Mechanical Properties of Composites Based on Thermosetting Polymers

3 Mechanical Properties of Composites Based on Thermosetting Polymers 

Due to their wide range of industrial uses, thermosetting polymers, in general, and epoxy resins, in particular, have been widely studied as a potential matrix for nanotube-based composites/ In the following Sections, all matrices were epoxy resins unless otherwise stated. As in previous Sections, only those papers with values of dlVdKf above the median (18 GPa) are discussed. 

The first record of the formation of epoxy nanotube composites comes from a paper by Ajayan et in 1994. In this work, nanotubes were aligned within the epoxy matrix by the shear forces induced by cutting with a diamond knife. However, no quantitative mechanical measurements were made. 

The first true mechanical study was made by Schadler et al. in 1998. They measured the stress-strain properties of a MWNT poxy composite during both tension and compression. In tension, the modulus increased from 3.1 GPa to 3.71 GPa on the addition of 5 -wt% nanotubes, a reinforcement of d I7d V( = 18 GPa. However, better results were seen in compression, with an increase in the modulus from 3.63 to 4.5 GPa, which corresponds to a reinforcement of 26 GPa. No significant increases in the strength of toughness were observed. The difference between tension and compression was explained by Raman studies which showed significantly better stress transfer to the nanotubes in compression than in tension. This can be explained by the fact that load transfer in compression can be thought of as a hydrostatic pressure effect, whereas load transfer in tension relies on the matrix-nanotube bond. However, it should be pointed out that later studies showed the reverse to be true, Le. load transfer in tension but none in eompression. In further contrast, work by Wood et al has shown that the mechanical response of SWNTs in tension and compression are identical. 

Finally, Li et al studied SWNT-epoxy composites by nano-hardness testing. They observed an increase in modulus from 4 to 7 GPa with 5 wt% SWNT (d T/dFf 94 GPa). Increases in hardness from 0.35 to 0.45 GPa were also observed. 

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