Study On Aggregation Structure Of Polyimide Based On BTDA Monomer

Four types of poly(amic acid)s were synthesized using 3,3' ,4,4' -benzophenone tetracarboxylic dianhydride (BTDA) as dianhydride monomer and meta-phenylene diamine (m-PDA), para-phenylene diamine (p-PDA), 4,4' -oxydianiline (ODA), benzidine (BZD) as diamine monomers respectively. Based on these poly(amic acid)s, four type of corresponding polyimides were prepared through different imidization procedures. The effects of macromolecular chain on the formation of aggregation structures, crystallinity, and morphology were studied using WAXD, PLM, SEM, etc. Also, the effects of imidization procedures on the formation of aggregation structures were studied as well.Three different imidization procedures were used to study BTDA-m-PDA, BTDA-ODA and BTDA-p-PDA systems, such as: imidization in PAA solution at 180°C (procedure 1) ; imidization of PAA coating film at high temperature (procedure 2); swell of the PI films and then processed at 300°C (procedure 3). Semicrystalline polyimide powders were prepared by procedure 1 for all three types of polyimides studied. By using procedure 2, banded spherulite was obtained for BTDA-m-PDA system, and underdeveloped spherulite was observed for BTDA-ODA system. For BTDA-p-PDA system imidized through procedure 2, no ordered aggregation structure was observed. For procedure 3, polyimide films with low-order aggregation structure were prepared for all the systems studied. The semicrystailine polyimide powders showed that all of the three polyimides could form crystal structure and have a crystallinity of 52.2%, 40.0%,64. 8% for system BTDA-m-PDA,BTDA-ODA and BTDA-p-PDA, respectively. Study on polyimides prepared through procedure 2 demonstrated that the ability to form order aggregation structure of these polyimides decreased in an order of BTDA-m-PDA, BTDA-ODA, BTDA-p-PDA. Compared with amorphous polyimide films, the films which were prepared by prepared by procedure 3 and bared low-order aggregation structure had a higher tensile strength and modulus, meanwhile, a lower elongation at break.Study on BTDA-BZD system showed that the process of crystallization consisted of two steps. Firstly, after held in 100°C oven for lh, the poly(amic acid) of BTDA-BZD formed a low-order aggregation structure. When the time in 100 °C oven was prolonged to 7h, the order of the aggregation structure disappeared. Secondly, after processed for 200 "C/lh, the molecular chains formed an order aggregation structure, and the process of 300°C/lh strengthened the order of aggregation structure. During the preparation of BTDA-BZD polyimide by two-step method, these steps of crystallization would affect the aggregation structure of final polyimide. The result of copolymerization illustrated that the polyimides had an order aggregation structure for alternative copolymer and irregular copolymer, but the order of the aggregation structure of polyimide copolymers was not so good as that of BTDA-BZD polyimide. BTDA-BZD polyimide had a lower tensile strength and elongation at break than those of polyimide copolymers, but a higher tensile modulus. There was no difference between the tensile properties of alternative copolymer and irregular copolymer.At last, computer molecular modeling was adopted to study the differences between the ability to crystallize and order of the aggregation structure of polyimides with different backbones. Simulation of conformation and energy needed to change conformation of single polyimide chains offered a kinetic reference to judge the ability of different polyimides to crystallize. Simulation of 3-D periodic boundary,from a thermodynamic point of view, explained the differences between the order of the aggregation structure of different polyimides.