Correspondence kart@photon.chitose.ac.jp Disciplines Biophysics Keywords Pollen Biophysics Electron Beam Pollen-like particles can be prepared by exposure of polymer microparticles to an electron beam Antonia Herzog, Konomi Uchiya, Olaf Karthaus Chemie, Freie Universität; Applied Chemistry and Bioscience, Chitose Institute of Science and Technology Abstract Plant pollen shows a wide variety of surface structures that develop during pollen sporogenesis. Some of the structures are similar to wrinkle structures that are formed when a layered material with different Young s moduli is compressed. Here we report on the formation of wrinkled surface structures in polymer particles that is caused by electron beam irradiation of poly(methyl methacrylate) microparticles. Type of Observation Standalone Type of Link Orphan Data Submitted Nov 28, 2015 Published Mar 18, 2016 3 x Triple Blind Peer Review The handling editor, the reviewers, and the authors are all blinded during the review process. Introduction Wrinkles occur in layered systems in which the components have different Young s moduli and can swell or can be compressed. Wrinkled polymer microparticles, for example, can be prepared in situ during polymerisation. Polyimide microparticles were prepared by electrospray of a precursor solution, and the evaporation of the solvent during imidisation lead to wrinkles [1]. Using another approach, Polydimethylsiloxane (PDMS) particles show dimple and wrinkles after ozone treatment and swelling with ethanol. The ozone treatment forms a hard silicon oxide layer and the difference in Young s modulus then leads to wrinkles after swelling [2] [3]. Instead of ozone, a wet chemical oxidation can also be used [4]. Simple swelling and de-swelling of a polymer particle fixed on a substrate also caused a buckling and the formation of surface dimples [5]. Inorganic core-shell microparticles can show dimple structures by thermal treatment, for example, particles with an Ag-core and SiO 2 -shell [6]. A general theory to describe labyrinth and hexagonal patterns on spherical particles has been published outlining the generality of the processes [3]. Poly(methyl methacrylate) (PMMA) is a known electron beam resist [7] in which a main chain scission of PMMA molecules by ionising radiation occurs via a stable radical species [8]. Thus, a PMMA bead should decompose and shrink upon electron beam irradiation. We have reported on the fabrication of polymer microparticles by a rapid evaporation of an oil-in-water emulsion [9] [10]. With this method, both particles from a single polymer as well as from polymer blends can be produced. Full Open Access Supported by the Velux Foundation, the University of Zurich, and the EPFL School of Life Sciences. Objective In this paper, we describe the formation of wrinkles on PMMA microbeads after coating with a thin metal layer and irradiation with a 12 kv electron beam in a scanning electron microscope. The beads were imaged in situ and we could show that a characteristic wrinkle structure appeared after a few minutes that got more pronounced over time. 4.0 Creative Commons 4.0 This observation is distributed under the terms of the Creative Commons Attribution 4.0 International License. DOI: 10.19185/matters.201603000009 Matters (ISSN: 2297-8240) 1
a b DOI: 10.19185/matters.201603000009 Matters (ISSN: 2297-8240) 2
c Figure Legend Figure 1. (A) Temporal development of the wrinkle structure during the irradiation with 12 kev electrons. The scale bar is 5 µm. (B) Dependence of the wrinkle structure upon the metal layer thickness. The metal layer thickness was not directly determined, but the thickness is directly proportional to the sputtering time. Thus a threefold longer sputtering time gives a three times thicker metal layer. The irradiation time with 12 kev electrons in the electron microscope was 10 min for all samples. Cut-out quadrants show the difference of wrinkle sharpness. The scale bar is 5 µm. (C) Left: Wrinkle patterns for core-shell and Janus particles. The scale bar is 5 µm. Right: Fluorescence microscope images of particles before metal coating that reveal the polymer phase separation for both types of particles. Polystyrene fluoresces red, PMMA green.the scale bar is 20 µm. The particles were prepared by rapid evaporation of an oil-in-water emulsion (0.2 ml of ethyl acetate solution, 1 ml aqueous solution) for which