8OCB and 8CB Liquid Crystals Confined in Nanoporous Alumina

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8OCB and 8CB Liquid Crystals Confined in Nanoporous Alumina. Effect of Confinement on the Structure and Dynamics. Aristoula Selevou, George V. Papamokos, Martin Steinhart, and George Floudas J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05042 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017

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The Journal of Physical Chemistry

8OCB and 8CB liquid crystals confined in nanoporous alumina. Effect of confinement on the structure and dynamics Aristoula Selevou, a George Papamokos, a Martin Steinhart b and George Floudas*a

a

b

Department of Physics, University of Ioannina, P.O. Box 1186, 451 10 Ioannina, Greece

Institut für Chemie neuer Materialien, Universität Osnabrück, D-49069 Osnabrück, Germany ABSTRACT

The effect of oxygen substitution is studied in two homologous compounds of ncyanobiphenyls with n=8 in the bulk and under confinemnet within self-ordered nanoporous alumina (AAO). Oxygen substitution in 8OCB increases the dipole moment and stabilizes the crystalline, smectic and nematic phases to higher temperatures relative to 8CB. Within their smectic– A (SmA) phase both 8CB and 8OCB behave as weak viscoelastic solids with low shear moduli reflecting the underlying supramolecular defect structure. Dielectric spectroscopy assisted by DFT calculations identified strong dipolar associations within the isotropic phases characterized by a Kirkwood-Fröhlich interaction parameter, g~0.36. Dielectric spectroscopy further identified a slow process (~ kHz) of low dielectric strength. The proximity of this process to the rheology time scale suggests as common origin a cooperative relaxation of the defect structure. Confinement alters the phase diagram by stabilizing certain crystalline phases and by reducing the N-I transition temperature in agreement with surface tension effects. However, the N-I transition seems to retain its first order character. Surface treatment with ndecyltrichlorosilane results in destabilization of the SmA phase at the expense of the N phase. This is consistent with a picture of surface anchored LC molecules at the pore walls that stabilize the nematic phase.

*Corresponding Author E-mail: [email protected] 1 ACS Paragon Plus Environment


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I.

INTRODUCTION

Understanding the phase state and dynamics of soft materials in contact with hard confining surfaces is important for the physics of phase transitions and has several technological implications.1 Thermotropic liquid crystals (LC) with several phase transitions within a narrow temperature range are model systems for testing confinement effects.2,3 Under confinement, the interplay of finite size effects and surface anchoring modifies the phase behavior relative to the bulk.4,5 Theory predicts the rounding of phase transitions within long cylindrical pores.6 On the other hand, second order transitions have a diverging length scale, but within pores there is no diverging length scale except, perhaps, along the cylinder axis. Addressing issues related to the precise nature of phase transitions as well as the effect of confinement on molecular orientation and dynamics requires model systems (such as LCs) and model surfaces where the confinement is uniform as well as a range of pore diameters. Earlier experimental studies explored the effect of confinement on several alkylcyanobiphenyls of the nCB type (where n is the number of carbons in the alkyl chain) mainly by diffraction, calorimetry (DSC) and dielectric spectroscopy (DS) techniques.7-14 For n=8 (4-n-octyl-4'-cyanobiphenyl, 8CB) in particular, the effect of cylindrical confinement within Anopore membranes of a single diameter (nominal diameter of 200 nm) on the phase behavior was investigated with DSC for both untreated and lecithin treated pores.15 Confinement exerted a strong influence both on the phase state and alignment. With respect to the former, the weakly first-order N-I transition under confinement was suppressed, rounded, broadened and shifted to lower temperatures for both untreated and treated walls. With respect to the latter, a distinct change from axial orientation of the director in the untreated pores to a homeotropic (radial) orientation in the treated pores was found. Smaller effects of confinement were found for 8CB confined in a controlled-pore glass (CPG) with a mean pore diameter of 400 nm with untreated and silanized pore surfaces when examined by deuteron NMR.16 Furthermore, the solid state polymorphism of 8CB confined within disordered porous silica matrices (composed from interconnected porous network with an irregular pore shape) was studied by neutron diffraction and Raman spectroscopy.17,18 Three metastable solid phases were identified, one crystalline and two frozen-in smectic-like phases. Optical birefringence measurements of 8CB located inside parallel silica channels with 10 nm diameter and 300 micrometer length revealed a weakly first order or even continuous paranematic-to-nematic transition in contrast to the bulk discontinuous N-I transition.19 Subsequently, the

