Abstract: A process of producing a structural network from a semicrystalline, melt crystallizable polyarylene ether or polyarylene thioether comprising providing a molten mixture of at least one semicrystalline polymer in the presence of a melt plasticizer for the polymer which melt contains at least some polymer entities having a crystalline form, the total polymer concentration being between 5 and 50% by weight of the mixture of polymer and melt plasticizer, and heating the mixture of polymer and melt plasticizer at a heating rate of between 1.degree. C./hour and 1000.degree. C./hr, preferably between 10.degree. C./hour and 100.degree. C./hour until the mixture is at a temperature in the range 10.degree. C. below to 20.degree. C. above its clearing point and cooling the mixture until it solidifies whereby an open cell structure essentially free from granular superstructure is obtained.

Abstract: An anisotropic-melt-forming polymer having improved stiffness retention at elevated temperatures comprising residues selected from residues (A) of dihydric phenols, aromatic dicarboxylic acids, hydroxyaromatic acids, aromatic diamines, aminophenols and aminoaromatic acids wherein at least 40 mole % of the residues (A) have their chain extending bonds in either coaxial or parallel and oppositely directed relationship and sufficient of non-linear residues (B) selected from at least one of: ##STR1## to give a polymer having an improved retention of stiffness at elevated temperature compared with an otherwise similar polymer not containing said residues (B).