A Study of the Physical and Mechanical Properties of Aerogels

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A study of the physical and mechanical properties of aerogels obtained from bacterial cellulose Victor V. Revin, Nikolay A. Pestov, Michael V. Shchankin, Vladimir M. Mishkin, Vladimir I. Platonov, and Dmitriy A. Uglanov Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01816 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Biomacromolecules

A study of the physical and mechanical properties of aerogels obtained from bacterial cellulose Victor V. Revin1, Nikolay A. Pestov1*, Michael V. Shchankin1, Vladimir M. Mishkin1, Vladimir I. Platonov2, Dmitriy A. Uglanov2 1

National Research Ogarev Mordovia State University, Saransk, 430005, Russia

2

Samara National Research University, Samara, 443086, Russia

KEYWORDS: Bacterial cellulose, Aerogel, Porosity, Thermal conductivity, Sound absorption.

ABSTRACT: Aerogels with a density of 4.2-22.8 kg/m3 were obtained from bacterial cellulose synthesized in static and dynamic cultivation conditions on a molasses medium. The strength properties and porous structure of the aerogels strongly depended on their density. With a 22.8 kg/m3 aerogel density, the modulus of elasticity at 80% compression of the sample was 0.1 MPa. The decrease in the density of aerogels led to an increase in the pore sizes ranging from 20 µm to 1000 µm and a decrease in the modulus of elasticity. These characteristics were more pronounced in aerogels obtained from bacterial cellulose in static cultivation conditions. The aerogels had a low coefficient of thermal conductivity (0.0257 W/m*°С), which is comparable with the thermal conductivity of air, and moderate thermal stability since the degradation processes of the aerogels began at 237°C. The aerogels obtained from bacterial cellulose had

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high sound absorption coefficients in the frequency range of 200-5000 Hz, which makes it possible to use as heat- and sound-insulating materials.

INTRODUCTION Bacterial cellulose is an extracellular metabolic product of certain bacteria. The most highpotential producers of bacterial cellulose are bacteria of the genus Komagataeibacter (formerly genus Gluconacetobacter)1, which synthesize it to provide cells with a sufficient amount of oxygen by moving the gel film to the surface of the liquid medium. In addition to bacteria of the genus

Komagataeibacter,

gram-negative

and

gram-positive

bacteria

of

the

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Agrobacterium2,3, Achromobacter4, Aerobacter5, Enterobacter6,7, Rhizobium4, Salmonella8 and Gram-positive bacteria Sarcina ventriculi5 and Rhodococcus9 have been used. Bacterial cellulose has the same chemical structure as plant cellulose, but it differs in its structure from the plant polymer. Bacterial and plant cellulose differ because bacterial cellulose is synthesized in a chemically pure form, the thickness of its fibrils is much smaller than the plant polymer and bacterial cellulose has a higher degree of crystallinity10. The unique properties of bacterial cellulose have led researchers to find its application in a wide variety of commercial products. Bacterial cellulose has considerable potential in medicine, for example, in tissue engineering, as an ideal bandage, which can be used in skin grafts, wound treatment, postoperative sutures, ulcers and for purulent inflammation, chafes and bed sores11-14. This gel film can be modified to provide it with antimicrobial, anti-inflammatory and cicatrizing properties1518.

This polymer can be used to produce high-quality paper, textiles and composite materials19-21.

Among the materials obtained from cellulose, there is an increasing emphasis on aerogels.


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Aerogels are highly porous nanostructure materials that were first obtained by Kistler in 193122. This material has high porosity, extremely low density and a high surface area. Cellulose aerogels can be an alternative to silica aerogels in some areas where their properties, such as biodegradability and biocompatibility, are required. This may be an area of medicine where organic aerogels can act as drug delivery vehicles23. The high sorption capacity of cellulose aerogels in relation to heavy metals and toxic organic compounds allows them to be used to create technologies for environmental remediation of pollution24. Due to the formation of the open-cell structure of the aerogel, this material can act as a thermal insulation material25. The usage of bacterial cellulose for aerogels stemmed from the fact that individual nanoscale bacterial cellulose fibrils have a high modulus of elasticity and high strength, which are more than 10 and 17 MPa, respectively26. Therefore, it could be expected that the resulting aerogels would have high strength properties. However, the aerogels obtained from this polymer do not always have satisfactory strength properties27, which would allow the use of this material in conditions where they would be mechanically restrained. Bacterial cellulose can be synthesized under different conditions of cultivation, which can affect the properties of the cellulose and the aerogel obtained from it. Different cultivation conditions include static or dynamic cultivation, which determine the form of synthesized bacterial cellulose and cultivation on media of different compositions, including media consisting of wastes from food processing28. The use of different media also determines some properties of bacterial cellulose fibrils, namely, the thickness of bacterial cellulose fibrils and the degree of its crystallinity10,29. The use of by-products from food processing as growth medium can reduce the cost of bacterial cellulose


