I generated several stress-strain curves and I am finding a slight decrease in the modulus and the tensile strength of the CNT/epoxy systems. Are there any explanations as to why this may be happening?
My suspicion is that at 3 wt%, the composite has reached a critical loading. Is this critical loading impacting the the adhesion of the CNT to the epoxy interface?
Any feedback would be greatly appreciated.
It appears that your system is suffering from the typical problem of nano filler aggregation. When you increase the filler content that has poor surface treatment, the filler-filler interaction overcome the dispersion role of the matrix and aggregate. Thus, the filler is no longer acting as nano fillers, which would show high aspect ratio if they are all homogeneously dispersed. Now, the filler aggregates are much bigger and approaching the micrometer size. This reduces the available surface area of the nano filler and in the extreme case, acts as the defects if the aggregate size exceed the Griffith's crack, leading to the reduction of the tensile strength. By treating the MWCNT surface either by covalent species or physical interaction of surfactant, you will improve the filler/matrix interaction thus raising the limit of the concentration at which the aggregation starts. You may wish to examine the sample at low concentration and high concentration by transmission electron miscoscopy and see if the nano fillers are well dispersed. Aggregation of nano fillers that is not surface treated typically starts around 3-5 wt%.
It appears that your system is suffering from the typical problem of nano filler aggregation. When you increase the filler content that has poor surface treatment, the filler-filler interaction overcome the dispersion role of the matrix and aggregate. Thus, the filler is no longer acting as nano fillers, which would show high aspect ratio if they are all homogeneously dispersed. Now, the filler aggregates are much bigger and approaching the micrometer size. This reduces the available surface area of the nano filler and in the extreme case, acts as the defects if the aggregate size exceed the Griffith's crack, leading to the reduction of the tensile strength. By treating the MWCNT surface either by covalent species or physical interaction of surfactant, you will improve the filler/matrix interaction thus raising the limit of the concentration at which the aggregation starts. You may wish to examine the sample at low concentration and high concentration by transmission electron miscoscopy and see if the nano fillers are well dispersed. Aggregation of nano fillers that is not surface treated typically starts around 3-5 wt%.
Dear Jamel,
Most of the degradation of physical or mechanical properties of composites (PMC, MMC, CMC) arises mainly due to poor dispersion of nanomaterials (Ex: CNTs). If a proper dispersion route is not chosen then they remain in very dense agglomerates mainly due to strong van der Waals forces between them. Most of them adopt high energy sonication, ultrasonic blending or high energy ball milling to obtain uniform dispersion. Second possiblility is lack of chemical compatibility among matrix and reinforcement which can solved by functionalization and electroless metal coating.
Refer following paper for more details
1.Effect of Nanofillers Dispersion in Polymer Matrices: A Review, Science of Advanced Materials 3, 1–25, 2011.
2. Fabrication of metal coated carbon nanotubes by electroless deposition for improved wettability with molten aluminum, Surface and Coatings Technology
Volume 212, November 2012, 207–213
3. Electroless Plating of Metals onto Carbon Nanotubes Activated by a Single-Step Activation Method, Chemistry of Materials, 1999, 11 (8), 2115–2118
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