Thermal-Annealing-Induced Self-Stretching: Fabrication of Anisotropic Polymer Particles on Polymer Films.

Designing anisotropic particles of various shapes draws great attention to scientists nowadays. We develop a facile and simple method to fabricate anisotropic polymer particles from spherical polymer particles. Poly(vinyl alcohol) (PVA) films spin-coated with polystyrene (PS) microspheres are confined on both sides using binder clips and are heated above the glass-transition temperatures of the polymers. During the thermal annealing process, the PS particles sink into the PVA films and transform to anisotropic particles. Depending on the distances to the bound regions, oblate spheroid PS particles or prolate spheroid particles with different aspect ratios can be obtained. The transformation of the particles is mainly driven by the stretching forces and the squeezing forces. The main advantage of this method is that anisotropic particles with different shapes can be fabricated simultaneously on a single film. We expect that this novel method can be helpful to various fields including colloids science, suspension rheology, and drug delivery.