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production. Molasses, which is a by-product of sugar production, can be used as growth medium for bacterial cellulose cultivation10,28. Taking the aforementioned characteristics into consideration, the main purpose of this study is to analyse the structure of bacterial cellulose and investigate the strength properties of aerogels with different densities obtained from this polymer synthesized under static and dynamic cultivation conditions. Several problems can be addressed based on the following objectives: 1) to study the microporous structure of aerogels depending on the biosynthetic conditions of bacterial cellulose, 2) to research the impact of preliminary treatments and aerogel density on the porosity of the resulting material and 3) to assess the heat conductivity and sound isolation characteristics of the material. MATERIALS AND METHODS Production of bacterial cellulose and cultivation conditions. Bacterial cellulose was produced by a strain of Gluconacetobacter sucrofermentans H-11030-31, which was grown on Hestrin & Schramm (HS) medium with the following composition: 2.0% glucose, 0.5% yeast extract, 0.5% peptone, 0.27% Na2HPO4*12H2O and 0.115% citric acid monohydrate32. Biosynthesis and purification of bacterial cellulose. To obtain inoculum, Gluconacetobacter sucrofermentans H-110 was grown periodically on a medium of HS in a 250 ml flask containing 100 ml medium on an ES-20/60 incubator shaker (Biosan, Latvia) at 250 rpm for 24 hours at 28°C. Inoculum size of 10% of fresh medium was used for inoculation of fresh medium with the following conditions: 5% molasses and pH 4.5. Cultivation was conducted in shallow trays (capacity of 7 L) under static conditions for 7 days or in dynamic conditions of cultivation on an ES-20/60 incubator shaker (BIOSAN) at 250 rpm and 28°C for 3 days.


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The obtained bacterial cellulose was processed with 0.5% NaOH solution at 80°C for 60 minutes for cell lysis. After alkaline treatment the bacterial cellulose was thoroughly washed with distilled water and then treated with 0.5% aqueous solution of acetic acid for 30 minutes, after which bacterial cellulose was thoroughly washed with distilled water. If necessary, the gel film was dried at 60°C for 24 hours. 13C NMR spectroscopy. The measurements of solid-state 13C NMR spectra were performed on a JEOL JNM-ECX400 spectrometer (9.39 T, 100.5 MHz) in the solid phase at room temperature using a cross-polarization technique (CPMAS) with a rotation speed of 10 kHz in 7 mm zirconium dioxide rotors. The angle of rotation of the sample (VUS) was defined at a rotation speed of 10 kHz. All MAS experiments were carried out at room temperature. Proton decoupling was performed by double-pulse phase modulation (TPPM). The measurements of 13C MAS NMR spectra were performed using the rotor synchronization of the echo sequence (RSE) or the one-setting pulse (SP) at a Larmor frequency of 100.6 MHz. To optimize the process of spectrum detection, the relaxation time of carbon nuclei was identified. The impulse duration for the angle of 90° was 6 ms and 12 ms for 180°, with a total number of 256 scans. Spectra were processed using ACD/NMR Processor Academic Edition software, Ver. 12.01. Infrared spectroscopy. IR spectra of bacterial cellulose were recorded on an IR Fourier spectrometer IRPrestige-2 (Japan) at a frequency of 400-4000 cm-1. The measurement was carried out using KBr discs containing 2-4 mg of bacterial cellulose. KBr discs were obtained using a manual hydraulic press. The crystallinity index of bacterial cellulose was calculated as follows33: CI=A1430/A897, where А1430 and А897 are absorption at 1430 and 750 сm-1.


