THERMOREVERSIBLE GELS FROM MIXTURES OF A HIGH-MELTING IONIC LIQUIDS AND A POLAR POLYMER FOR SELF-HEALING MATERIALS
Joonsung Yoon and Christopher M. Stafford
Polymers Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
Over the past few decades, considerable effort has been expended to develop self-healing polymeric materials, and a wide variety of strategies have been explored. While quite a few strategies exist for self-healing, materials based on an epoxy resin consisting of a catalyst and encapsulated reactive monomers dispersed in the epoxy matrix is especially notable. More recently, self-healing rubber was developed using supramolecular assembly, in which reversible hydrogen bonds were used to form both macromolecular chains and cross-links. In polyurethane-based self-healing coating systems, scratch resistance was controlled by the density of the constituent molecules (cross-link density) while low to moderate glass transition temperature (Tg) facilitated thermal flow under mild heating to heal small scratches. Another possibility to implement reversible interactions and low Tg for self-healing materials could be found in thermoreversible physical gels that are composed of supramolecular networks based on non-covalent van der Waals interactions. For example, poly(vinyl alcohol) (PVOH) gels are a well-known type of physical gels exhibiting soft, elastic properties. We investigated a new kind of PVOH physical gel in which the volatile water is replaced with a non-volatile ionic liquid (IL). High melting IL containing Br- anion was chosen so that the hydrogen-bond accepting Br- can enhance miscibility with PVOH while the high melting point can lead to solid-like mixtures at room temperature. The thermal and mechanical properties of the mixtures of IL and PVOH were investigated and the results are discussed in terms of the interaction between the two components. Thermal healing properties were evaluated visually by optical microscopy for macroscopic scratches after mild thermal treatment.