Phase and dielectric behaviors of a polymorphic liquid crystal doped with graphene
nanoplatelets
Po-Chang Wu and Wei Lee
Citation: Applied Physics Letters 102, 162904 (2013); doi: 10.1063/1.4802839 View online: http://dx.doi.org/10.1063/1.4802839
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/16?ver=pdfcov Published by the AIP Publishing
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Phase and dielectric behaviors of a polymorphic liquid crystal doped with
graphene nanoplatelets
Po-Chang Wu and Wei Leea)
College of Photonics, National Chiao Tung University, Guiren, Tainan 71150, Taiwan
(Received 17 March 2013; accepted 7 April 2013; published online 24 April 2013)
We report on the phase behavior and dielectric properties of the liquid crystal (LC) 40 -n-octyloxy-4-cyanobiphenyl dispersed with graphene nanoplatelets (GNPs). The temperature-dependent dielectric permittivity at 104Hz and its derivative with respect to the temperature reveal that the incorporation of GNPs in a LC cell leads to the modification of crystalline polymorphism and shift in phase transition temperature owing to the enhanced positional and orientational order. Additionally, the dielectric data between 1 and 103Hz show that the dopant reduces the ionic concentration and alters the diffusivity in the LC mesophases.VC 2013 AIP Publishing LLC
[http://dx.doi.org/10.1063/1.4802839]
Colloids of nanoscale materials dispersed in liquid crys-tals (LCs) have been a subject of extensive research during the last decade. From the point of view of practical applica-tions, manipulation of the physical properties of existing LCs by doping with various types of nanomaterials has been evidenced for the improvement of electro-optical perform-ance in LC devices.1,2Moreover, investigations demonstrat-ing the suppressed ionic effect by nanodopants have also been clarified for solving image problems in LC displays caused by ubiquitous impurity ions.3–7 Depending on the intrinsic characteristics of the dopant and mutual interaction with LC molecules, researchers have studied the individual effects of nanomaterial additives on the behaviors of phase transitions in LCs in terms of the molecular ordering,8,9 dielectric relaxation,10 and electrophysical properties of LCs.11–13
Among currently available nanomaterials, graphene, classified as a two-dimensional (2-D) carbon allotrope, has attracted immense attention for potential applications in electronic devices and nanocomposites since its successful isolation in 2004.14An alluring feature of graphene for opto-electronic uses is its high transparency and high conductiv-ity, suitable for replacing indium–tin oxide (ITO) as an alternative electrode material.15As the interest in LC suspen-sions grows readily in current nanoscience, researchers have investigated the optical manipulation in a LC/graphene nano-structure16 as well as applications of graphene in choles-teric17and polymer-dispersed18LCs very recently.
In the present work, we consider several colloidal sus-pensions of polymorphic LC impregnated with various con-tents of graphene nanoplatelets (GNPs) and look into the phase behavior and ionic properties of the hybrids based on temperature-dependent dielectric spectroscopy and polariz-ing optical microscopy. Our observations reveal that the pro-cess of crystallization in the solid phase, the order of LC molecules, and the transition temperature between two adja-cent phases are all significantly influenced by the GNP dopant. In addition, the dielectric spectra in the low-frequency regime (f 1 kHz), characterized by space-charge polarization, allow
one to investigate the impurity-ion properties of GNP-doped LC cells in comparison with their neat counterpart. The data manifest the reduced ionic concentration and modified diffusivity.
