Carbon Nanotube Reinforced Polymer-Stabilized Liquid Crystal

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Carbon nanotube reinforced polymer stabilized liquid crystal device: Lowered and thermally invariant threshold with accelerated dynamics Subbarao Krishna Prasad, Marlin Baral, Adhigan Murali, and Sellamuthu N. Jaisankar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08825 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Applied Materials & Interfaces

Carbon nanotube reinforced polymer stabilized liquid crystal device: Lowered and thermally invariant threshold with accelerated dynamics S. Krishna Prasad1*, Marlin Baral1 Adhigan Murali2 and Sellamuthu N. Jaisankar2 1 Centre for Nano and Soft Matter Sciences, Jalahalli, Bengaluru 560013, India 2 Polymer Science & Technology Division, Council of Scientific and Industrial Research (CSIR)-Central Leather Research Institute (CLRI), Adyar, Chennai 600 020, India

Abstract Polymer stabilized liquid crystal (PSLC) devices comprise a polymer matrix in an otherwise continuous phase of liquid crystal. The fibrils of the polymer provide, even in the bulk, virtual surfaces with finite anchoring energy resulting in attractive electro-optic properties. Here we describe a novel variation of the PSLC device fabricated by reinforcing the polymer matrix with polymer-capped single-walled carbon nanotubes (CNTs). The most important outcome of this strengthening of the polymer strands is that the threshold voltage associated with the electro-optic switching becomes essentially temperature independent in marked contrast to the significant thermal variation seen in the absence of the nanotubes. The reinforcement reduces the magnitude of the threshold voltage, notably accelerates the switching dynamics and the effective splay elasticity. Each of these attributes is quite attractive from the device operation point of view, especially the circuit design of the required drivers. The amelioration is caused by the polymer decorating CNTs being structurally identical to that of the matrix. The resulting good compatibility between CNTs and the matrix prevents the CNTs from drifting away from the matrix polymer, a lacuna in previous attempts to have CNTs in PSLC systems. Difference in the morphology, perhaps the primary cause for the effects seen, is noted in the electron microscopy images of the films.

Keywords: CNT-reinforcement, Polymer stabilization, Liquid Crystal Device, Threshold voltage, Fast switching, Haze

*Corresponding author Email: [email protected] ACS Paragon Plus Environment


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1. Introduction Composites of polymers and liquid crystals (LC) form an important class of materials, especially for smart glass applications based on tunable electro-optic properties1-3. In LC devices not having the composite, the LC material is contained between two substrate surfaces, often coated by surface aligning agents, which provide the necessary forces to act on the molecules for purposes of orienting them along the required direction. Application of an external field, magnetic, electrical or optical, could result in the reorientation of the LC molecules out of the surface plane, a process known as Freedericksz transformation. The associated changes in the optical properties of the medium lie at the heart of the hugely successful display devices4. The recovery of the original orientation direction from the fielddriven one is dictated by the surface forces. This method has an inherent feature that the “surface forces” are present only at the substrate, and can therefore result in undesirable behaviour such as back-flow, defect generation, etc. Although improvements such as having a finite pre-tilt at the surface, incorporation of a chiral component overcome these problems, stabilization with a polymer component has attracted much attention5. Depending on the concentration of the polymer component, two categories exist: polymer dispersed liquid crystal (PDLC) and polymer stabilized liquid crystal (PSLC). The former is rich in polymer and the liquid crystal is present as droplets confined in the polymer matrix. In the PSLC, on the other hand LC is the major component and is confined by strands (or networks) of the minority polymer material. The preparation of these devices is based on a phase separation process to create immiscibility of LC and the polymer through thermal induced phase separation, polymerization-induced phase separation, or solvent-induced phase separation. The phase separation of the two components, and a consequent difference in refractive indices in conjunction with the electrically-operated change in birefringence of LCs, form the basis of the device operation. For instance, in the field-off state, the chosen polymer and the LC in its equilibrium orientation direction present a finite difference in refractive indices. This mismatch and the right size of the LC droplet result in significant scattering of the incident light. An applied electric field coupling to the dielectric anisotropy of the LCs can, above a certain threshold, reorient the LC molecules reducing the mismatch. In the fully switched state the mismatch disappears and the device becomes completely transparent. Thus the reversible and effective switching between the scattering and transparent states provides the principle of the operation of the device.

