Fabrication, characterization, and dynamic behavior of polyester/TiO2 nanocomposites

By Victor M.F. Evora and Arun Shukla

Materials Science and Engineering, A 361 (2003) 358-366

Purpose of Study

Use experimental techniques to study the effect of nanosized particles on the bulk mechanical properties of nanocomposites.

Methods

Polyester resins were used as the matrix material and were loaded with TiO2 nanoparticles. The nanocomposite was fabricated using a casting process which included mechanical mixing, deaeration  and ultrasonication. Ultrasonification was employed to produce nanocomposites with uniform distribution of nanofibers which was verified by using TEM. Mechanical properties including tension, compression, quasi-static fracture toughness (3-point bending test) and dynamic fracture toughness (split Hopkinson pressure bar) were measured. Scanning electron microscopy is used to characterize the fracture surfaces.

Key Findings

1. It is observed that agglomeration of nanoparticles leads to significant decrease in quasi-static fracture toughness. There is a slight decrease in properties beyond a volume percentage of 1 % (nanofillers)which is attributed to clustering of nanoparticles. This is different from previous researchers who show a consistent increase in fracture properties with increase in volume percent of nanofillers.
2. Fracture toughening mechanisms are identified to be crack pinning and crack trapping using SEM photography. Thumbnail type markings and out-of-plane flakings exist in nanocomposites (in contrast with neat polyester where they are absent) which indicate the requirement of greater energy to form fracture surfaces.
3. The dynamic fracture toughness is observed to be higher than its quasi-static counterpart for all volume fractions. This was attributed to the rate dependency of the material.
4. Only moderate changes were observed in case of quasi-static mechanical properties like compression and tension. This is due to the weak interfacial bonding between the matrix and filler.

Prediction of effective moduli of carbon nanotube-composites with waviness and debonding

L.H.Shao, R.Y.Luo, S.L.Bai, and J.Wang. Composite Structures 87 (2009) 274-281

Purpose of the Study

To investigate the effects of waviness of CNT and interfacial debonding (between CNT and matrix) on the effective moduli. Analytical methods are presented to study the effect of these parameters on the effective moduli of the nanocomposite.

Methods

Micromechanics models are presented to analyze the influence of nanfiber waviness and debonding. In each case an RVE is considered in the analysis. For waviness, the nanofiber is projected onto the vertical and horizontal directions, and then the effective properties are calculated. In case of debonding, the total number of fibers is divided into partially debonded, fully debonded, and perfectly bonded fibers.

Key findings

1) It is shown that debonding and waviness can significantly reduce the stiffening effect of nanotubes, inspite of their high stiffness.

2) The effective elastic constants of the composites are shown to be very sensitive to the waviness in case of small waviness.

3) Other important mechanical properties including effective shear modulus and effective bulk modulus are calculated using the analytical method presented. While the effective shear modulus increases with volume fraction for partial debonding, it decreases when all the fibers are assumed to be debonded.

4) The load transferring mechanisms between the nanotube and matrix are acknowledged to be complicated when waviness and debonding simultaneously exist.

Key Terms

CNT: Carbon NanoTube

RVE: Representative Volume Element

Fundamental aspects of nano-reinforced composites

Bodo Fiedler, Florian H. Gojny, Malte H.G. Wichmann, Mathias C.M. Nolte, Karl Schulte

Composites Science and Technology, 66 (2006) 3115-3125

Purpose of Study

To understand the importance and challenges of using CNT’s as nanofillers in polymers. Particle size relations, seperation and volume content are analyzed analytically. The influence of the manufacturing process is assessed and the resulting mechanical properties are investigated.

Methods

1) Analytical methods are used to characterize fundamental aspects of particle size and separation. An average separation distance for randomly arranged particles is calculated based on volume percent of fibers.

2) CNT-polymer composites are manufactured utilizing chemical functionalization of the particle surface. This is achieved by oxidative treatment of the CNT  to enable direct bonding to the matrix. Further, dispersive techniques of sonication, stirring, and calendering are used to prevent agglomeration of fibers.

3) Fracture toughness values are measured for polymer-CNT specimens. TEM microscopy images are observed to show the phenomenon of crack bridging.

Key Findings

1) An increase in fracture toughness is observed with increased filler content. A significant enhancement is also observed in composites containing spherical nano-particles. The increase is attributed to nanofiber bridging of the matrix cracks and enhanced interfacial stresses.

2) It is seen that dispersion of nanofibers aids in increase of mechanical properties. This is due to the prevention of agglomeration which will cause weak spots inside the matrix.

3) Increase in particle content volume is seen to reduce the surface separation between spherical particles. Analytical methods show that spheres have a greater seperation compared to hexagonal fibers.

Key Terms

CNT-Carbon Nanotube

Mechanical Property Characterization of a Polymeric Nanocomposite reinforced by Graphitic Nanofibers with Reactive Linkers

L.R. Xu, V. Bhamidipati, W. Zhong, J. Li, C. Lukehart, E. Lara-Curzio, K.C. Liu and M.J. Lance

Journal of Composite Materials, Vol. 18, No. 38, 2004

Purpose of the study

Systematic mechanical property characterization for a newly functionalized nanofiber/epoxy nanocomposite including bending, tensile and fracture properties was carried out.

Methods

Experiments were carried out on polymeric matrix nanocomposites with GCNF’s as reinforcements. Chemical modification of the nanofiber surface sites and ultrasonic processing methods were used to in order to improve interfacial bonding and to disperse the nanofibers uniformly within the matrix respectively. Test specimens of standard dimensions were prepaed and tested for fracture strength, bending and tensile strength.

Key Findings

1. Experimental results show that there was little increase in mechanical properties of the nanocomposites inspite of the use of reactive linkers to improve interfacial strength. In some cases (like in bending strength determination), a decrease in strength was observed. Interestingly, even the addition of 10 wt % of nanofiber does not increase the bending strength of the nanocomposite significantly. It is concluded that a strong interface is a necessary condition but not a sufficient one for stronger nanocomposites.
2. Agglomeration of nanofibers was found to be a major cause of the decrease in strength. Low power sonication was utilized to decrease agglomeration.
3. The interfacial stress level of nanocomposites is determined to be much higher than that of traditional composites and this is explained as due to the high stress mismatch between the carbon nanofibers and the epoxy matrix.
4. The authors conclude that aligned nanofibers with a large weight or volume percent must be utilized in designing strong and stiff nanocomposites. They also suggest that the length of the nanofiber be long and the diameter not too small.

Key Terms

GCNF – Graphitic Carbon NanoFiber

Nanocomposites – Are a novel class of materials in which one of the constituents has a dimension in the range of 1 nm – 100 nm.