Role of rheological properties on physical chitosan aerogels obtained by supercritical drying

Journal Pre-proof Role of rheological properties on physical chitosan aerogels obtained by supercritical drying Antonio Tabernero, Lucia Baldino, Alexander Misol, Stefano Cardea, Eva M. Mart´ın del Valle

PII:

S0144-8617(20)30024-2

DOI:

https://doi.org/10.1016/j.carbpol.2020.115850

Reference:

CARP 115850

To appear in:

Carbohydrate Polymers

Received Date:

25 November 2019

Revised Date:

1 January 2020

Accepted Date:

8 January 2020

Please cite this article as: Tabernero A, Baldino L, Misol A, Cardea S, del Valle EMM, Role of rheological properties on physical chitosan aerogels obtained by supercritical drying, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115850

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Role of rheological properties on physical chitosan aerogels obtained by supercritical drying Antonio Tabernero1,*, Lucia Baldino2, Alexander Misol3, Stefano Cardea2,*, Eva M. Martín del Valle1,4 1 Department of Chemical Engineering, University of Salamanca, Plaza los Caídos s/n, 37008, Salamanca (SA), Spain 2 Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II,

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132, 84084 Fisciano (SA), Italy 3 Department of Inorganic Chemistry, University of Salamanca, Plaza los Caídos s/n, 37008, Salamanca (SA), Spain

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4 Instituto de Investigación Biomédica de Salamanca, Hospital Virgen de la Vega,

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Corresponding author information:

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Paseo San Vicente, 58-182, 37007, Salamanca (SA), Spain

*Stefano Cardea, [email protected], 0039-089964091

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*Antonio Tabernero, [email protected], 0034-923294479

Research highlights

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Chitosan aerogels were obtained after SC-CO2 drying of physical hydrogels. Initial chitosan concentration modifies physical hydrogel entanglement. Aerogel textural and morphological properties do not depend on hydrogel rheology. Hydrogel elastic modulus modifies aerogel porosity and mechanical resistance. Stress-strain response was successfully modeled with Yeoh´s model.

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Graphical ab stract

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Abstract

Chitosan aerogels were obtained after using supercritical carbon dioxide to dry physical hydrogels, studying the effect of the rheological behavior of hydrogels and solutions on

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the final aerogels properties. An increase on the solutions pseudoplasticity increased the

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subsequent hydrogels physical entanglement, without showing a significant effect on aerogels morphology (nanoporous) and textural properties (pores of about 10 nm).

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However, an increase of hydrogel physical entanglement promoted the formation of aerogels with a higher compressive strength (from 0.2 to 0.80 MPa) and higher thermal

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decomposition range, while decreasing the porosity (from 90% to 94%). Aerogels stress-strain responses were also successfully fitted using a hyperelastic equation with

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three adjustable parameters (Yeoh), showing that this type of models must be taken into

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account when large stresses are studied.

Keywords: Chitosan, Aerogels, Rheology, Physical hydrogels 1. Introduction The development of advanced biomaterials for biomedical applications is an important task for achieving a successful use. Tissue regeneration and wound healing applications involve a proper interaction of the biomaterial with the human body. The used 2

biomaterial should promote a 3D tissue formation and should have interconnected pores and proper surface properties for cell adhesion and cell proliferation as well as having suitable mechanical properties. Moreover, this material has to be biodegradable and biocompatible to avoid side effects and has to be easily manufactured in different shapes and sizes (Nikolova and Chavali, 2019; Vasconcelos et al., 2019). In this context, aerogels are suitable candidates to be used as scaffolds for these applications. This type of materials (inorganic, organic, organic-inorganic hybrid

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materials, …) has a 3D porous structure with low density, high surface area, high porosity and also with a structure that can be chemically modified (García-González and

Smirnova, 2011; Wang et al., 2019) . An aerogel can be obtained using different

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techniques, but basically a gel is obtained by physical (modifying pH, temperature, …)

or chemical (adding a crosslinker to promote a covalent bond) methodologies. After

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that, the liquid of the gel is substituted by a gas using a drying process, to produce the

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corresponding aerogel. This drying step is relevant with respect the final aerogel morphology and can be performed by different techniques under ambient conditions or using supercritical CO2 (SC-CO2).

