Article pubs.acs.org/Biomac
Bulky Quaternary Alkylammonium Counterions Enhance the Nanodispersibility of 2,2,6,6-Tetramethylpiperidine-1-oxyl-Oxidized Cellulose in Diverse Solvents Michiko Shimizu, Tsuguyuki Saito, and Akira Isogai* Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan S Supporting Information *
ABSTRACT: The degree of nanodispersion of cellulose in diverse solvents is a significant primary criterion for the preparation of bulk nanocelluloses and nanocellulose-containing composites. Here, high degrees of nanodispersion of fibrous 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose (TOC) were achieved in various solvents by efficiently incorporating quaternary alkylammoniums (QAs) as counterions of TOC carboxyl groups via simple ion-exchange treatment in water. Tetramethyl-, tetraethyl-, tetra-n-propyl-, and tetra-n-butylammoniums were used as the QAs. The TOC-QAs were converted to TOC nanofibrils (TOCN-QAs) with a high nanofibrillation yield via mechanical disintegration in not only water but also methanol and other organic solvents after solvent-exchange treatment. Fourier transform infrared spectra of cast TOCN-QA films and the electric conductivities of the TOCN-QA dispersions indicated that the TOCNs-QAs were dispersed primarily through dissociation of the bulky QA carboxylate groups. Moreover, the TOC-QAs were nanodispersible in water even after being oven dried at 105 °C, which is advantageous for their practical application.
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of nanocelluloses reviewed elsewhere.5−9 However, such surface modifications need, in most cases, large amounts of chemicals and multistage treatments, including washing and purification with large amounts of solvents, and discharge large amounts of effluents that must be treated to protect the environment. Consequently, the reaction efficiencies for such nanocellulose surface modification methods are low. Fibrous TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized celluloses (TOCs) can be converted to completely nanodispersed TOC nanofibrils (TOCNs) in water through a gentle mechanical disintegration treatment.11 TOCNs prepared from wood bleached kraft pulps of papermaking grade have a homogeneous width of ∼3 nm, a high aspect ratio, high crystallinity, and a high modulus.9,12 TEMPO-mediated oxidation causes sodium C6-carboxylate groups to abundantly and uniformly form on the surfaces of the crystalline cellulose microfibrils in fibrous wood cellulose. The TOCN elements are stably dispersed at the individual nanofibril level in water because of the efficient osmotic effect between the wood cellulose microfibrils, which have anionically charged surface carboxylates present in high densities. The structure of the carboxylate counterions is supposed to play an important role in the nanodispersion behavior or degree of nanofibrillation of such TOCs in water and organic solvents, as well as matrix polymers.
INTRODUCTION Native cellulose microfibrils are the smallest fibrous elements next to cellulose molecules in wood cell walls. Nanocelluloses isolated from plant cellulose are classified as nanofibrils and nanocrystals, which have high and low aspect ratios, respectively, and prepared through mechanical disintegration with or without pretreatment. Nanocelluloses are attracting a great deal attention as biodegradable and renewable nanomaterials with excellent properties such as high moduli, low coefficients of thermal expansion, and high mechanical strengths.1−4 Extensive studies of efficient preparation methods and applications of nanocelluloses, such as biobased composite materials, which can be substituted for petroleum-based materials, have been reported.5−9 Nevertheless, the dispersion of hydrophilic nanocelluloses is still a challenge, especially at the individual nanofibril level in hydrophobic matrix polymers. High degrees of cellulose nanodispersion are necessary in matrix polymers to achieve efficient nanocomposite effects at addition levels that are as low as possible.10 Because abundant hydroxyl groups are present on the surface of nanocellulose, efficient surface hydrophobization using simple and environmentally friendly procedures should be established to improve the degree of cellulose nanodispersion that can be achieved in hydrophobic matrix polymers, and also to increase the moisture and water resistance of the resulting composites. Numerous studies including silylation, esterification, and grafting have been reported for the surface hydrophobization © 2014 American Chemical Society
Received: March 12, 2014 Revised: April 18, 2014 Published: April 21, 2014 1904
dx.doi.org/10.1021/bm500384d | Biomacromolecules 2014, 15, 1904−1909
Biomacromolecules
Article
