Anisotropic Extended-Chain Polymer Nanocrystals

Abstract

As a concept for distinct shape polymer nanoparticles, nanoscale single crystals composed of a crystallizable chain with lyophilic end groups are explored. This differs from much studied block copolymer nanoparticles and nanostructures, in which the second (noncrystalline) blocks’ spacial demand impacts the overall structure and blurs the cores’ anisotropic shape. For precise C48 polyethylene telechelics X(CH2)46X (X = COO–M+ or CH2SO3–M+, with M+ = Na+, K+, or Cs+) as a relevant model system, a combined experimental and atomistic-level simulation study reveals them to form extended-chain, single-crystalline nanoparticles sandwiched by a layer of head groups. Their microscopic structure, order, and the resulting overall shape are decisively impacted by the mutual repulsion of the head groups, itself determined by the degree of ion pairing with the counterions and the size of the head groups. This leads to the bending of the chains at the lateral side of the crystal, preventing the particles from agglomeration, and to a chain tilt of the monolayer, thus reducing its thickness. By comparison, for a shorter analogue Cs+ –OOC(CH2)21COO– Cs+, the attractive van der Waals interactions between the hydrocarbon chains are not sufficient to overcome the head group repulsion, resulting in nanoparticle break up. These insights are instrumental for understanding and designing anisotropic organic polymer particles exploiting the principles of polymer crystallinity, which are also predestined for particle assembly.