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Among these methodologies, the use of SC-CO2 provides several advantages. As it is well-known, SC-CO2 is characterized by a negligible value of surface tension and, as a

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consequence, the pore collapsing phenomenon that can be caused by other drying

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processes is avoided, obtaining aerogels with interconnected pores. Moreover, the drying step is faster (between 4 and 6 hours) in comparison with the other methodologies (sometimes days or weeks) (Christophe et al., 2012; Cardea et al., 2018). In order to perform a correct drying with SC-CO2, it is important to take into account the affinity between the SC-CO2 and the solvent in which the polymer gel is immersed. Due to the previous fact, if a hydrogel is produced, the water of this hydrogel must be

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replaced by another solvent (ethanol or acetone, for instance) to produce an alcogel that can be subsequently dried by SC-CO2. In this context, the effect of experimental conditions (above the mixture critical point solvent-CO2 to avoid surface tension problems with liquids), such as pressure or temperature, has been studied previously in different articles (Cardea et al., 2013; De Marco et al, 2015). Generally, conditions are mild, such as low temperature (30-45 ºC) and pressure larger than 200 bar. Moreover, SC-CO2 has also been used to produce interpenetrated and composite

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materials using two different polymers (Baldino et al., 2016) and to entrap drugs inside the polymeric matrix (Prosapio et al., 2015; Prosapio et al., 2016). This drying step has

been also numerically studied in different articles (Selmer et al., 2018; Selmer et al.

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2019).

As was mentioned above, aerogels can be obtained from different type of materials.

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Generally, aerogels are mainly produced from inorganic compounds, such as silica.

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However, in order to develop aerogels for biomedical applications, the use of new and natural biomaterials has been explored (Novak et al., 2003). Among them, polysaccharides are good candidates for that purpose. These materials are

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biocompatible, biodegradable and they have special and useful properties for biomedical applications (Barclay et al., 2019). For instance, chitosan fungicide

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properties (Nilsen-Nygaard et al., 2015; Xing et al., 2018) or alginate special properties

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for developing wound dressing systems are well-known in the literature (Mndlovu et al., 2019). Moreover, the typical structure of polysaccharides provides the possibility of crosslinking their functional groups to produce a gel, or even surface functionalization for a subsequent gelation or for modifying the future applications. Based on this possibilities, different polysaccharides, such as cellulose (Zaman et al., 2019), alginate

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(Robitzer et al., 2008) and pectin (Chen and Zhang, 2019), have been processed with SC-CO2 drying to obtain aerogels. Particularly, Alnaif et al., 2011 produced microspheres of alginate using different crosslinking methodologies (diffusion and internal) followed by solvent exchange and SC-CO2 drying. They showed that the gelation method produced an influence on surface area as well as pore radius and volume. Moreover, it was also demonstrated that an increase in alginate concentration (from 1% to 3% w/w) increased specific surface

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area, up to a value of 600 m2·g-1, with a pore radius of 9 nm since there were more groups available for crosslinking, and, as a consequence, hydrogel (and then the final aerogel) network was improved.

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On the other hand, Baldino et al., 2014 produced chitosan hydrogels by a freezegelation methodology. Water substitution with different alcohols was performed,

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followed by different drying techniques. These authors showed that acetone and SC-

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CO2 drying provided the best results concerning mechanical resistance and pore interconnectivity for chitosan aerogels. Chitosan beads aerogel were also obtained from physical hydrogels by Quignard et al., 2008.