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polymorphism of 8CB was investigated within the usual Anopore membranes (nominal diameter of 200 nm) and within porous silicon (nominal diameter of 30 nm) with neutron diffraction and X-ray diffraction.20 It was shown that both templates induce strong unidirectional confinement, and in addition, a rich polymorphism especially within porous silicon, with four low temperature phases apart from the SmA phase. With respect to the dynamics, 8CB was studied by means of dielectric spectroscopy within CPGs,21, Anopore templates22 and SBA-type molecular sieves.23 In the former study,21 with CPGs having randomly oriented and interconnected pores with diameters in the range of 10 to 100 nm, apart from two bulk-like processes associated with of rotation of molecules around the short axis and due to tumbling, two slower processes were also evidenced: One at ~ 1 Hz associated with the interfacial material and another at 105 Hz, associated with the rotation of molecules at the surface layer. In the study of 8CB confined in Anopore membranes the surface was treated with lecithin resulting in molecules oriented perpendicular to the pore walls (radial alignment).22 Under these conditions a single dielectrically active process ascribed to the librational motion with a peculiar temperature dependence was reported. The latter medium (SBA sieves)23 was composed either from hexagonally packed cylindrical pores with a mean pore diameter of 8.4 nm or from cellular foam-like pores with a mean pose size of 17.3 nm. In both cases two processes with a Vogel-Fulcher-Tammann (VFT) temperature dependence could be resolved; one bulk-like process associated with molecules in the pore center and a slower process associated with molecules at the surface layer. Despite these important efforts in understanding the effect of confinement on the phase state and dynamics of 8CB as well as other members of the series, much less is known about the structurally similar 4-n-octyloxy-4'-cyanobiphenyl (8OCB) bearing an oxygen atom. As we will see below this structural modification alters the phase diagram and stabilizes the same liquid crystalline phases to higher temperatures as compared to 8CB. The bulk thermodynamics and dynamics of 8OCB were studied with pressure-volume-temperature (PVT),24 DSC25,26 and DS,27 respectively. A single study28 reported on the effect of confinement (inside an aerosol matrix) on the dynamics by proton spin-lattice relaxation rate measurements in its nematic and smectic phases. An increase in the relaxation rate in the low frequency regime (sub-MHz) was found and was assigned to slow reorientational motion of molecules undergoing diffusion process within an adsorbed layer.



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Investigating confinement effects of the nature of 1st order transitions – as in liquid crystals- requires confining media that are uniform in pore diameter and length. In the present work we compare the self-assembly and dynamics of the relatively unknown 8OCB with the much investigated 8CB, however, under the same confining medium. Different from earlier studies, as confining medium here we employ self-ordered nanoporous aluminum oxide (AAO) templates containing arrays of aligned cylindrical nanopores with a narrow diameter distribution and uniform pore length.29-32 At variance from earlier studies on Anopore membranes and controlled-pore glasses, these templates provide a unique 2D confinement where the phase state and dynamics can be investigated. Earlier studies on the lower member of alkyl-cyanobiphenyls, 4-pentyl-4'-cyanobiphenyl (5CB),33 confined within the same AAO revealed reduction and broadening of phase transitions with increasing confinement. Columnar discotic mesophases34 were also altered within AAO giving rise to arrays of aligned supramolecular wires. The work is organized as follows: We first investigate the bulk self-assembly and dynamics by employing respectively, X-ray diffraction, polarizing optical microscopy, differential scanning calorimetry and dielectric spectroscopy, rheology. Density functional theory is employed in calculating the effect of oxygen substitution on the dipole moment. The effect of confinement is studied next by dielectric spectroscopy in untreated and silanized pores. We find that confinement strongly affects the phase behavior by reducing the nematicto-isotropic (N-I) transition temperature and by stabilizing certain crystalline phases. The effect of surface treatment is even more pronounced. The smectic and crystalline phases are destabilized and the nematic phase becomes the dominant phase in the smaller pores. The results demonstrate the importance of interfaces and of confinement on the phase state and dynamics of LCs.

II. EXPERIMENTAL SECTION Materials and method of infiltration. Self-ordered nanoporous aluminum oxide (AAO) (pore diameters of 25, 35, 65, 200 and 400 nm; pore depth 100 µm) was prepared following the procedures reported in the literature.32 Infiltration of the liquid crystals was performed at 323 K for 8CB and at 363 K for 8OCB corresponding to the respective isotropic phases. Infiltration proceeded by capillary action for about 1day. Following this procedure the 4 ACS Paragon Plus Environment

