Materials World Network: Healing Polymers: The Self Assembly Approach

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A Materials World Network program, which consists of institutions from the US (Case Western Reserve University and Michigan State University) and the UK (University of Reading), has been created to investigate the potential of supramolecular polymers as a new class of healable materials. It is proposed that the dynamic nature of the non-covalent bonds that play a critical role in the structure of a supramolecular polymer offers the possibility of creating films and coatings that exhibit quick and easy thermally-induced healing capability. To achieve this goal this Network has brought together researchers from the fields of polymer chemistry, supramolecular chemistry, rheology, nanocomposites, engineering and microscopy. Traditional thermoplastics can be healed using a process called hot plate welding. It has been proposed that this process involves a number of steps (surface rearrangement, surface approach, wetting, diffusion and randomization). After the wetting stage the energy barriers associated with the interface essentially disappear and the polymer chains are free to diffuse and randomize. It is these last two stages which are believed to be important in order to restore the material's original mechanical strength. The limitations of this approach to heal polymer cracks are the temperatures and time, t, required for the crack to heal. In addition the large molecular weight, M, of the polymer can hinder full recovery of the material properties. For example, the healed fracture stress has been shown to be proportional to t/M. It is proposed that the use of supramolecular polymers will circumvents some of these problems as thermal annealing will result in a decrease in the molecular weight thus improving the healed fracture stress and the time required for the crack to heal. This Network will investigate a variety of different supramolecular motifs which will not only self-assemble into different architectures but will also exhibit different kinetic and thermodynamic binding characteristics to gain a better understanding of how self-assembly affects mechanical properties. Furthermore, once a better understanding of such systems is developed applications not only as healable materials but also in areas such as adhesives, low cost processing and recycling can be envisaged.