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Other aerogels based on polysaccharides have been obtained by a SC-CO2 drying step. Carrageenan aerogels with specific area of 200 m2·g-1 (Quignard et al. 2008) and

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cellulose aerogels of 200 m2·g-1 (Fischer et al., 2006) are other examples of

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polysaccharide aerogels that were obtained after performing a SC-CO2 drying. In this context, previous works were generally focused on the effect of different experimental conditions, such as pressure, temperature or how the gelation is performed, on sample specific surface areas. However, the formed polysaccharide gels are not characterized by a rheological point of view before SC-CO2 drying, and only a few works studied the polymeric solutions

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before the gelation process. In this context, Tkalec et al., 2016 proposed an alginate gelation process with different alcohols. Concerning the role of the viscosity, higher viscosity alginates provided aerogels with higher specific surface area. But the effect of the alginate pseudoplasticity solutions on the aerogels was not studied and was not taken into account. Also, Tkalec et al., 2015 obtained gels of xantham and guar that were subsequently dried to produce aerogels, obtaining different surface areas depending on the material (from 110 m2·g-1 to 500 m2·g-1). These authors also

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performed a strain sweep analysis that gave information about the crossover strain, showing that, due to the differences between the elastic and the viscous part of the gels, (at the beginning of the experiment the storage modulus was far higher than the loss

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modulus), they were “permanent” gels. However, no frequency sweep was performed

and the authors did not study the influence of the elastic and viscous nature of the gels

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on aerogel texture properties. Finally, Salgado et al., 2017 produced two different polysaccharides aerogels by SC-CO2 drying. They performed frequency sweep analysis

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on physical gels of β-glucan aerogels, but they did not study aerogels properties depending on the gel viscoelastic characteristics. Therefore, the role of the complex

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viscosity of gels and the effect of the pseudoplasticity of the polymeric solutions on the aerogel properties are still unexplored.

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Based on these previous facts, this work proposes the study of the effect of the

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rheological properties of the polymeric solutions and gels on surface area, morphology, pore size, compressive strength, hyperelasticity, thermal decomposition and adsorption curves of chitosan aerogels, obtained by a physical gelation and SC-CO2 drying. The polymeric solutions will be studied with a steady shear flow, fitting the results to the classical power law, characterizing the pseudoplasticity of the initial polymeric solutions. On the other hand, the physical gels will be characterized with an oscillatory

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shear flow to determine both elastic and viscous modulus and the complex viscosity. It should be specified that chitosan physical gelation was selected because those gels have been previously used for biomedical applications and also to avoid the use of crosslinking agents that can produce side effects due to their toxicity. Moreover, although few studies have been performed on rheological properties of chemically crosslinked hydrogels, the effect of elastic and viscous characteristics of physical hydrogels on the final aerogels properties is still unknown. This work will also provide

into account physical hydrogel rheological characteristics.

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2. Methodology

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some information that can be used to improve aerogel design and manufacture by taking

2.1. Preparation of chitosan gels

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Chitosan physical gelation was performed by following the procedure described by

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Rami et al., 2014. Basically, different solutions (from 1% to 3% w/w) of chitosan (molecular weight around 250 kDa by Sigma-Aldrich) were prepared in acidic water (3% v/v of acetic acid supply by Sigma-Aldrich). Then, the pH of these solutions was

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increased with a NaOH (Sigma-Aldrich) solution (1 M) until a value of 13-14. Using this procedure, a sol-gel transition was produced due to intermolecular and hydrophobic

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interactions. Finally, the gel was washed with water to eliminate residuals salt. These

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steps can be observed in Figure 1, describing the complete aerogel preparation.

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2.2. Aerogel preparation by SC-CO2 drying

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Figure 1. Steps for producing chitosan physical aerogels.

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In previous chitosan gel supercritical drying, a multistep solvent exchange procedure

with water and ethanol was performed. These gels were immersed in mixtures of water-

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ethanol at different concentrations (10, 30, 50, 70, 90 and 100% v/v), 30 min each (the

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last one 24 h) to substitute water with ethanol and to produce the alcogel. The alcogel was processed in a homemade laboratory plant to perform the SC-CO2

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drying. This equipment and the used procedure were described elsewhere (Baldino et al., 2014; Cardea et al., 2018). Basically, the alcogel was added to a mould of 2 cm

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diameter (thickness of 2 mm) that was introduced in a stainless steel cylindrical high pressure vessel of 200 mL. Then, the vessel was filled with CO2 using a high pressure

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pump until achieving the target pressure. The temperature was controlled using a PID controller and a rotameter was located at the exit of the vessel to measure the CO2 flow. Experimental conditions were set at 200 bar, 35 ºC with a CO2 flow rate of 1.5 kg·h-1. After 5 h (to ensure a complete solvent removal), the system was depressurized and the aerogel was recovered.

