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Mimicking the Humidity Response of the Plant Cell Wall by Using 2D Systems: The Critical Role of Amorphous and Crystalline Polysaccharides Elina Niinivaara, Marco Faustini, Tekla Tammelin, and Eero Kontturi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04264 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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Mimicking the Humidity Response of the Plant Cell Wall by Using 2D Systems: The Critical Role of Amorphous and Crystalline Polysaccharides Elina Niinivaara†, Marco Faustini‡, Tekla Tammelin§, Eero Kontturi*† †

Materials Chemistry of Cellulose, Department of Forest Products Technology, Aalto University, 02150, Espoo, Finland

‡

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005, Paris, France §

VTT – Technical Research Center of Finland, High Performance Fibre Products, 02150, Espoo, Finland

KEYWORDS biomimetics, cellulose, thin films, quartz crystal microbalance, ellipsometry

ABSTRACT. Of the composite materials occurring in nature, the plant cell wall is among the most intricate, consisting of a complex arrangement of semi-crystalline cellulose microfibrils in a dissipative matrix of lignin and hemicelluloses. Here, a biomimetic, 2-dimensional cellulose system of the cell wall structure is introduced where cellulose nanocrystals compose the crystalline portion and regenerated amorphous cellulose the dissipative matrix. Spectroscopic ellipsometry and QCM-D are used to study the water vapor uptake of several two layered systems. Quantitative

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analysis shows that the vapor-induced swelling of these ultrathin films can be controlled by varying ratios of the chemically identical ordered and unordered cellulose components. Intriguingly, increasing the share of crystalline cellulose appeared to increase vapor uptake but only in cases for which the interfacial area between crystalline and amorphous area was relatively large and the thickness of an amorphous overlayer was relatively small. The results show that a biomimetic approach may occasionally provide answers as to why certain native structures exist.

1. Introduction Nature has been a source of inspiration for scientists for an age and as a result, the biomimetic approach to materials science has led to the development of numerous advanced materials. The idea of ultra-strong yet light weight composite materials1, controlled adhesion2 and self–cleaning surfaces3 for example, all originate from naturally occurring structures. Nature has an unmatched ability to adapt and thrive in varying circumstances by proficiently tuning the chemistries, structures, morphologies and even the sensitivity of individual components. Biomimetics strives to develop novel materials by replicating these finely tuned characteristics to produce applications with supreme properties, examples of which include tissue engineering4, controlled release5, adhesives6 and catalysts7 among others. As one of the most primitive and abundant natural composite materials, the plant cell wall provides an attractive structure for biomimetics as it is a mechanically strong and resilient hierarchical mixture of three biopolymers: cellulose, hemicelluloses and lignin. In the cell wall, cellulose exists in the form of semi-crystalline microfibrils, which contribute to its mechanical strength, and typically makes up ca. 40 % of the plant cell wall composition, while the amorphous


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hemicellulose and lignin components act as a dissipative matrix, accounting for ca. 60 % of the cell wall material (Figure 1b).8,9

Figure 1. Schematic representation of the biomimetic approach of this study with a) model of the layered structure of the plant cell wall, b) rough model of the arrangement of the cellulose microfibrils, hemicelluloses and lignin in the primary cell wall, c) the regeneration (or hydrolyzation) of TMSC to amorphous cellulose and d) a matrix of the samples studied; areal masses have been deduced from QCM-D data (Table S1). Due to the complexity of natural systems, biomimicry often focuses on a single aspect of the bigger picture, such as the structural hierarchy in ultra-strong composites1 or the morphological intricacies of highly adhesive surfaces.2,6 In light of this, a tempting angle from which to study the plant cell wall for biomimetic purposes is to focus on its hierarchical structure. In this study, however, a model system of crystalline and amorphous polysaccharides is used to examine their response to water vapor. The hierarchical morphology, in turn, is reduced to the 2-dimensional realm of ultrathin films.


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This fundamental study provides insight into the water vapor adsorption behavior of a cell wall like system; ultrathin films consisting of two consecutively deposited layers of highly crystalline and highly amorphous cellulose. Whilst this system mimics the physical structure of the cell wall it eliminates all chemical differences between the polysaccharides, allowing for a controlled study of the role of the ratio between the crystalline and amorphous components. Although chemically identical, the cellulosic materials differ significantly in that regenerated amorphous cellulose swells abundantly in water10,11 whereas crystalline cellulose is impenetrable by water,12,13 providing a perfect contrast to the water response of the two layers. The objective of the study is to investigate whether the swelling response of these biomimetic films can be tuned by varying the ratio of amorphous and crystalline celluloses, a vital piece of information when designing, for example, sensor materials14 or actuators15 for custom made functionalities. Six two layered systems were investigated in this study. They can be separated into two categories that differ by their fixed amounts of amorphous cellulose (regenerated from trimethyl silyl cellulose (TMSC)) in the film (Figure 1d). The first category is one in which the amorphous component is fairly low with an approximately 0.3, 1.2, or 1.9 µg cm-2 cellulose nanocrystal (CNC) layer embedded in a 2.4 μg cm-2 amorphous cellulose film. In the second category the portion of the amorphous component is much larger than the crystalline with an approximately 0.3, 1.2, or 1.9 µg cm-2 CNC layer embedded in a 8.4 μg cm-2 regenerated amorphous cellulose film (Figure 1d). CNCs are rigid nanosized rods isolated from the cellulose microfibril using a strong acid hydrolysis16-18 and represent the crystalline region of the plant cell wall. In contrast, cellulose regenerated from TMSC by an acid vapor treatment19,20 (Figure 1c) has been found to be highly amorphous,10 and as such represents the amorphous regions of the plant cell wall. The results revealed that water–induced swelling is highly distinct for the system where the amorphous–


