Preparation And Properties Of Polyimides Based On Pyridine Group

Aromatic polyimides are well known as high performance polymeric materials for theirexcellent comprehensive properties, and have been widely employed in the different fields.With the fast development of different high-tech fields, the performances of the polymermaterials are simultaneously put forward to high requirements. Through structuralmodification, PI can make it with different functionality, adapt to different areas to get awider range of applications. This paper briefly introduces the polyimides development history,classification, properties, applications and preparation methods，and summarizes the modifiedmethod of PI, in particular structurally modified method.From the viewpoint of molecular design to improve the solubility of polyimide,incorporation of specific functionality into the polyimide backbones results in variousadvanced functional materials. Aromatic pyridine heterocycle possesses many goodcharacter, which is incorporated into the polymer backbone can effectively improve theheat resistance, chemical stability of the polymer, and improve the original mechanicalproperties. The C-N=C bond in pyridine possesses relatively high molar refraction(4.10) compared to a C=C bond (1.73). Consequently, it is just an efficient way thatincorporating of pyridine ring into polymer backbones to increase the refractive indexand optical transparency while not deteriorating thermal properties. Moreover, thepresence of a nitrogen atom in the polymer structure offers a polarized bond whichincreases dipole-dipole interactions in the polymer solvent system and improves thesolubility of the derived polymers.Fluorine atom with small polarization rate, can reduce the cohesion between the molecules, and reduces bulk density, and increases the free volume of the polymer, sothe solvent molecules can penetrate into the intermolecular to increase the solubility ofthe polymer; low surface free energy of fluorine atom can improve water and oilrepellency，and reduce water absorption and coefficient of friction. Fluorine atom hasthe strong electronegativity (4.0) and can cut off the conjugated electron clouds in themolecular structure of the PI, so that it has good optical transparency。C-F bondpolarization rate is small (0.66×10-24cc), which can weaken electromagnetic waveimpedance, and lower refractive index and dielectric constant of the polymer. Wedesigned and synthesized two novel diamines containing pyridine heterocyclic andfluorinated group,2,2’-bis[4-(5-amino-2-pyridinoxy)phenyl] hexafluoropropane (6FBAPDP)and2,2’-bis[4-(5-amino-2-pyridinoxy)phenyl] propane (BAPDP). The diamines wereincorporated into the backbone of polymer leads to great benefits for improving polymersolubility, thermal, optical property and dielectric performance. Meanwhile, BAPDP was usedto prepare polyimides with dianhydride for performance comparison. We explain the effect offluorinated groups incorporating into polymer. Moreover,2,2’-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (6FBAPP) was used to preparepolyimide which was used for performance comparison.Recently, a series of novel aromatic polyimides containing-O-,-SO2-and-S-groupshave been developed in an attempt to overcome the disadvantages of conventionalpolyimides, such as difficult to process for poor solubility in organic solvents, pooroptical properties, thus limiting their applications. Incorporation of oxygen-connectedether linkage into polyimides through diamine or dianhydride can often improve thesolubility and increase the transmittance. Sulfone group-containing polyimides exhibithigh glass transition temperature (Tg), mechanical strength, and optical transparency.Sulfur-connected ether linkage is more flexible for the existence of a3d orbit of sulfuratom which was incorporated into polyimides backbone can improve solubility andprocessability. Three novel monomers,4,4’-bis(5-amino-2-pyridinoxy)diphenylthioether,4,4’-bis(5-amino-2-pyridinoxy)diphenyl sulfone and 4,4’-bis(5-amino-2-pyridinoxy)diphenyl ether were synthesized to prepare polyimideswith3,3’,4,4’-oxydiphthalic anhydride (ODPA) and3,3’,4,4’-thiodiphenyltertracaboxylic dianhydride (TDPA), respectively. The influences of the molecularstructures on the thermal stability, optical transparency, mechanical property, solubilitywere systematically investigated. The resulting polyimides exhibited solubility in polarsolvents, such as DMAc, NMP, DMF, DMSO and m-cresol at room temperature.These polyimides with inherent viscosities of0.84-1.26dLg-1exhibited high enoughmolecular weight to give tough and flexible thin films by solution casting. The glasstransition temperature (Tg) values measured by differential scanning calorimetric andthermal stabilities based on5%weight loss temperature of PI films varied from226to305oC and from453to483oC, respectively. Interestingly, the influence of oxygen andsulfur-connected ether linkage in diamine and dianhydride on Tgexhibits the reverseorder. The polyimides also have good mechanical properties, such as tensile strengthsof101-114MPa, tensile modulus of2.8-3.2GPa, and elongations at break of8.8-33.3%.The cutoff wavelengths and transparency of PI films were ranged in338-400nm and72-84%at500nm, respectively. The water uptakes of the resulting PIfilms were in the range of0.5-1.4%.In the high-performance polymer materials, the molecular chain comprising biphenylenemay effectively change some properties of the polymer, resulting in high crystallinity, thermalstability, mechanical properties, but low solubility. For investigating the structure-propertyrelations of isomeric PIs, two series of novel isomeric diamine monomers,2,2’-bis(5-amino-2-pyridinoxy)biphenyl (2a),4,4’-bis(5-amino-2-pyridinoxy)biphenyl (2b),and4,4’-bis(5-amino-6-methyl-2-pyridinoxy)biphenyl (2c),4,4’-bis(5-amino-3-methyl-2-pyridinoxy)biphenyl (2d),4,4’-bis(5-amino-4-methyl-2-pyridinoxy)biphenyl (2e) which were employed to react withvarious aromatic dianhydrides to yield a series of novel polyimides through a typical two-steppolymerization method. The experimental results show that the incorporation of methylgroups at the ortho-position of pyridine diamines is an effective way to improve the glass transition temperature, mechanical properties, solubility and optical transparency of PIs. Allthe polyimides containing methyl substituted pyridine show good solubility in DMAc，DMF，NMP，DMSO，m-cresol. Glass transition temperatures (Tg) of the polyimides were in therange of249-313oC and241-348oC, as obtained by DSC and DMA, respectively.5%weightloss temperatures were located in the range of507-533oC in nitrogen. The residual weightretentions at800oC were in the range of58.7-65.1%in nitrogen. The coefficients of thermalexpansion (CTEs) of the polyimides ranged from21to62ppmoC-1。Polyimide films had thetensile strength as high as134MPa, the tensile modulus in range of2.1-3.6GPa, and theelongation at break of5.4-21.3%, indicating that the obtained polyimide films were tough andstrong. All the polyimides containing methyl substituted pyridine show good optical property,and λcut-offvalues were in the range of310-455nm. Polyimide films have the transmittance (%)as high as82. We discussed the different performance effects of heterocyclic pyridine and thebiphenyl structure of polyimide.