Article pubs.acs.org/Biomac
Aldehyde Approach to Hydrophobic Modification of Chitosan Aerogels Satoru Takeshita,*,† Arata Konishi,‡ Yoshihiro Takebayashi,† Satoshi Yoda,† and Katsuto Otake‡ †
Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *
ABSTRACT: Biobased nanofiber aerogels are ones of the attractive emerging materials in the fields of biochemistry and materials chemistry, but their poor humidity stability due to high hydrophilicity has limited their practical uses. In this paper, a new series of hydrophobic nanofibrous aerogels made from regenerated chitosan and alkyl aldehydes were prepared via a simple one-pot reaction followed by supercritical drying. Hexanal-modified chitosan aerogel shows excellent hydrophobicity with a water contact angle of ∼136°, a low density of 0.04−0.07 g cm−3, and structurally homogeneous threedimensional nanofiber network at the nanoscale. Systematic investigations using various alkyl aldehydes revealed that pentanal-modified aerogel has similar high hydrophobicity and low density compared to the hexanal-modified material, while heptanal- and octanal-modified aerogels show drastic shrinkage during gelation. The aldehyde modification also suppresses permeation of water droplets into aerogel monoliths as well as reducing shrinkage under high humidity conditions.
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aerogels reported by Isogai’s group.23 We reported another example of translucent and flexible aerogel made of regenerated chitosan nanofibers synthesized through a cross-linking gelation.24,25 These nanofibrous aerogels have highly porous mesoporous structures consisting of three-dimensionally entangled nanofibers. They show sufficient mechanical toughness not to break under deformation and could be good candidates for diverse applications. In particular, their ultralow density and nanoscale structural homogeneity are suitable for flexible thermal and acoustic insulators.24 Although the nanofibrous aerogels have promising features for the above-mentioned applications, their practical uses are inhibited by one fundamental nature of polysaccharides; they exhibit poor stability against humidity because of their high hydrophilicity. Several attempts to make hydrophobic modification have been reported mainly on cellulose cryogels and xerogels having micrometer-sized pores.26−28 However, it is still challenging to maintain the structural homogeneity at the nanoscale, which is essential for high-performance thermal insulation,29 throughout the modification process. In this paper, we report a facile method for hydrophobic modification of nanofibrous aerogels without changing the structural homogeneity of three-dimensional nanofiber networks. The key point of our approach is combination of the nanofiber material and hydrophobization agents: chitosan and
INTRODUCTION Biomass-derived polysaccharides have attracted much attention in various fields in chemistry as sustainable ingredients for functional materials.1−4 In particular, polysaccharide nanofibers have been extensively studied in the past decade for both their fundamental scientific interest and potential practical applications. Many researchers have focused on cellulose nanofibers and revealed the diverse potential of the materials from the viewpoints of abundant resources, environmental friendliness, and extremely high mechanical strength per unit of density.5−7 Chitin, chitosan, and regenerated chitosan nanofibers have also received particular attention because of their biocompatibility in addition to high mechanical performance.8−12 In recent years, polysaccharide nanofibers have also played a significant role in the research field of aerogels, that is, supercritically dried highly porous solids. After the first discovery by Kistler,13 silica aerogel had long been the most attracting aerogel material because of its unique physicochemical properties and wide potential applications.14−16 However, practical uses of silica aerogel have still been limited due to its extremely fragile nature. Several groups showed that polysaccharide nanofibers can provide mechanically tough matrices for supporting silica aerogel skeletons,17−22 but reinforcing silica aerogels with the nanofibers inevitably results in lost transparency and structural homogeneity at the nanoscale. More recently, aerogels made of only polysaccharides nanofibers have emerged as a new category of translucent and flexible aerogels. The first breakthrough was 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-oxidized cellulose nanofiber © XXXX American Chemical Society
Received: April 19, 2017 Revised: June 1, 2017
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DOI: 10.1021/acs.biomac.7b00562 Biomacromolecules XXXX, XXX, XXX−XXX
Article
Biomacromolecules Scheme 1. Synthesis Procedure of Aldehyde-Modified Chitosan Aerogels