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crystalline ratio is close to that occuring in plant cell walls. The swelling was observed with spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation monitoring (QCM-D) which provide sub-nanometer21 and nanogram22 precision, respectively, when observing the changes due to vapor uptake. The results revealed that the amorphous-crystalline ratio is certainly not the only factor governing vapor-induced swelling even in a very simplified system.

2. Experimental Section Materials. Water was purified in a Milli-Q system (Millipore Corporation, resistivity 18.2 MΩ cm). Dichlorodimethylsilane (≥ 98.5%) was purchased from Aldrich Chemistry (Germany) and used as received. Toluene (99.9%) were purchased from VWR Prolab Chemicals (France) and used as received. 37% hydrochloric acid was also purchased from VWR Prolab Chemicals (France) but was diluted to 2M. The materials used for TMSC synthesis were cellulose powder (from spruce), anhydrous lithium chloride (≥ 98.0%) and hexamethyldisilazane purchased from Fluka Analytics (Germany) and used as received. Dimethylacetamide (≥ 99.9) and tetrahydrofuran (≥ 99.5%) purchased from Sigma-Aldrich (USA). Methanol (100%) purchased from VWR Prolab Chemicals (France) and used as received. CNCs were prepared from ashless Whatman filter paper (Whatman, GmbH, Dassel, Germany). Sulfuric acid (H 2 SO 4 ), sodium chloride (solid) (NaCl) and 0.1 M sodium hydroxide (NaOH) were purchased from Sigma-Aldrich Finland Oy, Helsinki, Finland. Lithium chloride (solid) (LiCl), magnesium chloride hexahydrate (solid) (MgCl 2 ·6H 2 O), magnesium nitrate hexahydrate (solid) (Mg(NO 3 ) 2 ·6H 2 O) and potassium sulfate (solid) (K 2 SO 4 ) were purchased from VWR Chemicals (VWR International, Helsinki, Finland). Ethanol (Aa grade 99.5 % w/v) was purchased from the Altia Corporation (Rajamäki, Finland).


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CNC preparation. CNCs were prepared from cotton-based Whatman 1 filter paper using an established procedure with a 64 w-% (45 °C, 45 min) sulfuric acid hydrolysis.23 10 g of finely ground Whatman filter paper was hydrolyzed in 175 ml of 64 % w/w sulfuric acid at 45 °C for 45 min. Hydrolysis was quenched by the addition of 1800 ml milliQ water. The resulting suspension was then centrifuged to remove all of the excess water, after which the CNCs were dialyzed using milliQ water until their conductivity was < 5 µS cm-3. The dialyzed CNC dispersion was then counter-ion exchanged by adding 0.1M [OH-] sodium hydroxide until the dispersion reached pH 7. The dispersion was then freeze dried. The dry CNCs were then Soxhlet extracted with ethanol for 48 h, using a modified version of a method originally reported by Labet and Thielemans,24 in order to remove any surface impurities. After purification the CNCs were oven dried at 80 °C. The surface charge of CNCs (due to sulfate esters on their surfaces) was determined by conductometric titration to be 0.6 e nm-2. TMSC preparation. TMSC (degree of substitution 2.2) was synthesized and characterized according to a method described in detail elsewhere.20 The TMSC had an average molecular weight of 215000 g mol-1, and a polydispersity of 3.3 as characterized by size exclusion chromatography using chloroform with 2% triethyleneamine as eluent; elution speed was 1ml/min through the following system: PLgel precolumn and PLgel, 104, 105, 103, and 102 Å columns supplied by Polymer Laboratories; relative changes were determined with a Waters RI-detector (refractive index) against polystyrene standards at 35ºC. Thin film preparation. The two layered cellulose thin film samples were prepared using a WS-650SX-6NPP/LITE spin coater (Laurell Technologies Corporation, North Wales, PA, USA). Before spin coating, the substrates were cleansed in an UV Ozone cleaner (Bioforce Nanosciences Inc., California USA) for a minimum of 15 min. Prior to spin coating the CNC films, the substrate


