Simulation of bulk phases formed by polyphilic liquid crystal dendrimers

Abstract

A coarse-grained simulation model for a third generation liquid crystalline dendrimer (LCDr) is presented. It allows, for the first time, for a successful molecular simulation study of a relation between the shape of a polyphilic macromolecular mesogen and the symmetry of a macroscopic phase. The model dendrimer consists of a soft central sphere and 32 grafted chains each terminated by a mesogen group. The mesogenic pair interactions are modelled by the recently proposed soft core spherocylinder model of Lintuvuori and Wilson [J. Chem. Phys, 128, 044906, (2008)]. Coarse-grained (CG) molecular dynamics (MD) simulations are performed on a melt of 100 molecules in the anisotropic-isobaric ensemble. The model LCDr shows conformational bistability, with both rod-like and disc-like conformations stable at lower temperatures. Each conformation can be induced by an external aligning field of appropriate symmetry that acts on the mesogens (uniaxial for rod-like and planar for disc-like), leading to formation of a monodomain smectic A (Sm A ) or a columnar (Col) phase, respectively. Both phases are stable for approximately the same temperature range and both exhibit a sharp transition to an isotropic cubic-like phase upon heating. We observe a very strong coupling between the conformation of the LCDr and the symmetry of a bulk phase, as suggested previously by theory. The study reveals rich potential in terms of the application of this form of CG modelling to the study of molecular self-assembly of liquid crystalline macromolecules.

Relation between the grafting density of liquid crystal macromolecule and the symmetry of self-assembled bulk phase: coarse-grained molecular dynamics study

“…All these runs are performed at non-zero external pressure, as far the system is mostly density driven (out of all the non-bonded interactions, equations (1)- (3), only the mesogen-mesogen pair potential has an attractive contribution). The pressure of ¿ ØÑ is found to be quite adequate for this purpose, as was found in an earlier study [22].…”

“…It is assumed that the generic model for LC macromolecule (introduced in the previous section and shown in figure 1) is capable of self-assembling into the following bulk phases: lamellar smectic (macromolecules adopt a rod-like conformation), hexagonal columnar (macromolecules adopt a disc-like conformation) and cubic phase of possibly various symmetries. As already mentioned above, the grounds for this are to be found in both experimental [8,10,12,17] and simulation [22] studies.…”

“…The same scenario holds for at least AE ½ attached chains, and in all these cases the lamellar smectic phase is observed only. At the range of values of AE ¾ ¼, the model displays conformational bistability, discussed earlier in [22]. In this case, the symmetry of the aiding field acts as a conformation switcher.…”

“…The grounds for such coarse-graining (besides general arguments given above) are to be found in some previous simulation studies [18][19][20], where, in particular, it was found that the central core of the generation three carbosilane dendrimer is on average spherically symmetric in all (isotropic, nematic and smectic A) phases of LC solvent [18]. The models that exploit this fact have already been considered, namely in the form of a sphere with attached chains, each containing a mesogen [21,22], as well as a sphere decorated by Gay-Berne particles directly on its surface [23]. The number of bulk phases have been found in these simulation works.…”

“…Essential speed-up for microphase separation can be achieved by using soft potentials [e.g., equations (1)- (3)], since in this case the beads are semi-transparent and may overlap and cross each other during the equilibration (see, e.g., [21,24,[28][29][30][31][32]). However, for the case of the model depicted in figure 1, the spontaneous self-assembly was still found to typically lead to the polydomain (globally isotropic) phase, both in the case of slow cooling down or slow compressing (the results for AE ¿¾ chains are discussed earlier [22]). Similarly to these findings, spontaneous self-assembly at a broader interval of values of AE turns out to be also more "hit and miss".…”