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STARCH AEROGELS: A MEMBER OF THE FAMILY OF THERMAL SUPER-INSULATING MATERIALS Lucile Druel, Richard Bardl, Waltraud Vorwerg, and Tatiana Budtova Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01272 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Submitted to Biomacromolecules 3 September 2017 Revised 15 October 2017

STARCH AEROGELS: A MEMBER OF THE FAMILY OF THERMAL SUPERINSULATING MATERIALS

Lucile DRUEL1, Richard BARDL1, Waltraud VORWERG2, Tatiana BUDTOVA1*

1 - MINES ParisTech, PSL Research University, Centre for Material Forming (CEMEF), UMR CNRS 7635, CS 10207, 06904 Sophia Antipolis, France 2 - Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstrasse 69, D-14476 Potsdam, Germany

Corresponding author: Tatiana BUDTOVA: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT Starch aerogels were prepared via dissolution in water (thermomechanical treatment), retrogradation, solvent exchange and drying with supercritical CO2. Amylose content in starches was varied from 0 to 100%. Aerogels’ bulk density, morphology, specific surface area, thermal conductivity and mechanical properties under compression were investigated. Pea starch aerogels turned to have one of the highest specific surface area and lowest density and thermal conductivity (0.021 – 0.023 W/m.K), the latter indicating that a new thermal super-insulation material was obtained. A detailed study of the influence of processing parameters on pea starch aerogels properties showed the importance of retrogradation time which decreases specific surface area and increases mechanical properties and thermal conductivity. Finally, a comparison of starch aerogels’ thermal conductivity with that of other bio-aerogels is performed.

Key words: starch, amylose, amylopectin, aerogels, thermal conductivity, morphology


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1. INTRODUCTION Aerogels are lightweight materials (density < 0.2 g/cm3) with open porosity; they are usually mesoporous with small macropores and with high specific surface area up to 800 – 1000 m2/g. Classical aerogels are synthesized via sol-gel route either from inorganic (for example, silica)1 or organic (for example, resorcinol-formaldehyde)2 systems. In most of the cases the drying of gels is performed with supercritical (sc) CO2 in order to prevent pore collapse which usually occurs if drying at ambient pressure. One of the most extraordinary properties of silica aerogels is their thermal conductivity which can be as low as 0.012 - 0.015 W/m.K, much lower than that of air (0.025 W/m.K).3 This unique property is due to their low density and pore size below the mean free path of air molecules, which is around 70 nm at 25 °C and 1 atm. The latter leads to the conduction of gas phase lower than that of ambient air, according to the Knudsen effect. Silica aerogels are, for the time being, the most performing materials in the family of thermal super-insulators. Low thermal conductivity makes them very promising insulation materials for lowering energy losses which occur because of heat conduction in buildings, pipelines and refrigerators. Despite the extremely low thermal conductivity, silica aerogels are not widely used yet because of their mechanical fragility, release of small-particles “dust” and relatively high production cost. A new generation of aerogels was developed in the 21st century: they are biomass based and thus can be called bio-aerogels. Inspired by the synthesis of classical aerogels, bioaerogels are prepared via dissolution of a polysaccharide, solution gelation (in some cases this step is omitted) followed by sc drying with CO2. Because in most cases polymer solvent is not miscible with CO2, solvent exchange must be done. This step usually leads to polysaccharide coagulation; however, due to certain chain rigidity polymer does not collapse and results in a 3D network. 3 ACS Paragon Plus Environment

