**2. Experiments**

### **2.1 The sample preparation**

The chapter uses the two-step method to prepare the polyimide films. Pyromellitic dianhydride (PMDA) and 4,4<sup>0</sup> -diaminodiphenyl ether (ODA) are added into dimethylacetamide (DMAc). The polyamic acid (PAA) was prepared by polymerization reaction in the process. The PAA was coated on the glass plate, placed in a vacuum environment to remove air inside PAA, which was placed in the environment where the temperature was 60°C (1 h), 120°C (2 h), 150°C (1 h),
