Morphology, phase behavior, and structure/property relationships of nylon 6-aromatic polyimide graft copolymers as potential molecular composites

A series of Nylon 6-g-polyimide graft copolymers has been recently synthesized in Professor F. W. Harris' laboratory. Several molecular parameters were controlled in order to fully understand the morphology, phase behavior, and structure/property relationships of these graft copolymers. These parameters include polyimide block length, polyimide composition, chemical structure of different types of polyimide, and grafting density. In addition, in-situ blends have also been prepared to provide a comparison to the pure Nylon-6 and Nylon 6-polyimide graft copolymers.;The motivation for this research has two aspects. The first one is to characterize the Nylon 6-polyimide graft copolymers and to determine if these graft copolymers are miscible on the molecular level and exhibit molecular composite behavior. Another is to identify the location of the polyimide in the Nylon-6 matrix in order to determine the reinforcement mechanism.;From Dynamic Mechanical Analysis (DMA) results, the storage moduli E' for the graft copolymers are 2-3 times higher than that of the pure Nylon-6. This synergistic performance is probably caused by molecular level dispersion resulting in molecular composite behavior. The results from non-isothermal differential scanning calorimetry (DSC) experiments also show a shift in the glass transition (Tg) and cold crystallization (Tc) temperatures to higher temperatures, and a depression of the melting temperature (Tm). Further investigations via isothermal DSC, wide angle x-ray diffraction (WAXD), and transmission electron microscopy (TEM) are combined to examine the graft copolymers' structure/property relationships in order to determine the reinforcement mechanism.;WAXD fiber patterns of the graft copolymers are identical to those of pure Nylon-6. In addition, both pure Nylon-6 and graft copolymers exhibit the same WAXD powder patterns. However, what appears to be a Brill transition in Nylon-6 at high temperatures, but cannot be found in the graft copolymers. The morphological observations from TEM indicate that the cryrstalline morphology differences between graft copolymers and pure Nylon-6 are the crystal spherulitic size and the kinetics of crystallization. Based on measurements from WAXD and TEM experiments, it is clear that the crystal structure does not change when the polyimide is incorporated. This suggests that the polyimide is excluded from Nylon-6 crystalline region during the crystallization and is located in the interlamellar region of the Nylon-6 matrix. Isothermal crystallization measurements support this conclusion. However, the polymorphism phenomena still exist in the copolymer systems regardless of the incorporated polyimide composition. The overall crystallization kinetics are retarded because the polyimide prohibits the reorganization of the Nylon-6 crystals. Furthermore, varying the molecular parameters results in different degrees of retardation of crystallization.