Cross-linked and shapeable porous 3D substrates from freeze-linked

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Cross-linked and shapeable porous 3D substrates from freeze-linked cellulose nanofibrils Johan Erlandsson, Hugo Françon, Andrew Marais, Hjalmar Granberg, and Lars Wågberg Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01412 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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Biomacromolecules

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Cross-linked and shapeable porous 3D substrates

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from freeze-linked cellulose nanofibrils

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Johan Erlandsson a*, Hugo Françon a, Andrew Marais a, Hjalmar Granberg b, Lars Wågberg a,c*

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a. Division of Fibre Technology at the Department of Fibre and Polymer Technology, School of

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Engineering Sciences in Chemistry, Biotechnology and Health KTH Royal Institute of

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Technology, Teknikringen 56-58, SE-100 44, Stockholm, Sweden

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b. Papermaking & Packaging, RISE Bioeconomy, Box 5604, SE-114 86, Stockholm, Sweden

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c. Wallenberg Wood Science Center, KTH Royal Institute of Technology, Teknikringen56, SE-

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100 44, 8 Stockholm, Sweden

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ACS Paragon Plus Environment

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Biomacromolecules 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

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Chemically crosslinked highly porous nanocellulose aerogels with complex shapes have been

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prepared using a freeze-linking procedure which avoids common post activation of crosslinking

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reactions and freeze-drying. The aerogel shapes ranged from simple geometrical three

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dimensional bodies to swirls and solenoids. This was achieved by moulding or extruding a

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periodate oxidized cellulose nanofibril (CNF) dispersion prior to chemical crosslinking in a

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regular freezer, or by reshaping an already prepared aerogel by plasticizing the structure in water

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followed by reforming and locking the aerogel into its new shape. The new shapes were most

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probably retained by new crosslinks formed between CNFs brought into contact by the

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deformation during reshaping. This self-healing ability to form new bonds after plasticization

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and redrying also contributed to the mechanical resilience of the aerogels, allowing them to be

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cyclically deformed in the dry state, reswollen with water and redried with good retention of

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mechanical integrity. Furthermore, by exploiting the shapeability and available inner structure of

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the aerogels, a solenoid shaped aerogel with all surfaces coated with a thin film of conducting

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polypyrrole was able to produce a magnetic field inside the solenoid demonstrating

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electromagnetic properties. Also, by biomimicking the porous interior and stiff exterior of the

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beak of a toucan bird, an aerogel functionalization was created by applying a 300 µm thick stiff

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wax coating on its moulded external surfaces. This composite material displayed a ten times

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higher elastic modulus compared to the plain aerogel without drastically increasing in density.

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These examples show that it is possible to combine advanced shaping with functionalisation of

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both the inner structure and the surface of the aerogels, radically extending the possible use of

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aerogels.

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Biomacromolecules

Introduction

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Cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) are renewable, bio-derived

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nanomaterials that can easily be converted into several types of macroscopic advanced materials

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such as gas barrier films,1-3 high strength filaments,4-5 low density foams

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Nanocellulose aerogels are commonly prepared by freeze-drying (FD) or critical point drying

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(CPD), and properties such as density, specific surface area, pore structure and mechanical

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properties can be tuned by altering the preparation procedure.8,

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content of native hydroxyl groups and carboxyl groups introduced during pre-treatments, to

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facilitate the fibril liberation,13-14 makes CNF aerogels post-functionalizable both chemically and

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physically to produce new functional materials.6, 15 However, as cellulose is sensitive to water,

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nanocellulose aerogels tend to easily disintegrate when soaked in water, limiting both the

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possibility of using subsequent water-based functionalization methods and final applications.16-17

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To address this issue several methods for chemical crosslinking of nanocellulose aerogels

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through e.g. ester bonds by using tetrafunctional carboxylic acids or by commercial crosslinking

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agents such as Kymene, have been developed, giving rise to water-stable aerogels.15, 17-18 The

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wet-stability also allows functionalization methods such as the water-based Layer-by-Layer

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(LbL) method, used to prepare aerogels with properties ranging from flame-retardancy,

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antibacterial activity, cell compatibility and energy storage.18-21

11-12

6-7

and aerogels.8-10

Furthermore, the high

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Despite the versatility of nanocellulose aerogels as templates for functionalization and the

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broad range of applications they offer, upscaling remains a challenge because of the use of FD

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and CPD. Moreover, their use in shape demanding applications is limited because of their lack of

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shapeability, mainly due to their inherent brittleness and friability. The achievable shapes are

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generally limited to relatively simple geometries due to the use of moulds that need to be


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removed without damaging the aerogel. Hence, complex shapes such as spirals and loops cannot

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be achieved. In a recent study a procedure to fabricate and shape wet-stable CNF aerogels

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without any freeze-drying, using a simple freeze-thawing and solvent exchange procedure was

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reported and opened up possibilities of an easier and more versatile production.22 The produced

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aerogels could be soaked in water and compressed to 70% without losing their ability to recover

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their original shape and size. The wet stability was achieved by crosslinking through hemiacetal

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bonds between aldehydes introduced to the CNFs by sodium metaperiodate oxidation and

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hydroxyls on neighbouring CNFs.23

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Following up this work, we herein describe three methods how these freeze-linked aerogels

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can be prepared into complex shapes using different shaping procedures. We also show that the

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structural properties (density, porosity, pore size) of the aerogels can be tuned by varying the

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CNF concentration. Furthermore, we provide new properties to the aerogels through

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functionalization of both the inner structure and the outer surface of the aerogels. The

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functionalization is done with both inactive and active components such as natural waxes and

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conducting polymers. Inspired by the toucan bird, we use a biomimicking approach to prepare

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stiff and lightweight core-shell composites resembling the toucan beak. Finally by

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polymerisation of pyrrole within the porous structure of a solenoid-shaped freeze-linked aerogel,

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we show that it is possible to create a device that can generate a magnetic field.