The LC host used in this study was 4-n-octyloxy-40 -cyanobiphenyl (8OCB), a polar single compound exhibiting polymorphism with the following phase transition sequence: Crystal (Cr)–55C–Smectic A (SmA)–67C–Nematic (N)–80C–Isotropic (I). The GNP (xGNP-05), purchased from XG Sciences, contains more than 16 graphene layers with an average thickness of 10 nm and diameter of 5 lm. In order to promote homogeneity and uniform dispersion, the GNPs as received were first baked in a vacuum oven at 150C for 6 h to evaporate residual moistures, followed by ball milling for disaggregation and reduction in size. The condition of ball milling was optimized based on our previ-ous work.3The sizes of GNPs were reduced to the order of sub-micrometers. Various 8OCB/GNP mixtures were pre-pared with GNP concentrations ranging from 0.03 to 0.50 wt. %, as determined by a Mettler Toledo X26 analytical bal-ance. All mixtures were put in an ultrasonic bath for 2 h and then stirred for 1 h. By capillary action at 70C where the LC was in the N phase, pristine 8OCB and the well-dispersed colloids were individually introduced into glass cells, each composed of a pair of ITO-coated glass substrates covered with planar alignment films. The cell gap was 7.0 6 0.5 lm. Temperature-dependent dielectric spectra in the frequency range of 1 Hz–100 kHz were acquired with a high-precision LCR meter (HIOKI 3522-50). The probe volt-age for the dielectric measurement was 0.1 V in the sinusoi-dal waveform; this amplitude was smaller than the threshold voltage of 8OCB. The dielectric data were obtained in the temperature range controlled between 30 and 90C with an accuracy of 60.5C.
The complex dielectric function e*¼ e0 – ie00 depends
generally on the frequency (f) and temperature (T). In the fre-quency regime between 103and 105Hz, the real part e0, dic-tated by the orientation of 8OCB molecules, exhibits no dependence onf. Because of the strong T dependence of mo-lecular ordering, the variation of e0 withT in this frequency regime can be used to clarify the phase behavior and thermal a)
Electronic mail: [email protected].
stability of LC molecules.19,20Moreover, the derivative of e0 with respect toT, de0/dT, is a sensitive probe for determining the phase transition.20,21Figure1shows the temperature de-pendence of the real-part dielectric function e0(T) of a pure cell and a doped cell containing 0.50 wt. % GNPs at 104Hz. Starting from the crystalline phase in the case of pristine 8OCB, e0(T) increases with rising T from 45 to 55C until it reaches the SmA phase where e0(T) remains constant. This result is regarded as the pre-transitional behavior between the solid and LC states attributed to the increased kinetic energy of the molecules. Further raisingT above the SmA-to-N phase transition point (67C) causes the increase in e0(T) due to the thermal-induced reduction in LC molecular
order in the N phase. The value of e0(T) further reaches a sta-ble level at high temperatures (>80C) in the I phase. Compared with that of the pure counterpart, e0(T) of the GNP-doped cell varies similarly in the SmA, N, and I phases. Notably, the values of e0(T) for the doped sample are much lower especially at the temperatures covering the N phase. In the planar-aligned LC cell, e0corresponds to the perpendicu-lar component e?of the LC phases. Since the increase in e0
with increasing temperature is expected for the LC 8OCB with positive dielectric anisotropy, the lowered value of e0in
the N phase of GNP-doped LC cell indicates less variation of molecular ordering with temperature. This result implies that the presence of GNPs induces the reinforced structural order. In view of the intermolecular interaction between LC and carbon nanotubes (CNTs)8and the superior mechanical and thermal properties of GNPs, it can be deemed that the cou-pling between the basal plane of GNP and LC creates an in-plane anchoring force to the LC molecules and thus enhances the molecular order as well as its thermal stability. When the phase transforms from SmA to N, the 2-D layered structure of GNPs serves to consolidate the smectic layers, spatially maintaining the molecular orientation. Figure1also presents an abrupt drop of e0(T) between 37 and 42C in the crystal-line phase of the GNP-doped sample. This result is distinct from the neat 8OCB cell in which e0(T) retains almost unchanged untilT reaches the Cr–SmA pre-transition point. The drop implies that there is an additional Cr-to-Cr phase transition in the doped cell. Further examination of micro-scopic textures indicates that only the most stable powder-crystalline form (CrA) with perfect single domain can be observed within the crystalline phase of pure 8OCB as shown in Fig. 2(a). By contrast, poly-domain crystalline form (CrB) is exhibited in the GNP-doped cell at 30C (Fig. 2(b)). As the temperature increases to a specific value near 40C, a transformation from CrB to the more stable crystal form CrC takes place via the nucleation of crystal domains. As a matter of fact, CrB is a metastable form, spontaneously turning to the stable CrC form in several days at room tem-perature. The CrB-to-CrC transition temperature TCrB–CrCin
GNP-doped cells is dependent of the GNP concentration. In the case of the 0.50 wt. % content,TCrB–CrCis about 40C as
depicted in Fig. 2(b). Note that the appearances of the CrA and CrC forms are both opaque-white whereas the CrB form appears transparent. In 8OCB polymorphism, four crystalline forms—one stable state and three metastable states—have been observed by various methods.22,23In GNP-doped cells, we observed that the CrB form is generated by the expansion of platelet-like domains originating from the GNPs during the cooling process in the solid phase. It is explained that the enhanced order gradually declines as the LC molecules are
FIG. 1. Real-part dielectric constant of pure and GNP-doped 8OCB cells at 104Hz as a function of the temperature at the rate of 1C/min. The dopant concentration for the doped cell is 0.50 wt. %.
