This disclosure describes processes for producing polymer foams, and compositions of matter relating to these processes. A process includes the steps of dispersing an organic nucleating agent in a dispersing medium to obtain an additive dispersion, contacting the additive dispersion with a mineral nucleating agent to obtain a hybrid nucleating agent containing the organic nucleating agent localized on at least a portion of a surface of the mineral nucleating agent, and contacting the hybrid nucleating agent with a polymer in the presence of a foaming agent to obtain a polymer foam. A foamed polymer of the present disclosure includes a cellular matrix containing a polymer material, and a mineral matrix of particles containing a hybrid nucleating agent dispersed in the cellular matrix, in which the hybrid nucleating agent contains an organic nucleating agent localized on at least a portion of a surface of an inorganic material.