Figure 1. Quaternary alkylammonium (QA) hydroxides used in the ion-exchange treatment of fibrous TOC-H. aldehyde groups present at low levels in the oxidized cellulose to C6carboxylates. The resulting fibrous TEMPO-oxidized cellulose (TOC) had a sodium carboxylate content of 1.7 mmol g−1 determined using electric conductivity titration,21 suggesting that almost all of the C6primary hydroxyl groups exposed on the surfaces of the crystalline cellulose microfibril had been oxidized to C6-carboxylates.22 The resulting fibrous TOC with sodium carboxylate groups (TOC-Na) was converted to TOC-H according to a previously reported method.18 The wet and fibrous TOC-H obtained was stored at 4 °C without being dried before use. Preparation of Aqueous TOCN-QAs Dispersions. Four QA hydroxides, i.e., Me4NOH, Et4NOH, n-Pr4NOH, and n-Bu4NOH, were used in the ion-exchange treatment of the fibrous TOC-H (Figure 1). An aqueous 0.1−0.5 M QA hydroxide solution containing equimolar amounts of QA and TOC-H carboxyl groups was added to a 0.3% (w/v) TOC-H/water slurry. The aqueous TOC-QA slurry was then diluted to 0.1% (w/v) with water. The slurry was mechanically disintegrated using a double-cylinder-type homogenizer (Physcotron NS-56, Microtec Nition, Chiba, Japan) at 7500 rpm for 1 min and an ultrasonic homogenizer with a 7 mm probe-tip diameter (US-300T, Nihonseiki, Tokyo, Japan) at 19.5 kHz and 22 W for 4 min, to prepare an aqueous TOCN-QA dispersion. The unfibrillated fraction, if present in the dispersion, was removed by centrifugation at 12000g for 15 min, and the nanofibrillation yield of TOCN-QA was calculated from the dry weight of TOCN-QA present in the supernatant.18 Preparation of TOCN-QA/Organic Solvent Dispersions. An organic solvent (30 mL) was added to the aqueous 0.3% (w/v) slurry (10 mL) of the fibrous TOC-QA, and the slurry was then shaken several times and centrifuged at 12000g for 10 min in a poly(propylene) centrifugation bottle. After the supernatant was removed by centrifugation, fresh organic solvent (30 mL) was added, and the mixture was centrifuged. This solvent-exchange treatment was repeated three times with fresh organic solvent (30 mL each). Finally, the 0.3− 0.5% (w/v) TOC-QA/organic solvent slurry was mechanically disintegrated using the double-cylinder-type homogenizer at 7500 rpm for 2 min and the ultrasonic homogenizer for 16 min, which was performed with careful attention not to increase the temperature of the slurry. Nanofibrillation yields were calculated according to the same procedure described above. Analyses. The aqueous TOCN-QA dispersions were cast on glass Petri dishes and dried at 40 °C for 3 days to prepare TOCN-QA films. Fourier transform infrared (FTIR) spectra of the TOCN-QAs films were recorded using a JASCO (Tokyo, Japan) FT/IR-6100 spectrometer. The nitrogen content of fibrous TOC-QAs after freezedrying was determined using an elemental analyzer (Flash 2000, Thermo Scientific) and used to calculate the ion-exchange ratio from TOC-H to each of the TOC-QAs. A diluted TOCN-QA dispersion was dropped on a freshly exfoliated mica surface and dried in air, and atomic force microscopy (AFM) images of TOCN-QA were captured in tapping mode using a Nanoscope III Multimode (Digital Instruments). The widths of the TOCN-QAs were measured from height profiles obtained from ∼50 AFM images according to a previously reported method.4 The electric conductivities of the TOCN-QA/organic solvent dispersions were measured using the same system that was used to determine the carboxylate content of TOCs.21
Surface modifications of TOCNs from sodium carboxylates to free carboxyls and alkylammonium carboxylates through ionexchange treatments have been reported to yield high degrees of nanodispersion of TOCNs in organic solvents.13,14 In particular, the alkyl chain lengths of alkylammonium salts introduced into the TOCNs as counterions have strongly influenced their nanodispersibility behavior in organic solvents.15−17 One of the shortcomings of these surface hydrophobization treatments via ion exchange, however, is that multiple steps, including nanofibrillation of the TOCs in water and gelation of the TOCNs with protonated carboxyl groups (TOCN-H) using acid, are needed as pretreatments. In a previous study, fibrous TOCs with ammonium carboxylate groups (TOCs-NH4) were prepared from TOCs (not TOCNs) through ion-exchange treatment. The resulting fibrous TOCs-NH4 were directly converted to TOCNs-NH4 by mechanical disintegration treatment in water, whereas fibrous TOCs with protonated carboxyl groups (TOCs-H) could never be converted to TOCNs-H even after harsh disintegration treatment in water.18 These results indicate the possibilities that not only ammonium carboxylate groups but also quaternary alkylammonium carboxylate groups (QAs), once formed in fibrous TOCs (TOC-QAs) via ion-exchange treatment, have a high degree of dissociation, and that TOC-QAs can be directly converted to TOCNs-QAs. In this study, we report simple and efficient methods for preparing surface-hydrophobized TOCNs that can be nanodispersed in not only water but also various organic solvents. Commercially available tetramethyl-, tetraethyl-, tetra-n-propyl-, and tetra-n-butylammonium hydroxides were used as the QAs.
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EXPERIMENTAL SECTION
Materials. A never-dried softwood bleached kraft pulp (Nippon Paper Industries, Tokyo, Japan) was used as the original wood cellulose. The pulp had an α-cellulose content of 90%, and the rest was mostly hemicelluloses. The viscosity average degree of polymerization of the pulp was 1270, when measured using 0.5 M copper ethylenediamine.19,20 For demineralization, the pulp was soaked in dilute HCl for 1 h and then washed with water by filtration. TEMPO, sodium bromide, a 12% sodium hypochlorite solution, tetramethylammonium hydroxide (Me4NOH) pentahydrate, a 10% aqueous tetraethylammonium hydroxide (Et4NOH) solution, a 10% aqueous tetra-n-propylammonium hydroxide (n-Pr4NOH) solution, a 0.5 M aqueous tetra-nbutylammonium hydroxide (n-Bu4NOH) solution, and other chemicals were of laboratory grade (Wako Pure Chemicals, Tokyo, Japan) and used as received. All organic solvents used in this study, i.e., methanol (MeOH), isopropyl alcohol (iPA), acetone, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), were of superdehydrated grade (Wako Pure Chemicals). DMF and DMSO were used after further dewatering with molecular sieves. TEMPO-Mediated Oxidation of Wood Cellulose. TEMPOoxidized wood cellulose was prepared from the kraft pulp using the TEMPO/NaBr/NaClO system in water at pH 10, according to a previously reported method.11 The TEMPO-oxidized cellulose was further treated with NaClO2 in water at pH 4.8 to oxidize residual C61905
dx.doi.org/10.1021/bm500384d | Biomacromolecules 2014, 15, 1904−1909
Biomacromolecules
Article
Scheme 1. Preparation of Aqueous Dispersions (A) and Organic Solvent Dispersions (B) of TOCN-QAs
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RESULTS AND DISCUSSION Nanodispersion of Fibrous TOC-QAs in Water. Fibrous TOC-QAs were prepared by neutralizing the protonated carboxyl groups of TOC-H with the QA hydroxides (equimolar to the carboxyl groups) under aqueous conditions and then disintegrated in water according to Scheme 1A. The obtained aqueous dispersions were highly transparent and showed distinct birefringence at rest when observed between cross polarizers (Figure 2 and Figure S1 of the Supporting Information), which
the never-dried wet TOC-QAs (Figure S1 of the Supporting Information). This nanofibrillation behavior of the once-dried fibrous TOC-QAs may be advantageous in terms of the cost of delivery from a production mill to users, because it means that on-site nanofibrillation of dried TOC-QAs would be possible. FTIR spectra of the TOCN-QA cast films revealed that almost all protonated carboxyl groups had been converted to QA carboxylate salt-type structures, which had a CO stretching vibration band around 1600−1610 cm−1 (Figure 3 and Figure S2
Figure 2. TOC or TOCN dispersions [0.3% (w/v)] prepared through the disintegration treatment of fibrous TOC-H, TOC-NH4, and four TOC-QAs in water. Photos were taken with (bottom) and without (top) cross polarizers.
Figure 3. FTIR spectra of cast films of TOCN-H, TOCN-NH4, and four TOCN-QAs prepared from aqueous dispersions. TOCN-H and TOCN-NH4 cast films were prepared using the procedures reported previously.18,25
meets the definition of “nanodispersibility”.23 The nanofibrillation yield was more than 95% in all cases; the weight percentage of unfibrillated fractions present in the dispersions and separated by centrifugation was less than 5%. These results show that the fibrous TOC-QAs were almost completely converted to the corresponding aqueous TOCN-QA dispersions by the mechanical disintegration treatment. The TOCN-QAs elements formed self-aligned structures similar to nematic-type liquid crystals in water.23,24 In contrast, the fibrous TOC-H/ water slurry could not be converted to a transparent dispersion even after the harsh disintegration treatment.25,26 Moreover, once the fibrous TOC-H, TOC-Na, or TOC-NH4 was oven-dried at 105 °C for 3 h or freeze-dried, the aqueous dispersion of either of these dried fibrous TOCs had a light transmittance at 600 nm of 80% (Table 1). It is notable that the TOC-QAs were dispersed at the individual nanofibril level in an organic solvent such as MeOH, which has a quite low dielectric constant and low viscosity. Only the fibrous TOC-N(n-Bu)4 was nanodispersed in acetone and iPA with relatively high nanofibrillation yields of 30−80%, whereas the other TOC-QAs had low nanofibrillation yields of
