PSI - Issue 10

A. Sanida et al. / Procedia Structural Integrity 10 (2018) 257–263 A. Sanida et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 6. (a) Young’s modulus ; (b) tensile strength as a function of concentration, for the nanocomposites with SrFe 12 O 19 nanoparticles.

In heterogeneous polymer systems the mechanism of micromechanical deformations and consequently, the macro scopic properties of the polymers are determined by local stress distribution around the inclusions (Abraham et al. (2009)). Because the adherence of filler to polymer in the nanocomposites, the nanoparticles form a network that is able to carry the load. The tensile strength of the nanocomposites at low contents was similar, or even slightly improved than that of the neat epoxy thermoset. Stress distribution is created around the particles, increasing the nanocomposite strength further. The nanocomposites showed a small increment in its ultimate stress value, while higher filler content induced a decrease of tensile strength in most cases. A problem in particulate filled composites is the poor stress transfer at the filler-polymer interface because of the poor adherence of the filler to the polymer. The nanocomposites with high concentration in ceramic nanoparticles, exhibited lower strength than the neat epoxy thermoset which can be interpreted in terms of an increasing susceptibility of the aggregation of nanoparticles and weak interfacial adhesion between them and the epoxy matrix. From Figs.2-6 it is observed that higher nanoparticle content favours their aggregation, and the aggregates may cause a high stress concentration and premature failure. Overall, SrFe 12 O 19 nanocomposites exhibit the optimum mechanical behaviour in almost all examined cases. The mechanical improvement is also confirmed by DMA tests, as depicted in Figs.7 and 8. In the spectra of all systems, a step like decrease of storage modulus (Figs.7a, 8a) is observed at 45 to 70 o C indicating the presence of  - relaxation process, which is attributed to the glass to rubber transition of the polymer matrix. The incorporation of nanoinclusions resulted in an increase of the storage modulus over the whole temperature range for all examined systems. Indeed, all the nanocomposites showed a higher storage modulus than the neat epoxy thermoset, both in the glassy ( T < T g ) and rubbery states ( T > T g ). This kind of transitions are expressed with the formation of peaks in the loss modulus diagrams. The characteristic glass transition temperature was determined by peak maxima of the loss modulus diagram. The presence of nanoparticles at low concentrations seems to shift the transition range to higher temperatures. Apparently, glass transition temperature (T g ) is expected to increase with filler content. Shifting T g to higher values with the addition of nanometric particles is considered as a strong indication for good adhesion between matrix and inclusions and suggests the occurrence of attractive interactions between macromolecules and nanoparticles hindering the macromolecular motion. At high concentrations the results are mixed with ZnFe 2 O 4 and BaFe 12 O 19 systems exhibiting higher T g values, while SrFe 12 O 19, Fe 3 O 4 and Y 3 Fe 5 O 12 filled nanocomposites demonstrating lower values even than the ones of the neat matrix because the nanoparticles are adjacent to each other making particle-particle interactions stronger thus lifting part of the mobility hindrance.

4. Conclusions

Five different series of nanocomposites consisting of epoxy resin and ferrite nanoparticles (YIG, ZnFe 2 O 4 , Fe 3 O 4 , BaFe 12 O 19 and SrFe 12 O 19 ) were successfully fabricated and characterized morphologically via SEM. Their thermomechanical properties were studied via DMA and static tensile tests. From the experimental data, it seems that the incorporation of the ceramic nanoparticles enhances significantly both the storage and the tensile modulus of