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Bio-aerogels have low density (0.02 – 0.2 g/cm3) and rather high specific surface area (200 − 600 m²/g). Several polysaccharides have been used for making bio-aerogels via dissolutioncoagulation route: cellulose,4–9 marine polysaccharides,10–14 pectin15–17 and starch.18–22 A special case is bio-aerogels based on cellulose I (nano- or microfibrillated or bacterial cellulose): no polymer dissolution and gelation is involved; entangled cellulose nanofibrils dispersed in aqueous media are dried with sc CO2 after exchange of water to ethanol or acetone.23,24 Bio-aerogels are highly compressible without breakage up to strains of 70 − 80 %.8 As well as classical organic or inorganic aerogels, bio-aerogels can be used as matrices for catalysis when functionalised12 and/or pyrolysed,25 as carriers for controlled release,16,26 in electro-chemical applications when pyrolysed,27 and in adsorption and/or separation.13 The properties of bio-aerogels can be tuned due to a large amount of hydroxyl groups on polysaccharide chains which can be functionalised. However, practically nothing is known about thermal conductivity of bio-aerogels, with only few publications reporting the conductivity around 0.016 - 0.020 W/m.K of pectin,15 nanofibrillated cellulose24,28 and alginate13 aerogels. The first starch based aerogels were reported by Glenn and Irving in 1995 and called “microcellular foams”.29 These authors were the only ones who measured starch aerogel thermal conductivity. Wheat and corn starches were used, and the lowest thermal conductivity, 0.024 W/m.K, was obtained for high amylose corn starch aerogel made from 8 wt% solutions. Ten years later other starch aerogels were prepared. Most of them were made via dissolution-retrogradation-solvent exchange-sc CO2 drying route and suggested to be used as matrices for drug delivery applications.20,26 The increase of starch concentration from 5 to 15 wt% solutions leads to density increase, as expected: for example, from 0.12 to 0.23 g/cm3 for pea starch aerogels20 and from 0.04 to 0.015 g/cm3 for wheat starch aerogels,22 respectively. For starch concentrations below 10 wt% and as soon as it is well dissolved due


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to thermomechanical treatment, gelatinisation temperature (from 120 to 140 °C) does not influence aerogel density within experimental errors.22 The specific surface area of starch aerogels measured with nitrogen adsorption and BET method is usually not high, around 50150 m2/g.21,22,26,30 Some works report higher values, 200 – 230 m2/g, obtained for pea and corn starches.20 Interestingly, the highest specific surface area for starch aerogels made via dissolution-retrogradation route, 274 m2/g, was reported for native corn starch (amylose content 53 %, 15 wt% solutions) after very short drying time (1 h), when 92 % of ethanol was extracted from aerogel precursor.18 Longer drying decreased specific surface area. Depending on the application, not all starch aerogels are dried with sc CO2 and not all are prepared via dissolution-retrogradation route. Mesoporous carbons were obtained from pyrolysed high amylose corn starch prepared via dissolution-retrogradation, doped with acid and dried at ambient pressure from low surface tension fluid.25 The specific surface area varied from 200 to 500 m2/g for heating temperatures from 150 °C to 700 °C, respectively. It is also possible to obtain starch aerogels without retrogradation, i.e. from high amylose starch–sodium palmitate inclusion complexes, gelled at low pH, coagulated in ethanol and dried with sc CO2.19 The density of these aerogels varied from 0.12 – 0.18 g/cm3 and specific surface area was 310 – 360 m2/g. A detailed study of the influence of drying conditions showed that specific surface area is inversely proportional to the sc CO2 depressurization rate.19 The goal of this work was to explore the thermal conductivity of starch aerogels. Can they be thermal super-insulating materials, as indicated in the early work of Glenn et al?29 If yes, what is the influence of starch composition and processing conditions on aerogels’ morphology and properties? To answer these questions we used five types of starches with amylose content varying from 0 to 100 % and prepared aerogels via dissolutionretrogradation-solvent exchange-sc CO2 drying route. We performed a detailed analysis of


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aerogels’ density, morphology, thermal conductivity and mechanical properties, and showed that aerogels have a wide range of properties depending on starch origin and processing conditions. Pea starch aerogels possess thermal conductivity below that of air. We show that thermal conductivity is very sensitive to aerogel morphology which is controlled by processing conditions and we suggest using conductivity as a parameter describing the “finesse” of aerogel structure. 2. EXPERIMENTAL 2.1. Materials We used different types of starches in order to vary amylose content: waxy and regular potato, pea and high amylose corn starches were kindly donated by Emsland-Stärke GmbH (amylose content varied from 0 to 80 % as given by the provider, see Table 1). Pure amylose was purchased from Sigma-Aldrich (amylose, type III, from potato).

