K. Sigdel

High-resolution ac-calorimetry is reported on the weakly first-order isotropic to nematic (IN) an... more High-resolution ac-calorimetry is reported on the weakly first-order isotropic to nematic (IN) and the continuous nematic to smectic-A (N-SmA) phase transitions in the liquid crystal octylcyanobiphenyl (8CB) doped with a ferroelectric nanoparticle barium titanate, BaTiO 3 (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 IN and the N-SmA transitions evolve in character. Specifically, there appears an unusual change of the IN 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 c m = 0.002 as compared to that for φ m > φ c m. 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 c m 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 c m the mean distance is small enough for the LC to mediate coupling between the BT ferroelectric nanoparticles.

Measuring the Imaginary Part of the Permittivity Using Calorimetry

by K. Kashuri, Germano Iannacchione, and K. Sigdel

... 1:51 PM2:03 PM. Preview Abstract. Authors: Hektor Kashuri Krishna Sigdel Klaida Kashuri Germ... more

Study of the Isotropic-Nematic and the Nematic-Smectic-A Phase Transitions in Carbon Nanotubes and Liquid Crystal Composites

Thermal behavior of poly(ethylene-co-propylene) containing carbon nanofibers

A high-resolution ac-calorimetric study of the isotropic to nematic (\emph{IN}) and the nematic t... more

Polymer Engineering & Science, 2011

The effect of carbon nanofiber on the thermal behavior of poly(ethylene-co-propylene) (PEP) as re... more The effect of carbon nanofiber on the thermal behavior of poly(ethylene-co-propylene) (PEP) as revealed by differential scanning calorimetry and thermogravimetric analysis is reported. Analysis showed faster crystallization of PEP at higher temperature upon cooling with the increase of carbon nanofiber (CNF) content. The crystallization behavior changed to a single narrow crystallization peak as compared with the broad double crystallization peak of the neat polymer. This demonstrates the nucleation ability of CNF to induce crystals with more uniform distribution. The modified-Avrami approach was used to study the crystallization behavior. We found that the crystallization rate constant increased with addition of CNFs. The dimensionality of crystal growth was found not to depend significantly on the content of CNF. Thermal degradation in air was monitored using thermogravimetric analysis and observed that the incorporation of nanofiber greater than 2.4 vol% improves thermal stability of PEP. All these results indicate that CNFs can significantly help polymer processing and increase thermal stability of polymers. POLYM. ENG. SCI., 52:408-413, 2012.

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

Physical Review E, 2010

The continuous nematic to smectic-A (N -SmA) phase transition has been studied by highresolution ... more The continuous nematic to smectic-A (N -SmA) phase transition has been studied by highresolution ac-calorimetry in binary mixtures of the liquid crystal octylcyanobiphenyl(8CB) and a non-mesogenic, 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 non-monotonically 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 T CP hex ≃ 0.07 above which the distinct peak is observed, corresponding to a jump in both the real and imaginary enthalpy. A simple power-law analysis reveals an effective exponent that increases smoothly from 0.30 to 0.50 with an amplitude ratio

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

The Journal of Chemical Physics, 2013

High-resolution ac-calorimetry is reported on the weakly first-order isotropic to nematic (I-N) a... more 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, BaTiO 3 (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 c m = 0.002 as compared to that for φ m > φ c m . 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 c m is found to bex 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 c m the mean distance is small enough for the LC to mediate coupling between the BT ferroelectric nanoparticles.

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

The Journal of Chemical Physics, 2010

The first-order transition from the isotropic ͑I͒ to smectic-A ͑Sm A͒ phase in the liquid crystal... more The first-order transition from the isotropic ͑I͒ to smectic-A ͑Sm A͒ phase in the liquid crystal 4-cyano-4Ј-decylbiphenyl ͑10CB͒ doped with the polar solvent acetone ͑ace͒ 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 A͒ 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. Downloaded 02 Nov 2010 to 130.215.103.190. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 174501-2 K. P. Sigdel and G. S. Iannacchione J. Chem. Phys. 133, 174501 ͑2010͒ Downloaded 02 Nov 2010 to 130.215.103.190. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

Evolution of the isotropic to nematic phase transition in binary mixtures of octylcyanobiphenyl and n-hexane

by K. Sigdel, Germano Iannacchione, and Paul Clegg

The Journal of Chemical Physics, 2010

High-resolution calorimetry has been performed to study the effect of dilution by a nonmesogenic,... more High-resolution calorimetry has been performed to study the effect of dilution by a nonmesogenic, low-molecular weight solvent ͑linear n-hexane͒ on the isotropic-nematic ͑I-N͒ phase transition in the liquid crystal ͑LC͒ octylcyanobiphenyl ͑8CB͒ as a function of n-hexane concentration. Numerous temperature scans were performed without continuous mixing for pure 8CB and all mixture samples of n-hexane mole fraction ranging from x hex = 0.02 to 0.12. The I-N specific heat peak remains first-order for all samples and shifts toward lower temperature nonlinearly while the two-phase I + N coexistence width broadens linearly with increasing x hex . Multiple heating and cooling scans are reproducible and indicate phase separation, if it occurs, must be on very short length scales and is reversible. These results may be a consequence of a competition between random dilution effects and the tendency to phase separate. It is shown that solvent dilution of a LC, if miscible and depending on solvent structure, can lead to a controlled altering of the intermolecular potentials and softening of the LC viscoelastic properties.

