Molecular Design And Properties Of（A₂+B₄）-type Hyperbranched Polyimides

As an important kind of special engineering plastics, polyimides possess excellent thermal stability、mechanical property、dimensional stability、chemical stability and so forth, and become a kind of most popular materials for application among all thermal resistant polymers. Now polyimides have been widely used in many high technology fields and exhibited excellent performance in every field, such as aerospace、electronics、nuclear industry、info-communications and so on. However, the rigid molecular structure and the strong interaction among molecular chains lead to poor solubility and fusion property, resulting in the restriction of applications. So, to prepare the polyimides with excellent comprehensive performance and outstanding machinability has become a focus for the investigators in the future.Hyperbranched polyimides(HBPIs) possess excellent properties for the three-dimensional global molecular structure、highly branched molecular chain and amounts of terminal groups which linear polyimides do not own, such as low viscosity、outstanding solubility、chemical versatility and so forth. Since Moore began to study the HBPIs in 1999, the unique molecular structure、excellent physical and chemical property、flexible synthesis method render the HBPIs potential application value in many fields, including proton exchange membrane、gas separation membrane、optical waveguide、fluorescence and dielectric fields. However, researchers have mainly focused on the AB2 or A2+B3 type hyperbranched polyimides, as a result, to study the hyperbranched polyimides with larger amount of terminal groups will be a novel and interesting work. This thesis which based on the synthesis and functionalization of hyperbranched polyimides from the point of molecular design has studied the physical and chemical properties of the A2+B4 type fluorinated HBPIs deeply and scientifically, and tried to do some useful exploration for exploiting the new fields of HBPIs.In chapter 1, a novel kind of tetra-amine 4,4’-di[3,5-di(2-trifluoromethyl-4-aminophenoxy) phenoxy] sulfone has been designed and synthesized successfully. Through the MALDI-TOF mass spectrum、FTIR and 1H NMR measurement, the structure of the synthesized monomer has been proved to be accordant with designed molecule structure. The tetra-amine has been employed to react with commercial dianhydrides(PMDA、ODPA、BPDA、BTDA and 6FDA) to prepared polyimides through chemical imidization method. The HBPIs were anhydride-terminated through the control of monomer feeding ratio and feeding sequence, and the structure was proved by FTIR and 1H NMR spectra. In addition, the solubility、thermal、mechanical、optical properties of HBPIs have been explored carefully. The polymers exhibited excellent solubility in the strong polar solvents(such as NMP、DMF and DMAc) and less polar solvents(such as pyridine and THF) for the CF3 groups in the tetra-amine. Also the CF3 groups endow the polymers low moisture absorption(0.16 %~0.67 %). The polymers possess high glass transition temperature(Tg ≥ 229 ℃) and thermal decomposition temperature(T5% ≥ 469 ℃, nitrogen atmosphere). The molecular structure of hyperbranched polymers was isotropic, which could weaken the molecular chain orientation along the substrate; as a result, the polymers possess low birefringence values(Δn < 0.01). However, the poor mechanical property of HBPIs caused by the low intermolecular entanglements should be improved significantly.In chapter 2, to resolve the problem in chapter 1, a series of long chain dianhydride monomers have been synthesized to increase the entanglement ability of polymer chains, subsequently, the dianhydrides have been reacted with tetra-amine to synthesize anhydride-terminated hyperbranched polyimides, and the relevant mechanical、thermal and optical properties have been investigated. It has been discovered that, the mechanical property has been improved obviously. The tensile strength was in the range of 91~112 MPa while the modulus was in the range of 2.14 ~ 2.57 GPa, indicating the HBPIs could compare favorably with linear polyimides. At the same time, the polymers showed coefficient of thermal expansion in the range of 34.4 ~ 66.9 ppm·K-1. The introduction of ether groups in the dianhydride did not deteriorate the thermal stability and improve the solubility of polymers, in detail, the Tg ≥ 230 ℃, and T5% ≥ 480 ℃(in nitrogen atmosphere). However, the flexible ether groups could decrease the birefringence, indicating that the spatial structure would be more isotropic. At the same time, the refractive indices could be adjusted through the change of the dianhydrides(1.5780 ≤ nav ≤ 1.6270). Especially, the polyimide derived from fluorinated long chain dianhydride exhibited the outstanding comprehensive properties.In chapter 3, to employ large amount of terminal groups in the HBPIs sufficiently, the fluorinated long chain dianhydride was reacted with the tetra-amine to get the anhydride terminated hyperbranched polyamic acid, then the aniline-type monomers were added to modify the polyamic acid, at last, the hyperbranched polyimides with different terminal groups were obtained through chemical imidization method. The structure of polymers was affirmed through FTIR and element analyzation method, and the mechanical、thermal、optical properties were carefully characterized. It has been discovered that, the polymers whose terminal groups were modified still possess good mechanical property, low birefringence values. And the solubility was improved obviously, while the glass transition temperatures were decreased slightly(191 ℃ ≤ Tg ≤ 201 ℃). It was noteworthy that the thermal stability could be increased obviously(T5% ≥ 565 ℃, nitrogen atmosphere) through the modification. In addition, the refractive index could be adjusted through the change of terminal groups. The polyimide films showed excellent transmittance in the UV-visible region(λcutoff < 360 nm), and the transmittance at 800 nm was in the range of 86 %~90 %. Through the measurement of near infrared spectra for polymer films, the polymers showed low absorption at communication bands(1310 nm and 1550 nm), indicating that the polymers possess potential application prospect in optical communication field.In conclusion, the thesis designed and synthesized a series of hyperbranched polyimides which were centered around the tetra-amine 4,4’-di[3,5-di(2-trifluoromethyl-4-aminophenoxy) phenoxy] sulfone. The polymers not only maintain the inherent excellent properties of polyimides, but also possess the unique property of hyperbranched polymers through rational structure design. The results of optical measurements indicate that the polymers possess the potency for the optical application.