Figure 1. Photographs of chitosan aerogels modified with different concentrations of hexanal (left) and change in the degree of modification with the starting volume fraction of hexanal (right). water and then supercritically dried in a CO2−methanol system at 80 °C and 20 MPa to give aldehyde-modified chitosan aerogel. Detailed synthesis procedure is described in the Supporting Information. We focused on hexanal-modified chitosan aerogel and investigated the effect of the alkylation degree on hydrophobic properties by changing the initial amount of hexanal from 0 to 13.3 vol % (see Table S1 for detailed preparation conditions). We also tried various alkyl aldehydes with different lengths and structures at 13.3 vol % (see Table S2 for detailed preparation conditions) to investigate the potential versatility of this aldehyde approach. Characterization. The apparent density of aerogel was calculated from the diameter, height, and weight of the aerogel. The Fouriertransform infrared (FT-IR) transmission spectra were measured in pressed KBr disks on a spectrometer (JASCO, FT/IR-660 plus). The 1 H NMR spectra were measured on an NMR spectrometer (Bruker, Avance 500 MHz). The NMR specimen was prepared by dissolving the aerogel in concentrated DCl/D2O solution. The chemical shift was calibrated using the methyl-1H of acetone (2.18 ppm) as an internal standard. The C/N atomic ratios were determined using an elemental analyzer (PerkinElmer 2400) at the combustion and reduction temperatures of 925 and 640 °C, respectively. The microstructure of aerogel was observed with a field-emission scanning electron microscope (SEM; Hitachi, SU9000). The sample for SEM observation was coated with a thin Pd−Pt conductive layer. The brightness and contrast of the SEM images and photographs were optimized using PowerPoint 2013. The nitrogen gas sorption isotherms were measured on an automatic gas sorption instrument (MicrotracBel, BELSORP-max) at 77 K. The compression stress− strain curves were recorded on a table-top universal mechanical tester (Shimadzu, EZ Test EZ-LX) equipped with 5 kN load cell at the compression rate of 16% min−1. The compression elastic modulus was calculated from the slope of the linear part in the 0−25% strain region. The water contact angle was measured using an automatic contact angle analyzer (Kyowa Interface Science, DMs-401) with 1.0 μL of water for each measurement. The humidity test was carried out in a thermo-hygrostat chamber (Yamato Scientific, IG420) under 70% relative humidity (RH) at 30 °C and 85% RH at 60 °C. The change in the appearance of samples during the test was recorded on a fiber
alkyl aldehydes. Chitosan is easily dissolved in aqueous acidic solutions because of its amino groups, which has the approximate pKa value of 6.5. This makes it easier to fabricate homogeneous nanostructures without applying chemical or mechanical forces such as nanofibrillation. Amino groups also react with various aldehydes to form Schiff’s bases even at room temperature with a high selectivity.30,31 As shown in Scheme S1, lone pairs on amino groups of chitosan attack carbonyl groups of aldehydes to form NC bonds through dehydration. In the case of formaldehyde, the NC species react with another amino group to form cross-linking. In our approach (Scheme 1), a part of the amino groups of chitosan are consumed in cross-linking reaction with formaldehyde to form a physically stable gel, while some of the rest are used in hydrophobic modification by reacting with alkyl aldehydes. These cross-linking and alkylation reactions can be achieved in one-pot process. We also note that alkyl aldehydes, which are found naturally in plant leaves and fruits, are suitable as alkylation agents from the viewpoints of low cost and high biocompatibility, for example, hexanal is the main component of the smell of fresh cut grass and is used industrially as a perfume and food preservation agent.32
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EXPERIMENTAL SECTION
Synthesis of Alkyl-Aldehyde-Modified Chitosan Aerogels. For a typical synthesis, a certain amount (0−1.00 mL) of alkyl aldehyde was mixed with methanol to prepare 3.00 mL of methanolic solution of alkyl aldehyde. This solution was mixed with 1.5 mL of aqueous formaldehyde solution (36.5 wt %) and 3.00 mL of 10.0 g L−1 chitosan solution under constant stirring and aged at 60 °C overnight in a sealed container. The concentrations of chitosan, formaldehyde, and alkyl aldehyde in the final mixture were 4.00 g L−1 (0.0236 mol L−1 as a monomer concentration), 78.8 g L−1 (2.63 mol L−1), and 0− 13.3 vol %. After cooling to room temperature, the obtained gel was washed by soaking in methanol with constant stirring to eliminate B
DOI: 10.1021/acs.biomac.7b00562 Biomacromolecules XXXX, XXX, XXX−XXX
Article
Biomacromolecules
peak for C−C, C−N, C−O−C at ∼1050 cm−1 (peak 8) are not significantly changed by hexanal modification. The NMR spectrum (Figure 3) for the hexanal-modified sample contains
scope recorder (Panrico, PST-2486), and the brightness and contrast were optimized using VideoStudio Pro X9.5 (Corel). We defined the degree of modification, D [mol·fr], of aldehydemodified aerogels as follows: D = {number of introduced alkyl groups}/{number of glucosamine units}
(1) We estimated D from C/N atomic ratios based on the following assumptions: (i) N-acetylglucosamine units, 20.5% of the monomers in chitosan reagent used, or impurities, such as a small amount of proteins from crab shells, do not participate in the modification process, (ii) alkyl aldehydes interact only with NH2 groups of glucosamine units, and (iii) increase in the C/N ratio of aldehydemodified sample compared to unmodified sample is attributed to only introduced alkyl groups, and the degree of cross-linking does not change by modification. On the basis of the above-mentioned assumptions, D is calculated by the following equation: D = [{(C/N)modified − (C/N)unmodified }/n]/0.795
(2)
where (C/N)modified and (C/N)unmodified are C/N atomic ratios of aldehyde-modified and unmodified samples, respectively, and n is the number of carbon atoms in the alkyl aldehyde. {(C/N)modified − (C/ N)unmodified}/n means the number of introduced alkyl groups per unit of N atom. The value 0.795 is the degree of deacetylation, that is, the number of glucosamine units per unit of N atom.
