Synthesis and properties of thermotropic polyesters modified with a non‐mesogenic rigid group

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.