Cocrystalline Copolyimides of Poly(ethylene 2,6-naphthalate)

Copolycondensation of N,N′-bis­(2-hydroxyethyl)-biphenyl-3,4,3′,4′-tetracarboxylic diimide (5–25 mol %) with bis­(2-hydroxyethyl)-2,6-naphthalate affords a series of cocrystalline, poly­(ethylene 2,6-naphthalate) (PEN)-based poly­(ester imide)­s. The glass transition temperature rises with the level of comonomer, from 118 °C for PEN itself to 148 °C for the 25% diimide copolymer. X-ray powder and fiber diffraction studies show that, when 5 mol % or more of diimide is present, the α-PEN crystal structure is replaced by a new crystalline phase arising from isomorphic substitution of biphenyldiimide for PEN residues in the polymer crystal lattice. This new phase is provisionally identified as monoclinic, C2/m, with two chains per unit cell, a = 10.56, b = 6.74, c = 13.25 Å, and β = 143.0°.