J Porous Mater DOI 10.1007/s10934-017-0371-8
A novel starch-enhanced melamine-formaldehyde aerogel with low volume shrinkage and high toughness Yong Zhang1,2,3,4 · Jiayi Zhu2 · Hongbo Ren2 · Yutie Bi2 · Xianpan Shi2 · Bin Wang3 · Lin Zhang1,2,4
© Springer Science+Business Media New York 2017
Abstract A new hybrid aerogel with low volume shrinkage and high toughness was prepared by aqueous condensation of melamine, starch and formaldehyde and extracting the solvent from the wet gel with carbon dioxide under the supercritical condition. The sizes of the wet gel and aerogel were measured by vernier calipers and the volume shrinkage of the aerogel was as low as 1.8%. The results from BET and SEM indicated that the hybrid aerogel belonged to the mesoporous material, which showed a typical threedimensional porous structure with a specific surface area about 366.2 m2/g and pore diameter about 12.9 nm. To study the mechanical properties of the hybrid aerogel, we measured the compressive stress as a function of strain and the hybrid aerogel demonstrated excellent elasticity and mechanical durability.
* Jiayi Zhu
[email protected] * Lin Zhang
[email protected] 1
Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China
2
Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology and Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621010, People’s Republic of China
3
School of National Defense and Techonology, Southwest University of Science and Technology, Mianyang 621010, People’s Republic of China
4
Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, People’s Republic of China
Keywords Starch-enhanced · Melamine-formaldehyde aerogel · Sol–gel preparation · Porous materials · Shrinkage rate · High toughness
1 Introduction Aerogels are porous solid nanomaterials with the high inner surface area and high porosity, which are made by forming a three-dimensional wet gel network and removing the solvent without the pore collapse and shrink [1–6]. Due to the characteristics, such as high surface area, high porosity, low dielectric property, and low density, aerogels are attractive for the use as low dielectric substrates, catalyst supports, thermal insulators, biomedicine, sensors, adsorbents and building and construction materials [7–13]. However, most aerogels have poor mechanical properties and so there comes the inevitable volume shrinkage. Thus, many efforts [14–16] had focused on improving the volume shrinkage and mechanical properties of aerogels all the time. Furthermore, as a kind of typical organic aerogels, melamine-formaldehyde aerogel has high-strength but poor-toughness, which limits its many applications. More recently, polymer aerogels [17–22] with high-toughness, such as chitin, starch, polyvinyl alcohol-cellulose nanofiber, polypropylene and polyester, had been fabricated through the formation of either chemically or physically cross-linked networks. The controlled factors, such as polymer chain length, monomer type and cross-link density, make it possible to generate a broad spectrum of properties. Among them, starch, as an abundant, easily available, low toxicity, low-cost and environmentally benign biomaterial, is found in the leaves, seeds and tubers of many vegetables (e.g., wheat, potato, corn, and pea). The starch aerogel obtained through an effective method is composed of starch
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fibers and presents porous structure. Thus, considering the fibrous structure and high toughness of the starch aerogel, it could believed that the addition of starch would not only suppress the inevitable volume shrinkage, but also enhance the toughness of the hybrid aerogel. To the best of our knowledge, there had not been such a report so far about fabrication of the starch-enhanced melamine-formaldehyde (SEMF) aerogel. Herein, we facilely fabricated a novel SEMF aerogel by the sol–gel synthesis and supercritical CO2 drying. The SEMF aerogel exhibited the low volume shrinkage and high toughness. Furthermore, the differences of the structure and mechanical properties between the SEMF aerogel and pure MF aerogel were also discussed in details.
2 Experimental 2.1 Synthesis of the SEMF aerogel A typical process to prepare the SEMF aerogel was as follows: First, 2 g soluble starch was added into 20 mL deionized water at 75 °C under stirring to form a clear solution. Second, 10 g melamine and 0.1 g sodium carbonate were dissolved in the weighing bottle at 75 °C with 100 ml deionized water. Then, the two kinds of solution were totally mixed together and 1 ml formaldehyde solution (37%wt) was added into the mixture as a cross-linking agent. After gelation, the wet gel was moved to a beaker. After that, the drying of the was a three-step process. At first, water was replaced by ethanol for 24 h. During the second step, the hybrid gels were then dipped into highly pure acetone. At last, the acetone-saturated hybrid gels were placed in a high-pressure stainless steel autoclave of 1000 mL, and dried with supercritical C O2 extraction at 40 °C and 12 MPa. When the autoclave was opened at the ambient condition, supercritical C O2 was converted to the gaseous form and subsequently replaced by air by the following pressure release at the constant temperature (40 °C). For comparison, the pure MF aerogel, which had the same precursor concentration as the SEMF aerogel, was prepared by the similar procedure without the addition of starch.