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was rinsed twice with milliQ water. After rinsing, a volume of CNC solution sufficiently large enough to cover the substrate was dropped onto the surface of the substrate after which spin coating was carried out at 3000 rpm with an acceleration of 2130 rpm s-1 for approximately 60 sec. Before spin coating, the CNC dispersions were subjected to a 15 min ultrasonic disruption to ensure homogeneity throughout the solutions. The duration of coating was determined by the time required for the disappearance of the Newtonian rings (approximately 30 sec) with an additional 30 sec drying time. In order to anneal the CNCs to the substrate surface, the freshly spin coated samples were placed into an 80 ºC oven for 15 min. TMSC was then spin coated on top of the annealed CNC layer (using the same spin coating parameters as mentioned above) after which it was hydrolyzed (regenerated) back to cellulose. Hydrolyzation was carried out in acid vapor for 120 sec in a vacuum sealed desiccator with 2M HCl (Figure 1c).19,20 Six samples were prepared, three samples with an underlying film of CNC spin coated from solutions with concentrations of 5, 10 and 20 g l-1 underneath a regenerated cellulose film from a 10 g l-1 TMSC solution (resulting in a 2.4 μg cm-2 film of regenerated amorphous cellulose as determined by QCM-D) and three samples with the same CNC films underneath a regenerated cellulose film from a 20 g l-1 TMSC solution (resulting in an 8.4 μg cm-2 film of regenerated amorphous cellulose as determined by QCM-D). Films were prepared on 2 × 2 cm2 pieces cut from a silicon wafer (Si 100 with native oxide layer from Okmetic, Vantaa, Finland) for the SE measurements and AT–cut silicon dioxide coated QCM–D sensors (Q–Sense, AB, Gothenburg, Sweden) with a fundamental resonance frequency (f 0 ) of 5 MHz for the QCM–D measurements. Spectroscopic ellipsometry. SE measurements were performed on a UV–visible (from 401.39 to 998.85 nm) variable angle spectroscopic ellipsometer (VASE–2000U) from Woollam, and data analyses were performed with the Wvase32 software using a Cauchy model. A sample was placed


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inside an atmospheric chamber onto a sample plate and a beam of polarized light was reflected off the surface and focused and aligned into the light collector. To start, a spectroscopic data measurement was carried out in order to determine the initial theoretical thickness and refractive index of the film by using the Cauchy model. To validate the use of the classic Cauchy model, the measured refractive indices were fitted to model data (Figure S2 – S4). The good fit between the model and the experimental data also confirms the optically isotropic nature of the CNC films previously reported by Cranston et al.25 The RH cycle was then started and the ellipsometric measurements were performed dynamically throughout the duration of the cycle. Each RH cycle lasted for ca. 45 min; 20 min for water vapor adsorption, 1 – 2 min stabilization at the highest RH and 20 min for desorption (RH was incremented by 2% RH every 20 sec). To attain the desired RH, a continuous flux of air containing a fixed partial pressure of water was injected in the atmospheric chamber and the real RH was measured using a humidity sensor. During the measurements, the chamber temperature was fixed at 23 °C and was continuously filled by 3 dm3 min-1 flux of air with a controlled partial water pressure. Swelling of the CNC films was assessed by plotting the normalized thickness variation of the film as function of the measured RH in the closed chamber.26 Normalization was done in order to be able to provide a more complete comparison of the effects of water vapor on the films and was done so in respect to the initial thickness (h i ) of the CNC film (normalized Δthickness= (h – h i )/h i ). Quartz crystal microbalance with dissipation monitoring. The initial areal mass of each spin coated double layer cellulose film system was determined as described elsewhere27 by determining the frequency response in air (normal atmospheric conditions) before and after film deposition on the QCM–D sensor substrate. The collected frequency data was then stitched together using


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QTools software and the areal mass was calculated according to the Sauerbrey equation28 (Equation 1) ∆𝑚𝑚 = −𝐶𝐶

∆𝑓𝑓

(1)

𝑛𝑛

where Δf = f – f 0 is the resonance frequency, n is the overtone number (n = 1, 3, 5, 7…) and C is a constant which describes the sensitivity of the device to changes in mass (C ≈ 0.177 mg m-2 Hz1

). Prior to the mass change determinations, each sample was first dried in an oven at 80°C for

