Phase Behavior of Acetylated Cellulose Nanocrystals and Origins of

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Phase Behavior of Acetylated Cellulose Nanocrystals and Origins of the Cross-hatch Birefringent Texture Mingzhe Jiang, Matt McMillan, Virginia A. Davis, and Christopher L. Kitchens Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00746 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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Figure 1. Cross polarized image (bulk view) showing the phase separation of CNC suspensions of variable concentrations after 4 weeks of sedimentation. CNC-AA dispersion in scintillation vials: 1: 0.003 %vol, 2: 0.006 %vol, 3:0.012 %vol, 4:0.019 %vol, 5:0.031 %vol, 6:0.043 %vol, 7:0.06 %vol, 8:0.12 %vol, 9:0.19 %vol, 10:0.31 %vol, and 11:0.43 %vol.

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Figure 2. CNC –AA apparent hydrodynamic diameter (■) and apparent translation diffusion coefficient (▲) as a function of CNC-AA suspension concentration measured by DLS. Error bars represent the standard deviations of three replicate measurements.


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Figure 3. SANS intensity curves (I vs q) of CNC-AA suspensions in D2O fit to a parallelpipe model to determine assembly dimensions in Table 1. The inset is a power law fitting of the low q region to determine the fractal dimensions listed in Table 1.



Figure 4. Cross polarized microscope images of CNC-AA suspension droplet in 120 μm spacer with different angles. Scale bar: 100 μm. (a) 0.788 %vol; (b) 2.1 %vol; (c) 3.85 %vol.


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Figure 6. Cross polarized microscopy images of a) CNC-AA dried film under 10X magnification (Scale bar: 50µm); b) CNC-AA dried film under 63X magnification (Scale bar: 10µm)


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Figure 7. SEM images of the CNC-AA dried film fractured cross sections (a,b, c, d) and upper surface (e, f). Images a, b) of the fractured cross section inline the film plane; c, d) are images of the fractured cross section perpendicular to the film plane; e, f) Images of the film upper surface.



Figure 8. Polarized microscope time elapsed imaging of the CNC-AA dispersion with 45o angle between shear direction and analyzer/polarizer: a) 0.3 %vol, shear rate at 50 s-1 at 0, 30 and 60 seconds after shear cessation; b) 0.788 %vol, shear rate at 50 s-1, at 0, 60, and 600 seconds after shear cessation. Scale bar: 100 μm.


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Figure 9. Comparisons of polarized optical micrographs of CNC-AA aqueous suspension during steady shear test. (a) concentration at 0.788 %vol; (b) concentration at 2.1 %vol; (c) concentration at 3.85 %vol. Shear rates of 0, 10, and 50 s-1 are presented with the shear stage oriented at 0° and 45o angle between shear direction and analyzer/polarizer. Scale bar: 100 μm.


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Phase Behavior of Acetylated Cellulose

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Nanocrystals and Origins of the Cross-hatch

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Birefringent Texture

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Mingzhe Jiang1, Matt McMillan1, Virginia Davis2,* and Christopher L Kitchens1,*

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Department of Chemical and Biomolecular Engineering, Clemson University

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Department of Chemical Engineering, Auburn University

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KEYWORDS: Cellulose nanocrystals, liquid crystal phase behavior, birefringent glassy phase,

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fine-scale nematic, cross-hatch texture


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ABSTRACT

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Cellulose nanocrystals hydrolyzed by hydrochloric acid and esterified by acetic acid produces

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acetylated cellulose nanocrystals (CNC-AA) with acetyl ester surface functional groups. While

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much attention has been paid to understanding the phase behavior (liquid crystal) of aqueous

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dispersions of sulfonated nanocrystals, relatively few studies have focused on CNC-AA

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dispersions. CNC-AA dispersions exhibit multiple phase regimes and markedly different phase

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behavior due to their lower surface charge. At concentrations above 5.0 x 10-4 %vol, a decrease

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in the apparent diffusion coefficient indicates the onset of interparticle interactions and a

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transition from the dilute regime.

From 0.003 to 0.31 %vol, biphasic behavior is observed,

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consisting of a birefringent lower phase and disordered or isotropic upper phase. Small-angle

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neutron scattering was used to measure the growth of fractal structures with increasing

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concentration, and indicates a two dimensional assembly with short range order in plate-like

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assembled geometry.

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birefringent texture, which is believed to exist as a fine-scale nematic that possesses frozen-in

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flow shear behavior, consistent with a glassy phase. This cross-hatch pattern is maintained in

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dried films, where atomic force microscopy and scanning electron microscopy reveal a layered

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sheet-like structure. Imaging also indicates that the basic unit of CNC-AA microstructure in the

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film consists of 0.5-1.5 µm scale aligned nanorod domains, which agrees with neutron scattering

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and the dimensions of each individual “hatch” in the birefringent texture observed by cross

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polarized microscopy. The assembly of the nanorods into this layered structure and the fine-scale

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nematic birefringent cross-hatch texture is of significant fundamental interest, particularly since

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it differs greatly from cellulose nanocrystals with other surface chemistry and offers potential

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opportunities in other applications owing to the unique assembly.

Above 0.31 %vol, the dispersion transitions to exhibit a cross-hatch


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Introduction:

Cellulose nanocrystals (CNCs) are crystalline, elongated cuboidal-shaped nanoparticles

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obtained from cellulosic materials by acid hydrolysis.1, 2, 6-8 CNCs have been used for

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reinforcement and enhancement of polymer composites, targeted drug delivery, optical devices,

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nanoparticle assembly template and many other advanced materials applications.2-11 The diverse

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range of CNC applications are influenced by the size, shape, polydispersity and colloidal phase

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behavior, which are dictated by the raw material source, acid hydrolysis and surface chemistry.

