Synthesis and properties of multi-block semi-alicyclic polyimides for thermally stable transparent and low CTE film

Abstract

Highly transparent polyimide (PI) with a low coefficient of thermal expansion (CTE), comparable to that of copper foil (17 ppm/K), has been developed in conjunction with a relatively rigid and symmetrical monomer, trans-cyclohexyl diamine (CHDA), and in so doing we clarified the effectiveness of a multi-block copolymer system in terms of maximizing the degree of CTE reduction as well as mechanical strength without sacrificing other PI film properties such as high thermal stability (Tg > 280 °C) or high transparency. Multi-block co-PIs were synthesized through the formation of end-functional oligomers, anhydride-terminated oligo-imides (OIs), and amine-terminated oligo(amic acid)s (OAAs). Although there is a large polarity difference between OI and OAA, the multi-block copolymerization still took place well enough to produce a highly transparent PI film as well as a clear PI precursor solution. For comparison, random co-PIs and homo-PIs derived from the same monomer species were synthesized. We found that the difference in the polymer sequence – block vs. random – influenced the film properties so that even the respective PIs had the same monomer composition. Multi-block co-PIs effectively decreased CTE with less CHDA content, and marked the highest tensile modulus (Et: 5.1 GPa) and tensile strength (σt: 208 MPa) which surpassed that of homo-PIs. This can probably be attributed to the micro-aggregation for the multi-block system. It is also suggested that they have a close relationship between the micro-aggregation and the cut-off wavelength of the film while keeping a high light transmittance in the visible region.