Mesomorphic behaviour in copoly(ester-imide)s of poly(butylene-2,6-naphthalate) (PBN)

Copolycondensation of N,N’-bis(4-hydroxybutyl)-biphenyl-3,4,3',4'-tetracarboxylic diimide at 20 and 25 mol% with bis(4-hydroxybutyl)-2,6-naphthalate produces PBN-based copoly(ester-imide)s that not only crystallise but also form a (smectic) mesophase upon cooling from the melt. Incorporation of 25 mol% imide in PBN causes the glass transition temperature (measured by DSC) to rise from 51 to 74 °C, a significant increase relative to PBN. Furthermore, increased storage- (G'), loss- (G'') and elastic (E) moduli are observed for both copoly(ester-imide)s when compared to PBN itself. Structural analysis of the 20 mol% copolymer by X-ray powder and fibre diffraction, interfaced to computational modelling, suggests a crystal structure related to that of α-PBN, in space group P-1, with cell dimensions a = 4.74, b = 6.38, c = 14.45 Å, α = 106.1, β = 122.1, γ = 97.3°, ρ = 1.37 g cm-3.