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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new opportunities for applications in the fields of emerging technologies (Ramajo et al. (2009); Frickel et al. (2011)). Magnetite or iron oxide (Fe 3 O 4 ) appears as a strong reinforcing candidate because of its strong magnetic response, biocompatibility and low-cost (Yang et al. (2008)). On the other hand, polyvinylidene fluoride (PVDF) is a semi crystalline polymer suitable for matrix, with thermal stability, good chemical resistance, high elasticity and extra ordinary pyroelectric and piezoelectric properties (Kepler and Anderson (1978)). PVDF crystallises in five different phases related to different chain conformations, namely α , β , γ , δ and ε phases (Lovinger et al. (1982)). The present work investigates the mechanical, thermal and electrical response of Fe 3 O 4 /PVDF nanocomposites in order to provide a deeper insight of the structure-properties relationship.

Nomenclature α α

relaxation process of glass to rubber transition

α c

relaxation process of amorphous phase restricted in the crystalline phase

 *

complex dielectric permittivity

real part of complex dielectric permittivity imaginary part of complex dielectric permittivity



 

E′ E ″ M *

storage modulus loss modulus

complex electric modulus

2. Experimental protocol

Nanocomposites were prepared by employing commercially available constituents. In particular, PVDF SOLEF 1008 was supplied by Solvay Solexis, while Fe 3 O 4 nanoparticles were obtained from Sigma-Aldrich, with average size less than 50 nm. Nanocomposites were manufactured by employing a lab scale twin screw compounder (Thermo Scientific) with counter running screws. Details for the preparation procedure can be found elsewhere (Tsonos et al. (2015)). Filler’s content in the prepared samples was 0, 5, 10, and 15% w/w. Dynamic mechanical analysis (DMA) on all prepared specimens was conducted via a TA Q800 device provided by TA Instruments, in the temperature range from 30 o C to 100 o C at 5 o C/min heating rate. The DMA experiments were carried out in the three-point bending configuration using suitable rectangular shaped specimens at f =1 Hz. Static tensile mechanical tests were made by employing an Instron 5582 device operating at 5 mm/min strain rate. Specimens’ thermal response was examined by means of Differential Scanning Calorimetry (DSC), using a TA Q200 device. Samples from all examined systems were put into aluminum crucibles, and an empty one was serving as reference. In the applied thermal cycles, temperature was varied from -40 o C to 200 o C and backwards with heating/cooling rate 10 o C/min. Each sample was subjected to two successive thermal cycles. Dielectric response was investigated by means of Dielectric Relaxation spectroscopy (DRS) in a wide frequency (10 -1 - 10 7 Hz) and temperature (30 - 120 o C) range. Specimens were placed in a two, gold plated, parallel electrode dielectric cell (BDS 1200) and measurements were conducted via an Alpha-N Frequency Response Analyzer. Temperature was controlled by Novotherm system and data recording was performed with Windeta software. Software and devices employed in the dielectric set up were all supplied by Novocontrol Technologies. DRS has been proved to be a powerful tool for the investigation of molecular mobility, phase changes, conductivity mechanisms and interfacial effects in polymers and complex systems (Psarras (2010)). Its principal operation is based on the reorientation of dipoles and the translational diffusion of charged particles by an oscillating electric field which involves measurements of the complex dielectric permittivity (  * =  ΄ - i  ΄ ) in the frequency or time domain and at constant or varying temperature. The real part of the permittivity expresses the ability of the dielectric medium to store energy and the achieved level of polarization, while the imaginary component describes the energy losses (MacDonald JR (1987)). Concerning the other techniques employed in this study, dynamic mechanical analysis (DMA) is the most similar one to the isochronal (at constant frequency) dielectric analysis. DMA measures mechanical stiffness (E ΄ ) and energy absorption (E ΄΄ ) by subjecting a specimen to an oscillating mechanical stress or strain within the linear viscoelastic region, thus, recording a variety of molecular motions