# Preparation and Properties of Poly(imide-siloxane)
## 3 Results and Discussion
### Properties of PI/Al\({}_{2}\)O\({}_{3}\) Composite Films  
Because the PI-3, PI-4, and PI-5 films were free-standing and flexible, they were used to prepare PI/Al\({}_{2}\)O\({}_{3}\) composite films. Figure 4 shows the thermal conducting properties of neat PI films and PI/Al\({}_{2}\)O\({}_{3}\) composite films, with different Al\({}_{2}\)O\({}_{3}\) loadings along an in-plane direction (\(D_{\perp}\)) at room temperature (see Tables S1-S3). The films' thermal diffusivity (\(\alpha\)) was determined at room temperature and under ambient pressure. From the equation K = \(\alpha\times\varrho\times\mathrm{C_{p}}\), the thermal conductivity (K) value can be calculated. In the equation, \(\varrho\) is the measured film density, and \(\mathrm{C_{p}}\) is the specific heat capacity of the film [5, 49]. The thermal conductivities of neat PI-3, PI-4, and PI-5 were 0.11, 0.13, and 0.14 W/m\(\cdot\)K, respectively. It is known that thermal conductivities of polyimide and polydimethylsiloxane are 0.11 and 0.25 W/m\(\cdot\)K at room temperature, respectively [22]. The incorporation of Al\({}_{2}\)O\({}_{3}\) fillers increased thermal diffusivity and thermal conductivity of PI/Al\({}_{2}\)O\({}_{3}\) composite films (Figure 4a,b). When the  
Figure 3: Photographs of (**a**) PI-3; (**b**) PI-4; (**c**) PI-5 films. Figure 2: TGA curves of PIs. Al\({}_{2}\)O\({}_{3}\) loading was 80 wt%, PI-3 and PI-4 composites showed high thermal conductivity values, greater than 1.3 W/m-K. The thermal diffusivity and conductivity data are quite reproducible as presented in Tables S1-S3. In addition, thermal and mechanical properties of the PI/Al\({}_{2}\)O\({}_{3}\) composite films were studied (Table 3 and Figures S8 and S9). Among the flexible PI films, PI-3 was selected because it exhibited the best thermal conducting properties. The \(T_{5}\) and \(T_{10}\) values of the neat PI-3 and PI-3/Al\({}_{2}\)O\({}_{3}\) composite films ranged from 437 to 454 \({}^{\circ}\)C and 456 to 474 \({}^{\circ}\)C, respectively. Generally the decomposition temperatures of the PI/Al\({}_{2}\)O\({}_{3}\) composite films were higher than those of the neat PI film. The improvement in thermal stability can be attributed to restrained polymer chain mobility by Al\({}_{2}\)O\({}_{3}\) particles [17]. Furthermore, it should be noted that the PI/Al\({}_{2}\)O\({}_{3}\) composites showed much higher thermal stability compared to polysiloxane/Al\({}_{2}\)O\({}_{3}\) composites [17; 50]. From the char yield data, each mass value retained around 800 \({}^{\circ}\)C and was almost identical to the corresponding Al\({}_{2}\)O\({}_{3}\) content in each composite. The neat PI-3 film showed a tensile strength of 14.6 \(\pm\) 2.7 MPa, and a high elongation at a break of 210.4 \(\pm\) 38.4%. However, the PI/Al\({}_{2}\)O\({}_{3}\) composite films exhibited decreased tensile strength and elongation at break, and the mechanical properties decreased with increasing Al\({}_{2}\)O\({}_{3}\) loadings. It is well known that the incorporation of an inorganic filler to the polymer matrix reduces the polymer's flexibility. Furthermore, the polymer's strength is reduced if there are no binding sites between the polymer phase and inorganic material phase [51]. Nevertheless, the composite films with up to 75 wt% Al\({}_{2}\)O\({}_{3}\) are sufficiently flexible, as shown in Figure 5. They may be useful as a flexible heat-radiating film in flexible electronic devices requiring a high operating temperature. Even though the composite films with 80 wt% Al\({}_{2}\)O\({}_{3}\) loading are not flexible, they could also be used in high-temperature electronic devices. Figure 4: (**a**) Thermal diffusivity (\(D_{\perp}\)), and (**b**) thermal conductivity (\(D_{\perp}\)) of the PI and PI/Al\({}_{2}\)O\({}_{3}\) composite films with different Al\({}_{2}\)O\({}_{3}\) loadings.