Syntheses and properties of polyimides based on bis(p-aminophenoxy)biphenyls

Three biphenyl unit-containing diamines,4,4′-bis(p-aminophenoxy)biphenyl (III_a), 2,2′-bis(p-aminophenoxy)biphenyl (III_b), and 3,3′,5,5′-tetramethyl-4,4′-bis(p-aminophenoxy)biphenyl (III_c), were prepared by the chlorodisplacement of p-chloronitrobenzene with 4,4′-biphenol (I_a), 2,2′-biphenol (I_b), and 3,3′,5,5′-tetramethyl-4,4′-biphenol (I_c), respectively, giving the corresponding bis(nitrophenoxy) compounds II_a-c, followed by catalytic reduction with palladium (Pd) and hydrazine. Three series of polyimides p-PI, o-PI, and Me-PI were prepared from diamines III_a-c and aromatic tetracarboxylic dianhydrides via a two-stage procedure that included ring-opening polyaddition to give poly(amic acid)s followed by thermal cyclodehydration to polyimides. The resultant three series of poly(amic acid)s had inherent viscosities of 1.09-2.83, 0.78-1.93, and 1.55-3.09 dL/g, respectively. Almost all the poly(amic acid)s could be solution-cast and thermally converted into transparent, flexible, and tough polyimide films. All the polyimides were characterized by solubility, tensile test, wide-angle X-ray scattering measurements, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Effects of the structures of aromatic diamines and dianhydrides on the properties of polyimides were investigated.