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2.3. SEM characterization SEM microscopy was used to study aerogels morphology. Aerogels were previously cryofractured using liquid nitrogen. These samples were coated with gold at 30 mA for 150 s. Then, a scanning electron microscope (SEM mod. LEO420, Assing, Italy) was used to analyze aerogels morphology.

2.4. Thermogravimetric analysis

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A TA instrument (SDT Q600, USA) equipped with TGA-DTA was used to perform a thermogravimetric analysis. Analysis were done under a continuous nitrogen (L’Air

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Liquide, 99.999%) flow (50 mL·min-1) from 20 ºC to 900 ºC at 10 ºC·min-1.

2.5. Specific surface area analysis

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Aerogel specific surface area and pore size was studied with a Micromeritics Gemini

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VII 2390t apparatus. The sample was previously degassed at 110 ºC for 2 hours under a N2 stream in a Micromeritics FlowPrep 060 Sample Degass System and the adsorptiondesorption isotherms was evaluated with nitrogen (L’Air Liquide, 99.999%) at -196 ºC.

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Barret-Joyner-Halenda method was used to determine pore characteristics.

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2.6. Rheological measurements

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A rheometer (AR 1500 ex from TA Instruments) was used to perform the different rheological measurements, equipped with an aluminum plate of 4 cm of diameter at a gap size of 1 mm. This geometry was used for all the rheological experiments.

2.6.1. Shear flow experiments

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Firstly, different solutions of chitosan (1, 2 and 3% w/w) in acidic water were prepared and the sample was loaded on the peltier. After that, a temperature of 25 ºC was selected for all the experiments and a shear rate from 0.2 to 250 s-1 was set for all the experiments. The pseudoplasticity of the solutions was determined using this methodology (usually polymeric solutions are pseudoplastic). Then, the solutions rheological behavior was fitted to the classical power law equation (Eq. 1), determining the different semiempirical parameters. 𝜎 is the shear stress, K is the consistency index,

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𝛾̇ is the shear rate and n is the flow behavior index. 𝜎 = 𝐾𝛾̇ 𝑛

(Eq. 1)

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2.6.2. Oscillatory flow experiments

After producing the chitosan physical gels at different chitosan concentrations, the gels

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were studied by means of a frequency sweep. This sweep should be performed at a

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strain that should be in the linear viscoelastic region. In order to determine that region, a strain sweep analysis (from 0.5% to 500% strain) at a constant angular frequency of 1.0 rad·s-1 and at 25 ºC was performed. This angular frequency was used to determine

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chitosan hydrogels rheological properties (Argüelles-Monal et al., 1998). According to these results, a 1% strain was selected as the strain for all the different frequency sweeps

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for all the investigated gels oscillatory analysis.

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Frequency sweeps were conducted at a fixed strain of 1% and 25 ºC, from an angular frequency of 0.5 to 500 rad·s-1. These sweep results also provide information about the loss tangent (tanδ) that is calculated (Eq. 2) using frequency-dependent storage modulus (G’(w)) and loss modulus (G’’(w)). This value provides information about the relationship between storage and loss energy of the material and, as a consequence, can be used for determining if the material is solid-like or viscous-like. Due to the nature of

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the oscillatory analysis, a delta lower than 90º involves an elastic nature (the material response is in phase with the produced stress and the energy is storaged), whereas a delta higher than 90º shows a viscous-like gel because the energy is dissipated. tanδ = 𝐺 ′′ (𝑤)⁄𝐺′(𝑤)

(Eq. 2)

Finally, complex viscosity (η*) is calculated by Eq. 3 and depends on the real and imaginary components of the viscosity (η’ and η’’, respectively (Eq. 4 and Eq. 5) that are related with storage and loss modulus and with angular frequency w. η∗ = η′ − 𝑖η′′

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(Eq. 3)

η′ = 𝐺 ′′ (𝑤)/𝑤

(Eq. 4)

η′′ = 𝐺′(𝑤)/𝑤

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(Eq. 5)

2.7. Compression tests

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Mechanical properties of the aerogels were studied using an INSTRON 4301 (Instron

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Int. Ltd., High Wycombe, UK). Aerogel cylinders of 10 mm of diameter and 2.0 mm of height were cut and compressed at a cross-head speed of 1 mm·min-1.

