Liquid crystalline behavior of a coarse-grained model of shape-persistent macrocycles with flexible attractive chains

Abstract

Shape-Persistent Macrocycles (SPMs) are synthetic organic macromolecules characterized by a rigid open hoop or ring with flexible side groups pointing either towards the inside or outside of the structure. In a previous work, we have studied the liquid crystalline behavior of a Coarse-Grained model of SPMs using Molecular Dynamics simulations [C. Avendaño and E. A. Müller, Phys. Rev. E, 2009, 80, 061702]. In the aforementioned model, the system tends to organize in a rather unusual smectic-A phase in which the particles are aligned perpendicularly to the layers, i.e. the system presents antinematic order. In this work, this model is extended to study the effect of flexible chains attached to the macrocycles on the self-assembly of SPMs. Considering flexible chains linked to the SPM backbone allows a qualitative comparison with real systems where the chains play an important role in the supramolecular organization of the system. In our model particles are composed of a rigid hexagonal arrangement of 24 soft-repulsive spheres with chains of attractive spheres grafted into the cycle. Two different cases (isomers) have been considered in this work depending whether the attached chains are pointing inward (intraannular substituents) or outward (extraannular substituents) from the rigid structure. The isomers self-assemble into contrasting liquid crystalline phases upon temperature quenching from an isotropic state. It is observed how systems with extraannular substituents tend to order at high densities into a lamellar phase with the chains laying in between the layers. The orientation of the rings in the layers shows antinematic order. For SPMs with intraannular substituents, the systems exhibit a transition from an isotropic phase to an hexagonal-disordered columnar phase, where the attractive chains fill the inner voids of the ring and help to stabilize the columns. The liquid crystalline phases in both models are influenced by the amphiphilic nature of the different components of the model.