Critical Phenomenon During Drying of Semiaromatic, Transparent and Soluble Polyimide Cast Films: Real-Time Observation of Birefringence and Other Integrated Parameters

A newly developed real-time multifunctional monitoring system was used to track in-plane and out-of-plane birefringence, weight, thickness and surface temperature of semiaromatic, transparent, and soluble polyimide cast films during drying. At a critical point during drying, out-of-plane birefringence develops rapidly when the cast film undergoes vitrification. The rapid development of out-of-plane birefringence is attributed mainly to polymer chain orientation caused primarily by in-plane confined drying induced by thickness reduction and secondarily by shrinkage stress development in the process of vitrification. Shrinkage stress and its magnitude depend on the details of freezing-in process as dictated by the coating and drying variables. Some drying solutions also found to develop a small negative in-plane birefringence with the higher refractive index in the direction transverse to the air flow direction and it is attributed primarily to the residual stress development. In addition, the influence of solvent type on this phenomenon was studied using series of polar organic solvents.