Graphene Oxide Liquid Crystal Domains: Quantification and Role in

Subscriber access provided by BUFFALO STATE

Article

Graphene Oxide Liquid Crystal Domains: Quantification and Role in Tailoring Viscoelastic Behavior Md. Joynul Abedin, Tanesh D Gamot, Samuel T Martin, Muthana Ali, Kazi Imdadul Hassan, Meysam Sharifzadeh Mirshekarloo, Rico F. Tabor, Micah J. Green, and Mainak Majumder ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b02830 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 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

ACS Nano

Graphene Oxide Liquid Crystal Domains: Quantification and Role in Tailoring Viscoelastic Behavior Md. Joynul Abedin†¥, Tanesh D. Gamot†¥, Samuel T. Martin†, Muthana Ali‡, Kazi Imdadul Hassan†, Meysam Sharifzadeh Mirshekarloo†¥, Rico F. Tabor‡, Micah J. Greenϯ, Mainak Majumder*†¥

†Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia ‡School of Chemistry, Monash University, Clayton, VIC 3800, Australia ¥ARC Research Hub for Graphene Enabled Industry Transformation, Monash University, Clayton, Victoria 3800, Australia ϯArtie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA.

*E-mail: [email protected]

ACS Paragon Plus Environment

ACS Nano 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

ABSTRACT Graphene oxide liquid crystals (GOLCs) were exfoliated in a wide variety of solvents (water, ethylene glycol, NMP and DMF) by high speed shearing of graphite oxide. Quantitative polarized light imaging of the equilibrium nematic phases of the lyotropic GOLCs sheds insights into the extent of aggregation and quantifiable textural features such as domain size, d. Large nematic domains >100 μm with a high overall degree of order were obtained in water and ethylene glycol, in contrast to ~ 5–50 μm domains in NMP and DMF at comparable volume fractions. Comprehensive rheological studies of these GOLCs indicate that larger domains correlate with higher viscosity and higher elasticity, and scaling analysis shows a power-law dependence of Ericksen number (Er) with domain size (𝐸𝑟 ∝ 𝑑3.09). The improved understanding of the relationship between microstructure and flow properties of GOLCs leads us to an approach of mixed solvent-based GOLCs as a means to tune viscoelastic properties. We demonstrate this approach for the formation of shear-aligned GOLC films for advanced flexible electronic applications such as all-carbon conductive films and thermal heaters.

KEYWORDS graphene oxide, liquid crystal domain, isotropic, nematic, viscoelasticity, coating


Page 2 of 34

Page 3 of 34 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

ACS Nano

Colloidal dispersions containing two-dimensional (2D) sheets such as graphene or graphene oxide (GO) are inherently anisotropic and have been shown to exhibit concentration-dependent phase transitions from isotropic to liquid crystalline (LC) mesophases.1-6 These phase transitions occur spontaneously in a solvent possessing moderate to high solvating properties, and are primarily determined by the relative concentration and the dimensional anisotropy of the 2D particles. The self-alignment of the 2D sheets in nematic phase enables the colloidal system to be better and effectively aligned by external shear force than the isotropic phase itself. This means that the LC phases are suitable precursors for fabrication of macroscopic objects such as fibers, films, membranes and foams by typical forming processes such as 2D and 3D printing, coating, and extrusion.2,

7-13

More importantly, the higher degree of

orientational order and increased packing of the 2D sheets in these macroscopic objects lead to enhanced mechanical, electrical and mass transport properties.7,

10

For example, we have

demonstrated the scalable fabrication of large-area membranes of ~ 13 × 14 cm2 with highly enhanced nanofiltration properties.10 Onsager’s theories 14 and subsequent improvements by others for plate-like systems provide a reasonable qualitative prediction of LC phase formation, but the experimental nature of these phase transitions are affected by factors contributing to electrostatic interactions such as pH, surface charge density and ionic strength as well as the inherent polydispersity of the particle dimensions.6, 15 The latter has been shown to critically influence the phase transition in various 2D colloidal systems.16-18 It is quite clear that the phase transitions in GO systems are rather complex because the associated surface charged groups are prominent, polydispersity is usually large, and the morphology of the platelet may change over time.19


ACS Nano 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

For GO-based liquid crystals (GOLCs), water is the most widely reported medium to form the liquid crystals because of extensive H-bonding.2 It has also emerged that a wide range of polar organic solvents can support the formation of GOLCs. Jalili et al.2 suggested that the formation is subject to additional solvophobic interactions. It was proposed that the influence of solvent can be predicted by the Gordon parameter - a measure of the solvent cohesiveness. The authors further showed that because of the use of extra-large GO sheets, and a concomitant increase in solvophobic effects, GOLCs can be formed even in polar protic solvents such as ethanol and isopropanol with very low Gordon parameter values (0.54–0.56 J m-3). On the other hand, Gudarzi et al.20 suggested that GOLCs are primarily formed in polar aprotic solvents, and the solvation characteristics are determined by chemical transformations on the surface induced by solvents such as NMP. However, there has been very little work aimed at understanding the microstructure and morphology of the GOLCs, a pre-requisite for developing structure-processing-property relationships. To do this we employ quantitative polarized light microscopy and produce combined retardance and slow-axis orientation maps of GOLCs to measure and elucidate features in the polydomain texture. We study steady and dynamic flow properties of GOLCs and propose scaling behavior between flow and equilibrium microstructure, helping one to draw analogies to universally observed grain-grain boundary interactions in materials. With the enhanced understanding of these relationships, GOLCs with widely varying rheological features from gels to flowable fluids can be systematically produced by controlling the domain sizes. We demonstrate that the tailored GOLCs can form highly conductive, all-carbon coatings and electrical heating elements on polyimide substrates using shear-induced film formation techniques.


Page 4 of 34

Page 5 of 34 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

ACS Nano

RESULTS AND DISCUSSION Comparison of LC Behaviour of GO in Aqueous and Non-Aqueous Solvents

Figure 1. Orientational ordering of GOLCs. I-N phase transition of plate-like GO colloids as a function of concentration in different solvents. The birefringent nematic domains emerged in the isotropic fluids after a critical concentration as shown in the inset. Orientational ordering in water and EG occurs at lower concentration than DMF and NMP. The lyotropic LC phase transition of the GOLCs produced in various solvents is shown in Figure 1. It can be seen that the onset of nematic phase formation (𝐶𝑖) in water occurs significantly earlier compared to other solvents with the following sequence: water < EG < DMF ≈ NMP. Regardless of the solvent, with the onset of the I–N transition, the nematic domains start to appear in the isotropic solvent and spread over the entire volume of the


