Phase behavior of lyotropic rigid-chain polymer liquid crystal studied by dissipative particle dynamics

Abstract

The phase behavior of lyotropic rigid-chain liquid crystal polymer was studied by dissipative particle dynamics (DPD) with variations of the solution concentration and temperature. A chain of fused DPD particles was used to represent each mesogenic polymer backbone surrounded with the strongly interacted solvent molecules. The free solvent molecules were modeled as independent DPD particles, where each particle includes a lump of solvent molecules with the volume roughly equal to the solvated polymer segment. The simulation shows that smectic-B (S(B)), smectic-A (S(A)), nematic (N), and isotropic (I) phases exist within certain regions in the temperature and concentration parameter space. The temperature-dependent S(B)∕S(A), S(A)∕N, and N∕I phase transitions occur in the high concentration range. In the intermediate concentration range, the simulation shows coexistence of the anisotropic phases and isotropic phase, where the anisotropic phases can be the S(B), S(A), or N phases. Mole fraction and compositions of the coexisted phases are determined from the simulation, which indicates that concentration of rigid rods in isotropic phase increases as the temperature increases. By fitting the orientational distribution function of the systems, the biphasic coexistence is further confirmed. From the parameter α obtained for the simulation, the distribution of the rigid rods in the two coexistence phases is quantitatively evaluated. By using model and simulation methods developed in this work, the phase diagrams of the lyotropic rigid-chain polymer liquid crystal are obtained. Incorporating the solvent particles in the DPD simulation is critical to predict the phase coexistence and obtain the phase diagrams.

Steady shear viscosity and oscillatory complex viscosity of poly(p-phenylene terephthalamide) solutions in sulfuric acid

“…In biphasic region, an important factor needs to be considered, where the proportion of nematic phase to isotropic phases is strongly influenced by the temperature variation. The temperature rise in this range will increase the proportion of the isotropic phase to the nematic phase (Zhao and Wang 2011). As a consequence, the solution viscosity increases as the proportion of the isotropic phase increases.…”

Distortion and flow of nematics simulated by dissipative particle dynamics

“…The rods represent the mesogenic units in lyotropic systems and the normal DPD particles are used to represent coarse-grained solvent. 23 The AlSunaidi's framework for mesogens is adopted in this model. 19 After each time-step, the total force and torque on each rigid rod are computed as the sum of the forces on its constituent particles; therefore, the particles in a rigid rod move as a single rigid-body.…”

“…Phase behavior of this binary DPD fluids has been studied in our previous work. 23 Nematic phase is formed in an appropriate concentration and temperature range. In this study, rigid rods containing 9 DPD particles are used for the simulations typically under proper concentration (c = 0.9) and temperature (T = 1.0), which has been proven to exhibit the nematic phase.…”

“…In this study, rigid rods containing 9 DPD particles are used for the simulations typically under proper concentration (c = 0.9) and temperature (T = 1.0), which has been proven to exhibit the nematic phase. 23 The rodsolvent binary system at this state is used as a representative case to investigate static and dynamic behavior of the DPD nematic fluids. In some cases, the solution concentration is also adjusted to study the dependence of shear viscosity on concentration.…”

“…[19][20][21][22] We have used binary mixtures composed of rodlike chains and normal DPD particles to model lyotropic LCs, which are in nematic phase under certain concentration and temperature range. [23][24][25][26] Theoretically, DPD method exhibits the potential to model nematic fluids and can be further explored to simulate other important characteristics of the anisotropic fluids.…”