Isotropization of nematic liquid crystals by TMDSC

Conference · Mon Dec 01 04:00:00 EST 1997

OSTI ID:552780

Chen, Wei; Dadmun, M; Zhang, Ge; Boller, A; Wunderlich, B ^[1]

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1. Univ. of Tennessee, Knoxville, TN (United States). Dept. of Chemistry

Temperature-modulated differential scanning calorimetry (TMDSC) and traditional DSC are used to study the transition between the nematic liquid crystalline state and the isotropic liquid for two small molecules [4,4{prime}-azoxyanisole and N,N`-bis(4-n-octyloxybenzal)-1,4-phenylenediamine] and one macromolecule (4,4{prime}-dihydroxy-{alpha}-methylstilbene copolymerized with a 1:1 molar mixture of 1,7-dibromoheptane and 1,9-dibromononane). The DSC measurements with 4,4{prime}-azoxyanisole were used for temperature calibration with varying heating and cooling rates. Quasi-isothermal TMDSC with small temperature amplitude and standard TMDSC with underlying heating and cooling rates were utilized to analyze the breadth of the transitions. It could be verified that the isotropization transition of a nematic liquid crystal is, indeed, reversible for all three molecules. The nature of the transition changes, however, from relatively sharp, for small, rigid molecules, to about three kelvins wide for the small molecule with flexible ends, to as broad as 20 K for the macromolecule. It was also demonstrated that quantitative heats of fusion of sharp transitions can be extracted from TMDSC, but only from the time-domain heat-flow signal.

• Research Organization: Oak Ridge National Lab., TN (United States)
• Sponsoring Organization: USDOE Office of Energy Research, Washington, DC (United States); National Science Foundation, Washington, DC (United States)
• DOE Contract Number: AC05-96OR22464
• OSTI ID: 552780
• Report Number(s): ORNL/CP--95019; CONF-9709121--; ON: DE98001071
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
CALIBRATION
COOLING
HEATING
LIQUID CRYSTALS
ORGANIC COMPOUNDS
PHASE TRANSFORMATIONS
TRANSITION TEMPERATURE