ACS Nano 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

dispersion at a critical concentration (𝐶𝑛). The co-existence region of the I–N phase demonstrates a vivid interface with high optical contrast in retardance imaging (Figure 2a). These images represent pixel-by-pixel measurements of the optical retardance, 𝑅 = ∆𝑛 × 𝑡 × 𝑐𝑜𝑠2𝜎, where ∆n is the differential refractive index or birefringence of GO platelets, t is the optical path length or thickness of the sample, and 𝜎 is the inclination angle of the optic axis with respect to the plane of view. It is worth mentioning that the differential transmittance (also known as diattenuation) along the two principal axes of GO can also contribute to the measured retardance.21 The isotropic phase in the capillary showed an average optical retardance of 3.57 ± 0.42 nm, whereas the nematic phase hosting the self-assembled birefringent GO platelets showed an average optical retardance of 18.45 ± 10 nm (Figure 2b). This reveals that the birefringence of GO platelets in the nematic phase in water is ― 0.2 and in DMF is ― 0.3 at 𝜆 = 546 𝑛𝑚 (green light) considering the optical path length equivalent to the cell thickness (200 µm) and assuming an average orientation factor of 0.5. These values are consistent with previously reported values of birefringence of GO films.22 These estimations of birefringence also implies that the GO platelets in DMF have very stronger in-plane anisotropy. The optical retardance in both phases depends on the colloid concentration; however, DMF- and NMP(aprotic solvents) based GOLCs showed large deviation from the mean optical retardance. Note that despite the isotropic phase showing no visible texture, the presence of GO platelets in the isotropic phase is still quantifiable using this imaging technique. The retardance is truly above the background retardance of ~ 2 nm produced by the shot noise of camera electronics and the birefringence arising from the distortion of optic lenses (see Figure S6).


Page 6 of 34

Page 7 of 34 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

ACS Nano

a

b

Figure 2. Optical retardance of isotropic and nematic phases. (a) LC-PolScope micrographs (combined retardance and slow-axis orientation) of the I–N phase transition in water and DMF solvents for the determining the volume percentage of nematic phase where the solid contents were ~2 g/L and ~12 g/L, respectively. The brightness of the micrograph represents optical retardance and hue represents slow-axis azimuth. (b) Quantification of the optical phase retardation of the polarized light indicates the magnitude of anisotropy in nematic and isotropic phases, and a clear I-N interface. AFM measurements of >1000 GO platelets were undertaken to ensure statistically significant measurement of dimensions of GO, and this shows high extent of exfoliation of graphite oxide to single layer or a few layer GO in all four solvents (Figure 3).


ACS Nano 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

a

b


Page 8 of 34

Page 9 of 34 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

ACS Nano

c

d

Figure 3. Polydispersity of GO sheets in solvents. AFM images and corresponding lateral size distribution plots (calculated from area) of GO platelets in (a) water, (b) EG, (c) DMF and (d) NMP. The height profiles corresponding to the lines on the GO surface in the AFM images are given in the inset. The solid lines are log-normal fit of the particle size distributions. The polydispersity (𝝈𝑫) is the relative standard deviation of the distribution and the aspect ratio ( < 𝑫/𝒕 > ) is the ratio of mean lateral size to mean thickness of the GO platelets. Very high polydispersity (𝜎𝐷) was observed for the lateral sheet dimension in all cases, and large differences in the aspect ratio ( < 𝐷/𝑡 > ) from solvent to solvent is noticeable. The highspeed shearing method used for producing the GOLCs breaks the lateral sizes of the GO platelets depending on the shearing speed and the solubility parameters of the solvents.23 The


ACS Nano 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

< 𝐷/𝑡 > of GOLC in the solvents decreases as: water > EG > DMF ≈ NMP, and is qualitatively consistent with the observations of the onset of I–N phase transition (Figure 1), as 𝐶𝑖 and 𝐶𝑛 scale linearly with < 𝑡/𝐷 > (equation S8 and S9). Our GO platelets have an average thickness of 1.0 nm (from AFM measurements) and assuming an average density of 1.3 g/cm3,24 the onset concentrations from equations S8 and S9 are compared with our experimental results and experimentally determined values in other 2D systems in Figure 4.

Figure 4. Effect of polydispersity and aspect ratio on I–N phase transition of plate-like materials. The black symbols denote isotropic to biphasic transition and the corresponding red symbols mark the transition to complete nematic phase. The solid lines are estimated theoretical transitions. The horizontal dotted lines illustrate the broadening of the biphasic region. It can be deduced from Figure 4 that at large aspect ratio (or small < 𝑡/𝐷 > ), the transition from I to N phase occurs at lower concentration (water and EG) while for small aspect ratio (or large < 𝑡/𝐷 > ) this transition occurs at higher concentration (NMP and DMF). It can also be seen that the experimental critical phase transition concentrations showed large deviation from


Page 10 of 34

Page 11 of 34 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

ACS Nano

the theory for most of the plate-like systems.4-5, 16-18, 25-26 These discrepancies can be attributed to the flexible nature of the platelets in contrast to the theoretical assumption of a true hard body type, and the variation in the physical dimension of the platelets. Moreover, the theories and the simulation studies only considered the interaction between two platelets, whereas at higher concentrations the possible long-range interactions can be significant when many particles are present instead of two particles.27 The GO platelet thickness (t) is estimated to be ~ 0.8 – 1.5 nm according to the literatures 4-5 and the measured average GO thickness by AFM in this report is ~ 1 nm. However, the possibility of 3D shape of GO in the bulk are often overlooked. The AFM measurements were undertaken using samples from the LC phases and gives an estimation of the lateral size of GO sheets in different solvents. AFM shows the particles when ironed flat. In the bulk, they are clothed in solvent, and likely to have solventdependent 3D shapes such as wrinkling, rippling, tortuosity, stacking and any solvent intercalation that may contribute to their effective thickness.28-29 However, we can interpret subtle variation in the 3D shape of GO sheets in the GOLCs from the global nature of the retardance measurements, which is not possible with AFM measurements. We note that higher average retardance and large deviation from the mean retardance intensity in the nematic phase (Figure 2b) is observed in DMF. The retardance in the nematic phase of DMF-based GOLC showed log-normal distribution in contrast to a normal distribution for water-based GOLC (Figure S3) indicating higher degree of face-to-face stacking in the DMF-based GOLC because particle aggregation can be characterized by the skewness in the size distribution.30 The dotted lines in Figure 4 illustrate the broadening of the isotropic-to-nematic phase transition as well as the deviation of our experimental observations from theoretical estimations. The rather large deviation for GO-NMP and GO-DMF system can therefore be attributed to the higher degree of stacking in these solvents.


ACS Nano 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

Quantification of Nematic Texture a

b

c

d

Figure 5. Quantification of nematic texture in GOLCs. (a-d) Each row illustrates the elliptically polarized light image (left), circularly polarized retardance image (middle) and combined retardance and slow-axis image (right) of the circularly polarized light of GOLCs in (a) water (b) EG (c) DMF & (d) NMP. The brightness of these images represents the optical retardance and the hue portrays the slow-axis azimuth of the nematic domains. The solid content was ~ 20 g/L in each solvent.


