SYNTHESIS AND PROPERTIES OF THERMOTROPIC POLYESTERS MODIFIED WITH A NONMESOGENIC 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'-bi benzoate (BE) with a dimethyl phthalate (DMT: dimethyl terephthalate o r 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 t he non-mesogenic rigid phthalate unit. All BB6-DMT copolyesters remain crystalline. As x, the molar fraction of the phthalate units in the d iacid units, greater than or equal to 0.7 the mesophase of the BB6-DMT copolyesters is destroyed completely. For BB5-DMT copolyesters, the m esophase disappears as x greater than or equal to 0.4, and the copolye sters become amorphous as 0.5 less than or equal to x less than or equ al to 0.8. The mesophase and the crystallinity of the BB6-DMI copolyes ters are destroyed completely as x > 0.5. The BB5-DMI copolyesters los e the mesophase as x greater than or equal to 0.3, and become amorphou s as x greater than or equal to 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 un it.