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Materials and Methods

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Materials

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Carboxymethylated CNFs were produced according to a previously reported method

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and

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were provided by RISE Bioeconomy AB, Stockholm, Sweden, as a 20 g/L gel. Sodium

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metaperiodate, sodium chloride and hydroxylamine hydrochloride, carnauba wax, pyrrole,


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anhydrous iron(III) chloride, para-toluenesulfonic acid were all purchased from Sigma Aldrich.

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All chemicals were used without further purification unless otherwise stated.

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Methods

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Preparation of crosslinked CNF aerogels by freeze-induced crosslinking

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CNF aerogels were prepared by mixing sodium periodate and CNF of varying concentrations

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in a 0.7:1 dry weight ratio. The oxidation reaction was carried out for 1 hour after which the

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CNF-periodate mixture was transferred to a pre-selected mould and then placed in a freezer (-18

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°C) over night. The frozen samples were then thawed at room temperature, solvent exchanged to

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acetone and dried under ambient conditions.

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Total charge determination

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Conductometric titration was used to determine the total carboxyl content of the material and

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was performed with a Titrino 702 SM (Metrohm AG, Herisau, Switzerland) according to a

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previously described method 24. Prior to the measurements, the CNF aerogels were washed with

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water to remove any residual salt remaining after the oxidation reaction. The aerogels were

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washed until the conductivity of the washing water became lower than 5 µS/cm. The carboxyl

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groups present in the materials were subsequently converted into their proton form by

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equilibration in water at pH 2 for 30 min after which they were again washed with water until the

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conductivity of the washing water became lower than 5 µS/cm. 0.1 g of CNF aerogel material

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(dry weight), was added to a total volume of 500 mL of water containing 0.1 mM HCl and 2 mM

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NaCl. The solution was purged from oxygen and carbon dioxide with nitrogen gas for 15 min

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prior to the titration. The titration was performed with 0.1 M NaOH and the amount of titrant


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used for calculation of the total charge was determined from the plateau region for the

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conductivity in the titration curve 24.

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Aldehyde content determination

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The amount of aldehydes introduced into the CNFs by the periodate oxidation was determined

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by titration with sodium hydroxide after reaction with hydroxylamine hydrochloride, which

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reacts with the aldehydes forming oximes while releasing a stoichiometric amount of protons 25.

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The oxidized material was added to 25 mL of water containing 10 mM NaCl to ensure that the

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pH on the surface of the fibrils was the same as that in the bulk, and the pH was adjusted to 4.

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The sample mixture was then mixed with 25 mL of 0.25 M hydroxylamine hydrochloride

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solution (also containing 10 mM NaCl) adjusted to pH 4, and the mixture was allowed to react

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for 2 h prior to a back titration at pH 4 using 0.1 M NaOH. The amount of aldehydes in the

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sample was calculated from the number of moles of NaOH needed to reach pH 4 and the results

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were normalized with respect to the dry weight of the CNF sample.

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Morphology

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The microstructure and morphology of the aerogels produced was studied using an S-4800

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field emission scanning electron microscope (SEM) (Hitachi, Tokyo, Japan). Aerogels samples

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were carefully cut and glued onto a sample holder using conducting carbon tape. Prior to

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imaging the samples were coated with Pt/Pd in a Cressington 208 HR sputter coater (Cressington

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Scientific Intruments, Watford, UK) for 40 s to limit specimen charging during imaging in the

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SEM. The pore size was estimated from SEM images as an average of the pore diameter

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measured in two perpendicular directions.

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Biomacromolecules

Mechanical properties

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The aerogels were tested at 50% RH and 23 °C in the dry state using an Instron 5566 universal

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testing machine (Norwood, MA, USA) equipped with a 500 N load cell. The samples consisted

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of cubic aerogel samples of approximatively 20 mm side (the exact dimensions were determined

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with a caliper before testing). The rate of compressive strain was 10 %/min, and the samples

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were compressed until they reached 80% compression. Unless stated otherwise, six samples were

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tested for each aerogel type.

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Wax coating

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Cubic aerogels (17.5 g/L, 2x2x2 cm3) were coated with carnauba wax on four adjoining sides

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by spin coating, thus leaving two opposite uncoated faces. The wax was melted and set to a

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temperature of 105 °C and subsequently added dropwise, a total of five drops per side, to the

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spinning aerogel (3600 rpm for 20 s). Excess wax extending off the sides of the aerogel was

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removed using a heated spatula. The coated aerogels were tested in compression with the coated

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sides in the compression direction.

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Polypyrrole functionalization

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The polypyrrole (PPy) functionalization of the porous aerogel structure was performed by

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soaking the aerogel in a water solution containing pyrrole, p-Toluenesulfonic acid and

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iron(III)chloride and allowing the polymerization to continue for 24h. The initial concentration

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of pyrrole was 0.033 M and the ratio of p-Toluenesulfonic acid/pyrrole was 0.3 and,

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iron(III)chloride/pyrrole 2.3 respectively. Following the polymerization, the aerogel was washed

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extensively with water and subsequently solvent exchanged to acetone and ambiently dried. Any

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electrical connections to the aerogel were managed by gluing copper wires to the aerogel using


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silver paint. The electrical properties of the aerogel were evaluated using a Keithley 2400 series

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Source-Meter (Keithley Instruments Inc., OH, USA).

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Magnetic properties

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The magnetic properties were measured using an Adafruit HMC5883L tripe axle

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magnetometer connected to an Arduino Uno microcontroller with the Adafruit sensor library.

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The magnetometer was inserted into the middle of a solenoid shaped aerogel functionalized with

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PPy with one axis parallel to the solenoid. The magnetic field was subsequently recorded

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continuously while current passing through the spiral was alternatively turned on and off.