Page 12 of 34

Page 13 of 34 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

ACS Nano

Although GOLCs at 20 g/L formed complete nematic phase in all the four solvents, but only the water- and EG-based GOLCs exhibited gelation, where other two GOLCs maintained good flowability. The self-aligned microstructures formed by the GO platelets in these solvents could explain such behavior. In Figure 5, we report a set of three optical images which comprises an elliptically polarized light image (left), corresponding circularly polarized retardance (middle) and the combined retardance and slow-axis orientation image (right) for each solvent-based GOLCs. These images typically exhibit thread-like nematic textures - an optical motif characteristic of nematic LCs. The pixel brightness indicates the local retardance within the LC domain unaffected by the specimen orientation in the plane of the microscope stage. Our imaging technique generates consecutively four different elliptically polarized light images (one of the images is provided in left panel) which is converted to a circularly polarized light retardance image (middle panel) algorithmically within a few seconds.31 When elliptically polarized light travels through GO, the slow-axis of GO parallel to the major axis of ellipse experiences high retardance, whereas it experiences low retardance while traversing parallel to the minor axis of ellipse. A benefit of the elliptically polarized light is that it permits one to focus at different planes of the specimen providing more accurate optical features.32 The circularly polarized light images (middle and right panel) provide the average optical properties (retardance and slow-axis azimuth) of the GOLC domains in the plane of view. We note that GOLC experiences homeotropic alignment induced by surface anchoring of LCs, where the local director (n) is normal to the surface. Therefore, the slow-axis aligned parallel to the circularly polarized light (side wall) resulted in high retardance, whereas if it is aligned normal to the circularly polarized light, such as on the microscopic slide or the cover slip, it results in almost no retardance. Due to high viscoelastic nature of GOLCs, the anchoring effect brings elastic distortion to the nematic domain. To avoid the influence of surface anchoring and flow induced distortion of the domains, we undertook the measurements in specimen points far from


ACS Nano 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

Page 14 of 34

the walls of the cover slip surface when the GOLC is placed between two cover slips on a glass slide. Further discussions on surface anchoring are provided in section S6.

a

b

Figure 6. Pseudo-lamellar ordering of GOLCs in solvents. (a) The histogram (solid lines are normal fit) illustrates the average domain size distribution at a concentration of 20 g/L in four different solvents- the inset shows the spatial variation of the local director (n). (b) Water- and EG-based GOLCs with larger domains formed gels, where the GOLCs in other two solvents showed flowability though each are in a complete nematic phase. We quantified the size of the nematic domains (Figure 6a) by combining the characteristics of uniform orientation of slow-axis (represented by the hue) and of retardance (represented by the brightness) as illustrated by the local directors (n) in the slow-axis map (Figure 6a (inset)). n represents the direction of slow-axis of GO and therefore the localized orientation of the GO sheets.33 In low intensity domains, n is oriented in more parallel fashion to the optic axis (completely parallel orientation will result in zero intensity) and in high intensity domains, n


Page 15 of 34 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

ACS Nano

is oriented in more perpendicular fashion. The local director n represents the average direction in each domain, but changes spatially throughout the specimen. Conceptually, one can argue that a boundary layer is formed when n changes its direction. In metallurgy, this boundary layer formed between grains having similar crystallographic orientation and direction, is known as the grain boundary. Various properties of the metal such as strength and hardness depends on the grain size, and such behavior has analogy to GOLC systems. These domain boundaries will influence the flow behaviour of the structured GOLCs and given the elastic energies associated with the domain boundaries34-35 may provide a physical basis for understanding the flow behaviour of GOLCs (Figure 6b). Nematic domains larger than 100 µm were observed in both the water- and EG-based GOLCs, in contrast to much smaller sizes of 5 to 50 µm in case of NMP and DMF, indicating longer range order in water- and EG-based GOLCs. The spatial variation of n resulted in an average order parameter (S) of 0.75 ~ 0.76 in water- and EG- based GOLCs, where it was in the order of 0.5 ~ 0.6 for NMP- and DMF-based GOLCs. These S values were calculated using a scalar order parameter, S = < 2cos2θ ― 1 > , where 𝜃 is the angle between the orientation (azimuth) of slow-axis at each pixel and the mean orientation (azimuth) from the distribution of slow-axis azimuth in Figure 5. Note that water and EG have a higher propensity for hydrogen bonding in contrast to NMP and DMF.2 The domain structures of the polymeric mesogens such as the one used in the display technology are independent of the size of the mesogens and well-described by the continuum theories.36 In contrast, the continuum descriptions are expected to break down where the mesogens are very large (~ µm) such as graphene oxide platelets.37 In these LCs, the domain size is dependent on the finitesize of the mesogens and larger platelets are better packed in the domains, usually attributed to the lower surface charge density in bigger platelets.6, 38 It has been observed in 2D niobate colloids that the larger platelets are better packed and arguably with larger degree of order


ACS Nano 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

Page 16 of 34

shows higher viscosity at a fixed solid fraction.38 It is consequences of these aspects that the larger domains and higher degree of order exhibit increased viscoelastic properties. Rheological Behavior of GOLCs

a

b

c

d

Figure 7. Elasticity and flowability of GOLCs. (a) The GOLCs exhibit linear viscoelasticity with typical plateau like behavior. (b) Frequency dependent viscous and elastic behavior reveals the total resistance [|𝑮 ∗ | = (𝑮′𝟐 + 𝑮′′𝟐)] of the GOLCs in a dynamic environment. (c) Dependency of complex modulus on angular frequency in the range of [0–10 rad/s] and (d) Pseudo-plastic (shear thinning) behaviour of GOLCs. The GO concentration was 20 g/L in each solvents (a - d).


Page 17 of 34 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

ACS Nano

Table 1. Key findings from the rheological measurements Solvent

GO conc. (g/L)

Crosska over stress (Pa)

ma

Zero-shear viscosityb (Pa.s)

Water

20

20.08

155.09

0.05

834

EG

20

25.76

110.89

0.11

853

No Flowability

DMF

20

02.70

6.40

0.23

28.4

Flowability increases

NMP

20

00.46

1.84

0.30

7.73

∗ 𝒎 k (degree of network connectivity) & m (exponent) are the fitted value of power law equation 𝑮 (𝝎) = 𝒌𝝎 to the frequency b -1 sweep data in a range of (0 - 10) rad/s; measured at .001 s a

To obtain further insights into the flow properties of GOLCs, oscillatory shear measurements were performed for the same solid content of 20 g/L as shown in Figure 7a. After a critical stress, the elastic/storage (𝐺′) and viscous/loss (𝐺′′) moduli cross over and become stress dependent due to the breakdown of GO elastic network.39-40 After that critical stress, the GOLC undergoes a transition from elastic to viscous-dominant behaviour where GO sheets align in the direction of applied shear. Further, we note that the elastic moduli are higher for water and EG than DMF and NMP, consistent with Figure 6b. Figure 7b shows that the total deformation resistance (|𝐺 ∗ |) of GOLC is not influenced by the frequency change for a critical range depending on the type of solvent used- water ≈ EG > DMF > NMP. This suggests the formation of stronger elastic network which once again supports our inferences that binding forces in water and EG are large as compared to DMF and NMP. To predict the flowability of the GOLCs, we have fitted the frequency data with a power law relationship |𝐺 ∗ (𝜔)| = 𝑘𝜔𝑚 in the low frequency region (0–10 rad/s) (Figure 7c). From a more physical perspective, a higher exponent (m) is indicative of a network which is fluid


