Liquid crystalline behavior of a semifluorinated oligomer

Monte Carlo simulations with a coarse-grained model were performed to study the microstructure of a semifluoroalkane C20 diblock oligomer [F(CF(2))(10)(CH(2))(10)H]. The coarse-grained model adopted is based on previously reported united-atom force fields for alkanes and perfluoroalkanes and was first validated by simulating the phase behavior of a mixture of hexane and perfluorohexane. These preliminary simulations established the need of a significant correction factor in the Berthelot mixing rule between alkane and perfluoroalkane groups. Using such a force field, the semifluorinated C20 oligomer liquid was simulated using efficient Monte Carlo moves to sample different molecular arrangements and box dimensions so as to allow different layering structures to form. In qualitative agreement with experimental observations, a smectic-to-isotropic phase transition occurs as temperature is increased but the transition point and the structure of the smectic phase depend on the stiffness of the torsional potential and the model of van der Waals interactions adopted. We identify two smectic phases LC1' and LC2', whose structures do not agree with those that have been postulated before to explain x-ray diffraction data, namely, LC1 and LC2. LC1' has a layer spacing similar to LC1 but the antiparallel packing is not observed with individual chains but with groups of chains producing a checkerboard pattern. LC2' has fully microsegregated blocks such as LC2 but the alkyl tails are not fully stretched or interdigitated. Despite these inconsistencies, and considering that reported experimental data also reveal the presence of mixed phases, the simulated structures suggest other plausible ways how the semifluorinated chains could pack and microsegregate to best negotiate energetic and entropic constraints.