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Commonly, sulfuric acid is used as the hydrolyzing agent to yield negatively-charged surface

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sulfate half-ester groups (CNC-SA) that promote a stable colloidal dispersion of CNCs in water

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with well-defined cholesteric liquid crystal phase behavior that has been widely studied.12-15

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While fundamentally intriguing and with many proven applications, the lower thermal stability

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of the CNC-SA and hydrophilic nature leads to difficulties in certain applications, particularly

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those in thermoplastic polymer composites.16,17 In attempt to overcome these obstacles, covalent

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(eg. oxidation, silylation, polymer grafting18-20), non-covalent surface modification (eg.

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adsorbing surfactants and polymers) 21 and CNC surface sulfate group neutralization by NaOH

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solution17 have been studied. Other CNCs preparation methods have also been introduced to

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solve this problem, such as acid vapour22, electron beam irradiation23 , esterification24,

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oxidation25, acetylation in ionic liquids26 and microwave-assisted hydrothermal treatment27.

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Dorgan and co-workers originally developed a one-pot reaction methodology for the isolation of

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acetylated CNC (CNC-AA) in a single-step reaction where Fischer esteriﬁcation of hydroxyl

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groups by acetic acid is achieved simultaneously with the HCl hydrolysis of amorphous


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cellulose. Compared with CNC-SA, this synthesis method is potentially more economical and

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involves less hazardous reagents that can be derived using renewable resources.28,29

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Aqueous colloidal dispersions of surface-modified CNCs theoretically should behave

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similarly to other rod-like particle dispersions such as tobacco mosaic virus and DNA

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fragments.30-32 According to classical DLVO theory,33,34 the stability of a dispersion of charged

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colloidal particles is a balance between repulsive electrostatic inter-particle forces resulting from

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electric double layer repulsion, attractive van der Waals forces, and Brownian motion. For non-

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spherical or anisotropic particles, this balance and thus colloid stability also depends on the

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mutual particle orientation. To model the liquid crystal phase behavior of charged anisotropic-

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shaped particles, Onsager’s monodisperse hard rod theory 35 was modified in the Stroobants,

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Lekkerkerker and Odijk (SLO) theory 36,37 to account for particle aspect ratio and surface charge

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density. However, in low surface charge systems, it has been observed that neither Onsager or

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SLO theory effectively predict the dispersion phase behavior.38

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CNC-SA dispersions are categorized as an electrostatic colloidal system due to the high

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surface charge density and the lyotropic phase behavior that exists as isotropic, biphasic, liquid

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crystal, glassy, gel phase with dependency on the solution concentration, ionic strength, or

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surface chemistry.39,40 41 At high concentrations where excluded volume or attractive interactions

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take effect, the glassy phase consists of dispersed densely packed structures, while the gel phase

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consists a percolated network of particles and/or clusters, which show greater rigidity than the

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glassy phase.42 In addition to cholesteric, other birefringence patterns have been observed in


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concentrated CNC solutions. 16 43 Araki et. al. 44 discovered a “birefringent glassy phase” with

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“cross-hatch” birefringent texture for post-sulfonated HCl-hydrolyzed CNCs in aqueous

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suspension. Poly(ethylene oxide) grafted CNCs were also found to exhibit this cross-hatch

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birefringent pattern. 45 This structure is clearly different from the chiral nematic phase of the

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H2SO4-hydrolyzed CNC-SA suspension. This cross-hatch birefringent pattern was also observed

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in suspensions of boehmite rods by Buining et. al.38 The boehmite rod dispersion was found to be

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a glassy phase with unrelaxed birefringent texture after shear or shaking cessation, indicating a

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so called “frozen-in” shear phase with the cross-hatch birefringent texture. The origin of the

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cross-hatch birefringence pattern and “frozen-in shear structure” is not fully understood or

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characterized. It is believed to be associated with the low aspect ratio and low surface charge of

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the nanorod colloidal system. Araki et. al. postulated that as the surface charge of CNC-SA

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decreases to a certain level, the CNCs will self-assemble into a structure resulting in the cross-

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hatch birefringent pattern.44 Despite the significant potential and unique properties of these

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materials, a detailed mechanism of the glassy phase formation with cross-hatch birefringent

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texture and a correlation with microstructure has not been explored.

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This work focuses on an in-depth characterization of CNC-AA nanoparticle colloidal

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structure formation and phase behavior, which shows distinct phenomena that contrasts to

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highly-charged CNC (sulfuric acid hydrolyzed) dispersions. CNC-AA dispersions do not show

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typical liquid crystal texture. Instead, a ‘fine-scale’ aligned microstructure exists that manifests

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as a birefringent cross-hatch texture. We have used small angle neutron scattering (SANS) to

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determine the dimensional lateral assembly with increasing concentration, which eventually

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leads to a layered structure with micron-scale features. While the assembled structure does not


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possess the long-range order of a classic liquid crystal, it can be classified as a glassy gel phase

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or fine-scale nematic as defined by Windle et. al46. This structure possesses nematic orientation

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that persists with short range order on the order of one to several μm2,46 Shear alignment of the

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CNC-AA dispersions was also studied and found to relax within 10 to 60 seconds at intermediate

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concentrations. At higher concentrations, the dispersion shows frozen-in flow shear behavior

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that is unchanged for more than 2 days after the shear has ceased. Overall, the CNC-AA

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dispersions demonstrate fundamentally different behavior compared to other CNC systems with

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different surface functional groups and thus have potential for unique applications.

