Synthesis And Characterization Of Thermoplastic And Thermosetthing Copolyimide From Isomeric Biphenyltetracarboxylic Dianhydrides (i-BPDA/s-BPDA)

Aromatic polyimides are well known as polymers with outstanding mechanical and dielectric properties and high temperature capabilities. They can be utilized for a wide range of applications:such as aerospace and microelectronic industries. Polyimid comprise two types:thermoplastics polyimid and.thermoset polyimid. They have advantages and disadvantages. The thermoplastics polyimid have good toughness, however, most of the commercial available aromatic thermoplastics polyimides are based on phthalimide unit which owns a five-membered imide ring, they are difficult to process, because of their insolubility in organic solvents as well as high glass transition and melt temperature. And the thermoset polyimid have good process-ability, now the widespread use of thermosetting polyimide mainly PMR-15, PETI-5, TriA-PI and other types, but these materials have a lower elongation at break and poor toughness.Herein, a series of organosoluble copolyimides was successfully synthesized from the 2,2',3,3'-Biphenyltetracarboxylic dianhydride (i-BPDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) with aromatic diamine 4,4',-ODA. The copolyimides were characterized by DSC, TGA, DMA and tensile apparatus. And they hold superior mechanical and thermal properties. The i-BPDA with a non-linear and non-coplanar but rigid structure, so the polyimids from i-BPDA have bent and non-coplanar but rigid poiymer backbones, it can effectively reduce the molecular chain packing density and destroy the symmetry and regularity of the molecular chain. So the polyimides showed improved solubilities, depending on the i-BPDA content. A regular increase of glass transition temperatures, a decrease of inherent viscosities of PAA and a decrease of the tensile modulus, tensile strength and elongation of the films at break with increasing content of i-BPDA were also observed.In recent years, TriA-PI, a new phenylethynyl terminated addition-type a-BPDA/4, 4'-ODA with PEPA imide oligomer, has been exploited as a high-performance materials with higher Tg, good mechanical properties and lower melt viscosity. But a-BPDA is very expensive, limiting their application.In this paper, new kinds of all-aromatic phenylethynyl-terminated imide oligomers were prepared by the reaction of 3,3',4,4'- biphenylenetetracarboxylic dianhydride(s-BPDA) and isomeric 2,2',3,3'-biphenylenetetracarboxylic dianhydride (i-BPDA) with 4,4'-ODA and PEPA. The molar ration of i-BPDA and s-BPDA was 50/50 and designed the molecular weights of imide oligomers were about 1000,2000, 3000,5000g/mol (n=1,3,5,9). The Olig-5(n=5) showed good tensile properties with strength at break of 112MPa, modulus of 2.2GPa and 17.0% elongation. It also had excellent thermal stability, the temperature of 5% weight loss was 558℃in N?, the Tg of the cured polymer was 339℃, and the oligomer possessed low melt viscosity above 300℃. Most of properties i.e. the mechanical properties, the thermal stability properties and the melt viscosity, were almost comparable with TriA-PI having molecular weight of 2500g/mol. The thermal curing process of the oligomers was also investigated by differential scanning calorimetry(DSC) by monitoring the change of Tg and the exothermic peak. Melt viscosity, thermal and tensile properties of oligomers with different molecular weight were also investigated. After the deeply evaluation of the oligomers, the Olig-5 was used for a long term thermal-stablity test. The oligomer was exposed at a 177℃air atmosphere for 500 hours, the thermal and mechanical properties of the Olig-5 didn't change. Then the good thermal stability of the oligomer was proved.