ACS Paragon Plus Environment

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Devices based on the above described principle are already commercially available, especially for smart switchable e-glass applications6,7. However, among the drawbacks of both PDLC and PSLC categories, a notable one is the behaviour of Vth, the threshold voltage needed to initiate the reorientation from the surface dictated one: It not only becomes much higher than the value for the pristine LC, but retains the same temperature dependence. Owing to the importance of these polymer-LC composite devices there has been a continual effort to improve their operating characteristics, such as threshold voltage (Vth), abovethreshold slope (dC/dV), response time (τ), and contrast ratio (CR). One such ongoing effort is to incorporate nanoparticles (NPs) in the PDLC medium. Several types of NPs have been employed. For example, Hinojosa and Sharma8 introduced gold NPs, and found a small decrease in Vth, while Masutani et al.,9 were able to increase the viewing angle. Incorporating silica NPs Kim et al10 report that while the driving voltage shows a marginal (~ 2%) improvement, the other operating parameters deteriorate. characteristics of PSLC devices also have been reported

11.12

Attempts to improve the

although the method employed

seems tedious in some cases. The incorporation of carbon nanotubes (CNTs) into PDLC has indeed been expected to improve the properties13. Efforts have been made to consolidate CNTs in liquid crystals, although their inclusion into the PSLC architecture has not been attempted. Nevertheless, a few reports exist wherein the composites consist of CNTs and LC, but without any polymer; the CNTs were included as independent entities either in their pristine form or functionalised form. The results of these studies reporting influence on electrooptic characteristics in systems comprising CNTs and host LC, but without the polymeric matrix, is summarized below. In all the cases the concentration of CNT is quite low < 1%. Employing 4-npentylcyanobiphenyl (5CB) as the host LC and 0.005 wt % of MWCNT, the threshold field for the planar-homeotropic transformation was found to be higher by ~ 25% than for the pure LC14. Further, the slope dC/dV was seen to drastically reduce for the composite with consequent increase in the response-saturation field from 60 kV/m for pure LC to 80 kV/m for the CNT composite 15. These features are true in a dye-doped nematic system also16. Lu and Chien investigated composites of CNT with four different commercially available liquid crystal mixtures found hardly any variation in Vth17. Rahman and Lee, however, found that the threshold decreases by ~ 50% for high load factor of CNT18, but hardly altered the dynamics. For a Merck LC mixture, while there was hardly any change or at best a small



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decrease for the MWCNT doped systems, with SWCNT doping a clear increase was seen in Vth

19

. Using two commercially available liquid crystals, including the eutectic mixture E7

used here and DC bias for reorientation, Schymura and Scalia20 reported a slight increase in Vth and a small reduction in dC/dV when CNTs (SWCNTs or MWCNTs) were added to the LC. To summarize these observations, with the addition of CNT (i) generally Vth is not lowered, and (ii) the ratio dC/dV diminishes and (iii) no improvement in response time. These features are important from a device point of view since a lower Vth would reduce the required operating voltage, a larger dC/dV helps in better grey capability. The faster responding system would enhance the screen refreshing time and thus provide flicker-free images. A major disadvantage in this scheme, which may be limiting the improvements possible, is the following. Owing to the physical mixing of CNTs and LC, the nanotubes are free to diffuse through the medium, especially the fluid LC regions and therefore not to be largely beneficial. Employing SiO2 nanoparticles Busbee et. al.,21 have shown that sequestration of NPs is important from the viewpoint of enhancing the optical characteristics. The investigation described in the present article overcomes the problem in the previous investigations, viz., the lack of proper functionalization of CNTs so as to retain in the polymer matrix, in addition to bringing in a polymeric material to create polymer stabilization as well. The CNTs are functionalized in such a way as to be fully compatible with the polymer matrix by decorating the CNT surface with polymer. This helps in preventing CNTs from diffusing away from the polymer areas, and provides perfect structure-compatibility with the host polymer owing to the usage of an identical polymer chain as the rest of the polymer matrix.

