(71a) Global Fluid Phase Diagram of a Model Dendron

Authors: 
Muller, E. A. - Presenter, Imperial College London
Crane, A. J. - Presenter, Imperial College London


We present a coarse-grained molecular dynamic 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 ?pizza slice' arrangement, with a single apex beads and four interior beads defined to have self attraction, whilst remaining periphery beads and cross interactions were made softly repulsive, in essence bestowing an incompatibility between the three bead types. A parametric study was performed over a domain of temperature and pressure spanning from the fully isotropic fluid, down to the onset of crystallisation. Three micro segregated thermotropic liquid crystalline (LC) phases were observed in this region; a hexagonal columnar (Colh) phase, a crystalline ordered aggregate (CryAgg) phase and an isotropically ordered aggregate (IsoAgg) phase. By studying metrics based on dendron clustering distributions, as well as dendron orientational order, it proved possible to fully characterise each phase, facilitating the phase diagram mapping. The LC nature of both the Colh and aggregate type phases was confirmed visually, with dendron diffusion along and around the columns in the former, and intra and inter aggregate dendron diffusion in the later two. Self assembly simulations of both the Colh and CryAgg phases from quenched isotropic states furthermore proved the fluidity of these structured phases, and by tracking our metrics gave insight into their formation mechanisms. First order phase transitions were seen, with increasing temperature, from the Colh phase to the IsoAgg phase at higher pressures, and to the CryAgg and then IsoAgg phase at lower pressures. For this latter transition a Gibbs Duhem integration was used to trace the coexistence curve from a point determined by thermodynamic integration. The other transitions, along with an observed continuous transition from the IsoAgg to fully isotropic phase, were fit from isothermal and isobaric simulation runs. It is of note that both the Colh and CryAgg LC phases resemble those inferred in a family of synthesised dendrons molecules [1,2]. While some of the observed phases have been reported experimentally as a unique cases of self-organized supramolecular soft matter [3], the intermediate liquid-phase is an undiscovered prediction of our model, others, observed in the simulation are predictions. The simulations presented give physical insight on the relation between the apparent distinct phases observed.

1. V. Percec, J. Am. Chem. Soc., 1997, 119, 1539 1555. 2. V. Percec, G. Johansson, G. Ungar, and J. Zhou, J. Am. Chem. Soc., 1996, 118, 9855 9866. 3. G. Ungar and X. Zeng Soft Matter, 2005, 1, 95-106