59th Annual Report on Research 2014

We have developed a new approach to high-performance multiferroic polymer composites via utilizing hydrogen bonding interaction to enhance the piezoelectricity of the ferroelectric polymers. Al(NO₃)₃•9H₂O (8 wt%) was added in the N,N-dimethylformamide (DMF)/acetone (1:1 v/v) solution of poly(vinylidene fluoride-co-hexafluoropropylene), P(VDF-HFP) (10 wt.% HFP). The cast ferroelectric polymer films were dried at 135 ºC under vacuum and then rapidly quenched in liquid nitrogen, followed by uniaxially stretching at 135 ºC with a draw ratio of 4 to yield the films denoted as P2. For the purpose of comparison, the pristine P(VDF-HFP) was processed under the same condition and referred to as P1. P2 was further annealed at 110 ºC for 3 hr to produce the films named as P3.

The presence of hydrogen bonding interaction between O-H from the Al(NO₃)₃•9H₂O salt and C-F from P(VDF-HFP) was clearly manifested by the emergence of a broad absorption band centered at 3400 cm^-1 in the FTIR spectra of the Al(NO₃)₃•9H₂O doped P(VDF-HFP) films P2 and P3. The fraction of the β phase was further increased from 0.62 of P1 to 0.77 and 0.82 of films P2 and P3, respectively, indicating that the addition of hydrate and subsequent annealing favored the crystallization of the β phase. Consistent with the FTIR results, the characteristic peaks at 20.3º and 36.5^o attributable to the (110, 200) and (020) diffractions, respectively, of the crystalline β phase became increasingly pronounced from the film P1 to P3 in the WXRD spectra. The crystallinity of the polymer was increased from 67% of P1 to 75% of P2 and 80% of P3. Concurrently, the crystallite size of the β phase was improved from 5.4 nm of P1 to 5.7 nm of P2 and 6.5 nm of P3. These results could be rationalized that the hydrogen bonding interactions between the salts and P(VDF-HFP) result in the interchain registration of the all-trans conformations, which consequently function as nucleation sites to improve the crystallinity and crystallite size and the content of the polar β phase of the polymer from the films P1 to P2 and P3.

In 2D WXRD patterns, the pristine P(VDF-HFP) films exhibited sharp isotropic rings, indicative of a random orientation of the crystals with respect to the normal direction of the film. For P1, P2 and P3, the c-axes of the β phase crystalline regions were preferentially oriented parallel to the drawing direction (y) because the (110/200)_β reflection appeared in the horizontal x direction in the z 2D WAXD pattern. The Herman’s orientation function, f, was increased from -0.30 of P1 to -0.33, and -0.35 of P2 and P3, respectively, signifying the improvement of the crystalline chain orientation by hydrogen bonding and post-annealing. The dichroic ratio (DR) values were found to increase monotonously from 0.6 of P1 to 0.72 and 0.74 of P2 and P3, respectively, at 842 cm^-1 and from 0.35 of P1 to 0.44 and 0.48 of P2 and P3, respectively, at 880 cm^-1, clearly manifesting that higher degrees of chain orientation were achieved by hydration and post-annealing in P2 and P3.

The enhancement of polarization ordering induced by hydrogen-bonding interaction has been substantiated in the frequency dependence of the dielectric constant of the polymer films. The dielectric constants of the polymers measured along the poling direction were reduced after poling due to the alignment of the dipoles along the direction of the applied field. The dielectric constant of P1 measured at 1 kHz decreased from 11.9 to 10.9 after poling under 200 MV/m for 5 min. at room temperature. Comparatively, the dielectric constants of P2 and P3 at 1 kHz were reduced from 13.9 to 9.8 and from 15.2 to 10.1, respectively, after poling under the same condition. A more pronounced reduction in the dielectric constant upon poling suggests that P2 and P3 films possess a better alignment of dipoles and polarization ordering. Additionally, it is evident from the dielectric relaxation spectra that the peak corresponding to the transition from the polar to nonpolar phases shifted progressively from approximately 44 °C in P1 to 82 °C in P2 and 127 ^oC in P3, again verifying the existence of higher ordering of the polar phase in P2 and P3.

Compared to the pristine P(VDF-HFP) with a P_r of 15 mC/m², P1 exhibited an improved P_r value of 47 mC/m². P_r was increased markedly to 78 and 87 mC/m² in P2 and P3, respectively, originating from the constant increase in the polar β phase and the polarization ordering from P1 to P2 and P3. Analogously, the increase of storage modulus measured by dynamic mechanical analysis (DMA) from 1.26 GPa of P1 to 1.66 and 1.87 GPa of P2 and P3, respectively, is well correlated with the enhancement of crystallinity, crystallite size and chain orientation. Since the piezoelectric coefficient is directly proportional to the remnant polarization in the PVDF based ferroelectric polymers, higher piezoelectricity was thus anticipated in P2 and P3.

The multiferroic (ME) laminate composites were fabricated by attaching the transversely poled P(VDF-HFP) films on the central part of iron-based Metglas sheet. For the P1 - Metglas composites, a ME voltage coefficient (α_ME) value of 10.5 V/cm Oe was obtained, which was increased to 19.1 and 22 V/cm Oe for P2 and P3 based composites, respectively. The achieved α_ME are compared favorably to the values of the reported multiferroic polymer composites. For example, the P(VDF-TrFE) based composites exhibit the maximum value of α_ME of 6.9 V/cm Oe, and the α_ME of Metglas/PVDF and Terfenol-D/PVDF composites are 7.2 and 3 V/cm Oe, respectively. It is important to note that the prepared composites required an applied magnetic field of only ~4 Oe to achieve a maximum ME coefficient, which is more than 50% lower than the magnetic bias of the PVDF/Metglas laminates. A low magnetic bias field is highly desirable for highly sensitive magnetic sensors and miniaturized transducers. A colossal α_ME of 320 V/cm Oe was obtained at a frequency of 68 kHz, which is attributed to the electromechanical resonance enhancement of elastic coupling interaction between the P(VDF-HFP) and Metglas layers.