1 Critical -like behavior of ionic -related low-frequency dielectric properties in compressed liquid cryst alline 8OCB and its nanocolloid

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Critical-like behavior of ionic-related, low-frequency dielectric properties

in compressed liquid crystalline 8OCB and its nanocolloid

J. Łoś, A. Drozd-Rzoska, S. J. Rzoska

Institute of High Pressure Physics, Polish Academy of Sciences,

ul. Sokołowska 29/37, 01-142 Warsaw, Poland

Joanna Łoś:

ORCID: 0000-0001-6283-0948 ; e-mail: joalos@unipress.waw.pl

Aleksandra Drozd-Rzoska:

ORCID: 0000-0001-8510-2388 ; e-mail: Ola.DrozdRzoska@gmail.com

Sylwester J. Rzoska:

ORCID: 0000-0002-2736-2891 ; e-mail: sylwester.rzoska@gmail.com

Abstract

The report presents pressure-related broadband dielectric spectroscopy (BDS) studies in

liquid crystalline octyloxycyanobiphenyl (8OCB) and its nanocolloid with BaTiO3

nanoparticles, focused on the ionic-related low-frequency (LF) domain and the influence of

pretransitional fluctuations. Hence, basic exogenic (pressure) and endogenic (nanoparticles)

impacts on the dielectric properties are addressed. The innovative derivative-based analysis

revealed functional ‘critical-like’ descriptions of ionic- contributions to dielectric permittivity

and electric conductivity. The supplementary dielectric constant scan, yielding insight into the

preferable dipole-dipole arrangements, is also presented.

Studies cover the puzzling case of complex liquid (Smectic A) – Solid crystal phase transition,

revealing relatively strong, critical-like, premelting effects, which have hardly been observed

for the discontinuous ‘melting transition’ so far.

Key Words: liquid crystals, nanocolloids, high pressures, critical behavior, broadband

dielectric spectroscopy, premelting, ionics-related dielectric properties

Introduction

Soft matter constitutes the category of systems linking the dominance of mesoscale

elements, often multi-molecular species, and the enormous sensitivity to perturbations [1-3]. In

this family, rod-like liquid crystalline (LC) compounds play the reference model role [2, 4]. It

is associated with the simple molecular ‘geometry’, well approximated by rods or ellipsoids,

and the significance of pretransitional, multi-molecular, fluctuations [4-7]. Their size

(correlation length,



) and lifetime 



 exhibit a critical-type growth on approaching

mesophasic phase transitions [7]:







  









  



(1)

where  denotes the (extrapolated) hypothetical continuous phase transition temperature,



the universal correlation length exponent, and  stands for the scaling dynamical exponent.

Values of critical exponents are described by the space and order parameter dimensionalities.

The uniqueness of phase transition in LC systems is worth stressing: they are often related to

single elements of symmetry freezing or releasing [7]. The continuous or weakly discontinuous

character of phase transitions in LC systems leads to the significant influence of pretransitional

fluctuations on material properties. It can be exemplified by dielectric constant changes [8-22]:







      







for    (2)

where



is the heat capacity ‘critical’ exponent;



  

.

Eq. (2) shows that pretransitional effects depend on the distance from the continuous phase

transition temperature ( and are governed by universal critical exponents. For weakly-

discontinuous phase transitions at   , the discontinuity metric



 is

introduced. The appearance of pretransitional effects is possible if a ‘contrast factor’ between

fluctuations and their surrounding exists. For instance, in the isotropic liquid phase of nematic

LC materials dielectric constant within prenematic fluctuations can be significantly lesser than

for the isotropic surrounding. It occurs for LC molecules with the permanent dipole moment

parallel to the long molecular axis. In such a case, the uniaxial prenematic ordering within

fluctuations leads to the accidental ‘up’ and ‘down’ positioning of permanent dipole moments

and the cancellation of the dipolar contribution to dielectric constant, which becomes lesser

than for the isotropic surrounding. Consequently, on approaching the Isotropic - Nematic (I-N)

transition, the crossover 



  







 

 , described by Eq. (2), occurs. The

contrast factor is absent for rod-like LC molecules with the permanent dipole moment

approximately perpendicular to the long molecular axis. In such a case,



pretransitional

effect does not appear [12]. Notable that fluctuations-driven changes of dielectric constant in

the isotropic liquid phase are significant even 100 K above I-N transition [14-17, 21, 22]. They

also dominate the whole range of LC mesophases, as shown recently [20- 22].

