Role of specific interactions in determining the mesogenic behavior of side-chain liquid crystal polymers

A new series of side-chain liquid crystal copolymers has been prepared, poly[4-[[[[[(4-methoxyphenyl)azo]phenyl-4]oxy]-8-octyl]oxy]styrene]-co-poly[4- [[α-[[[(4-nitrophenyl)azo]phenyl-4]oxy]-ω-alkyl]oxy]styrene]s, containing electron rich and electron deficient mesogenic groups. The length of the spacer attached to the electron rich group, 4-methoxyazobenzene, is held at 8 methylene units, while that attached to the electron deficient group, 4-nitroazobenzene, is varied in length from 3 to 12 methylene units. The thermal properties of the polymers have been characterized using polarized light microscopy and differential scanning calorimetry. All the copolymers exhibit a smectic A phase. The thermal properties of the copolymers are compared with those of the corresponding homopolymers. The glass transition temperatures of the copolymers are typically within experimental error of the weighted average of those of the respective homopolymers. By contrast, the smectic A-isotropic transition temperatures of the copolymers are significantly higher than the average of those of the corresponding homopolymers. These differences are expressed as reduced deviations and rationalized in terms of a specific interaction between the unlike mesogenic units in the copolymer.