Thermodynamic Characteristics of Chain Molecules in the Nematic State:  PVT Studies of Main-Chain Liquid Crystal Comprising a Long Oxyethylene-Type Spacer

Abstract

Thermodynamic characteristics of a dimer liquid crystal comprising a relatively long spacer -O(CH 2 -CH 2 O) 6were investigated. The contour length of the flexible spacer (∼25 Å) exceeds that of the hard segments (∼18 Å). The compound exhibits a nematic mesophase (N) between the crystal (C) and the isotropic melt (I) over a sizably wide temperature range (∼80 °C). Thermodynamic quantities such as thermal expansion coefficient R, isothermal compressibility β, thermal pressure coefficient γ ()R/β), and the volume change ∆V at the CN and NI interphase were carefully determined by the PVT measurements. The transition entropies due to volume expansion were estimated by the well-known relation ∆S V ) γ∆V. The latent entropies (∆S tr ) P for both CN and NI transitions were estimated according to the Clapeyron relation from the PVT and DTA data. The magnitude of the constant-volume entropy changes (∆S tr ) V calculated was found to be about 50-60% of the corresponding values of (∆S tr ) P . These results were consistent with the previous 2 H NMR observations that the chain segments adopt a nematic conformation in the LC state. The thermodynamic role of the flexible segment in determining the phase transitions has been critically discussed.

On the Stability of the Nematic Order Observed During the Cold-crystallization of PET

“…The van der Waals attractive potential effective in the nematic mesophase may be expressed in a generalized form: 25,28,29 …”

“…[30][31][32] In order to optimize the anisotropic interactions ( aniso ðs 2 Þ) in the mesophase, the cooperative bond rotation of the EG segments -OCH 2 -CH 2 Ois inevitable, accordingly the C-C bond being required to pay certain entropy penalty. A recent analysis of mainchain liquid crystals 25,28,29 suggests that (1) flexible segments (spacer) cooperatively participate in the anisotropic attractive interaction by adjusting relative orientation of hard cores, (2) the mitigation of the excessive repulsive interactions may lead to an increase of the volume fraction of unoccupied sites (free volume), and (3) as explicitly indicated by the orientational order parameter of the mesogenic cores, incorporation of the anisotropic interactions aniso ðs 2 Þ causes substantial enhancement of the thermal expansion coefficient in the nematic state. 29 As known from various thermodynamic theories of polymer liquids, is closely related to the free volume of the liquid.…”

Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase

“…It should be noted that the order-disorder characteristics inherent to the primary structure of the flexible spacer are precisely controlled in order to develop the LC mesophase. Combined use of spectroscopic and thermodynamic technique has been extended to treat the trimer models CBA-T9 and CBA-T10, and MBBE-6 [26,38]. As a result, it has been confirmed that 50-60% of the transition entropy (ΔS tr ) p (tr ¼ NI or CN) arises from the variation in the conformational distribution of the spacer at the phase boundary.…”

“…A comparison between the two different types of spacer, BCBOn (designated as CBA-n in this paper) [25] and MBBE-x with the oxyethylene (OE)-type spacer [26,27] is shown in Fig. 5, where the latent entropy ΔS NI /R is plotted against the number of constituent atoms (n) of the spacer.…”

“…The conformer distribution thus estimated leads to the configurational partition function Z N for the nematic state. Since the partition function Z I for the isotropic state is available [25] and MBBE-x ( filled circles) [26,27]. Note that the OE spacers of MBBE-x are expressed by the number of constituent atoms.…”