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Materials and Methods:

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Whatman cellulose filter aid in 1 cm squares was soaked overnight in acetic acid

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(≥99.8%, Fisher Chemical). HCl (36 wt%, Fisher Chemical) and deionized (DI) water were

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added to yield a 17.5 M acetic acid (90 wt%), and 0.027 M HCl suspension. The suspension was

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heated in a round bottom flask by heating mantle at 105 °C for 15 hours with stirring and then

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cooled in an ice bath for 15 minutes following the reaction. The suspension was centrifuged at

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8,600 rpm for 3 minutes to precipitate the hydrolyzed cellulose. The acidic supernatant was

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decanted and the solids were redispersed in DI water to the original volume with vortex mixing.

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The centrifugation and redispersion washing cycles were conducted three times until the

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suspension pH reached 6.5. This CNC-AA isolation protocol is a modification of Dorgan’s

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original method14, using ultra-sonication instead of agitating blender or homogenizer. A sonic

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dismembrator (Fisher Scientific™ Model 550) was used to sonicate the dispersion for 40


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minutes at a power of 150 watts while immersed in an ice bath to maintain temperature. The

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sonication power level and time was systematically determined to acquire the CNC-AA

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dispersion with minimal agglomerations observed as bright spots with cross polarized

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microscopy (Figure S1 in Supporting Information). Sonication powers from 100 to 200 watts

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and times ranging from 10 to 40 minutes were investigated. It should be noted that cross

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polarized microscopy of the dispersed phase is an extremely useful method of identifying the

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presence of aggregates and complete CNC isolation. Aggregates are easily identified as bright

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birefringent points. DI water was added in order to approximately double the volume, followed

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by two additional washings until the final nanocrystals were recovered and dispersed in the

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supernatant after centrifugation. Conductometric titration with sodium hydroxide (0.5 × 10 -4 M)

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was used to determine the surface charge density.

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The CNC dispersion concentrations were determined by thermal gravimetric analysis

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(TGA) (TA Instruments SDT-Q600). A 60 μg sample was heated from 30°C to 120°C at 10

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°C/min, holding for 20 min followed by a cool-down to 35°C at 20 °C/min. The weight

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differential is used to calculate the CNC volume fraction, assuming no total volume change and a

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CNC density of 1.6 g/ml.32 The sample was then heated from 30°C to 600°C at 10 °C/min in air

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and the weight as a function of temperature was recorded to determine the CNC thermal stability.

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A decomposition temperature of 280oC was identified by the downturn of TGA weight loss

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curves, which is compared to 160°C for CNC-SA17 and 340oC for the cotton cellulose47 raw

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material (Supporting Information Figures S2-S3).


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1

The CNC diffusion coefficient and average intensity-weighted hydrodynamic diameter

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were obtained as a function of concentration by dynamic light scattering (DLS) using a Wyatt

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Technologies DAWN HELEOS II model 337-H2. CNC dispersions were prepared using DI

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water as the dilution media. In each case, three independent replicates of each dispersion (5.0 x

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10-5 %vol to 0.01 %vol) were used to give an average hydrodynamic diameter. The CNC zeta-

6

potential was measured using a Malvern instrument (Zeta-sizer Nano series).

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A Veeco Dimension 3100 atomic force microscope (AFM) equipped with a Nanoscope

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3A controller in tapping mode in ambient air was used to observe the CNCs dried film topology.

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Nanoscope III 5.12r3 software was used for imaging. The AFM tip (HQ:NSC15/AL BS)

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purchased from MikroMasch had an 8 nm nominal radius, typical force constant of 40 N/m and

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typical resonance frequency of 325 kHz. The CNC-AA particles are considered as elongated rods

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with rectangular cross sections48 characterized by an average length of 193 nm (± 66 nm),

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average width of 30 nm (± 8 nm) and average height of 9.1 nm (± 3.2 nm). Transmission

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electron microscope (TEM) observations were performed using a Hitachi H7600 operated at 110

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kV. A 0.01%vol droplet of CNC dispersion was placed on a copper grid covered by a thin carbon

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film and allowed to dry at room temperature overnight.

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CNC-AA dispersions ranging in concentration from 0.3 to 15 %vol were characterized by optical

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microscopy using a Nikon Eclipse 80i optical microscope with cross polarizers. Samples were

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prepared in glass capillary tubes (VitroTubes™ 5015) or conventional microscope slide and

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cover glass with 120 µm spacer (CoverWell™ Perfusion Chambers) and sealed by fast-dry


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fingernail polish. Pictures were taken at 5 to 20X magnification between cross polarizers at room

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temperature. To understand the shear response and shear-induced structure formation, cross

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polarized optical microscopy was conducted with a Linkam Cryo-CSS450 temperature

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controlled shear stage. Shear rates of 10 and 50 s-1 were applied to the dispersion at 25oC with

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100 μm gap.

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The CNC dried films were prepared by slow evaporation over several days from 1 - 2

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vol. % aqueous dispersions under ambient conditions in a flat-bottomed glass petri-dish.

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Scanning electron microscopy (SEM) images of dried CNC-AA films with magnification up to

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300K were acquired on a Hitachi S-4800 Type II Ultra-High Resolution Field Emission SEM

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operating at 2 keV and working distance 5.7 mm. Free-standing CNC films were fractured by

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bending. The film upper surface and cross-sections were coated with a thin (~2 nm) Pt layer.