2. Experimental The liquid crystal used is the commercially available (from Synthon GmBH) eutectic mixture E7,which

comprises

biphenyl (25%),

of

4-cyano-4'-n-pentyl-biphenyl (51%), 4-cyano-4'-n-octyloxy-biphenyl(16%),

4-cyano-4'-n-heptyland

4-cyano-4'-n-pentyl-p-terphenyl (8%). The monomer, methyl methacrylate was obtained from Sigma Aldrich, USA. The monomer was polymerized as per the reported procedure15. The novel ingredient incorporated into the polymer matrix is the modified PMMA having the PMMA polymer strands grafted on to single-walled carbon nanotubes. The “graft from” method, previously described 22, in which the monomer chain grow up in-situ from the nano surface, was employed to prepare this PMMA-capped SWCNTs. The brief synthetic steps are depicted in Figure 1. The CNT used has an aspect ratio >1000, and exhibits an electrical


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conductivity of 103 Scm-1. The materials used for all the experiments described in this article contained,by weight 5% of polymer and 95% (by weight) of liquid crystal. The two constituents were first dispersed separately in chloroform and then mixed, followed by sonication (Probe sonicator, Ultra sonic processor VCX-750 W) and removal of the solvent at low vacuum. The placement in vacuum continued for extended duration to ensure removal of any residual chloroform. The mixture was then maintained at 150 oC for a short duration and subsequently cooled to room temperature. Hereafter, we refer to the mixture in which the polymer content is entirely the regular PMMA, as PMMA-b. SET-LRP Cu(0) PMDETA

CH2

H3C C C

O

+

O

H2 C

CH3

Br C

Br

C

CH3

o Methyl methacrylate (MAA)

CNT Initiator

o

SWCNT-g-PMMA CH3

Synthesis of SWCNT-g-PMMA O

O

i) DMF/Et3N

C

C

+

NH2

Amide-SWCNTs

Br

CH3 C

Br

ii) 0 oC

Br

CH3

bromoisobutyryl bromide

CNT Initiator

Synthesis of SWCNTs based initiator Figure 1: Scheme of the synthesis (22) of single walled-carbon nanotubes-graft-polymethyl methacrylate (SWCNT-g-PMMA) and SWCNT based initiator. The novel mixture employed here -labelled PMMA-CNT contained the regular PMMA and the PMMA with tethered carbon nanotube material in 4:1 ratio, by weight. For the preparation of this mixture, after the 150oC treatment mentioned above, the sample was cooled to 70 oC and subjected to magnetic stirring for approximately 2 h to prevent any sedimentation of SWCNTs. For the electro-optical measurements, the samples were sandwiched in a cell made of indium tin oxide-coated (ITO) glass plates with a nominal inter-electrode gap of ~ 9 µm. To



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promote planar orientation of the molecules the inside surfaces of the glass plates are pretreated with a polymer aligning agent (PI2555 from HD Microsystem). Electro-optical characteristics were obtained by using an apparatus that essentially consists of optical microscope (Leica DMRXP) white light source, focusing optics that focuses the light transmitted through the sample onto a silicon photo-detector connected to a transimpedance amplifier. The output of the amplifier was fed to an oscilloscope, the digitized output of which was acquired on a PC. To electrically drive the sample a function generator (Agilent 33401A) in conjunction with a voltage amplifier (Trek, mode-50/750) was employed. The electrical parameters such as voltage threshold, slope of the threshold profile, etc. were determined by employing the electric field driven Freedericksz transition. The host LC exhibits a positive dielectric anisotropy ∆ε (= ε|| – ε⊥) owing to which, an electric field well above a certain threshold voltage, Vth, applied normal to the substrate plane imposes a positive torque on the nematic director causing reorientation of the molecules to the homeotropic (vertical to the substrate plane) state. Consequently the permittivity changes from ε⊥ for V < Vth to ε|| for V >> Vth. This field induced reorientation was brought about by utilizing the oscillating output of a precision LCR meter (Agilent E4284A), which also probed the capacitance of the sample. The Field emission scanning electron microscope (SEM) images were taken using MIRA 3 LHU model (company-Tescan). For this purpose, the LC was leached out from the PSLC films.

