquardt analysis. NR measurements were made in D2 O and H2 O-based buffers to take advantage of the difference in neutron scattering lengths between H (−3.74 fm) and D (6.67 fm). Making measurements in D2 O-based buffers followed by re-measurement of the same sample in H2 Obased buffers, where the system of interest is not disturbed by the buffer exchange, allows for water-containing components of the system to be resolved and reduces ambiguity in measurements. 3. Results 3.1. Preparation of PMMA films and measurement in air Contact angle measurements were made in order to assess wettability of surfaces and to verify that the prepared films were comparable with previously obtained values. A drop volume of 15 ␮L and a residence time of 5 s were used in all measurements. We found a mean contact angle across 10 datasets of 73.9 ± 2.1◦ . Ellipsometric thickness for PMMA films prepared using the ˚ which was a value averaged using above conditions was 138 ± 2 A, at least 3 measurement points each on 25 substrates. Over the 25 substrates prepared and measured in this study, the thickness at each of the different points did not vary by more than 2%, and ellipsometric imaging of the surface showed high uniformity (see Fig. 1). One representative PMMA film was selected for subsequent XRR and NR measurements, which had an ellipsometric thickness of ˚ Data from the XRR measurements made on the rep138 ± 1.2 A. resentative film were analysed using a model that consisted of a single PMMA layer of 91.9 ± 0.4 A˚ with an xSLD of 10.9 × 10−6 A˚ −2 , which matches the theoretical value for PMMA calculated from bulk polymer density (see Fig. 2) 3.2. Neutron reflectometry measurements in aqueous solution NR measurements were made on the representative PMMA film against D2 O and H2 O buffered with 50 mM sodium phosphate at pH 7. The data from NR measurements on were simultaneously analysed with a 2 layer model, which shows a highly hydrated (51.4 ± 1.6% water) PMMA layer of 7.7 ± 0.3 A˚ at the ˚ which conSiOx interface and a thicker bulk layer of 76.7 ± 0.3 A, tained 3.5 ± 0.1% water (see Fig. 3). A comparison of the fitted values from XRR in air and NR in aqueous solution is given in Table 1. Five other films prepared identically and measured by NR contained the same hydrated layer at the PMMA/SiOx interface with thickness ranging from 8 to 18 A˚ and percentages of water between 44 and 56%. We also attempted to analyse the data with a single

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Fig. 1. (A) PMMA coated Si wafer used in this experiment; (B) image taken from ellipsometer of PMMA film on Si wafer at 76 ◦ , showing section of uniform PMMA film.

Fig. 2. X-ray reflectivity and datafit (A) and xSLD plot of PMMA film (B) measured against air. The model shows a smooth, uniform PMMA film with no observed defects.

Fig. 3. (A) Neutron reflectivity data and fits of PMMA film in D2 O (red) and H2 O-based (blue) pH 7 phosphate buffers; (B) neutron scattering length density plot for PMMA film in D2 O (red) and H2 O (blue) sodium phosphate buffer pH 7. The PMMA/SiOx interface contains a highly hydrated sublayer region (∼50% water by volume) and the main film contains around 4% (v/v) H2 O. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

layer model, but the 2 values for the single layer model were 9.2 as opposed to 1.6 for the 2-layer model. Models where the hydrated layer was adjacent to the PMMA/water interface were not as good as the models where the hydrated layer is located at the SiOx layer, resulting in an increase in the 2 values to 10.7.

Another PMMA film was cured at 125 ◦ C (the TG for PMMA is 105 ◦ C) under vacuum. Modelling showed a 7.7 ± 0.1 A˚ thick hydrated PMMA sublayer which contained 49 ± 0.8% water, which is comparable to the representative PMMA film and to other identically prepared PMMA films cured at 80 ◦ C (see Fig. 4).

Table 1 Fitted parameters from modelling of XRR (air) and NR (aqueous solution) measurements made on the representative PMMA film.

