This work aims some challenges in the manufacturing of flexible substrates which will be used in solar cells as a substrate. Poly(imide siloxane) block copolymers were produced with the same bis(aminopropyl) polydimethylsiloxane (APPS). The polyimide hard blocks were composed by using 4,4'-oxydianiline (ODA) and benzofenon-3,3,4,4-tetrakarboksilik dianhydride (BTDA). Besides, the polysiloxane soft blocks were derived by using APPS and BTDA. APPS and BTDA formed the polysiloxane soft block in the structure. The length of polysiloxane soft block increased with increase in the length of polyimide hard block. Hence, it was possible to obtain copolymer structure and the changes in physical properties of the copolymers. These copolymers were characterized by using FT-IR analysis to evaluate the structure of flexible substrates.