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2.8. Modeling stress-strain curves

Yeoh hyperelastic model was used to fit stress-strain response of the different aerogels.

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Yeoh equation (Yeoh, 1993) (Eq. 6) is a hyperelastic equation that depends on the first

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strain invariant (I1). This model is an expanded series, which is usually truncated after the third term to define three adjustable parameters C1, C2 and C3. Yeoh´s equation has been already used to fit stress-strain response of different materials that have been processed with SC-CO2 (Tabernero et al., 2019a; Tabernero et al., 2019b). Results will provide information about the capability of hyperelastic models to fit physical aerogels depending on the initial chitosan concentration. The procedure for developing the series

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has been published in Rackl, 2015. In this context, it is important to specify that stretch ratio (λ) was calculated with Eq. 7, accounting li for the deformed length and Li for the undeformed length. Finally, compression stress was considered negative in the y axis and the true stress was calculated with Eq. 8. Since the material is compressed, the true stress will be lower than the unit. (Eq. 6)

𝜎𝐸 = 𝜎 · (1 + 𝜆)

(Eq. 7)

𝜆𝑖 = 𝑙𝑖 /𝐿𝑖

(Eq. 8)

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𝑊 = ∑𝑛𝑖=1 𝐶𝑖 · (𝐼1 − 3)𝑖

2.9. Porosity calculation

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Aerogels porosity was determined by weighing a known volume of an aerogel using Eq. 9. True density of the chitosan, excluding closed and open pores, was taken from

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Courrier et al., 2002, with a value of 1.48 g·cm-3.

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3. Results and discussion

(Eq. 9)

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𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦 (𝑃) = 1 − 𝐷𝑒𝑛𝑠𝑖𝑡𝑦/𝑇𝑟𝑢𝑒 𝑑𝑒𝑛𝑠𝑖𝑡𝑦

3.1. Rheological results

First results concern rheological results on polymeric solutions and gels, which will be

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subsequently correlated with aerogel properties.

3.1.2. Shear flow results of initial chitosan polymeric solutions Shear flow results from different chitosan solutions are shown in Figure 2. Moreover, fitting results with Power law are given in Table 1. All the investigated solutions behave as pseudoplastic fluids, which is the typical behavior of polymeric solutions for shear flow studies. These results are explained by taking into account chitosan structure. At

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the beginning, for all the investigated polymeric solutions, a higher shear stress is required for increasing the shear rate (Figure 2a). However, when the shear stress achieves a certain value, due to the alignments of the polymeric chains of chitosan solutions, a lower shear stress is required to increase the shear rate. Finally, for high shear stresses, there is a linear relationship between shear stress and shear rate because the polymeric structure is aligned and the solution behaves as a Newtonian fluid. These results indicate that the viscosity is higher at the beginning because the chitosan

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structure is against its own alignment (Figure 2b). However, for higher shear rates (and higher shear stresses), the structure is aligned and, as a consequence, the viscosity is

Chitosan 1.0% Chitosan 2.0% Chitosan 3.0% Power law fit (Chitosan 1.0%) Power law Fit (Chitosan 2.0%) Power law Fit (Chitosan 3.0%)

600

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Viscosity (Pa·s)

12 10

Chitosan 1.0% Chitosan 2.0% Chitosan 3.0%

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400

300

200

100

0 0

50

100

150

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Shear stress (Pa)

500

16

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lower and the force that is against the movement is also reduced (shear-thinning effect).