ACS Nano 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

Page 18 of 34

and a higher value of the intercept (k) represents a higher degree of network connectivity in the gel. It can be seen that the complex moduli are almost independent of frequency in water- and EG-based GOLCs, which agree the formation of the gel like structure due to long range interaction and orientation order as quantified by the polarized images. At the same solid content of 20 g/L, the other two GOLCs showed greater frequency dependency and as a result maintained excellent flowability despite showing fully nematic phases (Figure 7c). Note that the gel-like phase of GOLC in water and EG are of weak physical type since 𝐺′ > 𝐺′′ (elasticitydominant physical network) whereas strong chemical gelation usually shows 𝐺′ < 𝐺′′.41 We note that disk-shaped anisotropic materials with morphology similar to GO also exhibits I-N phase transition before higher concentration of particles produces physical gelation.26, 42 The steady flow characteristics of all GOLCs showed non-Newtonian shear-thinning behaviour (Figure 7d) due to alignment of the nematic director normal to the flowing direction. As a matter of comparison, the zero-shear viscosity is compared for the four different GOLCs in Table 1 and shows an order of magnitude difference even with similar particle content, but at this point it is worth recalling the dramatic differences in the nematic microstructures for these systems (Figure 5 and 6). To determine the origin of such behavior, we utilized a dimensionless quantity Ericksen number (𝐸𝑟 =

𝜂 𝛾 𝑑2 𝑘 ),

where 𝜂 is the characteristic viscosity, 𝛾

is the deformation rate, 𝑘 is the Frank elastic constant.43 The development of Er in shear flow of a nematic system is delineated in section S6. The elastic constant in Er is the average of the three elasticity constants typically associated with distortional energy of the nematic boundaries. The magnitude of this elastic constant is in the order of 10 ―13 𝑡𝑜 10 ―11 N for nematic LCs.34, 44 We have calculated Er for GOLCs which reflects their hydrodynamic forces in terms of a characteristic stress density (𝜂 𝛾) & microscopic domain size (d) at a low deformation rate of 1 s-1. Assuming a Frank elastic constant of 10-12 N, the GOLCs based on NMP and DMF yields Er ~ 102 whereas the GOLCs based on water and EG has Er ~ 105. To


Page 19 of 34 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

ACS Nano

include additional data points and determine a co-relation between low shear viscosity and domain size, we prepared blended GOLCs and measured their domain sizes (Figure S4) and flow characteristics (Figure 8a). The co-relation between experimentally measured GOLC domain sizes scales as 𝐸𝑟 ∝ 𝑑3.09 (Figure 8b). In this correlation, we have assumed that the nematic elastic constant remains unchanged for the GOLCs. This scaling law is in good agreement with the polymeric LCs, where scaling such as 𝐸𝑟 ∝ 𝑑3~4

45-46

are reported in

contrast to the theoretical prediction of 𝐸𝑟 ∝ 𝑑2. We note that most of the prior studies such scaling relationships are determined by measuring the changes in domain structure and boundaries with an application of shear rate. The scaling analysis shows that the GOLCs with larger domain sizes and longer-range structural order form gel and the GOLCs with smaller domain sizes maintain the flowability. The existence of scaling relationship between domain size and flow properties of GOLCs can be brought into perspective with more universal relationship of viscous and elastic properties of various material systems. For example, polycrystalline metallic grains contain single crystals oriented in a particular direction and each grain possesses distinctive orientation of crystals separated from each other by grain boundaries. Movement of dislocations, key to many properties such as strength and elasticity, is dependent on the interaction of the grain and grain boundaries.47 Elastic strength of a material can be related to the grain size by Hall-Petch relation,48 𝜎𝑦 = 𝜎0 +𝐾𝑑 ―1/2 where 𝜎0 is the frictional stress, 𝐾 is the materials constant and 𝑑 is the average diameter of the grain. The Hall-Petch relation states that the yield strength is higher in smaller grains as smaller the grain, smaller the dislocation pile-up.49 While this relation holds true for all classes of metallic materials and is applicable to grain sizes from 1 µm to 100 µm. As the grain size reduces below 100 nm, the mechanism reverses and the yield strength 𝜎𝑦 becomes directly proportional to 𝑑. This is called Inverse Hall-Petch effect (IHPE) and states that the smaller the grains, the lesser the yield strength.50 Among all models proposed


ACS Nano 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

to explain the IHPE, the one closest to our two-phase (solvent-mesogen) system is the grainboundary-shearing model. This model predicts the grain boundary and crystallites in grains as two separate phases and grain boundaries deform by an athermal shearing mechanism leading to the IHPE.51 The elastic strength of the GOLCs and their blends showed a power law trend with the characteristic domain size, where the larger domain exhibited higher elastic strength (Figure 8c). Based on these observations, we believe this two-phase system exhibits IHPE. Lower probability of hydrogen bonding and lower repulsive forces in DMF and NMP serves as a weakly interacting grain boundary allowing easy stress transfer. While in case of water and EG, hydrogen bonding and van der Waals attraction prevails, giving rise to the situation of dislocation pinning at the grain boundaries, giving high yield strength and greater elastic character. This type of behaviour has also been observed in other liquid crystalline materials.52

Exploitation of Domain Size Control of GOLC in Conductive Coating Formation


Page 20 of 34

Page 21 of 34 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

ACS Nano

a

b

c

d

e

f


ACS Nano 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

g

Figure 8. Domain size effect of GOLCs on coating properties. (a) Viscosity is tailored while different solvent based GOLCs were blended (corresponding to 1 s-1). (b) Ericksen number follow a power law relationship with the domains of GOLCs and their blends. (c) Elastic modulus of GOLC as a function of domain size. The elastic modulus was measured at a frequency of 10 rad/s. (d) Water based GOLC de-wetted from the substrate. (e) Blended water- and organic-based GOLC significantly improved the roughness and morphology of the film on the same substrate. (f) A GOLC blend enabled casting of a thin flexible film on PI substrate at Er ~103. (g) Transition of GOLCs to viscous liquid and their elastic relaxation behaviour significantly affected the morphology of the film [red dotted line indicates elastic (𝜹 < 𝟒𝟓°) to viscous (𝜹 > 𝟒𝟓°) transition]. GO concentration was ~20 g/L. We and others have shown that GOLCs can be shear aligned to form films, coatings, patterns and membranes of GO.10, 53 When these GOLCs were employed for forming coatings on PI


