T. Dollase et Bm. Fung, TRICRITICAL BEHAVIOR OF THE SMECTIC-A TO SMECTIC-C-ASTERISK TRANSITION FOR BINARY-LIQUID CRYSTAL MIXTURES, Liquid crystals, 21(6), 1996, pp. 915-921
The tricritical behaviour for the smectic A (S-A) to chiral smectic (S
-C) or smectic C (S-C) phase transition of four binary liquid crystal
systems was investigated by C-13 NMR spectroscopy. The compounds hlor
o-3-methylpentanoyloxy)-4-hexylo-1,1'-biphenyl (denoted C6) and oro-3-
methylpentanoyloxy)-4-octyloxy-1,1'-biphenyl (C8) were used in their e
nantiomeric (L) and racemic (DL) forms. L-C6 and L-C8 exhibit a chiral
smectic C (S-C) to S-A phase transition, and DL-C6 and DL-C8 exhibit
a S-C to S-A transition, which are first order. Information about the
orientational ordering of the aromatic core of C6 and C8 was obtained
from measurements of C-13 chemical shifts. C6 displays a significant
jump at the phase transition, whereas the discontinuity of C8 is much
weaker. The enantiomeric and racemic forms have similar order paramete
rs and phase behaviour. Various mixtures of C6 and C8 with 4-butylolxy
phenyl-4'-decyloxybenzoate (<(10)over bar>O (4) over bar), which has a
second order S-C to S-A transition, show a crossover from a first ord
er to a continuous second order transition via a tricritical point. Th
e tricritical mole fraction X(TCP) of <(10)over bar>O (4) over bar in
the mixtures was found to be 0 . 14+/-0 . 01 for DL-C6 and 0 . 13+/-0
. 01 for L-C6. For mixtures containing C8, the tricritical mole fracti
ons were much smaller, and it was found that X(TCP)less than or equal
to 0 . 02 for DL-C8 and X(TCP)less than or equal to 0 . 03 for L-C8.