M. Laus et al., SEMIFLEXIBLE LIQUID-CRYSTALLINE POLYESTERS BASED ON TWIN BIS(P-OXYBENZOYL)UNITS .2. EFFECT OF MOLAR-MASS ON MESOMORPHIC BEHAVIOR, Journal of materials chemistry, 4(3), 1994, pp. 437-443
The effect of the molar mass (M(n)) and molar mass distribution (M(w)/
M(n)) on liquid-crystalline phase-transition behaviour was investigate
d for four polymers of the general class C.C., which were based on two
mesogenic bis(p-oxybenzoyl) units and two linear alkylene spacer segm
ents containing m (m = 5 or 7) and n (n = 5, 7 or 10) methylene groups
, respectively. Each polymer exhibited smectic and nematic mesophases.
The polymers were fractionated into various samples having different
M(n) and M(w)/M(n) values. The liquid-crystalline phase-transition par
ameters of samples C7C7 and C7C10 increased with M(n) until a limiting
value of M(n) almost-equal-to 6000-8000 g mol-1, and then remained co
nstant. In complete contrast, for samples C5C5 and C5C7 the stability
of the smectic mesophase increased, whereas that of the nematic mesoph
ase decreased with increasing M(n). In particular, for the former samp
les the nematic mesophase turned from enantiotropic to monotropic with
increasing M(n), and then disappeared in correspondence to a critical
value of M(n) almost-equal-to 7000 g mol-1. In addition, wide M(w)/M(
n) values were found to increase the stability of the nematic mesophas
e.