Facile preparation of clay reinforced konjac glucomannan aerogels

Abstract

Low density and foam-like aerogels based on abundant renewable konjac glucomannan and montmorillonite clay were prepared by a facile freeze-drying process. The amount of montmorillonite and alkali added into the aerogel and the freezing temperature greatly influenced the properties of the aerogel.

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As renewable resources, polysaccharides have been widely used because they are environmentally friendly and possess great potential for agricultural, industrial and medical applications as substitutes for some petrochemical products.¹ Konjac glucomannan (KGM) is a kind of natural polysaccharide from the tuber of the Amorphophallus konjac plant. It is a high molecular weight water-soluble neutral polysaccharide, which is composed of β-(1 → 4) linked D-mannose and D-glucose in a molar ratio of 1.6 : 1 or 1.4 : 1.² The glucomannan backbone possesses 5–10% acetyl-substituted residues.³ It is known that the removal of acetyl groups on the molecular chains of KGM with addition of alkaline and heating results in the formation of a thermoirreversible gel.⁴

Aerogels could be classified as inorganic aerogels, organic aerogels and inorganic/organic aerogel composites according to their compositions.⁵ Aerogels are usually synthesized by a sol–gel process and dried under supercritical fluid (SCF) extraction/exchange conditions.^6,7 In recent years, freeze drying has been proved as an ideal drying method to substitute traditional supercritical drying. There have been a number of reports on the preparation of polymer/clay materials through a freeze-drying process from aqueous mixtures.^8–11 Freeze-drying has been widely used for its safety and minimal effect on both product and the environment.¹² Addition of clay into polymers has been shown to improve aerogel properties such as mechanical properties and thermal stability. Aerogel structures and properties can be controlled by varying polymer/clay ratios, each composition concentration and type and freezing condition.^12–14

In this study, foam-like materials based on konjac glucomannan (KGM) and sodium montmorillonite (Na⁺-MMT) was reported. The effects of Na⁺-MMT and freezing conditions on mechanical and physical properties of polysaccharide-based materials were discussed. The effect of alkali amount was also investigated.

To prepare aerogels, Na⁺-MMT was mixed with DI to obtain a clay aqueous suspension. Sodium carbonate was then added to the clay aqueous suspension under constant stirring. KGM was added slowly into the mixture and stirring was maintained for 1 min. The mixture was poured into cylindrical vials, covered with preservative film and allowed to stand for 4 h at room temperature. The vials with mixture were placed into a thermostat water bath kept at 90 °C for 1 h. Polymer/clay suspensions were then frozen by immersing the vials into low-temperature thermostat bath for 8 h after being cooled to room temperature. The samples were then freeze-dried using a LGJ-10 lyophilizer (Songyuanhuaxing Technology Develop Co., Ltd, Beijing). To simplify the expressions, KxMy is used in figures and tables where K stands for konjac glucomannan, M stands for sodium montmorillonite, and the number (x and y) after letters represents weight percentage of each component in the initial solution. The ratios of alkali to KGM are all 0.12 : 1 unless otherwise specified. Table 1 lists the effect of KGM and Na⁺-MMT contents on mechanical property and density of KGM/Na⁺-MMT aerogels. As it is shown in Table 1, pure KGM aerogels exhibited very poor mechanical properties. Even the initial KGM weight percentage was 3 wt%, K3 had a low modulus of 0.787 MPa. The specific modulus (modulus divided by density) of pure KGM aerogels increased significantly as the density increased, indicating the enhanced modulus was attributed to factors beyond increasing density. Since KGM content was greater than 3 wt%, the aerogel shrinked severely during the freeze-drying process. The addition of Na⁺-MMT obviously improved the mechanical properties of composite aerogels. When the KGM content was constant, all compression modulus of composite aerogel significantly increased with the increase of the content of montmorillonite. For instance, with 2 wt% nano montmorillonite added to the KGM solution, the compressive modulus of K2 increased from 0.273 to 1.320 MPa, meanwhile the specific modulus increased from 10.48 to 29.78 MPa cm³ g⁻¹ in despite of the increased sample densities. When the initial content of KGM and Na⁺-MMT was 2 wt% and 4 wt% respectively, compression modulus and specific modulus of composite aerogel reached the maximum of 3.123 MPa and 45.08 MPa cm³ g⁻¹, respectively. As the initial content of KGM was increased to 3 wt%, the aerogel would shrink obviously during the freeze-drying process if the adding amount of Na⁺-MMT was greater than 2 wt%, which may result from the aggregation of the MMT and enhanced hydrogen bonding interaction between MMT and KGM.^15,16