15 min. The thin film systems were each stabilized overnight (ca. 18 h) inside the humidity module at 11% RH by passing a saturated solution of LiCl through the module at a rate of 100 µl min-1.29 After stabilization, water vapor adsorption experiments were carried out in six steps by gradually increasing the RH within the chamber using 5 different saturated salt solutions and milliQ water (Table S2). Each saturated salt solution was passed through the QHM 401 humidity module at a rate of 0.1 ml min-1 for 30 min at 23 °C. The QCM-D device is able to detect both changes in frequency (Δf) and dissipation (ΔD) using several overtones simultaneously (Q-Tools Software) however, for the sake of comparability, mass change analyses as a result of water vapor adsorption were performed on experimental values taken from the third overtone (15 MHz) of each measurement. In addition, frequency change and dissipation change data from all of the overtones was compared to gain more information on the viscoelastic properties of the thin films when exposed to different RH levels. The change in dissipation energy (ΔD = D – D 0 ) is defined as (Equation 2): 𝐷𝐷 =

𝐸𝐸𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

(2)

2𝜋𝜋𝐸𝐸𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

where E diss is the total energy dissipated during one oscillation cycle and E stor is the total energy. Qualitatively, it is a measure of the softness or rigidity of the film adhered onto the sensor surface.


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A thin film can be considered fully elastic and rigid when ΔD ≤ 1 × 10-6 and no clear spreading of the overtones in the Δf and ΔD graphs can be detected. Under such conditions the Sauerbrey equation is valid to estimate the mass changes on the sensor surface. Atomic force microscopy. Sample morphologies were characterized using a MultiMode 8 scanning probe microscope by Bruker AXS Inc. (Madison, WI, USA). Images were taken using an E scanner in tapping mode with NSC15/AIBS silicon cantilevers by Ultrasharp μmasch (Tallinn, Estonia). According to the manufacturer the radius of curvature of the tips were less than 10 nm with a typical cantilever resonance frequency of 325 kHz. A minimum of four images were taken per sample. Other than a simple flattening, no image processing was carried out. AFM images were analyzed using Nanoscope Analysis 1.20 and Scanning Probe Image Processor (SPIP) 6.0.6 softwares.

3. Results and discussion Figure 2 shows atomic force microscopy (AFM) height images of representative samples of each of the components in the two layered cellulose systems studied. Upon spin coating, CNCs form optically isotropic,25 uniform films in which voids are present between the individual crystals of the network, both of which can clearly be distinguished in AFM images (Figure 2a). Regenerated amorphous cellulose, on the other hand, forms smooth films from which no distinct morphologies can be determined (Figure 2b).


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Figure 2. 1 × 1 μm2 AFM height images of representative films of a) CNCs and b) regenerated amorphous cellulose.

The morphologies of all films prepared by combining CNCs and amorphous cellulose layers strongly suggested that the amorphous cellulose was not a discrete layer on top of the CNCs; instead, it appeared to fill in the voids in the CNC layer below and leave the CNCs embedded in the amorphous material, much like in the plant cell wall itself. In the samples with a lower amount of amorphous component, the two layered systems with a 0.3 and 1.2 µg cm-2 CNC film underneath the 2.4 μg cm-2 amorphous cellulose film, the morphologies resembled that of a pure CNC film (Figure 3a and 3b, respectively). However, there was a significant shift in the system when the thickness of the underlying CNC layer was increased to 1.9 µg cm-2 and the morphology resembled more that of a typical regenerated amorphous cellulose film (Figure 2b). With increasing CNC dispersion concentration, the porosity of the CNC network decreased; the porosities of the underlying CNC films were 27, 21 and 5 vol-% for the 0.3, 1.2, and 1.9 µg cm-2 CNC films, respectively, as shown in a previous study with ellipsometry.30 AFM images revealed that a 0.3 µg cm-2 CNC film spin coated from a 5 g l-1 dispersion did not form a fully covering film on the surface of the substrate unlike dispersions of higher concentrations. A 10 g l-1 CNC dispersion


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formed a 1.2 µg cm-2 thick uniform and fully covering network whereas a 20 g l-1 CNC dispersion formed a very densely packed network with overlapping layers of CNCs with an areal mass of 1.9 µg cm-2 (Figure S1). Evidently, when the porosity of the underlying CNC network is sufficiently low, less TMSC is required to fill the voids of the CNC network structure, leaving enough polymer to form a smooth film atop the CNC network (Figure 3c). However, when the porosity is high, more TMSC material is required to fill the voids in the CNC network structure during spin coating, resulting in a top layer of regenerated amorphous cellulose that conforms to the topography of the underlying CNC film. The larger the porosity of the CNC network, the more evident its morphology underneath the regenerated cellulose film. Due to the shear gradients present during spin coating, CNCs roughly align on the substrate radially31-33 and although the individual crystals of the 1.9 µg cm-2 CNCs network were indistinguishable underneath the amorphous cellulose layer, a clear orientation could still be seen in the systems morphology. This slight orientation could in fact be seen for all of the samples with a 2.4 g μg cm-2 amorphous cellulose layer. When a significantly thicker top layer of amorphous cellulose is present (8.4 μg cm-2), all evidence of the underlying CNC layer disappears (Figure 3d – df). The morphologies of all three of the two layered films in this category resembled that of a typical regenerated amorphous cellulose film (Figure 2b). A quantitative idea of the films is presented in the ellipsometry results of Table 1. CNCs and amorphous cellulose form a bottom layer with an overlayer of homogeneous amorphous cellulose on top.


