Influence of internal flexibility on the double glass transition in a series of odd non-symmetric liquid crystal dimers characterised by dielectric measurements

Diego, J. A. de; Sellarès, J.; Diez‐Berart, S.; Salud, J.; Cañadas, J. C.; Mudarra, M.; López, David; Fuente, M. R. de la; Ros, M. Blanca

doi:10.1080/02678292.2017.1309099

Cited by 11 publications

(23 citation statements)

References 43 publications

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“…A very interesting result that we have recently presented is the presence of two close glass transition temperatures in the same supercooled mesophase. This result has been obtained by calorimetric measurements, dielectric spectroscopy and TSDC measurements in the series of nonsymmetric liquid crystal dimers α-(4-cyanobiphenyl-4'yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) alkanes (CBOnO.Py) with n being an odd number [25,26]. The rigid units of these dimers are a cyanobiphenyl group and a pyrene group, and they were first synthesized with the idea of obtaining a liquid crystalline compound (as the cyanobiphenyl group is pro-mesogenic) which could vitrify (triggered by the bulky pyrene unit) [27].…”

Section: Introductionmentioning

confidence: 70%

“…The rigid units of these dimers are a cyanobiphenyl group and a pyrene group, and they were first synthesized with the idea of obtaining a liquid crystalline compound (as the cyanobiphenyl group is pro-mesogenic) which could vitrify (triggered by the bulky pyrene unit) [27]. In these dimers three dielectric relaxation modes have been identified [4,25,26,28,29]: the one at higher frequencies,  2 , due to precessions of the cyanobiphenyl rigid units around their long axes (the pyrene groups do also perform similar precessional motions, but they cannot be detected dielectrically); the ones at intermediate,  1H , and lowest frequencies,  1L , due to the flip-flop reorientations around their short axes of the cyanobiphenyl groups and the pyrene units, respectively. In these dimers, regardless of being nematogenic (n=11) or smectogenic (n=9), both  1H and  2 converge at some temperature above the glass transition in one unique mode and are responsible of one T g , whereas  1L vitrifies at a close but different temperature.…”

Section: Introductionmentioning

confidence: 99%

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Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer

et al. 2018

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In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py) throughout both nematic and smectic-A mesophases by means of broadband dielectric spectroscopy has been performed. CBO7O.Py shows three different dielectric relaxation modes and two glass transition (T_{g}) temperatures: The higher T_{g} is due to the freezing of the molecular motions responsible for the relaxation mode with the lowest frequency (μ_{1L}); the lower T_{g} is due to the motions responsible for the two relaxation modes with highest frequencies (μ_{1H} and μ_{2}), which converge just at their corresponding T_{g}. It is shown how the three modes follow a critical-like description via the dynamic scaling model. The two modes with lowest frequencies (μ_{1L} and μ_{1H}) are cooperative in the whole range of the mesophases, whereas the highest frequency mode (μ_{2}) is cooperative just below some crossover temperature. In terms of fragility, at the glass transition, the ensemble (μ_{1H}+μ_{2}) presents a value of the steepness index and μ_{1L} a different one, meaning that fragility is a property intrinsic to the molecular motion itself. Finally, the steepness index seems to have a universal behavior with temperature for the dielectric relaxation modes of liquid crystal dimers, being almost constant at high temperatures and increasing drastically when cooling the compound down to the glass transition from a temperature about 3/4T_{NI}.

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Section: Introductionmentioning

confidence: 70%

Section: Introductionmentioning

confidence: 99%

Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer

et al. 2018

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“…So, if the material state is modified because of many factors such as, processing conditions, crosslinking and thermal history its fingerprint will change. Compensation phenomenon has been found intensively in the area of chemical kinetics and has been published in many TSDC studies [37][38][39][40]. It has been assumed that, at the compensation temperature, Tc, all the relaxation process will obtain at a single compensation relaxation time, c.…”

Section: Ts-rmamentioning

confidence: 99%

Rate Theory and Relaxation Map Analysis of Iodine-Doped Poly(Ethyl Methacrylate) composite films using Thermally Stimulated Depolarization Current-Thermal Sampling (TSDC-TS) Technique

Elsharkawy

2021

Egypt. J. Chem.