PMMA was dissolved in ethyl acetate (3 mg/ml), and the emulsion was stabilised by adding sodium dodecyl sulphate (0.1 mg/ml) to the aqueous phase, without controlling its ph [9]. The emulsion was cast on a glass substrate and allowed to evaporate at ambient temperature. Optical microscopy (Olympus BX-51) confirmed the presence of polymer beads on the substrate after evaporation. The substrate was then covered with a thin metal film by ion sputtering (Hitachi E-1010, Pd/Pt target). The sputtering time was set between 50 and 200 s at 15 ma, and then irradiated in situ in an electron microscope (Keyence VE-8800) at various acceleration voltages and for various duration. The phase separated particles were prepared by dissolving PMMA and polystyrene in ethyl acetate, each at a concentration of 3 mg/ml, and a trace of TCNQ (purchased from TCI, Tokyo) as a fluorescence marker. The emulsion was prepared described as above. The polymer phase separation was monitored by fluorescence microscopy (Olympus BX-51, blue-violet excitation). TCNQ forms a red-fluorescing charge transfer complex with polystyrene, but the TCNQ shows a weak green fluorescence in PMMA. Results & Discussion DOI: 10.19185/matters.201603000009 Matters (ISSN: 2297-8240) 3
Figure 1A shows the temporal development of the wrinkle structure on a PMMA particle that had been metal-sputtered for 100 s. The acceleration voltage of the e-beam was 12 kv and we estimate the penetration depth to be about 1 µm for an organic polymer that only contains elements with a low atom number- in our case, hydrogen, carbon, and oxygen [11]. First wrinkles appear nearly immediately when the irradiation started. The wrinkles are most pronounced on the top of the particle, because there is the highest flux of electrons per surface unit area of polymer. The sloped sides of the particles develop wrinkles at longer irradiation times. Once a wrinkle is formed, it does not change in position or in width, but it becomes deeper with ongoing irradiation. Wrinkles depend on the difference in Young s modulus of the bulk material and the hard skin layer [5]. Thus, the thickness of the metal layer should have an effect. In order to observe this, we coated the polymer particles at the same ion sputtering conditions for different times. Figure 1B shows the results of four samples in which the coating time was between 50 and 300 s. The absolute thickness of the metal layer was not directly observed, but it should be around 5 nm for 50 s, and 25 nm for 300 s sputtering, according to the operation manual. Figure 1B shows that there is a slight dependence of the wrinkle width and periodicity upon sputtering time. The longer the time, the smaller and more crispy (i.e. with sharper edges and deeper grooves) the pattern becomes, but the effect is not very pronounced, and the wrinkle structure does not depend critically on the thickness of the skin layer. In order to elucidate the effect of the thickness of the soft polymer layer, we produced core-shell particles for which polystyrene-core/pmma-shell particles had been synthesised by rapid emulsion evaporation as already reported in the literature [9]. Polystyrene is inert towards electron beam irradiation and does not decompose. Thus, polystyrene particles do not show wrinkle structures. Figure 1C shows that a thin PMMA layer on top of a polystyrene core leads to a shorter wavelength of the wrinkle structure. On the other hand, Janus-type particles show a surface structure in which the PMMA hemisphere shows the usual, large wrinkle wavelength and the polystyrene hemisphere is wrinkle-free. Conclusions In this paper, we could show that electron beam decomposition of PMMA containing polymer microparticles that had been coated with a thin metal layer can be used to prepare wrinkled particle surfaces. Even though wrinkled particles have been prepared by other methods described in the introduction, an interesting aspect of our e-beam irradiation is the particle irradiation of particles. Thus, in contrast to other methods, even the wrinkling of a selected area on a single particle should be possible in principle. This will lead to a tailor-made hierarchical surface structure in which micron-size particles are covered with a nanometer-sized surface structure. Applications may range from biocompatible surfaces to superhydrophobic coatings. Additional Information Methods The