TRICRITICAL BEHAVIOR OF THE SMECTIC-A TO SMECTIC-C-ASTERISK TRANSITION FOR BINARY-LIQUID CRYSTAL MIXTURES

Authors
Citation
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
Citations number
40
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Crystallography
Journal title
ISSN journal
0267-8292
Volume
21
Issue
6
Year of publication
1996
Pages
915 - 921
Database
ISI
SICI code
0267-8292(1996)21:6<915:TBOTST>2.0.ZU;2-B
Abstract
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.