Global phase behaviour of polyphilic tapered dendrons

Abstract

We present a coarse-grained molecular dynamics study into the fluid phase behaviour of a model dendron like polyphilic molecule. Our model consists of 11 spherical beads, held in a semi flexible planar tapered arrangement, with a single apex bead and four interior beads defined to have self attraction, whilst the remaining periphery beads and all cross interactions are made softly repulsive, in essence bestowing an incompatibility between the three bead types. A parametric study was performed covering around 1000 state points in a domain of temperature and pressure spanning from regions where the fully isotropic fluid is observed, to the onset of crystallisation. Three micro-segregated thermotropic liquid crystalline phases are detected; a hexagonal columnar (HC) mesogenic phase, a supramolecular sphere body-centred cubic (SSbcc) mesogenic phase and a supramolecular sphere fluid (SSf) mesogenic phase. Each phase is characterized by order parameters based on dendron clustering distributions and spatial orientations, facilitating the phase diagram mapping. The mesogenic nature of the HC, SSbcc and SSf structured phases was confirmed visually, presenting dendron diffusion along and around the columns in the former, and intra and inter supramolecular sphere dendron diffusion in the latter two. HC and SSbcc phases were obtained by self-assembly by quenching from isotropic states, proving the stability and reproducibility of the liquid crystalline phases. Tracking of appropriate metrics gives insight into their formation mechanisms. Temperature driven first order phase transitions are seen from the HC phase to the SSf phase at higher pressures, and to the SSbcc and then SSf phase at lower pressures. For this latter transition, a Gibbs Duhem integration is used to trace the coexistence curve from a point determined by thermodynamic integration. The other transitions, including a continuous transition from the SSf to fully isotropic phase, are estimated from isothermal and isobaric simulation runs. It is of note that both the HC and SSbcc mesogenic phases resemble those reported experimentally however, the intermediate liquid-phase is an undiscovered prediction of our model. The simulations presented give physical insight on the relation between the apparent distinct phases observed. Our results shed light on the underlying relationships between these phases of self-organized supramolecular soft matter and showcase how the different experimental instances are local manifestations of an underlying generic phase diagram, which we present here.