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SANS Measurements

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Neutron scattering was performed on the General-Purpose Small-Angle Neutron

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Scattering (SANS) Diffractometer (GP-SANS | CG-2 | HFIR ) at the Oak Rdige National

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Laboratory. Neutrons with a wavelength, λ, of 5 Å and resolution range δλ/λ of 9 to 45% were

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collimated using a 12.5 mm diameter aperture. The Q range from 0.00115 (or lower) to 0.5 A-1

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was measured. CNC-AA dispersions in D2O with concentrations from 0.05 to 1.5% were filled

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in banjo cells with a path length of 1 or 2mm. The 2-D SANS results were radially averaged and

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analyzed with SASView and Igor packages. 49 50


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To determine the CNC self-assembled structural dimensions, several fitting models were

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explored. The parallelepiped form factor model with a rectangular cross-section, averaged over

3

all space orientations was found to best fit the scattering data, which agrees with Cherhal et al.51

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The form factor is given in Eq. 1 where , , and are the length, width and height of the CNC

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parallelepiped. In the model fitting, the scattering length density of D2O was set at 6.35 x 1010

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cm-2, and the CNC was set to 1.86 x 1010 cm-2. A Gaussian polydispersity of 0.3 on the length,

7

width and height suppressed the oscillations present in the simulated model of the scattering for

8

all of the samples and increased the accuracy of the size measurements as compared to the TEM.

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This polydispersity was then assumed for all measurements.

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/

(, , , ) = !

/ ( )

!

×

( ) ( )

sin "

Eq. 1

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Results and Discussion

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1.

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CNC Phase Behavior Figure 1 shows CNC-AA aqueous dispersion viewed in 20 ml scintillation vials between

15

cross polarized films after 4 weeks of settling. The biphasic region ranging from 0.006 to 0.043

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% vol. distinguishes the phase behavior into three regions: an initial single phase region below

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0.006 %vol., a biphasic region, and a turbid phase region above 0.043 %vol. For comparison,

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Onsager’s hard rod model24 predicts a 16.7 %vol. biphasic transition and 22.5 %vol. anisotropic

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phase transition for the measured CNC dimensions. The classic Onsager model far overestimates


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these concentrations and fails to account for the specific solvation and inter-particle interactions

2

involved (inter-particle is discussed below). Furthermore, it must be mentioned that classic liquid

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crystal phase behavior is not exhibited, as classic liquid crystal birefringence texture is not

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observed. In the biphasic region, gentle shaking of the vial indicates a less viscous upper phase

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and thus differences in CNC-AA concentration. In addition, slight shaking results in complete

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mixing that separates back into two phases over 3 days. The lower phase shows weak

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birefringence between cross polarized films and increased shear birefringence intensity when

8

moderately stirred. The upper phase does not exhibit visual birefringence. Visualization of the

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lower phase (0.031 % vol.) by polarized microscopy does not show birefringence due to the

10

weak intensity and small sample thickness. Above 0.043 % vol, the dispersion is single phase

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and becomes more turbid with increasing concentration.

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Figure 1. Cross polarized image (bulk view) showing the phase separation of CNC

2

suspensions of variable concentrations after 4 weeks of sedimentation. CNC-AA

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dispersion in scintillation vials: 1: 0.003 %vol, 2: 0.006 %vol, 3:0.012 %vol, 4:0.019

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%vol, 5:0.031 %vol, 6:0.043 %vol, 7:0.06 %vol, 8:0.12 %vol, 9:0.19 %vol, 10:0.31

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%vol, and 11:0.43 %vol.

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2. CNC Colloidal Phenomena and Assembly

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The apparent diffusion coefficient (Dt) and corresponding apparent hydrodynamic diameter

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(Figure 2) were measured by DLS to define the colloidal dilute region (region 1). In solution,

9

long range interactions between the CNCs will introduce a cooperativity in diffusion at

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concentrations above the dilute regime.52 As inter-particle interactions arise with increasing

11

concentration, Dt does not represent the true single particle diffusion, but rather reflects the

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relative differences in mobility of the CNCs as a function of concentration. Following Stokes-

13

Einstein relation,30 the hydrodynamic diameter is inversely proportional to Dt. Figure 3 shows

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that below 5.0 x 10-5 % vol, the apparent hydrodynamic diameter of the CNC-AA is constant at

15

103 ± 11 nm. This indicates that particles in solution are isolated and independent from one

16

another (dilute dispersion) below 5.0 x 10-5 %vol. Above this concentration, Dt begins to

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decrease as a result of changes in mobility due to the CNC cooperativity, identifying a semi-

18

dilute region. A surface charge density of 0.04 e·nm-2 CNC has been found by a conductometric

19

titration with sodium hydroxide, which is much lower than sulfuric acid hydrolyzed CNC (0.48 e·nm-

20

2

). Additionally, CNC-AA particles possess a Zeta potential of -33.1±0.5 mV, as compared to -


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62.8 mV for CNC-SA.39 The decreased magnitude of CNC-AA surface charge corresponds to

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weaker electrostatic repulsive interparticle interactions as compared to CNC-SA and an onset of

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cooperativity at significantly lower volume fractions. Additionally, it should be noted that the

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small volume fraction dispersions were made by diluting a 1.5 %vol sample, which demonstrates

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that flocculation is reversible at low concentrations without the need of added sonication. This

6

observation is significant as related to the agglomeration formation discussed below.

