Relaxor ferroelectric (RFE) behavior is attractive for various application such as electrostriction, electrocaloric cooling, and electric energy storage, because it shows high dielectric constant and low loss. Current state-of-the-art RFE polymers include e-beam irradiated poly(vinylidene fluoride-<i>co</i>-trifluoroethylene) P(VDF-TrFE) and P(VDF-TrFE)-based terpolymers such as P(VDF-TrFE-CTFE) (CTFE is chlorotrifluoroethylene); however, their costs are too high for real applications. Here, we ask a question: can we achieve the RFE behavior in a much cheaper copolymer, i.e., P(VDF-CTFE)? Learning from the study on P(VDF-TrFE-CTFE) terpolymers, the bulky CTFE units must be included into the crystalline structure to achieve nanosized ferroelectric domains for the RFE behavior. To realize this goal, both high-pressure crystallization and low-temperature stretching were employed in this study. Under high-pressure crystallization, CTFE units could enter the crystalline phase via the paraelectric hexagonal phase. However, they were found to be excluded from large ferroelectric domains. As a result, no RFE behavior was achieved. Although low-temperature stretching decreased the crystallite size as well as the domains size due to plastic deformation, large CTFE units were still excluded outside the crystals. As a result, the RFE behavior was still not observed, although relatively slim hysteresis loops were seen. These results demonstrate that CTFE units were too large to be included into the crystalline structure of PVDF, and the initial introduction of the TrFE defects into PVDF crystals to expand the interchain spacing is important for the subsequent inclusion of CTFE.

Can P(VDF-CTFE) achieve relaxor ferroelectric behavior by inclusion of CTFE units in the crystalline phase?

256th National Meeting (2018)

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