Table 1. Amylose content of starches used and aerogels’ density and specific surface area for samples made from the initial starch concentration 8 wt%, retrogradation time 48 h except for waxy potato starch (168 h), solvent exchange water→ethanol. Starches

Amylose content, %

Density, g/cm3

Specific surface area, m2/g

Waxy potato

0

0.2

88

Regular potato

20

0.23

85

Pea

40

0.14

221

High amylose corn

80

0.14

254

Pure amylose

100

-

-


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Distilled water was used as starch solvent. Two starch non-solvents were purchased from Fisher Chemicals: absolute ethanol (purity > 99 %) and acetone (purity > 99 %).

2.2. Methods 2.2.1. Synthesis of starch aerogels Starch aerogels were prepared via dissolution-retrogradation-solvent exchange-sc CO2 drying route (Figure 1). First, moisture content was determined for each starch and taken into account in the calculation of starch concentration. All concentrations of starch (5, 8 and 11 wt%) are given on dry weight basis.

Figure 1. Schematic representation of starch aerogels synthesis

The goal is to completely destroy starch granular structure to avoid granule’s remnants that would increase density and thus increase thermal conductivity. Starch dissolution was performed in two steps. First, starch was mixed with water (typically around 100 mL), heated up to 95 °C and stirred at 1000 rpm during 1 – 1.5 h. This solution was then placed into an autoclave and progressively heated during 40 minutes up to 130 °C or 140 °C under stirring at 250 rpm and under pressure of about 1.5 bar. High temperature was maintained during 20 minutes, before cooling down the vessel and opening it at 90 °C.


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The dissolution of pure amylose was done in only one step: amylose and distilled water were placed in the autoclave, mixed under magnetic stirring (250 rpm), heated up to 160 °C during 40 minutes, kept for 20 minutes at 160 °C before cooling down and opening at 90 °C. After this thermomechanical treatment, solutions were immediately poured in polypropylene cylindrical containers of various dimensions depending on the characterisation method to be used (see below samples’ sizes after drying). Aerogels were stored at 4 °C for the gel formation by retrogradation. Different retrogradation times were used: from 24 h to 240 h. To perform drying with sc CO2, water must be replaced by a fluid which is miscible with CO2. Thus the as-obtained starch gels were placed in ethanol or acetone which led to polymer coagulation. The amount of non-solvent was approximately six times starch gel volume. The exchange was carried out several times every 24 h in order to fully replace water by nonsolvent. Drying with sc CO2 was performed as described previously.15 Briefly, the system was pressurized at 50 bar and 37 °C with gaseous CO2 while the non-solvent was slowly drained. Afterwards, pressure in the autoclave was increased to reach 80 bar to be above CO2 critical point. The sc CO2 solubilized the residual non-solvent inside samples pores. A dynamic washing step was then performed at 80 bar and 37 °C, at output of 5 kg of CO2/h for 1 h. It was followed by a static mode of 1 - 2 h at the same pressure and temperature and then by dynamic washing again for 2 h. Finally, the system was slowly depressurized at 4 bar/h and 37 °C, and cooled down to room temperature before being opened. Typical samples were disks of diameter around 20 - 22 mm and height around 8 – 9 mm. For mechanical testing the samples were cylinders with the same diameter and height around 17 mm. For thermal conductivity measurements samples were disks of diameter around 34 mm and height of around 9 mm.


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2.2.2. Characterization Moisture content in starches was measured with an Excellence Plus HX204 Mettler Toledo halogen desiccator. Starch was heated up to 120 °C with halogen lights and loss in weight was measured until variation was inferior to 1 mg per 140 sec. This loss in weight referred to the moisture content of starch. The morphology of starch aerogels was studied with a Supra40 Zeiss SEM FEG (Field Emission Gun). Acceleration voltage was 3 kV and diaphragm 20 µm. Before observations, a thin layer of platinum (about 14 nm) was sputtered onto the samples’ surface with a Q150T Quarum metallizer. This operation is required to prevent accumulation of electrostatic charges on the surface of samples. Samples’ dimensions were measured with a calliper with precision of 0.03 mm to calculate the shrinkage. The bulk density (ρbulk) was determined with a Micrometics Geopyc 1360 powder densitometer and the powder was Dryflo®. The chamber was 19.1 mm diameter and the force applied was 25 N. The specific surface area (SBET) was measured with ASAP 2020 from Micrometics by using nitrogen adsorption and BET method. Prior to measurements, samples were degassed at 70 °C and high vacuum for 6 h. To determine thermal conductivity (λ), we used a Fox150 Thermal Conductivity Meter (Laser Comp) equipped with a custom “micro flow meter cell” developed for small samples by CSTB (Grenoble, France).15 The minimum diameter of disk-shaped samples should be around 25 mm for an accurate measurement. Young’s modulus was determined from the stress-strain curves under the uniaxial compression using Zwick mechanical testing machine with a 2500 N load cell. The tests were