Page 22 of 34

Page 23 of 34 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

ACS Nano

substrate by shear-alignment, it was observed that the GOLCs based on NMP and DMF demonstrated very little coating ability due to their high flowability (see Table 1) and rapid dewetting. Water- and EG-based GOLCs showed some extent of coating ability; however, this also resulted in elastic de-wetting and accumulation of non-uniform island-like structures (height ~ 2 µm) as shown in Figure 8d. However, a blended GOLC with moderate Er number was less susceptible to hydrodynamic instabilities and produced film (thickness ~ 0.5 µm) with low roughness (~ 0.1 µm) (Figure 8e & f). To explain this coating behavior, we take insights into the subtle variation of viscoelastic properties such as elastic-to-viscous transition and relaxation behaviour of GOLCs (Figure 8g). During oscillatory rheological measurements, elastic (𝐺′) and viscous (𝐺′′) resistance along with the phase shift (𝛿°) between the applied deformation and material response can be measured, noting that 𝛿 = 0° is purely elastic and 𝛿 = 90° is purely viscous. A plot (𝛿 𝑣𝑠 |𝐺 ∗ |), first proposed by van Gurp and Palmen54 for polymeric materials and also used for plate-like materials55 is shown in Figure 8g. The transition from elastic-to-viscous dominant regime occurring at 𝛿 = 45° is only observed for pure DMF- and NMP-based GOLCs. Therefore, these two GOLCs are more susceptible to hydrodynamic forces, which results in liquid-like behavior and fast de-wetting of the coated film. On the other hand, water-based GOLC is an elastically-dominated gel even at high frequency, indicating that the relaxation time-scales are short (𝑡 =

2𝜋

𝜔). When a velocity

gradient is not present, typically after the deformation stress from the coating process is removed, viscous stress cease to be significant56 and elastic stress starts to have stronger influence. When water-based GOLC is blended with DMF and NMP, the phase angle shifts to a moderate viscoelastic zone and the relaxation timescale increases by ~30 times over waterbased GOLC, enabling the stabilization of the liquid film. Blending GOLCs are therefore a means to tune the rheological properties such as viscosity, viscoelasticity and elastic relaxation, and a useful strategy to form GOLC films. This capability of tuning viscoelastic properties can


ACS Nano 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

Page 24 of 34

also be articulated in terms Er and guide the choice of an appropriate fabrication technique. For example, at low Er (< 102) GOLC behaves almost as a viscous liquid (𝛿 > 45°), making it suitable for casting thin films by spin coating and spray coating. The intermediate region (Er ~ 103 - 104) could be useful for inkjet printing, dip coating and doctor blade coating. It is also worth mentioning that a solvent with high boiling point (> 130 ℃) such as DMF/NMP, can have additional benefits by partially improving the surface morphology of the film.57

a

b

c

Figure 9. Thin-film heaters based on GOLC coatings. (a) XRD patterns of a PI film, a GOLC coated PI and the conducting coating of the reduced GOLC on PI. (b) GOLCcoated flexible PI film demonstrated high conductivity. (c) GOLC-coated PI film working as an electro-thermal thin film heater demonstrated excellent heating capability (~25 ℃/s at 16V). To demonstrate the stability, flexibility and conductivity of the GOLC coatings, we have applied hydroiodic (HI) acid reduction process on the coatings. The GOLC coated PI showed excellent stability and uniformity after going through the harsh HI reduction process. The sheet resistivity of the coating was reduced from ~1G Ω/sq. to ~100 Ω/sq. after HI reduction demonstrating its potential in wide range of flexible electronic systems. The characteristic 2𝜃 peak of GO moved from 11.5° towards the graphitic peak at 24.5° (Figure 9a). HI reduction


Page 25 of 34 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

ACS Nano

removed the oxygenated functional groups from the GO surface shown here by the reduction of interlayer distance and increased electrical conductivity. We demonstrated this by using the GOLC coated film to light up an LED bulb (Figure 9b). The PI substrate is well known for its excellent flexibility and thermal stability (> 450 ℃), making it widely used as a substrate for flexible electronics. Flexible electro-thermal heaters are being used in many applications such as in wearable medical devices & vehicle window defrosters. The GOLC coated PI film showed excellent heating capacity and a very high steady state temperature (Figure 9c). It is worth noting that heat generation was much faster with GOLC coated PI films compared to the previously reported value.58

CONCLUSIONS In summary, we have demonstrated that a wide range of polar solvents can allow the formation of lyotropic GOLCs. Quantitative polarized light imaging and phase equilibrium studies of these systems show that NMP and DMF have higher levels of aggregation and consequently a broader isotropic–nematic transitions than GOLCs in water and ethylene glycol. Larger and more cohesive domains with a higher degree of structural order were observed for GOLCs in water and ethylene glycol as compared to GOLCs in NMP and DMF. The GOLCs with larger domain sizes naturally exhibited greater viscoelasticity. The large variation of the GOLC properties with domain size and the general miscibility of the solvents also meant that strategies such as blended GOLCs for controlling nematic domain sizes and the consequent ability to tune viscoelasticity of GOLCs are proposed in this research. These strategies allow the tuning of the viscoelasticity of GOLCs to meet a wide range of fabrication processes exemplified here by coating of polyimide films.

MATERIALS AND METHODS


ACS Nano 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

GOLC Production by High Shear Mixing Graphite oxide (GtO) was purchased from Sixth Element Inc. (China) and purified in the laboratory using established methods such as centrifugation and dialysis. GOLCs were produced by liquid phase exfoliation of GtO in a high shear laboratory mixer, Silverson-L4RT, as described in Paton et al.23. Four different solvents have been used for this study including polar protic (water) and aprotic (EG, DMF and NMP) solvents. The shearing speed was varied for different solvents to enable the formation of the LC phase within 1 h of mixing and centrifuged at 1000 rpm for upto 0.5 hr to separate un-exfoliated GtO. It was observed that gelation occurred in case of water- and EG-based GOLCs, whereas DMF and NMP-based GOLCs were flowable. GOLCs were produced at an initial concentration of ~ 20 g/L and later diluted to the desired concentration. Morphological Study using AFM The size distribution of the GO platelets was characterized using atomic force microscopy (JPK Nanowizard 3 AFM). Images were obtained using Bruker NCHV cantilevers in alternating contact mode in air. Diluted samples (0.25 g/L) were prepared by spin coating (Laurell technologies, WS-400BZ-6NPP/LITE) onto microscope slides. Afterwards, the AFM images were analysed using the software Gwyddion59 to obtain statistical distribution of GO lateral sizes and thickness. I-N Phase Transition and LC Domain Characterization Optical anisotropy arises in GO because of different refractive indices in the in-plane and out-of-plane direction of the 2D sheets. The axis of symmetry of the 2D sheets, which coincides with the out-of-plane direction, is known as the ‘optic axis’ or the ‘fast axis’. If the plane of the polarized light is perpendicular to the optic axis it will experience the ordinary index (𝑛𝑜), whereas if it is parallel, it will experience the extraordinary index (𝑛𝑒).31 The difference of these


