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A combined analytical and molecular dynamics model for the electrocaloric effect in ferroelectric poly(vinylidene difluoride‐co‐trifluoroethylene) copolymer (poly(VDF‐co‐TrFE)) is developed. The model calculates the polymer polarization, P, and temperature change under adiabatic electric field variation, ΔT, as functions of temperature. An analytical component of the model is based on the Landau phenomenological theory adapted for modeling of the first order phase transitions in a polymer crystal from a ferroelectric β phase to a paraelectric conformationally disordered (condis) phase. Parameters of the free energy functional are calibrated through molecular dynamics simulations of poly(VDF‐co‐TrFE) perfect crystal. Random orientation and the scatter of the phase transition temperature for various crystallites in a real amorphous‐crystalline polymer are incorporated into the model. Comparison of the model prediction with experimental data shows good agreement for PT while the model overestimates the value of ΔT by approximately 2.5 times. We attribute this discrepancy to the presence of structural defects in real polymer crystallites, which reduces the entropy change under the phase transition compared to the perfect crystal simulated in our molecular dynamics approach. The theoretical limit of ΔT calculated by the model indicates that ΔT can be increased up to 3 times compared to the currently observed experimental value of ΔT. [ABSTRACT FROM AUTHOR]

Molecular-dynamics simulations were used to study the structure of water clusters in different perfluorinated ionomers. The ionomers included in this work were Perfluorosulfonic Acid (PFSA) and Perfluoroimide Acid (PFIA). Different shapes and sizes of water domains in PFSA and PFIA were observed. In PFSA, ionomer water domains have spherical-like shape while water domains have complicated branch structure in PFIA ionomer. The mean water-cluster size for PFSA and PFIA ionomers as a function of water content lie on one universal curve, which indicates that the size of water clusters depends primarily on water content and weakly on ionomer chemistry. In PFSA, almost all hydronium ions are located predominantly close to the water cluster/backbone interface for both large and small values of water content; while in PFIA a relatively large fraction of hydronium ions are distributed within the bulk of the water clusters. A similar distribution of acidic groups was observed in both ionomers. In PFSA ionomer, all sulfonic groups are located in the surface water layer at the distance from 3 Å to 6 Å from the backbone. In PFIA ionomer, approximately 60% of sulfonic groups are located in the surface water layer and 40% of sulfonic groups are immersed into the bulk of water cluster. [ABSTRACT FROM AUTHOR]