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It has been previously shown that the two peaks at 750 and 710 cm-1 are typical for Iα and Iβ polymorphic modifications of cellulose34-36. The contents of Iα and Iβ cellulose were identified as follows: %Iβ = А710/(А710+А750), where А710 and А750 are absorption at 710 750 сm-1. Scanning electron microscopy. Scanning electron microscopy of bacterial cellulose aerogels was performed using a Quanta 200 I 3D FEI scanning electron microscope. Average pore sizes were studied using the image analysis software FEI xT microscope Server. At least size of 100 pores on each SEM image were measured. To determine pore area total area occupied by pores of the same size was calculated. The area of a single pore was determined based on the linear dimensions of each pore. Aerogel production. The bacterial cellulose aerogels were obtained using the freeze drying method. The hydrogel was produced by grinding cellulose, obtained in static or dynamic conditions from the cultivated bacteria using a laboratory homogenizer for 5 minutes. If necessary, the bacterial cellulose hydrogel was subjected to ultrasonic treatment for 6 minutes by a QSONICA Sonicator Q500 ultrasonic homogenizer (USA). Freezing of bacterial cellulose hydrogel was carried out by single-stage at a foil container with dimensions (length-widthheight) 90*60*25 mm. To do this, the resulting bacterial cellulose hydrogel was placed in a freezer with a temperature -50°C for 24 hours. Freeze drying was conducted using a FreeZone Plus freeze dryer lyophilizer (Labconco, USA) for 72 hours. Determination of aerogel properties. The density of the aerogels was calculated by determining the volume and weight of aerogel samples.


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To define the modulus of elasticity of an aerogel sample, which was a cube with an edge length of 15 mm, we compressed it by 80% of the original size using a HAAKE MARS III modular rotational rheometer (Thermo Fisher Scientific). The thermal conductivity coefficient was measured based on the following equation: λ= d*q/ ∆T, where d is the thickness of the sample (m), q is the heat flow density, ∆T is the temperature difference between the sample facets and λ is the thermal conductivity coefficient of the sample. The measurement of the heat flow density and the temperatures of the opposite face sides was performed using a digital device for measuring heat flow density (HFM-2, Russia). We considered the heat flow, which flowed through a test sample with dimensions of 60×40×10 mm, to be established if the values of the sample thermal conductivity, calculated from the results of five consecutive measurements, differed from each other by less than 1%. Thermogravimetric analysis was performed using a TGA/SDTA 851e instrument from Mettler Toledo (USA). The normal sound absorption coefficient was determined with an impedance tube from the Spectronics Company. To determine the normal sound absorption coefficient, we propagated a sound signal of varying frequencies through the aerogel sample using a NI USB-4431 module. The signal level that was propagated through the sample was measured using 2 PSB 377B02 microphones. The measurement results were processed using LabVIEW software with the sound and vibration toolset. RESULTS AND DISCUSSION Analysis of bacterial cellulose obtained in static and dynamic cultivation conditions. Biosynthesis of bacterial cellulose was conducted in static and dynamic cultivation conditions.


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Under static cultivation conditions, bacterial cellulose was obtained by cultivating the bacteria in sterile containers at 28°C for 5-7 days. At the same time, a gel film of bacterial cellulose appeared on the surface of the growth medium in the shape of the container. This bacterial cultivation method allowed us to obtain films with a predetermined shape. Under dynamic cultivation conditions, the synthesized bacterial cellulose has varying flake-like morphology (Figure 1).

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Figure 1. Gel film of bacterial cellulose obtained in static cultivation conditions (A) and bacterial cellulose flakes obtained in dynamic cultivation conditions (B). Both forms of bacterial cellulose can be used to produce aerogels. Bacterial cellulose obtained in static and dynamic cultivation conditions was analysed by IR spectroscopy (Figure 2). Chemically, bacterial and plant cellulose consist of glucose moieties, and the biosynthetic methods of this biopolymer do not affect the chemical structure. Analysis of the IR spectra of bacterial cellulose obtained under different cultivation conditions allowed identifying the changes in the absorption spectra. Therefore, in the IR spectra of bacterial cellulose obtained in static and dynamic cultivation conditions, we identified a peak in the area of 3352 cm-1, which is


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due to stretch vibrations of the cellulose hydroxyl groups involved in the formation of intramolecular and intermolecular hydrogen bonds37.

Static cultivation conditions Dynamic cultivation conditions

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Figure 2. IR spectra of bacterial cellulose obtained in static and dynamic cultivation conditions. In dynamic cultivation conditions, this peak became narrower compared to the static cultivation conditions, which indicates differences in the number of hydrogen bonds formed. The peak in the 2900 cm-1 area is due to the stretch vibrations of the C-H groups37. The peak in the 1650 cm-1 area is caused by deformation vibration of OH groups from adsorbed water. The peak in the 1431 cm-1 area was caused by symmetric CH2 bending38. This peak characterizes the degree of cellulose crystallinity, and its intensity decreased in proportion to the decrease in the degree of cellulose crystallinity. This peak was more intense under static cultivation conditions, which is indicative of a higher degree of crystallinity of bacterial cellulose synthesized in these conditions. The band at 897 cm-1 assigned as C–O–C stretching at the β-(1-4)-glycosidic linkage39. This peak characterizes the amorphous state of cellulose. The calculation of the crystallinity index on the basis of the absorption data at 1431 and 898 cm-1 indicated that in static cultivation conditions the crystallinity index is 2.59, while in dynamic conditions, it is 1.95.