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Figure 2. CNC –AA apparent hydrodynamic diameter (■) and apparent translation diffusion

2

coefficient (▲) as a function of CNC-AA suspension concentration measured by DLS. Error

3

bars represent the standard deviations of three replicate measurements.

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At concentrations above the biphasic region, the CNC-AA interparticle interactions give

5

rise to self-assembly and the formation of dispersed assemblies with structure. The structure in

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the CNC-AA assemblies was monitored by small angle neutron scattering (SANS), where the

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intensity versus wave vector, q, for concentrations ranging from 0.05 to 1.5 %vol is shown in

8

Figure 3. The q range between 0.00115 and 0.44 Å−1 probes the length scales in direct space

9

from 550 nm (∼2π/qmin) down to 1 nm. The curves were fit with a parallelepiped form factor

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containing a rectangular cross-section, averaged over all space orientations.53 Given that the

11

three fitted length parameters a, b, and c have different distinct lengths, the influences on the

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scattering curves are evident in different q ranges, which allows for decorrelation in fitting. The

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height parameter, c, contributes to the scattering curve in the medium q range and the width

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parameter, b, is present in the low q range. The length parameter, a, cannot be precisely fit at

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higher concentrations because the Guinier range54 for the longest length scale is at the edge of

16

the measured q range. A simulated parallelepiped form factor plot contains oscillation peaks that

17

arise from destructive interferences in the high q range. Experimentally, these oscillations are not

18

observed due to size polydispersity, and a Gaussian polydispersity of 0.3 for each dimension

19

suppresses these oscillations and increases accuracy in the fit parameters. This polydispersity

20

was confirmed from the TEM size analysis and assumed for all samples. The fitted structure

21

dimensions are listed in Table 1. At the lowest concentration where SANS signal intensity was

22

measurable, 0.05%vol ( biphasic region), the dimensions of the assembled CNC-AA structures is


14

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already larger than the single CNCs nanorod length and width as determined by TEM

2

(Supporting Information Figures S4-S6). This indicates that CNC-AA assemblies are already

3

formed at the start of the turbid region and the lower limit of SANS signal detection.

4 5

With increasing concentration, growth was observed in two dimensions, the width (b) and

6

length (a), while height (c) remains relatively constant around 10 nm, as shown in Table 1.

7

Comparatively, the width growth is greater than length growth with 110% vs. 75% increase

8

respectively from 0.05 to 0.82 %vol. Samples at 0.3 and 0.5 %vol have similar aggregate

9

dimensions, which indicates a potential equilibrium stage in this concentration range before

10

continued growth. It must be noted that the probed q range does not probe length measurements

11

that exceed 550nm and thus the model is insensitive to changes in the length (a) measurement at

12

concentrations above 0.8 %vol. The growth in the two dimension of the assembly cross section

13

suggests plate formation (both linear and lateral packing) of CNC-AAs, which is different from

14

the end-to-end linear assembly in CNC-SA dispersions (dilute regime) and chiral nematic tactoid

15

formation (concentrated regime), reported by Cherhal51 and Andy55, respectively.

16

Although, the Guinier regime extends beyond the q range probed, the fractal dimension,

17

Df , of the CNC-AA assemblies56 was obtained from the exponent of the power law scattering in

18

the low q range (below 0.007 A-1). This is indicative of the compactness of the CNC-AA

19

assembly. The Df was found to increase with concentration, possessing a step increase above 0.5

20

%vol (Table 1). At concentrations below 0.5 %vol, the fractal dimension is consistent with

21

values below 1.3 but increases for concentrations above 0.5 %vol., reaching a value of 1.69 at

22

1.5 %vol.


15


1

Page 26 of 47

In general, the assembled colloidal systems may be categorized by one of two colloidal

2

assembly or structure formation processes: a diffusion-limited cluster−cluster aggregation

3

(DLCA) model33 or a reaction-limited cluster−cluster aggregation (RLCA) model.34 Both

4

processes lead to an assembly with an internal self-similar structure and Df that is well-described

5

for spheres interacting with an isotropic potential. DLCA is a fast process with loose and

6

ramified clusters and a Df ≈ 1.8. RLCA is slower, promoting more compact clusters and

7

possesses a Df ≈ 2.1. For anisotropic particles it is reported that Df increases from 1.8 to 2.3 with

8

an increase in aspect ratio, eliminating the structural distinction between DLCA and RLCA for

9

highly anisotropic rods.57 At concentrations below 0.5 %vol, the Df is below 1.3, suggesting

10

CNC-AA assembly is approximated in between one dimensional (line-like) and two dimensional

11

(square-like) growth. This observation indicates disproportionate growth in one direction, which

12

is consistent with the aggregate dimensions determined by fitting with the parallelepiped model.

13

The Df has a maximum value of 1.69 at 1.5%vol, which is below the isotropic DLCA typical

14

value of 1.8.58 This indicates a more complex assembly where CNC-AA percolation in the

15

concentrated dispersions is not achieved. This observation is consistent with experimental and

16

Monte Carlo (MC) simulation findings in Laponite dispersions that form layered structures as

17

well. 59 For concentrated colloids with isotropic short-range attractive interactions, gel-like

18

percolation network will be eventually formed.60 For CNC-AA and Laponite dispersions, it

19

appears that at relatively high concentration, intermediate sized clusters are stabilized in a glassy

20

phase where the assembled structure hampers assembly into a larger network or aggregation

21

state, evidenced by the relatively low fractal dimension value.


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2

Figure 3. SANS intensity curves (I vs q) of CNC-AA suspensions in D2O fit to a

3

parallelpipe model to determine assembly dimensions in Table 1. The inset is a power law fitting

4

of the low q region to determine the fractal dimensions listed in Table 1.

