a foam structure having a density of less than about 20 lbs / ft 3 . the method of forming the same comprising forming a homogeneous melt blend under pressure of a polymer member selected from the group consisting of polyetherimide resin particles , polycarbonate resin particles , a polymer blend of polyphenylene oxide and polystyrene in particle form imbibed with a blowing agent , followed by extrusion of the same into a lower pressure atmosphere . a composition comprising said particles imbibed with methylene chloride , chloroform , 1 , 1 , 2 - trichloroethane or mixtures thereof . a method of thermoforming said foam in sheet form .