Self Healing of Polymer Systems

Self Healing of Polymer Systems (An Abstract)

Adwait M. Deshpande

I. Introduction and Background

Self healing materials are materials which have the ability of autonomous damage recovery. However, most of the man-made self healing systems require an external stimulus and are therefore non autonomous self healing systems, wherein, most of the self healing systems encountered in nature are truly autonomous in their functionality (Ghosh, 2008).

Self healing is essential in nanomaterials because: -

• It prolongs the life of the nanomaterial being used
• Human detection of defects and intervention is difficult/impossible in some applications like spaceships, etc., especially in environments where harsh chemical and physical conditions exist
• Kinetic and thermodynamic conditions which are required for the nanosystems to function are favorable for the formation and propagation of defects

Nano-systems comprise of various functional nanostructures, thus, we can impart self healing properties to nano-systems, if we incorporate elements which have a self healing function (Drexler, 1992). The purpose of this paper is to review in brief, several self healing processes in nature and their man-made counterparts.

If we look around us, we can observe that nature had employed many self healing strategies, for the preservation of both inanimate and animate objects. Some of the most common strategies for the protection of materials are autonomous bond restoration, formation of an oxide layer on the surface, and monolayer self assembly, all of which have surface free energy minimization as the driving force.

Self healing in biological systems is a result of evolution, and therefore we cannot isolate a specific function and hope to replicate it, as it is a part of a more complex process. The overall self healing process in biological systems involve sub-processes like multifunctionality, the self-replication ability, the self-assembly ability and the markers for the activation of specific processes, to occur in coordination with each other, to ultimately achieve self healing.

Artificial self healing strategies are extremely primitive as compared to their biological counterparts. Some of the more interesting self healing approaches invented by humans for materials are auto-assembling materials, shape-memory materials and materials capable of responsive chemical reactions or reversible bonding.

II. Self Healing in Nature

Self healing via autonomous bond restoration can be observed in natural materials with nanostructure components, like the abalone shell, which is composed of multi-layers of hard aragonite lamellae and soft protein lamellae (Meyers et al., 2008). If two nanoscale surfaces are placed in contact with each other, under the right conditions of temperature and pressure, we see a formation of chemical bonds between the interfaces to form a single unit. This is not possible in macroscopic materials because they have irregular interfaces, which leads to a limited number of points of contact. Nanostructures on the other hand have soft boundaries which can adapt to reduce the interfacial energy.

Self healing in nature also occurs via the self assembly of monolayers as seen in phospholipids, micelles and thiols (Neves et al., 2001). However, in these materials, excess building blocks are required for self repair. Carbon nanotubes,  rigid macromolecules or metal nanoparticles also respond to scratches and defects by self assembly at their liquid - solid and solid - solid interfaces.

Living organisms exhibit the most complex self healing systems. Cells have microscopic dimensions and their components are sized in the order of hundreds of nanometers or less. Therefore, studying the processes within a cell can tell us a lot about how self healing occurs at a nanoscale level which then manifests itself on the macroscopic scale.

What happens in a typical biological process is that a molecule attached to a protein has to change its conformation (Barber and Anderson, 1992). However, it must get back to its original conformation in order for it to be reused. We can treat the molecule with the conformational change as "damaged", and restoring it is analogous to self healing.

Some of the processes involved in this nanoscale cycle are: -

1. Assembly of functional nanostructures
2. the part which is to be repaired moves toward the repairing site
3. the part is repaired with the help of specific nanostructures with repairing functions, i.e. the enzymes
4. the whole system has an excess of these molecules and continues to operate if a part is always in the repairing stage

Thus, using this knowledge, we can engineer nanostructures with repairing functions (enzymes) to heal damaged nanoscale materials, and through the use of an ensemble of such nanostructures, tackle a variety of different damage events.

On a macroscopic level, the repair of biological tissues in living organisms occurs by the replacement of the building blocks of the tissue, which is a single cell. On closer observation, we can conclude that the replacement of a damaged single cell by a new one is obtained by a system which has functional components working in the nanoscale, namely, biomarkers and stem cells (Fratzl, 2007).

III. Artificial Self Healing Systems

Man made self healing systems primarily consist of three approaches: -

• auto-assembly of materials,
• shape-memory materials, and
• materials capable of responsive chemical reactions or reversible bonding

Auto-assembly is the simplest technique used for achieving self healing in materials. On the nanoscale, it consists of layer-by-layer assembly of polymeric monolayers with a metal nanoparticle intralayer. After a damage event, the materials exhibit complete and autonomous recovery of their properties by means of a reversible polymer–nanoparticle self-organization process (Gupta et al., 2006).

Shape-memory materials have the ability of restoring their initial form after a plastic deformation. Shape memory materials are an original, non-biomimetic approach to self healing. However, they require an external stimulus, like increased temperature, in order for the self healing to begin (Lendlien, 2002).