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performed at room temperature (20 - 22 °C), atmospheric pressure and 30 − 40 % relative humidity on samples of a cylindrical shape as mentioned in the “Synthesis” section. The compression tools were covered with Teflon in order to allow free sliding and avoid the “barrel effect”. A displacement rate of 1 mm/min was applied on samples, until 75 % of deformation. For each formulation, at least three samples were tested.

3. RESULTS AND DISCUSSION 3.1. Screening of starch aerogel morphology and properties as a function of starch type All starches as described in Materials section were used to prepare aerogels, the initial concentration was 8 wt%. The photos of the representative samples are shown in Figure 2. Because waxy potato aerogel retrograded over 24 h and 48 h did not result in samples with a stable shape, this starch was allowed to retrograde for 168 h. For all other starches retrogradation time was 48 h. Pure amylose did not lead to monolithic aerogels at any retrogradation times studied. The reason can be highly intensive retrogradation process in the amylose gel at rather high concentration used, leading to heterogeneous samples which did not sustain pressure during drying.

a

b

c

d

e

Figure 2 Starch aerogels based on: waxy potato starch (a), regular potato starch (b), pea starch (c), high amylose corn starch (d) and amylose (e). The preparation conditions are as given in Table 1.


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It is well known that during solvent exchange and drying the precursors of bio-aerogels are shrinking. The extent of shrinkage mainly depends on the type of polysaccharide and its concentration (higher concentration lead to lower shrinkage).20,31 The influence of amylose content on the shrinkage of starch aerogel precursors after solvent exchange and after drying is shown in Figure 3. The shrinkage ∆V is determined as follows:

∆ =

× 100%

(1)

where V0 corresponds to the volume of starch gel before coagulation and Vi is the volume either after solvent exchange or after drying. Figure 3 shows that the most severe contraction occurs during solvent exchange, and that higher amylose content (up to 80 % in high amylose corn starch), more the precursor is “resistant” to solvent exchange and drying. Two reasons can explain this result. On one hand, amylopectin cluster structure and stronger water immobilisation, as compared to amylose, results in higher amylopectin affinity to water. On the other hand, at the same retrogradation time higher amylose content leads to more developed supramolecular structure and stronger network, as compared to amylopectin-rich samples. Both reasons result in higher contraction of potato starches, in particular during solvent exchange. This is the first indication that amylose/amylopectin ratio will influence final aerogel properties.


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∆V, % after coagulation total

80

60

40

20

0 0

20

40

80

amylose content, %

Figure 3. Shrinkage of aerogel precursors as a function of amylose content. The preparation conditions are as given in Table 1.

The density of starch aerogels as a function of amylose content is shown in Table 1. As expected from the results on shrinkage, density decreases from waxy potato to high amylose corn starch. The specific surface area of starch aerogels is presented in Table 1 and the corresponding morphology at low and high magnifications in Figure 4. The increase of amylose content leads to threefold increase in SBET, from about 90 m2/g for waxy potato to about 250 m2/g for high amylose corn starch. The latter is similar to what was reported for cellulose II4,31 and starch aerogels,21,26,30 and it is lower than that of some bio-aerogels based on marine polysaccharides.10 The significant difference in SBET between lower and higher amylose aerogels is clearly seen on high magnification SEM images in Figure 4. Potato-based starch aerogels’ morphology is rather heterogeneous, it is a “mixture” of continuous and fibrous phases, with pores up to several microns. Pea and high amylose corn starch aerogels are made of a network of strands which themselves consist of nanometer-size beads assembled together 12 ACS Paragon Plus Environment

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(Figure 4D). As seen by SEM, pore sizes vary from few tens to few hundreds nanometers. It should be noted that till now, standard methods, such as BJH approach from nitrogen adsorption or mercury porosimetry, used for the measurements of pore size distribution in porous materials, are not applicable to bio-aerogels. The first does not consider pores above 200 nm and thus measures only a small fraction of pore volume,15,28 and in the second mercury compresses aerogels without penetration. Fine structure of pea and high amylose corn starch aerogels shown on SEM images explains their higher specific surface area as compared to their potato-based counterparts.


