Directing Polymorphism of Colloid Crystals Using Conformational Entropy of Polymer Chains

Controlling the polytypes of close-packed structures of spherical colloids is still a challenging problem despite their wide occurrence. In this work, we show that systematic engineering of the polytype structures of close-packed colloids is possible by using the conformational entropy of polymer chains confined in the interstitial space of colloid crystals. Our interstitial space analysis shows that the hexagonal close-packed (HCP) structures offer larger local interstitial space domains, and the structure director chains in favor of HCP counteract the entropic advantages of the face-centered cubic (FCC) lattices. Using model block copolymer colloids and the known lattice entropy of FCC, a proportionality parameter, β_CP = 1.91 × 10^-3 ± 3.67 × 10^-4, for quantifying the conformational entropy contribution toward HCP structures is extracted. This work demonstrates that the interstitial space of colloid crystals serves as a new structure engineering tool for the self-assembly of colloids.