3. Results 3.1 Microscopy

Optical microscopy Visual inspection of the sample vial after the preparation procedures described in the previous section showed homogenous mixing of the components, in the PMMA-CNT composite as well, a feature corroborated by observation of the samples under a polarizing microscope (POM).

The microscopic scenario of PSLC in the absence of, and with

SWCNT/PMMA strands may be envisaged as represented schematically in Figure 2. To be noted are the aspects that (i) the orientation of liquid crystal molecules in the vicinity of the polymer strands is dictated by the nature of their mutual interaction and (ii) whereas for the PMMA-b sample the polymer fibrils consist of entirely simple PMMA chains, for the PMMA-CNT case, the fibrils could also contain SWCNTs-tethered PMMA. The formation of the fibres and the network are clearly seen in POM images obtained after leaching the liquid


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crystal out (Figure 2(a)); an image with larger area coverage at lower resolution is shown in supporting information as Figure S1. Electron microscopy To further characterize the network we imaged the LC-leached out films using SEM. The image for the PMMA-CNT film, presented in Figure 2(g), exhibits Swiss-cheese architecture, and bears much resemblance to the POM image shown in figure 2(a). Images at other different resolutions are shown as Figure S2 and S3, which corroborate the well formed networks. Surprisingly, a similar film of the PMMA-b sample yielded a non-descriptive image under POM. However, SEM imaging showed that the network does not sustain the leaching out process accounting for the fibrous morphology seen (Figure S4). This difference in the morphology between the PMMA-b and PMMA-CNT films could be taken to support the idea that the reinforcement with CNT makes the polymer network mechanically stronger preventing the network collapse during the leaching out process.

Figure 2: (a) Photomicrograph obtained after leaching the liquid crystal out, showing the formation of the polymer chains in the polymer stabilized system. (b) Schematic representation to illustrate the confinement of the liquid crystal molecules (blue ellipses) between the polymer fibrils. (c) Depiction that in the case of PMMA-b sample, the polymer fibrils (yellow wiggles) are entirely made up of the bare PMMA chains (d), whereas in the case of PMMA-CNT sample, the hierarchically created fibrils contain additionally (gray wiggle), the PMMA tethered SWCNTs (e) with the capping units shown in (f). The polymer matrix formed around the LC molecules remains intact in the case of PMMA-CNT, even after the LC is leached out as seen in the SEM image shown in (g); also see SEM images in SI.

3.2 Orientational switching Figure 3 shows the Freedericksz transformation behaviour for the two PSLC mixtures presenting the variation of sample capacitance (C) as a function of the applied voltage; for



better illustration the data are shown normalized with respect to the equilibrium capacitance value (C⊥) obtained in the planar orientation of the LC molecules. The PMMA-CNT sample has a lower threshold voltage (Vth) than the mixture without CNT (PMMA-b). More importantly, above Vth the increase in capacitance, which is a direct measure of the molecular reorientation, is sharper for the PMMA-CNT sample. This feature, is associated with the magnitude of the bend elastic constant, is a useful parameter for better grey scale capability of the device. This steepness, quantified in terms of the ratio dC/dV is a factor of 2 higher for the PMMA-CNT mixture than its non-CNTs counterpart.

0.9 PMMA-CNT PMMA-b

0.6 PMMA-CNT

Cnorm

Cnorm

0.2

0.3

PMMA-b

0.0

0.6 1.3 Applied voltage (V)

0.0 0

5

10

15

20

Applied voltage (V)

Figure 3: The voltage dependence of the normalized capacitance [Cnorm = (C-C⊥)/C⊥], where C⊥ is the value measured in the equilibrium planar state, presenting the Freedericksz transformation. Both samples exhibit complete reorientation to the homeotropic state, as seen by the saturation of the capacitance at high fields. The PMMA-CNT sample shows a lower threshold as well as a higher slope post-threshold, features which are clearly seen in the data presented on an enlarged scale in the inset. The most salient advantage of the PMMA-CNT device is, however, the thermal behaviour of Vth. The data, shown in Figure 4, covering a useful operating thermal range, bring out the drastic difference in the behaviour of PMMA-b and PMMA-CNT samples.