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Figure 4 SEM images of starch aerogels at low and high magnifications: waxy potato (A), regular potato (B), pea (C) and high amylose corn starch (D). The preparation conditions are as given in Table 1. 14 ACS Paragon Plus Environment

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As mentioned in the introduction, one of the amazing properties of silica aerogels is their very low thermal conductivity making them thermal super-insulating materials, and some bioaerogels were reported to also belong to this class of materials. We measured thermal conductivity of aerogels based on all four types of starches. These results are shown with filled points in Figure 5 as a function of amylose content. Pea aerogel falls in the region of conductivities below that of air, i.e. it is a material with thermal super-insulating properties. This exceptional result is due to pea starch aerogel low density and fine microstructure, as shown in Table 1 and Figure 4C. It seems that a certain amylopectin to amylose ratio is needed to obtain the lowest thermal conductivity: amylose is recrystallizing into crystalline domains of the B-type structure32,33 and thus “too much” of amylose may lead to the increase of solid phase conduction, while too much amylopectin does not lead to fine morphology (see Figure 4).

0.04

thermal conductivity, W/m.K

0.035

0.03

0.025

0.02

0.015 0

20

40

60

80

100

amylose %

Figure 5 Thermal conductivities of starch aerogels as a function of amylose content. Filled points: samples prepared with conditions as in Table 1; open points: pea starch aerogels prepared 15 ACS Paragon Plus Environment

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from 5 wt% initial starch concentration and with different processing parameters (retrogradation times and non-solvent fluid, see details below). Dashed line corresponds to the conductivity of air.

The next question is if thermal conductivity of starch aerogel can still be decreased and how it depends on the processing conditions. We used pea starch which showed the lowest value, varied starch concentration, retrogradation time and non-solvent and obtained a set of conductivity values shown by open points in Figure 5. The following section is devoted to the understanding of the influence of processing conditions on pea starch aerogel properties. We will use thermal conductivity data not only as values per se, but as a measure of morphology “finesse”.

3.2. Influence of processing conditions on pea starch aerogel properties It is well known that aside of starch composition, the properties of its solutions and gels are controlled by numerous physical parameters among which are starch concentration, way of cooking, retrogradation conditions (cooling rate, stirring) and dewatering solvent. It is also known that thermal conductivity of classical silica aerogels makes a U-curve as a function of density: high density leads to high conductivity because of solid conduction, and low density leads to higher gas phase conductivity because of the presence of large pores which do not contribute to the Knudsen effect.34 We varied the density of pea starch aerogels by varying initial starch concentration (5 and 8 wt%), retrogradation time (24, 48, 68 and 96 h) and dewatering solvent (ethanol or acetone). Acetone is better miscible with CO2 than ethanol (Hansen solubility parameters of ethanol: 26.5 MPa1/2, acetone: 19.9 MPa1/2 and CO2: 17.4 MPa1/2)35 which may help better preserving mesoporosity during drying. All results are shown in Figure 6 as a function of pea starch aerogel density.


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0.03

thermal conductivity, W/m.K

0.025

0.02

0.015 0.05

0.1

0.15

0.2

density, g/cm3

Figure 6 Thermal conductivity as a function of density of pea starch aerogels; filled points are for solvent exchange water→acetone, open points for water→ethanol. Dashed line corresponds to the conductivity of air.

It is evident that density is one of the main parameters controlling thermal conductivity. However, several conductivity values can be observed for the samples of the same density, which points on additional parameters influencing conductivity. The main one is retrogradation time which changes starch aerogel morphology and crystallinity. For aerogels of the same density longer retrogradation increase thermal conductivity (Figure 7) most probably due to a) increase of crystallinity32 and b) thicker pores’ walls due to the increasing chains’ aggregation, both phenomena augmenting the conduction of the solid phase. For the samples of the same density, if pore walls thickness increases, pore size should also increase and specific surface area decrease (see a schematic illustration in the Supporting Information, Figure S1). This is confirmed in Figure 7 which shows the decrease of SBET with retrogradation time for aerogels of the same density. 17 ACS Paragon Plus Environment

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SBET, m²/g

thermal conductivity, W/m.K 0.032

250

0.03

200

0.028 150 0.026 100 0.024 50

0.022

Conductivity Sp. surface area

0.02

0 0

50

100

150

retrogradation time, h

Figure 7 Pea starch aerogels’ thermal conductivity and specific surface area as a function of retrogradation time. Open points correspond to solvent exchange water→ethanol, filled to water→acetone. Aerogels’ density is 0.14 g/cm3.

Another example confirming the hypothesis that retrogradation time changes starch aerogels’ morphology, leading to thicker and stronger pore walls, is shown in Figure 8 for Young’s modulus of pea starch aerogels of the same density but retrograded at different times. The representative stress-strain curves together with an image of a compressed sample are shown in Figure S2 of the Supporting Information. The modulus obtained from the uniaxial compression increases with retrogradation time despite that density remains the same. The values obtained are similar to those reported for other bio-aerogels of a similar density.8 The evolution of Young’s modulus of starch aerogels with retrogradation time is in line with the continuous increase of shear modulus and crystallinity of starch gels.32


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Young's modulus, MPa 1.8

1.5

1.2

0.9

0.6

0.3

0 0

50

100

150

200

retrogradation time, h

Figure 8 Young’s modulus of pea starch aerogels as a function of retrogradation time. Aerogels’ density is 0.1 g/cm3.

3.3. Thermal conductivity of starch aerogels vs other bio-aerogels Finally, we compare the conductivity of starch aerogels with that of other bio-aerogels. As mentioned in the Introduction, not much is known, and the correlations between the type of polysaccharide, processing conditions and thermal conductivity are only empirical and not well understood yet. Figure 9 shows thermal conductivity of bio-aerogels in the region around and below the conductivity of air plotted as a function of aerogel density for starch (this study), pectin15 and nanofibrillated cellulose dried under sc conditions24 and freeze dried.28 Cellulose II aerogels (not shown) do not possess thermal super-insulation properties whatever cellulose concentration, solvent, processing conditions or chemical cross-linking used.36 The most probable reasons are too thick pore walls and the presence of large macropores. Some pectin and nanofibrillated cellulose aerogels show low conductivity, around 0.018 - 0.020 W/m.K (Figure 9). Similar values were reported for alginate aerogels but density values were not provided and thus the results for alginate are not shown in Figure 9.13 Till now, the lowest 19 ACS Paragon Plus Environment

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value obtained for starch aerogels is 0.021 - 0.022 W/m.K. It may be possible that further optimisation of starch aerogel processing will lead to finer morphology and thus lower conductivity values. Another way of decreasing the values of bio-aerogel thermal conductivity is “filling the pores” of polysaccharide network with super-insulating silica aerogel. In this case the conduction of gaseous phase is strongly decreased due to mesoporous silica aerogel, however, the conduction of the solid phase is increased because of density increase. A delicate balance is needed to reach low conductivity values. For example, nanofibrillated cellulose (NFC) was silylated, freeze-dried, impregnated with SiO2 sol which was gelled, hydrophobised and all together dried with sc CO2.37 Cellulose skeleton reinforced the composite aerogel and silica phase provided thermal super-insulation properties. Thermal conductivity of this hydrophobic cellulose/silica interpenetrated network was slightly higher than that of silica aerogel, the latter being around 0.012 - 0.013 W/m.K (see points corresponding to “NFC/SiO2” aerogel in Figure 9).37 A somewhat similar approach was taken in ref.38 but cellulose matrix was based on cellulose II: cellulose was hydrophobised in homogeneous conditions, and, when coagulated, impregnated with silica sol which was then gelled and hydrophobised inside cellulose matrix, and dried with sc CO2 (see points for “tritylcellulose/SiO2” aerogels in Figure 9). This example shows that it is possible to obtain rather low conductivity at “high” density, compare the results for starch aerogels at density 0.2 - 0.25 g/cm3 with those for tritylcellulose/SiO2. A special case is hybrid aerogels based on “one-pot” approach: pectin solutions at pH 1.5 were mixed with silicic acid, both phases gelled, silica hydrophobised and interpenetrated pectin/SiO2 dried with sc CO2.39 Mechanically strong and thermal superinsulating aerogels with conductivity of 0.014 – 0.016 W/m.K were obtained (see points “pectin/SiO2” aerogels in Figure 9).