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Figure 3. 3 × 3 μm2 AFM height image of each two layered cellulose system, a) 10 nm CNC, b) 15 nm CNC and c) 26 nm CNC films all underneath a 2.4 μg cm-2 regenerated amorphous cellulose film and d) 10 nm CNC, e) 15 nm CNC and f) 26 nm CNC films all underneath a 8.4 μg cm2

regenerated amorphous cellulose film.


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Table 1. Thickness of the mixed films of CNCs and amorphous cellulose and their individual components as measured by SE at 0% RH. Initial thickness Initial thickness of Thickness of of filmb underlying CNCc overlying amorphous CNC/amorphous layer cellulosea (nm) (nm) (nm) 0.3 / 2.4 34 10 24 1.2 / 2.4 36 15 21 1.9 / 2.4 43 26 17 0.3 / 8.4 1.2 / 8.4 1.9 / 8.4

64 70 83

10 15 26

a

Areal mass ratios deduced from QCM-D data (Table S1)

b

Measured by SE before the deposition of amorphous cellulose

c

54 55 57

Thickness of amorphous cellulose above the mixed CNC/amorphous matrix; deduced by

subtracting the CNC layer thickness from total film thickness Although the morphologies of the systems with a 2.4 μg cm-2 regenerated amorphous cellulose layer and an underlying 0.3 and 1.2 µg cm-2 CNC film resembled that of a pure CNC film, their swelling behaviors (Figure 4b and 4c), as seen by spectroscopic ellipsometry (SE), were almost identical to that of the 2.4 μg cm-2 amorphous cellulose film (Figure 4a). SE allows for the precise monitoring of minute changes occurring in the two layered system with increasing relative humidity (RH). It is a spectroscopic technique used to monitor changes in thin film refractive index upon water vapor adsorption. Using the classic Cauchy model, these monitored changes can then be modeled to provide information on changes in film thickness with increasing RH (see Supporting Information for raw SE data and data fittings to the Cauchy model).26 The extent of swelling in these two layered systems, 43% and 49% respectively, were almost identical to that of the 2.4 μg cm-2 amorphous cellulose reference film, which had a swelling ratio of 1.5 upon hydration (100% RH in the case of the SE measurements) – swelling ratio being the ratio between


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the hydrated thickness and the initial (dry) thickness of the film. For the amorphous cellulose reference films, the extent of the normalized change in film thickness was dependent on the initial thickness of the film; the thicker the film, the larger the degree of swelling upon hydration, as was expected (Figure 4a and 4e).34

Figure 4. Normalized change in thickness of both amorphous reference thin films and each two layered cellulose system as a function of relative humidity during SE measurements, a) 2.4 μg cm-2 regenerated amorphous cellulose and e) 8.4 μg cm-2 regenerated amorphous cellulose and b) 0.3 µg cm-2 , c) 1.2 µg cm-2 and d) 1.9 µg cm-2 CNC films all underneath a 2.4 μg cm-2 regenerated amorphous cellulose film and f) 0.3 µg cm-2 , g) 1.2 µg cm-2 and h) 1.9 µg


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cm-2 CNC films all underneath a 8.4 μg cm-2 regenerated amorphous cellulose film. Swelling ratios (± 0.1%) for each sample are also shown. However, it would appear that for the system with a 1.9 µg cm-2 CNC layer underneath the 2.4 μg cm-2 amorphous cellulose layer, the presence of the CNCs in fact promoted swelling of the entire system, as can be seen with the significant increase in swelling ratio to 1.7 (Figure 4d). Previously, it has been shown that upon hydration, the individual crystals in the CNC network become encompassed with 3 monolayers of water, causing a film thickness increase of ca. 2 nm per layer of CNCs in the network.30 The diameter of a single CNC (ca. 7 nm)35 can be used to approximate the thickness of a single layer of CNCs in the network. As such, the 10, 15 and 26 nm CNC networks consist of ca. 1, 2 and 3 layers of CNC respectively resulting in a thickness increase of 2, 4 and 6 nm upon hydration. As a result, if the CNC layer promoted the extent of swelling in each system, a systematic increase in the swelling ratio would be detected with an increasing number of CNC layers. No such increase was seen in the extent of swelling between the systems with 1 and 2 CNC layers in the underlying film. However, a significant jump in the swelling ratio was detected in the system when a third CNC layer was introduced. Based on these results, it can be assumed that when embedded in a relatively thin layer of amorphous cellulose, a sufficiently thick layer of CNCs will promote the swelling of the system. When

the

amount

of

amorphous

cellulose

was

tripled

and

thus

the

crystalline to amorphous ratio became considerably less than that of the plant cell wall, the trend in the swelling behavior of the systems was reversed. From the results it can be seen that where the swelling ratios of the samples with a 2.4 μg cm-2 amorphous layer increased with increasing initial thickness, the opposite was true for the second category of systems. However, the degree of swelling of the films with an underlying 0.3 µg cm-2 and 1.2 µg cm-2 CNC layer was identical to


