Journal Club Theme of Sept. 1 2008: Self Healing Polymers
There is a global need to extend the lifetime of materials in order to reduce maintenance and energy costs. One route to achieve this for polymeric materials is through self-healing. There are two routes for activating a self-healing mechanism in polymer composites. Outside stimuli such as heat, light, mechanical, or chemical agents may be applied after damage has occurred in the polymer. This is effective, but requires an infrastructure (more $) to monitor and then fix the damage. A second route is to incorporate the healing mechanisms within the framework of the polymer matrix. In this case, when the material is damaged the localized damage is repaired and the crack no longer propagates through the material. For this week, I focus on two mechanisms of self-healing.
In the first paper, a mechanism for self healing using dispersion of unreacted monomer microcapsules in an epoxy is described. A stable dicyclopentadiene monomer is encapsulated within a stiff shell (urea-formaldehyde). A Grubbs catalyst is also dispersed within the epoxy matrix. The mechanism of healing occurs when a crack fractures the microcapsule to release the monomer into the crack region. The exposed catalyst reacts with the monomer to form a cross-linked network that fills the crack. The authors investigate the healing efficiency by measuring the ratio of the fracture toughness of the healed material to the fracture toughness of the virgin material.
The fracture toughness is measured using the Tapered Double Cantilever Beam (TDCB) test. The advantage of this geometry is that stable crack growth is maintained during testing.
The authors find that they can achieve about 60 % healing efficiency by using the microcapsule method. This method for self-healing is multi-functional because the presence of the microcapsules, without catalyst, improves the epoxy fracture toughness. Also, the release of healing agent may slow the crack via hydrodynamic crack-tip shielding . An interesting note is that if the epoxy is fractured again after healing the new crack propagates at the interface between the epoxy and the healing material.
The second paper is an extension of the first using a different mechanism for self-healing. In this paper an epoxy monomer/solvent mixture is encapsulated and dispersed within the epoxy. It is postulated that during fracture of the capsule, the solvent swells the surrounding matrix which promotes diffusion of residual amine sol to the crack interface. The amine reacts with the epoxy to heal the crack. The benefit of this method is that the solvent in non-toxic and the authors demonstrate 100 % healing efficiency at much lower microcapsule loadings. Analysis of the fracture surface of a healed epoxy shows that the crack again propagates around the healed region.
The third paper discusses an elegant route to promote crack healing of brittle films (silicon oxide) on ductile polymer nanocomposites (PMMA). This application would be more suitable to thin film applications. The authors utilize nanoparticle ligands and nanoparticle size to use enthalpy and entropy to alter the dispersion of nanoparticles within the polymer in response to a crack at the interface. Selecting the right ligand and nanoparticle diameter will drive the nanoparticles towards the free surface created when a crack forms between the polymer and the brittle film.
The fourth paper utilizes the concepts of the third paper to investigate the effect of nanoparticle concentration on craze formation and growth in polystyrene. While this is not a self-healing application, it illustrates how nanoparticles may impact fracture behavior in a self-healing application. The authors report that the nanoparticles do not affect craze initiation, growth, or propagation, but they do affect the maximum strain required to cause material failure. Increasing the nanoparticle concentration causes a maximum in the strain required for material failure. This maximum is attributed to nanoparticles trapped within the craze that inhibit the formation of cross-tie fibrils .
I have a few questions that might spur discussion:
- For the first two papers, the initial pre-crack in the virgin material was created using a sharp razor blade. When the material is tested after healing, the crack tip is no longer sharp. Would this be a significant contribution to the healed fracture toughness?
- Would a similar healing efficiency occur if there was a mixture of Mode I and Mode II loading?
- For the third paper, would filling the crack between the materials with nanoparticles toughen the interface?
Fracture Testing of a Self-Healing Polymer Composite, E.N. Brown, N.R. Sottos, and S.R. White, Experimental Mechanics, 42, 372-379, 2002
- Full Recovery of Fracture Toughness Using a Nontoxic Solvent-Based Self-Healing System, M.M. Caruso, B.J. Blaiszik, S.R. White, N.R. Sottos, J.S. Moore., Advanced Functional Materials, 18, 1898-1904, 2008
- Entropy-driven segregation of nanoparticles to cracks in multilayered composite polymer structures, S. Gupta, Q. Zhang, T. Emrick, A.C. Balazs, T.P. Russell, Nature Materials, 5, 229-233, 2006
- Failure Mechanism of Glassy Polymer-Nanoparticle Composites, J-Y. Lee, Q. Zhang, J-Y. Wang, T. Emrick, A. J. Crosby, Macromolecules, 40, 6406-6412, 2007