Langmuir-Blodgett films of photochromic polyglutamates, 10. The influence of the side chain architecture on the thermal and monolayer forming properties of “hairy rod”-like polymers

The phase behavior of so called “hairy rod”-like polymers with mesogenic elements in the side chains was investigated by means of differential scanning calorimetry and X-ray diffraction and correlated with the properties of their monolayers at the air/water interface. The side chains of the polymers form a highly ordered (crystalline?) structure. The degree of crystallinity and the temperature of the phase transition to a less ordered liquid crystalline structure depend strongly on the side chain architecture. A higher degree of crystallinity is found for a decyloxy tail group than for a hexyl tail group, since alkoxy groups at the mesogenic elements can be packed more tightly. X-ray diffraction proves that the side chains with a hexyl tail form a liquid crystalline phase, which may provide a fluid matrix for the rigid rod like backbones. The monolayers of these polymers with the side chains in a liquid crystalline phase at room temperature consist of soft domains and can be transferred to homogeneous Langmuir-Blodgett films. By way of contrast the polymers in which the side chains are in a more highly ordered (crystalline) phase form stiff domains in the monolayers, resulting in isotherms with a shallow increase in surface pressure. Furthermore, a strong temperature dependence is found for the isotherms which can be interpreted to be a result of domain softening. The monolayers of the polymers with highly ordered side chains can be transferred only after inducing some disorder either photochemically or by increasing the temperature.