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infiltrated AAO templates were brought to ambient temperature. The infiltration procedure was repeated about 2 times to ensure complete filling of pores. Typically, the sample mass was ~11 mg (400 nm), ~2 mg (200 nm), ~6 mg (65 nm), ~4 mg (35 nm) and ~2 mg (25 nm). Prior to DSC, excess amount of LCs was removed from the surface of the self-ordered AAO membranes. AAO Surface modification. Literature procedures were followed.35,36 AAO membranes were heated in aqueous H2O2 (30 wt%, VWR) at 323 K for 2 hrs and were then rinsed with water and dried under vacuum at 373 K overnight. The surface was modified using n-decyltrichlorsilane (C10H21Cl3Si, Alfa Aesar GmbH & Co KG) by heating the templates in a closed environment at 363 K for 4 h and at 403 K for additional 4 h in the presence of 100 µL of the silane. Differential Scanning Calorimetry (DSC). A Q2000 (TA Instruments) was used for thermal analysis with a cooling/heating rate of 10 K/min at a temperature range from 173 K to 363 K. The instrument was calibrated for best performance on the specific temperature range and heating/cooling rate. The calibration sequence included a baseline calibration for the determination of the time constants and capacitances of the sample and reference sensor using a sapphire standard, an enthalpy and temperature calibration for the correction of thermal resistance using indium as standard (∆H=28.71 J/g, Tm=428.8 K), and a heat capacity calibration with sapphire standard. Temperature modulated DSC (TMDSC) was made with an amplitude of 1 K and for periods in the range from 20 to 200 s with corresponding heating rates from 10 to 1 K/min. Prior to any DSC measurement, the Al substrates to which the AAO layers had been connected were etched with solutions containing 1.7 mg CuCl2•2H2O, 50 ml deionized H2O and 50 ml concentrated HCl(aq) under cooling with ice water. Subsequently, the samples were further milled to powder, and 2.1 – 4.4 mg sample material was sealed in aluminum pans (100 µl). DSC traces of infiltrated self-ordered AAO were recorded using reference pans containing empty AAO pieces of the same pore diameter. All samples were first cooled at a rate of 10 K/min from ambient temperature to 173 K and then heated to 328 K and 363 K for 8CB and 8OCB, respectively, under a nitrogen atmosphere. The same cycle was repeated two times. Melting and crystallization points, as well as heats of fusion and of crystallization were determined from the second heating and cooling thermograms, respectively. Wide Angle X-Ray Scattering (WAXS). Two set-ups (1D and 2D) were employed (University of Ioannina (UoI) and Max Planck Institute for Polymer Research (MPI-P)). 2D 5 ACS Paragon Plus Environment


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Wide angle X-ray scattering (WAXS) measurements were made using CuKα radiation from a Rigaku MicroMax 007 x-ray generator, using Osmic Confocal Max-Flux curved multilayer optics (MPI-P). Samples in the form of fibers were prepared with a mini-extruder at 313 K for 8OCB. Temperature dependent WAXS measurements were performed by inserting the fibers into glass capillaries (1 mm diameter) at temperatures in the range from 292 K to 358 K on cooling and subsequent heating. A waiting (equilibration) time of 1800 s and a measurement time of 1800 s was set in the temperature program. Diffraction patterns were recorded by a 2Ddetector (Mar345 Image Plate). 1D X-ray diffraction (XRD) patterns were obtained with a D8 Advance diffractometer (Bruker AXS GmbH) (UoI). The system employs the Bragg-Brentano geometry in a θ-θ configuration, using CuKα radiation (λ=1.5406 Å) generated at 36 kV/36 mA and a NaI scintillation detector. The X-ray optics include a 2 mm divergence slit and 0.6 mm antiscatter/receiving slits to condition the incident and scattered beam, respectively; the axial divergence of both the incident and scattered beam is further controlled by Soller slits. The CuKβ radiation is reduced by a Ni filter, while fluorescence radiation excited in the sample is suppressed by a pyrolitic graphite-monochromator, positioned between the receiving slit and the detector. Bulk samples were loosely packed into the circular cavity of standard glass holders, 14 mm in diameter and 1 mm deep. Samples confined to AAO were supported on a wax mount and carefully aligned with a glass slide to reach the focusing circle. All samples were scanned over a 1-50º 2θ range, in steps of 0.03º (2θ), at a rate of 2 s per step. All diffraction patterns were corrected for background scattering (glass substrate and empty AAO templates, for the bulk and infiltrated samples, respectively). For the AAO background in particular, the same discs were used that were later infiltrated and examined with WAXS. Polarizing Optical Microscopy (POM). A Zeiss Axioskop 40 polarizing optical microscope, in crossed polarizers configuration, was employed in obtaining images of both (birefringent) samples’ liquid crystalline textures. The microscope is connected to a Linkam (THMS600) heating stage and liquid nitrogen supply dewar thus precisely controlling sample temperature. Each compound is initially melted between two glass substrates the distance of which is held constant by insertion of 50 µm Teflon spacers. Liquid crystalline textures are identified through a 20× objective on cooling and subsequent heating of samples at rates from 0.1 to 10 K/min. Dielectric Spectroscopy (DS). The sample cell consisted of two electrodes, 20 mm in diameter and a thickness of 50 µm. Dielectric measurements were made at different temperatures in the range 248.15 to 383.15 K, at atmospheric pressure, and for frequencies in the range from 1 × 10-2 to 1 × 106 Hz using a Novocontrol Alpha frequency analyzer with an 6 ACS Paragon Plus Environment

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active sample head. The complex dielectric permittivity ε*=ε′-iε′′, where ε′ is the real and ε′′ is the imaginary part, is a function of frequency ω, temperature T, and in general pressure P, ε*=ε*(ω, T, P).37,38,39 In the analysis of the DS spectra we have used the empirical equation of Havriliak and Negami (HN) ∗ , = +

∙

+

!