5 6

Table 1. CNC-AA assembly dimensions and fractal dimension at different concentrations from parallelpipe model and power law fitting

CNC-AA Concentration Length a (nm) Height c (nm) Width b (nm) Fractal dimension

0.05%vol 314 10.7 50.2 1.24

0.30%vol 414 9.0 80.7 1.30

0.50%vol 418 8.6 80.8 1.31

0.82%vol 550 9.0 105 1.57

1.50%vol 550 9.0 130 1.69


17


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3. Cross-Hatch Birefringence Pattern

2

Cross polarized microscopy was used to characterize the birefringent texture of the CNC-

3

AA dispersions. Different from the cholesteric birefringence and finger print texture observed for

4

CNC-SA, a “cross-hatch” patterned texture was observed for CNC-AA above 2.75 %vol. A

5

similar pattern was previously observed by Jun Araki et. Al, which was described as a

6

“birefringent glassy phase”.44. In their work, CNCs is syhthesized by HCl-hydrolysis, and

7

sulfated afterwards to plant sulfate group on the surface postsulfating the HCl-hydrolyzed CNCs,

8

This phase behavior was considered to be immobilized by strong long-range repulsion forces that

9

arise from surface charge and possesses long term stability. Cross polarized microscopy of a

10

droplet of 0.788 %vol dispersion with a polarized microscope in Figure 4 shows obvious

11

birefringence on the droplet edge. Rotating the CNCs orientation with respect to the analyzer

12

polarization results in a transition from bright to dark, which is evidence of CNC alignment.

13

This behavior is also observed with greater prominence at higher concentrations of 2.1 and 3.85

14

%vol, shown in Figure 4b, c. This edge birefringence is observed in isotropic and biphasic

15

systems61,62 63 and is attributed to capillary flow that causes free nanorods to accumulate at the

16

interface, resulting in a concentration gradient. The transparent area between the birefringent

17

texture and the interface (arrow in Figure 4a) is an isotropic phase that results from the nanorod

18

accumulation.

19 20

To emphasize the orientation and alignment of the CNC-AA assemblies, a first-order retardation plate was applied as shown in Figure 5, where the birefringent texture of orientated


18

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CNC-AAs is enhanced. The blue areas indicate predominant orientation along the retardation

2

slow axis; yellow areas indicate orientation perpendicular to the retardation slow axis. Figure 5

3

shows mixed colors within the ‘cross-hatch’ texture indicating that the cross-hatch pattern is not

4

reflecting a single directionally aligned structure, but is rather a more complex polydomain

5

structure without continuous long-range order.

6

7


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1

Figure 4. Cross polarized microscope images of CNC-AA suspension droplet in 120 µm spacer

2

with different angles. Scale bar: 100 µm. (a) 0.788 %vol; (b) 2.1 %vol; (c) 3.85 %vol.

3

4

5

Figure 5. Polarized images with first order retardation plate of 3.38 %vol CNC-AA dispersion at

6

0° and 90° orientation with respect to the first order retardation plate slow axis (Arrow shown in

7

the figure). Scale bars: 200 µm.

8


20

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Figure 6. Cross polarized microscopy images of a) CNC-AA dried film under 10X

2

magnification (Scale bar: 50µm); b) CNC-AA dried film under 63X magnification (Scale bar:

3

10µm)

4 5

6


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Page 32 of 47

1

Figure 7. SEM images of the CNC-AA dried film fractured cross sections (a,b, c, d) and

2

upper surface (e, f). Images a, b) of the fractured cross section inline the film plane; c, d)

3

are images of the fractured cross section perpendicular to the film plane; e, f) Images of

4

the film upper surface.

5 6

In order to elucidate the relation between the cross-hatch birefringent texture and the

7

assembled structures, CNC-AA dispersions were dried into films. The CNC-AA dried films were

8

prepared by slow evaporation in order to minimize interfacial drying effects and external shear

9

forces. Figure 6 shows that the cross-hatch birefringent texture is maintained in the dried film at

10

low magnification (10X), consistent with the aqueous dispersion. Magnification at 63X (Figure

11

6b) shows that the detailed cross-hatch textured pattern is lost and a fine-scale version of the

12

thread-like Schlieren texture appears.64 Local birefringence domains on the order of 0.8~1.5 µm

13

in size are distributed across the image and this texture appears across the entire CNC-AA dried

14

film. This fine-scale Schlieren texture has been reported in thermotropic random co-polyesters

15

by Windle et al.46 Though still under debate, some characterization results by Windle suggest

16

optical biaxial orientation in the microstructure as the cause of this special texture. The co-

17

polyester molecular domains assemble in layers stacked together to form the thin film. Although

18

the molecules with each layer are aligned locally, the orientations of the individual layers normal

19

to the substrate are not particularly correlated. For the CNC-AA system, optical microscopy

20

does not provide sufficient evidence to claim this texture as a nematic orientation structure, thus

21

a closer look of the film structure is needed. The optical microscope resolution limits (on the

22

order of 1 µm), prevent identification of obvious point singularities in the birefringent image.

23

Thus, scanning electron microscopy (SEM) was applied for higher resolution examination.