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that of the 8.4 μg cm-2 amorphous cellulose reference sample with a swelling ratio of 1.7 (Figure 4e, 4f and 4g). Interestingly however, with increasing initial thickness of the system, the extent of swelling decreased. A more noticeable inhibition of swelling came into play when the CNC layer was sufficiently thick, as can be seen by the significantly decreased swelling ratio (1.6) of the system with a 1.9 µg cm-2 CNC layer underneath 8.4 μg cm-2 amorphous cellulose (Figure 4h). Swelling of the 8.4 μg cm-2 amorphous cellulose film was decreased by ca. 5.5% upon the addition of the underlying 1.9 µg cm-2 CNC film. It would seem here that within a very narrow window of opportunity, it is possible to control the swelling behavior of these predominantly amorphous two layered systems. The comparability of the two layered systems to their respective references may not yield any vital information due to their distinctly different behavioral patterns upon water vapor adsorption as the introduction of the CNC layer into the system provides it with an unexpected complexity. To complement the SE findings on the behavior of the double layered cellulose systems, their ability to uptake water vapor mass was studied using a quartz crystal microbalance with dissipation monitoring (QCM–D). The mass of water vapor adsorbed by the system is inversely proportional to changes in resonance frequency (Δf) detected by the QCM–D through the Sauerbrey equation (Equation 1).36 Interestingly, for the cell wall -like samples with fairly equal amounts of amorphous cellulose and CNCs it would appear that the underlying CNC layer provides the system with a more pronounced flexibility upon water vapor adsorption. For these three systems, a higher swelling ratio was achievable with less water vapor adsorption compared to the 2.4 μg cm-2 amorphous cellulose reference film. Although the SE results show that the systems tend to swell more than the 2.4 μg cm-2 amorphous cellulose film, the mass of water vapor adsorbed by the two layered systems, was systematically less at hydration. The mass of water vapor adsorbed by the


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systems with 0.3, 1.2, and 1.9 µg cm-2 CNC films embedded in 2.4 μg cm-2 amorphous cellulose, were 0.31, 0.33 and 0.42 mg H 2 O/mg of cellulose, respectively (Figure 5b – 5d) whereas the corresponding value for the 2.4 μg cm-2 amorphous cellulose reference film was 0.45 mg H 2 O/mg of cellulose (Figure 5a) (see Supporting Information for example calculation of mg H 2 O/mg of cellulose). It should be noted that for the QCM–D device, the highest achievable RH without the possibility of water condensation effects was 97% RH; the assumed point of hydration for the two layered cellulose systems. Due to this possible condensation, any changes in Δf or dissipation energy of the sensor above 97% RH are considered exaggerated and as such, unreliable. The water vapor uptake behavior, measured by the QCM–D, of the samples in the second category follow the same trend as seen with the SE. The samples with 0.3 µg cm-2 and 1.2 µg cm2

underlying CNC layers with a 8.4 μg cm-2 amorphous cellulose top layer adsorbed approximately

the same mass of water as did the reference 8.4 μg cm-2 amorphous cellulose film; 0.39 and 0.37 mg H 2 O/mg of cellulose compared (Figure 5f and 5g) to 0.39 mg H 2 O/mg of cellulose (Figure 5e), respectively. As seen previously, when the CNC layer is sufficiently thick underneath the 8.4 μg cm-2 amorphous cellulose film, an inhibition of water vapor uptake occurs; the uptake of water mass upon hydration of this film was ca. 14% less than for the 8.4 μg cm-2 reference film. Swelling of the system could successfully be inhibited by the addition of the underlying CNC layer; the extent of swelling systematically decreased as the fraction of CNC in the system was increased. The significant difference between the water vapor uptake trends of the two sample categories further underlines the fact that the composition of the films plays a key role in the behavior of the samples. For all of the samples, including the reference samples, the mass ratio of adsorbed water vapor to cellulose at hydration point were in the same order of magnitude as reported for cellulose films elsewhere.34,37 Even when compared to values in actual plant-based fibers, the water uptake


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ratios are similar, if a little higher than in the native materials.38,39 The reasons for the consistently higher water uptake in our films in relation to natural fibers may lie in the sulfate groups that create charge on the CNC surface once in contact with water or in the rigid hierarchical construction of the plant cell wall that is likely to suppress the swelling in fibers.

