PSI - Issue 10

A. Sanida et al. / Procedia Structural Integrity 10 (2018) 91–96 A. Sanida et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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The thermal and crystalline properties of the nanocomposites were obtained from DSC measurements, Fig.3. For all samples, polymorphism could be ascertained from the broad melting peaks with a low-temperature shoulder along side the main melting peak (Prabhakaran and Hemalatha (2013)). A systemic lowering of the melting temperature, T M , was observed as a function of increased filler loading wherein the pristine PVDF showed a T M of ~ 175 o C which reduced to 173.5 o C (5 wt% Fe 3 O 4 ) to a value of ~172.8 o C (10, 15 wt% Fe 3 O 4 ), respectively. Similarly, during the first cooling scan from the melt, a single crystallisation peak was observed with a crystallisation temperature ( T C ) of 138 o C for pristine PVDF, while for nanocomposites the narrower peaks were upshifted by 3-5 o C. This variation of the T M and T C parameters are indicative of the changes occurring in the samples upon the addition of Fe 3 O 4 nanoparticles to the polymer which, at least at lower concentrations (0-10 wt%), leads to preferential β -phase and enhanced crystallinity. At still higher concentration of Fe 3 O 4 (15 wt%), a reduction in the crystallinity was observed which could be ascribed to the inhibition effect of nanofillers on polymer chain conformation (Bhatt et al. (2011); Xu et al. (2009)).

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(a)

(b) Fig. 3. DSC thermographs depicting (a) melting during first heating and ( b ) crystallization process upon cooling.

The dielectric behaviour of a material is usually described in terms of the dielectric function  * =  ΄ - i  ΄΄ . In Figs. 4(a,b) results on PVDF nanocomposites from dielectric relaxation spectroscopy (DRS) measurements are depicted. Fig.4a shows frequency spectra of the real part ε  of the complex permittivity for all the samples at 40 ο C. It is observed that the dielectric permittivity reduces with increasing frequency and it depends on the filler content i.e. augments with the increase in the Fe 3 O 4 filler content. The enhancement of permittivity values attributed to the increment in dipolar contribution has been reported in PVDF composites (Thirmal et al. (2013)). In the MHz frequency region a “step - like” dispersion of the ε  values is recorded, which is attributed to the glass to rubber transition mechanism of PVDF matrix ( α α – relaxation ). This is a contribution to the segmental dynamics associated with the glass transition arising, from polymer chains of amorphous PVDF phase (Linares et al. (2007)). The high values of ε  at low frequencies indicate the existence of space-charge polarization and charge motion within the material related with conductivity and/or inter facial polarization, (Tuncer et al. (2005)). A mechanism giving rise to the shoulder at Hz region is discussed below. Fig.4b presents ε  (f) spectra for all samples at fixed temperature 40 ο C. These results show two dielectric dispersions. A main dielectric relaxation mechanism recorded at high frequencies whose position and magnitude allows us to relate it to the main glass-rubber transition ( α α -relaxation) of the PVDF phase (Bello et al. (1999)) and a slower relaxation peak at about 3 Hz associated with the molecular motions in the crystalline region of PVDF (Tuncer et al. (2005)). This relaxation, namely α c -process, it is attributed to the amorphous phase restricted in the crystalline phase, or to defects and chain loops or twisting (Rekik et al. (2013)). The a a relaxation process affected slightly by the presence of filler particles (Patsidis and Psarras (2008); Raptis et al. (2010)) while the magnitude of α c -relaxation peak increases slightly by the addition of filler. These results provide the evidence that the presence of nanoparticles in the composites does not apply restrictions to the mobility of amorphous chains while affects the crystalline phase of PVDF (Prabhakaran and Hemalatha (2013)).