High-resolution Ac Calorimetry Research Articles

Blue phase stabilization by CoPt-decorated reduced-graphene oxide nanosheets dispersed in a chiral liquid crystal

We report on the effect of reduced-graphene oxide nanosheets decorated by CoPt nanoparticles on the blue phase range of a chiral liquid crystal. By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that a small concentration of these nanosheets induces the stabilization of a single blue phase structure in comparison to three blue phases existing in the pure compound. The results are compared with other liquid crystal-dispersed graphene studies, and, moreover, a short theoretical discussion of the stabilization effect is included.

Implementation of a programmable electromechanical chopper with adjustable frequency and duty cycle for specific heat measurements

A hard disk mechanism was used as an electromechanical light chopper controlled by software developed in LabVIEW with an accuracy of ±0.01Hz in frequency to realize specific heat measurements. This chopper can be used in applications in experimental techniques in which low-frequency light excitation is required. The software provides an user interface to set the excitation frequency and its duty cycle. The evaluation of the efficiency was measured by installing the device in a fully automated high-resolution AC calorimetry system. The device provides a light cutter signal with a frequency varying from 1mHz to 40Hz. Employing the desired wave function, the excitation mode provided by this design can be used under theoretical models proposed for the specific heat technique, obtaining an increased efficiency in the measurements of the specific heat response for the studied materials. This paper reports the performance characteristics of the technology of an asymmetric CA-chopper, which can optimize efficiency in capturing the measurement of the response of specific heat of any given study material in a high-resolution ac calorimetry system. The chopper consists entirely of the positioning mechanism of a hard disk.

Blue Phase Range Widening Induced by Laponite Nanoplatelets in the Chiral Liquid Crystal CE8

We report on the widening of blue phase range induced by surface-functionalised laponite nanoplatelets dispersed in a chiral liquid crystal. The results are obtained by means of high-resolution ac calorimetry and optical microscopy and they are reproducible upon heating and cooling, identifying thermodynamically stable blue phases. It is shown that nanoplatelets target mostly the cubic structure of blue phase I and increase its temperature stability range by a factor of two. The present results are in agreement with recent findings that anisotropic nanoparticles tend to effectively stabilise blue phase I.

Nanoparticle-induced twist-grain boundary phase.

By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that surface-functionalized spherical CdSSe nanoparticles induce a twist-grain boundary phase when dispersed in a chiral liquid crystal. These nanoparticles can effectively stabilize the one-dimensional lattice of screw dislocations, thus establishing the twist-grain boundary order between the cholesteric and the smectic-A phases. A Landau-de Gennes-Ginzburg model is used to analyze the impact of nanoparticles on widening the temperature range of molecular organizations possessing a lattice of screw dislocations. We show that in addition to the defect-core-replacement mechanism, the saddle-splay elasticity may also play a significant role.

Calorimetric study of phase transitions in ocylcyanobiphenyl-barium titanate nanoparticle dispersions

High-resolution ac-calorimetry is reported on the weakly first-order isotropic to nematic (I-N) and the continuous nematic to smectic-A (N-SmA) phase transitions in the liquid crystal octylcyanobiphenyl (8CB) doped with a ferroelectric nanoparticle barium titanate, BaTiO3 (BT). Measurements were performed as a function of BT concentration and over a wide temperature range well above and below the two transitions. From the thermal scans of all samples (having BT mass fraction φ(m) = 0.001 to 0.014 and pure 8CB), both the I-N and the N-SmA transitions evolve in character. Specifically, there appears an unusual change of the I-N specific heat peak shape on heating as φ(m) increases. Both the transitions shift to lower temperature at a different rate for φ(m)<φ(m)(c)=0.002 as compared to that for φ(m)>φ(m)(c). The effective transition enthalpies are essentially constant and similar to that seen in the bulk. Using a simple geometric model, the mean distance between the BT particles at the cross-over φ(m)(c) is found to be x(c)~3 μm, which is consistent with an estimated surface extrapolation length b for the nematic director. This suggests that the low φ(m) regime is dominated by an impurity/disorder effect while for φ(m)>φ(m)(c) the mean distance is small enough for the LC to mediate coupling between the BT ferroelectric nanoparticles.

Memory-controlled smectic wetting of liquid crystals confined to controlled-pore matrices

The impact of severe spatial confinement on the phase transition behavior of smectic liquid crystals has been systematically studied. Dodecylcyanobiphenyl (12CB) and octylcyanobiphenyl (8CB) liquid crystals were confined to controlled-pore glasses of various pore radii R. The phase transition behavior was studied by means of high-resolution ac calorimetry. The results show that the carbon tails length of the liquid crystal molecules strongly influences the phase behavior. In 8CB samples strong pretransitional effects were observed; however, these effects were substantially weaker in 12CB samples. Moreover, a crossover between first and second order phase transition character was observed in 12CB samples upon decreasing R. The observed behavior is explained in terms of surface-memory effects and a dimensional crossover.

Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation

We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range [Formula: see text] T (BP) of liquid-crystalline (LC) blue phases. Using a mesoscopic Landau-de Gennes type approach we obtain that the defect core replacement (DCR) mechanism yields in the diluted regime [Formula: see text] T (BP)(x) [Formula: see text] 1/(1 - xb) , where x stands for the concentration of NPs and b is a constant. Our calculations suggest that the DCR mechanism is efficient if a local NP environment resembles the core structure of disclinations, which represent the characteristic property of BP structures. These predictions are in line with high-resolution ac calorimetry and optical polarising microscopy experiments using the CE8 LC and CdSe or aerosil NPs. In mixtures with CdSe NPs of 3.5nm diameter and hydrophobic coating the BPIII stability range has been extended up to 20K. On the contrary, the effect of aerosil silica nanoparticles of 7.0nm diameter and hydrophilic coating is very weak.

Calorimetric study of the nematic to smectic-Aphase transition in octylcyanobiphenyl-hexane binary mixtures

The continuous nematic to smectic-A (N-SmA) phase transition has been studied by high-resolution ac-calorimetry in binary mixtures of the liquid crystal octylcyanobiphenyl (8CB) and a nonmesogenic, low-molecular weight, solvent n-hexane (hex) as a function of temperature and solvent concentration. Heating and cooling scans about the N-SmA transition temperature were repeatedly performed on pure and six 8CB+hex samples having hexane molar concentration ranging from x(hex)=0.02 to 0.12. All 8CB+hex samples in this range of x(hex) remain macroscopically miscible and exhibit an N-SmA heat capacity peak that shifts nonmonotonically to lower temperature and evolves in shape, with a reproducible hysteresis, as x(hex) increases. The imaginary part of heat capacity remains zero up to x(hex)(TCP) ≃ 0.07 above which the distinct peak is observed, corresponding to a jump in both the real and imaginary enthalpy, indicating the onset of first-order behavior. A simple power-law analysis reveals an effective exponent that increases smoothly from 0.30 to 0.50 with an amplitude ratio A-/A+→1 as x(hex)→x(hex)(TCP). This observed crossover of the N-SmA toward a tricritical point driven by solvent concentration is consistent with previous results and can be understood as a solvent softening of the liquid crystal and concument promoting of nematic fluctuations.

Study of the isotropic to smectic-A phase transition in liquid crystal and acetone binary mixtures

The first-order transition from the isotropic (I) to smectic-A (Sm A) phase in the liquid crystal 4-cyano-4(')-decylbiphenyl (10CB) doped with the polar solvent acetone (ace) has been studied as a function of solvent concentration by high-resolution ac-calorimetry. Heating and cooling scans were performed for miscible 10CB+ace samples having acetone mole fractions from x(ace)=0.05 (1 wt %) to 0.36 (10%) over a wide temperature range from 310 to 327 K. Two distinct first-order phase transition features are observed in the mixture whereas there is only one transition (I-Sm A) in the pure 10CB for that particular temperature range. Both calorimetric features reproduce on repeated heating and cooling scans and evolve with increasing x(ace) with the high-temperature feature relatively stable in temperature but reduced in size while the low-temperature feature shifts dramatically to lower temperature and exhibits increased dispersion. The coexistence region increases for the low-temperature feature but remains fairly constant for the high-temperature feature as a function of x(ace). Polarizing optical microscopy supports the identification of a smectic phase below the high-temperature heat capacity signature indicating that the low-temperature feature represents an injected smectic-smectic phase transition. These effects may be the consequence of screening the intermolecular potential of the liquid crystals by the solvent that stabilizes a weak smectic phase intermediate of the isotropic and pure smectic-A.

Calorimetric studies on isotropic-B 4 phase transition in the mixture of bent-shaped and rod-like molecules

Calorimetric studies have been performed on a mixture of achiral bent-shaped and rod-like molecules using a high-resolution ac calorimetry and a high-sensitivity differential scanning calorimetry (DSC). Both ac calorimetry and DSC yielded essentially the same result for B 4–B X transition, while a significant difference has been observed at the isotropic-B 4 transition during cooling. Particularly, DSC measurements revealed the characteristic heat capacity behavior accompanying distinct many peaks at the isotropic-B 4 transition. A comparison with recent reports strongly suggests that the obtained data reflect the growth processes of the helical nanofilaments in the isotropic 5CB molecules.

