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Paraffin Wax Crystal Coarsening: Effects of Strains and Wax Crystal Shape

We developed a model of paraffin wax crystal coarsening that well describes our experimental results and allows behavior of the paraffin films to be predicted on the basis of the extracted kinetic parameters. Wax crystalline films were evaporated on different substrates (silicon wafer, glass slide, thin layer of gold on silicon), thermally treated at different temperatures, and investigated by powder X-ray diffraction, high-resolution scanning electron microscopy, and optical confocal imaging of the surfaces. Preferred (110) crystal orientation of all deposited wax films, independently of substrate type, was observed from the start and increased during heat treatment. The change in preferred orientation was accompanied by changes in crystal morphology and shape, resulting in surface nano-roughening. We modeled the process as the coarsening of oriented C36H74 crystal islands driven by the decrease in total surface energy. Coarsening kinetics was controlled by diffusion of single molecular chains along the substrate. Evolution of nano-roughness during annealing time was well described by a surface coarsening law. Two additional factors influenced the evolution rate: strains accumulated in wax crystals during deposition, and divergence of initial crystal shape from the shape at equilibrium. Both factors lowered activation energy and effectively shortened coarsening time.