Polymer-Mediated Polymorphic Control over Open Colloidal Crystals

Globally ordered colloidal crystal lattices have broad utility in a wide range of optical and catalytic devices, for example, as photonic bandgap materials. However, the self-assembly of stereospecific structures is often confounded by polymorphism. Small free energy differences often characterize ensembles of different structures, making it difficult to produce a single morphology at will. Current techniques to handle this problem adopt one of two approaches: that of the “top-down,” or “bottom-up” methodology, whereby structures are engineered starting from the largest or smallest relevant length scales, respectively. However, recently a third approach for directing high fidelity assembly of colloidal crystals has been suggested which relies on the introduction of polymer co-solutes into the crystal phase. [1] By tuning the polymer’s morphology to interact uniquely with the void symmetry of a single desired crystal, the entropy loss associated with polymer confinement has been shown to strongly bias the formation of that phase. [2-3] However, previously this approach has only been demonstrated in the limiting case of close-packed crystals. Here we show how this approach may be generalized and extended to complex open crystals, illustrating the utility of this third “structure-directing agent” paradigm in engineering the nanoscale structure of ordered colloidal materials. [4] The high degree of transferability of this paradigm’s basic principles between relatively simple crystals [1-4] and much more complex ones indicates this represents a fundamental addition to presently known self-assembly techniques, and opens new avenues to better engineer the structure of these materials.

References

[1] Mahynski, Panagiotopoulos, Meng, and Kumar, Nature Communications 5, 4472 (2014).
[2] Mahynski, Kumar, and Panagiotopoulos, Soft Matter 11, 280-289 (2015).
[3] Mahynski, Kumar, and Panagiotopoulos, Soft Matter 11, 5146-5153 (2015).
[4] Mahynski, submitted (2016).