A polymer foam body comprising a polymeric matrix containing gas-filled cells, the foam body having a porosity of at least 50% and a number average cell size of 50 to 1000 nm determined as indicated in the description, wherein the polymeric matrix comprises: A) a phosphorus-containing, thermoplastic random copolymer of at least one dialkyl (meth)acryloyloxyalkylphosph(on)ate and one or more comonomers, the phosphorus-containing copolymer being characterized in having a glass transition temperature of 70 to 150°C as measured by dynamic scanning calorimetry and reported as the half-height of the endothermic peak, a weight average molecular weight of at least 60,000 g/mol as measured by gel permeation chromatography relative to polystyrene standards, and 3 to 8% by weight phosphorus contributed by the dialkyl(meth)acryoloxyalkylphosph(on)ate; and B) at least one thermoplastic poly(methyl methacrylate) or thermoplastic non-phosphorus-containing copolymer of methyl methacrylate in which the comonomer(s) each have a solubility parameter lower than 20 (megaPascals) 0.5 and the chemical composition of the non-phosphorus-containing copolymer is such that twice the mass fraction of oxygen plus the mass fraction of each of nitrogen, fluorine and silicone in the monomers taken together is greater than 0.2, wherein the thermoplastic poly(methyl methacrylate) or thermoplastic non-phosphorus-containing copolymer of methyl methacrylate has a glass transition temperature of at least 70°C as measured by differential scanning calorimetry and reported as half-height of the endothermic peak, and wherein the polymeric matrix contains an amount of the phosphorus-containing thermoplastic random copolymer to provide the polymeric matrix with 2.5 to 5% by weight phosphorus. A) a phosphorus-containing, thermoplastic random copolymer of at least one dialkyl (meth)acryloyloxyalkylphosph(on)ate and one or more comonomers, the phosphorus-containing copolymer being characterized in having a glass transition temperature of 70 to 150°C as measured by dynamic scanning calorimetry and reported as the half-height of the endothermic peak, a weight average molecular weight of at least 60,000 g/mol as measured by gel permeation chromatography relative to polystyrene standards, and 3 to 8% by weight phosphorus contributed by the dialkyl(meth)acryoloxyalkylphosph(on)ate; and B) at least one thermoplastic poly(methyl methacrylate) or thermoplastic non-phosphorus-containing copolymer of methyl methacrylate in which the comonomer(s) each have a solubility parameter lower than 20 (megaPascals) 0.5 and the chemical composition of the non-phosphorus-containing copolymer is such that twice the mass fraction of oxygen plus the mass fraction of each of nitrogen, fluorine and silicone in the monomers taken together is greater than 0.2, wherein the thermoplastic poly(methyl methacrylate) or thermoplastic non-phosphorus-containing copolymer of methyl methacrylate has a glass transition temperature of at least 70°C as measured by differential scanning calorimetry and reported as half-height of the endothermic peak, and wherein the polymeric matrix contains an amount of the phosphorus-containing thermoplastic random copolymer to provide the polymeric matrix with 2.5 to 5% by weight phosphorus.