2.2 Characterization Unless stated otherwise, all the tests described below were carried out in triplicate and the average results as well as the standard deviations were reported. The volume of the aerogels were calculated by measuring the sizes of the samples after processing the monolith to a uniform shape. The microstructures of the aerogels were observed by scanning electron microscopy (SEM) on a Sirion 200 electron microscope. High resolution transmission electron microscopy (HRTEM) micrographs were obtained using a JEOL JEM2010 microscope. The chemical properties of the samples were characterized by Fourier transform infrared spectroscopy (FTIR). The specific surface area and pore size distribution of the aerogels were commonly measured by the multipoint BET method on the basis of nitrogen adsorption–desorption isotherms at 77 K with a Quadrasorb SI instrument. The sample was degassed for at least 24 h at a temperature of 150 °C in vacuum in order to remove all the possible absorbed species. The mechanical properties of the aerogels were conducted by using TA company Q800 DMA. The compression strain rate was set at 0.5 N min−1 for the tests at 25 °C.
3 Results and discussion The volume shrinkage of the aerogels was calculated through dividing the volume change after supercritical drying by the volume before solvent exchange and the data was listed in Table 1. As seen from Fig. 1 and Table 1, the volume shrinkage of the SEMF aerogel (1.8%) was much lower than that of pure MF aerogel (28.7%). As far as we know, uniform volume shrinkage is usually observed during the solvent exchange and supercritical C O2 drying, which is generally attributed to the chain relaxation or macro-syneresis [23, 24]. In a word, the SEMF wet gel underwent the little uniform shrinkage during solvent exchange and supercritical CO2 drying and its shrinkage was closed to zero. Such extra low volume shrinkage might be attribute to the introduce of the fibrous structure constructed by the starch, inhibiting the chain relaxation and reinforce the network of the hybrid aerogel.
Table 1 Physical properties and BET surface characterizations of the SEMF aerogel and pure MF aerogel Sample ID
The volume before sol- The volume after vent exchange ( cm3) supercritical drying (cm3)
The volume shrinkage (%)
Bulk density (mg cm− 3)
Surface area Total pore vol(m2 g− 1) ume (cm3 g− 1)
Average pore diameter (nm)
SEMF MF
10.85 10.67
1.8 28.7
93.6 129.5
366.2 634.7
12.9 11.5
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10.65 7.61
1.181 1.827
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Fig. 1 The photos of the SEMF aerogel (left) and the corresponding gel (right)
Furthermore, the morphology and structure of the SEMF aerogel and pure MF aerogel were elucidated by SEM and TEM observations (Fig. 2). From the SEM images of the SEMF aerogel (Fig. 2a) and the pure MF aerogel (Fig. 2b), it was found that both of them had a typical organic cluster texture with three dimensional cobweb-like networks with numerous voids. The result demonstrated that in the SEMF aerogel, melamine and starch had been homogeneously mixed with formaldehyde to form uniform porous structure.
Even though, it could be seen that compared with the pure MF aerogel, the SEMF aerogel had relatively ordered voids and uniform pore size. As shown in Fig. 2c, d, compared with the relatively dense porous structure of the pure MF aerogel, the SEMF aerogel exhibited a relatively uniform cellular structure with the bigger pore size. It was known that the denser porous structure of the pure MF aerogel could be attributed to the structure collapse caused by the chain relaxation, resulting in the obvious volume shrinkage. However, the SEMF aerogel exhibited the low volume shrinkage and this could be attributed to two reasons. On one hand, the introduce of the fibrous structure constructed by the starch could inhibit the chain relaxation and significantly reinforce the network of the hybrid aerogel. On the other hand, the increase of pore size in the SEMF aerogel could reduce the capillary pressure inside the matrix of aerogels and improve the resistance to collapse during the solvent exchange and drying process. In order to shed additional light on the chemical composition of the aerogel, the FTIR spectrum of the SEMF aerogel was carried out. It was seen that the absorbance peak at 3440 cm−1 was due to the –OH stretching vibration, and the peak at 2900 cm−1 was due to the –C–H stretching vibration. The stretching vibration peaks of C=N appeared at 1485 and 1553 cm−1 were the characteristic peaks of thiotriazinone, and the
Fig. 2 The FESEM images of a the SEMF aerogel and b the pure MF aerogel. The HRTEM images of c the SEMF aerogel and d the pure MF aerogel
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Transmittance/%
C–N stretching vibration was at 1358 cm−1. The absorbance peaks at 1157 and 1054 cm−1 were attributed to the stretching vibration and frame vibration of C–O–C groups, respectively. The C–O–C groups might be attributed to the condensation reaction between the starch molecules and hydroxymethyl intermediates, which were resulted by the addition reaction between melamine and formaldehyde. Thus, the FTIR spectrum could reveal the existence of chemical cross-linking between reaction components, which was also beneficial for the low volume shrinkage (Fig. 3). Meanwhile, measurements of the surface area and pore size distribution of the aerogels were carried out by using nitrogen sorption and analyzed by using the BET method [25]. Figure 4 showed the nitrogen adsorption/desorption