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IR analysis also allows the determination of the ratio between 1α and 1β cellulose allomorphs. The ratio between 1α and 1β bacterial cellulose forms obtained in both static and dynamic cultivation conditions is 40%:60%. The crystal structure of bacterial cellulose was investigated by 13C NMR spectroscopy (Figure 3).

Figure 3. 13C NMR spectrum of solid bacterial cellulose. We were able to trace four spectral regions in the bacterial cellulose 13C NMR spectrum. According to a previous study40, one region (105.57 ppm) corresponds to the acetal С-1 carbon signal. The second region contains C-4 carbon signals. In this region, there is a double peak, which corresponds to the crystalline (89.4 ppm, approximately 85%) and amorphous forms (84.47 ppm, approximately 15%) of bacterial cellulose41,42. The third region contains the C-2, C3, and C-5 atomic signals in the pyranose ring, whereas the fourth region (65.76 ppm) contains the C-6 atomic signal. The degree of bacterial cellulose crystallinity was also determined by X-ray crystallography analysis (data not provided), and it was determined to be 87% crystalline.


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Therefore, this data indicates that bacterial cellulose synthesized in static and dynamic conditions has a high degree of crystallinity. Strength properties of aerogels made from bacterial cellulose obtained in static and dynamic cultivation conditions. The aerogels composed of bacterial cellulose, were obtained by freeze-drying of the frozen bacterial cellulose hydrogel. Crystallization of water can determine the pore size of the aerogel sample and such conditions as the sample size and the freezing mode can affect the properties of the aerogels from bacterial cellulose. All samples of hydrogel of bacterial cellulose were frozen under the same conditions. Freezing of bacterial cellulose hydrogel was carried out during a single-stage process in foil containers with dimensions 90*60*25 mm (length-width-height). Rapid freezing of hydrogel in liquid nitrogen or multistage cooling of the sample did not allow to obtain aerogels with satisfactory strength properties (data are not presented). To obtain a homogeneous hydrogel, we had to grind the bacterial cellulose gel film. The density and porosity and average pore size of bacterial cellulose-based aerogels can be changed by producing hydrogels with varying concentrations of bacterial cellulose. Therefore, aerogel properties can be easily controlled during the preliminary treatment stage of bacterial cellulose. Mechanical grinding of bacterial cellulose was combined with ultrasonic treatment as it is widely used for polymer disintegration. Furthermore, disintegration does not lead to a change in the chemical structure of the processed polymers but does decrease their molecular weight. From bacterial cellulose obtained in static or dynamic cultivation conditions, we obtained aerogel samples with densities ranging from 4.2-22.8 kg/m3 (Figure 4). The aerogel with the lowest density exceeded the air density (1.204 kg/m3) by a factor of 3.5. The strength


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properties of aerogels obtained under static and dynamic cultivation conditions were also examined (Figure 5).

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Figure 4. Aerogels with various densities obtained from bacterial cellulose synthesized under static (A) and dynamic (B) cultivation conditions that was not subjected to ultrasonic treatment and subjected to ultrasonic treatment at the hydrogel formation stage.

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The density reduction of aerogels obtained from bacterial cellulose synthesized in static cultivation conditions from 20 kg/m3 to 11.39 kg/m3 and led to a decrease in the aerogel’s modulus of elasticity from 0.08 to 0.024 MPa. Further aerogel density reduction to 4.02 kg/m3 led to a further decrease in the modulus of elasticity to 0.006 MPa. Ultrasonic treatment of bacterial cellulose hydrogels did not have any significant influence on the strength properties of the obtained aerogels (Figure 5B). The strength properties of bacterial cellulose aerogels synthesized in dynamic cultivation conditions were also examined (Figure 6). 0.09

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Figure 6. Changes in the strength properties of aerogels obtained from bacterial cellulose in dynamic cultivation conditions that was not subjected to ultrasonic treatment (A) and subjected to ultrasonic treatment (B) at the hydrogel formation stage. At an aerogel density of 19.22 kg/m3, the strength was 0.095 MPa, i.e., the strength was comparable with the strength of aerogels obtained from the bacterial cellulose in static cultivation conditions. The decrease in aerogel density of aerogels from bacterial cellulose synthesized under dynamic cultivation conditions did not lead to a significant decrease in the modulus of elasticity, as was observed in the case of aerogels produced from cellulose


