Introduction

Thermosetting polymers (e.g. epoxy systems) are being used by industry in paints, coatings, adhesives, industrial tooling, and composites, as well as in the semi-conductor and electronic packaging industries. These find applications in electronics, aerospace vehicles, civil infrastructure, and energy harvesting (e.g. wind turbine blades). The applications could directly or indirectly lead us to a greener, cleaner, and safer world.

Using thermosetting polymers, however, has its limitations. These limitations stem from the brittle failure, low toughness, excessive creep at high loads or temperatures, and poor resistance to fatigue of epoxies. These deficiencies were addressed in many studies. For example, toughness in thermosetting polymers was enhanced by the addition of rubber fillers and plasticizers. However, these fillers decrease the stiffness of the composite and lower its glass transition temperature.

The macroscopic behavior of the epoxy system depends on its molecular network architecture (e.g. cross-link density). In this project, the macroscopic mechanical behavior of epoxy systems was tuned by nano-engineering; specifically, by introducing novel carbon nanofillers in the form of 1D (Carbon nano-tubes (CNT)) or 2D (Graphene platelets (GPL)) phases in the polymeric matrix. The size and quality of dispersion of the second phases are important parameters controlling the mechanics of these composites.

Results to Date

Fatigue, fracture, and crazing in nanocomposites

By addition of Carbon nanotube fillers to epoxy system, the fracture behavior of thermosetting polymers is fundamentally altered by promoting crazing. Such an approach can significantly enhance the toughness of thermosetting polymers without incurring adverse effects such as loss of Young’s modulus or strength. Significant improvement in fracture toughness and fatigue crack growth rate in epoxies reinforced with carbon nanotubes was observed. Typical results for fatigue crack propagation (i.e. crack propagation rate vs. stress intensity factor amplitude) are shown in Fig. 3 for the baseline epoxy, epoxy with 0.25% weight pristine multiwall carbon nanotubes (MWNT) and epoxy with 0.25% weight amine-functionalized MWNT (A-MWNT).

Microscopy and modeling indicated that crack bridging by MWNT is responsible for the toughening observed at low applied stress intensity factors. The toughening observed in the A-MWNT sample is attributed to a change of the local material behavior of the epoxy. Surprisingly, crazes connecting the two crack faces are observed in these samples. Crazing is generally not observed in thermosetting polymers such as epoxies due to the very high cross-link density of the epoxy chains which inhibits fibril formation. Figure 4 shows a side view close to the crack tip. Probing indicated that these ligaments are epoxy and not MWNT. Therefore, it appears that improving the cohesion of the fillers to the matrix (via functionalization) leads to substantial modification of the mechanical behavior of the epoxy on the nanoscale. This reflects in the macroscopic mechanical behavior as the toughening observed in Fig. 3.

Thin film nanocomposites

To be able to mechanically probe the second phases, the epoxy and epoxy-GPL nanocomposites were prepared in form of thin films on silicon substrates by spin coating. This offers the advantage that the dispersed nanofillers are easy to locate in the transparent matrix, using optical and atomic force microscopy (AFM), see Figure 5. These protrusions were probed by nanoindentation and subjected to cyclic loading. The epoxy base was also probed away from inclusions. This analysis led to the conclusion that the fillers stick well to the matrix.

Creep (time-dependent) behavior in nanocomposites

The addition of graphene platelets (GPL) as the second phase, to the epoxy had an effect on the creep response at high loads and high temperatures. The creep rate is significantly reduced at higher loads and higher temperatures, while no such reduction is observed at lower loads. The steady state creep response can be approximated with

\[\varepsilon(t) \sim t^{1/q}\]

This type of power function is usually interpreted as a manifestation of hierarchical relaxation processes taking place in the material. This observation is important as the reduction of the creep rate due to GPL happens in conditions in which creep resistance is more critical, i.e. at high temperatures. Figure 5 shows the creep response of the two materials at three different temperatures.

To investigate the local creep response a flat punch (50 μm in diameter) indenter was utilized. In agreement to macroscopic behavior, nanocomposite shows less creep in the nanoscale mechanical probation. The indentation displacement can be related to the creep compliance of the viscoelastic material. Hence, this nanoscale test allows identifying a material property which is usually accessible only in macroscopic uniaxial tests. It is interesting to observe that the initial creep response of the composite and the pristine epoxy are identical.

