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.