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synthesized under static cultivation conditions. Therefore, the decrease in density from 19.22 kg/m3 to 13.56 kg/m3 led to a decrease in the aerogel strength from 0.095 to 0.048 MPa. An additional ultrasonic treatment of a bacterial cellulose hydrogel obtained in dynamic cultivation conditions led to the production of a more homogeneous bacterial cellulose hydrogel and did not lead to a change in the strength properties of the aerogel. Micromorphology of aerogels produced from bacterial cellulose synthesized in static and dynamic cultivation conditions. The bacterial cellulose gel film is an interlaced matrix of nanoscale bacterial cellulose fibrils. In the process of bacterial cellulose hydrogel preparation, the gel film must be grinded. During the formation of the hydrogel, the interaction between individual fragments of bacterial cellulose is facilitated through numerous hydrogen bonds, which stabilize the final structure of the aerogel and ensure the strength of this material43,44. We conducted a study of the microporous structure of aerogels obtained from bacterial cellulose synthesized in both static and dynamic cultivation conditions (Figures 7-10). The decrease in the aerogel density obtained from the bacterial cellulose synthesized under static conditions led to an increase in the pore size. In the aerogels with a density of 20.55 kg/m3, we observed pores with sizes ranging from 11-164 µm, and pores with a size of 27 µm were quantitatively prevalent. Despite that pores with a size of 27 µm represented the majority of the pores, these pores are present in approximately 15% of the occupied area (Figure 7A). The major portion of the space is occupied by pores with large sizes. The aerogel density reduction of up to 11.39 kg/m3 did not result in pore formation larger than 164 µm (Figure 7B). However, the decrease in aerogel density led to more pores with sizes from 38-110 µm, which occupied 67% of the total pore area. Further aerogel density reduction of up to 9.91 kg/m3 led to an increase in the pore size to 274 µm (Figure 7B).


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Figure 7. Scanning electron microscopy of aerogels with a density of 20.55 (A), 11.39 (B) and 9.91 (C) kg/m3 obtained from bacterial cellulose synthesized in static cultivation conditions. Pores with sizes from 137-274 µm comprised 87% of the total area occupied by the pores. The research of the strength properties showed that the decrease in the aerogel density from 20.55 to 11.39 kg/m3 led to a significant decrease in their strength properties, which could be associated with a change in the porous structure. The porous structure of aerogels with a density of 20.55 and 11.39 kg/m3 were fairly homogeneous, and the differences were consistent upon the emergence of a large number of pores with sizes ranging from 38-164 µm. There were no pores with a diameter greater than 164 µm in the aerogel with a density of 13.34 kg/m3.


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As evidenced by the strength properties of aerogels, ultrasonic treatment of bacterial cellulose hydrogels did not result in a change of the strength properties of aerogels obtained from bacterial cellulose. However, the examination of the microporous structure of aerogels obtained from bacterial cellulose, in which the hydrogel was treated with ultrasound, showed the appearance of pores, with the maximum diameter exceeding the corresponding diameter of the pores of aerogels prepared without ultrasound treatment by a factor of 2-3 (Figure 8). Similar to aerogels from bacterial cellulose not treated with ultrasound, aerogels obtained from bacterial cellulose treated with ultrasound had a large number of small pores (22 µm). Despite the presence of a large amount of small pores, they occupied a small volume of aerogel. A comparison of the pore distribution of aerogels with similar densities obtained from bacterial cellulose synthesized under static cultivation conditions either with or without ultrasonic treatment determined ultrasonic treatment led to an increase in the pore size of the aerogels. For instance, an aerogel with a density of 20.55 kg/m3 primarily had pores with dimensions of 16-27 µm (Figure 7). The ultrasonic treatment of bacterial cellulose hydrogels resulted in pore sizes from 22-110 µm. The decrease in the aerogel density from 13.34 to 10.97 kg/m3 did not lead to pores with diameters greater than 600 µm (Figure 8). The study of the porous structure of bacterial cellulose aerogels synthesized under dynamic cultivation conditions showed that the porous structure of the aerogels is characterized by the presence of a large number of small pores and the presence of larger pores which account for the bulk of the aerogel (Figure 9). For example, an aerogel with a density of 19.21 kg/m3 had pores with a size of 330 µm, which were twice as large as the pores of aerogels obtained from bacterial cellulose synthesized in static cultivation conditions. Furthermore, these large pores with a size from 165 to 330 µm occupied 58% of the total pore area.


