58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

We have prepared high performance multiferroic polymer composites based on the chain-end cross-linked ferroelectric poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE). The synthetic strategy involves the copolymerization of VDF and TrFE using the triethoxylsilane-based benzoyl peroxide 1 as the initiator in anhydrous acetonitrile. The ¹H NMR spectrum of the resulting P(VDF-TrFE) clearly shows the peaks at 1.18 and 3.82 ppm attributed to the methyl and methylene protons from ethoxyl groups and the resonances at 7.78 and 8.03 ppm assigned to the aromatic protons, indicating the presence of triethoxylsilane end groups. The cross-linking reaction were performed by heating the cast triethoxylsilane-terminated P(VDF-TrFE) films in vacuo to yield polysilsesquioxanes in the films, which was evidenced by the emergence of the characteristic bands at 1100 cm^-1 corresponding to asymmetric stretching of Si-O-Si in the FTIR spectra. As expected, the elastic modulus revealed in dynamic mechanical analysis (DMA) increases from 2.83 GPa of the pristine P(VDF-TrFE) (75/25 mol%) to 3.14 GPa of the cross-linked films.  

The cross-linked P(VDF-TrFE) films with a thickness of ~25 μm were then sputtered with 50 nm thick gold at both sides and transversely poled by a 100 MV/m electrical field at 55 ^oC for 5 min. The two-layer ME laminate composites were obtained by attaching the poled P(VDF-TrFE)-polysilsesquioxane thin films which is 7 mm long and 4 mm wide on the central part of 25-μm thick iron-based Metglas using epoxy resin. The dimension of Metglas was set at a length of 30 mm and a width of 4.5 mm to employ the magnetic flux concentration effect. Metglas was chosen as the magnetostrictive phase because of its highest piezomagnetic coefficient (~ 4 ppm/Oe) among the known materials. For the purpose of comparison, the pristine P(VDF-TrFE)/Metglas laminate composites were also prepared following the same procedure.

 The dependence of the ME voltage coefficient α_ME on H_dc for the longitudinal-transverse two-layer laminate composites has been measured at 20 Hz. For the cross-linked P(VDF-TrFE) - Metglas composites, a α_ME value of 17.7 V/cm Oe was achieved under a very low dc magnetic field of 3.79 Oe, whereas the pristine P(VDF-TrFE) based composites exhibit the maximum value of α_ME of 6.9 V/cm Oe under the same dc magnetic field. A low magnetic bias field is highly desirable for high sensitivity magnetic sensors and miniaturized transducers. In addition, a large phase shift from -180^o to 0^o occurs as the direction of the magnetic bias is changed on the order of 0.4 Oe; this offers a great potential of the measurement of small moment or spin of the magnetic field. The frequency dependence of the ME voltage coefficient of the laminates has been studied in the frequency range from 20 to 100 kHz. It can be observed that the composites have a dramatically increased coefficient at the frequency of 65 kHz, as high as 383 V/cm Oe, corresponding to the longitudinal electromechanical resonance that significantly enhances the elastic coupling interaction between the P(VDF-TrFE) and Metglas layers. The measured ME coupling and field sensitivity from the cross-linked P(VDF-TrFE) also compares favorably with other multiferroic polymer composites. For example, the ME voltage of Metglas/PVDF has been reported to be α_ME = 7.2 V/cm Oe under a dc magnetic bias of 8 Oe Hz and up to 310 V/cm Oe in the resonance frequency, while the PZT/PVDF and Terfenol-D/PVDF composites have been shown a α_ME value of 3 V/cm Oe.

To understand the giant ME effect observed in the cross-linked P(VDF-TrFE) based composites, we first examined the polarization of the copolymers under the electric field. The polarization-hysteresis loops have been measured at 100 MV/m on the poled cross-linked and pristine P(VDF-TrFE) films, respectively. Compared to the pristine P(VDF-TrFE) with a remanent polarization (P_r) of 0.064 C/m², the cross-linked P(VDF-TrFE)s exhibit a square-shaped D-E hysteresis loop with a much increased P_r of 0.096 C/m². The average slopes of the ε-E loops gave the longitudinal piezoelectric coefficient d₃₃ of -38.9 pm/V for the cross-linked P(VDF-TrFE) and -32.5 pm/V for the pristine copolymers.

The enhancement of the remanent polarization in the cross-linked copolymers is attributable to the improvement of polarization ordering in the cross-linked films. The dielectric constant of the pristine copolymers measured at 20 Hz decreases from 11.4 to 10.0 after poling at room temperature, and is reduced further to 9.6 for the samples poled at 55 ^oC. Comparatively, the cross-linked copolymer exhibits a slightly higher dielectric constant of 12.3 at 20 Hz than the pristine copolymer, probably due to the dipolar contributions by the presence of polar groups such as O-H and Si-O in the polysilsesquioxane segments. The dielectric constant of the cross-linked copolymers is reduced to smaller values of 9.0 after poling at room temperature and 8.6 after poling at 55 ^oC. A more pronounced decrease in the dielectric constant upon poling suggests that the cross-linked copolymers possess a better alignment of dipoles and polarization ordering. This same trend is also reflected in the dependence of the dielectric constant on temperature of the copolymers. Additionally, it can be clearly seen from the dielectric relaxation spectra that the Curie transition is elevated from approximately 126 °C in P(VDF-TrFE) to 134 °C in the cross-linked P(VDF-TrFE). This further indicates that the chain-end cross-linking tends to favor the formation of higher ordering of the polar phase in the copolymers.

It has been suggested that intermolecular hydrogen bonding and dipolar interactions can preferably lead to the formation of the ordered all-trans conformation and the β phase. The presence of the hydrogen bonding interactions between the remaining O-H from the polysilsesquioxanes and C-F from P(VDF-TrFE) is manifested by the broad absorption band from 3210 to 3660 cm⁻¹, corresponding to the hydrogen bonded O–H stretching, in the FTIR spectra of the cross-linked films. Furthermore, the presence of a strong vibration peak at 1715 cm^-1 assigned to C=O vibration implies the hydrogen bonding interaction between C=O and C-F. It is found that the contents of the all-trans conformation are nearly identical (~77%) in the cross-linked and pristine P(VDF-TrFE)s.