XRR (single layer) NR (sublayer) NR (bulk layer)

Thickness (Å)

SLD (×10−6 A˚ −2 )

Solvent %

Roughness (Å)

91.9 (0.4) 7.7 (0.3) 76.7 (0.3)

10.9 1.07 1.07

0 51.4 (1.6) 3.5 (0.1)

4 3 6

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Fig. 6. A cartoon of the fitted layer structure of the PMMA film in air, and after being placed under water.

Fig. 4. Neutron scattering length density plot for the PMMA film cured above TG (125 ◦ C) in D2 O and H2 O (black dashed lines) sodium phosphate buffer pH 7. No significant difference was found between this and the films cured below TG (D2 O plots shown in red and H2 O plots shown in blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

A separate PMMA film (cured at 80 ◦ C) was prepared and measured at 25 ◦ C in aqueous solution, heated to 50 ◦ C where it was measured again and cooled back to 25 ◦ C where it was remeasured a final time. The modelled data is shown in Fig. 5. At 50 ◦ C, the ˚ and hydrated layer at the SiOx interface grows from 7.7 A˚ to 14 A, becomes rougher. The density of PMMA on the surface remains constant (5.4 mg m−2 ) and the surface remains stable over 5 h. On cooling to 25 ◦ C the hydrated layer does not revert to the original state; instead it expands further with loss of material from the surface, with only 3.4 mg m−2 remaining (see Fig. 5). 4. Discussion 4.1. PMMA films measured in air The PMMA films that were prepared were characterised using contact angle measurements, ellipsometry, XRR and NR. Contact angle results were in agreement with previously reported values [9,22] and indicate that although the films prepared in this study are thinner than most in the literature, the surface energetics are the same. The film uniformity was assessed using ellipsometry and indicated that the films were uniform and that the spin coating conditions used produced an even distribution of PMMA, and moreover that the ellipsometric thickness of the representative film was identical to the mean value across all 25 wafers. The thickness of the underlying SiOx layer was assigned at the time of ellipsometry measurements by measuring selected substrates before spin coating of PMMA films, which is necessary as the refractive indices (ni ) used

as fitting parameters for the SiOx and PMMA layer are very similar, meaning that the two layers cannot be clearly resolved. Although ellipsometric thickness measurements for SiOx films below 20 A˚ can be inaccurate [23], the SiOx layer was fit to a model of 15 ± 1 A˚ which was corroborated by NR measurements which produced a ˚ SiOx layer thickness of 15 ± 0.5 A. The thickness obtained by XRR for the PMMA film was lower than the ellipsometric thickness, but the data was well modelled by a single homogeneous PMMA layer with low interfacial rough˚ at both the PMMA/air and PMMA/SiOx interfaces. This ness (4 A) layer had an xSLD which matches the theoretical xSLD of PMMA, indicating that the film does not contain voids or significant defects.

4.2. PMMA films measured in aqueous solution using NR The representative PMMA film characterised in air was measured in aqueous solution at 25 ◦ C along with other identically prepared films to test whether the observed homogeneity is affected by exposure to water. Data analysis of the representative film required a two layer PMMA model, illustrated in Fig. 6, comprising of a hydrated layer at the SiO2 interface containing ∼50% water, above which was a bulk layer of which contained almost no water, as would be expected for the hydrophobic PMMA film. The film was left in aqueous solution for ∼30 h after which the thickness and hydration in the hydrated layer had increased by ˚ and the thickness of the bulk layer by ∼4%. The approximately 4 A, increase in the hydrated layer for the representative wafer was higher than other films measured, with thickness values for other hydrated layers measured on identically prepared PMMA films having a mean increase across 5 datasets of 2 ± 1 A˚ with matching increases in hydration. Overall, hydrated sublayers were observed in every PMMA film measured under water, and were very consistent (see Fig. 4). The water sorption properties of the bulk PMMA layers measured in this study were also very consistent with an