Related Publications

1. [1]A. Zandiatashbar, C. R. Picu, and N. Koratkar, “Control of Epoxy Creep Using Graphene,” Small, vol. 8, no. 11, pp. 1676–1682, Jun. 2012, doi: 10.1002/smll.201102686. Show/Hide Abstract

   The creep behavior of epoxy–graphene platelet (GPL) nanocomposites with different weight fractions of filler is investigated by macroscopic testing and nanoindentation. No difference is observed at low stress and ambient temperature between neat epoxy and nanocomposites. At elevated stress and temperature the nanocomposite with the optimal weight fraction, 0.1 wt% GPLs, creeps significantly less than the unfilled polymer. This indicates that thermally activated processes controlling the creep rate are in part inhibited by the presence of GPLs. The phenomenon is qualitatively similar at the macroscale and in nanoindentation tests. The results are compared with the creep of epoxy–single-walled (SWNT) and multi-walled carbon nanotube (MWNT) composites and it is observed that creep in both these systems is similar to that in pure epoxy, that is, faster than creep in the epoxy–GPL system considered in this work.

2. [2]A. Zandiatashbar, C. R. Picu, and N. Koratkar, “Mechanical Behavior of Epoxy-Graphene Platelets Nanocomposites,” Journal of Engineering Materials and Technology, vol. 134, no. 3, pp. 031011–031011, May 2012, doi: 10.1115/1.4006499. Show/Hide Abstract

   Various aspects of the mechanical behavior of epoxy-based nanocomposites with graphene platelets (GPL) as additives are discussed in this article. The monotonic loading response indicates that at elevated temperatures, the elastic modulus and the yield stress are significantly improved in the composite as compared to neat epoxy. The activation energy for creep is smaller in neat epoxy, which indicates that the composite creeps less, especially at elevated temperatures and higher stresses. The composites also exhibit larger fracture toughness. When subjected to cyclic loading, fatigue crack growth rate is smaller in the composite relative to neat epoxy. This reduction is important by at least an order of magnitude at all stress intensity factor amplitudes. Optimal property improvements in the monotonic, cyclic, and fracture behaviors are obtained for very low filling fraction of approximately 0.1 wt. %. Similar differences in the mechanical behavior are observed when the composite is probed on the local scale by nanoindentation.

3. [3]A. Zandiatashbar, C. R. Picu, and N. Koratkar, “Depth sensing indentation of nanoscale graphene platelets in nanocomposite thin films,” in Symposium HH/II/JJ – Polymer-Based Materials and Composites—Synthesis, Assembly and Applications, 2011, vol. 1312, doi: 10.1557/opl.2011.125. Show/Hide Abstract

   ABSTRACTSignificant improvement of mechanical properties was observed recently in graphene platelet-epoxy nanocomposites relative to unfilled epoxy, such as an increase of the fracture toughness by 50% and dramatic decrease of fatigue crack growth rate. In this work, thin films of 0.1 wt.% of graphene platelet (GPL) – epoxy nanocomposites were fabricated and the nanoscale mechanical properties of the nanocomposite were investigated by nanoindentation. This provides information about the presence of characteristic length scales induced by the microstructure and the strength of the filler-matrix interface.

4. [4]W. Zhang, I. Srivastava, Y.-F. Zhu, C. R. Picu, and N. A. Koratkar, “Heterogeneity in epoxy nanocomposites initiates crazing: significant improvements in fatigue resistance and toughening,” Small, vol. 5, no. 12, pp. 1403–1407, 2009, Accessed: Jan. 24, 2016. [Online]. Available at: http://onlinelibrary.wiley.com/doi/10.1002/smll.200801910/pdf.
5. [5]W. Zhang, C. R. Picu, and N. Koratkar, “The effect of carbon nanotube dimensions and dispersion on the fatigue behavior of epoxy nanocomposites,” Nanotechnology, vol. 19, no. 28, p. 285709, 2008, Accessed: Jan. 24, 2016. [Online]. Available at: http://iopscience.iop.org/0957-4484/19/28/285709.
6. [6]W. Zhang, C. R. Picu, and N. Koratkar, “Suppression of fatigue crack growth in carbon nanotube composites,” Applied Physics Letters, vol. 91, no. 19, p. 193109, 2007, Accessed: Jan. 24, 2016. [Online]. Available at: http://scitation.aip.org/content/aip/journal/apl/91/19/10.1063/1.2809457.