0.7 Vth (V)

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PMMA-b

0.6

PMMA-CNT 0.5 30

35 40 o Temperature ( C)

45


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Figure 4: Thermal dependence of the threshold voltage for the two polymer stabilized samples presenting a striking difference between the two. While PMMA-b exhibits a sizeable increase on lowering the temperature, behaviour typical of nematics, including polymer stabilized ones, for the SWCNTs-reinforced PMMA Vth is essentially independent of temperature; the data for neat LC (E7) and another composite with PMMA-CNT are shown in SI. The PMMA-b device exhibits the temperature dependence typical of nematic materials, increasing monotonically and substantially as the temperature is lowered.. In stark contrast, the mixture incorporating PMMA strands tethered to CNT leads to, even at the small concentration used, an essential temperature independence of Vth. This assumes importance in terms of designing the driving circuit23 since such a feature obviates the need to account for ambient temperature variation of Vth. It may be mentioned that the temperature dependence for PMMA-b is itself weaker than that for pure LC (E7). Further, a slightly lower concentration(0.5%) of the PMMA-CNT in the mixture places the behaviour to be in between those for PMMA-b and the 1%PMMA-CNT composite, data for which are depicted in figure 4.These results are presented in Figure S5 and 6.Meaurements on higher concentrations greater than 1%PMMA-CNT could not be carried out owing to electrical conductivity. The Frank elastic constant K11 associated with the splay deformation is related to Vth and the dielectric anisotropy ∆ε through ε ∆ε V 2

K = o 11

th

(1),

π2

where εo is the permittivity of free space. It should be mentioned that eq. (1) ignores any alteration due to the presence of the network, and therefore the evaluated K11 is only an effective splay elastic constant. With this caveat and using the Vth data from C-V profiles (such as the ones in Figure 3), and ∆ε from capacitance measurements, K11 was estimated. The temperature dependence of K11 for the neat LC, PMMA-b and PMMA-CNT composites are shown in Figure S7. It is seen that K11 decreases substantially on the incorporation of the polymer network, with a further diminution, specifically at low temperatures for the CNT reinforced case. It must be noted that K11 is lowered as the concentration of the reinforced CNT is increased.



3.3 Dynamic response The dynamic Freedericksz switching behaviour of both PMMA-b and PMMA-CNT samples obtained on turning the electric field is presented in Figure 5 . Interestingly, the PMMA-CNT sample has a 7% higher change in the total intensity variation than the PMMA-b sample, i.e., a better throughput. The oscilloscope profiles after the electric field is switched off are shown in Figure 5 (b).

(a)

1.0

PMMA-CNT

Normalised optical output


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PMMA-b 0.5

0.0

(b) 90%

0.8

PMMA-b 0.4 PMMA-CNT 10%

0.0

0.0

0.1

0.2

Time (s)

0

40

Time (ms)

Figure 5: Dynamics of the Freedericksz transition behaviour for both PMMA-b and PMMACNT samples during the (a) switch-on and (b) switch-off process, shown in terms of the normalized optical response. While the time taken for the switch-on process (low to high intensity) appears to be similar, the switch-off response is definitely faster for the PMMACNT sample. To aid better comparison, we have shown these profiles by normalizing the as obtained data by the on-state intensity. For further quantitative description we define switch-on response time τON as the time required for the transmission through the device to change from 10% to 90% of the final (electric field on) saturated value of the intensity, and the switch-off time τOFF for the transmission through the device to reduce from 90% to 10% of the saturated equilibrium value. While the time taken for the switch-on process appears to be similar (Figure 5a), the switch-off response is definitely faster for the PMMA-CNT sample (Figure 5b). The evaluated τON and τOFF values, tabulated in Table 1, show that the off-response is accelerated by a factor of 2 for the PMMA-SWCNTs sample. This acceleration is found to be true over the entire thermal range of the nematic phase (30-55oC). Even for applications


