Dielectric relaxation spectroscopy of amorphous and liquid-crystalline side-chain polycarbonates

The molecular dynamics of amorphous and liquid-crystalline (LC) side-chain polycarbonates was studied by dielectric spectroscopy at frequencies from 10(-2) to 10(6) Hz and at temperatures from -160 to 180 degrees C. 'Model' compounds containing no mesogenic side-groups showed two relaxations, which originate from the carbonate group (alpha, beta(m)-relaxation). By contrast, in LC-polycarbonates bearing a mesogenic nitrostilbene side group around and above the glass transition temperature Tg UP to three relaxation modes were distinguished (alpha-, lambda(1)-, lambda(2)-process); below T-g four secondary relaxations (gamma-, beta(m)-, beta(s)-, beta(sc)- relaxation) were observed. The gamma-relaxation was found only in compounds possessing an aliphatic spacer linked to the backbone by an ether bond. Apart from beta(m)-, two additional beta-processes were identified as relaxations associated with the mesogenic unit in the glassy (beta(s)) or in the crystalline state (beta(sc)).