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Abstract
It is well known that the crystallization of liquids often initiates at interfaces to foreign solid surfaces. In this study, using polarized light optical microscopy, atomic force microscopy (AFM), and wide-angle X-ray scattering (WAXS), we investigate the effect of substrate–material interactions on nucleation in an ensemble of polyethylene oxide (PEO) droplets on graphite and on amorphous polystyrene (PS). The optical microscopy measurements during cooling with a constant rate explicitly evidenced that the graphite substrate enhances the nucleation kinetics, as crystallization occurred at approximately an 11 °C higher temperature than on PS due to changes in the interactions at the solid interface. This observation allowed us to conclude that graphite induces heterogeneous nucleation in PEO. By employing the classical nucleation theory for analysis of the data with reference to the amorphous PS substrate, the obtained results indicated that the crystal nuclei with contact angles in the range of 100–117° were formed at the graphite interface. Furthermore, we show that heterogeneous nucleation led to a preferred orientation of PEO crystals on graphite, whereas PEO crystals on PS had isotropic orientation. The difference in crystal orientations on the two substrates was also confirmed with AFM, which showed only edge-on lamellae in PEO droplets on graphite compared to unoriented lamellae on PS.
Highlights
We investigated the effects of the substrate–material interactions on crystal nucleation and crystal orientation in an ensemble of polyethylene oxide (PEO) droplets formed via dewetting of thin films in the molten state on highly oriented pyrolytic graphite (HOPG) and amorphous PS substrates using optical microscopy, wide-angle X-ray scattering (WAXS), and atomic force microscopy (AFM)
PEO droplets on PS and on HOPG have similar length scales and the experiments were performed under identical conditions, nucleation in PEO droplets occurred in distinctly different temperature ranges on the two substrates
To demonstrate the effects of the substrate–material interactions on nucleation kinetics and the resulting crystal orientation, we investigated crystallization from the melt in an ensemble of PEO droplets with similar base areas on HOPG and amorphous PS
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The crystallization of liquids typically starts at interfaces to foreign solid surfaces, such as substrates, the walls of containers, or small particles, e.g., impurities or nucleating agents. A solid substrate can induce the crystallization of liquids either by heterogeneous nucleation [1,2] or by prefreezing [3,4]. Prefreezing, i.e., the formation of a stable crystalline layer at the substrate interface above the melting temperature (Tm ) of the material, is an equilibrium phenomenon and does not require a nucleation event [5,6,7,8,9]
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