A foam structure having a density of less than about 20 lbs/ft.sup.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.