FIG. 2. Microscopic textures of (a) pure and (b) 0.50 wt. % GNP-doped 8OCB cells in the crystalline phase at different temperatures. Here, P, A, and R repre-sent the transmission axes of the polar-izer, analyzer, and the rubbing direction, respectively.
aligned farther away from the GNPs. In other words, the dis-persed GNPs serve as the nuclei, causing the change in energy for the LC molecules surrounding the GNPs during the crystallization process. The breaking in symmetry of mo-lecular ordering by the scattered GNPs thus hinders the crys-tallization into the single-domain form CrA and gives rise to the poly-domain CrB form.
The phase transition temperatures of the undoped and doped cells can be discussed more explicitly in terms of de0/dT as a function of the temperature as shown in Fig.3. Figure3(a) exhibits three peaks at 54, 67, and 78C in the case of the pure 8OCB cell. These values are quite consistent with the knownTCr–SmA,TSmA–N, and TN–I, respectively, in
accordance with the phase diagram of the pure 8OCB. Doping GNPs in 8OCB results in an additional feature—upside-down peak—corresponding to the CrB–CrC phase transition. Depending onc, the temperature for this upside-down peak is located around 40 to 44C as illustrated in Figs. 3(b)–3(f). One can see thatTCrB–CrC decreases with increasingc,
pre-sumably due to the domain-size dependence of transition energy. Since the dispersed GNPs are nuclei of these platelet domains, more GNPs contained in the cell are considered to create smaller domain sizes in the restricted cell space. Once theT is elevated (>30C), 8OCB molecules in the CrB form characterized by numerous smaller domains can transform to CrC much more easily. Obviously, Fig. 3 also reveals the shifts of both TSmA–N and TN–I to higher temperatures as c
goes beyond 0.10 wt. %. The extension ofTN–Ican be referred
to as the enhanced orientational order by GNPs which has also been observed in LC/CNT dispersions.11 Figure 4
summarizes the phase transition temperatures deduced from Fig.3. It is obvious thatTCrB–CrC,TSmA–N, andTN–Iare
modi-fied from 44 to 40C, 67 to 71C, and 78 to 83C, respec-tively, asc is increased up to 0.50 wt. %.