(1)

where τHN(T) is the characteristic relaxation time, ∆ε(T)=ε0(T)-ε∞(T) is the relaxation strength of the process under investigation, m, n (with limits 0 < m, mn ≤ 1) describe, respectively, the symmetrical and unsymmetrical broadening of the distribution of relaxation times, σ0 is the dcconductivity and εf is the permittivity of the free space. In the fitting procedure, we have used the ε′′ values at every temperature and in some cases the ε′ data were also used as a consistency check. From, τHN the relaxation time at maximum loss, τmax, is obtained analytically following:

 πm  1/ m  π mn   ⋅ sin    2(1 + n)   2(1 + n) 

τ max = τ HN ⋅ sin −1/ m 

(2)

The measured ε'' spectra have been used for the analysis except at high temperatures where the derivative of ε' has been employed (dε'/dlnω ~ -(2/π)ε''). This method is useful in fitting relaxation processes which are hidden under the conductivity, provided that the system is free of surface polarization effects. Therefore the latter representation was employed in the analysis of the slower processes. In the temperature range where two relaxation processes contribute to ε* a summation of two HN functions was employed. Density Functional Theory (DFT). The octyl- groups of molecules 8OCB and 8CB were initially set in all trans conformation (dihedral angles of the backbone was set to 180o). Subsequently, they were optimized with tight optimization criteria (RMS force criterion was set to 1*10-5) applying the DFT-B3LYP40 level of theory and cc-pVTZ41 basis set. This combination has recently shown to reproduce experimental values of dipole moments with high accuracy.42 The stationary points found were true minima since the subsequent frequency calculations did not produce negative frequencies. The potential energy surface of such molecules is expected to be relatively flat; consequently, a 99.590 grid was adopted to obtain smoother convergence to minima (the optimized geometries of the two molecules are shown in figure 4, below). For all computations Gaussian0943 software package was employed. Rheology. A TA Instruments AR-G2 rheometer was used to record the mechanical properties of 8CB and 8OCB in response to a time dependent oscillatory stress with angular frequencies (ω) ranging from 0.1 to 100 rad/s. Measurements were carried out in different



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parallel plate geometries (with diameters 40 mm, 25 mm and 8 mm). Samples were prepared directly onto the geometries’ lower plates by heating up to the isotropic state and then lowering the upper plate to a gap ensuring uniform sample coverage of the entire cross section. The behavior of the storage (G') and loss (G") moduli was monitored in three types of experiments. Initially, strain sweep measurements within each phase were used to identify the linear and nonlinear viscoelastic ranges in the response of the complex shear modulus to varying strain amplitudes. It was found that Cr and SmA phases exhibit a nonlinear response at very small strain amplitudes. The following strain amplitudes ensured that subsequent experiments were within the linear viscoelastic range. For 8CB, γ0=0.03% and γ0=0.07-0.1% within the Cr and SmA phases, respectively. For 8OCB, γ0=0.005% and γ0=0.07-0.1% within the Cr and SmA phases, respectively. Subsequent experiments involved (i) isochronal (ω=10 rad/s) temperature ramps at a rate of 2 K/min, and (ii) isothermal frequency sweeps at temperatures within the SmA phases of 8CB (T=299 K) and 8OCB (T=335 K). To facilitate the comparison of the frequency sweeps made in rheology with DS, the former were performed following cooling and subsequent heating (at 2 K/min) at their respective temperatures.

III.

RESULTS AND DISCUSSION

(a) Bulk The bulk self-assembly can be discussed in view of the X-ray results. WAXS from macroscopically oriented fibers was employed to identify the molecular structure within the liquid crystal phases. The anisotropic 2D diffraction patterns of neat 8OCB shown in Fig. 1 at four temperatures are typical for orientationally ordered crystalline, smectic, nematic and isotropic phases, respectively. In all cases, the molecules were oriented vertically (fiber in the vertical direction) and the X-ray beam was perpendicular to the fiber axis. Within the nematic phase, the WAXS image was re-oriented due to flow. The scattered intensity maxima within the crystalline phase are assigned by the hk indices corresponding to an oblique (2D) lattice. The corresponding d-spacing (d=2.88 nm) suggests a layered structure composed of domains with interdigitated aliphatic chains and of head-groups in a double layer. The meridional scattered intensity distribution exhibits also a sharp peak at q~17 nm-1 with a spacing of 0.37 nm characteristic of π-π stacking. Within the smectic phase the layered structure has a similar spacing (d~3.1 nm). In the nematic phase the 2D patterns comprise a set of spots along the meridional direction that correspond to low wavevectors, q||=2π/d||, with a characteristic spacing of d||~3.1 nm that, in principle, provide the dimensions of rod-like molecules. Here, this spacing 8 ACS Paragon Plus Environment

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exceeds the length of a single rod-like molecule suggesting the formation of larger entities, i.e. dimers. In the isotropic phase albeit the broad peak the average spacing suggests also the formation of dimers. This finding will be discussed subsequently with respect to the dielectric results.