Figure 5. Change in resonance frequency of both amorphous cellulose reference films and each two layered cellulose system as a function of time (and RH) from the QCM–D measurements, a) 2.4 μg cm-2 regenerated amorphous cellulose and e) 8.4 μg cm-2 regenerated amorphous cellulose and b) 0.3 µg cm-2, c) 1.2 µg cm-2 and d) 1.9 µg cm-2 CNC films all underneath a 2.4 μg cm-2 regenerated amorphous cellulose film and f) 0.3 µg cm-2, g) 1.2 µg cm-2 and h) 1.9 µg cm-2


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CNC films all underneath a 8.4 μg cm-2 regenerated amorphous cellulose film. Inset values of mass ratio of adsorbed water vapor to cellulose (± 16%) at hydration point (97% RH). The QCM–D not only provides information on the water vapor uptake of a system but also on the changes in its physical properties upon swelling. The physical properties of the film are monitored through changes in the dissipation energy (ΔD) of the sensor with increasing RH. ΔD is a measure of the viscoelastic properties of the film, the larger the response in dissipation energy and more prevalent the spreading of the measurement overtones, the more viscoelastic the film. From the results, it can be seen that neither of the amorphous cellulose reference films exhibited viscoelastic behavior at any time during the RH cycle, indicating full elasticity (Figure 6a and 6e). The results are contradictory to a recently published study where a notable dissipation change along with a spreading of the measurement overtones due to water adsorption was detected from amorphous cellulose films.37 The cellulose used in this study was from a different source (cotton vs. wood), which may play a role in the distinction. The viscoelastic properties of the two layered cellulose systems are quite interesting; a significant response in ΔD was only seen in the samples in which the thickness of the amorphous layer of cellulose was twice that of the CNC layer (Figure 6b and 6h), however, in the other samples such a clear sign of viscoelastic behavior was not detected. It would seem – that as was the case with tuning the extent of swelling and water vapor uptake of these systems – control of their viscoelastic properties, within the given set of parameters, occurs over a very small range.


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Figure 6. Change in dissipation for both amorphous cellulose reference films and each two layered cellulose system as a function of time from the QCM–D measurements, a) 2.4 μg cm-2 regenerated amorphous cellulose and e) 8.4 μg cm-2 regenerated amorphous cellulose and b) 0.3 µg cm-2, c) 1.2 µg cm-2 and d) 1.9 µg cm-2 CNC films all underneath a 2.4 μg cm-2 regenerated amorphous cellulose film and f) 0.3 µg cm-2, g) 1.2 µg cm-2 and h) 1.9 µg cm-2 CNC films all underneath a 8.4 μg cm-2 regenerated amorphous cellulose film.


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The main findings of this study have been summarized schematically in Figure 7 and quantitatively in Table 2 which clearly shows that the crystalline to amorphous ratio does not exclusively determine the swelling of the system by water vapor. The size of the amorphous overlayer above the mixed CNC/amorphous matrix (Figure 7, Table 1) appears to be a central issue. Evidently, the presence of a relatively thick (>50 nm) armophous overlayer (Table 1) leads to a behavior where a thicker underlying CNC network – i.e., higher crystalline to amorphous ratio – appears to slightly mitigate swelling. With a thinner amorphous overlayer (ca. 20 nm (Table 1)), by contrast, a higher crystalline to amorphous ratio leads to an increased uptake of water vapor (Table 2). Another way to approach this is by examining swelling as a function of the interfacial area of CNCs (Figure 8). Because of the nanoscale size of CNCs, their total surface area is huge resulting in significant differences between the samples. When the interface between CNCs and amorphous cellulose is large, the trend in swelling is reminiscent of that in the cell wall: as shown in model studies of plant cell wall mimicking systems, water vapor adsorption at the crystalline/amorphous interface is excessive in comparison to that occuring solely in the amorphous region.40 Therefore, a higher interfacial area results in a greater extent of swelling (Figure 8). However, Figure 8 also suggests the existance of a water vapor uptake minimum as the interfacial area (or the amount) of CNCs is decreased. After this point of minimum swelling, a decrease in interfacial area leads to increased swelling. This small set of data leaves us with two alternative explanations: either (i) a substantially thick amorphous overlayer, not embedded within the crystallites, or (ii) a low interfacial area between crystalline and amorphous cellulose shifts the swelling behavior further from that of the plant cell wall. This suggests that not only the crystalline to amorphous ratio, but also the sheer amount of “self-standing” amorphous material, or the interfacial contact between the amorphous and crystalline materials (or both), in the cell wall has


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a physiological meaning in the plant. Unfortunately, the factual interfacial area between the crystalline cellulose microfibrils and the amorphous hemicellulose/lignin materix in the plant cell wall is not known due to the unknown extent of microfibril aggregation that certainly occurs when a plant is removed from its growth environment41 but which may already occur during biosynthesis42.

Figure 7. Schematic representation of the water vapor uptake behavior of the two different samples with a) lower and b) higher amounts of amorphous cellulose. The interfacial swelling is more pronounced when the amount of amorphous overlayer is smaller.