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Results & Discussion

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Principles of CNF aerogel preparation

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The freeze-linked CNF aerogels all shared one common production procedure shown in a flow

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chart in Figure 1a. The only difference is found in the shaping where one of three different

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methods, moulding, extruding and reshaping, were used to achieve the final shape of the

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material. The moulding and extruding methods (Fig 1b) shaped the CNF dispersion into the

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desired shape prior to crosslinking the structure by freezing. The third shaping method involved

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reshaping and locking an already prepared aerogel, i.e. an already shaped aerogel (through

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moulding or extrusion) into a new shape, as exemplified in Fig 1c.

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The freeze-linking technique relies, similarly to freeze-casting,26-28 on the exclusion of the

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CNFs and the solutes from the growing ice crystals and their packing in the thin lamellae

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between the crystals during freezing, displayed schematically in Fig 1d. It is in these lamellae in

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the frozen state that the CNFs crosslink through hemiacetal linkages and produce a wet-stable


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structure.29 The creation of the 3D structure by ice-templating a colloid is dependent on the

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nucleation and growth of the ice crystals which in turn are dependent on for example the

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ice/solid interactions, the degree of super cooling

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both the amount of periodate used during the oxidation and the shape of the extruded body and

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mould affect the freezing behaviour and therefore yield different pore structures. The wide range

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of herein reported aerogel shapes therefore give rise to pore structures which are strongly related

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to their initial shape, CNF concentration and degree of oxidation.

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and the ionic strength and ion type.31 Thus

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Figure 1 (a) Schematic representation of the preparation of the aerogels, (b) shaping by moulding

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or extrusion (c) post production reshaping of aerogels, (d) schematic view of the ice crystals

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excluding the CNFs from the ice-phase and tightly packing them together in the lamellae.


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Aerogels were prepared from CNF dispersions at concentrations ranging from 7.5 g/L to

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20 g/L, which are well above the overlapping concentration (0.4-0.7 g/L) 32 of the CNFs used in

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this study, also after the periodate oxidation. Figure 2a shows a 5 cm diameter spherical aerogel

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shaped by moulding of a 20 g/L CNF dispersion together with its porous microstructure which

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consists of 2D sheets of compressed CNFs (Fig 2b), also seen at higher magnification in

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Figure 2c. Below 7.5 g/L, the solvent exchanged samples collapsed during ambient drying. Since

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a decreased CNF concentration resulted in fewer CNFs packing together in the lamellae during

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freezing, the collapse was presumably due to insufficient mechanical strength of the formed

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structure. It has also been reported that freezing of CNFs at 5 g/L and lower concentrations

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results in a weak fibrillar structure that collapses (even if freeze-drying was used) rather than in a

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stronger structure of interconnected 2D sheets.33 When soaking aerogels with the fibrillar and

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sheet like structures in water the aerogels with the fibrillar structure collapsed due to capillary

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forces while the aerogels with the 2D sheets remained intact34. Examination of a 0.1 g/L CNF

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dispersion that was oxidized and freeze-dried showed that the oxidized CNFs also formed a

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fibrillar structure, as shown in Figure S1. The collapse of the aerogels with a concentration

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below 7.5 g/L is therefore presumably due to a combination of both low mechanical strength of

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the formed CNF structure and possibly also to a different 3D structure resulting from the low

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CNF concentration. The effect of the CNF dispersion concentration on how the CNF is

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templated in the lamellae between the crystals clearly demonstrates the wide range of pore

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structures possible to produce by ice templating . The formation of the 2D-sheets was evidently

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required to obtain a structure with sufficient strength to withstand the ambient drying step

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following solvent exchange.


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Biomacromolecules

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Figure 2d shows the evolution of the aerogel density as a function of the initial CNF

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concentration. A natural consequence of using a range of CNF concentrations was that the

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density of the aerogels increased, from 12 kg/m3 to 34 kg/m3, (between 98-99% porosity) as the

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concentration increased from 7.5 g/L to 20 g/L. Although the aerogels were dried at room

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temperature and subjected to capillary forces, the average volumetric retention of the aerogels

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was above 75% regardless of the initial CNF concentration. This value is comparable to

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volumetric retentions reported for CNF based aerogels produced from similar CNF

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concentrations and dried by freeze-drying 12. All samples displayed wet-integrity when soaked in

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water and were able to be redried after a second solvent exchange, In Figure S2 two aerogels

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(7.5 g/L and 17.5 g/L) that have been completely soaked in water without disintegrating are

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displayed. This suggested that the crosslinking through hemiacetals between the CNFs in the

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lamellae was effective also at the lowest CNF concentration. This is in line with the aldehyde

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content of all samples, being between 1.0 mmol/g and 1.1 mmol/g, which exceeded the

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0.6 mmol/g, previously reported as the threshold value needed for effective crosslinking 29. The

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total charge (carboxylic acid content) of the final material was on average 350 µeq/g,

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independent of CNF concentration, which is significantly lower than the initial 600 µeq/g. This

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agrees with previous studies on aerogels with similar aldehyde contents produced using the same

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freeze-linking procedure. The loss of charged groups in this case was attributed to dissolution of

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the charged and highly oxidized cellulose chains on the surface of the fibrils 22, 35-36.


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Figure 2 (a) Photograph of a spherical cross-linked CNF aerogel made from 20 g/L CNF gel

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(scale bar = 1 cm), (b) SEM-image of the cross-section of the spherical aerogel (scale bar =

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500 µm), (c) SEM image showing the pore walls (scale bar = 10 µm), (d) The aerogel density as

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a function of the initial CNF concentration.

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Shaping the aerogels through moulding limited the geometries to simple geometrical bodies

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such as spheres (Fig 2a), however, by using a CNF dispersion with a concentration of 17.5 g/L or

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above shaping of the CNF dispersion by extrusion was possible. The viscosity of the dispersions

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above 17.5 g/L was sufficient to allow the dispersion to retain the nozzle shape also after

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extrusion. Extrusion of the CNF gel through a star-shaped nozzle yielded shapes such as the one

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displayed in Fig 3a. The possibility to extrude the dispersions into a specific shape opens up the


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Biomacromolecules

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possibility of 3D-printing advanced shapes and subsequently crosslink the printed structure in a

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consecutive freezing step.

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The shape of the aerogels could also be altered post-preparation by soaking the aerogel in

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water to plasticize the structure, reshaping it and subsequently locking the structure into a new

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shape. The new shape can be locked by submitting the strained structure to either a new solvent

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exchange step with subsequent drying or by another freezing, solvent exchange and drying

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sequence. To illustrate this reshapability property, a solenoid shaped aerogel was prepared, see

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Figure 3b. The solenoid spring was obtained by soaking the aerogel in water and wrapping it

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around a tube before solvent-exchange and drying. The ability to be re-soaked, solvent

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exchanged and dried indicates that the structure is largely undamaged by the physical treatment

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involved in reshaping, and that the strength of the pore walls remained also after being reshaped.

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The reshapeability was further tested by soaking an aerogel in water and inserting the softened

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structure in a different mould, freezing, solvent exchange and drying. Figure 3c shows a

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cylindrical aerogel that was reshaped into a prism following this method.


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Figure 3 Photographs of (a) an extruded swirl aerogel, (b) a solenoid- shaped aerogel prepared by

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reshaping a long cylindrical aerogel , (d) a cylindrical aerogel (left) reshaped into a prism (right).

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The scale bars are 1 cm.

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Given that the fibrils in the aerogel are crosslinked and therefore fixed in space with respect to

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each other and that the structure is relatively inelastic in tension, a change of shape requires a

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change in volume. Hence, a change in density naturally follows due to the densification of the

259

cylinder when it was reshaped into a prism. The final density of the prism was 77 kg/m3 and the

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initial of the cylinder 34 kg/m3. Note that the density increase was due to the deformation and

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densification of the outer parts of the cylinder and not due to an overall densification. The

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densified parts of the aerogel were visible as skin on the outer parts of the aerogel. This

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demonstrates the possibility of creating aerogels with density gradients. As the new structure

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contains cell walls that have been deformed in a plasticized state and subsequently pressed

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together by both the mechanical force induced by the decreased volume of the mould and by the

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ice forming in the pores, the aerogel is crosslinked into the new shape. This was evident from


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Biomacromolecules

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soaking the aerogel in water where it maintained its prism structure and did not revert to its

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original cylindrical shape even after soaking in water for several hours.

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Microstructure and morphology of the moulded freeze-linked aerogels

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All moulded, extruded or reshaped aerogels, displayed an open pore structure with

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interconnected 2D sheet-like pore walls comprising oxidized CNFs tightly packed into a CNF

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nano sheet structure3, irrespective of CNF concentration from 7.5 g/L and above. Overall, the

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pores were not found to be aligned, of a specific shape or oriented. Instead all were of random

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size and orientation (Fig 4). This suggested that the nucleation of ice crystals was randomly

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distributed with no preferred growth direction in the dispersion during the freezing, unlike the

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freeze casting method27 where the crystals are continuously growing parallel to a temperature

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gradient.

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In the aerogels prepared from the lowest CNF concentration (7.5 g/L) the pores were

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significantly larger compared to the pores found in the aerogels made from higher CNF

280

concentrations, see Fig 4a. Presumably, at lower solid contents the CNFs are not blocking each

281

other and are easily excluded from the propagating ice front as the ice crystals grow larger.30

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Smaller pores of similar size were observed in the aerogels prepared at higher concentrations

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(Fig 4b and c).

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Figure 4. SEM images of cross sections of moulded (cubic) aerogels prepared from (a) 7.5 g/L,

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(b) 17.5 g/L and (c) 20 g/L. Scale bars represent 1 mm.


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At higher CNF concentrations the excluded CNFs accumulate at the interface of the growing

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ice front and start forming pore walls earlier which hinders the propagation of the ice front and

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hence creates smaller pores. In addition a higher CNF content also allows for an increased

290

number of sites where heterogeneous nucleation of ice crystals can occur, which also would

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decrease the pore size. As discussed earlier, several parameters affects the formation of the pore

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structure in the aerogel and as the ionic strength varied (since it is based on the CNF solid

293

content) and the oxidation reaction altered the CNF/CNF and CNF/water interactions29, it is

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difficult to isolate and quantify the contribution of each of those parameters in the formation of

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the pore structure.

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Compressive properties of freeze-linked aerogels

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The mechanical properties of the freeze-linked aerogels were evaluated in uniaxial

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compression of cubic aerogel samples. All samples displayed a typical aerogel behaviour with

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three different regions in the compression curve: an elastic deformation at 60% where opposite

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cell walls come into contact and the stress increases due to the densification of the overall

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structure. Figure 5a displays typical stress-strain curves in uniaxial compression.


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Figure 5. (a) Compressive stress strain curves for the freeze-linked aerogels prepared from

308

different initial CNF concentrations. The inset shows the initial part of the compression test and

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(b) the cell wall properties calculated using the Ashby and Gibson scaling laws.

310

In general, as the CNF concentration increased, the compressive strength of the aerogel is

311

increased. This is similar to the mechanical behaviour reported for CNF aerogels previously11

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and a summary of the mechanical properties is presented in Table 1. However, aerogels prepared

313

from 17.5 g/L dispersions exhibited both higher elastic modulus and yield stress compared to the

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20 g/L aerogels. This observation of the significantly higher modulus, ~1000 kPa (17.5 g/L),

315

which is twice that at 15 g/L and 20 g/L (~500 kPa), suggests that it is not only the concentration

316

of CNFs that is contributing to the final mechanical characteristics of the aerogel. This somewhat

317

unexpected result was further tested by comparing the results from cubic aerogels with

318

cylindrical aerogels where the same sharp increase in mechanical properties was found for

319

aerogels prepared from a 17.5 g/L CNF dispersion. This excluded the possibility of it being a

320

result of only the geometry of the sample and thus the pore structure governing the mechanical

321

properties. In favor of another mechanism rather than the concentration and geometry is that all

322

samples had the same amount of aldehydes which should allow for equal number of crosslinks


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323

between the CNFs and thus should the property scale only with the number of CNFs. To test this

324

hypothesis the Gibson and Ashby

325

structures were employed.

326

Table 1 Bulk mechanical properties of CNF aerogels prepared from different CNF

327

concentrations, presented as the average value and standard deviation of six samples.

CNF concentration Elastic modulus

37

scaling laws for the mechanical properties of open cell

7.5 g/L

10 g/L

15 g/L

17.5 g/L

20 g/L

101 ± 39

240 ± 83

495 ± 81

1033 ± 240

501 ± 86

4.5 ± 1.6

15.9 ± 3.5

28.1 ± 4.4

38.6 ± 7.7

30.5 ± 5.9

(kPa) Yield strength (kPa) 328 329

The Gibson and Ashby scaling laws suggest that the yield stress and elastic modulus of the cell

330

walls of open cellular materials can be related simply to the relative density of the bulk material (

331

𝜌 ∗ ) and the solids (𝜌𝑠) according to the following relationships:

332

3/2

𝜎∗ 𝜌∗ = 0.3 𝜎𝑠 𝜌𝑠

( )

333

334

𝐸∗ 𝜌∗ = 𝐸𝑠 𝜌𝑠

2

( )

335

Where 𝜎 ∗ , 𝐸 ∗ are the yield stress and elastic moduli of the bulk material and 𝜎𝑠 and 𝐸𝑠 are the

336

yield stress and elastic moduli of the cell walls. The cell wall density (𝜌𝑠) is estimated to be

337

1500 kg/m3 for the CNF walls. This assumption overestimates the cell wall property as the cell


18

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338

wall is not perfectly packed cellulose I but given the resemblance of the cell wall to a CNF-nano

339

sheet which has densities close to 1500 kg/m3 3 this assumption is reasonable. Furthermore, the

340

changes in morphology and complexity of the ice-templated structure also results in that only

341

qualitative estimates of 𝐸𝑠 and 𝜎𝑠were calculated by using the measured bulk densities and

342

mechanical properties. The results are presented in Figure 5b.

343 344

The estimated cell wall modulus reached a maximum value of approximately 4 GPa for the

345

aerogels prepared at 17.5 g/L while the aerogels from 7.5 g/L, 10 g/L, 15 g/L are all resulted in

346

approximately around 2 GPa and the aerogel prepared from 20 g/L had a cell wall modulus of 1

347

GPa. The trend visible in the yield stress of the cell walls also display a maximum at 17.5 g/L

348

and a significant drop for the aerogels prepared at 20 g/L CNF concentration. The almost

349

constant modulus observed below 17.5 g/L is reasonable, given that the CNFs have the same

350

aldehyde content and hence the same properties on an individual CNF level and therefore the

351

global performance only scales with the number of CNFs, i.e. the density as suggested by the

352

scaling laws. Given that the mechanical performance was unaffected by the geometry/pore

353

structure, the sudden increase and subsequent drop observed for the 17.5 g/L and 20 g/L aerogels

354

is presumably due to the CNFs interacting differently at higher concentrations during the

355

freezing process. A possible explanation is that at the lower concentration, the packing and

356

interaction between the CNFs are not limited by interactions with other CNFs when they are

357

excluded from the ice, and can therefore pack together and crosslink to a large extent and the

358

property therefore scales with the amount of CNFs. At 20 g/L the movements of the CNFs

359

become limited as they are excluded from the ice at an earlier stage during freezing which might

360

inhibit their packing and interaction. The 17.5 g/L CNFs dispersions instead presents conditions


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361

such as ionic strength, CNF concentration and freezing rate that are optimal for the formation of

362

a strong 3D network. In addition, the smaller pores observed for the 20 g/L aerogels compared to

363

the 17.5 g/L aerogels would result in fewer CNFs in each cell wall which in turn also affects the

364

total strength of each wall.

365

Mechanical resilience

366

The ability of the aerogels to be resoaked, reshaped and redried suggests that the cell walls are

367

resistant to the mechanical deformation during the reshaping and can maintain their structure.

368

The mechanical properties of 17.5 g/L aerogels were therefore evaluated after several cycles of

369

dry compression, wet shape recovery, and drying. Figure 6(a) shows photographs of the shape

370

recovery of a compressed aerogel upon contact with water. The relative mechanical properties

371

and volume of the cycled aerogels, compared to the first cycle, are shown in Figure 6b.


20

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Biomacromolecules

372 373

Figure 6. (a) A sequence of photographs showing the shape recovery of a compressed aerogel,

374

(b) plot of the relative bulk modulus, yield stress and volume retention of a 17.5 g/L aerogel as a

375

function of consecutive compression, reswelling and drying cycles, (c) SEM image of the cross

376

section of an aerogel after 5 cycles of dry compression, reswelling and drying (scale bar 500 µm)

377

and (d) SEM image showing buckling and cracking (arrows) of the pore walls after 5

378

compression cycles (scale bar 1 µm)

379

The sequential compression, shape recovery and drying of the aerogels had no significant

380

effect on the density which remained almost constant, since the aerogels displayed a volumetric


21


Page 22 of 41

381

retention of above 90% even after 5 compression cycles. The modulus and yield strength on the

382

other hand were largely affected by the cycling, and dropped after one cycle, as can be seen in

383

Figure 6b. The modulus dropped to a value approximately 60% of the initial value and

384

subsequently remained stable during cycles 2-4 before dropping to around 30% of the initial

385

value in the fifth cycle. The yield stress displayed an initial drop to just over 20% of the initial

386

yield stress after which it increased in cycle 3 and remained stable in cycle 4, to approximately

387

55-60% of the initial, before it decreased in cycle 5. The overall decrease in mechanical

388

performance of the aerogels can be ascribed to the changes occurring in the structure during the

389

compression. Figure 6c shows that the overall pore structure changed compared to the initial

390

structure. The pores were significantly less regular and the pore walls were more disordered. The

391

examination of individual pore walls revealed a buckled surface, with some local cracks (Fig 6d).

392

In addition, the majority of the pore walls had creases on their surfaces, see Fig S3 in the

393

Supporting Information. The regain in yield stress and the stable value of the modulus can be

394

suggested to be due to the reformation of new adhesive contacts between the fibrils after each

395

cycle which is reasonable due to the close proximity of the fibrils in the cell wall. Since the

396

material becomes weaker after the mechanical deformation cycles and since water has been

397

shown to greatly plasticize hemiacetal crosslinked structures 22 it is possible that the material can

398

reorganize in the swollen state and subsequently reform new bonds when redried and hence show

399

self-healing properties. This is consistent with the reshaping and re-crosslinking of the

400

cylindrical aerogel into a prism shape where the cell walls can be reformed in the plasticized

401

state to adopt the new prism format when dried. This reshaping ability is however not unlimited

402

since the mechanical performance drops significantly after 5 cycles. This is supposedly due to

403

the cracks observed in the pore walls after 5 cycles and this is probably due to that the fibrils


22

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Biomacromolecules

404

have been permanently separated and no longer can form adhesive contacts as the fibrils have to

405

be in very close proximity to react and form hemiacetals.23

406 407

Functionalization of the aerogels using a biomimicking approach and possible applications Several structures found naturally such as cancellous bone 38, the interior of porcupine quills 39-

408 409

40

410

presented in the present work. All these natural materials also share a core shell structure where

411

the interior porous structure provides weight reduction and structural integrity while the outer

412

stiff layer provides the mechanical strength of the composite. A covering of the outer surface of

413

the aerogels with a stiff wax would therefore create a core-shell composite similar to the

414

structure that has been optimized by nature. The spin coating of molten carnauba wax on the

415

aerogels resulted in complete coverage of four adjoining sides of the porous aerogel. The average

416

thickness of the coated layer was 300 µm but variations in the structure of the aerogel surface

417

naturally resulted in variations of the wax coating layer. Since cellulose has well documented

418

amphiphilic nature, i.e. a high water wettability and a high dispersive surface energy 42-43 the wax

419

wets the pore walls and can therefore penetrate into the aerogel to different extents depending on

420

the local structure. The majority of the wax is however located on the surface where it sealed the

421

pores and created a solid wax crust (see Figure 7a). The penetration of the wax into the structure

422

is also indeed similar to how the keratin shell found in porcupine quills extends into the interior

423

of the quill39. The density of the wax coated aerogels increased fourfold from 31.8 kg/m3

424

(17.5 g/L aerogels) to 122.8 kg/m3, a value similar to that of a toucan beak (100 kg/m3) 41. The

425

effect of wax coating on the mechanical properties was tested in uniaxial compression and the

426

elastic modulus increased from approximately 1 MPa for the 17.5 g/L aerogel to 10 MPa for the

and the beak of the toucan bird

41

resemble the porous structure found in the aerogels


23


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427

corresponding coated aerogels. Figure 7b displays the initial part of the compression curves for a

428

wax coated and an uncoated aerogel. The tenfold increase in modulus is substantial considering

429

that the wax coating is only approximately 300 µm thick, which is comparable to the 500 µm

430

thick keratin layer of the toucan beak 41. The wax coating effectively increased the modulus and

431

the strength of the material and the composite displayed the dual effect of a porous interior and

432

stiff exterior. To exemplify the versatility of this core/shell aerogel, a toucan beak was moulded,

433

mechanically functionalized by a wax coating, see inset in Figure 7b, and spray painted to mime

434

the stiff and light weight shape and colour of a real toucan beak.

435 436

Figure 7 (a) SEM image of a cross section of a wax-coated aerogel (scale bar 250 µm), and (b)

437

Plot of the mechanical compressive stress vs. strain of reference and wax-coated aerogels, inset

438

is a photograph of a 16 cm long wax coated toucan beak inspired aerogel.

439

The porous inner structure of the aerogel also provides a large specific surface area that could

440

be utilized for further functionalization. By soaking the wet stable aerogels in a solution

441

containing pyrrole, a dopant and an initiator, a thin film of PPy was synthesized to cover the

442

entire aerogel structure. A linear relationship between the achieved PPy loading and the initial

443

pyrrole concentration was observed and aerogels with PPy loadings ranging from 11.3 wt%


24

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Biomacromolecules

444

(0.02 M Py) to 44 wt% (0.06 M Py) were prepared. The coating did not affect the external

445

dimensions of the aerogels which remained intact also after solvent exchange and drying under

446

ambient conditions. The polymerization of PPy turned the aerogels completely black and the

447

dark colour extended throughout the entire interior of the aerogel suggesting an even PPy

448

coverage. The coating itself consisted of closely packed PPy nanoparticles, displayed in

449

Figure 8a, with an average diameter of 35 nm which created an interconnected network verified

450

by the electrical conductivity of the aerogels. The electrical conductivity increased with PPy

451

loading up to a maximum value of 7 mS/cm for the aerogel containing 44 wt% PPy. The

452

conductivity value was calculated by considering the total aerogel volume including air. The

453

conductivity of the low density PPy composite was enough to light up a red LED connected by

454

two pieces of PPy coated aerogels exposed to a potential of 6V, see Figure 8b.

455

To further exemplify the versatility of crosslinked aerogel, a complex electrically conducting

456

solenoid, similar to that which was prepared from a CNF nanosheet with conducting polymers, 44

457

was prepared by reshaping a 50 cm long, 4 mm diameter aerogel cylinder, prior to

458

polymerisation of pyrrole on the surfaces within the porous structure. The entire solenoid had a

459

resistance of 7 kOhm and created an average magnetic field of 0.4 µT in the direction of the

460

solenoid by passing a current (3 mA) through the solenoid, see Figure 8c-d.