22

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1 2

SEM imaging of the film cross section with a curved fracture was used to visualize the

3

microstructure and correlated network in focus at two angles of orientation and on the film upper

4

surface, as depicted in Figure S6 in Supporting Information. A cross section imaging direction

5

in-line with the film plane shows stacked layers of CNC-AA (Figure 7a). A similar stacked

6

structure was also observed in fractured films of microfibrillated cellulose (MFC) nanofiber (50

7

wt%) composites in glycerol (25 wt%) and amylopectin (25 wt%) matrix.65 Imaging of the cross

8

section perpendicular to the film plane (Figure 7b) shows protruding leaf-like flat pieces that

9

overlap in a layered structure. Figure 7c shows the CNCs orientation on the surface of the dried

10

film. Unlike biaxial assembly structure explained by Windle et al46, a high density of

11

disclinations are observed in the CNC film surface layer, which indicates low orientation order

12

along the surface plane. In a 6 x 6 µm scanning area, several defects are observed, which

13

frustrate orientation and inhibit long-range orientation at this scale. At higher magnification

14

within a 3 x 3 µm scanning area, the CNC alignment can be better observed with oriented

15

domains on the order of 0.5~1 µm in width and length. Interestingly, this domain length scale is

16

consistent with the local birefringence feature sizes observed in the polarized microscopy

17

images.

18 19

This local alignment of CNCs has not been identified in prior literature and we hypothesize

20

that the concentrated CNC-AA dispersions possesses similar alignment and orientation as the

21

dried film due to the consistent cross-hatch birefringence texture observed in the dispersion and

22

dried film. From a colloidal phenomena standpoint, we propose that the particle – particle

23

interactions and dispersion forces are adequate to afford colloidal assembly but are insufficient to


23


Page 34 of 47

1

result in long range order observed in true liquid crystals. Due to the lower surface charge and

2

electrostatic repulsive force, the microstructure is likely more strongly flocculating (aligned)

3

assembly. Similar behavior was observed by Lagerwall et al. with bacterial cellulose and was

4

referred to as a supramolecular polymerization of lyotropic liquid crystals.14 With the onset of

5

microstructure formation and lateral assembly, the aspect ratio of the aligned domains decreases

6

compared to the single nanorods, which prevents longer range alignment. The cross-hatch

7

texture thus arises from the layered structure with non-correlated orientation of the individual

8

domains. We postulate that as polarization is rotated for the concentrated suspensions with

9

stacked layers of CNC-AA assemblies (demonstrated by SANS), some cross-hatch areas go dark

10

and others become bright in appearance, which indicates the different optical axis direction

11

correlated with different layer orientation.

12

4. Shear Response

13

Owing to the anisotropic geometry of the CNC rods, application of shear force will further

14

induce alignment in the direction of shear. The extent of alignment is driven by the magnitude of

15

the shear rate as well as the aspect ratio and interparticle interactions. It has been reported that a

16

birefringent glassy phase can possess “frozen-in” shear texture where the birefringence generated

17

by shear will sustain for more than 2 days after the shear ceased.38 In Figure 8, this phenomenon

18

is observed for CNC-AA in the Linkam shear stage. At 0.30 %vol (Figure 8a), the birefringence

19

brightness is enhanced immediately with shear application (50 s-1 shear rate) indicating nanorod

20

alignment in the shear direction. Cessation of the shear results in birefringence relaxation back to


24

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1

the original dark color in less than 60 seconds. However, at a concentration of 0.788 %vol

2

(Figure 8b) and above the shear induced birefringent texture is observed and remains unchanged

3

for more than 1 hour after shear cessation. The CNC-AA dispersion loses its ability to relax, and

4

the frozen-in glassy phase is maintained. The shear induced order can however be disrupted by

5

sonication of the sample.

6

7

Figure 8. Polarized microscope time elapsed imaging of the CNC-AA dispersion with 45o angle

8

between shear direction and analyzer/polarizer: a) 0.3 %vol, shear rate at 50 s-1 at 0, 30 and 60

9

seconds after shear cessation; b) 0.788 %vol, shear rate at 50 s-1, at 0, 60, and 600 seconds after

10

shear cessation. Scale bar: 100 µm.


25


Page 36 of 47

1

Figure 9 shows the birefringent texture pattern in the glassy phase after applying shear to the

2

suspension at various concentrations and shear rates on the shear stage. This illustrates increasing

3

image brightness with increased shear as a result of increased alignment as observed on the

4

macro scale. Rotating the sample stage such that the shear direction is parallel to the analyzer

5

filter axis blocks the bright birefringence of the oriented phase and results in the appearance of

6

the cross-hatch texture which indicates a coexistence the fine-scale nematic with the oriented

7

phase. At 0.788 %vol. (Figure 9 a) and 10 s-1 shear rate, the cross-hatch is observed with the

8

shear direction both perpendicular (not shown) and parallel to the analyzer axis. Increasing the

9

shear rate to 50 s-1 eliminates the cross-hatch, giving uniform birefringence that is brighter than

10

10 s-1 with 0°, 45°, and 90° shear/analyzer orientations. At 2.1 and 3.85 % vol (Figure 9 b, c), the

11

cross-hatch pattern is sustained at all shear rates and most noticeably observed with 0° and 90°

12

(not shown) shear orientation to the analyzer polarization. The cross-hatch pattern does not

13

appear at 45o shear direction w.r.t. analyzer, may be due to the layer by layer packing structure.

14

When shear is applied to the suspensions, upper layers will experience the greatest shear and be

15

forced to align, while lower layers will experience decreased shear, due to slipping between

16

layers. This could result in a coexistence of oriented regions with varying alignment, where the

17

highly aligned regions, presumably the upper layers, give the most intense birefringence are

18

filtered at 0° and 90° orientations to the analyzer polarization. Thus, revealing the weaker cross-

19

hatch birefringence pattern of the lower layers that experience decreased shear and do not align.