Fig. 5. (A) PMMA data (including fits) of PMMA measured at 25 ◦ C (blue), 50 ◦ C (red) and after cooling to 25 ◦ C (blue squares); (B) corresponding volume fractions of PMMA from each dataset. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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average layer thickness increase of 2.2 ± 1.8% averaged across five datasets each measured after 30 h in solution. Published work on nanometre-scale thickness PMMA [8] films using AFM and optical profiling techniques reported similar effects. In the PMMA study, a film was spin coated onto N,Ndimethylaminopropyltrimethoxysilane (DMAPS) substrates and the growth of ‘bubbles’ at the surface was observed over a timescale of 3 h. Over the measured timescale the film swelled and roughened considerably with the thickness increasing from 47 to 70 nm with reported length of the bubbles 1250 nm after 3 h. Without lateral information in our study it is not possible to unambiguously determine whether the sublayer we observed is due to even delamination of the film from the surface or formation of bubbles as noted in the previous study. However if the bubbling effect observed in the previous study were present in our films this would cause significant increases in the interfacial roughness at the PMMA/water interface, which we did not observe. The thickness increase of 47–70 nm measured over 3 h with the presence of the bubbles is much greater than what we observed over 30 h, with none of the ˚ Observed bulk layers showing thickness increases higher than 5 A. differences between the previously reported PMMA films and our data may be due to the differences in the underlying DMAPS and SiOx surfaces. To test whether this dewetting effect is related to annealing temperature, a substrate was prepared and annealed at 125 ◦ C, which is higher than the theoretical TG of bulk PMMA (105 ◦ C), and higher still than the expected TG of thin PMMA films [24,25]. The substrate was measured using NR against water, and modelling of the data showed the presence of a hydrated sublayer and low-hydration bulk layers not significantly different to those observed from films annealed below TG , showing that the hydrated layer we observed at the SiOx/PMMA interface is not dependent on exposure of the film to temperatures higher than TG . Roughness at the PMMA/H2 O interface was also unaffected by the difference in annealing temperature. To test the effect of temperature on the hydrated and bulk PMMA layers a single film was measured at 25 ◦ C, heated to 50 ◦ C and measured, and cooled back to 25 ◦ C and re-measured. In aqueous solution to 50 ◦ C, the amount of water in the hydrated layer increased from 53 to 74% and the overall thickness in the layer ˚ This was accompanied by an increase increased from 8 A˚ to 14 A. in the thickness, water content and roughness in the bulk layer, which could not be fitted to a model of a single layer of higher hydration and roughness, but multiple layers which increased in roughness and water content with increasing distance from the SiOx/PMMA interface. Although the surface delamination effect appeared higher at 50 ◦ C after 5 h than after 30 h at 25 ◦ C, no material was lost from the film with the overall mass of PMMA at the surface remaining unchanged within experimental resolution. Upon cooling back to 25 ◦ C over a period of 5 h further delamination of the film from the SiOx interface was observed, with the hydrated layer thickness increasing to ∼30 A˚ and containing almost 100% water. The bulk layer swelled to approximately 250 A˚ with an accompanying increase in roughness from an original thickness of ˚ Upon cooling almost half of the PMMA in the original film was 44 A. lost from the surface.

5. Conclusion ˚ were characterised in air using Ultrathin PMMA films (50–150 A) contact angle measurements, ellipsometry and XRR and in aqueous solution using NR. While films appear smooth and homogenous as dry films, close analysis of their structure under water shows formation of a hydrated layer containing approximately 50% water underneath a bulk layer, which typically contains 1–5% water. The