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λ, W/m.K all starches aeropectin NFC sc dried NFC freeze dried tritylcellulose/SiO2 pectin/SiO2 NFC/SiO2

0.04

0.03

0.02

0.01 0

0.05

0.1

0.15

0.2

0.25

density, g/cm3

Figure 9 Thermal conductivity of various bio-aerogels and their composites as a function of density: “all starches” (this work), “aeropectin” (data from ref.15), “NFC sc dried” (data from ref.24), “NFC freeze dried” (data from ref.28), “tritylcellulose/SiO2) (data from ref.38), “pectin/SiO2” (data from ref.39) and “NFC/SiO2” (data from ref.37)

The examples in Figure 9 show that bio-aerogels offer a lot of opportunities in terms of variation of morphology and properties. Several factors must be taken into account. Polysaccharide intrinsic parameters such as composition and chain structure are very important. For example, one of puzzling questions is why, despite the similarity between cellulose and amylose/amylopectin structure and composition, cellulose II aerogels are not thermal super-insulating materials while starch aerogels are? The understanding of supramolecular assemblies of polysaccharide chains during aggregation at solvent exchange (which could be analysed using molecular modelling) may help answering this question. Obviously many external parameters such as polymer concentration, pH and ionic strength (for charged polysaccharides), type of solvent and non-solvent also influence aerogel properties. Numerous hydroxyl groups allow polysaccharide’s chemical modifications leading 21 ACS Paragon Plus Environment

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to targeted aerogel functions. Finally, the possibility of making composite and/or hybrid aerogels opens new ways of producing organic/inorganic porous materials. All said above is applicable to starch aerogels which still remain largely unexplored.

4. CONCLUSIONS The influence of starch type and processing conditions (concentration, retrogradation time, and type of dewatering solvent) on aerogel density, morphology, specific surface area, mechanical properties and thermal conductivity was studied. Low amylose containing starches (potato-based) resulted in aerogels with higher density and lower specific surface area. Pea starch aerogels have a fine morphology, low density, high specific surface area and the lowest thermal conductivity, below that of air, making them a new thermal superinsulating material. We hypothise that a “compromise” in amylose to amylopectin ratio is needed to obtain low thermal conductivity values. Thermal conductivity was used as a parameter describing “structure finesse” to understand the influence of processing parameters on pea starch aerogel morphology and properties. We showed that for aerogels of the same density, the increase in retrogradation time decreases specific surface area and increases thermal conductivity and Young’s modulus. Finally, the potential of starch aerogels in terms of thermal super-insulating material is discussed by comparing the results obtained with thermal conductivity of other bio-aerogels. Hydrophobisation is the obvious next step to further decrease thermal conductivity of starch aerogels. Making hybrid organic-inorganic composite aerogels and functionalization of starch opens numerous ways of producing new versatile and functional starch materials.


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Acknowledgements Authors are grateful to Emsland Stärke GmbH for providing different starches, to Pierre Ilbizian (PERSEE, MINES ParisTech) for drying with sc CO2 and help in thermal conductivity measurements, to Suzanne Jacomet (CEMEF, MINES ParisTech) for SEM and Gilbert Fiorucci (CEMEF, MINES ParisTech) for help in compression testing.

Supporting Information Figure S1. A schematic illustration of morphology evolution in a material of the same density but with different pore wall thickness. Figure S2. Stress-strain curves of pea starch aerogels retrograded for 24 h (1), 96 h (2) and 168 h (3), solvent exchange water→acetone.


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