Table 1 Modulus, density, and specific modulus of aerogels composited by various contents of KGM and sodium montmorillonite^a

	Property	M0	M1	M2	M3	M4
a K stands for konjac glucomannan, M stands for sodium montmorillonite, and the number after letter represents weight percentage of each component in the initial solution. Modulus in MPa, density in g cm^−3, specific modulus in MPa cm^3 g^−1 – means no available data.
K1	Modulus	0.033 ± 0.006	0.067 ± 0.006	0.210 ± 0.026	0.540 ± 0.044	1.347 ± 0.087
	Density	0.014 ± 0.001	0.024 ± 0.002	0.032 ± 0.001	0.042 ± 0.001	0.052 ± 0.001
	Specific modulus	2.41 ± 0.31	2.73 ± 0.08	6.58 ± 1.05	12.92 ± 1.13	25.87 ± 0.99
K2	Modulus	0.273 ± 0.031	0.623 ± 0.025	1.320 ± 0.095	1.920 ± 0.128	3.123 ± 0.106
	Density	0.026 ± 0.001	0.037 ± 0.002	0.044 ± 0.001	0.053 ± 0.001	0.069 ± 0.004
	Specific modulus	10.48 ± 0.86	16.69 ± 0.28	29.78 ± 1.75	35.89 ± 1.87	45.08 ± 1.23
K3	Modulus	0.787 ± 0.059	1.233 ± 0.215	2.283 ± 0.180	—	—
	Density	0.039 ± 0.001	0.056 ± 0.004	0.083 ± 0.009	—	—
	Specific modulus	20.16 ± 1.08	22.11 ± 2.22	27.68 ± 1.37	—	—

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Fig. 1 shows the SEM images of aerogels composed of KGM and Na⁺-MMT with different ratio observed by means of a scanning electron microscope (SEM, JSM-6390/LV, Japan). All of the aerogels exhibited a network structure. As the initial content of Na⁺-MMT was constant, the additive amount of KGM influenced the microstructure of aerogels that the pore wall thickness increased with the increasing amount of KGM. Likewise, the pore wall thickness increased with the increasing initial Na⁺-MMT content.

Fig. 1 SEM images of aerogels, (a) K1M2, (b) K1M4, (c) K2M2, (d) K2M4, (e) K3M1, (f) K3M2, scale bar = 50 μm.

In freezing process, the formation of crystalline ice causes every precursors originally dispersed in the aqueous medium to be expelled to the boundaries between adjacent ice crystals.¹⁷ A higher precursor concentration leads to the thicker pore wall due to the retarded ice crystal growth with increased mixture viscosity. The structural change could influence mechanical properties that K2M4 with a network microstructure and the thickest pore wall exhibited the highest modulus of 3.123 MPa.

Fourier transform infrared (FT-IR) spectra were recorded at room temperature using a Nicolet (USA) Nexus 470 FTIR spectrometer. Fig. S1† shows the FT-IR spectra of the raw montmorillonite clay, KGM and aerogels. The stretching vibration absorption peaks at 3627 cm⁻¹ and 3451 cm⁻¹ are assigned to the Al–O–H in montmorillonite clay and –OH in H₂O, respectively. The bending vibration absorption peak at 1644 cm⁻¹ is assigned to –OH in H₂O. The stretching vibration absorption peak at 1036 cm⁻¹ is assigned to Si–O. The peaks from 400 to 800 cm⁻¹ are the internal vibrations of silica tetrahedron and alumina octahedron in montmorillonite clay. The peak around 3427 cm⁻¹ is assigned to the –OH in KGM. The stretching vibration absorption peaks and bending vibration absorption peaks around 2925 cm⁻¹ and 1381 cm⁻¹ are assigned to C–H. The peak at 1727 cm⁻¹ is assigned to stretching of C–O in acetyl groups. The peak at 1639 cm⁻¹ is the absorption peak of intramolecular hydrogen bonds. The peak at 1023 cm⁻¹ is assigned to the stretching of C–O. Compared to KGM, the peak at 1727 cm⁻¹ for pure KGM aerogel disappeared in the spectra, which means all acetyl groups have been removed as a result of the addition of alkali. The schematic of deacetylation of konjac glucomannan is represented in Fig. S4.† As for the montmorillonite clay added composite aerogel, the peak at 3627 cm⁻¹ almost disappeared, which may be caused by the masking of the stretching absorption peak of –OH. The disappearance of carbonyl peak of acetyl group at 1727 cm⁻¹ shows the total removal of acetyl groups, along with the appearance of stretching and bending vibration absorption peaks of C–H around 2925 cm⁻¹ and 1381 cm⁻¹.