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RESULTS AND DISCUSSION Hexanal Modification. Figure 1 shows typical photographs of chitosan aerogels modified with different amounts of hexanal. In the range of 0−13.3 vol % of hexanal, which is equivalent to glucosamine unit/hexanal = 1:0−58 in molar ratio, the samples form stable gels and aerogels without any drastic or systematic changes in shape and density (0.04−0.07 g cm−3, see Table S3). As the initial amount of hexanal increases, a pink color of the aerogel becomes stronger while their translucent appearance still remains to some extent. At the same time, the FT-IR signal at 2865−2920 cm−1 corresponding to the CH stretching vibration (peak 2 in Figure 2) becomes strong, indicating
Figure 3. 1H NMR spectra of hexanal reagent, unmodified and hexanal-modified (13.3 vol %) chitosan aerogels (see Table S5 in the Supporting Information for the detailed assignments).
signals from CH3 (0.9 ppm, peak 6) and CH2 groups (1.3−2.5 ppm, peaks 2−5) in hexyl groups. The broadening of these peaks indicates that their movement is restricted by being connected to chitosan chains. The degree of modification (the right plot in Figure 1), the amount of hexyl groups per one glucosamine unit calculated from the C/N ratio, increases roughly with increasing initial hexanal and saturates ∼1 for hexanal concentrations over 10.0 vol %. Although quantitative analysis of the cross-linking degree is difficult, we suggest a certain fraction of amino groups must be used in the cross-linking with formaldehyde to form a gel. The degree of modification exceeding 1 thereby indicates multialkylation to one glucosamine unit. Fink et al. reported that dialkylated amino groups are responsible for a degree of modification over 1 in the synthesis of alkylated chitosan.33 We also suggest that in the case of excess aldehydes a part of them are oxidized by heat and oxygen in the air and form a carboxylic acid, which possibly makes an ester bond with OH groups of chitosan. Such oxidation is often accompanied by radical intermediates that cause coloration by dehydrogenating polysaccharide chains even though their amount is quite small.34 Further work will be needed to simplify and analyze the whole reaction system in order to suppress the coloration. The nanoscale structural homogeneity is an important item for potential applications of nanofibrous aerogels such as flexible thermal insulators. For example, an ideal highperformance thermal insulator requires a homogeneous nanoporous structure to realize a thermal conductivity lower than that of the stationary air by confining gas molecules.29 SEM observation (Figures 4 and S1) and gas adsorption profiles
Figure 2. FT-IR spectra of unmodified and hexanal-modified chitosan aerogels (see Table S4 in the Supporting Information for the assignments).
successful introduction of hexyl groups. A small peak at 1725 cm−1 (peak 3) also appears in the spectra of samples with high hexanal amounts, but the origin of this peak has not yet been clarified. We propose that it could be attributed to CO in ester species (this will be discussed later). The peak for CO in N-acetylglucosamine units at ∼1660 cm−1 (peak 4), the small peaks for NH2 scissoring and CH2 scissoring/bending/wagging in the region of 1380−1590 cm−1 (peaks 5−7), and the broad C
DOI: 10.1021/acs.biomac.7b00562 Biomacromolecules XXXX, XXX, XXX−XXX
Article
Biomacromolecules
Conventional particulate aerogels such as silica aerogels have poor mechanical toughness, and they normally break at low compression strain of ∼10%.35 In our previous work,24 the unmodified nanofibrous chitosan aerogels show remarkable mechanical toughness not to break under compression, which is favorable for the flexible thermal insulators. According to the compression test (Figure S3), unmodified and hexanalmodified chitosan aerogels show similar compression stress− strain curves. Both samples were compressed up to the limit of the instrument without forming any cracks, indicating that their mechanical toughness remains after the modification. The compression elastic moduli are 0.085 and 0.072 MPa for the unmodified and hexanal-modified samples, respectively. Influence of Alkyl Chain Structure. Figure 6 and Table 1 summarize typical photographs, densities, and degrees of modification of samples modified with various alkyl aldehydes. The samples modified with butanal and isopentanal do not form a stable gel under the present conditions. The other samples form homogeneous gels and aerogels. As shown in Table 1, heptanal- and octanal-modified samples have a high density over ∼0.3 g cm−3 because they show drastic shrinkage during the gelation process. We note that there are two types of shrinkages at the different stages of preparation: (i) shrinkage during gelation observed only in heptanal- and octanalmodified samples and (ii) shrinkage during supercritical drying observed in all the samples; the latter might be related to a volumetric phase transition of the cross-linked chitosan gel at the high temperature and pressure.25 Pentanal-, heptanal-, and octanal-modified samples have high degrees of modification, ∼1 alkyl group for one glucosamine unit, while pivalaldehyde- and cyclohexanecarbaldehyde-modified samples have low degrees of modification of
