Switchable Liquid Crystal Composite Windows Using Bacterial Cellulose

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Letter

Switchable Liquid Crystal Composite Windows Using Bacterial Cellulose (BC) Mat Substrates Michael McMaster, Fei Liu, William P Christopherson, Richard A Gross, and Kenneth Singer ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00007 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Applied Polymer Materials

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Title: Switchable Liquid Crystal Composite Windows Using Bacterial Cellulose (BC) Mat Substrates Authors: Michael McMaster†*, Fei Liu‡, William Christopherson†, Richard A. Gross‡, and Kenneth Singer† Author Addresses: † Department of Physics, Case Western Reserve University 2076 Adelbert Road, Cleveland, OH 44106, USA ‡ Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, United States *[email protected] Keywords: Bacterial Cellulose (BC), electro-optical, switchable window, liquid crystal (LC), network dispersed liquid crystal, complex geometry Abstract: Performance properties were investigated for transparency switching devices fabricated with nematic LC filled bacterial cellulose (BC) mats as electro-optical components. Bio-based BC is a nanoporous bicontinuous network of cellulose fibers produced by bacteria and cultured to form mats of desired thickness. Devices made from BC mats with thicknesses of 6, 20, and 40

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μm were compared. Furthermore, the electro-optical response of the devices provides insight into the ordering of the nematic LC within the bicontinuous BC matrix. Main Body: Bacterial cellulose (BC) mats are nanoporous membranes that consist of highcrystallinity inter-connected fibers of self-assembled poly(β-(1→4)-D-glucose), cellulose, produced by aerobic bacteria such as Gluconacetobacter, Rhizobium, Agrobacterium, and Sarcina.1,2,3,4,5,6,7,8 The biosynthesis of BC is both sustainable, eco-friendly, and results in highpurity, high-crystallinity 3-D nanostructured matrices consisting of 40-60 nm diameter ribbonshaped cellulose.1 Previously we demonstrated BC produced from a static culture results in BC mats in desired thicknesses in the submicron regime by controlling production parameters (e.g. time, culture depth, etc.).9 In addition, BC has high tensile strength and Young’s modulus (200300MPa and ~78 GPa respectively).4 Also, BC has low density, low coefficient of thermal expansion, is insulating, and has exceptionable flexibility akin to that of pulp-based paper.1,7 Additionally, BC has been successfully incorporated as a structural component of electronic devices.10,11 Given these properties, we consider BC mats as an alternative substrate for nematic liquid crystal (LC) based switchable window devices. Electrically switchable windows (a.k.a. smart glass, dynamic glass, etc.) are constituent technologies used for privacy, safety, and aesthetic purposes in modern buildings.12,13 The electro-optical component of liquid crystal based switchable windows is typically a polymerdispersed LC, comprised of LC droplets trapped in a polymer film, or a LC-confined porous network/membrane.12,14 To obtain optical switching, the refractive indices of the substrate and liquid crystal are such that, when the LCs are aligned by an electric field, the reorientation is accompanied by a change in opacity.


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The BC mats as window substrates are most comparable to network systems with interconnected pores. Network dispersed LCs provide a rich environment for studying the physics of confinement in nematic and smectic liquid crystals.14 Well studied network dispersed LC substrates include silica aerogels,15,16 porous Vycor glass,17,18 and Synpor membranes19,20 (cellulose nitrate). The confinement results in shifts and broadenings in the nematic to isotropic (N-I) phase transition of the constituent LCs and the appearance of slow relaxations due to alignment in the pores.14 The bulk properties reveal information concerning the alignment of liquid crystals throughout the porous media, and, consequently, structure-property relationships can inform future improvements of the media. BC is particularly attractive because of ongoing work to control the porosity,21,22 and diverse literature pertaining to surface modification1, which could be implemented to alter the LC-BC surface interactions. This will be the focus of future research. This paper reports the performance properties of switchable window devices made using crystal 4′-pentyl-4-biphenylcarbonitrile (5CB), and using BC mats as a bio-based alternative to conventional substrates. Devices were fabricated using BC mats with various thicknesses (6 μm, 20 μm and 40 μm). The electro-optical effects of LC confined BC composites, including the switching time and contrast ratio of the devices are described. Furthermore, the electro-optical response of the LC-BC switching devices provides insight into LC ordering within the network structure. BC production followed the procedure described by Liu et. al (see supplementary information).9 Switchable window devices were prepared by filling the nanoporous cellulose membrane with 5CB, purchased from Sigma-Aldrich and used as received. The BC was filled with 5CB in the isotropic phase (60 °C) under vacuum for a minimum of 24 hours. The



