A detailed structural, morphological, and magnetic characterization of the samples has previously been published [8
]. Particles of about 100 nm diameter were chosen for this study. The magnesia coating not only protects the high magnetic moment (170 ± 8 emu/g) Co-fcc core from corrosion, but also brings about further improvement by providing polar surfaces for attachment of polymers.
Figure shows FTIR measurements for samples crystallized from the melt. It is observed that increasing the concentration of the nanoparticles induces nucleation of the γ-polymorph [9
]. In contrast, when the solution crystallizes at room temperature, the electroactive β-phase is obtained; though the presence of nanoparticles modifies the polar crystal PVDF structure, as can be surmised from the presence of α-domains in Figure .
FTIR spectra of the Co-MgO PVDF nanocomposites crystallized from the melt (a), and crystallized at room temperature (b).
These results suggest that the Co-MgO nanoparticles have an influence on polymer crystallization. A large number of studies have been focused on the crystallization of PVDF resin. For example, addition of hydrate salt Mg(NO3
O to PVDF was shown to promote the formation of the β-phase [11
]. In the case of this study, hydroxylated MgO surfaces are obtained after exposure of our particles to normal air conditions. Thus, we surmise hydrogen bonding at the interface between PVDF molecules and the Mg-OH pair can preferably lead to better-oriented packing of CH2
dipoles, which is the trans
conformation. We surmise on increasing the particle loading, the stronger polarity of the hydroxyl groups. Evidences are shown in Figure , the broad absorption band attributed to the hydrogen-bonded O-H stretch.
Infrared spectroscopy and thermal analysis in nanocomposites obtained by crystallization from the melt. (a) FTIR absorption band attributed to O-H bonds (b) DSC thermograms.
The presence of the different phases and their nature was further confirmed by DSC. Thermograms for samples crystallized from the melt are shown in Figure . The samples containing nanoparticles show two melting peaks. The first one is related to the melting of the α-phase of the polymer, the second one occurring at higher temperatures relates to the melting of the γ-PVDF. It is also observed that the incorporation of the Co-MgO nanoparticles into the polymeric matrix increases its thermal stability, as surmised from the slightly higher melting temperature of the nanocomposite. In this way, a method to obtain γ-PVDF at high or moderate cooling rates from the melt is obtained, without the need to apply thermal annealing at temperatures above 160°C for long term, the usual way to obtain this phase [9
Figure shows the decline of relative dielectric constant according to increasing frequency from 100 Hz to 1 MHz, which revealed the typical model of anomalous dispersion. At a closer inspection, it is demonstrated that incorporating Co-MgO particles increase the dielectric constant for the nanocomposite samples when compared to the pure α-PVDF matrix. The maximum is obtained for the lowest concentration of 0.02 wt.% the decrease in the dielectric constant between 0.2 and 2 wt.% loadings may be a signature of dipolar interactions influencing the nanoparticles intrinsic magnetic properties [12
Electrical and mechanical behavior in nanocomposites obtained by crystallization from the melt. (a) Dielectric response as a function of frequency and composition (b) Dynamical mechanical response
For samples crystallized at room temperature (data not shown), the dielectric response of the polymer is quite similar, and shows that the nanoparticles do not improve the dielectric response of the β-PVDF. With respect to the dynamical mechanical response, on the other hand, the pure α-phase matrix has highest mechanical properties when compared with the nanocomposite samples (Figure ), which can be related to defect formation around the nanoparticles, due to the interaction with the nanoparticle shell, and a decrease in the degree of crystallinity as observed in the DSC scans.