Controllable Aggregation Structure Preparation And Films Properties Study For Polyimide Based On PMDA-ODA

With the fast development of high and new technology and industry, polyimides (PIs) become a kind of indispensable material, and it is also important for civil and military use. PIs are a typical rigid chain polymer, which has regular primary structure, and possesses few kinds of conformation. Theoretically speaking, it is easy to form stable ordered aggregation structure. However, since their T_g or T_m is usually ultra-high, it is soluble to a few kinds of organic solvent, their processabilities are seriously confined, and the formation and control of ordered aggregation structures are also disturbed, which directly affects their properties. This contribution mainly investigated the polyimide based on PMDA-ODA from both thermodynamic and kinetic ways. The aim is to realize its controllable preparation of aggregation structure and properties by controlling technology, introducing regular rigid structure into molecule chains and filling nucleating agent, to systematically understand structure evolution and controlling during imidization, to reveal some science and technology problems for polyimide preparation, and to found some semi-quantitative ways and measures.Generally speaking, there are two ways to introduce rigid structures into the molecular chain: one is in-situ, the other is through copolymerization. In this paper, polyamic acid (PAA) based on PMDA-ODA was first partly imidized to bring rigid structures in situ, and formed irregular copolyamic acid-imide (PA-I). When imidization degree reached a certain point, phase separation of the copolymer solution occurred. A systematic investigation was done by rotation viscometer and FT-IR. In chemical imidization, with the increasing of acetic anhydride amount or precursor concentration, phase separation speed became fast, and imidzation degree at critical point of phase separation (ID_c) was almost content. In thermal imidizaiton, with temperature rising, phase separation speed became fast and ID_c increased, the effect of precursor concentration is the same as in chemical imidization. Imidization degree of PA-I solution could be controlled by using adequate acetic anhydride amount (or temperature), precursor concentration and imidization time by 3Ddiagram. Therefore, aggregation structure and properties of polyimide films can be controlled. As imidization degree of solution increased by chemical imidization, ordering extent was increased, and self-reinforced role was played since modulus and tensile strength were improved. While in thermal imidization, mechanical properties were decreased due to reduced molecular weight. Synthesis of block-copolymer is another important work in this paper to bring in rigid structure. Block-copolyimide film from pPDA/ODA-PMDA (b-PI) was studied by TEM, SEM, PLM, ultra-deep three-dimensional microscope, TMA, and WAXD, which showed that the extensile strength and modulus of b-PI film were higher than that of irregular copolyimide film (r-PI). It was also revealed that there was a large amount of crystallite formed in b-PI film, which was mainly attributed to its chain structure: rigid pPDA-PMDA chains (PO) were easy to form crystal nucleus because of phase separation, based on which semi-rigid PMDA-ODA molecular chains began to crystallize. The crystallization of ODA-PMDA chains and imidization were simultaneous on the whole. During crystallization, the amount of nucleus is mainly determined by the ratio of pPDA/ODA and the chain length of PO, while the crystal size only depends on the latter. Higher ratio of pPDA/ODA and shorter PO resulted in more crystal nucleus. Shorter PO led to less crystal size. Thus, it is to realize controllable preparation of aggregation structure and properties by regulating the ratio of pPDA/ODA and sequence length of two block segment. In order to study the relationship between technology conditions and the aggregation structure and properties of PI films, two different solvents: NMP(b.p.=202℃) and DMAc(b.p.=166.1℃) were used, and films were prepared through differefit motheds:R1(direct thermal imidization of PAA solution), R2 (thermal imidization of partial imidized PA-I film) R3 (thermal imidization of partial imidized PA-I solution) and heating rate (1℃/min, 3℃/min, 5℃/min, 7℃/min and 10℃/min). TGA and FTIR revealed that the imidization degree and residual solvent were largly affected by heating rate. In the earlier stage of imidization, the main process is solvent evaporation; during the middle stage, imidization and solvent evaporation are undergoing synchronously, and which one is the main factor is determined by heating rate; in the late stage, solvent has almost all evaporated, and there is only imidization process. WAXD study betokened that both too fast and too slow heating rate go against the formation of ordered aggregation structures in the film; while moderate heating rate made imidization temperature, solvent evaporation speed and chain mobility cooperate well and facilitated chains arrange into regular crystalline structure. The higher the boiling temperature of solvent is, the lower of optimal heating rate is to obtain regular structure. For R1, when NMP and DMAC are used as solvent, the optimal heating rates are 5℃/min and 7℃/min, respectively. When solvent is NMP, the difference of optimal heating rate for R1, R2 and R3 differ in ordering extent of polymer molecular chains (R1≈R2＜R3) and imidization degree (R1＜R2＜R3) before thermal imidization. If molecular chains are irregular arranged before imidization, optimal heating rate is higher when imidization degree is higher. The optimal heating rates are 5℃/min, 7℃/min and 1℃/min, respectively for R1, R2 and R3. Higher ordering extent of aggregation structure resulted in higher tensile strength and moldulus.In order to understand the relationship between crystal structure and imidization degree, a series of PA-I powders with different imidization degree were prepared from PAA based on PMDA-ODA, and a systematic investigation was carried out by X diffractometer and Pawley method. X-ray diffraction (XRD) results of these powders indicate that PA-I powders are crystals with high-imperfection and that higher imidization degree, more imidization time or higher imidization temperature in solution is favored for perfection of their crystals. Reflections of crystalline PA-I in XRD patterns deducted amorphous PA-I bump was used to index and calculate crystalline structure. The structure is triclinic and space group is P1. XRD patterns of crystalline portion was refined with Pawley method and Pawley refinement indicated that unit cell parameters, crystallite size and microstrain of PA-I powders changed regularly as imidization time prolonged or imidization temperature rose. SEM showed that the morphology of these powders was like sheaf, and similar to spherulite. Higher imidization degree led to greater crystal and more regular shape. Higher imidization temperature resulted in less crystal and greater crystal.When studying the effect of nucleating agent to aggregation structure, it was found that when talc was filled into PAA based on PMDA-ODA, its lamella structure can not only act as nucleating agent to induce PMDA-ODA molecular chains crystallize, but it also can well distribute in the polyimide system and form net-like texture during the conversion from PAA film to PI one. PLM and WAXD study revealed that the appearance of net-like structure was caused by the formation of hydrogen bond between talc and PAA. In PAA solution, PAA forms hydrogen bond with the solvent, while the aggregated talc particles does not, which results in poor distribution of talc in the solution. When PAA film was treated at 50℃for 30min, and at 100℃for 1h, most of the solvent has evaporated, and part of the hydrogen bond between PAA and solvent was destroyed, while hydrogen bond formed between talc and PAA, which resulted in good distribution of talc particles in the PAA matrix. It is suggested that the lamella structure of talc can act as crosslink point that make the PAA film exhibit net-like texture. As temperature rises, solvent keeps on losing, and PAA gradually turns into rigid structure PI. The fine distribution of talc particles and net-like structure are both kept, and crystallization is more obvious. AFM and SEM investigation showed that lamella talc particle had regular shape (the dimension is about 0.8～1μm). They arranged in parallel with the film surface, and distributed equally in the film. Considering all properties, the most suitable filled amount of talc is 3％.