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infiltrated BC mat was sandwiched between electrodes and then cooled to room temperature, within the nematic phase of 5CB (Figure 1a). The 5CB molecules are disordered by the bicontinuous structure. This results in a randomly oriented director such that the LC-BC device is turbid at room temperature. Micrographs recorded between crossed polarizers at temperatures between 29 and 30 oC (Figure S1) reveal that the LC-BC composite scatters light when the 5CB is in the nematic phase. The observed scattering is typical of a disordered nematic. The nematogen, 5CB, has a positive dielectric anisotropy, so the director field aligns in the direction of an applied AC electric field (Figure 1a). The refractive indices of 5CB and BC are sufficiently close that the device becomes more transparent when an electric field is applied (Video S1). First, the electro-optical response of the LC-BC switching devices was investigated to assure that the windows are fully switched. Figure 1b shows the normalized transmission (𝑇𝑜𝑛 = 1) versus electric field for samples made from 6, 20, and 40 μm thick BC mats, measured using a HeNe laser (632 nm). Second, the critical electric field, 𝐸𝑐, that is the minimum electric field required to produce switching, was determined. The hydrophilic BC fibers promote homeotropic anchoring, and some minimum energy is required to overcome the effect of the surface anchoring for the device to switch.14 The critical field may be used to estimate the extrapolation length of 5CB in the system.20


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Figure 1. a) Schematic of switchable window device, showing 5CB filled BC sandwiched between ITO-coated glass slides in the nematic state (bottom), and the reorientation of the positive dielectric anisotropic liquid crystal in the presence of an electric field (top). b) Normalized transmission measured at 632 nm plotted against the applied electric field (10 kHz) for 6, 20, and 40 μm thick BC mats. The inset shows the low field (𝐸 < 0.5 𝑀𝑉/𝑚) transmission in arbitrary units (T (a. u.)) with respect to temperature for the 5CB filled 6 μm BC mat. The critical electric field is inversely related to the temperature. From the graph in Figure 1b, the 6 and 20 μm LC-BC switching devices require an electric field of 1.25 MV/m to fully switch, but the 40 μm BC mat requires 4 MV/m, where fully switched refers to reaching 90 % of the maximum transmission. Therefore, the 6 μm and 20 μm BC mat switching devices have low switching voltages of 7.5 and 25 V, respectively, when compared with the less-preferred switching devices made from thicker (120 μm) cellulose nitrate membranes infiltrated with 5CB, which require switching voltages greater than 200 V.20



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Equation 1 relates the critical electric field, 𝐸𝑐, which is the minimum electric field to change the transparency, to the electric coherence length ξ𝑒𝑙𝑒𝑐,20 the scale characterizing the profile of molecular axis orientation in the transition layers formed at a boundary when an electric field is applied: 1

ξ𝑒𝑙𝑒𝑐 =

( )( ) 𝐾

2

𝜀0 𝛥𝜀

1

𝐸𝑐

(1)

K is the average of the splay and bend elastic constants, (𝐾11 + 𝐾33)/2, and ∆𝜖 is the dielectric anisotropy. The elastic constants and dielectric anisotropy are temperature dependent. The values for the elastic constant (K), 6.1, 6.9, and 7.45 pN, and dielectric anisotropy, 10.5, 11, and 11.6, for neat 5CB at temperatures of 28, 26, and 24 °C, respectively, were found in literature23 and used for subsequent calculations. The electric coherence length is also related to the extrapolation length ξ (Eqn. 2):24

ξ𝑒𝑙𝑒𝑐 =

ξ=

𝑑 + 2ξ 𝜋

𝐾 𝑊

(2)

(3)

where d is a structural length (i.e. pore diameter). The extrapolation length is defined as the ratio between the effective elastic constant 𝐾 and the surface anchoring energy 𝑊 (Eqn. 3). The structural constant (d) is reported as either the width of a LC cell,24 or the pore size for a confined system.20 From the inset of Figure 1b, we see that the critical electric field changes with temperature. The electric coherence length, ξ𝑒𝑙𝑒𝑐, calculated using Eqn. 1, is greater than 10 μm for each measurement, which is large compared to the pores seen for room temperature dried


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cellulose films, which are at most 100 nm.9 Thus, for purposes of calculating ξ from ξ𝑒𝑙𝑒𝑐, we can assume the same argument as Dierking et. al.25 and let 𝑑 = 0. Thus, we calculate ξ as 68, 79, and 130 μm at 28, 26 and 24 °C respectively. Thus, the calculated extrapolation length is much larger than either the pore size or the distance between electrodes, and is much larger than what would be calculated using Eq. 3 using the 𝑊 of typical –OH covered materials (e.g. glass). It is the case that for very small pores, with 𝐾

