Multiblock Copolyimide Exhibiting High Flexibility and Low Thermal Volumetric Expansion owing to Negative Thermal Expansion of PDMS Nanodomain

Multiblock copolymers comprising 92 wt % aromatic polyimide (PI) and 8 wt % polydimethylsiloxane, PI/PDMS, exhibited not only superior ductility and toughness but also unexpectedly lower coefficients of volumetric thermal expansion (CVE) than the corresponding homopolyimide. Segregated nanodomains (NDs) of the PDMS block in the PI matrix are observed in the cross-sectional transmission electron microscopy images of the unstained PI/PDMS film. The evaluated weight density of the PDMS-ND in PI/PDMS (ρ PDMS ) was significantly lower than that of PDMS homopolymer. Furthermore, the ρ PDMS and CVE of the PI/PDMS samples were positively correlated, whereas the CVE of typical materials were negatively correlated with density. Synchrotron small-angle X-ray scattering measurements also revealed that the average volume of the PDMS-ND decreases/increases with heating/cooling. These unusual results can be explained by a model in which the extremely low-density state is memorized in/on the PDMS-NDs during the film preparation process from the precursor solution, and the NDs reversibly contract/expand during heating/cooling, leading to a lower CVE of the PI/PDMS. A balance between the negative pressure in the NDs and expansion stress from the PI matrix is likely to induce reversible volumetric deformation with temperature. These PI/PDMS films have great potential as high-performance flexible and transparent substrates.