Abstract
Four series of copolyesters, namely BB6‐DMT, BB5‐DMT, BB6‐DMI and BB5‐DMI series, were prepared by melt polycondensation of dimethyl 4,4′‐bibenzoate (BB) with a dimethyl phthalate (DMT: dimethyl terephthalate or DMI: dimethyl isophthalate) and an alkanediol (1,6‐hexanediol or 1,5‐pentanediol). The homopolyesters poly(hexamethylene 4,4′‐bibenzoate) (BB6) and poly(pentamethylene 4,4′‐bibenzoate) (BB5) exhibit a smectic phase. The thermotropic liquid crystalline and crystalline properties of the copolyesters are significantly influenced by the presence of the non‐mesogenic rigid phthalate unit. All BB6‐DMT copolyesters remain crystalline. As x, the molar fraction of the phthalate units in the diacid units, ≧ 0.7 the mesophase of the BB6‐DMT copolyesters is destroyed completely. For BB5‐DMT copolyesters, the mesophase disappears as x ≧ 0.4, and the copolyesters become amorphous as 0.5 ≦ x ≦ 0.8. The mesophase and the crystallinity of the BB6‐DMI copolyesters are destroyed completely as x > 0.5. The BB5‐DMI copolyesters lose the mesophase as x ≧ 0.3, and become amorphous as x ≧ 0.4. The results indicate that the non‐linear isophthalate unit destroys mesophase and crystallinity of the copolyesters to a greater extent than the para‐linked terephthalate unit.