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Journal Club Theme of Sept. 1 2008: Self Healing Polymers

For this journal club, I have chosen a topic that might be fresh on everyone’s mind considering it was the focus of the August issue of the MRS Bulletin. The general topic is 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:

  1. 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?
  2. Would a similar healing efficiency occur if there was a mixture of Mode I and Mode II loading?
  3. For the third paper, would filling the crack between the materials with nanoparticles toughen the interface?

zhan-sheng guo's picture

gone with the wind

Zhigang Suo's picture

Please take a look at this post, "Having difficulty with posting comments?"  This post is also placed on the right side of iMechanica, under Quick guide.

zhan-sheng guo's picture

thanks prof Suo, i got that.

Thanks Aaron. This is a very interesting topic. I think that Mother Nature has aleady developed some recipes for self-healing. Do you know any good papers that describe the self-healing mechanisms in natural materials?

Thanks alot!

MichelleLOyen's picture

The answer of course is that in nature things are more complicated!  The complicating factor is the living element, the cells that can "hear" signs of distress, such as a fracture, go to the area, clean it up, deposit new material and then remodel it.  The whole idea of "self-healing" systems is fundamentally different in natural versus engineered systems because the mechanisms of healing are so different--at least for now.  In the grand world of science fiction nanotechnology where there are tiny self replicating robots, in the future a more biologically-active self-healing response may be possible.  For the moment, however, we are stuck with what are some very clever engineering solutions that take a different route!

 Michelle you are exactly right.  The natural self healing process is quite complicated. It involves a cascade of biological signals that build upon each other to locate damage, dispatch the appropriate response to stop trauma, then rebuild tissue.  In the engineering sense, the natural response is facilitated by cells that are multifunctional which are difficult to replicate in structural materials.  1 and 2 below are included for further reading on the bridge between biomimetic and synthetic engineering.  Michelle's journal club (http://www.imechanica.org/node/2509) was another great look at the bridge between biology and engineering. Right now the dispersion of monomer and crosslinker within the matrix is a first active approach to healing fracture.  3 is a review of nanotube applications for sensing and actuation.  These could be coupled with active methods to find and replace fractures or crazes before they propagate further.  Certainly, science is getting closer to creating synthetic scaffolds that allow cells to rebuild tissues.  Hopefully as this research progesses further we can make links between the two fields.  

 

 1.  Biomimetics for next generation materials (http://journals.royalsociety.org/content/xqk3gk6647k17852/)

Focus on bimimetic materials in engineering that were inspired by nature.

2. Biomimetic materials research: what can we really learn from nature’s structural materials?

(http://apps.isiknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=1&SID=1ALD2N8bG3lMP1bM1Go&page=1&doc=4)

Discussion of the hierarchical structures in nature and their relationship to engineering materials.

 3.  Review of nanotube composites for sensors and actuators

http://apps.isiknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=4&SID=2Ap53F9biMMooL9efG4&page=1&doc=1

 

 

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