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Figure 1. (Right) 2D-WAXS patterns of neat 8OCB at temperatures corresponding to the different phases; Cr (T=293.15 K), SmA (T=328.15 K), N (T~343 K) and I (T=358.15 K) indicate crystalline, smectic, nematic and isotropic phases, respectively. (Left) Corresponding equatorial and meridional scattered intensity distributions from the 2D diffraction patterns. The first scattered intensity maxima are assigned by the hk indices corresponding to an oblique (2D) lattice. More informative of the phase transformations are combined results from DSC, DS and rheology studies presented in Figs 2 and 3 for 8CB and 8OCB, respectively. The DSC trace of 8CB shows two exothermic peaks at 314 and 269 K on cooling and two (at 295 and 314 K) on subsequent heating associated with the Cr-SmA and N-I transitions. However, the SmA-N transition cannot be easily identified from the DSC traces alone. Contrast this with the temperature dependence of dielectric permittivity that exhibits discontinuous changes at the transitions. This is nicely depicted in the absolute derivative of dielectric permittivity with temperature. On cooling, discontinuous changes of |dε'/dT| exists at 314 and 269 K and a weaker one at 307 K. On heating, discontinuities appear at 293, 305 and 312 K. Evidently, cooling through the Cr phase is important in identifying the SmA-N transition in DS. The dielectric permittivity curves provide with additional valuable information. First, we note that 9 ACS Paragon Plus Environment


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the N-I transition in DS appears some 2 K lower on heating as compared to cooling. Secondly, the permittivity within the nematic phase is not constant but exhibits a bell-like shape. Both these findings call for slow kinetic effects within the nematic phase. The slow reorganization kinetics within the nematic phase are explored in Supporting Information section (with respect to Figs S1 and S2) by recording the evolution of dielectric permittivity following a Tjump within the nematic phase of 8OCB. Thirdly, within the isotropic phase the permittivity is nearly independent of temperature contrary to theoretical predictions. Analytically, the static dielectric permittivity of polar liquids with short-range interactions between molecules was treated theoretically by Kirkwood and later by Fröhlich.37 Fröhlich, in particular, considered an infinite continuum of dielectric permittivity, εS', and within this a spherical region containing No elementary dipoles that were treated explicitly. Based on these assumptions, the dielectric permittivity can be expressed as:

′# = +

$%

&'

()

*+ ,

(3)

Here, F=ε'S(ε∞+2)2/[2(ε'S+ ε∞)] is the local field, N/V is the number density of dipoles expressed as (ρ/M)NA, where ρ is the mass density and M is the molar mass, µ is the dipole moment and g is the dipole orientation correlation function defined as

' =1+

〈∑034 ∑ 021 (0 ∙(1 〉 ()

(4)

Considering only nearest neighbor orientation correlations, Eq. 4 for the reference dipole surrounded by z equivalent nearest neighbors reduces to g=1+z. Here, θ is the angle between the reference dipole and one of its z nearest neighbors. According to Eq. 3, εS' should scale with temperature as T-1 whereas a nearly temperature independent permittivity is observed within the isotropic phases of 8CB (Fig. 2) and 8OCB (Fig. 3). This finding reflects dipolar association (with different levels of dipolar interactions) within the isotropic phase identified in X-rays as dimers. The experimental εS'(T) data allow for estimating g and of its T-dependence within the isotropic phase, once the gas phase dipole moment is known. To this end we employ DFT calculations of the dipole moment (Fig. 4). The calculated dipole moments were 6.2 debye and 7.3 debye for 8CB and 8OCB, respectively. There are two dipoles in 8OCB (cyano group and the oxygen atom of the ether group) which act additively and increase the total dipole moment relative to 8CB. The dipole moment vector lies parallel to the chemical bond of the nitrile group in 8CB, while in 8OCB this vector is slightly off parallelicity and this is attributed to the presence of the ether group.


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Figure 2. 8CB results from DSC, DS and rheology obtained on cooling (left) and subsequent heating (right) at a rate of 2 K/min. (a) DSC trace, (b) dielectric permittivity at a frequency of 105 Hz, (c) absolute derivative of dielectric permittivity with temperature, |dε'/dT| and (d) storage (filled symbols) and loss (open symbols) moduli at an angular frequency of 10 rad/s. Cr, SmA, N and I indicate crystalline, smectic, nematic and isotropic phases, respectively. Dashed lines indicate phase boundaries. POM images at two temperatures corresponding to the Cr (263.15 K) and N (312.65 K) phases are shown. Images exhibit spherulitic formation over a pre-existing planar SmA texture (Cr, T=263.15 K) and Schlieren textures (N, T=312.65 K), respectively. The scale bar indicates 250µm. From Eq. 3, using the gas phase dipole moments, the density (ρ=0.98 g/cm3)24 and the measured εS'(T) we obtain g ~ 0.36 within the isotropic phase of 8CB and 8OCB. The complete g(T) dependence in the isotropic phases of 8CB and 8OCB is provided in the Supporting Information section (Fig. S3). This is a clear indication that the “isotropic” phase of both compounds is not entirely isotropic but contains molecules with preferred orientation correlations (also suggested by X-rays).



