Examples of the solvent used for the solvent heating treatment include water, an aromatic hydrocarbon-based solvent such as toluene and xylene, a halogenated hydrocarbon-based solvent such as chlorobenzene and o-dichlorobenzene, an alcohol-based solvent such as isopropanol and isobutanol, a polar aprotic organic solvent such as N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidon