Versatile cellular foams derived from CNC-stabilized Pickering emulsions

Abstract

Lightweight cellular foams were prepared by freeze-drying oil-in-water emulsions stabilized with cellulose nanocrystals (CNCs). Oil and water were removed while preserving the cellular emulsion template without any shrinkage or deformation both at the internal cell and bulk foam scales. This versatile process proved to be relevant for controlling the pore size of the foam with respect to emulsion droplets. Moreover, the composite foams could easily be obtained as shown here by the addition of polyelectrolytes. Mechanical properties of the foam were investigated and they were found to be reinforced by chitosan addition. Thus, our finding opens new routes to a broad range of cellulose-based porous materials.

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Porous materials have attracted a great deal of attention in recent years due to their many applications that include, in particular, catalysis,^1,2 immobilization of enzymes and proteins,³ kinetic energy absorbers, thermal and acoustic insulating materials, reinforcing platforms and scaffolds for nanocomposites, as well as pharmaceutical and biomedical applications.⁴ Porous materials and solid foams were initially produced primarily by using template syntheses of self-assembled micelle structures and low-cost material sources such as silica and carbon. Increasing environmental concerns have led to intense research involving bio-sourced materials, among which cellulose is an attractive candidate due to its outstanding mechanical properties, renewable character and low environmental impact. Thus, efforts have been made to develop cellulose-based porous materials such as aerogels. Following the pioneering work of Kistler who reported the first cellulose-based aerogels from liquid gels of cellophane by water exchange with propanol in 1931 (ref. 5), many cellulose-based aerogels have been obtained.^6,7 However, the field is now undergoing changes due to the use of nanocelluloses. Indeed, nanocelluloses have created attractive new opportunities for aerogel production since they offer an improvement of the material properties. For example, colloidal suspensions of pure or mixed nanofibrillated cellulose (NFC) can form porous structures through the careful control of the preparation procedure, resulting in highly homogeneous and porous structures.^8–10 A strengthening effect was also demonstrated by the introduction of nanocellulose in the case of biomimetic starch/NFC nanocomposites.¹¹ Pure cellulose structures have been obtained from NFC suspensions or cellulose nanocrystals (CNC) through acid hydrolysis of cellulose fibrils. In most cases, nanocellulose-based aerogels are obtained either by freeze-drying^12–14 or super critical drying.^12,15 However, all these preparations may induce shrinking. Another strategy to obtain highly porous materials is based on the polymerization of a high internal phase emulsion (HIPE) derived from Pickering-type emulsions that are emulsions stabilized by solid colloidal particles. These materials referred to as poly Pickering-HIPE^16–20 are used as templates for final porous materials. The continuous phase is physically or chemically crosslinked, and the removal of the liquid internal phase gives rise to a porous replica of the emulsion. In the literature, poly Pickering-HIPEs are based on solid particles such as silica,²¹ polystyrene²² or PMMA.¹⁹ Emulsion-based porous cellulose materials have recently been prepared by drying aqueous foams stabilized by carboxymethyl surface-modified NFC.²³

CNCs are able to stabilize Pickering emulsions that are resistant to creaming or centrifugation.^24,25 In the present work we report the formation of foams from a dense creamed phase composed of CNCs based Pickering emulsions that can be further freeze-dried to produce aerogels. The aim is to describe the elaboration pathway of this new cellulosic foam and how their structure can be tuned. Indeed, the porous open structure obtained is composed of cells displaying cellulosic walls with variable CNC content according to the surface coverage of the starting emulsions interface. Moreover, the variation of the droplets size using an original swelling process makes it possible to control foam cell sizes. Finally, composite foams can be produced by addition of polymers to the emulsions such as chitosan reinforcing mechanical properties. Thus, this new and easy to handle procedure offers versatile route for producing lightweight cellulosic foams.

Production of pure cellulose-based cellular foams

The process for producing homogeneous cellular foams composed only of CNC is illustrated in Fig. 1. In the first step, oil-in-water emulsions stabilized by an aqueous CNC dispersion were prepared.²⁴ Cyclohexane was used as hydrophobic phase because its low triple point allows it to be removed by lyophilization at the same time as water sublimation. The emulsion showed a rather narrow distribution of droplet sizes with diameters of approximately 4 to 20 μm, as expected for these Pickering emulsions. These emulsions that contained 20% oil proved to be extremely resistant to compression and were consequently centrifuged in order to remove the excess water. Since cyclohexane is lighter than water, a dense cream was obtained at the top of the tube. The internal phase fraction after centrifugation was measured and always found to be around 0.74 which is the close packing concentration as already noticed in similar conditions.²⁵ At this stage, it leads to oil microbeads in an aqueous continuous phase in which all of the CNCs are adsorbed at the oil/water interface and are thus responsible for the stabilization of the emulsions. The centrifugation step could possibly lead to coalescence, but some centrifuged samples were redispersed in a large volume of water and the average drop size, measured by the light scattering technique, was found to be unchanged. The concentrated cream was directly freeze-dried in the tube, giving rise to a dry foam with a preserved shape made exclusively of cellulose. The solid foam was fractured and the surface was metalized in order to visualize the internal organization by scanning electron microscopy (SEM). The images shown in Fig. 2 reveal a cellular structure composed of cells with sizes similar to the initial emulsion droplets. It is then shown that the interactions involved in the stabilization of the CNCs at the interface are strong enough to maintain the structure of the foam during the freeze-drying procedure, without alteration of the macroscopic organization. Organized pure cellulosic foam with a density of 1.2 10⁻² g cm⁻³ is produced without any change in the former template according to the emulsion drop size.

Fig. 1 Schematic representation of the preparation of CNC cellular foam, including the preparation of oil-in-water emulsion stabilized by CNC, and subsequently concentrated by centrifugation. The emulsion is frozen at −18 °C and lyophilized to produce pure cellulosic unshrunk cellular foam.

Fig. 2 SEM images of cellular foams made of CNCs alone at two concentrations, (a–c) 5 g L⁻¹ and (d–f) 10 g L⁻¹.

According to the weight of CNC dispersed in the aqueous phase, two different coverage modes can be observed.²⁵ At low concentration, according to the limited coalescence process, coalescence is stopped when the minimum coverage required is reached. This critical coverage leads then to the modulation of the drop size and a non-complete coverage is always observed, with about 84% of covered surface. Above this limit, the drop size of the emulsion does not vary anymore and the coverage of the interface increases with CNC concentration.²⁵ In order to modulate the wall density and thereby its porosity, several CNC concentrations corresponding to different interface coverages were tested. All concentrations tested below or at the limited concentration (corresponding to 84% coverage) produced no cellular foams, and it was not possible to remove the template from the tube without disrupting it. Thus, it is likely that a densely packed organization of CNCs at the interface is required. Two CNC concentrations of 5 g L⁻¹ and 10 g L⁻¹ that correspond to a coverage above 100% showed excellent template stability and the same foam cell diameters of 11.9 ± 1.8 μm and 13.2 ± 1.8 μm, respectively. SEM images (Fig. 2) showed that at 5 g L⁻¹, the cellulose interfacial thin films display a visible regular porosity, whereas at 10 g L⁻¹, the walls of foam cells are formed by more homogeneous and continuous films. Thus, it is likely that the density of the wall can be modulated by the initial CNC concentration.

In order to further explore the possibility provided by the Pickering pathway to produce foams, the variation in drop size before foam formation was investigated. As already developed in the previous paragraph, the lower CNC concentration domain that allows a drop size variation led to unstable foams. However, we recently revealed the ability to control the drop size of such emulsions by incorporating additional internal oil phase.²⁶ The process consists in a two-step emulsification, beginning with the production of Pickering emulsions with a monomodal drop size distribution, and then followed by the addition of oil and mixing with a homogenizer.

The oil is included in the drops without desorption of the CNCs from the interface, leading to a swelling of the already existing droplets. The new drop size is controlled by the volume of oil added to the emulsion by simple mixing using a rotor–stator at low speed. Cyclohexane is then added to initial emulsions already containing 20% of cyclohexane and mixed to reach 29%, 47% and 54% of internal phase (Fig. 3). In that case, some initial droplets of 10 μm in diameter can coexist with a population of larger size droplets of up to 100 μm in diameter. The global emulsions maintained the same stuck form as well as structural internal organization after freeze-drying as the one observed in the liquid state. This CNC-stabilized emulsion templating therefore provides a simple and versatile methodology to generate materials of predetermined cell sizes.

Fig. 3 Comparative SEM images of cellular foams made of an emulsion stabilized by CNCs at 5 g L⁻¹ (a) and upon addition of various amounts of oil after the emulsion preparation (b–d), leading to increasing cell diameter. The cyclohexane fraction in the emulsion was (a) 20%; (b) 29%; (c) 47%; (d) 54%.

Toward biobased composite cellular foams

Since the emulsion droplets are highly stable, functional foams may also be produced using simple processes such as the modification of the wall functionality of the individual droplets via coatings. CNC stabilized emulsions were thus prepared and subsequently mixed with two different water-soluble charged polysaccharides: carboxylated alginate (negatively charged) and partially aminated chitosan (positively charged). This process allowed an over layer without any competing effect with CNCs. The alginate coating did not allow the formation of cellular foam even in presence of 0.05 M or 0.1 M NaCl or CaCl₂. SEM images (Fig. 4a–d) showed that coalescence occurred during the drying process leading to a networking organization. Since the CNCs and the alginate are both negatively charged, alginate occurred as a filler of the aqueous phase without forming interactions with the interface of the droplet. The centrifugation step then led to a highly viscous continuous aqueous phase. Furthermore, increasing the global ionic strength decreases van der Waals interactions. The droplets deformed by centrifugation led to a relatively disorganized structure with thick walls, and the original droplet-shape template disappeared.

Fig. 4 SEM images of cellular foams made of CNCs at 5 g L⁻¹ and coated with alginate (a–d) or chitosan (e–h) at 1 g L⁻¹ at different magnifications.

On the other hand, chitosan dissolved in NaCl 0.1 M and HCl 0.01 M led to the formation of cellular foam that was identical to the former one. Since chitosan carries some amine groups, it is positively charged at the pH 5 chosen for the coating step. It interacted with the negatively charged CNCs, thus reinforcing the existing cellular template (Fig. 4). As a result, no modification of the template was observed with the same average drop size of the emulsion and cell size of the foam as measured directly on the SEM images. A thorough examination of the surface showed more isolated CNCs for foam made of CNC alone (Fig. 2c), compared to films in the presence of alginate (Fig. 4d), that formed a more continuous gel-like film with hardly observable contrast. On the opposite, foam images with chitosan (Fig. 4h) were hardly distinguishable from the former pure cellulosic foam, suggesting that chitosan adsorption occurs as a very thin layer that reverses the charge surface of the cellulose whiskers, preventing the adsorption of aggregates. It is then shown that the chemistry and functionality of the surface of the foam might easily be changed from negative (CNC alone) to positive (chitosan) by electrostatic interactions.

Fig. 5 Compression stress–strain curves of the cellular foams made of CNs at 5 g L⁻¹ and coated with chitosan at 0.5 and 1 g L⁻¹.

Table 1 Mechanical test results the for various compositions at 57% relative humidity

Sample	Density (×10^2) (g cm^−3)	Young modulus (kPa)	Specific modulus (MPa g^−1 cm^3)
CNC5 g L^−1	1.88 ± 0.02	Not measurable	Not measurable
CNC5 g L^−1 + chitosane 0.5 g L^−1	1.95 ± 0.03	41 ± 18	2.1 ± 0.9
CNC5 g L^−1 + chitosane 1 g L^−1	2.51 ± 0.04	97 ± 12	3.9 ± 0.5

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The presence of the over layer clearly reinforced the resistance to deformation of the foam. Indeed, it was not possible to evaluate the mechanical properties of the pure cellulosic cellular structure using a conventional dynamic mechanical analyzer. The resistance was too low and no contact was detectable by any of the three dynamic mechanical analyzers tested. However, when chitosan was added even at very low concentration such as 0.5 g L⁻¹ and 1 g L⁻¹ the strain was measurable upon compression stress (Fig. 5). The foam samples had the same cylindrical shape and since cellulose is susceptible to humidity, they were all conditioned at 57% relative humidity at room temperature prior to compression test. Typical compression stress–strain curves composed of two distinct regions were obtained: the linear elastic domain at low strains followed by a plasticity domain. The foam Young modulus (E), calculated as the slope of the initial elastic region, increases with the chitosan concentration (Table 1). The values obtained are low, showing that using CNC stabilized at an oil/water interface is a powerful way to maintain such controlled cell foam of low density without disruption during the process. On such basis, this coating appeared to be an efficient agent for reinforcing wall mechanical properties. It illustrates also that some various properties such as electrical or magnetic should be obtained using different specific coating.

Conclusions

We have successfully prepared lightweight pure cellulosic unshrunk foam with controlled cell dimension via CNCs based Pickering emulsions. This organization leads to a porous network with a density in the 10⁻² g cm⁻³ range as a result of the preservation of the original emulsion template. The global foam shape and internal homogeneity in cell dimensions is preserved due to the use of CNCs that allow monodisperse drop-size emulsions. The process proposed is easy to handle and allows a modulation of the cell wall rigidity by adjusting the concentration of the CNCs in the aqueous phase of the starting emulsions. Furthermore, simple modifications of the surface of the droplets, like shown here by chitosan over layer, are possible in order to control the surface chemistry or change the physico–chemical properties, demonstrating the versatility of such foam. Finally, since this process requires a low amount of water, it limits the ice crystals formation and does not require a precise control of the freezing step, which is the critical parameter of the freeze-drying procedure. Indeed we never observed any hole due to crystal formation during SEM observation. As a result, it could be adapted to any functionality such as electrical or magnetic coating without the destructuring of the cellular organization. Overall, the ultralow density and highly porous cellulose nanocrystal foam presented here opens the way to the formation of new cellulose-based cellular materials.

Experimental section

Materials

Cyclohexane at 99% or greater purity was purchased from Fluka was cleaned thoroughly both with water and alumina to remove any remaining surfactants. Chitosan (highly viscous) was purchased from Fluka and alginate (M_w = 151 500 g mol⁻¹ and M_w/M_n = 2.11) was obtained from FMC (USA).

Preparation of cotton cellulose nanocrystals (CNCs)

25 g of cotton Whatman filters (grade 20 Chr) was mixed with 700 mL distilled water (Milli-Q®) until a homogeneous dispersion was obtained. The mixture was then hydrolyzed in sulfuric acid at 64% for 30 min at 70 °C. Immediately after acid hydrolysis, the reaction was quenched by diluting the suspension 10-fold with deionized water. The suspension was then washed by repeated centrifugations and dialyzed to neutrality against water. The residual electrolytes were removed with ion-exchange resin (TMD-8 Sigma-Aldrich) for 4 days at 4 °C to remove residual counterions from sulfate ester groups. The final dispersion was sonicated for 15 min (ultrasonic processor XL 2020; Misonix, NY, USA), filtered and stored at 4 °C. The surface charge density of 0.64 e nm⁻² was measured by conductometric titration with 0.1 mM NaOH. The length and width of 185 nm and 13 nm, respectively, were determined by image analysis of the TEM images using Image J software.

Emulsion preparation

The oil-in-water (o/w) emulsions were prepared using cyclohexane and a nanocrystal aqueous suspension at the required concentrations without further dilution, to match an oil/water ratio of 20/80, with 50 mM NaCl to prevent electrical repulsion. A quantity of 1.6 mL of cyclohexane was added to 6.4 mL of aqueous suspension in a plastic vial and sonicated at a power level of 50–55 J. Droplet diameters were measured by laser light diffraction using a Malvern 2000 granulometer apparatus equipped with a He–Ne laser (Malvern Instruments, U.K.) with Fraunhofer diffraction.

High internal phase emulsions that lead to increased drop sizes were prepared by adding oil to the former Pickering emulsion and mixing with a rotor stator homogenizer for one minute at 20 000 rpm.

Preparation of foams

The emulsions were centrifuged for 5 minutes at 1500–4000g. The tube was pierced in order to remove the water at the bottom. The concentrated emulsion was then deposited on a plane surface and frozen at −18 °C for 24 h and then lyophilized for 16 h in order to remove both water and cyclohexane, leading to a homogeneous cellulose cylinder of foam (diameter: 13.1 ± 0.1 mm; height: 12.5 ± 1.0 mm). Composite foams were prepared plunging the all-prepared emulsion in alginate (prepared in NaCl 0.05 M and 0.1 M) or chitosan (prepared in 0.1 M NaCl and HCl 0.01 M) at the required concentration.

Scanning electron microscopy

Scanning electron microscopy images of foams were visualized after 120 s of metalization with platinum with a JEOL 6400F instrument. Pore sizes were measured directly on the images using image J software from an average of 50 to 100 measurements on two representative images.

Mechanical test

Foams dimensions were 13.1 ± 0.1 mm of diameter and 12.5 ± 1.0 mm high. They were kept one week at 57% relative humidity in saturated NaBr salts. The compression tests were performed using a Bose 3100 with Win Test 7 software. Contact was fixed at 0.5 N followed by a compression at 0.004 mm s⁻¹ with a maximum deformation of 20%. The Young modulus was calculated from the stress vs. deformation curve calculated according to:

σ = F/A

and

ε = (l₀ − l)/l₀

where σ is the stress (Pa), F is the applied force (N), A is the surface area in contact with the measuring device (m²), ε is the strain, l₀ is the initial length and l the length when measured (m). The measure was stopped before the densification region was reached.

Acknowledgements

The authors would like to thank Adelin Barbacci for the mechanical analyses, Joelle Davy for SEM visualization and Nicolas Stephan for SEM assistance (IMN, Nantes, France).

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