Relationship Between Composition, Structure and Dynamics of Main-Chain Liquid Crystalline Polymers with Biphenyl Mesogens

Abstract

Semiflexible polyesters prepared by an alternating arrangement of biphenyl mesogenic units and aliphatic spacers represent an ideal model for studying the behavior of main-chain liquid crystalline polymers (MCLcp). The transition temperatures, type of mesophase and rate of mesophase–crystal transformation can be tailored by the utilization of a suitable flexible spacer, so that polymers with a stable mesophase at room temperature can be prepared. Furthermore, if the structure of the spacer is adequately chosen, the liquid-crystallization can be slowed down, and sometimes it is possible to quench the amorphous state by cooling the isotropic melt at not very high rates. Following these strategies a rich variety of glass forming liquid crystalline polymers can be prepared. The dynamical behavior of a MCLcp is closely related to the phase present. The segmental dynamics (α relaxation) is the result of cooperative motions near the glass transition of repeating units of the chains built in the structure. The α process behaves differently in the amorphous than in the liquid crystalline (or crystalline) states. Here, the preparation of the polymers and the analysis of their mesomorphic behavior by differential scanning calorimetry and X-ray diffraction are reviewed. The discussion of mechanical/electrical relaxations related to different phases is also included. Eventually, the link between molecular composition, structure and dynamics of the MCLcps will be established.