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Figure 8. Scanning electron microscopy of aerogels with a density of 20.28 (A), 13.34 (B) and 10.97 (C) kg/m3 obtained from ultrasound-treated bacterial cellulose synthesized in static cultivation conditions. The strength properties of an aerogel with a density of 19.21 kg/m3 obtained from bacterial cellulose synthesized under dynamic cultivation conditions are comparable to the strength properties of the same aerogel obtained from bacterial cellulose synthesized under static cultivation conditions. The aerogel density reduction to 13.55 kg/m3 led to the formation of 527 µm pores, which account for 30% of the occupied area.


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110 154 176 220 242 264 286 308 330 374 396 418 440 550 659 725 835 989

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Figure 9. Scanning electron microscopy of aerogels with densities of 19.21 (A), 13.55 (B) and 10.24 (C) kg/m3 obtained from bacterial cellulose synthesized under dynamic cultivation conditions. However, the appearance of these large pores did not lead to a significant decrease in the strength properties of the aerogel, as was observed in aerogels obtained from bacterial cellulose in static cultivation conditions with a density of 11.39 kg/m3. Pores, 1 mm in size, appeared in aerogels with a density of 10.24 kg/m3 (Figure 9B). Nevertheless, a comparison of its strength properties to an aerogel with a density of 9.91 kg/m3 produced from bacterial cellulose


18

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synthesized in static cultivation conditions demonstrated increased strength properties despite the presence of larger pores. These results did not demonstrate a connection between the pore size and strength properties of bacterial cellulose aerogels. Figure 10 shows the porous structure of aerogels obtained from ultrasound-treated bacterial cellulose synthesized under dynamic cultivation conditions.

A

35

35

30

30 25

Pore area, %

Quantity, %

25 20 15 10

20 15 10 5

5

0

0 22

33

55

77

110

165

220

22

330

33

55

B

77

110

165

220

330

Pore size, μm

Pore size, μm 18

35

16

30

14

Pore area, %

Quantity, %

25 20 15 10

12 10 8 6 4

5

2 0

0 22

33

55

77

110

165

220

275

330

385

22

495

33

55

C

77

110

165

220

275

330

385

495

Pore size, μm

Pore size, μm 25

35 30

20

Pore area, %

25

Quantity, %

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

Biomacromolecules

20 15 10

15

10

5

5 0

0 22

33

55

77

110

165

220

330

440

659

Pore size, μm

22

33

55

77

110

165

220

330

440

659

Pore size, μm

Figure 10. Scanning electron microscopy of aerogels with a density of 22.8 (A), 12.82 (B) and 9.28 (C) kg/m3 obtained from ultrasound-treated bacterial cellulose synthesized in dynamic cultivation conditions.


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Biomacromolecules

The ultrasonic treatment of bacterial cellulose hydrogels synthesized in dynamic cultivation conditions led to the production of aerogels whose porous structure did not differ from the porous structure of aerogels obtained from bacterial cellulose that were not subjected to ultrasonic treatment. Therefore, ultrasonic treatment did not affect the strength properties of bacterial cellulose aerogels but led to an increase in the pore sizes of aerogels obtained from bacterial cellulose synthesized in static cultivation conditions. Heat- and sound-insulating properties of aerogels. Aerogels composed of bacterial cellulose are materials with an open-cell structure and should have good heat- and sound-insulating properties. Therefore, we conducted a study of the heat- and sound-insulating properties of aerogels obtained from bacterial cellulose synthesized under static cultivation conditions, which were not subjected to subsequent ultrasonic treatment. This type of aerogel was chosen primarily because these aerogels have a more homogeneous porous structure. Figure 11 shows the dependence of the thermal conductivity of aerogels with different densities. 0.03

Thermal conductivity W/m*°C

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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0.025 0.02 0.015 0.01 0.005 0 10.61

11.9

22.76

Density, kg/m3

Figure 11. The coefficient of thermal conductivity of aerogels from bacterial cellulose synthesized in the static conditions of cultivation.