80

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wherein the birefringence and the transmission of the sample in the forward direction are important, the PMMA-SWCNTs device works better than that without CNT. Figure 6 presents the response for the device placed between crossed polarizers 1.2

(a)

1.0



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


PMMA-CNT

0.5 PMMA-b

0.0 0.0

0.1

0.2

(b)

PMMA-CNT

0.8 PMMA-b 0.4

0.0

0.3

Time (s)

0

200

Time (ms)

Figure 6: Dynamics of the Freedericksz transition behaviour for both PMMA-b and PMMACNT samples during the (a) switch-on and (b) switch-off process under crossed polarizers. Specifically to be noted is that the off-response is two-stepped for PMMA-b, but has only one step for PMMA-CNT. The net effect is a faster response for the latter sample. It is observed that owing to the birefringence effect the profiles in this case appear as mirror reflections of those in Figure 5. Again, the with-CNT case has a larger (~ 30% more) change in intensity. But the highlight in the devices operated between crossed polarizers is that in the PMMA-CNT sample the relaxation (after the voltage is switched off) seems to be governed by a single process, whereas in the PMMA-b case two-time scales are present, with the second one being quite long. This latter is definitely undesirable as it not only affects the contrast characteristics, but also makes the addressing protocols more complicated owing to the two time scales. The presence of the tethered CNT obviously eliminates the second process yielding a faster responding device. The corresponding τON and τOFF values are shown in Table 1, with the improvement being about a factor of 4. The last parameter that we compare here between the PMMA-b and PMMA-CNT devices is the haze of the device. It is defined as the see-through quality, and total transmittance of a material, based on how much visible light is diffused or scattered when passing through a material, and quantitatively given by

T T  Haze (%) =  4 − 3  ×100  T2 T1 



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Here T1 to T4 are the transmitted intensities measured by an integrating sphere detector under the conditions, T1: no sample, but only the standard white reflector at the back; T2: sample in front of the reflector; T3: no sample, no reflector and T4: only sample. We employed the detector provided with a UV-visible spectrometer (Perkin Elmer) and covered the wavelength range 400-700 nm. Under the conditions employed the measured Haze is 0% for a perfectly transparent sample and 100% for a perfect white diffuser. In the off state, where haze is most important, the two samples have similar haze values, with the PMMACNT having a slightly higher (~ 14%) value, again a useful parameter; the values are tabulated in Table 1.

Table 1: Switching response times (t) and the haze factor for composite with, and without CNT component. Components Without polarizers With polarizers τON

τOFF

(msec)

(msec)

PMMA-CNT

22

38

30

51

54

PMMA-b

22

63

31

194

47

τON (msec)

τOFF (msec)

Haze

3.4 Device The operation of the fabricated PMMA-CNT device (glass) kept in front of a printed text, and exhibiting significant contrast between the field-off scattering and field-on transparent device, is shown in Figure 7. The next section, we consider the possible cause for the improvement in the device parameter when the tethered CNT is present in the polymer matrix. As depicted in Figure 3, the liquid crystal molecules get contained in the network formed by the polymer fibres. It is known that in nematic devices, upon switching the reorientation field off, the return of the molecules to the equilibrium orientation direction is controlled by the surface forces also. (a)

(b)

Figure 7: The PMMA-CNT in the field-off (left) scattering (a) and field–on (right) transparent states (b), exhibiting significant contrast for the text kept behind the device.


