The Formation Of Banded Spherulites In Polyimide And Effects On Their Growth

Polyimide is a kind of high thermal stable polymeric material, and it is widely used in many areas, and its physical property is closely related to its aggregation morphology. The study of polyimide crystallization is of important application value since it could provide theory guidance to the control of polyimide aggregation morphology. Further, the study of rigid chain polyimide crystal structure and morphology could contribute to the whole knowledge of crystallography: it could test the availability of polymer crystalline theories and models that are constructed upon soft back bone polymers. Our group has first reported the growth of banded spherulite in BTDA/m-PDA polyimide. The main purpose in this contribution is to further investigate the inner structure of these banded spherulites, and study various factors that could influence its growth and morphology.Both free surface and sandwiched films were prepared through two-step thermal imidization. The free surface sample was first ultra-thin sliced to about 200nm, and then observed using Transmission Electron Microscopy (TEM), which revealed that the banded spherulite is constructed by S-profile radial grown fibers, and the crystal density is low in the center of the ball. The sandwiched film was gently detached and directly observed by Atom Force Microscopy (AFM). It was noticed that those fibers are lamella bundles. Smaller region scanning displayed that bands are caused by lamella twisting: the valley and the peak correspond to the edge-on and flat-on lamellae, respectively. SEM observation associated with a permanganic etching technique was carried out to further identify these observed morphology features.PLM observation discovered that spherulites grown in different regions of the sandwiched film had different morphologies: spherulites grown near the center of the sample have larger band spacing compared with those near the fringe. AFM study showed that the lamella in the former is wider, and twisted less vigorous. This difference is suggested to be caused by different solvent evaporation speed between the center and fringe of the film. To further improve this conjecture, the morphology features of spherulites grown in three different situations were reviewed : (1) in the ultra-thin sandwiched film; (2) in the sandwiched film of diluted PAA solution; (3) in the thick sandwiched film. The morphology of spheruiltes grown in the third situation was compared with that of crystal particles grown in the solution, and X-ray diffraction study revealed that the spheruilte and the crystal particle have the same crystal structure.It was discovered in this paper that when imidization temperature is high than 250°C , ODPA/m-PDA polyimide could also form banded spherulites, the morphology of which was investigated by AFM. This is the second polyimide system that could form banded spherulites according to our knowledge. We believed that the formation of banding in polyimide was determined by its molecular structure. In this paper, irregular copolymerization, homopolymerization using different monomers, and introducing into side-chains were carried out to change molecular structures and study the relationship between the banding formation and chain characters.Molecular simulation technique revealed that the BTDA/m-PDA polyimide molecular chain exhibits helical conformation, ODPA/m-PDA polyimide also takes helical conformation, except that it is much looser compared with the former, while other polyimide systems exhibit extended or zig-zag chain conformations. We suggested that polyimide has helical chain conformation could banded spherulite form, and the more compact the chain helix is, the more easily band structure could form.