Crystaline/Amorphous Transitions in Printed Polymeric Droplets

The relationship between structure of a material and cooling rate has been well studied at macro scale. However, this relationship at micro scale is not fully understood. We focus on the solidification of a droplet of polyethylene glycol (PEG) printed onto substrate at different cooling rates. Our data shown that the amorphous PEG appears in small PEG droplet while in the larger droplets, PEG spherulites locates at the center and surrounded by amorphous PEG layer which occupies the thinner, near-contact line region. Phase contrast and DSC techniques have been applied to detail the mechanism of droplet solidification. It was found that the crystal structure appears when the polymer droplet reaches a critical size. A non-steady state heat transfer model has been proposed to interpret this phenomenon.