200

250

300

Shear rate (1/s)

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a)

8 6 4 2 0

0

50

100

150

200

250

300

Shear rate (1/s)

b)

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Figure 2. a) Shear stress-shear rate relationship (shear flow experiments) with Power

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law fit results and b) viscosity-shear rate experiments (shear flow experiments)

Modeling results indicate always a flow behavior index (parameter n) lower than 1. That means that the solutions are pseudoplastic and this parameter can be a quantitative indicator of the pseuplasticity of the solutions. As can be seen in Table 1, a decrease in chitosan concentration increases the flow behavior index (value closer to the unity). That means that solutions behavior get closer to the ideality (Newtonian fluid) and the 13

pseudoplasticity is reduced. This is explained based on the effect of the chitosan chains on the solution pseudoplasticity. A higher concentration increases the number of chitosan chains, reducing the ideal behavior and increasing the pseudoplastic character.

Table 1. Fitting results. K (Pa·s)

n

Standard error

1% Chitosan

0.57

0.74

10.14

2% Chitosan

7.40

0.57

15.35

3% Chitosan

28.39

0.52

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Solution

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3.1.2. Oscillatory results

Frequency sweep results for the different gels are shown in Figures 3 and 4. Figure 3a

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shows the storage modulus (G’) for different physical gels of chitosan. As can be seen

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in Figure 3a, storage modulus shows a weak dependence of the frequency, whereas the loss modulus (G’’) is almost constant (around 1000 Pa) for all the investigated concentrations of the physical gels. Moreover, the storage modulus is always higher

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(more than order of magnitude) than the loss modulus (G’’), as can be observed in Figure 3b. Both phenomena indicate that the formed physical gels provide a permanent

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network and can be considered as “permanent” gels; therefore, the gel elasticity nature

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(solid-like) always dominates over the gel viscous nature. Figure 3a also shows that the weak dependence of the storage modulus with the angular frequency is mathematically given by a small increase of the storage modulus with the angular frequency. This small increase highlights that there are still interactions between the chitosan structure of the physical entanglement.

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Finally, the storage modulus increases with chitosan concentration. This result can be explained by taking into account the increase in the gel entanglement due to the increase in chitosan concentration. This phenomenon increases the elastic nature of the physical gels.

10000

10000

Chitosan 1.0% Chitosan 2.0% Chitosan 3.0%

1000

100 0.1

1

10

100

1000

100

10 0.1

1000

Ang. Frequency (rad·s-1)

a)

Chitosan 1.0% Chitosan 2.0% Chitosan 3.0%

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Loss modulus G'' (Pa)

Storage Modulus G' (Pa)

100000

1

10

100

1000

Ang. Frequency (rad·s-1)

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b)

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for the different chitosan solutions.

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Figure 3. a) Storage modulus for the different chitosan solutions and b) Loss modulus

Finally, Figure 4a and Figure 4b show the complex viscosity and the loss angle for the

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different chitosan physical gels. These results demonstrate that an increase in the chitosan concentration increases the complex viscosity. However, loss tangent is around

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10º for all the investigated concentrations. That phenomenon also highlights the predominant elastic nature of the obtained chitosan physical gels. In this context, a delta

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degree lower than 90º indicated a predominant solid like gel.

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100 Chitosan 1.0% Chitosan 2.0% Chitosan 3.0 %

Chitosan 1.0% Chitosan 2.0% Chitosan 3.0%

10000

Delta (degrees)

Complex Viscosity (Pa·s)

100000

1000

10

100

10 0.1

1

10

100

1 0.1

1000

Ang. Frequency (rad·s-1)

1

10

100

Ang. Frequency (rad·s-1)

a)

b)

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Figure 4. a) Complex viscosity of the different chitosan gels and b) Delta for the different chitosan gels.

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3.2. Aerogels characterization. 3.2.1. Morphology.