Calorimetric study of the Paranematic-to-Nematic transition of polydomain side-chain liquid-crystalline elastomers with different mesogen composition

The phase transition behaviour of various nematic side-chain liquid-crystalline elastomers with different mesogen composition has been explored by means of high-resolution ac calorimetry. Polydomain samples of the same crosslinking density and different type of mesogens have been investigated. The results show a strong dependence of the phase transition features upon the composition of the mesogen. The distance from the critical point, reflected in the sharpness of the heat capacity anomalies, increases when adding a shorter-length mesogen. The results provide new insight for the impact of mesogens on the thermodynamic behaviour and, thus, on the thermomechanical response of nematic liquid-crystalline elastomers.

Controlling the Critical Behavior of Paranematic to Nematic Transition in Main-Chain Liquid Single-Crystal Elastomers

We investigated the paranematic-to-nematic phase transition of nematic main-chain liquid single-crystal elastomers by means of high-resolution ac calorimetry and deuteron quadrupole-perturbed nucle...

High-resolution calorimetric study of the nematic to smectic-Atransition in aligned liquid crystal–aerosil gels

High-resolution ac calorimetry has been used to study the nematic to smectic- A (N-SmA) phase transition in the liquid crystal octylcyanobiphenyl (8CB) confined in aligned colloidal aerosil gels. A stable and robust nematic alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field. In some ways (such as transition temperature and integrated enthalpy), the dependence of the specific heat peak associated with the N-SmA transition on the aerosil density for aligned gels is consistent with that observed in unaligned (random) gel samples. However, a power-law analysis reveals that the behavior of the critical exponent alpha is quite different. For random gels, alpha varies gradually with aerosil density, whereas we find that alpha for aligned gels shifts abruptly to an XY -like value for the lowest aerosil density studied and remains essentially constant as the sil density increases. This aerosil density independence of alpha is consistent with the critical behavior of the smectic correlation lengths obtained from an x-ray scattering study of 8CB in aligned aerosil gels. The combined calorimetric and x-ray results indicate that the role of quenched randomness in aligned gels of 8CB+sils differs significantly from that in random gels.

Dynamics at the Nematic-Isotropic Phase Transition in Aerosil Dispersed Liquid Crystal

Photopyroelectric high resolution ac calorimetry has been used to study the nematic-isotropic phase transition in octylcyanobiphenyl liquid crystal incorporating an aerosil soft gel. We have found that the sample is not in thermodynamic equilibrium in the two phase coexistence region, as shown by the frequency dependence of the double peak in the specific heat and by its thermal hysteresis. The results suggest that the dynamics of the system becomes slower as it is cooled over the transition temperature. An explanation for the formation of a nematic glassy phase in liquid crystals with quenched disorder based on the similarity of these results with the ones obtained in other disordered materials is attempted.

Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions.

High-resolution ac calorimetry has been carried out on dispersions of aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+aerosil system and so general quenched random-disorder effects and liquid crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an aerosil independent overlap of the heat capacity wings far from the transition and having a nonmonotonic variation of the transition temperature. A crossover between low and high aerosil density behavior is observed for 8OCB+aerosil. These features are generally consistent with those on the 8CB+aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low aerosil density regime is apparently more orientationally disordered than the high aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed. Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low aerosil density samples but does not influence high-density samples.

Fluctuation study of the specific heat ofMg11B2

The specific heat of polycrystalline ${\mathrm{Mg}}^{11}{\mathrm{B}}_{2}$ has been measured with high-resolution ac calorimetry from 5 to 45 K at constant magnetic fields. The excess specific heat above ${T}_{c}$ is discussed in terms of Gaussian fluctuations and suggests that ${\mathrm{Mg}}^{11}{\mathrm{B}}_{2}$ is a bulk superconductor with Ginzburg-Landau coherence length ${\ensuremath{\xi}}_{0}=26\mathrm{\AA{}}.$ The transition-width broadening in field is treated in terms of lowest-Landau-level (LLL) fluctuations. That analysis requires that ${\ensuremath{\xi}}_{0}=20\mathrm{\AA{}}.$ The underestimate of the coherence length in field, along with deviations from three-dimensional LLL predictions, suggests that there is an influence from the anisotropy of ${B}_{c2}$ between the c axis and $a\ensuremath{-}b$ plane.

