Effects of Ether Linkage and Benzene Ring Substitution Structures on Thermal and Optical Properties of 6FDA/3DDS-Based Transparent Polyimide Films

Abstract

This study reports the thermal and optical properties of a transparent polyimide (PI) film system incorporating trifluoromethyl (CF₃-), ether (-O-), and sulfone (-SO₂–) functionalities into the polymer backbone by co-polymerizing 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and bis(3-aminophenyl) sulfone (3DDS)-based transparent PI with additional diamine monomers such as bis[4-(4-aminophenoxy)phenyl] sulfone (p-BAPS) and bis[4-(3-aminophenoxy)phenyl] sulfone (m-BAPS). Our findings clearly demonstrate that the introduction of p-BAPS and m-BAPS monomers into the 6FDA/3DDS-based PI system significantly influences the thermal and optical properties of the resulting PI films. Specifically, the glass transition temperature (T_g) of the 6FDA/3DDS/p-BAPS-based PI film increased by approximately 10 °C compared to the pristine 6FDA/3DDS-based PI film, while the yellowness index (YI) decreased from 12.75 to 3.76. On the other hand, the T_g of the 6FDA/3DDS/m-BAPS-based PI film decreased by about 5 °C compared to the pure 6FDA/3DDS-based PI film, but nevertheless, the YI value decreased from 12.75 to − 1.85 and the optical properties were improved compared to the 6FDA/3DDS-based PI film. Based on these observations, it was reasonably concluded that the simultaneous introduction of flexible ether bonds and rigid, linear benzene rings would not only minimize the thermal degradation of the final PI film due to the introduction of comonomers but also improve its thermal and optical properties simultaneously. Thus, we believed that this study will provide valuable insights on the simultaneous enhancement of the thermal and optical properties of PI system through the combination of various monomers, such as introducing flexibility into the polymer backbone and reinforcing the rigid ring structure and substituent configurations.