Modeling Impact of Regiodefects on the Electrocaloric Effect in Poly(VDF-co-TrFE) Copolymers.

Impact of regiodefects in the ferroelectric poly(vinylidene difluoride-co-trifluoroethylene) copolymer [poly(VDF-co-TrFE)] on the electrocaloric effect was studied with use of a developed analytical model and molecular dynamics (MD) simulations. It was shown earlier that the electrocaloric effect in these polymers is caused by the first-order phase transition from the ferroelectric ß phase to the paraelectric conformationally disordered (condis) phase. MD simulations performed in the current work show that the presence of regiodefects in polymer chains makes this phase transition more gradual, the second order. The proposed analytical model is based on the Landau phenomenological theory of phase transitions, modified to ensure the correct behavior of polymer polarization at low temperatures. The model calculates the polymer polarization, P, and temperature change under adiabatic electric field variation, ΔT, as functions of temperature, applied electric field, and regiodefect concentration. Parameters of the free energy functional are calibrated through MD simulations of the poly(VDF-co-TrFE) crystal. The obtained results show that presence of regiodefects substantially changes the form of dependencies P(T) and ΔT(T), which significantly shifts them closer to experimental data.

Surface tension model for surfactant solutions at the critical micelle concentration.

A model for the limiting surface tension of surfactant solutions (surface tension at and above the critical micelle concentration, cmc) was developed. This model takes advantage of the equilibrium between the surfactant molecules on the liquid/vacuum surface and in micelles in the bulk at the cmc. An approximate analytical equation for the surface tension at the cmc was obtained. The derived equation contains two parameters, which characterize the intermolecular interactions in the micelles, and the third parameter, which is the surface area per surfactant molecule at the interface. These parameters were calculated using a new atomistic modeling approach. The performed calculations of the limiting surface tension for four simple surfactants show good agreement with experimental data (~30% accuracy). The developed model provides the guidance for design of surfactants with low surface tension values.