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The thermal conductivity of bacterial cellulose aerogels synthesized under static cultivation conditions did not significantly depend on their density and was 0.0257 W/m*°C for the aerogel with a density of 22.76 kg/m3 and 0.0251 W/m*°C for the aerogel with a density of 10.61 kg/m3. The thermal conductivity of bacterial cellulose aerogels is comparable with the thermal conductivity of air, which is 0.0251 W/m*°C at 10°C. We compared the thermal conductivity coefficient parameters of aerogels obtained from regenerated plant cellulose in the NaOH/urea system, which were between 0,029-0,032 W/m*°C25,45. The aerogels obtained from bacterial cellulose are characterized by high porosity, which reaches 99%; therefore, air occupies the main volume of the aerogel. Consequently, bacterial cellulose aerogels are materials with excellent heat-insulating properties. The thermal conductivity of a material also depends on factors such as the density of the material, its porosity and the structure and the shape of the pores. Not only the total porosity but also the shape, size and orientation of the pores play an important role in thermal conductivity since the directional flow of heat and radiance within the pores has a great influence on the overall thermal conductivity of the material. Studies of the microporous structure of aerogels have shown that this material has a large number of small pores, which do not exceed 1 mm, even in aerogels with a low density. However, aerogels with a more developed porous structure and with lower density had a lower coefficient of thermal conductivity. Unfortunately, an increase in aerogel porosity led to a decrease in their strength properties, which makes it not possible to use aerogels with low densities as heat-insulating materials. For a variety of functional materials, an important characteristic is their temperature stability, especially in high-temperature applications. Therefore, we also conducted a thermogravimetric study of bacterial cellulose and aerogels obtained from bacterial cellulose (Figure 12). The


21

Biomacromolecules

weight loss of bacterial cellulose started at 57°С and upon reaching a temperature of 200°C, bacterial cellulose lost 5% of its mass. 100 Aerogel, 20.22 kg/m3 Bacterial cellulose

80

Weight loss, %

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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60

40

20

0 25

75

125

175

225

275

325

375

425

475

Temperature, оС

Figure 12. Thermogravimetric analysis of bacterial cellulose and aerogel derived from bacterial cellulose synthesized in the static conditions of cultivation.

This weight loss is caused by the evaporation of bacterial cellulose-related water46. Thermogravimetric analysis showed that the degradation processes of the bacterial cellulose film began to appear at 290°C, and when the temperature reached 300°C, there was a loss of 10% of the bacterial cellulose film mass. This result is consistent with other studies on the thermal stability of bacterial cellulose47,48. The weight loss process of aerogels began at a lower temperature and the loss of 10% of the mass was observed at 237°C. The decrease in the temperature stability of the aerogel could have been caused by the preservation of the bacterial cellulose nanofibrillar structure as a result of freeze-drying, since it has been previously demonstrated that the size of cellulose fibrils determines their reaction capacity49. The porous structure of the aerogel should provide not only high thermal insulation properties but also sufficient sound absorption. Therefore, the sound absorption coefficient was measured in the range of 200-5000 Hz (Table 1).


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Table 1. Sound absorption coefficients of bacterial cellulose aerogels and assorted construction materials. Material Frequency (Hz)

Aerogelbacterial cellulose (this study)

Concrete

Foamed plastic

Acoustic foam rubber

Mineral wool

200

0.33

0.01

0.08

0.2

0.56

250

0.34

0.01

0.09

0.3

0.65

315

0.41

0.01

0.11

0.45

0.77

400

0.54

0.02

0.15

0.5

0.95

500

0.67

0.02

0.19

0.65

0.98

630

0.74

0.02

0.18

0.71

1

800

0.72

0.02

0.175

0.78

0.98

1000

0.65

0.02

0.16

0.8

0.96

1250

0.77

0.025

0.155

0.8

0.95

1600

0.99

0.031

0.15

0.73

0.88

2000

0.92

0.04

0.14

0.7

0.86

2500

0.84

0.04

0.138

0.68

0.83

3150

0.87

0.04

0.13

0.66

0.82

4000

0.93

0.04

0.12

0.6

0.78

5000

0.95

0.041

0.115

0.55

0.77

According to the data, aerogels obtained from bacterial cellulose have a sufficiently high sound absorption coefficient in the entire measured range. Table 1 depicts the sound absorption coefficients of some materials used for construction applications.