25


Page 26 of 41

461 462

Figure 8 (a) SEM image of the PPy coating on the surface of the functionalized aerogel (scale

463

bar 250 nm), photographs of (b) a lit LED connected by PPy-coated aerogels, (c) a PPy-

464

functionalized solenoid aerogel and the magnetometer (scale bar 5 mm), and (d) plot of the

465

magnetic field generated inside the solenoid as the current is turned on and off, the solid lines

466

represents the average magnetic field.

467

Conclusions

468

It has been shown that it is possible to mould, extrude, and reshape wet-stable CNF based

469

aerogels into complex shapes based on the freezing-induced crosslinking technique. Depending

470

on the shaping method CNF aerogels with densities ranging from 12 kg/m3 to 34 kg/m3 (7.5 g/L

471

– 20 g/L) could be obtained. All samples, independent of concentration, displayed a volumetric

472

retention of 75% after solvent exchange and ambient drying. The dispersions in the higher end of


26

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Biomacromolecules

473

the concentration range had a viscosity high enough to allow the periodate/CNF mixture to be

474

extruded through a nozzle and retain the shape in the liquid state before being crosslinked by

475

freezing. This opens up the possibility of using the freeze-linking method in combination with

476

3D-printing to produce unconventional wet-stable 3D structures. Additionally by re-soaking a

477

dry aerogel in water it became plasticized and could be reshaped into new macro structures such

478

as solenoids that are not possible to achieve by moulding. The formed macro structure became

479

locked and crosslinked into its new shape following either a solvent exchange or a freezing-

480

solvent exchange step which introduced new crosslinks between the CNFs.

481

The compressive modulus and yield strength scaled with an increasing density of the aerogels

482

formed at a concentration of up to 15 g/L after which the material property first increased

483

significantly for the samples formed at 17.5 g/L and subsequently decreased for the aerogels

484

formed at 20 g/L. The sharp increase and subsequent drop was attributed to the conditions in the

485

dispersion and interactions between the CNFs during freezing rather than simply the density or

486

the microscopic internal structure of the aerogels. At higher concentrations the CNF movements

487

became restricted earlier during freezing which inhibits an efficient crosslinking. The aerogels

488

showed shape memory effects exemplified by the 17.5 g/L dry aerogels that could be

489

compressed to 80%, re-soaked in water and re-dried 5 times without any significant shape or

490

volumetric loss. The mechanical performance was affected by the cycling and a decrease in

491

modulus and yield stress was observed after 1 cycle. However, the retained and in the case of

492

regained mechanical property during cycles 2-4 is argued to be due to the softening of the cell

493

walls when reswollen in water allowing the CNFs to rearrange and hence to allow for a renewed

494

crosslinking upon re-drying.


27


Page 28 of 41

495

One of the main advantages of aerogels is that both the inner porous structure and the outer

496

surface can be functionalized for different applications. The current work illustrates that it is

497

possible to completely coat the inner structure of the aerogel with the conducting polymer PPy

498

introducing enough conductivity to be able to light up an LED and to generate a 0.4 µT magnetic

499

field inside a functionalized solenoid shaped aerogel. Another example shows that by applying a

500

thin coating of carnauba wax to the surface of a toucan beak shaped aerogel, it is possible to

501

mimic a natural stiff and light weight structure and increase the modulus by a factor of ten. These

502

three examples show that it is possible to combine advanced shaping with functionalization of

503

both the inner structure and the surface of the aerogels, radically extending the possible use of

504

aerogels.

505

Associated content

506

SEM image of a freeze-dried 0.1 g/L oxidized CNF dispersion; photographs of a 7.5 g/L and a

507

17.5 g/L aerogel soaked in water; SEM image of the cross section of a 5 times cyclically

508

compressed aerogel displaying the creases observed on the surfaces of the 2D sheets comprised

509

of CNFs.

510

Author information

511

Corresponding Authors

512

*(J.E.) E-mail: [email protected]

513

*(L.W.) E-mail: [email protected]

514

Notes

515

The authors declare no competing financial interest.

516 517


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Biomacromolecules

518 519 520 521

Acknowledgements

522

Johan Erlandsson acknowledges the Swedish Energy Agency through the Modulit project

523

(grant number 37716-1) and Vinnova through the Digitial Cellulose Centre for financial support.

524

Hugo Françon and Hjalmar Granberg acknowledge Stiftelsen för Strategisk Forskning through

525

the 0D-3D project, Andrew Marais acknowledges Knut and Alice Wallenberg foundation for

526

financial support. Lars Wågberg acknowledges the Wallenberg Wood Science Centre for

527

financial support.