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350

Temperature (K)

Figure 3. 8OCB results from DSC, DS and rheology obtained on cooling (left) and subsequent heating (right) all obtained at a rate of 2 K/min. (a) DSC trace, (b) dielectric permittivity at a frequency of 105Hz, (c) absolute derivative of dielectric permittivity with temperature, |dε'/dT| and (d) storage (filled symbols) and loss (open symbols) moduli at an angular frequency of 10 rad/s. Cr, SmA, N and I indicate crystalline, smectic, nematic and isotropic phases, respectively. Dashed lines indicate phase boundaries. POM images obtained on cooling show the existence of focal conic defects within the smectic phase (327.15 K); on subsequent heating a coexistence of the smectic phase with a planar nematic texture is seen at 340.15 K; at 354.15 K a nematic phase with the typical Schlieren texture is shown. The scale bar indicates 250µm.

µ=7.3 D

µ=6.2 D

Figure 4. Optimized geometries and calculated dipole moments of molecules 8OCB (left) and 8CB (right). Level of theory: DFT-B3LYP, basis set: cc-pVTZ. (Color code: carbon=gray, nitrogen=blue, oxygen=red. Hydrogens not shown.)


Page 13 of 33

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In addition to DSC and DS, POM images are used to identify the nematic phase from the typical Schlieren texture exhibiting four point disclinations (s=±1) as well as the SmA and Cr phases, the latter with a spherulitic superstructure. Similar findings exist for the 8OCB (Fig. 3). In fact, although transition temperatures are increased in 8OCB relative to 8CB, both LCs exhibit comparable N and SmA ranges (Figs 2 and 3) with a similar McMillan ratio44 defined as the ratio of the transition temperatures TS/N/TN/I (0.98 and 0.96 for 8CB and 8OCB, respectively). The viscoelastic properties of both compounds are discussed next based on the results from the isochronal measurements on cooling and subsequent heating with respect to Figs. 2 and 3. First-order transitions involving the Cr, SmA and N phases are evident as step-like discontinuities in the storage and loss moduli. Within their respective Cr phases, both mesogens exhibit an elastic response with G'>G" independent of temperature whereas, within their SmA phases, G' is only slightly higher than G" at the chosen frequency. Additional information on the viscoelastic properties can be obtained from the frequency dependent response of the respective smectic phases (obtained at very small strain amplitudes following heating from the Cr phase). The result of such frequency sweeps for 8CB and 8OCB within their SmA phases is shown in Fig. 5. Both data sets exhibit a similar frequency dependence characteristic of the underlying smectic phase. Unaligned smectics are known to behave as “weak” viscoelastic solids.45 This results from the interplay between differently aligned smectic monodomains and supramolecular defect structures. In accord with this expectation we obtain a (low) modulus of 160 Pa and 260 Pa for 8CB and 8OCB, respectively. The former is in agreement with an earlier estimate for 8CB.45 At higher frequencies the storage and loss moduli approach suggesting their crossing at a frequency outside our experimental window (ω~600 rad/s). A characteristic length of defect structure can be estimated from the value of storage modulus at their (extrapolated) crossing as G'=kBT/d3(ωcτ)2, giving a characteristic length of a defect structure of about 19 nm for both 8CB and 8OCB. Such length scales exceed the characteristic spacing of the layered smectic phases (d~3.25 nm and 3.1 nm for 8CB and 8OCB, respectively), suggesting a supramolecular defect structure.



3

10

G''

8OCB

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

8CB 2

10

1

10

10

-1

0

10

10

1

10

2

-1

ω (rad s ) Figure 5. Frequency dependence of the storage (filled squares) and loss (open circles) moduli of 8CB (red) and 8OCB (blue) within the respective SmA phases (8CB: T=299 K, γo=0.0007; 8OCB: T=335 K, γo=0.0008). With respect to the dynamics, the dielectric relaxation of liquid crystals - assuming that they reorient as rigid rods - has been calculated by employing orientational probability distribution functions.46 Models predict in total four dielectrically active processes with intensities that depend on the direction of the probing electric field relative to the nematic director. When the probing electric field is parallel to the nematic director, theory predicts two modes; a “slow” mode associated with reorientations of the mesogenic unit about the short molecular axis and a “faster” mode associated with reorientations about the long axis. Similarly, when the electric field is perpendicular to the nematic director, two modes exist; one associated with fluctuations of the longitudinal component of the dipole moment in the direction of the probing field and another one dominated by molecular rotation about the long axis. On the other hand, experimental studies on the dynamics of liquid crystals usually identify two processes; a slower process ascribed to the rotation of molecules about the short molecular axes (usually called δ); and a faster process associated with the precession of long molecular axes about the director (usually called α); the rotation about the long molecular axis is even faster. These dynamic features are very distinct within the nematic phase because the nematic potential hinders rotation about the short axis. In the present study we were able to identify two dynamic processes in bulk 8CB and 8OCB termed as δ- and σ- processes. Representative loss curves for the two compounds at some selected temperatures in the dielectric loss and derivative representations are shown in