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4 Discussion While in the normal non-PSLC devices the surface forces act only at the two substrate planes, it is argued that in the presence of PSLC architecture the polymer fibres mimic the role of virtual surfaces, creating an additional force to accelerate the relaxation to the equilibrium situation. The dynamics of the device, especially the switch off response, is known to be faster, and can be explained by the virtual surface phenomenon. Concerning the threshold voltage, till recently it was thought that the presence of the polymer always increases Vth. However, we showed that this is true only above a certain critical concentration, below which the presence of the polymer actually lowers Vth with respect to that for the non-polymeric material24. We proposed an argument, based on atomic force microscopy images that the lowering is owing to the very short size of the polymer strands at these concentrations. Now let us look at the system studied here. The improvements seen are with respect an already polymer stabilized system. Thus, any explanation for the observed behaviour has to go beyond the mere creation of virtual surfaces. Keeping in view the fact that the capping units on the CNT are structurally identical to the rest of the polymeric structure, we can envisage that the tethered SWCNTs get into the polymeric fibres. The lower concentration of these materials (25% of the total PMMA) would make them to be present only in a few fibres. However, the properties that concern us here are on scales much larger than a few diameters of the fibres, and therefore can be taken to be spatially averaged out. Compared to the floppy polymer chains the CNTs are far more rigid, and hence their presence would make the fibres much stronger. This increase in the strength of the fibres may be expected to translate to an enhanced anchoring strength. Several studies have been performed to understand the influence of the anchoring strength of the solid surfaces supporting the liquid crystal on the nematic switching characteristics25-28. A feature that is relevant from the viewpoint of present studies is the temperature dependence of anchoring strength. Here we consider a calculation proposed by Seo et al.,29 for the polar anchoring strength of polystyrene films. The anchoring strength is given by the ratio of the splay elastic constant K1 to the surface extrapolation length de. The latter parameter has two contributions in the polymer-LC interfacial region, de(1), owing to the order parameter variation and de(2), due to a simple and direct interaction of the LC molecules with the surface. Considering the situation wherein only the de(2) contribution is significant, the anchoring energy is given by



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A∝

K1S o S b2

, where So and Sb are the magnitudes of the nematic orientational order at the

surface and bulk of the sample, respectively. Making two fair assumptions that (i) So is only dependent on the nature of the polymeric surface but not on the temperature, and (ii) mean field arguments applicable according to which K1~ S b2 , the anchoring energy becomes independent of temperature. Now, translating these features we can propose the following. For the case of PSLC with only PMMA, contribution from de(1) may not be negligible in comparison with de(2) and for such a case, as found by Seo et al.,29 the anchoring energy increases with decreasing temperature causing Vth to increase on lowering the temperature. On the other hand, when the CNTs are incorporated into the PMMA architecture, as the mechanical strength of the polymer increases, de(2) dominates the behaviour. Thus the anchoring energy and consequently Vth become independent of temperature, as is observed experimentally. Another argument that can be proposed in favour of the increased mechanical strength of the CNT-reinforced polymer architecture is the following. It may be recalled that droplet morphologies are a hallmark of polymer-dispersed liquid crystals (PDLC) in which the polymer is the major constituent, which should necessarily improve the rigidity of the device. Let us recall that the PMMA-b sample exhibits fibrous morphology, known for PMMA, and in contrast, the PMMA-CNT material presents Swiss-cheese or droplet type of architecture. Considering the architecture to be evidence of the mechanical strength of the medium, we would like to suggest that the CNT-reinforced system has a larger mechanical strength, at least comparable to that of PDLC systems.

5

Summary We have developed a novel type of polymer stabilized liquid crystal device

incorporating functionalized SWCNTs into the PMMA matrix. The novelty is owing to the fact that the nanotubes are tethered with the same polymer material that forms the host matrix. The most important outcome of this is the temperature independence of the threshold voltage, a feature quite attractive from a device operation point of view. Also seen are the interesting features that at ambient temperature, the presence of CNT reduces the threshold voltage as well as accelerates the dynamics of the device. We argue that the cause for the improvement is that the polymer decorated CNT creates a good structure-compatibility with the host polymer, and is prevented from diffusing into the liquid crystal regions. In


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conjunction with this feature, the presence of CNT enhancing the mechanical properties of the polymer perhaps is responsible for the experimental observations. Supporting Information POM and SEM images, Thermal variation of the threshold voltage for the neat and different polymer-LC composites, Temperature dependence of the splay elastic constant References 1.

2. 3. 4. 5. 6.

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Table of Content Graphics Carbon nanotube reinforced polymer stabilized liquid crystal device: Lowered and thermally invariant threshold with accelerated dynamics S. Krishna Prasad1, Marlin Baral1 Adhigan Murali2 and Sellamuthu N. Jaisankar2


Carbon Nanotube Reinforced Polymer-Stabilized Liquid Crystal

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