The addition of montmorillonite changed the thermal properties of the composite aerogel.^10,11 Thermogravimetric analysis (TGA) was used to analyse the thermostability of aerogel with different ratio of KGM to Na⁺-MMT, as can be seen in Fig. 2.

Fig. 2 Thermogravimetric analysis (TGA) and differential thermal gravimetry (DTG) curves of Na⁺-MMT and aerogels.

The onset decomposition gravimetric temperature, termination gravimetric temperature and the weight loss ratio under a temperature can be obtained in TGA curves. According to differential thermal gravimetry (DTG), the mass was taken a derivative with respect to temperature, giving the relationship between the rate of mass change and temperature. The TGA curves displayed two main steps of weight loss. The first weight loss, observed from starting temperature to 100 °C was the result of the evaporation of free water and bound water, regardless with thermostability. After that, no weight loss was available in pure Na⁺-MMT, which demonstrated its good thermostability. The second weight loss step began from about 200 °C, with a sharp weight loss showed in every group of aerogel (pure Da-KGM showing the greatest weight loss), which was caused by the thermal degradation of KGM. When the initial content of KGM stayed constant, the rate of weight loss decreased with the increase of the mass of montmorillonite clay, which demonstrated that montmorillonite clay played a positive role in the thermostability of aerogel, which may due to the continuous network structure between montmorillonite clay and Da-KGM. The lamellar structure of nano montmorillonite clay possessed excellent barrier property, leading to better thermostability of aerogel by cutting off heat conduction.

Alkali plays an important role in the formation of Da-KGM gel. The removal of acetyl groups on the molecular chains of konjac glucomannan by addition of alkaline and heating results in the formation of a thermoirreversible gel.⁴ The effect of different dosage of alkali on composite aerogel was studied. As shown in Table 2, when the initial content of Na⁺-MMT kept constant, the addition of alkali affected the density of aerogel insignificantly but the mechanical property obviously. When the initial additions of KGM and Na⁺-MMT were 2 wt%, the modulus of compression and specific modulus of composite aerogel with addition of alkali were respectively 0.357 MPa and 8.46 MPa cm³ g⁻¹. When the addition amount of alkali was 3 wt% of initial amount of KGM, modulus of compression and specific modulus increased to 0.990 MPa and 23.52 MPa cm³ g⁻¹ respectively. With the increasing addition of alkali, modulus of compression and specific modulus increased gradually and reached the maximum (1.320 MPa and 29.78 MPa cm³ g⁻¹, respectively) when adding 12 wt% KGM of alkali. The same trend occurred when the initial amount of KGM and Na⁺-MMT were 2 wt% and 4 wt%, respectively. The appearance of alkali gave rise to less acetyl groups in KGM molecular chain, stronger molecular chain rigidity, more molecular entanglement and increased intermolecular forces, thus led to improved mechanical property of aerogel.

