INFLUENCE OF LATERAL SUBSTITUENTS IN THE MESOGENS ON THE PROPERTIES OF LIQUID-CRYSTALLINE SIDE-GROUP COPOLYMETHACRYLATES - DIELECTRIC-RELAXATION SPECTROSCOPY ON HOMOPOLYMERS
A. Schonhals et al., INFLUENCE OF LATERAL SUBSTITUENTS IN THE MESOGENS ON THE PROPERTIES OF LIQUID-CRYSTALLINE SIDE-GROUP COPOLYMETHACRYLATES - DIELECTRIC-RELAXATION SPECTROSCOPY ON HOMOPOLYMERS, Macromolecular chemistry and physics, 196(5), 1995, pp. 1671-1685
Broad-band dielectric spectroscopy in the frequency range from 10(-2)
Hz to 10(6) Hz and in a temperature range from 170 K to 430 K was empl
oyed to study the molecular dynamics in polymethacrylates with novel c
alamitic side groups. As mesogenic units, derivatives of p-methoxyphen
yl benzoate were used where the hydrogen in the meta position of the b
enzoate ring is substituted by a methoxy group or by bromine. The subs
tituted samples do not show any mesophases. The following results were
obtained: (i) The beta-relaxation which corresponds to a rotational f
luctuation of the mesogen around its long axis is drastically suppress
ed by the lateral substitution. Moreover the activation parameter of t
his process depends strongly on the actual mesophase structure of the
material under study. (ii)) The alpha-relaxation is related to the dyn
amic glass transition of the system. It is shown that the value of the
glass transition temperature of the liquid-crystalline polymethacryla
te can be understood as competition of internal plasticization and mol
ecular ordering. (iii) The delta-relaxation corresponds to a rotationa
l fluctuation of the side group around its short axis. From comparison
of the relaxation rates of the alpha- and delta-relaxation, the tempe
rature dependence of the order parameter of the liquid-crystalline pol
ymethacrylate is estimated by dielectric spectroscopy.