Effect of flexible spacer length on the mesophase structures of main-chain/side-chain liquid crystalline polymers based on ethyl cellulose

Main-chain/side-chain liquid crystalline polymers (MCSCLCPs) based on ethyl cellulose (AzomEC, m=2, 4, 6, where m is the length of the spacer between the main chain and mesogens) were successfully synthesized by N,N′-dicylcohexylcarbodiimide (DCC) coupling. Molecular characterizations of the resultant polymers with different spacer lengths were performed by proton nuclear magnetic resonance, Fourier transform-infrared spectroscopy and gel permeation chromatography. The phase transitions and liquid crystalline (LC) behaviors of these polymers were investigated by differential scanning calorimetry, polarized optical microscopy and wide-angle X-ray diffraction. The results indicate that spacer length has a tremendous effect on the LC behavior of the polymer. The glass transition temperatures, phase transition temperatures and the corresponding enthalpy of transitions decreased as the flexible spacer length increased. The mesophase structures of polymers consisted of a large-scale ordered lamellar structure composed of the EC main chain and a relatively small-scale ordered structure formed by azobenzene side chains. All the polymers form a similar lamellar structure on a large scale; however, the small-scale ordered structure becomes relatively disordered, that is, from crystal (m=2) to smectic B (m=4) to smectic A (m=6) at low temperatures.