Starch
1054 1157
Melamine
2900
1358 3440
1485 1553
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Fig. 3 The FTIR spectra of the SEMF aerogel (inset the chemical formula of the starch and melamine)
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isotherms and pore size distributions of the SEMF aerogel (Fig. 4a) and the pure MF aerogel (Fig. 4b). Compared with that of the pure MF aerogel, nitrogen adsorption/desorption isotherms of the SEMF aerogel showed a soft slope for relative pressures (P/P0) in the range from 0.05 to 0.85, which was followed by a high volume uptake at P/P0 > 0.85 related to the presence of mesopores and macropores. The low volume of gas adsorbed at very low pressures (P/P0 < 0.05) indicated that microporosity had no significant contribution to the surface area of the SEMF aerogel. According to the IUPAC classification, the nitrogen adsorption/desorption isotherms at 77 K of the SEMF aerogel were of the type IV isotherms with a H3 hysteresis loop, associated with mesoporous materials. N 2 adsorption analysis of the SEMF aerogel showed a multi point BET surface area of 366.2 m2/g. As shown in Fig. 4a, the BJH-pore size distribution showed a broad pore range in the mesopore and macropore region for the SEMF aerogel, which had an average pore diameter about 12.9 nm. These results indicated the SEMF aerogel belonged to the mesoporous material. As a result, although the size broadening reduced the total pore volume and surface area, the bigger pore size and uniform cellular structure of the SEMF aerogel could not only surpress the volume shrinkage but also endow the probability of high toughness for the SEMF aerogel. Therefore, the tests of mechanical properties of the aerogels were carried out and exhibited an unexpectedly result. Figure 5 showed the non-linear compression stress–strain curve of the SEMF aerogel (Fig. 5a) and the linear curve of the pure MF aerogel (Fig. 5b) at ambient temperature. From Fig. 5a, it was seen that the SEMF aerogel could undergo relatively as high as 80% large deformations under mechanical compression and recover almost its initial material volume elastically. Moreover, the compressive
0.02 0.00 1
10
Pore Size(nm)
100
200
800 600
0.2 0.1 0.0 1
10 Pore Size(nm)
100
400 200
100 0
0
-100 0.0
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0.8
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Fig. 4 The nitrogen adsorption/desorption isotherms at 77 K and BJH analysis (inset) of pore size distributions of a the SEMF aerogel and b the pure MF aerogel
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materials, which showed a typical three-dimensional porous structure. Furthermore, the compressive stress of the SEMF aerogel at the strain of 80% could be as high as 0.525 MPa, comparing with those of the pure aerogel (strain ~4%, sterss ~0.225 MPa), showing that the SEMF aerogel had high compression strength and great deformability insulation. Therefore,it is believed that the SEMF aerogel would be used as the load-bearing materials. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51502274), the Doctoral Research Fund of Southwest University of Science and Technology (Nos. 15zx7137, 16zx7142) and the Research Fund for Joint Laboratory for Extreme Conditions Matter Properties (Nos.13zxjk04, 14tdjk03). Fig. 5 The stress–strain curves at 25 °C of a the SEMF aerogel and b pure MF aerogel
stress of the SEMF aerogel at the strain of 80% could be as high as 0.525 MPa, comparing with those of the pure MF aerogel (strain ~4%, sterss ~0.225 MPa in Fig. 5b). Moreover, it could be seen that in Fig. 5a, the stress–strain curve of the SEMF aerogel could be divided into three different stages according to the slope of the curve: the linear stage, the yielding stage and the densification stage [26]. In the first stage, the slope of the compression curve remained unchanged when the strain ranged from 0 to 5%. In the second stage (plastic deformation, strain ranging from 5% to about 40%), the stress increased at a fixed rate lower than that of the first stage. But in the third stage (inelastic hardening, strain higher than 40%), the slope of the stress–strain curve increased as the strain rised. According to the above analysis, it could be revealed that the high toughness of the SEMF aerogel could be attributed to the native toughness of the starch and the size broadening of the porous structure, which could supply sufficient compressing and releasing space. As a result, it could be concluded that the SEMF aerogel, which had high compression strength and great deformability insulation, would be suitable to perform as load-bearing materials.
4 Conclusion We had successfully prepared the SEMF aerogel with the low volume shrinkage and high toughness. The volume shrinkage of the SEMF aerogel (1.8%) was so extra low that the aerogel hardly underwent shrinkage during the solvent exchange and supercritical CO2 drying. This was attributed to the fibrous and porous structure of the SEMF aerogel constructed by the addition of starch. Moreover, investigations of FESEM and BET suggested the SEMF aerogel was a kind of mesoporous
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