Table 2 Effects of alkali contents and freezing temperature on modulus, density, and specific modulus of aerogels^a

	Property	Ratios of alkali to KGM				Temperature^b (°C)
		0 : 1	0.03 : 1	0.06 : 1	0.12 : 1	−20	−40	−80
a K stands for konjac glucomannan, M stands for sodium montmorillonite, and the number after letter represents weight percentage of each component in the initial solution; modulus in MPa, density in g cm^−3, specific modulus in MPa cm^3 g^−1.b The ratios of alkali to KGM are all 0.12 : 1.
K2M2	Modulus	0.357 ± 0.042	0.990 ± 0.069	1.203 ± 0.118	1.320 ± 0.095	0.873 ± 0.059	1.320 ± 0.095	1.560 ± 0.130
	Density	0.042 ± 0.002	0.042 ± 0.001	0.043 ± 0.001	0.044 ± 0.001	0.045 ± 0.001	0.044 ± 0.001	0.045 ± 0.001
	Specific modulus	8.46 ± 0.70	23.52 ± 1.55	27.92 ± 2.22	29.78 ± 1.75	19.53 ± 1.08	29.78 ± 1.75	34.97 ± 2.54
K2M4	Modulus	1.393 ± 0.074	2.597 ± 0.031	2.840 ± 0.089	3.123 ± 0.106	2.443 ± 0.133	3.123 ± 0.106	3.480 ± 0.235
	Density	0.058 ± 0.001	0.060 ± 0.001	0.063 ± 0.001	0.069 ± 0.004	0.064 ± 0.002	0.069 ± 0.004	0.065 ± 0.001
	Specific modulus	24.04 ± 0.92	43.50 ± 0.21	44.98 ± 0.98	45.08 ± 1.23	38.33 ± 0.71	45.08 ± 1.23	53.80 ± 3.10

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Comparing the TGA and DTG curves (Fig. S2†) of composite aerogel with different amount of alkali, it can be found that thermal evaporation-induced weight loss occurred in all samples. Sample A3 and A2 turned to the second step ahead of sample A1 and A0, however, the thermal weight loss of A3 and A2 was less than that of A1 and A0, which demonstrated that the increase of alkali amount can reduce the rate of thermal degradation of KGM. When the temperature reached 600 °C, the weight loss of A0 and A3 are respectively 46.6% and 54.2%, which means the addition of alkali improves the thermostability of aerogel.

Freezing temperature has an effect on the growth status and morphology of ice crystal. At the lower freezing temperature, large amount of crystal nucleus grow together, leading to an end of crystallization in a short time and large amount of small size ice crystal. On the contrary, relatively higher freezing temperature results in large-size ice crystal. The formation of crystalline ice causes every solute originally (inorganic substance and polymer) dispersed in the aqueous medium to be pushed to the boundaries between adjacent ice crystals.¹⁷ Table 2 shows the effect of different cryogenic temperature on modulus of compression, density and specific modulus of aerogel. When the initial content of KGM and Na⁺-MMT stayed constant, the modulus of compression and specific modulus of aerogel gradually increased with the decreasing cryogenic temperature. The modulus of compression of sample K2M2 rose from 0.873 MPa at −20 °C to 1.560 MPa at −80 °C; specific modulus increased from 19.53 MPa cm³ g⁻¹ to 34.97 MPa cm³ g⁻¹. Meanwhile, the modulus of compression of sample K2M4 increased from 2.443 MPa at −20 °C to 3.480 MPa at −80 °C; specific modulus increased from 38.33 MPa cm³ g⁻¹ to 53.80 MPa cm³ g⁻¹. It is demonstrated that the growth and size of ice crystal can affect the mechanical property of aerogel.

The XRD profiles of raw KGM, Na⁺-MMT and aerogels prepared under different freezing temperature are shown in Fig. S3,† The pattern of KGM showed a broad peak at 2θ = 21.37° while the pattern of Na⁺-MMT showed several strong diffraction peak. The pattern of composite aerogel showed inorganic characteristic. The peak of 2θ < 10° is the diffraction peak of d₀₀₁, representing the size of interlamellar spacing. The pattern of Na⁺-MMT showed a strong diffraction peak at 2θ = 7.07°. The distance between montmorillonite slice is 1.25 nm according to Bragg equation: 2d sin θ = nλ. As for the aerogel prepared under different temperature, the diffraction peak in d₀₀₁ shifted to small angle. The d₀₀₁ diffraction peak of precooled sample under −20 °C, −40 °C and −80 °C shifted respectively to 6.25°, 6.39° and 6.47°, which demonstrated that some organic ingredient came into the space between montmorillonite slice, giving rise to large lamellar distance. Fig. S4† is the schematic of interaction of deacetylated konjac glucomannan with montmorillonite.