Page 26 of 34

Page 27 of 34 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

ACS Nano

two indices is known as birefringence, ∆𝑛 = 𝑛𝑒 ― 𝑛𝑜 and when multiplied with the sample thickness (t) and an orientation factor (𝑐𝑜𝑠2𝜎) one obtain the optical retardance, 𝑅 = ∆𝑛 × 𝑡 × 𝑐𝑜𝑠2𝜎, where 𝜎 is the inclination angle of the optic axis with respect to the plane of view. Since the solvents selected exhibit no optical anisotropy, imaging methods using polarized light can detect the anisotropy from the GO platelets. We have used a quantitative polarization technique known as ‘LC-PolScope’.6, 10 The critical advantage of this system over traditional polarizing microscopy is that it can measure the optical parameters at all points of the image simultaneously without mechanical rotation of the specimen, which significantly improves the sensitivity and accuracy of the measurement. The imaging technique also reveals the orientation and distribution of the slow-axis (the axis for maximum retardation of the polarized light i.e. basal plane of GO) of GO sheets at the level of each pixel. A wide range of concentrations, 0.25–20 g/L (pH ~ 6.0 for aqueous dispersions) were loaded in rectangular capillary tubes of 0.2 × 2 × 75 mm for each of the solvents. The capillaries were sealed and kept vertical for 24 hours before studying the phase transition. The imaging starts with a calibration procedure, where a background image is captured in a clear area on the mounted specimen to remove all the background retardances in the measurement. In capillary cell, the background image was taken above the meniscus of the capillary column and in microscopic slide it was taken in a clear area free of any GOLC. For a particular area of interest in the specimen, five raw images – one circularly polarized light image and four elliptically polarized light images were captured within a few seconds. Subsequently, an image processing algorithm computes the optical properties from the raw images to generate an optical retardance image, a slow-axis orientation image and an image comprising both of these properties, where the brightness represents retardance and hue represents the azimuth of the orientation. A monochromatic illumination of 546 nm was used for all measurements. The exposure time was kept constant to 35 ms. The retardance was measured in the range of 0 – 273 nm and the slow-


ACS Nano 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

axis orientation was measured in the range of 0 – 360 degree. The volume fraction of the nematic phase was calculated by locating the interface of I–N phase transition in the capillary. The capillaries were retested after one month to observe any changes. The GOLCs were also loaded in traditional microscope slides for imaging the liquid crystals at higher magnifications (10×). Distinct and separated regions in the combined retardance and slow-axis orientation maps showed together the characteristics of similar orientation of slow-axis and uniformity of retardance. The size of the similar hue and uniform brightness was selected manually and measured in ImageJ. The size of this region is defined as the domain size (d) of the liquid crystal. Computer algorithms will be considered in future refinement of the method, but has not been utilized here. The slow-axis images have also been used to quantify the order parameter,21 S = < 2cos2θ ― 1 > , where 𝜃 is the angle between the orientation (azimuth) of slow-axis at each pixel (total ~1000 pixels) and the mean orientation (azimuth). Steady and Dynamic Flow Behaviour Rheological measurements of the GOLCs were performed using a HAAKE MARS II Rheometer (Thermo Electron Corporation, Germany) with a 35 mm parallel plate geometry. A constant temperature of 23.00 ± 0.01 ℃ was maintained throughout the experiment by using a Peltier system and a thermostat HAAKE Phoenix II (Thermo Electron Corporation). In a typical experiment, 1.5 mL of GOLC was transferred directly to the bottom plate maintaining a constant gap of 1 mm. The linear viscoelastic region was determined by performing a stress sweep in the range of 0.01–100 Pa at different frequencies. Afterwards, the frequency sweep was carried out in the range of 0.1–100 rad/s and at a constant stress of 0.1 Pa. The flow characteristics were determined by performing a rate sweep in the range of 0.01–100 s-1. Additional steady state measurement was conducted in Anton-Paar rheometer (Physica MCR 301) in the shear rate range of 0.0009 – 1 s-1 for measuring the zero-shear viscosity.


Page 28 of 34

Page 29 of 34 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

ACS Nano

Fabrication and Characterization of GOLC Coating on Polyimide GOLCs were coated on a commercial hydrophobic polyimide (PI) substrate by using a height-adjustable lab-scale doctor blade. Typically, 1 mL of GO dispersion was deposited on the substrate which was sheared in an appropriate gap (2 µm) to obtain a coated film. Subsequently, the coating was dried overnight at room temperature. Afterwards, the coated film was reduced using the following method. The coated films were immersed in a 250 mL beaker containing 1.25 mL of acetic acid and 0.5 mL of hydroiodic acid (HI). The beaker was heated at 40 ℃ for 20 h in an oil bath. After reduction, the films were washed in water and methanol several times to remove the acids. The surface morphology and roughness of the coatings were measured using an optical profilometer (Bruker Contour GT-I 3D). X-ray diffraction patterns for the PI substrate, and the GOLC coated films (before and after reduction) were recorded using a Bruker D2 phaser diffractometer with Cu-Kα radiation (generated at 30 kV and 10 mA) where step size of 0.01° and a scan rate of 1° per minute were used. The sheet resistivity was measured using a four-point probe (Everbeing SR 4). For thermo-electrical measurements, the coated films were connected to a voltage power source via two probes to enable Joule’s heating. A desired steady temperature was achieved by simply changing the voltage drop from a DC power supply. An infrared (IR) camera captured the change in the surface temperature of the coating.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.


ACS Nano 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

Electrostatic Stability of the GO Platelets in Water; Theoretical Calculation of I–N Phase Transition Concentration; The Details of the LC-PolScope System; Thicker Anisotropic Particles in DMF and NMP; GOLC Domains in Blended Systems and Films; The Effect of Confinement on GOLC Alignment and Optical Anisotropy in Isotropic Media; Zero-shear Viscosity of GOLC; Hydrodynamic Behaviour of GOLC in Shear Rheology

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Rico F. Tabor: 0000-0003-2926-0095 Mainak Majumder: 0000-0002-0194-9387

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS This research was partially funded by the Australian Research Council Research Hub for Graphene Enabled Industry Transformation (project number IH 150100003). The authors would also acknowledge the facilities, scientific and technical assistance of Monash Micro Imaging, Monash University, Victoria, Australia. The authors acknowledge extensive discussions with Dr. Rudolf Oldenbourg from the Marine Biological Laboratory, Woods Hole, MA, USA on the interpretation of the polarized light images. REFERENCES


