Polyimides containing trans-1,4-cyclohexane unit.: Polymerizability of their precursors and Low-CTE, low-K and high-Tg properties

The structure-polymerizability relationship has been investigated in a series of polyimide (PI) precursor, poly(amic acid) systems obtained from trans-1,4-cyclohexanediamine (CHDA) and various dianhydrides. The results have shown that the systems examined in this work can be classified into three groups on the basis of the solubility of the salt formed at the initial reaction stage: (A) polymerization proceeds at room temperature via gradual salt dissolution; (B) polymerization is initiated upon heating at an appropriate temperature; (C) no reaction takes place under any conditions owing to the formation of completely insoluble salt. The polymerizability based on the salt solubility in each system was considered from the points of view of the strength of acid in an initially formed low-molecular weight amic acid and their steric structures. In order to improve the non-polymerizability in the systems of group C, sequence-controlled copolymerization was carried out using a rod-like fluorinated diamine as a comonomer and this resulted in high-molecular weight CHDA-containing copoly(amic acid)s. The properties of a series of CHDA-based PIs have also been investigated. Some of them simultaneously achieved a low coefficient of thermal expansion (CTE), a low dielectric constant (K), a high glass transition temperature (T-g) and good film flexibility. The structure-property relationship indicates the molecular-design of low-K and low-CTE PI materials.