Fig. S5† shows the TGA and DTG curves of composite aerogel prepared under different temperature. In the first and second steps, samples with higher content of Na⁺-MMT exhibited slower rate of weight loss. In the second step, when the initial content of KGM and Na⁺-MMT stayed constant, lower precooling temperature led to slower rate of weight loss and reduce the thermal degradation speed of KGM in composite aerogel.

Conclusions

Konjac glucomannan/montmorillonite composite aerogel can be prepared using the freeze-drying method. Addition of montmorillonite into the aerogel greatly improved the mechanical properties and thermal stability and changed microstructure of the aerogel. The compression modulus and specific modulus of the composite aerogel increased with increase of Na₂CO₃, reaching the maximum when adding 12 wt% KGM of alkali. The compression modulus and specific modulus of the composite aerogel gradually increased with decreasing of freezing temperature when the initial content of KGM and Na⁺-MMT were fixed. Since the polysaccharide-based composite aerogels exhibited favorable mechanical properties, thermal stability and porosity, these materials are expected to be used in packaging, insulation, cushioning, engineering and adsorbing applications.

Acknowledgements

The authors gratefully acknowledge financial support from contract grant sponsors: The National Natural Science Foundation of China (Grant no. 31371841).

Notes and references

1. T. Köhnke, A. Lin, T. Elder, H. Theliander and A. J. Ragauskas, Green Chem., 2012, 14, 1864–1869.
2. Q. Xu, W. Huang, L. Jiang, Z. Lei, X. Li and H. Deng, Carbohydr. Polym., 2013, 97, 565–570.
3. L. Huang, R. Takahashi, S. Kobayashi, T. Kawase and K. Nishinari, Biomacromolecules, 2002, 3, 1296–1303.
4. X. Du, J. Li, J. Chen and B. Li, Food Res. Int., 2012, 46, 270–278.
5. J. Guo, B. N. Nguyen, L. Li, M. A. B. Meador, D. A. Scheiman and M. Cakmak, J. Mater. Chem. A, 2013, 1, 7211–7221.
6. A. C. Pierre and G. M. Pajonk, Chem. Rev., 2002, 102, 4243–4266.
7. G. Amaral-Labat, L. Grishechko, A. Szczurek, V. Fierro, A. Pizzi, B. Kuznetsov and A. Celzard, Green Chem., 2012, 14, 3099–3106.
8. M. D. Gawryla, M. Nezamzadeh and D. A. Schiraldi, Green Chem., 2008, 10, 1078–1081.
9. T. Pojanavaraphan, R. Magaraphan, B.-S. Chiou and D. A. Schiraldi, Biomacromolecules, 2010, 11, 2640–2646.
10. H.-B. Chen, Y.-Z. Wang, M. Sánchez-Soto and D. A. Schiraldi, Polymer, 2012, 53, 5825–5831.
11. H.-B. Chen, B.-S. Chiou, Y.-Z. Wang and D. A. Schiraldi, ACS Appl. Mater. Interfaces, 2013, 5, 1715–1721.
12. Y. Wang, M. D. Gawryla and D. A. Schiraldi, J. Appl. Polym. Sci., 2013, 129, 1637–1641.
13. C. A. Colard, R. A. Cave, N. Grossiord, J. A. Covington and S. A. Bon, Adv. Mater., 2009, 21, 2894–2898.
14. S. Bandi, M. Bell and D. A. Schiraldi, Macromolecules, 2005, 38, 9216–9220.
15. M.-F. Huang, J.-G. Yu and X.-F. Ma, Polymer, 2004, 45, 7017–7023.
16. S. K. Lee, B. C. Bai, J. S. Im, S. J. In and Y.-S. Lee, J. Ind. Eng. Chem., 2010, 16, 891–895.
17. M. C. Gutiérrez, M. L. Ferrer and F. del Monte, Chem. Mater., 2008, 20, 634–648.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03165b

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