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Iodine-doped Poly (Ethyl Methacrylate) composites have been prepared using casting method. Global TSDC of the composite samples has been investigated using TSDC-TS technique. It is found that, global TSDC spectra of pure PEMA are characterized by two relaxations. The first relaxation which is observed as a shoulder in the temperature range 324-333 K is attributed to the motion of side group and named -relaxation. Whereas, the second relaxation is observed in the glass temperature range of PEMA at Tm= 347 K and named as dipolar relaxation, i.e., -relaxation. On doping with iodine, a new relaxation peak has been obtained at high temperatures ~ 397-409 K, and named as space charge relaxation, i.e., -relaxation. Complex TSDC spectra of PEMA-Iodine composites have been resolved into its elementary peaks using TS technique. These elementary peaks have been used to construct relaxation map (RM) of all samples using Eyring transformation. The existence of compensation phenomenon for all samples is also verified and the coordinates of each compensation point have been estimated. These coordinates are used to calculate the density of disorder (DOD) and thermal expansion coefficient of the samples. The thermodynamic parameters of the samples, such as, enthalpy, entropy and Gibbs free energy have been calculated at different poling temperatures.

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“…The corresponding diether linked dimer series, the 4′‐{[ω‐(4‐{(E)‐[(pyren‐1‐yl)imino]methyl}phenoxy)alkyl]‐oxy}[1,1′‐biphenyl]‐4‐carbonitriles, referred to as CBO n O.Py, is shown in Figure 2 [33] . This series is well‐known and has been the subject of a number of prior investigations [36–40] . The phase behaviour of a selection of homologues from both series are reported and compared.…”

Section: Introductionmentioning

confidence: 99%

“…[33] This series is well-known and has been the subject of a number of prior investigations. [36][37][38][39][40] The phase behaviour of a selection of homologues from both series are reported and compared.…”

Section: Introductionmentioning

confidence: 99%

Twist‐Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene‐based Nonsymmetric Dimers

et al. 2021

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A selection of pyrene‐based liquid crystal dimers have been prepared, containing either methylene‐ether or diether linked spacers of varying length and parity. All the diether linked materials, CBOnO.Py (n=5, 6, 11, 12), exhibit conventional nematic and smectic A phases, with the exception of CBO11O.Py which is exclusively nematic. The methylene‐ether linked dimer, CBnO.Py, with an even‐membered spacer (n=5) was solely nematogenic, but odd‐members (n=6, 8, 10) exhibited both nematic and twist‐bend nematic phases. Replacement of the cyanobiphenyl fragment by cyanoterphenyl giving CT6O.Py, gave elevated melting and nematic‐isotropic transition temperatures, and SmA and SmCA phases were observed on cooling the nematic phase. Intermolecular face‐to‐face associations of the pyrene moieties drive glass formation, and all these materials have a glass transition temperature at or above room temperature. The stability of the glassy twist‐bend nematic phase allowed for its study using AFM, and the helical pitch length, PTB, was measured as 6.3 and 6.7 nm for CB6O.Py and CB8O.Py, respectively. These values are comparable to the shortest pitch of a twist‐bend nematic phase measured to date.

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Influence of internal flexibility on the double glass transition in a series of odd non-symmetric liquid crystal dimers characterised by dielectric measurements

Cited by 11 publications

References 43 publications

Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer

Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer

Rate Theory and Relaxation Map Analysis of Iodine-Doped Poly(Ethyl Methacrylate) composite films using Thermally Stimulated Depolarization Current-Thermal Sampling (TSDC-TS) Technique

Twist‐Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene‐based Nonsymmetric Dimers

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