format_quoteContinuous mixing of 8CB-cyclohexane shows linear decrease in transition temperatures and constant latent heat with higher solvent fractions.format_quote

Effect of carbon nanotubes on the isotropic to nematic and the nematic to smectic- A phase transitions in liquid crystal and carbon nanotubes composites

format_quoteThe I-N phase transition occurs at T IN = 313.21 K and N-SmA at T NA = 306.09 K, both slightly lower than previous literature values.format_quote

The European Physical Journal E, 2011

A high-resolution ac-calorimetric study on the weakly first-order isotropic to nematic (I-N )and ... more A high-resolution ac-calorimetric study on the weakly first-order isotropic to nematic (I-N )and the continuous nematic to smectic-A (N -SmA) phase transitions of the liquid crystal octyl-cyanobiphenyl (8CB) doped with well-dispersed multiwall carbon nanotubes (CNTs) as a function of CNT concentrations is reported. Thermal scans were performed for all samples having CNT weight fraction from φw =0.0005 to 0.0060 over a wide temperature range well above and below the two transitions in pure 8CB. Both the I-N and the N -SmA transitions evolve in character and have their transition temperatures qualitatively offset by ∼ 1.10 K lower as compared to that in pure 8CB for all 8CB+CNTs samples. The enthalpy change associated with each phase transition is essentially the same as that of pure 8CB and remains unchanged with increasing φ w . However, there is an evidence that the thermal transport properties of the composites differ from the pure LC upon cooling below a φw-dependent temperature within the nematic phase. In addition, a new Cp feature is resolved for intermediate φw samples that appears to be correlated to this onset temperature.

Effect of Solvent Concentration on the Liquid Crystal Phase Transitions of Octylcyanobiphenyl-Hexane Mixtures

The effect of a non-mesogenic, low-molecular weight, solvent on the phases of a liquid crystal ca... more

Study of the Isotropic-Nematic and the Nematic-Smectic-A Phase Transitions in Carbon Nanotubes and Liquid Crystal Composites

A high-resolution ac-calorimetric study of the isotropic to nematic (I-N) and the nematic to smec... more A high-resolution ac-calorimetric study of the isotropic to nematic (I-N) and the nematic to smectic-A (N-SmA) phase transitions of carbon nanotubes (CNTs) and liquid crystal octyl-cyanobiphenyl (8CB) composites (8CB+CNTs) as a function of CNTs concentration is reported. Scans were performed on heating and cooling for all samples (0.5-6 wt% of CNTs) over a wide temperature range well above and below the two transitions in pure 8CB. Both the I-N and the N-SmA transitions evolve in character and have their transition temperatures shift lower as the wt% of CNTs increases. For intermediate wt% values, new transitions features are observed, which suggest new phase ordering of the CNTs within the liquid crystal host.

Evolution 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 cry... more

Measuring the Imaginary Part of the Permittivity Using Calorimetry

by K. Kashuri, Germano Iannacchione, and K. Sigdel

Modulated or AC calorimetry is a well established technique for measuring the temperature depende... more Modulated or AC calorimetry is a well established technique for measuring the temperature dependence of the heat capacity of many complex fluids. Employing a dielectric or RF heating method, the heat capacity, thermal conductivity, and the dielectric properties of the sample are all probed simultaneously. Combining the results obtained by this technique for the liquid crystal 4-n-pentyl-4-cyanophenyl (5CB) with those obtained by our novel AC calorimetric technique employing RF (dielectric) heating, we have been able to directly measure the temperature dependence of the imaginary part of the permittivity of this liquid crystal. Measurements were performed over a temperature range from 303 to 313 K, spanning the nematic to isotropic phase transition, as well as radio frequencies from 10 to 30 MHz

Isotropic to Nematic Phase Transition In Carbon Nanotube Dispersed Liquid Crystal Composites

Study of Phase Transitions in Barium Titanate - Liquid Crystal Nano-colloidal Suspensions

Arxiv preprint arXiv:0905.2779, 2009

Rajratan Basu, Krishna P. Sigdel and Germano S. Iannacchione∗ Order-disorder phenomena laboratory... more

A high-resolution ac-calorimetric study of the isotropic to nematic (I-N) and the nematic to smec... more A high-resolution ac-calorimetric study of the isotropic to nematic (I-N) and the nematic to smectic-A (N-SmA) phase transitions in nano-colloidal suspensions of Barium Titanate (BaTiO3) in the liquid crystal (LC) octylcyanobiphenyl (8CB) as a function of BaTiO3 concentration is reported. Heating and cooling scans were performed for all samples (0.1 - 1.4 wt% of BaTiO3 and pure 8CB) over a wide temperature range well above and below the two transitions. Both the I-N and the N- SmA transitions evolve in character and have their transition temperatures shift lower as the wt% of BaTiO3 increases. Increasing hysteresis with increasing concentration is also observed at the I- N transition between heating and cooling scans within the two- phase I+N coexistence region. These results will be contrasted with other LC colloidal dispersion systems.