Page 30 of 34

Page 31 of 34 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

ACS Nano

1. Aboutalebi, S. H.; Gudarzi, M. M.; Zheng, Q. B.; Kim, J. K., Spontaneous Formation of Liquid Crystals in Ultralarge Graphene Oxide Dispersions. Adv. Funct. Mater. 2011, 21, 2978-2988. 2. Jalili, R.; Aboutalebi, S. H.; Esrafilzadeh, D.; Konstantinov, K.; Moulton, S. E.; Razal, J. M.; Wallace, G. G., Organic Solvent-Based Graphene Oxide Liquid Crystals: A Facile Route Toward the Next Generation of Self-Assembled Layer-by-Layer Multifunctional 3D Architectures. ACS Nano 2013, 7, 3981-3990. 3. Dan, B.; Behabtu, N.; Martinez, A.; Evans, J. S.; Kosynkin, D. V.; Tour, J. M.; Pasquali, M.; Smalyukh, I. I., Liquid Crystals of Aqueous, Giant Graphene Oxide Flakes. Soft Matter 2011, 7, 11154-11159. 4. Kim, J. E.; Han, T. H.; Lee, S. H.; Kim, J. Y.; Ahn, C. W.; Yun, J. M.; Kim, S. O., Graphene Oxide Liquid Crystals. Angew. Chem., Int. Ed. 2011, 50, 3043-3047. 5. Xu, Z.; Gao, C., Aqueous Liquid Crystals of Graphene Oxide. ACS Nano 2011, 5, 29082915. 6. Tkacz, R.; Oldenbourg, R.; Mehta, S. B.; Miansari, M.; Verma, A.; Majumder, M., pH Dependent Isotropic to Nematic Phase Transitions in Graphene Oxide Dispersions Reveal Droplet Liquid Crystalline Phases. Chem. Commun. 2014, 50, 6668-6671. 7. Shim, Y. H.; Lee, K. E.; Shin, T. J.; Kim, S. O.; Kim, S. Y., Tailored Colloidal Stability and Rheological Properties of Graphene Oxide Liquid Crystals with Polymer-Induced Depletion Attractions. ACS Nano 2018, 12, 11399-11406. 8. Narayan, R.; Kim, J. E.; Kim, J. Y.; Lee, K. E.; Kim, S. O., Graphene Oxide Liquid Crystals: Discovery, Evolution and Applications. Adv. Mater. 2016, 28, 3045-3068. 9. Kim, I. H.; Yun, T.; Kim, J. E.; Yu, H.; Sasikala, S. P.; Lee, K. E.; Koo, S. H.; Hwang, H.; Jung, H. J.; Park, J. Y., Mussel‐Inspired Defect Engineering of Graphene Liquid Crystalline Fibers for Synergistic Enhancement of Mechanical Strength and Electrical Conductivity. Adv. Mater. 2018, 30, 1803267. 10. Akbari, A.; Sheath, P.; Martin, S. T.; Shinde, D. B.; Shaibani, M.; Banerjee, P. C.; Tkacz, R.; Bhattacharyya, D.; Majumder, M., Large-Area Graphene-Based Nanofiltration Membranes by Shear Alignment of Discotic Nematic Liquid Crystals of Graphene Oxide. Nat. Commun. 2016, 7, 10891. 11. Jalili, R.; Aboutalebi, S. H.; Esrafilzadeh, D.; Shepherd, R. L.; Chen, J.; Aminorroaya‐Yamini, S.; Konstantinov, K.; Minett, A. I.; Razal, J. M.; Wallace, G. G., Scalable One‐Step Wet‐Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide: Towards Multifunctional Textiles. Adv. Funct. Mater. 2013, 23, 5345-5354. 12. Shaibani, M.; Akbari, A.; Sheath, P.; Easton, C. D.; Banerjee, P. C.; Konstas, K.; Fakhfouri, A.; Barghamadi, M.; Musameh, M. M.; Best, A. S., Suppressed Polysulfide Crossover in Li–S Batteries Through a High-Flux Graphene Oxide Membrane Supported on a Sulfur Cathode. ACS Nano 2016, 10, 7768-7779. 13. Xu, Z.; Sun, H.; Zhao, X.; Gao, C., Ultrastrong Fibers Assembled from Giant Graphene Oxide Sheets. Adv. Mater. 2013, 25, 188-193. 14. Onsager, L., The Effects of Shape on the Interaction of Colloidal Particles. Ann. N. Y. Acad. Sci. 1949, 51, 627-659. 15. Tkacz, R.; Abedin, M. J.; Sheath, P.; Mehta, S. B.; Verma, A.; Oldenbourg, R.; Majumder, M., Phase Transition and Liquid Crystalline Organization of Colloidal Graphene Oxide As a Function of pH. Part. Part. Syst. Charact. 2017, 34, 1600391. 16. Miyamoto, N.; Nakato, T., Liquid Crystalline Nature of K4Nb6O17 Nanosheet Sols and Their Macroscopic Alignment. Adv. Mater. 2002, 14, 1267-1270.


ACS Nano 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

17. Gabriel, J.-C. P.; Camerel, F.; Lemaire, B. J.; Desvaux, H.; Davidson, P.; Batail, P., Swollen Liquid-Crystalline Lamellar Phase Based on Extended Solid-Like Sheets. Nature 2001, 413, 504. 18. van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N., Liquid Crystal Phase Transitions in Suspensions of Polydisperse Plate-Like Particles. Nature 2000, 406, 868. 19. Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G., Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101. 20. Gudarzi, M. M.; Moghadam, M. H. M.; Sharif, F., Spontaneous Exfoliation of Graphite Oxide in Polar Aprotic Solvents As the Route to Produce Graphene Oxide–Organic Solvents Liquid Crystals. Carbon 2013, 64, 403-415. 21. Tkacz, R.; Oldenbourg, R.; Fulcher, A.; Miansari, M.; Majumder, M., Capillary-ForceAssisted Self-Assembly (CAS) of Highly Ordered and Anisotropic Graphene-Based Thin Films. J. Phys. Chem. C 2013, 118, 259-267. 22. Kravets, V.; Marshall, O.; Nair, R.; Thackray, B.; Zhukov, A.; Leng, J.; Grigorenko, A., Engineering Optical Properties of a Graphene Oxide Metamaterial Assembled in Microfluidic Channels. Opt. Express 2015, 23, 1265-1275. 23. Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.; Khan, U.; O’Neill, A.; Boland, C.; Lotya, M.; Istrate, O. M.; King, P., Scalable Production of Large Quantities of Defect-Free Few-Layer Graphene by Shear Exfoliation in Liquids. Nat. Mater. 2014, 13, 624. 24. Sun, X.; Luo, D.; Liu, J.; Evans, D. G., Monodisperse Chemically Modified Graphene Obtained by Density Gradient Ultracentrifugal Rate Separation. ACS Nano 2010, 4, 33813389. 25. Lamarque‐Forget, S.; Pelletier, O.; Dozov, I.; Davidson, P.; Martinot‐Lagarde, P.; Livage, J., Electrooptic Effects in the Nematic and Isotropic Phases of Aqueous V2O5 Suspensions. Adv. Mater. 2000, 12, 1267-1270. 26. Paineau, E.; Antonova, K.; Baravian, C.; Bihannic, I.; Davidson, P.; Dozov, I.; ImpérorClerc, M.; Levitz, P.; Madsen, A.; Meneau, F., Liquid-Crystalline Nematic Phase in Aqueous Suspensions of a Disk-Shaped Natural Beidellite Clay. J. Phys. Chem. B 2009, 113, 1585815869. 27. Lekkerkerker, H. N.; Tuinier, R., Colloids and the Depletion Interaction. Springer: Dordrecht, Netherlands, 2011. 28. Dimiev, A. M.; Alemany, L. B.; Tour, J. M., Graphene Oxide. Origin of Acidity, Its Instability in Water, and a New Dynamic Structural Model. ACS Nano 2012, 7, 576-588. 29. Dimiev, A. M.; Tour, J. M., Mechanism of Graphene Oxide Formation. ACS Nano 2014, 8, 3060-3068. 30. Tandon, P.; Rosner, D. E., Monte Carlo Simulation of Particle Aggregation and Simultaneous Restructuring. J. Colloid Interface Sci. 1999, 213, 273-286. 31. Oldenbourg, R., Polarized Light Microscopy: Principles and Practice. Cold Spring Harb. Protoc. 2013, 2013, pdb. top078600. 32. Da Silva, A.; Deumié, C.; Vanzetta, I., Elliptically Polarized Light for Depth Resolved Optical Imaging. Biomed. Opt. Express 2012, 3, 2907-2915. 33. Ware, T. H.; McConney, M. E.; Wie, J. J.; Tondiglia, V. P.; White, T. J., Voxelated Liquid Crystal Elastomers. Science 2015, 347, 982-984. 34. Wissbrun, K. F., Rheology of Rod‐Like Polymers in the Liquid Crystalline State. J. Rheol. 1981, 25, 619-662. 35. Moldenaers, P.; Mewis, J., On the Nature of Viscoelasticity in Polymeric Liquid Crystals. J. Rheol. 1993, 37, 367-380. 36. Campbell, M. G.; Tasinkevych, M.; Smalyukh, I. I., Topological Polymer Dispersed Liquid Crystals with Bulk Nematic Defect Lines Pinned to Handlebody Surfaces. Phys. Rev. Lett. 2014, 112, 197801.


