Polyaryletherketone is a kind of special engineering plastics with excellent comprehensive properties, high strength, good stability, and almost insoluble in any common solvent except concentrated sulfuric acid at room temperature. However, its high processing temperature also hinders its application and development in the foam. Here, a series of polyetheretherketone (PEEK) and diphenyl polyetheretherketone (PEDEK) composite foams are prepared by temperature induced phase separation, using diphenyl sulfone as solvent. The microstructure, thermal and mechanical properties of PEEK and PEDEK foam are compared. Highly porous PEEK and PEDEK foams with densities ranging from 0.13 to 0.25 g cm−3, with compressive strength ranging from 0.68 to 2.6 MPa, respectively, are produced. Meanwhile, the introduction of diphenyl structure can effectively improve the performance of PEEK foams at high temperature. It is found that PEDEK foam has a higher operating temperature than PEEK, and the compressive strength of 0.24 g cm−3 foam is still ≈1 MPa at 200 °C.

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The preparation of the electromagnetic wave (EMW) absorbing materials with 3D porous structural, light weight, and high mechanical strength has been receiving greater attention for a long time. A series of light‐weight polyetheretherketone/graphite/FeCo ternary heterogeneous composite foams (PEEK/C/FeCo) with high mechanical strength are prepared via thermally induced phase separation. Based on the high compressive strength of PEEK resin and its stable skeleton formed by thermally induced phase separation, the composite foam has high compressive strength and low density. In addition, during the thermally induced phase separation process, porous structures are formed inside the PEEK, and graphite and FeCo particles are uniformly dispersed between the pore structures, the composite foams possess excellent porous structure, moderate dielectric loss, and magnetic loss, exhibiting good absorbing properties. With the low density of 0.49 g cm−3 and high compressive strength of 6.40 MPa, the as‐synthesized optimal sample (PEEK‐50/C‐40/FeCo‐10) achieves a minimum reflection loss up to −35.9 dB covering a bandwidth of 3.6 GHz for a thickness of 1.5 mm. In a word, this study provides a novel, economical, and efficient way to obtain a light foam with high mechanical strength and good EMW absorbing property.