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Table 2. Tabulated data of crystalline to amorphous ratio of each sample, total surface area of the CNCs within each film in square meters and normalized to the mass of the amorphous component along with the mass (of the whole film) to CNC charge ratio, swelling ratio and mass of water vapor adsorbed per mass of cellulose for each sample. Crystalline to Normalized Surface charge swelling ratiod mg H 2 O/ amorphous surface to mass ratioc mg cellulosee area of CNCsb ratioa 2.4 μg cm-2 0.3 / 2.4 1.2 / 2.4 1.9 / 2.4

0 0.10 0.49 0.77

(cm2/mg) 0 730 1300 1400

(×1016 e/mg) 0 4.4 7.8 8.3

1.5 1.4 1.5 1.7

0.45 0.31 0.33 0.42

8.4 μg cm-2 0.3 / 8.4 1.2 / 8.4 1.9 / 8.4

0 0.03 0.14 0.22

0 330 480 530

0 2.0 2.9 3.2

1.7 1.7 1.7 1.6

0.39 0.39 0.37 0.33

a

Ratio between dry mass of reference corresponding CNC film and dry mass of corresponding reference amorphous film (as measured by QCM-D). b

Total surface area of CNCs in sample normalized to dry mass of entire composite film (as measured by QCM-D). For calculation example of total surface area of CNCs refer to explanation in Supporting Information and Table S3. c

Ratio between total surface charge of CNCs in corresponding reference film and dry mass of entire composite film (as measured by QCM-D). d

Ratio between the hydrated thickness and the initial (dry) thickness of the film.

e

For calculation example refer to Supporting Information.


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Figure 8. Mass of water vapor adsorbed per mass of cellulose as a function of the normalized surface area of CNCs. 4. Conclusions From a biomimetic standpoint, the study presented here further underlines the complexity with which nature has designed the plant cell wall and it provides a comprehensive insight into the vapor-induced swelling of ultrathin cellulose films. With a relatively small set of data, we were able to demonstrate two opposing trends. When the interfacial area between crystalline and amorphous cellulose was small and the thickness of an amorphous overlayer in the system was large, increasing the crystalline to amorphous cellulose ratio led to decreased swelling. In contrast, when the interfacial area was larger and the thickness of the amorphous overlayer significantly smaller, increasing the crystalline to amorphous cellulose ratio led to increased swelling. This latter behavior is in agreement with the trends found in model studies in which water vapor is adsorbed not only by the amorphous component but also in excess at the crystalline/amorphous interface.36 However, when the ratio is significantly decreased, the presence of the crystalline fraction inhibits swelling. All this suggests that evolution may have controlled the ratios and the amounts of crystalline and amorphous components in the plant cell wall in order to optimize the integrity and swelling response of the cell. From the perspective of biomimetic materials design, these results clearly indicate that the mass of water vapor uptake and the viscoelastic properties of two layered cellulose systems can be controlled by fine-tuning the ratio between the crystalline and amorphous


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components of the systems but that the window of opportunity within which such control is possible is remarkably small. This could prove beneficial in applications requiring extreme precision. In addition, these types of cellulosic systems could potentially provide a novel platform for the development of smart materials which require constituents with distinctly different physical properties but with chemical identicality. All in all, the study makes a point that biomimetics is not only about learning and mimicking structures from nature but at times, it may provide answers as to why certain native structures exist.

ASSOCIATED CONTENT Supporting Information. AFM images of 10, 15 and 26 nm CNC thin films with height profiles. Fits between the model (Cauchy) and experimental data for each two layered cellulose sample and each reference sample along with the raw ellipsometric data (Ψ and Δ values) for 5 different wavelengths as a function of the experiment time for each sample. Graphs of the change in refractive index (700 nm) of both amorphous reference thin films and each two layered cellulose system as a function of relative humidity during SE measurements. Table showing the relative humidities of each salt solution used in the QCM-D measurements. Example calculations for the ratio of water vapor mass adsorbed to cellulose for the 2.4 μg cm-2 amorphous cellulose reference sample. Example calculation of the total surface area of CNCs in the 1.9 μg cm-2 CNC reference sample (and its normalization). QCM-D data on the water uptake behavior of pristine silicon dioxide. Quantitative data on the determination of CNC dimensions. Masses of amorphous and crystalline reference films and composite films as measured by QCM-D. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENTS We acknowledge Academy of Finland (259500) and Aalto University (Aalto University Starting Grant) for funding as well as WoodWisdom–Net (Tunable Films project). We also thank Reeta Salminen, Dr. Jessie Peyre, Katja Pettersson and Ritva Kivelä for their laboratory assistance and Dr. Benjamin Wilson for useful discussions.

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Mimicking the Humidity Response of the Plant Cell ... - ACS Publications

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