The Synthesis And Structure-Properties Study Of Polyimides With Fully Aromatic Ester Side Chains

The technological applications of many polyimides are somewhat limited by processing difficulties owing to their insolubility in organic solvents. Therefore, the preparation of soluble or thermoplastic polyimides has been a major interest in research work. Current and prior attempts to produce soluble and processable aromatic polyimides involved the introduction of mesogenic bulky side groups. Thus the synthetic chemistry and structure-properties relationship of polyimides with mesogenic side chains is of great interest. The studies in this dissertation have focused on the synthesis, characterization and structure-properties relationship of polyimides with various side chains. Based on two different kind of newly prepared diamines containing biphenyl or fully aromatic ester mesogenic units, several novel homopolyimides, copolyimides and copoly(ester)imides were synthesized by polycondensation. Many fundamental questions related to the synthetic chemistry and physics were studied, such as: the methodology of introducing side chains, the effect of length/diameter ratio of side-chain, the control of macromolecular structure and architecture, and the formation and control of ordered structure. And the methodology of side chain modification and functionalization has made the polyimides combination of processability and outstanding properties. In the second and fourth chapters, two different type of novel diamines containing biphenyl or aromatic esters groups 3,5-diamino-benzonic-4'-diphenyl ester, 4-(3'',5''-diamidobenzoxy)benzonic-4'-diphenyl ester, and 4'-(p-methoxybenzoyl ) -phenyl -3,5-diaminobenzoate, 4 ' -[4 ' ' -(p-methoxybenzoxy)-benzoxy]-phenyl -3,5-diaminobenzoate were synthesized using 3,5-dinitrobenzoic acid as starting material. All the monomers and the intermediates were characterized by melt points, Fourier transform infrared and 1H-nuclear magnetic resonance spectroscopy; and the results were in good agreement with the proposed structures. Based on these newly synthesized diamine monomers, a series of polyimides with various side mesogenic units were prepared through classical two-step thermal imidization process or one-step high temperature solution imidization process. The influences of the approach adding monomer, the imidization conditions and imidization proceed on the resulted films were studied. It has been recognized that the aggregation structures of polymer are largely affected by the chemical structures. The combination of WAXD and POM studies revealed that all four side-chain homopolyimides derived from 4 , 4 ' -oxy-diphthalic anhydride (ODPA) form different aggregation structures due to different length-diameter ratio of side-chain. With side-chain longer, a regular sequence was formed in homopolyimide with aromatic ester side-chain end-capping of methoxy group. While biphenyl side group was introduced onto homopolyimide main-chain, indicated that the polyimides backbones were in the extended conformation and packed paralleled to each other, and the pendent biphenyl groups occupied the space between the layers, perpendicular to the main chains. The influence of length-diameter ratio of side-chain on the aggregation structures of polymer was also investigated, and results showed that single-layer structure was formed by shorter side-chain in the crystalline region, and double-layer structure with lower orderliness was made by longer side-chain in the amorphous region. As the same time, bis-(trimellitic anhydride) hydroquinone involving ester structure was introduced in the polyimide backbone, two co-polyester-imides were prepared based on the two novel diamines with aromatic ester end-capped by methoxy group: MBP and MBBP. These two kinds of polyester-imides and polyimides films were preliminary characterized by polarizing optical microscope, X-ray diffraction and differential scanning calorimeter. The results indicated that, due to the different components of backbones and/or side chains, the aggregationstructures of these polyester-imide are different. Especially, the polyester-imide with shorter side-chain from MBP shown an apparent liquid-crystal schlieren texture when treated above glass transition temperature, and 3D crystalline structure was formed by longer side-chain at room temperature. By SAXD spectras,while fully aromatic ester side groups were introduced onto polymers main-chain, layer-structure was formed, and indicated that single-layer structure belonging in crystalline domain was formed by shorter side-chain with biphenyl group, and double-layer structure with lower orderliness was made by longer side-chain of biphenyl group or fully aromatic ester side-chain end-capped by methoxy group in amorphic domains. The influence of chemical structure and aggregate on the mechanical properties, thermal stability, solubility, and density were investigated through stress-strain, DSC, and TGA. The stress-strain tests indicated that the tensile strength of the polyimide films with biphenyl side-chain was maintained, even improved because of its more regular structure. But the tensile strength of the polyimides with fully aromatic esters and end capping of methoxy group somewhat decreased due to the lower reaction ability of the diamines. By incorporation of side-chain, the interaction of polyimide backbones was lowered. Thereby, the density and Tgs were minished and the solubility was enhanced. And with longer side-chain,the trends were more obvious. TGA analysis revealed that the decomposing temperature at 5% weight loss of about 400 ℃, and displayed a multi-step degradation behavior corresponding to the degradation of side chains and main chains respectively.