In regard to the dielectric response in the frequency range between 1 and 103Hz, the complex dielectric function is characterized by the motion of mobile ions in which e0and e00 are inversely proportional to f3/2and
f1, respectively. Because of the large scatter in the dielectric data for the solid phase, the T-dependent ionic properties were investigated within the temperature range from 60 to 90C, covering the SmA to I phases of 8OCB. Following the model estab-lished by Uemura,24,25 the complex dielectric function can be written as e0ðf Þ ¼ 2nq 2 D32 e0 ffiffiffip p dkBT f32 (1) and e00ðf Þ ¼ 2nq 2D e0kBT f1; (2)
wheren is the ionic concentration, q is the electric charge, D is the diffusivity of the ions,d is the cell gap, e0is the
permit-tivity in free space, kB is the Boltzmann constant, and T is
the absolute temperature. Consequentially, n and D can be obtained by fitting the experimental data of e0and e00 accord-ing to Eqs.(1)and(2). Figure5(a)shows the fitted ionic con-centrations in undoped and doped 8OCB cells. Noticeably,n in either phase is nearly invariant to T for each cell. Such a T-independent ionic concentration has been reported in the N phase and explained by the absence of the ionic association– dissociation reaction.3Here, the approximately invariantn in the individual SmA and N phases is attributable to the simi-lar molecusimi-lar orientation across the continuous (namely, sec-ond-order) phase transition. As T increases, n drops steeply just near the N–I phase transition temperature on account of the dipole interaction between the polar LC molecules rather than the dopant effect. Clearly from Fig. 5(a), all GNP-doped cells exhibit lower n in the entire temperature range, indicating that the dispersion of GNPs is responsible for trap-ping mobile impurity ions. For the 0.50 wt. % dopant con-tent, the value ofn is reduced, from 2.5 to 1.7 1012cm3, by 30%.
FIG. 3. The first-order derivative of the real-part dielectric constant with respect to temperature of various 8OCB cells.
FIG. 4. Phase transition temperatures of undoped and doped 8OCB cells with different GNP concentrations.
Figure5(b)presents the temperature-dependent diffusiv-ities in the pure and doped 8OCB. For all cells, D(T) increases with increasing T. In the doped cells with GNP contents of 0.03 and 0.07 wt. %,D is higher than that in the undoped cell in the entire temperature range. At these two dopant concentrations, the effect of GNPs on the molecular order is very weak since the phase transition temperatures of both SmA–N and N–I are identical to those of the pristine counterpart as displayed in Fig. 3. In consideration of the dimensions of GNPs used in this study, the distributed GNPs in a 7 lm-thick cell are likely to form percolated networks and, in turn, lead to the increase in diffusivity and electrical conductivity as well. On the other hand,D is lowered as c increases beyond 0.10 wt. %, indicating the remarkable abil-ity of hindering the ion transport. It is known that GNPs tend to conglomerate at higher concentrations, increasing the vis-cosity of LC colloids.18In this study, the loweredD seems to stem from the increasing flow viscosity, speculatively due to the pronounced enhancement of the molecular order by GNPs. With the limited variation in ionic concentration, the behavior of the temperature-varying dc conductivity is pri-marily governed by the ionic diffusivity.
In summary, temperature-dependent dielectric proper-ties of the polymorphic LC 8OCB doped with GNPs are investigated by dielectric spectroscopy. Our results indicate that, different from the conventional methods for obtaining metastable crystalline forms, a metastable poly-domain crys-talline form (CrB) can readily be generated in the solid phase for GNP-doped samples owing to the presence of the dopant
serving as nuclei during crystallization. The transformation from the metastable crystalline form to the more stable crys-tallize form (CrC) is achieved in the temperature-heating process. The transition temperature between these crystalline forms decreases with increasing dopant concentration. With the first derivative of e0(T), this study demonstrates that TSmA–NandTN–Ishift to higher temperatures by 4 and 5C,
respectively, as the dopant concentration increases from 0.03 to 0.50 wt. % due to the elevated positional and orientational order. By means of Eqs. (1) and (2), the temperature-dependent ion concentration and diffusivity are deduced for various cells using the dielectric data in the low-frequency regime (f 103Hz). Experimental results reveal that the GNPs as a dopant possess the ability to trap impurity ions, with efficacy to be of temperature independence. Our results also suggest that the diffusivity can be reduced when the dopant concentration is high enough (c¼ 0.30 or 0.50 wt. %) to presumably enhance the molecular order and increase the viscosity.
The authors thank Professor Victor Ya. Zyryanov of Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences for providing 8OCB. This work was financially supported by the National Science Council of the Republic of China (Taiwan) through Grant Nos. NSC 101-2112-M-009-018-MY3 and NSC 101-2811-M-009-059.
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