Page 32 of 34

Page 33 of 34 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

ACS Nano

37. Gârlea, I. C.; Mulder, P.; Alvarado, J.; Dammone, O.; Aarts, D. G.; Lettinga, M. P.; Koenderink, G. H.; Mulder, B. M., Finite Particle Size Drives Defect-Mediated Domain Structures in Strongly Confined Colloidal Liquid Crystals. Nat. Commun. 2016, 7, 12112. 38. Miyamoto, N.; Nakato, T., Liquid Crystalline Nanosheet Colloids with Controlled Particle Size Obtained by Exfoliating Single Crystal of Layered Niobate K4Nb6O17. J. Phys. Chem. B 2004, 108, 6152-6159. 39. Vasu, K.; Krishnaswamy, R.; Sampath, S.; Sood, A., Yield Stress, Thixotropy and Shear Banding in a Dilute Aqueous Suspension of Few Layer Graphene Oxide Platelets. Soft Matter 2013, 9, 5874-5882. 40. Del Giudice, F.; Cunning, B. V.; Ruoff, R. S.; Shen, A. Q., Filling the Gap Between Transient and Steady Shear Rheology of Aqueous Graphene Oxide Dispersions. Rheol. Acta 2018, 57, 293-306. 41. Kjøniksen, A.-L.; Nyström, B.; Lindman, B., Dynamic Viscoelasticity of Gelling and Nongelling Aqueous Mixtures of Ethyl (Hydroxyethyl) Cellulose and an Ionic Surfactant. Macromolecules 1998, 31, 1852-1858. 42. Michot, L. J.; Baravian, C.; Bihannic, I.; Maddi, S.; Moyne, C.; Duval, J. F.; Levitz, P.; Davidson, P., Sol− Gel and Isotropic/Nematic Transitions in Aqueous Suspensions of Natural Nontronite Clay. Influence of Particle Anisotropy. 2. Gel Structure and Mechanical Properties. Langmuir 2008, 25, 127-139. 43. de Gennes, P. G.; Prost, J., The Physics of Liquid Crystals. 2nd ed.; Clarendon Press: Oxford 1993. 44. Marrucci, G., Rheology of Liquid Crystalline Polymers. Pure Appl. Chem. 1985, 57, 1545-1552. 45. Walker, L.; Wagner, N., Rheology of Region I Flow in a Lyotropic Liquid‐Crystal Polymer: The Effects of Defect Texture. J. Rheol. 1994, 38, 1525-1547. 46. Burghardt, W.; Hongladarom, K., Texture Refinement in a Sheared Liquid-Crystalline Polymer. Macromolecules 1994, 27, 2327-2329. 47. Lu, K., Nanocrystalline Metals Crystallized from Amorphous Solids: Nanocrystallization, Structure, and Properties. Mat. Sci. Eng. R Rep. 1996, 16, 161-221. 48. Pande, C.; Cooper, K., Nanomechanics of Hall–Petch Relationship in Nanocrystalline Materials. Prog. Mater. Sci. 2009, 54, 689-706. 49. Nieman, G.; Weertman, J.; Siegel, R., Mechanical Behavior of Nanocrystalline Cu and Pd. J. Mater. Res. 1991, 6, 1012-1027. 50. Latapie, A.; Farkas, D., Effect of Grain Size on the Elastic Properties of Nanocrystalline Α-Iron. Scr. Mater. 2003, 48, 611-615. 51. Carlton, C.; Ferreira, P., What is Behind the Inverse Hall–Petch Effect in Nanocrystalline Materials? Acta Mater. 2007, 55, 3749-3756. 52. Philippe, A.; Baravian, C.; Bezuglyy, V.; Angilella, J.; Meneau, F.; Bihannic, I.; Michot, L., Rheological Study of Two-Dimensional Very Anisometric Colloidal Particle Suspensions: From Shear-Induced Orientation to Viscous Dissipation. Langmuir 2013, 29, 5315-5324. 53. Coskun, M. B.; Akbari, A.; Lai, D. T.; Neild, A.; Majumder, M.; Alan, T., Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate. ACS Appl. Mater. Interfaces 2016, 8, 22501-22505. 54. Van Gurp, M.; Palmen, J., Time-Temperature Superposition for Polymeric Blends. Rheol. Bull. 1998, 67, 5-8. 55. White, K. L.; Li, P.; Yao, H.; Nishimura, R.; Sue, H.-J., Effect of Surface Modifier on Flow Properties of Epoxy Suspensions Containing Model Plate-Like Nanoparticles. Rheol. Acta 2014, 53, 571-583.


ACS Nano 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

56. Maffettone, P.; Marrucci, G.; Mortier, M.; Moldenaers, P.; Mewis, J., Dynamic Characterization of Liquid Crystalline Polymers Under Flow‐Aligning Shear Conditions. J. Chem. Phys. 1994, 100, 7736-7743. 57. Voigt, M. M.; Mackenzie, R. C.; King, S. P.; Yau, C. P.; Atienzar, P.; Dane, J.; Keivanidis, P. E.; Zadrazil, I.; Bradley, D. D.; Nelson, J., Gravure Printing Inverted Organic Solar Cells: The Influence of Ink Properties on Film Quality and Device Performance. Sol. Energy Mater. Sol. Cells 2012, 105, 77-85. 58. Sui, D.; Huang, Y.; Huang, L.; Liang, J.; Ma, Y.; Chen, Y., Flexible and Transparent Electrothermal Film Heaters Based on Graphene Materials. Small 2011, 7, 3186-3192. 59. Nečas, D.; Klapetek, P., Gwyddion: An Open-Source Software for SPM Data Analysis. Open Phys. 2012, 10, 181-188.

“For Table of Contents Only”


Page 34 of 34

Graphene Oxide Liquid Crystal Domains: Quantification and Role in

Recommend Documents