Page 15 of 33

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Fig. S4, Supporting Information. Fig. 6 gives the corresponding relaxation times with filled symbols in the usual Arrhenius representation. Note, that the figure includes literature data for the faster α-process (open symbols) that is outside our experimental window.27,47 The main dielectrically active process within our experimental window is the δ-process whereas the slower σ-process can be obtained only in the derivative representation (Fig. S4). The low intensity of this mode and its proximity to the ion mobility motion (the latter times obtained from the crossing of the real and imaginary parts in any of the M*, ε* or σ* representations) explains its absence in earlier studies of 8CB. It could reflect a minority of ellipsoids where the nematic director is nearly perpendicular to the electric field. This assignment is based on the low dielectric strength. In 8CB, the dielectric strength of the δ-process is ∆ε~8, ~10 and ~7 within the isotropic, nematic and SmA phases, respectively. The strength for the σ-process is 0.05, 0.1 and ~0.7, within the respective phases, i.e., 10 to 100 times weaker than the δ-process. For 8OCB, the dielectric strength of the δ-process is 2-3, 10, ~2.5 and ~0.02 within the isotropic, nematic, SmA and Cr phases, respectively. Likewise, the corresponding strength for the σ-process is 10 to 100 times weaker than the δ-process within the different phases. In addition, all processes, except within the Cr phase, could be described with a Debye process (i.e., m=mn=1). The temperature dependence of the characteristic times for the δ- and σ-processes conform to an Arrhenius dependence

τ = τ 0 e E / RT

(5)

where τo is the relaxation time in the limit of very high temperatures and E the activation energy. For 8CB, the activation energy associated with the δ-process is E=70±7 kJ/mol and 44±3 kJ/mol, within the nematic and smectic phases, respectively, whereas within the σ-process corresponding values are 109±17 kJ/mol and 128±9 kJ/mol, respectively. The higher activation energies for the σ-process are consistent with the notion of a more cooperative process further corroborated by their proximity to the rheology time scale associated with the defect structure. For 8OCB, the activation energy associated with the δ-process is E=62±3 kJ/mol and 52±6 kJ/mol, within the nematic and smectic phases, respectively, whereas within the σ-process 71 kJ/mol and 45±7 kJ/mol, respectively.



α 8

δ

-log(τmax/s)

6 4

σ

2 0

I

2.6

N SmA 2.8

3.0

3.2

3.4

-1

Cr 3.6

3.8

1000/T (K ) 8

δ

6

-log(τmax/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

4

σ

2 0

-2 2.6

I 2.8

N SmA 3.0

Cr 3.2

-1

3.4

1000/T (K )

Figure 6. Arrhenius relaxation map for 8CB (top) and 8OCB (bottom). Filled symbols are from this work as follows; (circles): fast process, (filled squares): slow process. For 8CB the open blue and red circles are from ref[47] whereas green circles are from [27]. For 8OCB, the open green circles are from ref[27]. Rhombi give the characteristic times of defect diffusion within the SmA phases from rheology. Vertical lines separate the different phases: Cr, SmA, N and I indicate crystalline, smectic, nematic and isotropic phases, respectively.


Page 17 of 33

(b)

Effect of confinement

The effect of confinement on the phase state was studied by following the temperature dependence of the dielectric permittivity (DS) and of the heat flow (DSC). Results of the dielectric permittivity and of its absolute derivative with respect to temperature for the bulk and confined liquid crystals within AAO are given in Fig. 7. The effect of confinement is two-fold: first, to broaden practically all phase transitions and secondly to stabilize certain crystalline phases. More evident in the |dε'/dT| dependence is the stabilization of the CrB phase with the appearance of an additional peak at temperatures below the CrA phase.

(b)

(a) 12

ε'

18 16 14 12 10 8 6 4 2

Bulk 400 nm 200 nm 65 nm 35 nm 25 nm

14

10 8 6 4

(c)

A

Cr -1

|dε'/dT| (K )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr

280

SmA N

(d)

I

x1 Cr

x1

A

SmA

N

I

B

290

300

310

x11

x15

x50

x25

x119

x36

x128

x67

x115

x100

320

310

Temperature (K)

320

330

340

350

Temperature (K)

Figure 7. Dielectric permittivity (a, b) and absolute derivative of dielectric permittivity with temperature (c, d) for 8CB (left) and 8OCB (right) at f=0.1MHz with a heating rate of 2 K/min. Notice the different scaling factors of the vertical axis in the |dε'/dT| plots. Results obtained from DSC are depicted in Fig. 8. New endothermic peaks appear for 8CB and 8OCB located within AAO as compared to the bulk at low temperatures. In 8CB, in particular, two additional crystalline phases are stabilized in confinement (CrB and CrC). In 8OCB, already the bulk DSC curve suggests a hidden Cr phase other than the main CrA; On heating with 10 K/min (Fig. 8) or with 2 K/min (Fig. 3), bulk 8OCB undergoes a melting transition at 319.4 K followed by re-crystallization at 320.4 K and by a dual melting at 325.5 K and 328 K (the latter is more evident in Fig. 3 with the slower heating rate). This is suggestive 17 ACS Paragon Plus Environment