20


26

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2


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Page 38 of 47

1

2

Figure 9. Comparisons of polarized optical micrographs of CNC-AA aqueous suspension during

3

steady shear test. (a) concentration at 0.788 %vol; (b) concentration at 2.1 %vol; (c)

4

concentration at 3.85 %vol. Shear rates of 0, 10, and 50 s-1 are presented with the shear stage

5

oriented at 0° and 45o angle between shear direction and analyzer/polarizer. Scale bar: 100 µm.

6 7

Conclusion:

8

CNCs hydrolyzed by HCl and acetic acid produced nanocrystals that are surface modified by

9

acetyl ester groups (CNC-AA), which are larger in size and possess a lower surface charge

10

compared to CNCs hydrolyzed by sulfuric acid and other techniques. The presence of the acetyl

11

ester groups reduces the hydrophilicity of the CNCs due to the decreased surface charge density.

12

This results in stronger interparticle attraction and the onset of phase transitions at lower

13

concentrations. The CNC-AA dispersions in DI water show multiple phase regimes. At 5.0 x 10-

14

4

15

interparticle interactions and a transition from the dilute regime. From 0.0035 to 0.043 %vol,

16

biphasic behavior is observed which consists of a birefringent lower phase and disordered or

17

isotropic upper phase. At even higher concentrations, a transition to a glassy phase is observed

18

which possesses a “cross-hatch” birefringent texture owing to fine-scale nematic orientation.

19

SANS was used to characterize self-assembly formation with increasing concentration, which

%vol. an increase in the apparent diffusion coefficient is observed indicating the onset of


28

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1

occurs in a two dimensional assembly resulting in plate-like layered structures, as confirmed

2

with SEM imaging of dried films.

3

The cross-hatch texture is observed in both the birefringent dispersion and dried films,

4

which suggests preservation of order on drying. Visualization by SEM indicates a basic unit of

5

CNC-AA microstructure in the film consists of 0.5-1.5 µm scale aligned nanorod domains,

6

which is consistent with the dimensions of each individual “hatch” in the birefringent texture

7

observed by cross polarized microscopy. Cross-section SEM of the dried CNC-AA film indicates

8

a layered sheet-like structure. This characterization of the film structure can be defined as a fine-

9

scale nematic texture, which has been observed in other systems but not well characterized.

10

The anisotropic geometry of the CNC leads to shear induced orientation. Depending on

11

the magnitude of the shear, the fine-scale nematic can be aligned in the direction of shear, giving

12

uniform birefringence under high shear while exhibiting a coexistence of textures under low

13

shear. At low concentrations, the shear induced orientation can relax back to the original state,

14

due to the high particle diffusivity. At higher concentrations, a frozen-in (glassy phase) shear

15

behavior is observed, which does not show quick relaxation and maintains orientation for more

16

than an hour after cessation of shear. Furthermore, at higher concentrations, the fine-scale

17

orientation persists at higher shear rates. Compilation of these results provides a correlation

18

between the interparticle interactions, macro scale phase behavior, aggregation process, shear

19

response and resulting order within a dried film. Another significant outcome of this work is the


29


1

identification of the particulate orientation and structure that correlates with the cross-hatch

2

birefringent texture and fine-scale nematic order.

3

ASSOCIATED CONTENT

4

Supporting Information.

Page 40 of 47

5 6 7 8 9 10

Additional cross polarized microscopy of CNC suspension droplets to demonstrate the visualization of agglomerates in suspensions right after hydrolysis, before optimization of sonication process and after optimization of sonication process. Thermogravimetric curve of acetylated cellulose nanocrystals for thermal stability characterization. Atomic force microscopy and transmission electron microscopy images for CNC size measurement and distribution histograms.

11

AUTHOR INFORMATION

12

Corresponding Author

13

Christopher L. Kitchens

14

Phone (864) 656-2131. E-mail [email protected].

15

Present Addresses

16

127 Earle Hall

17

Department of Chemical and Biomolecular Engineering, Clemson University, SC, 29634

18 19

Funding Sources

20

Funding was provided by National Science Foundation, grant #NSF CMMI-1131633; CMMI-

21

1130825 and CMMI-1563435.

22

Notes

23

ACKNOWLEDGMENT


30

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1 2 3 4 5 6

Biomacromolecules

We acknowledge the Clemson Light Imaging Facility at Clemson University for optical microscope support and Electron Microscopy Laboratory at Clemson University for TEM support. We also acknowledge George Chumanov’s group in the Chemistry Department at Clemson University for Atomic Force Microscope support. Alexander Haywood is acknowledged for his assistance with initial experiments. We thank Dr. Lilin He of ORNL for the help with SANS experiments at GP-SANS.

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Table of Content Title: Phase Behavior of Acetylated Cellulose Nanocrystals and Origins of the Cross-hatch Birefringent Texture Authors: Mingzhe Jiang, Matt McMillan, Virginia Davis, and Christopher L Kitchens 308x113mm (150 x 150 DPI)



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Phase Behavior of Acetylated Cellulose Nanocrystals and Origins of the Cross-hatch Birefringent Texture

Mingzhe Jiang1, Matt McMillan1, Virginia Davis2,* and Christopher L Kitchens1,* 1

2

Department of Chemical and Biomolecular Engineering, Clemson University

Department of Chemical Engineering, Auburn University

Table of Content


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Phase Behavior of Acetylated Cellulose Nanocrystals and Origins of

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