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°.