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of two crystalline phases even in bulk 8OCB with slow kinetics. This is not surprising as four crystal phases have been identified in solution crystallized 8OCB samples by X-ray diffraction and POM.25,48 These comprised square-plate, needle (with a triclinic unit cell) and long parallelepiped crystals (with a monoclinic unit cell) that were all metastable with respect to the most stable crystal (triclinic) found in a powder sample. What is of interest here is the stabilization of crystal forms in confinement. In comparing DS and DSC results on the phase boundaries we find that the two methods have different sensitivities. Presumably, the dipole-dipole interactions are very similar within the two Cr phases so that they show only weak differences in dielectric permittivity but more evident changes are seen in the specific heat. On the other hand, the SmA-N boundaries are better resolved in DS, hence dashed lines in the figure indicate the anticipated phase boundaries from the latter method (Fig. 8). The results on the transition temperatures can be summarized in a composite plot with respect to the inverse pore diameter in Fig. 9. Although both DSC and DS results are included, solid lines represent linear fits to the latter data set. In all cases, the trend for decreasing transition temperatures on confinement can be parameterized according to Tm=Tm∞-A/d, where Tm∞ is the transition temperature (in K) in the absence of confinement and d is the pore diameter (in nm).48 For example, for the N-I transition, values of Tm∞=311.8±0.2 K and A=28±11and Tm∞=352.0±0.5 K and A=44±19 were obtained for 8CB and 8OCB, respectively. But the most prominent effect of confinement is the stabilization of the isotropic phase of 8CB within the smaller AAO pores and the appearance/stabilization of new crystalline phases. Presumably, these phases were metastable in the bulk and became stable upon confinement. The stability vs metastability of these phases was tested in DSC by the following protocol: Samples were cooled to sufficiently low temperatures, in order to crystallize all polymorphs, and subsequently heated past the melting point of the phase under investigation. After annealing for 1 hour at the corresponding temperature, samples were cooled and subsequently heated with 10 K/min to their isotropic phase. Divergences from the initial thermogram following ageing would indicate metastability of low temperature phases towards phases at higher temperature. The results of this investigation for the stability of CrA, CrB and CrC phases (ESI†, Fig. S5) demonstrated the stability of all crystal phases in confinement. The presence of a different crystal structure of confined 8OCB with respect to the bulk was confirmed by WAXS measurements at ambient temperature (Supporting Information, Fig. S6).


Page 18 of 33

0.02 0.02

65nm

35nm 270

280

1 -1

290

300

310

320

0.1

I

Heat Flow (Wg )

SmA N

Bulk

Cr

A

SmA

N

I

400nm

Cr 0.1

CrA

CrC CrB 400nm

0.02

-1

Heat Flow (Wg )

2

Bulk

260

65nm

35nm

280

B

300

Temperature (K)

320

340

360

Temperature (K)

Figure 8. DSC traces for 8CB (left) and 8OCB (right) as a function of temperature obtained on heating with a rate of 10 K/min. Dashed lines give the approximate temperatures of phase transitions. Colored areas indicate different phases as follows: (isotropic, I): magenta, (nematic, N): orange, (smectic, SmA): yellow, (crystalline phase A, CrA): dark cyan, (crystal phase B, CrB): cyan. 355

I

I

350

310

N

Temperature (K)

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

Page 19 of 33

300

SmA 290

Cr Cr

A

B

345

335

0.01

325 320 315

0.02

0.03

SmA

330

280 0.00

N

340

Cr 0.00

0.04

A

Cr 0.01

-1

B

0.02

0.03

0.04

-1

1/d (nm )

1/d (nm )

Figure 9. Effective phase diagrams for 8CB (left) and 8OCB (right) as a function of inverse pore diameter. Filled and open symbols represent transition temperatures from DSC and DS, respectively. In both experiments the heating rate was 10 K/min. Colored areas indicate different phases as follows: (isotropic, I): magenta, (nematic, N): orange, (smectic, SmA): yellow, (crystalline phase A, CrA): dark cyan, (crystal phase B, CrB): cyan.



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Page 20 of 33

The reduced N-I transition temperature on confinement can be discussed in terms of (i) an elastic deformation of the nematic director and/or (ii) surface tension effects.7 The former treats the effect of nematic material on the AAO surface as an imposed deformation on the inner part that must arrange so as to minimize the elastic energy. This produces a shift in the NI transition temperature as 7

6 =

89

8; 8

: :

(6)

/@

(7)

8OCB and 8CB Liquid Crystals Confined in Nanoporous Alumina

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