More thermal studies on the PVOH/H 3PO 2/H 2O solid proton conductor gels

Polymer blends based on poly(vinyl alcohol) (PVOH), hypophosphorous acid (H 3PO 2) and water behave as solid protonic conductors. The ionic conductivity at room temperature depends strongly on OH/P ([number of mols of vinyl alcohol monomeric units]/[number of mols of H 3PO 2]) ratios, x, showing a sensitive variation between 10 −6 and 10 −1 S cm −1, as the acid concentration is increased. Using the membranes as electrolyte separator in a fuel cell, voltages up to 726 mV have been obtained. Using high-resolution AC calorimetry and differential scanning calorimetry, we studied the phase behavior of the blends. We found that, independent of x, the glass transition ( T g) is about 167 K, compared with that of PVOH/H 2O at 298 K and pure PVOH at 318 K. However, the step change at the glass transition and other two thermal events (peaks) at higher temperatures (at about 220 and 255 K, respectively) representing the cold crystallization and the corresponding crystalline melting of the samples, increase with increasing acid content in the samples. These results are consistent with the interpretation of a two-phase mixture in the blends: one of them with constant composition PVOH/H 3PO 2/H 2O and a separate second phase with composition H 3PO 2/H 2O.

Specific heat of β-AgI single crystal at low temperatures

Using high resolution AC calorimetry between 10 and 300 K, we measured the specific heat of AgI single crystal grown at room temperature in the hexagonal (β) phase. In addition to the previously reported thermal anomaly at T 1∼60 K, a second peak in the specific heat was observed at T 2∼220 K. Although the thermal signal at T 1 was reversible and shows no dependence on the thermal history of the sample, the peak at T 2 was enhanced on successive temperature scanning of the sample through the transition point until it reached a constant area (enthalpy) of about 4.15 J mol −1. We discuss our observations near 220 K in terms of a phase transition in the silver ion sublattice in β-AgI due to disordering of the mobile ions on heating the sample through the transition temperature.

Phase behavior of complexes of PVA and acid salts

Polymer electrolytes of COLBON®(BASF TM) (based on poly vinyl alcohol (PVA)) complexed with KHSO 4 of different stoichiometric ratios have been characterized using high-resolution ac calorimetry, differential scanning calorimetry (DSC) and electrical conductivity measurements. These complexes have been prepared by solvent casting method using H 3PO 2 as solvent. All complexes have been confirmed to be amorphous up to 450 K by thermal and conductivity measurements. The glass transition temperature is maintained low (around T g≈213 K), without largely depressing conductivity up to the KHSO 4 weight fraction x=0.5. The electrolyte with x=0.09 exhibits the highest conductivity, of 1.1×10 −2 (Ω cm) −1 at room temperature. The activation energy of charge transport is independent of salt concentration ( x<0.5) and remains mainly constant at 0.23 eV in the 300–360 K temperature range.

Quasicritical heat capacity at a smectic-A–hexatic-Bphase transition

High-resolution ac calorimetry has been used to characterize the excess heat capacity $\ensuremath{\Delta}{C}_{p}(T)$ associated with the smectic-$A$ $(\mathrm{Sm}A)$ --hexatic-$B$ $(\mathrm{Hex}B)$ transition in $n$-hexyl-${4}^{\ensuremath{'}}\ensuremath{-}n$-pentyloxybiphenyl-4-carboxylate (65OBC). For temperature oscillations in the investigated frequency range 1.42--400 mHz, the $\ensuremath{\Delta}{C}_{p}$ data reveal no frequency dependence and correspond to static thermodynamic values. The present data resolve several discrepancies in previously reported ${C}_{p}$ values. This transition is very weakly first order, but power-law fits could be made to $\ensuremath{\Delta}{C}_{p}.$ Such fits yield an effective critical exponent ${\ensuremath{\alpha}}_{\mathrm{eff}}=0.65\ifmmode\pm\else\textpm\fi{}0.05,$ which agrees quite well with values of $\ensuremath{\sim}0.6$ reported by Huang and co-workers [Phys. Rev. Lett. 46, 1289 (1981); Phys. Rev. A 28, 2433 (1983); Phys. Rev. Lett. 56, 1712 (1986)]. Such an exponent value disagrees strongly with the theoretically predicted three-dimensional $\mathrm{XY}$ value ${\ensuremath{\alpha}}_{\mathrm{XY}}=\ensuremath{-}0.007.$ It is proposed that this ${C}_{p}$ behavior and also the reported order parameter variation, which yields an effective critical exponent ${\ensuremath{\beta}}_{\mathrm{eff}}$ of $\ensuremath{\sim}0.2,$ are consistent with quasitricritical or quasitetracritical (strain-smeared, very weak first-order) behavior arising from a coupling between the amplitude of the bond-orientational order and the in-plane positional strain.