particles were prepared by rapid evaporation of an oil-in-water emulsion (0.2 ml of ethyl acetate solution, 1 ml aqueous solution) for which PMMA was dissolved in ethyl acetate (3 mg/ml), and the emulsion was stabilised by adding sodium dodecyl sulphate (0.1 mg/ml) to the aqueous phase, without controlling its ph [9]. The emulsion was cast on a glass substrate and allowed to evaporate at ambient temperature. Optical microscopy (Olympus BX-51) confirmed the presence of polymer beads on the substrate after evaporation. The substrate was then covered with a thin metal film by ion sputtering (Hitachi E-1010, Pd/Pt target). The sputtering time was set between 50 and 200 s at 15 ma, and then irradiated in situ in an electron microscope (Keyence VE-8800) at various acceleration voltages and for various duration. The phase separated particles were prepared by dissolving PMMA and polystyrene in ethyl acetate, each at a concentration of 3 mg/ml, and a trace of TCNQ (purchased from TCI, Tokyo) as a fluorescence marker. The emulsion was prepared described as above. DOI: 10.19185/matters.201603000009 Matters (ISSN: 2297-8240) 4
The polymer phase separation was monitored by fluorescence microscopy (Olympus BX-51, blue-violet excitation). TCNQ forms a red-fluorescing charge transfer complex with polystyrene, but the TCNQ shows a weak green fluorescence in PMMA. Supplementary Material Please see https://sciencematters.io/articles/201603000009. Funding Statement A.H. acknowledges a RISE stipend of the DAAD. A part of this work was conducted in Chitose Institute of Science and Technology, supported by Nanotechnology Platform Program (Synthesis of Molecules and Materials) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Ethics Statement Not applicable. Citations [1] Jin Young Park et al. Facile fabrication of superhydrophobic coatings with polyimide particles using a reactive electrospraying process. In: Journal of Materials Chemistry 22.31 (2012), pp. 16005 16010. doi: 10.1039/c2jm32210b. url: http://dx.doi.org/10.1039/c2jm32210b. [2] Derek Breid and Alfred J. Crosby. Curvature-controlled wrinkle morphologies. In: Soft Matter 9.13 (2013), pp. 3624 3630. doi: 10.1039/c3sm27331h. url: http://dx.doi.org/10.1039/c3sm27331h. [3] Norbert Stoop et al. Curvature-induced symmetry breaking determines elastic surface patterns. In: Nature Materials 14.3 (Feb. 2015), pp. 337 342. doi: 10.1038/nmat4202. url: http://dx.doi.org/10.1038/nmat4202. [4] Jian Yin et al. Surface Wrinkling on Polydimethylsiloxane Microspheres via Wet Surface Chemical Oxidation. In: Scientific Reports 4 (July 2014), p. 5710. doi: 10.1038/srep05710. url: http://dx.doi.org/10.1038/srep05710. [5] Yifan Zhang et al. Surface-mediated buckling of core shell spheres for the formation of oriented anisotropic particles with tunable morphologies. In: Soft Matter 9.9 (2013), pp. 2589 2592. doi: 10.1039/c2sm27582a. url: http://dx.doi.org/10.1039/c2sm27582a. [6] Guoxin Cao et al. Self-Assembled Triangular and Labyrinth Buckling Patterns of Thin Films on Spherical Substrates. In: Physical Review Letters 100.3 (Jan. 2008), p. 036102. doi: 10.1103/physrevlett.100.036102. url: http:// dx.doi.org/10.1103/physrevlett.100.036102. [7] Alec N. Broers. Resolution Limits of PMMA Resist for Exposure with 50 kv Electrons. In: Journal of The Electrochemical Society 128.1 (1981), pp. 166 170. doi: 10.1149/1.2127360. url: http://dx.doi.org/10.1149/1.2127360. [8] M. Tabata and J. Sohma. Degradation of poly(methyl methacrylate) by ionizing radiation and mechanical forces. In: Developments in Polymer Degradation and Elsevier Applied Science Publishers (1987), pp. 123 163. [9] Yuji Kiyono et al. Preparation and Structural Investigation of PMMA-Polystyrene Janus Beads by Rapid Evaporation of an Ethyl Acetate Aqueous Emulsion. In: e-journal of Surface Science and Nanotechnology 10 (2012), pp. 360 366. doi: 10.1380/ejssnt.2012.360. url: http://dx.doi.org/10.1380/ejssnt.2012.360. [10] Olaf Karthaus et al. Pollen-Mimetic Multiphase Polymer Microparticles. In: e-journal of Surface Science and Nanotechnology 13 (2015), pp. 204 206. doi: 10.1380/ejssnt.2015.204. url: http://dx.doi.org/10.1380/ejssnt.2015.204. [11] H. Nykänen et al. Low energy electron beam induced damage on InGaN/GaN quantum well structure. In: Journal of Applied Physics 109.8 (2011), p. 083105. doi: 10.1063/1.3574655. url: http://dx.doi.org/10.1063/1.3574655. 5