The sensitivity of LC systems to perturbations can illustrate the strong impact of such

canonic exogenic factors as pressure. Even moderate-range compressing notably changes phase

transitions temperatures and



 values [15, 19, 21]. Regarding pretransitional effects, their

functional forms on cooling and compressing are isomorphic, in agreement with the Physics of

Critical Phenomena [7]. Notable that pressure-related studies in LC-based nanocolloids are still

very limited [23].

As for the endogenic impact factors, adding even a tiny amount of solid nanoparticles (NPs)

can significantly change the LC host properties [24, 25]. Unique features of LC-based

nanocolloids led to enormous interest in such materials and the creation of a new category in

the Physics of Liquid Crystals [24, 25]. However, studies on the impact of pretransitional

fluctuations on such nanocoloids are still limited [23, 26-32].

Changes in dielectric constant in LC systems illustrate the above discussion. Its is

associated with broadband dielectric spectroscopy (BDS) [33] monitoring in the frequency

range     . The impact of fluctuations is also explicitly manifested in high-

frequency BDS studies up to the GHz domain [14-17, 30-31].

Numerous research reports are focused on dielectric properties of LC compounds and

their nancocolloids in the ionic-related low-frequency (LF) domain, i.e., for frequencies below

the static domain [24, 25, 32, 34-56]. It is motivated by omnipotent applications of liquid

crystalline materials, for which the enormous sensitivity to the external electric field and LF

dielectric properties are essential [4, 6, 57]. For nanocolloids this interest is associated with new

generations of displays and photonic devices [23, 24, 57].

However, the insight into the impact of pretransitional fluctuations on dielectric properties

detected in the low-frequency BDS studies remains poor. It is associated with the practical lack

of temperature or pressure evolutions of reference dielectric properties in this domain. Such

evolutions were considered only in the isotropic phase of pentylcyanobiphenyl (5CB) [13] and,

very recently in undecylcyanobiphenyl (11CB) and its nanocolloids [32]. In 11CB the influence

of fluctuations was also detected in SmA mesophase, although only qualitatively.

This report presents pressure-related broadband dielectric spectroscopy (BDS) studies

in liquid crystalline octyloxycyanobiphenyl (8OCB) and its nanocolloid with BaTiO3

nanoparticles, focused on the ionic-related low-frequency (LF) domain and the influence of

pretransitional fluctuations. The innovative derivative-based analysis revealed the critical-like

form of the ionic-related contribution to dielectric permittivity and electric conductivity.

Studies cover Nematic, Smectic A mesophases and extend to the liquid SmA–Solid crystal

transition, introducing new results to the challenging field of the discontinuous liquid-solid

‘melting’ transition and premelting effects [58-66].

Experimental

Studies were carried out in 4'-octyloxy-4-cyanobiphenyl (8OCB,

octyloxycyanobiphenyl), the rod-like LC compound with  

       mesomorphism

[4, 31]. Its molecular structures is rod-like type, with the permanent dipole moment

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摘要：

1Critical-likebehaviorofionic-related,low-frequencydielectricpropertiesincompressedliquidcrystalline8OCBanditsnanocolloidJ.Łoś,A.Drozd-Rzoska,S.J.RzoskaInstituteofHighPressurePhysics,PolishAcademyofSciences,ul.Sokołowska29/37,01-142Warsaw,PolandJoannaŁoś:ORCID:0000-0001-6283-0948;e-mail:joalos@unipr...

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1 Critical -like behavior of ionic -related low-frequency dielectric properties in compressed liquid cryst alline 8OCB and its nanocolloid

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