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Aerogels were obtained after drying the physical gels by SC-CO2, and then SEM

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analyses were performed to study material morphology. Figure 5 shows the SEM images of the aerogel at 1% and 3% w/w of chitosan. As can be observed, aerogels are

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characterized by structures with interconnected pores. This morphology was observed for all the obtained aerogels. Therefore, gel rheological characteristics did not provide

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an effect on aerogel internal structure after the SC-CO2 drying step. In this case, aerogels morphology did not depend on the physical gel characterization and the drying

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step technique was the most important parameter to obtain a porous structure.

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(a)

(b)

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Figure 5. SEM images of aerogel from solutions of 1% w/w (a) and 3% w/w (b) of chitosan.

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3.2.2. Thermal analysis

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On the other hand, Figure 6 shows the thermal analysis results for chitosan aerogels starting from different concentrations of chitosan physical gels (1% and 3% w/w).

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These results indicate that after the first weight loss due to moisture removal, there was only one weight-loss step, as it is reported in Corazzari et al., 2015. These weight loss

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profile and temperature range are similar for all the aerogels samples (Figure 6). However, it is possible to observe that the main decomposition for the 3% w/w chitosan

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aerogel ranges from 200 to 400 ºC approximately, whereas the decomposition for the 1% w/w chitosan ranges from 200 to 250 ºC. Although not shown here, 2% w/w

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chitosan aerogels have intermediate properties between 1% and 3% w/w chitosan aerogels. That fact highlights that there is a relationship between aerogels thermal properties, physical gel elastic modulus and initial chitosan solutions pseudoplasticity.

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100 Chitosan 3.0% Chitosan 1.0%

% Weight

80

60

40

0 0

200

400

600

800

1000

Temperature (ºC)

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Figure 6. Aerogels TGA

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In this context, an increase of gel chitosan concentration increases the physical gel

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entanglement, according to the storage modulus (G’) from the oscillatory experiments

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(frequency sweep). As a consequence, a more solid-like gel will increase the temperature range for the main aerogel decomposition step. Therefore, this previous

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physical gel characterization can be used to predict in a qualitative way aerogel thermal properties. Moreover, since the frequency sweep results are dependent on the initial

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solution pseudoplasticity, it would be possible to know that the higher the

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pseudoplasticity is, the wider and the slower main decomposition step will be.

3.2.3. Aerogels textural properties Aerogel surface properties concerning pore size, BET surface area, BJH pore size, density and porosity are indicated in Table 2. As can be seen in this table, pseudoplasticity and gel elasticity do not produce a significant effect on pore size and specific surface area of the aerogels. Concerning textural properties, SC-CO2 drying is 18

not dependent on the rheological properties of the physical gel because pore size and surface area range from 10-11 nm and 110-125 m2·g-1 respectively. Similar specific surface areas have been obtained for other polysaccharides (Quignard et al., 2008). However, porosity was reduced (from 94% to 90%) when increasing chitosan concentration, indicating the existence of fewer pores in the aerogel structure. These results can be explained from a rheological point of view because a higher elasticity increases the gel entanglement and the number of pores is reduced. This fact is

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obviously related to aerogel density, ranging from 0.090 to 0.123 g·cm-3 depending on chitosan hydrogel entanglements.

Density results have a significant effect on aerogels porosity. Although porosity is

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always greater than 90%, viscoelastic gel properties had an effect on that property, since

porosity increases when high elastic modulus hydrogels were dried. Therefore, an

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increase in gel interactions promoted the formation of aerogels with a lower porosity.

Table 2. Aerogel surface properties. BET

concentration

area (m2·g-1)

(nm)

(g·cm-3)

(%)

115

11

0.090

94

125

10

0.112

92

110

11

0.137

90

1% Chitosan

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2% Chitosan

Surface BJH Pore size Density

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Solution

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3% Chitosan

Porosity

On the other hand, Figure 7 shows adsorption isotherms (type II isotherms) for aerogels starting from two different physical chitosan gels (1% and 3% w/w of chitosan). According to these results, the physical gel properties do not affect the isotherm type. Type II isotherms were always obtained, indicating that every aerogel was characterized

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by an unrestricted monolayer-multilayer adsorption up to a high p/po. The only difference observed was that 1% w/w chitosan concentration aerogels can adsorb more nitrogen (close to a value of 250 cm3·g-1 STP (standard temperature and pressure)) than the 3% w/w chitosan aerogels. That fact is explained by taking into account the different specific surface area and pore size (Thommes et al., 2015). A hysteresis curve was also observed for all the investigated aerogels due to the complex character of the pore structure. In this case, hysteresis is typical of materials

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with a delayed condensation and ink-bottle pores with similar withds of the neck size distribution and pore/cavity size distributions (Thommes et al., 2015).