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hydrated layer is consistently observed, and does not appear to depend on temperature of annealing. The bulk PMMA film remains coherent, has very low water content and does not increase in thickness significantly with time. However, on heating the amount of water in the hydrated layer next to the surface increases dramatically, and the surface eventually delaminates almost completely with no loss of material. This delamination is not reversible on cooling, with loss of material observed upon cooling to ambient temperature. The observed surface delamination in aqueous solution may affect the mechanical properties of PMMA films and could have implications for its use as a protective coating on devices exposed to aqueous environments where temperature may vary. Acknowledgements This work was supported by an AINSE Post-Graduate Research Award for Peter Akers, and AINSE travel support for neutron experiments. Additional thanks to Rayomand Shahlori for assistance during NR experiments and to Nam Cao Hoai Le and Joseph Nichols for advice on preparation of PMMA films. References [1] R.Q. Frazer, R.T. Byron, P.B. Osborne, K.P. West, PMMA: an essential material in medicine and dentistry, Journal of Long-Term Effects of Medical Implants 15 (2005) 629–639. [2] N.G. Semaltianos, Spin-coated PMMA films, Microelectronics Journal 38 (2007) 754–761. [3] M. Cristina, R. Gemma, B. Xavier, P.-M. Francesc, Nanolithography on thin layers of PMMA using atomic force microscopy, Nanotechnology 16 (2005) 1016. [4] D.G. Castner, B.D. Ratner, Biomedical surface science: foundations to frontiers, Surface Science 500 (2002) 28–60. [5] S.J. Kim, B. Choi, K.S. Kim, W.J. Bae, S.H. Hong, J.Y. Lee, T.-K. Hwang, S.W. Kim, The potential role of polymethyl methacrylate as a new packaging material for the implantable medical device in the bladder, BioMed Research International 2015 (2015) 8. [6] M.M. Shalabi, J.G. Wolke, V.M. Cuijpers, J.A. Jansen, Evaluation of bone response to titanium-coated polymethyl methacrylate resin (PMMA) implants by X-ray tomography, Journal of Materials Science. Materials in Medicine 18 (2007) 2033–2039. [7] J.-W. Byun, J.-U. Kim, W.-J. Chung, Y.-S. Lee, Surface-grafted polystyrene beads with comb-like poly(ethylene glycol) chains: preparation and biological application, Macromolecular Bioscience 4 (2004) 512–519. [8] B. Jing, J. Zhao, Y. Wang, X. Yi, H. Duan, Water-swelling-induced morphological instability of a supported polymethyl methacrylate thin film, Langmuir 26 (2010) 7651–7655. [9] K. Tanaka, Y. Fujii, H. Atarashi, K.-i. Akabori, M. Hino, T. Nagamura, Nonsolvents cause swelling at the interface with poly(methyl methacrylate) films, Langmuir 24 (2008) 296–301. [10] R.P. Berkelaar, P. Bampoulis, E. Dietrich, H.P. Jansen, X. Zhang, E.S. Kooij, D. Lohse, H.J.W. Zandvliet, Water-induced blister formation in a thin film polymer, Langmuir 31 (2015) 1017–1025. [11] R. Inoue, K. Kawashima, K. Matsui, M. Nakamura, K. Nishida, T. Kanaya, N.L. Yamada, Interfacial properties of polystyrene thin films as revealed by neutron reflectivity, Physical Review E 84 (2011) 031802. [12] R. Inoue, K. Kawashima, K. Matsui, T. Kanaya, K. Nishida, G. Matsuba, M. Hino, Distributions of glass-transition temperature and thermal expansivity in multilayered polystyrene thin films studied by neutron reflectivity, Physical Review E 83 (2011) 021801. [13] A. Diethert, E. Metwalli, R. Meier, Q. Zhong, R.A. Campbell, R. Cubitt, P. Muller-Buschbaum, In situ neutron reflectometry study of the near-surface solvent concentration profile during solution casting, Soft Matter 7 (2011) 6648–6659. [14] J. Perlich, V. Körstgens, E. Metwalli, L. Schulz, R. Georgii, P. Müller-Buschbaum, Solvent content in thin spin-coated polystyrene homopolymer films, Macromolecules 42 (2009) 337–344. [15] D.E. Aspnes, A.A. Studna, Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV, Physical Review B 27 (1983) 985–1009. [16] E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, 1998. [17] N.G. Sultanova, S.N. Kasarova, I.D. Nikolov, Characterization of optical properties of optical polymers, Optical and Quantum Electronics 45 (2013) 221–232. [18] M. James, A. Nelson, S.A. Holt, T. Saerbeck, W.A. Hamilton, F. Klose, The multipurpose time-of-flight neutron reflectometer “Platypus” at Australia’s OPAL reactor, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 632 (2011) 112–123.

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