chord length 𝑟, the effective extrapolation length, ξ𝑒𝑓𝑓, is very large (𝑟 ≪ 𝑊 →ξ𝑒𝑓𝑓 > 𝜉).26 In other words, for small pores, as the pores decrease in size, the influence of anchoring decreases. This indicates that there is extensive interpore connectivity in the plane of the mat, which agrees with the large (>10 μm) regions seen in the cross-polarized micrographs. The paramount qualities of a good switchable window are high contrast, fast switching time, and reproducible switching. We define the switching time with respect to the maximum transmission (Tmax) and off-state transmission (Toff) as the time for the transmission to go from 10% Toff to 90% Tmax. Figure 2 shows the switching time decreases as 1/𝑉2 for mats of all three thicknesses. The 20 μm switching device mat has the fastest switching times for the measured electric fields. We speculate that the slower switching time of the 6 μm sample is due to frustration in the system caused by the interplay between through pore and ITO surface interactions. Thus, the calculated extrapolation length, ξ, is much larger than the cell thickness, resulting in switching behavior of 5CB-filled BC that is more complex and energetically costly than simple rotating droplet systems. For the 40 μm thickness sample, we propose that the larger switching field, and slower switching-on time, is due to frustration of 5CB within long nanometer scale pores that exist throughout the bulk of the mat. This hypothesis is supported by



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the unique frustrated bicontinuous structure suggested by the slow switching-off time of the 40 μm mat when compared with the thinner mats (Figure S3).

06 m 20 m 40 m

-0.5 -1.0

Log[Time(s)]

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-1.5 -2.0 -2.5 -3.0 -3.5

Slope = -2.01 ± 0.02 5.5

5.8

6.1

6.4

Log[E(V/m)]

Figure 2. Log-log plot of the switching-on times vs. electric field, demonstrating a 1/𝑉2 relationship for the switching times of all three samples.


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Table 1. Contrast Ratio and Transmission for transparency switches made from various thickness BC mats. BC Mat

Contrast

Thickness

Ratio

% Toff

% Ton

6 μm

1.25

75

94

20 μm

1.74

43

75

40 μm

2.16

27

58

The contrast ratio (𝑇𝑜𝑛/𝑇𝑜𝑓𝑓) was measured for 632 nm light, and the results are reported in Table 1. The 6, 20 and 40 μm BC mat window samples were switched using voltages of 25, 40, and 120 V, respectively. These voltages were chosen such that the system would be fully switched (Figure 2). In this case, the best (highest) contrast ratio was obtained for the 40 μm mat. We see that the scattering in the on state (𝑇𝑜𝑛) decreases monotonically with thickness. Images of the LC-BC switching devices with and without voltage are available in the supplementary information (Figure S2). Results for 10 switching cycles of each sample are displayed in Figure 3. From Figure 3, we see no signs of hysteresis in the samples through the 10 switching cycles. Also, Figure S3 compares the switching-off times of switching devices with different thicknesses. The switching-off times are dominated by surface interactions between cellulose fibrils and LC molecules as the molecules reorient within the pores to minimize the system energy. Similar to the switching-on times, the 20 μm mat also switches off fastest. However, switching times of < 1 ms are generally required for switching applications, so further work as mentioned below is needed.



10

06 m

1

0

0

Norm. T

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

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10

20

30

40

50 20 m

1

0

0

1

2

3

4

5 40 m

1

0

0

500

1000

1500

2000

Time (s)

Figure 3. Transmission of LC-BC switching devices made from BC mats with different thickness over 10 cycles.

In conclusion, we have prepared electro-optical transparency switches that use bio-based BC mats as an alternative to traditional bicontinuous networks such as silica aerogels. The LC-BC switching devices displayed typical switching-on time dependence of 1/𝑉2, are fully switched with a reasonably small electric field, and have no apparent hysteresis. Mats of different


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thicknesses (6, 20 and 40 μm) were studied, and a trade-off between visually transparent 𝑇𝑜𝑛 and cloudy 𝑇𝑜𝑓𝑓 with respect to thickness was identified. This work lays a foundation for BC based network dispersed LC devices that may be improved via surface modification, tuning of pore size and pore distribution as well as the molecular structure of the liquid crystal. Additionally, through engineering advancements, BC could enable green, facile production of flexible, electrooptical, switchable windows. Furthermore, we are investigating materials and processes aspects (e.g. porosity and surface chemistry) to decrease switching time and improve contrast. Lastly, values of calculated extrapolation lengths of 5CB from the low-field electrooptical properties are much larger than the pore size and larger than the sample thickness. The effect of nanoporous confinement of the liquid crystal on their elastic properties will be investigated via dynamic light scattering and broadband dielectric spectroscopy in future work in order to shed light on their electro-optic response.

Acknowledgments The authors are grateful to Oleg Lavrentovich for helpful discussions and to the National Science Foundation Partnerships for International Research and Education (PIRE) Program (Award #1243313) for funding this work. The authors acknowledge the use of the Materials for Opto/electronics Research and Education (MORE) Center (Ohio Third Frontier grant TECH 09-021), a research facility at CWRU.



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12 Supporting Information. Experimental specifications, micrographs between crossed polars of the

composite made using 20 μm BC mat, and photographs of LC-BC switching devices in the on and off state.

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Switchable Liquid Crystal Composite Windows Using Bacterial Cellulose

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