Calculation of the Conformation Entropies of Dimer Liquid Crystals and Comparison with the Observed Transition Entropies at Constant Volume

Abe, Akihiro; Furuya, Hidemine; Shimizu, Renato Norio; Nam, Su Yong

doi:10.1021/ma00105a012

Cited by 41 publications

(20 citation statements)

References 7 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…To test this hypothesis, Blatch et al [74] studied two series of chiral dimers, one symmetric (S)2 MB.OnO. (S)2MB, 21 and the other non-symmetric, CBOnO. (S)2MB, 22 For the CBOnO.…”

Section: Chiral Liquid Crystal Dimersmentioning

confidence: 99%

Liquid Crystal Dimers

Imrie

1999

Structure and Bonding

230

104

View full text Add to dashboard Cite

A liquid crystal dimer is composed of molecules containing two conventional mesogenic groups linked via a¯exible spacer. These materials show quite different behaviour to conventional low molar mass liquid crystals and in particular their transitional behaviour exhibits a dramatic dependence on the length and parity of the¯exible spacer. In this review a comprehensive overview of the relationships between molecular structure and liquid crystallinity in dimers is provided. This includes a description of the novel modulated and intercalated smectic phases exhibited by dimers.

show abstract

“…To test this hypothesis, Blatch et al [74] studied two series of chiral dimers, one symmetric (S)2 MB.OnO. (S)2MB, 21 and the other non-symmetric, CBOnO. (S)2MB, 22 For the CBOnO.…”

Section: Chiral Liquid Crystal Dimersmentioning

confidence: 99%

Liquid Crystal Dimers

Imrie

1999

Structure and Bonding

230

104

View full text Add to dashboard Cite

show abstract

“…We designated these dimers BPA-n, which consist of both a flexible even-number polymethylene spacer of varying length (n= 8,10,12,14,16,18,20) and two terminal mesogens. Since the nematic-isotropic transition temperatures of the dimers with even numbers less 326 than n = 8 were so high that the precise thermodynamic quantities could not be obtained, they were not dealt with here.…”

mentioning

confidence: 99%

Effects of Spacer Length on Thermal Properties of Dimeric Model Compounds for Main Chain Liquid Crystalline Polymers

Ogawa

1996

Polym J

View full text Add to dashboard Cite

ABSTRACT:The thermal properties of a series of o:,w-bis[4-(4-biphenyloxycarbonyl)phenyl]alkanedioates (BPA-n) dimeric liquid crystals containing even-number methylene spacer from n = 8 to n = 20 were studied. All synthesized compounds showed the same crystal form at room temperature. They were all enantiotropic nematic except BPA-20 which was monotropic. The temperature range over which the nematic phase exists becomes smaller with increasing spacer length, and finally at n = 20 it no longer shows liquid crystal on heating. Thermodynamic quantities were classified as contributions from mesogens and polymethylene spacers. The overall entropies LlSK1 of transition from crystals at room temperature to the isotropic liquid were shown as a linear equation of the number, n, of methylene units as follows, LlSK1 =8.50n +95.41 K 1 1 . The coefficient (8.501 K 1 1 ), which means the increase in LlSK, value per methylene, corresponds well to melting entropy per methylene of n-alkanes. The entropies of transition from nematic to isotropic phase were almost constant (8.3-8.9 J K 1 1 ), independent of the number of methylene units in the flexible spacer.KEY WORDS Dimer I Liquid Crystal I Nematic Phase I Model Compounds I Spacer Length Effect I Thermotropic Polyester I Long Spacer I Mesogens I Thermal Properties I Dimeric liquid crystals formed by a sequence of are low molecules from the standpoint of the degree of polymerization, though these nematic-isotropic transition temperatures and heats of transition depend on odd or even numbers of methylene units in the spacer, very much the same as main chain liquid crystalline polymers with repeated units of mesogen-spacers. 1 -3 This is a quite different character from low molecular mass liquid crystals which have small odd-even oscillation. Therefore dimeric liquid crystals are considered as a proper model of liquid crystalline polymer. There are many reports stating how the thermodynamic quantity of dimeric liquid crystals depends on spacer length, 4 • 5 but there is no study that treats such thermodynamic quantity quantitatively with separation of contribution of mesogens and contribution of polymethylene spacer parts respectively.If spacer length becomes certain large and crystal structures of compounds show same form, then we consider that contribution caused by mesogens and polymethylene spacer would work almost independently for thermodynamic quantity of phase transitions, and then handle dimeric compounds with considerably long polymethylene chain.In this work, we synthesized and studied dimeric liquid crystals having long polymethylene chain, as shown below:We designated these dimers BPA-n, which consist of both a flexible even-number polymethylene spacer of varying length (n= 8, 10, 12, 14, 16, 18, 20) and two terminal mesogens. Since the nematic-isotropic transition temperatures of the dimers with even numbers less 326 than n = 8 were so high that the precise thermodynamic quantities could not be obtained, they were not dealt with here. The dimers with odd numbers wil...

show abstract

“…In the studies mentioned hereafter, detailed analyses were mostly carried out for oligomeric LCs. The knowledge gained through studies on the low DP analogs has been found useful in understanding the spatial arrangements and thermodynamic properties of polymer LCs [12,13].…”

Section: Dependence Of Thermodynamic Properties On Degree Of Polymerimentioning

confidence: 99%

“…The integrated distribution curves of the disorientation angle θ between the two successive mesogens (P(θ) À θ) were calculated for spacers comprising odd and even numbers (n) of methylene units in the isotropic conformation [13,22]. Definition of θ and the bond angles of the linking group (X) adopted in these calculations are shown in Fig.…”

Section: Ris Analysis Of the Orientational Correlation Of The Neighbomentioning

confidence: 99%

See 1 more Smart Citation

Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase

Abe

2013

Hierarchical Macromolecular Structures: 60 Years After the Staudinger Nobel Prize II

Self Cite

View full text Add to dashboard Cite

The physical picture of the nematic conformation has been discussed on the basis of the rotational isomeric state analysis for segmented liquid-crystal (LC)-forming molecules, comprising mesogenic units on both sides of a flexible spacer. The disorientation angles (θ) of the two terminal mesogenic units are calculated for given spatial configurations of the spacer. The observed odd-even character of the thermodynamic quantities at the nematic (N)-isotropic (I) phase transition T NI ¼ ΔH NI /ΔS NI has been interpreted in terms of the profiles of the calculated distribution curves P(θ) À θ. Combined use of rotational isomeric state and 2 H NMR techniques has led to an estimate of the conformer fraction in the nematic state. The transition entropies derived on this basis are favorably compared with the constant-volume transition entropies obtained from the pressure-volume-temperature measurement. The observed V-T relation indicates that the expansivity of the nematic LC phase is higher relative to that of the isotropic melt. It has been pointed out that the nematic conformation might possibly gain extra stability from the free volume effect in the LC state. These considerations offer an explanation why amazingly long flexible chain segments can be accommodated in the nematic fluid.

show abstract

Calculation of the Conformation Entropies of Dimer Liquid Crystals and Comparison with the Observed Transition Entropies at Constant Volume

Cited by 41 publications

References 7 publications

Liquid Crystal Dimers

Liquid Crystal Dimers

Effects of Spacer Length on Thermal Properties of Dimeric Model Compounds for Main Chain Liquid Crystalline Polymers

Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase

Contact Info

Product

Resources

About