Thermodynamic modelling of Phase Equilibrium in Nanoparticle-Filled Nematic Liquid Crystals

EZEQUIEL R. SOULÉ; JONATHAN MILETTE; LINDA REVEN; ALEJANDRO D. REY

Maribor

Congreso; European Conference on Liquid Crystals 2011; 2011

Liquid Crystal (LC) nanocomposites have received much attention as new functionalmaterials with interesting properties, due to the possible formation of different multi-scale structures,arising from the coupling between phase separation, texturing and defect formation of the LC matrix,and self-assembly of the nanoparticles1,2,3. Some major challenges in nanocomposite researchinvolves creating the means to control the extent of dispersion of the nanopartciles (NPs) in the hostmatrix, and to control the characteristics of the structures formed. Knowledge of the basicthermodynamic aspects of phase separation, phase ordering and structure formation are offundamental importance for this issue.The focus of this work is to analyze the fundamental aspects of phase separation and phaseordering, by means of a mean-field continuum thermodynamic model. This model takes into accountmixing, nematic ordering of the LC, crystalline ordering (self-assembly) of the nanoparticles, andLC-NP interactions. The nematic and crystalline orderings are represented by two non-conservedorder parameters, S and s, that measure the degree of order respectively. Four possible phases exist:Isotropic (S = s = 0), nematic (S>0, s=0) crystal (S=0, s>0), and nematic-crystal (S>0, s>0). Phasecoexistence between the possible combinations of these four phases are calculated. Phase diagramsare found to depend significantly on different model parameters, like particle size and specificinteraction parameters.The presented model can explain qualitatively some experimental observations, like theformation of NP aggregates in the isotropic phase (due to a I-I or I-C phase separation) or theformation of Schlieren textures or network structures in the nematic phase (due to different extents ofphase separation), when some experimental parameters like NP concentration or the nature of the NPsurface is changed. The model parameters that give rise to changes in these phase behaviours can bedirectly correlated with experimental variables.