Synthesis And Properties Of Soluble Copolyimides And Pyridine-containing Polyimides

Linear aromatic polyimides exhibit excellent thermal and thermo oxidative stabilities, chemical resistance, mechanical and electrical properties and have been used in gas separation, microelectronics, optoelectronics, aerospace devices, integrated electronic circuits, passivation coatings, substrate components, adhesives, composites and so on. However, aromatic polyimides are generally insoluble in organic solvents because of the rigidity of the backbone and strong interchain interactions, exhibit low optical transparency, and have intense yellow color. These facts may restrict their practical applications in some fields. Therefore, much effort has been concentrated on synthesizing soluble and transparent polyimides while retaining their desired properties. In this paper, the structural and performance features, synthesis methods and applications of polyimides were systemically reviewed, also the synthesis and modification of soluble and transparent polyimides as well as the pyridine-containing polyimides were detailedly introduced.Two kinds of copolyimides PI(BAPP-PBAP-BPADA) and PI(PBAP-ODPA-BPADA) were synthesized via the conventional two-step thermal or chemical imidization process by using2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) and4-phenyl-2,6-bis(4-aminophenyl) pyridine (PBAP) as the diamine monomers,4,4’-oxydiphthalic dianhydride (ODPA) and2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) as the dianhydride monomers in N,N-dimethylformamide (DMF). The structure of the polymers were characterized by FTIR. The results show that there were characteristic peaks of polyimides around1780cm-1、1720cm-1and725cm-1. The properties of the products were measured by solubility tests, X-ray diffraction (XRD), differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), tensile tests and water uptake tests. The resulting copolyimides were soluble in common organic solvents. They had good thermal stability with decomposition temperatures at initial above500℃in nitrogen atmosphere. The polyimide films had good mechanical properties. These films also possessed low water uptakes of less than1%.A novel pyridine-containing aromatic diamine monomer,4-(4-trifluoromethylphenyl)-2,6-bis[3-(4-aminophenoxy)phenyl]pyridine (m,p-3FPAPP), was successfully synthesized by a modified Chichibabin reaction of4-trifluoromethylbenzaldehyde and a substituted acetophenone,3-(4-nitrophenoxy)acetophenone (m,p-NPAP), followed by a reduction of the resulting dinitro compound4-(4-trifluoromethylphenyl)-2,6-bis[3-(4-nitrophenoxy)phenyl] pyridine (m,p-3FPNPP) with Pd/C and hydrazine monohydrate. The aromatic diamine was employed to synthesize a series of pyridine-containing polyimides by polycondensation with various aromatic dianhydrides in DMF via the conventional two-step method. The inherent viscosities of the resulting poly(amic acid)s and polyimides were0.62-0.89and0.58-0.69dL/g, most of the polyimides obtained by chemical imidization were readily soluble in common organic solvents such as DMF, N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), and tetrahydrofuran (THF), etc. Meanwhile, strong and flexible polyimide films were obtained, which had good thermal stability, with the glass transition temperature (Tg) of234.4-246.6℃and the temperature at10%weight loss of561.4-600.6℃in nitrogen atmosphere, and good optical transparency with the cut-off wavelengths of369-378nm, as well as outstanding mechanical properties with tensile strengths of89.5-96.4MPa, tensile moduli of1.49-1.63GPa and elongations at break of8.1%-9.8%. The polyimide films also were found to possess low water uptake of0.61%-0.79%.Thereafter, a novel pyridine-containing aromatic diamine monomer,4-phenyl-2,6-bis[3-(4-amino-2-trifluoromethyphenoxy)phenyl]pyridine (m,p-6FPAPP), was prepared in three steps starting from a modified Chichibabin reaction of3-hydroxyacetophenone and benzaldehyde to form4-phenyl-2,6-bis(3-hydroxyphenyl)pyridine (m-PHPP), followed by nucleophilic substitution reaction of m-PHPP with2-chloro-5-nitrobenzotrifluoride in the presence of potassium carbonate gives the dinitro compound4-phenyl-2,6-bis[3-(4-nitro-2-trifluoromethyphenoxy) phenyl]pyridine (m,p-6FPNPP). Finally, the novel diamine monomer m,p-6FPAPP was obtained in a high yield by the reduction of m,p-6FPNPP using hydrazine monohydrate catalyzed by Pd/C. The aromatic diamine was employed to synthesize a series of pyridine-containing polyimides by polycondensation with various aromatic dianhydrides in DMF via the conventional two-step method. The inherent viscosities of the resulting poly(amic acid)s and polyimides were0.72-1.09and0.59-0.78dL/g, all the polyimides obtained by chemical imidization were readily soluble in common organic solvents such as DMF, DMAc, NMP, DMSO, and THF, etc. Meanwhile, transparent, strong and flexible polyimide films were obtained, which had good thermal stability, with the glass transition temperature (Tg) of202.1-250.4℃and the temperature at10%weight loss of539.2-600.7℃in nitrogen atmosphere, and good optical transparency with the cut-off wavelengths of360-388nm, as well as outstanding mechanical properties with tensile strengths of92.3-99.1MPa, tensile moduli of1.58-1.89GPa and elongations at break of6.3%-9.4%. The polyimide films also were found to possess low water uptake of0.56%-0.69%.