Polydimethylsiloxane-Based Self-Healing Materials

Self-healing represents a new paradigm for active and responsive materials. As first demonstrated by White et al., and subsequently in additional publications, polymer composites can be engineered to chemically self-heal. However, the chemistry of previous systems possesses inherent shortcomings due to the potential side reactions with the polymer matrix and air. Here we present a new, chemically stable selfhealing materials system based on the tin-catalyzed polycondensation of phase-separated droplets containing hydroxy end-functionalized polydimethylsiloxane (HOPDMS) and polydiethoxysiloxane (PDES). The catalyst, di-n-butyltin dilaurate (DBTL), is contained within polyurethane microcapsules embedded in a vinyl ester matrix and is released when the capsules are broken by mechanical damage. This system possesses a number of important advantages over the previous self-healing methodology, including a) the healing chemistry remains stable in humid or wet environments, b) the chemistry is stable to an elevated temperature (> 100 °C), enabling healing in higher-temperature thermoset systems, c) the components are widely available and comparatively low in cost, and d) the concept of phase separation of the healing agent greatly simplifies processing, as the healing agent can now be simply mixed into the polymer matrix. Although inspired by our previous self-healing methodology, in which the monomeric healing agent was encapsulated and the catalyst was dispersed as particulate throughout an epoxy matrix, this new system contains a number of distinct differences. The siloxane-based healing agent mixture is not encapsulated, rather it is phase-separated in the matrix while the catalyst is encapsulated. The low solubility of siloxane-based polymers enables the HOPDMS–PDES mixture and catalystcontaining microcapsules to be directly blended with the vinyl ester prepolymer, forming a distribution of stable phase-separated droplets and protected catalyst. No reactions take place between the HOPDMS and PDES prior to exposure to the catalyst. When the matrix cracks, a mixture of catalyst released from microcapsules and the healing agent wets the entire crack plane. Addition of an adhesion promoter to the matrix optimizes wetting and bonding of the crack faces. After the healing agent mixture cures, the crack is self-healed (Figs. 1a–c). The polycondensation of HOPDMS with PDES occurs rapidly at room temperature in the presence of amine and carboxylic acid organotin catalysts. Because side reactions are limited, organotin catalysts are highly desirable for curing PDMS-based systems, even in open air. This stability to C O M M U N IC A IO N S