Polymer Evaporative Crystallization on Water Surface*

Solvent evaporation is one of the most fundamental processes in soft matter. Structures formed via solvent evaporation are often complex yet tunable via the competition between solute diffusion and solvent evaporation time scales. This work concerns the polymer evaporative crystallization on water surface (ECWS). The dynamic and two-dimensional nature of the water surface offers a unique way to control the crystallization pathway of polymeric materials. Using poly(L-lactic acid) (PLLA) as the model polymer, we demonstrate that both one-dimensional (1D) crystalline filaments and two-dimensional (2D) lamellae are formed via ECWS, in stark contrast to the 2D Langmuir-Blodget monolayer systems as well as polymer solution crystallization. Results show that this biphasic 1D/2D structure is tunable via chemical structures such as molecular weight and polarity of the polymer and processing conditions such as temperature, evaporation speed, and interfacial tension. Only 1D filaments are formed when poly(L-lactic acid)-b-poly(ethylene oxide) (PLLA-b-PEO) block copolymer is employed in ECWS, which is attributed to the strong pinning effect of the block copolymer imparted by the hydrophilic PEO block. Our work demonstrates that ECWS provides a rich platform to tune polymer crystallization pathways.