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Discipline
Biological
Keywords
Pollen
Biophysics
Electron Beam
Observation Type
Standalone
Nature
Orphan Data
Submitted
Nov 28th, 2015
Published
Mar 18th, 2016
  • 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.

  • Figure
  • 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. 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. Instead of ozone, a wet chemical oxidation can also be used. Simple swelling and de-swelling of a polymer particle fixed on a substrate also caused a buckling and the formation of surface dimples. Inorganic core-shell microparticles can show dimple structures by thermal treatment, for example, particles with an Ag-core and SiO2-shell. A general theory to describe labyrinth and hexagonal patterns on spherical particles has been published outlining the generality of the processes. Poly(methyl methacrylate) (PMMA) is a known electron beam resist in which a main chain scission of PMMA molecules by ionising radiation occurs via a stable radical species. 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. With this method, both particles from a single polymer as well as from polymer blends can be produced.

  • 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.

  • Results & Discussion

    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. 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. 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. 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.

  • 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. 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.

  • 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.

  • References
  • 1
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    Matters11/20

    Pollen-like particles can be prepared by exposure of polymer microparticles to an electron beam

    Affiliation listing not available.
    Abstractlink

    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.

    Figurelink

    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.

    Introductionlink

    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 SiO2-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]. With this method, both particles from a single polymer as well as from polymer blends can be produced.

    Objectivelink

    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.

    Results & Discussionlink

    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.

    Conclusionslink

    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.

    Methodslink

    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.

    Funding Statementlink

    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.

    Conflict of interestlink

    The authors declare no conflicts of interest.

    Ethics Statementlink

    Not applicable.

    No fraudulence is committed in performing these experiments or during processing of the data. We understand that in the case of fraudulence, the study can be retracted by ScienceMatters.

    Referenceslink
    1. Jin Young Park, Kyung Ok Oh, Jong Chan Won,more_horiz, Yong Seok Kim
      Facile fabrication of superhydrophobic coatings with polyimide particles using a reactive electrospraying process
      Journal of Materials Chemistry, 22/2012, pages 16005-16010 DOI: 10.1039/c2jm32210bchrome_reader_mode
    2. Derek Breid, Alfred J. Crosby
      Curvature-controlled wrinkle morphologies
      Soft Matter, 9/2013, pages 3624-3630 DOI: 10.1039/c3sm27331hchrome_reader_mode
    3. Norbert Stoop, Romain Lagrange, Denis Terwagne,more_horiz, Jörn Dunkel
      Curvature-induced symmetry breaking determines elastic surface patterns
      Nature Materials, 14/2015, pages 337-342 DOI: 10.1038/nmat4202chrome_reader_mode
    4. Jian Yin, Xue Han, Yanping Cao, Conghua Lu
      Surface Wrinkling on Polydimethylsiloxane Microspheres via Wet Surface Chemical Oxidation
      Scientific Reports, 4/2014, page 5710 DOI: 10.1038/srep05710chrome_reader_mode
    5. Yifan Zhang, Teng Lu, Xiping Zeng,more_horiz, Lei Jiang
      Surface-mediated buckling of core–shell spheres for the formation of oriented anisotropic particles with tunable morphologies
      Soft Matter, 9/2013, pages 2589-2592 DOI: 10.1039/c2sm27582achrome_reader_mode
    6. Guoxin Cao, Xi Chen, Chaorong Li,more_horiz, Zexian Cao
      Self-Assembled Triangular and Labyrinth Buckling Patterns of Thin Films on Spherical Substrates
      Physical Review Letters, 100/2008, page 036102 DOI: 10.1103/physrevlett.100.036102chrome_reader_mode
    7. Alec N. Broers
      Resolution Limits of PMMA Resist for Exposure with 50 kV Electrons
      Journal of The Electrochemical Society, 128/1981, pages 166-170 DOI: 10.1149/1.2127360chrome_reader_mode
    8. M. Tabata, J. Sohma
      Degradation of poly(methyl methacrylate) by ionizing radiation and mechanical forces
      Developments in Polymer Degradation, Elsevier Applied Science Publishers, 1987, pages 123-163 chrome_reader_mode
    9. Yuji Kiyono, Laszlo Szikszai, Junichi Watanabe,more_horiz, Hans-Gerd Löhmannsröben
      Preparation and Structural Investigation of PMMA-Polystyrene 'Janus Beads' by Rapid Evaporation of an Ethyl Acetate Aqueous Emulsion
      e-Journal of Surface Science and Nanotechnology, 10/2012, pages 360-366 DOI: 10.1380/ejssnt.2012.360chrome_reader_mode
    10. Olaf Karthaus, Akihiro Kikukawa, Pacal Acker, Philipp Polzin
      Pollen-Mimetic Multiphase Polymer Microparticles
      e-Journal of Surface Science and Nanotechnology, 13/2015, pages 204-206 DOI: 10.1380/ejssnt.2015.204chrome_reader_mode
    11. H. Nykänen, P. Mattila, S. Suihkonen,more_horiz, M. Sopanen
      Low energy electron beam induced damage on InGaN/GaN quantum well structure
      Journal of Applied Physics, 109/2011, page 083105 DOI: 10.1063/1.3574655chrome_reader_mode
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