Adsorption Desorption

100

50

0 0.0

Adsorption Desorption

150

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150

200

-p

Quantity adsorbed (cm 3/g STP)

200

250

0.2

0.4

0.6

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Quantity adsorbed (cm 3/g STP)

250

0.8

1.0

50

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

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Relative pressure (P/Po)

100

Figure 7. Type II isotherms for aerogels from 1% w/w (left) and 3% w/w (right)

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chitosan concentrations.

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3.2.4. Compression test results and modelling stress-strain response The results from compression tests indicate that 3% w/w chitosan aerogels were characterized by the greatest resistance to a compression stress. Specifically, the compressive strength increased 4 times when the initial chitosan concentration increased from 1% to 3% w/w. These results can be observed in Table 3 and Figure 8.

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Table 3. Aerogels compressive strength and Yeoh model parameters and fitting deviations. Compressive strength

Porosity C1

(MPa)

(%)

1% Chitosan

0.19

94

0.97

0.63

0.61

20.57

2% Chitosan

0.48

92

-0.067

1.42

-0.37

29.64

3% Chitosan

0.79

90

0.099

0.16

-0.045

10.32

C2

C3

Deviation (%)

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Solution

These results can be explained in terms of aerogel porosity and gel viscoelastic

properties. A higher elastic modulus produces a greater entanglement of the chitosan

-p

chains in the produced hydrogel. That fact shows an effect on the final aerogel porosity (Table 2). An increase in the porosity value reduces the compressive strength because

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there are more voids in the polymeric network and the material cannot tolerate high

lP

stresses.

Finally, Figure 8 shows as Yeoh model properly fits experimental material stress-strain response. Deviations range from 10 to 30%, obtaining the worst fit at low stresses for

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the different systems.

These results highlight that these chitosan aerogels can be considered as hyperelastic

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materials regarding their mechanical behavior and elasticity ideality cannot be applied

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here. Hyperelasticity theory must be considered if the material behavior at high stresses is known.

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-6 Chitosan 1% Chitosan 2% Chitosan 3% Chitosan 1% (Yeoh fit) Chitosan 2% (Yeoh fit) Chitosan 3% (Yeoh fit)

True stress (MPa)

-5

-4

-3

-2

-1

0.6

0.7

0.8

0.9

1.0

1.1

1+Lambda

-p

Figure 8. Stress-strain experimental data and Yeoh fit.

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0 0.5

4. Conclusions

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From the experiments reported in this work, it can be concluded that the rheological properties of physical chitosan hydrogels can be tuned to modify chitosan aerogel

lP

properties after a supercritical drying process. Different chitosan physical gels were obtained after promoting a network physical entanglement. Rheologically, a higher

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chitosan concentration produced a higher pseudoplastic polymeric solution that at the same time produced a physical gel with a higher elastic modulus (higher physical

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entanglement). A high hydrogel entanglement increased the thermal decomposition range of the different aerogels as well as reducing the aerogel porosity and, therefore,

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increased the compressive strength. Stress-strain response was successfully modelled with a hyperelastic model, demonstrating the hyperelastic behavior of these aerogels. However, solutions and hydrogels rheological behavior did not affect aerogel morphology and textural properties regarding pore size and surface area. In summary, chitosan solutions pseudoplasticity and gels viscoelasticity properties of chitosan physical gels can be tuned, to control thermal decomposition profile, porosity and 22

mechanical properties of the obtained aerogels. Taking into account these results, it can also be indicated that, since rheology did not show any effect on pore size, this parameter must be controlled by modifying the experimental conditions of the SC-CO2

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drying step.

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