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

In this article, we present detailed processing characteristics and st ructure development in a thermoplastic polyimide BTDA-DMDA in the soli d-state extrusion process. This fully imidized polyimide polymer is kn own to crosslink at fast rates when it is brought to a molten phase ev en for short periods of time. This characteristic makes it difficult t o process it in the molten phase and attempts at melt processing resul t in melt fracture and highly distorted extrudates. However, this poly mer can be shaped into high-quality extrudates when it is processed be low its melting temperature directly from its postpolymerization powde red 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 rheom eter used in this research. Above 320 degrees C, the extrudates were f ound to be of poor quality as a result of degradation and crosslinking in the molten phase. Structural characteristics of the samples produc ed by solid-state extrusion was investigated by the microbeam X-ray di ffraction technique. The thermal behavior of the extrudates was also c haracterized by differential scanning calorimetry (DSC). The DSC resul ts show that at low extrusion temperatures the samples exhibit dual en dothermic peaks and are highly crystalline in an extruded state. The h igher melting peak located at about 350 degrees C is due to the meltin g of the new crystalline phase that has developed partially during the solid-state extrusion process and partially during the recrystallizat ion process that takes place at temperatures at and slightly above the primary melting process during the DSC heating scan. This has been co nfirmed by DSC, depolarized light hot-stage video microscopy, and wide -angle X-ray diffraction studies. The long spacing of the higher melti ng 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.