Dynamic t–Tdyn–Tg diagram analysis of thermal isomerization of polyisoimide using complex electric modulus properties

A time–temperature–glass transition temperature diagram under a dynamic-heating condition (dynamic t–Tdyn–Tg diagram) was developed to analyze physicochemical transitions of a reacting polyisoimide system using 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3′-diaminodiphenyl sulfone (3,3′-DDS) as a model system. Under the dynamic-heating condition, the complex electric modulus properties exhibited two transitions at high frequencies but three transitions at lower frequencies, due to the coupled phenomena of the viscoelastic glass–rubber transitions and the kinetic increment of the glass transition temperature of the isomerizing polyisoimide. The dynamic t–Tdyn–Tg diagram was proposed to analyze those multiple peaks to identify the material status and the transition temperatures corresponding to the initial glass transition (Tg0), vitrification (Tvit), and final glass transition (Tg∞) temperatures. Using the developed methodology, the isomerization of polyisoimide, which is usually difficult to analyze, was successfully investigated to identify the relationships among the thermodynamic, viscoelastic, and kinetic processes.