| Summary: | Polymer foams are expanding their applications into functional materials. Partial foam structure has been fabricated in polyethylene terephthalate–polyethylene naphthalate (PET–PEN) copolymer by solid state foaming. Through SEM image analyses, a potential to fabricate gradient foam structures with micropores and unfoamed skin layers has been identified. The post-foaming temperature T<sub>f</sub> tune the pore size distribution. Radial distribution of micromechanical properties, indentation hardness and elastic modulus were measured for the partial foam and their values were around 0.12 GPa and 2.0 GPa, respectively, for the outer foamed region. Foaming temperature affects the glass transition temperature T<sub>g</sub>, the coefficient of thermal expansion and the flame spreading rate. For the range of T<sub>f</sub> ≤ 60°C, thermal expansion coefficients for T > T<sub>g</sub> are about 0.5 m/m°C (steep expansion group, SEG). When T<sub>f</sub> is above 80 °C, they are around 0.02 m/m°C (mild expansion group, MEG). The burning rate of SEG is 2.8 times higher than that of MEG.
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