Study on the Synthesis and Properties of Copolyimides Derived from ODPA/Isomeric ODA
|School||Beijing Institute of Clothing Technology|
|Keywords||copolyimide mechanical property thermal properties crystalline behavior soluble relations|
In this article, a series of copolyimides based on 3,3’,4,4’-oxydiphthalic dianhydride (3,3’,4,4’- ODPA) and isomeric oxydianiline (4,4’-ODA and 3,4’-ODA) were synthesized by thermal imidization and chemical imidization through two-step procedure. The molecular weights were controlled using phthalic anhydride (PA) as an end capping agent. We also synthesized the block copolyimides by controlling the addition order of reactants. The thermal properties, mechanical properties and crystalline behavior were investigated by means of TGA, DSC, Intron and WAXD. The thermal stability and crystallization kinetics of PIs were investigated by means of thermogravimetric analysis(TGA) and differential scanning calorimetry(DSC) in nitrogen, the PIs showed great thermal stability with a thermal decomposition temperature (Td,5%) of over 500℃. DSC analysis gave class transition temperature (Tg) ranged from 245℃to 247℃. Three kind of random copolyimides had no melting peak at room room temperature. Obvious fusion peaks appeared in the subsequent DSC heating scan after isothermal crystallization process. In addition, crystal domains become more perfect through prolonging the time of isothermal crystallization and/or elevating the crystallization temperature, demonstrated by higher melt temperature (Tm) and fusion enthalpy. And the double endothermic melting peak appeared during the melting process. However, the copolyimide with more 3,4’-ODA loading (over 10%, molar ratio) showed no crystalline behavior, even after isothermal treatment. Three of the polyimides dissolve well in m-cresol at room temperature. They can dissolve or partly dissolve in NMP and DMSO if giving their proper heating. But in DMAc and DMF, heating also cannot make them dissolved.The block copolyimides still maintained the high thermal stability. The thermal properties and crystalline behavior had not changed significantly, compared with the random copolyimides. But their mechanical properties were obviously improved. With the loading of 3,4’-ODA increases, the elastic modulus and tensile strength decreased. Their solubility, compared with the random copolymer imide, had not been significantly improved.PI powders obtained via chemical imidization show good thermal stability. A double melting peak appeared between 320℃and 375℃. The double-crystal melting peak is due to the different degree of perfection combined with the WAXD curve, not due to the different crystalline structure. The degree of crystallinity gradually decreased as the third monomer 3,4’-ODA content increased. The solubility of PI powder greatly improved compared with the PI films.