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Among these materials, bacterial cellulose aerogels could only be compared with mineral wool, which absorbs more sound than bacterial cellulose aerogel only in the frequency range of 200-1250 Hz, while in the region of 1600-5000 Hz, bacterial cellulose aerogels had a higher sound absorption coefficient. Sound waves in the frequency range of 3000-5000 Hz have the most irritant effect on humans and bacterial cellulose aerogels had the highest sound absorption coefficient in the frequency range between 3000-5000 Hz. CONCLUSION The current study was aimed at the comparison of strength properties and microporous structure of aerogels obtained by freeze drying of bacterial cellulose synthesized in static and dynamic conditions of cultivation. Analysis of the IR spectra of bacterial cellulose obtained under different cultivation conditions enabled to identify the changes in the absorption spectra. In dynamic cultivation conditions 3352 cm-1 peak which is due to stretch vibrations of the cellulose hydroxyl groups involved in the formation of intramolecular and intermolecular hydrogen bonds became narrower compared to the static cultivation conditions, which indicates differences in the number of hydrogen bonds formed. The strength properties of bacterial cellulose aerogels synthesized under different cultivation conditions strongly depended on the density of the obtained material. However, the use of bacterial cellulose synthesized under dynamic conditions of cultivation allows to obtain aerogels with slightly higher strength characteristics. With a 22.8 kg/m3 aerogel density, the modulus of elasticity at 80% compression of the sample was 0.1 MPa. The study of microporous structure revealed that bacterial cellulose synthesized under static conditions of cultivation allowed to obtain aerogels with a larger number of small pores compared to aerogels from bacterial cellulose synthesized under dynamic conditions of cultivation. Ultrasonic treatment of hydrogel of bacterial cellulose synthesized under static


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conditions of cultivation led to the formation of larger pores. It can be assumed that this may be attributed to the formation of gas bubbles during the ultrasonic cavitation. Based on the increase in the pore size of aerogels from bacterial cellulose synthesized under static conditions of cultivation, it can be assumed that the size of gas bubbles induced by ultrasonic cavitation was not more than 400-500 µm. The appearance of larger pores in aerogels from bacterial cellulose synthesized under static conditions of cultivation after ultrasonic treatment did not lead to a change in the strength properties of these aerogels. Ultrasonic treatment of bacterial cellulose synthesized in dynamic conditions of cultivation did not lead to the increase in pores size and changes in strength properties of the obtained aerogels. The appearance of larger pores in aerogels from bacterial cellulose synthesized under static conditions of cultivation and exposed to ultrasonic treatment makes them similar both in the distribution of pore sizes and in the occupied pore area to aerogels from bacterial cellulose synthesized under dynamic conditions of cultivation. However, the strength properties of aerogels from bacterial cellulose synthesized under static conditions of cultivation and exposed to ultrasonic treatment at the stage of hydrogel formation were lower in comparison with aerogels from bacterial cellulose obtained under dynamic conditions of cultivation. This could be explained by the fact that ultrasonic treatment led to the formation of a heterogeneous porous structure, which was most noticeable in aerogels from bacterial cellulose synthesized under static conditions of cultivation. The aerogels from bacterial cellulose had a low coefficient of thermal conductivity (0.0257 W/m*°С), which is comparable with the thermal conductivity of air, and moderate thermal stability since the degradation processes of aerogels began at 237°C. The aerogels obtained from bacterial cellulose had high sound absorption coefficients in the frequency range of 200-5000 Hz, which makes it possible to use them as heat- and sound-insulating materials.


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AUTHOR INFORMATION Corresponding Author Nikolay A. Pestov (Email: [email protected]). ORCID Nikolay A. Pestov: 0000-0002-5041-4354. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript FUNDING SOURCES The authors are grateful for the financial support by Ministry of Education and Science of Russian Federation (№ 11.10882.2018/11.12). REFERENCES (1) Yuzo, Y.; Pattaraporn, Y.; Vu, H. T. L.; Yuki M.; Duangjai O.; Yasuyoshi, N. Subdivision of the genus Gluconacetobacter Yamada, Hoshino and Ishikawa 1998: the proposal of Komagatabacter gen. nov., for strains accommodated to the Gluconacetobacter xylinus group in the α-Proteobacteria. Ann. Microbiol. 2011, 62, 849–859. (2) Barnhart, D. M.; Su, S.; Farrand, S. K. A signaling pathway involving the diguanylate cyclase CelR and the response regulator DivK controls cellulose synthesis in Agrobacterium tumefaciens. J. Bacteriol. 2014, 196, 1257–1274. (3) Matthysse, A. G.; Thomas, D. L.; White, A. R. Mechanism of cellulose synthesis in Agrobacterium tumefaciens. J. Bacteriol. 1995, 177, 1076–1081.


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Table of Contents graphic (TOC)

A study of the physical and mechanical properties of aerogels obtained from bacterial cellulose Victor V. Revin1, Nikolay A. Pestov1*, Michael V. Shchankin1, Vladimir M. Mishkin1, Vladimir I. Platonov2, Dmitriy A. Uglanov2


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A Study of the Physical and Mechanical Properties of Aerogels

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