Thermally robust capsule system, and composites including the capsules

A method of making capsules includes forming a mixture including a core liquid, a polyurethane precursor system, a first component of a two-component poly(urea-formaldehyde) precursor system, and a solvent. The method further includes emulsifying the mixture, adding a second component of the two-component poly(urea-formaldehyde) precursor system to the emulsified mixture, and maintaining the emulsified mixture at a temperature and for a time sufficient to form a plurality of capsules that encapsulate at least a portion of the core liquid. The capsules made by the method may include a polymerizer in the capsules, where the capsules have an inner capsule wall including a polyurethane, and an outer capsule wall including a poly(urea-formaldehyde). The capsules may include in the solid polymer matrix of a composite material.