PROCESSING CHARACTERISTICS AND STRUCTURE DEVELOPMENT IN SOLID-STATE EXTRUSION OF A NEW SEMICRYSTALLINE POLYIMIDE (BTDA-DMDA)

摘要

In this article, we present detailed processing characteristics and structure development in a thermoplastic polyimide BTDA-DMDA in the solid-state extrusion process. This fully imidized polyimide polymer is known to crosslink at fast rates when it is brought to a molten phase even for short periods of time. This characteristic makes it difficult to process it in the molten phase and attempts at melt processing result in melt fracture and highly distorted extrudates. However, this polymer can be shaped into high-quality extrudates when it is processed below its melting temperature directly from its postpolymerization powdered state. The solid-state extrusion of precompacted BTDA-DMDA powder was studied in the temperature range from 250 to 320 degrees C. At the temperatures from 290 to 320 degrees C, high-quality extrudates were obtained. Below 290 degrees C, solid-state extrusion was not possible due to the limitation of the load cell capacity of the capillary rheometer used in this research. Above 320 degrees C, the extrudates were found to be of poor quality as a result of degradation and crosslinking in the molten phase. Structural characteristics of the samples produced by solid-state extrusion was investigated by the microbeam X-ray diffraction technique. The thermal behavior of the extrudates was also characterized by differential scanning calorimetry (DSC). The DSC results show that at low extrusion temperatures the samples exhibit dual endothermic peaks and are highly crystalline in an extruded state. The higher melting peak located at about 350 degrees C is due to the melting of the new crystalline phase that has developed partially during the solid-state extrusion process and partially during the recrystallization process that takes place at temperatures at and slightly above the primary melting process during the DSC heating scan. This has been confirmed by DSC, depolarized light hot-stage video microscopy, and wide-angle X-ray diffraction studies. The long spacing of the higher melting crystals was found to be much larger than that of the lower melting crystals, as evidenced by the small angle X-ray scattering studies. (C) 1995 John Wiley and Sons, Inc. [References: 36]