528

References

529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548

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25. Zhao, H.; Heindel, N., Determination of Degree of Substitution of Formyl Groups in Polyaldehyde Dextran by the Hydroxylamine Hydrochloride Method. Pharm Res 1991, 8, 400402. 26. Munier, P.; Gordeyeva, K.; Bergström, L.; Fall, A. B., Directional Freezing of Nanocellulose Dispersions Aligns the Rod-Like Particles and Produces Low-Density and Robust Particle Networks. Biomacromolecules 2016, 17, 1875-1881. 27. Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L., Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide. Nat Nano 2015, 10, 277-283. 28. Deville, S.; Adrien, J.; Maire, E.; Scheel, M.; Di Michiel, M., Time-lapse, threedimensional in situ imaging of ice crystal growth in a colloidal silica suspension. Acta Mater. 2013, 61, 2077-2086. 29. Erlandsson, J.; Pettersson, T.; Ingverud, T.; Granberg, H.; Larsson, P. A.; Malkoch, M.; Wagberg, L., On the mechanism behind freezing-induced chemical crosslinking in ice-templated cellulose nanofibril aerogels. J. Mater. Chem. A 2018, 10.1039/C8TA06319B. 30. Li, W. L.; Lu, K.; Walz, J. Y., Freeze casting of porous materials: review of critical factors in microstructure evolution. Int. Mater. Rev. 2012, 57, 37-60. 31. Wu, S.; Zhu, C.; He, Z.; Xue, H.; Fan, Q.; Song, Y.; Francisco, J. S.; Zeng, X. C.; Wang, J., Ion-specific ice recrystallization provides a facile approach for the fabrication of porous materials. Nat. Commun. 2017, 8, 15154. 32. Naderi, A.; Lindström, T.; Pettersson, T., The state of carboxymethylated nanofibrils after homogenization-aided dilution from concentrated suspensions: a rheological perspective. Cellulose 2014, 21, 2357-2368. 33. Chen, W.; Li, Q.; Wang, Y.; Yi, X.; Zeng, J.; Yu, H.; Liu, Y.; Li, J., Comparative study of aerogels obtained from differently prepared nanocellulose fibers. ChemSusChem 2014, 7, 15461. 34. Osorio, D. A.; Seifried, B.; Moquin, P.; Grandfield, K.; Cranston, E. D., Morphology of cross-linked cellulose nanocrystal aerogels: cryo-templating versus pressurized gas expansion processing. J. Mater. Sci. 2018, 53, 9842-9860. 35. Guigo, N.; Mazeau, K.; Putaux, J.-L.; Heux, L., Surface modification of cellulose microfibrils by periodate oxidation and subsequent reductive amination with benzylamine: a topochemical study. Cellulose 2014, 21, 4119-4133. 36. Kim, U.-J.; Kuga, S.; Wada, M.; Okano, T.; Kondo, T., Periodate Oxidation of Crystalline Cellulose. Biomacromolecules 2000, 1, 488-492. 37. Gibson, L. J., Cellular solids : structure & properties / Lorna J. Gibson, Michael F. Ashby. Pergamon Press: Oxford [Oxfordshire] ; New York, 1988. 38. Meyers, M. A.; Chen, P.-Y.; Lin, A. Y.-M.; Seki, Y., Biological materials: Structure and mechanical properties. Prog. Mater Sci. 2008, 53, 1-206. 39. Yang, W.; Chao, C.; McKittrick, J., Axial compression of a hollow cylinder filled with foam: A study of porcupine quills. Acta Biomater. 2013, 9, 5297-5304. 40. Yang, W.; McKittrick, J., Separating the influence of the cortex and foam on the mechanical properties of porcupine quills. Acta Biomater. 2013, 9, 9065-9074. 41. Seki, Y.; Kad, B.; Benson, D.; Meyers, M. A., The toucan beak: Structure and mechanical response. Mater. Sci. Eng., C 2006, 26, 1412-1420.


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42. Lehtiö, J.; Sugiyama, J.; Gustavsson, M.; Fransson, L.; Linder, M.; Teeri, T. T., The binding specificity and affinity determinants of family 1 and family 3 cellulose binding modules. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 484-489. 43. Lindman, B.; Medronho, B.; Alves, L.; Costa, C.; Edlund, H.; Norgren, M., The relevance of structural features of cellulose and its interactions to dissolution, regeneration, gelation and plasticization phenomena. PCCP 2017, 19, 23704-23718. 44. Malti, A.; Tu, D.; Edberg, J.; Sani, N.; Rudd, S.; Evans, D.; Forchheimer, R., Electromagnetic devices from conducting polymers. Org. Electron. 2017, 50, 304-310.

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88x36mm (300 x 300 DPI)



Figure 1 (a) Schematic representation of the preparation of the aerogels, (b) shaping by moulding or extrusion (c) post production reshaping of aerogels, (d) schematic view of the ice crystals excluding the CNFs from the ice-phase and tightly packing them together in the lamellae.


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Figure 2 (a) Photograph of a spherical cross-linked CNF aerogel made from 20 g/L CNF gel (scale bar = 1 cm), (b) SEM-image of the cross-section of the spherical aerogel (scale bar = 500 µm), (c) SEM image showing the pore walls (scale bar = 10 µm), (d) The aerogel density as a function of the initial CNF concentration. 96x139mm (300 x 300 DPI)



Figure 3 Photographs of (a) an extruded swirl aerogel, (b) a solenoid- shaped aerogel prepared by reshaping a long cylindrical aerogel , (d) a cylindrical aerogel (left) reshaped into a prism (right). The scale bars are 1 cm. 150x128mm (300 x 300 DPI)


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Figure 4. SEM images of cross sections of moulded (cubic) aerogels prepared from (a) 7.5 g/L, (b) 17.5 g/L and (c) 20 g/L. Scale bars represent 1 mm. 384x88mm (300 x 300 DPI)



Figure 5. (a) Compressive stress strain curves for the freeze-linked aerogels prepared from different initial CNF concentrations. The inset shows the initial part of the compression test and (b) the cell wall properties calculated using the Ashby and Gibson scaling laws. 292x108mm (300 x 300 DPI)


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Figure 6. (a) A sequence of photographs showing the shape recovery of a compressed aerogel, (b) plot of the relative bulk modulus, yield stress and volume retention of a 17.5 g/L aerogel as a function of consecutive compression, reswelling and drying cycles, (c) SEM image of the cross section of an aerogel after 5 cycles of dry compression, reswelling and drying (scale bar 500 µm) and (d) SEM image showing buckling and cracking (arrows) of the pore walls after 5 compression cycles (scale bar 1 µm) 164x190mm (300 x 300 DPI)



Figure 7 (a) SEM image of a cross section of a wax-coated aerogel (scale bar 250 µm), and (b) Plot of the mechanical compressive stress vs. strain of reference and wax-coated aerogels, inset is a photograph of a 16 cm long wax coated toucan beak inspired aerogel.


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Figure 8 (a) SEM image of the PPy coating on the surface of the functionalized aerogel (scale bar 250 nm), photographs of (b) a lit LED connected by PPy-coated aerogels, (c) a PPy-functionalized solenoid aerogel and the magnetometer (scale bar 5 mm), and (d) plot of the magnetic field generated inside the solenoid as the current is turned on and off, the solid lines represents the average magnetic field. 180x136mm (300 x 300 DPI)


Cross-linked and shapeable porous 3D substrates from freeze-linked

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