A self-healing approach for optically transparent thermoplastic polymers, based on plasticizer-induced solvent welding, is reported. For the specific system investigated, dibutylphthalate (DBP) filled urea-formaldehyde capsules are dispersed in a polymethylmethacrylate (PMMA) matrix. Upon a damage event, DBP is released into the crack, and locally plasticizes and swells the polymer, enabling it to remend. Two challenges are addressed to maintain optical transparency: minimization of light scatter from the capsules in the polymer matrix and minimization of light scatter from the healed polymer. PMMA films containing DBP capsules have good transmissive properties as a result of the close index match between PMMA and DBP. The transmission properties are better than, for example, when DBP capsules are dispersed into a poorly index matched matrix, such as polystyrene. In the DBP PMMA system, the healed material is inherently index matched to the polymer matrix and thus the polymer's original optical properties are largely restored. Self-healing using both small capsules, 1.5 μm in diameter, and large capsules, 75 μm in diameter is demonstrated. Smaller capsules are particularly important for thin polymer films which are not thick enough to hold the larger capsules. Polymer films with smaller capsules also have very good transmission properties due to a minimization of light scattering by the small size of the capsules. Large capsules enable healing of larger damage events, but do inherently result in some light scattering. This plasticizer-based approach to self-healing is shown to enable recovery of the protective properties and a portion of the mechanical properties of a polymeric film.
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