Article pubs.acs.org/Langmuir
Films Prepared from Electrosterically Stabilized Nanocrystalline Cellulose Han Yang, Alvaro Tejado, Nur Alam, Miro Antal, and Theo G. M. van de Ven* Pulp & Paper Research Centre, Department of Chemistry, McGill University, 3420 University Street, H3A 2A7 Montreal, Quebec, Canada S Supporting Information *
ABSTRACT: Electrosterically stabilized nanocrystalline cellulose (ENCC) was modified in three ways: (1) the hydroxyl groups on C2 and C3 of glucose repeat units of ENCC were converted to aldehyde groups by periodate oxidation to various extents; (2) the carboxyl groups in the sodium form on ENCC were converted to the acid form by treating them with an acid-type ion-exchange resin; and (3) ENCC was crosslinked in two different ways by employing adipic dihydrazide as a cross-linker and water-soluble 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide as a carboxyl-activating agent. Films were prepared from these modified ENCC suspensions by vacuum filtration. The effects of these three modifications on the properties of films were investigated by a variety of techniques, including UV−visible spectroscopy, a tensile test, thermogravimetric analysis (TGA), the water vapor transmission rate (WVTR), and contact angle (CA) studies. On the basis of the results from UV spectra, the transmittance of these films was as high as 87%, which shows them to be highly transparent. The tensile strength of these films was increased with increasing aldehyde content. From TGA and WVTR experiments, cross-linked films showed much higher thermal stability and lower water permeability. Furthermore, although the original cellulose is hydrophilic, these films also exhibited a certain hydrophobic behavior. Films treated by trichloromethylsilane become superhydrophobic. The unique characteristics of these transparent films are very promising for potential applications in flexible packaging and other high-technology products.
■
electronic devices, such as flexible displays,14 and for oxygenbarrier layers,15,16 such as food packaging. To utilize the advantage of nanocelluloses completely, it is important to find an effective method to prepare them. However, each individual nanocellulose fiber inside a pulp fiber is firmly attached to others by hydrogen bonds17 because of the large numbers of hydroxyl groups on the natural fibers.18 Thus, it is not easy to achieve this goal. Many proposed methods have been developed recently. Fibrillation of cellulose fibers has been performed by mechanical disintegration,19,20 but most of the obtained fibers contain large bundles of nanofibers despite the huge amounts of energy consumed. The separation of cellulose nanofibers was also performed by treating the cellulose with sulfuric acid.7,21 However, the acid hydrolysis leads to a decrease in the length of nanofibers of up to 100−200 nm, resulting in NCC, as well as a decrease in the final yield to 30− 50%. The combination of enzymatic pretreatment of cellulose and then mechanical disintegration enables the preparation of CNF with reduced energy consumption,22 and the fiber length of CNF was well preserved by this method.23
INTRODUCTION Cellulose is a natural carbohydrate polymer consisting of repeating β-D-glucose monomer units1 and is considered to be an almost inexhaustible raw material.2 With the increasing demand for environmentally friendly products, over the last two decades a large amount of research has been focused on natural cellulose fibers.3 Recently, nanomaterials science has attracted a great amount of attention because of the large surface area to volume ratio and the unique properties of nanoparticles.4 Two main cellulose nanostructures are becoming more and more important in this category because they are “green” and a renewable biomass material: cellulose nanofibers (CNF) and nanocrystalline cellulose (NCC). CNF consists of alternating crystalline and amorphous domains, and NCC consists of rodlike crystalline nanoparticles.5,6 The lateral dimension of CNF and NCC is between 3 and 10 nm, and the length is a few micrometers5 for CNF and 100−200 nm for NCC.7 In addition, nanocellulose has an extremely high tensile strength of 2 to 3 GPa8,9 and high stiffness with an elastic modulus of up to 138 GPa.10,11 They also possess very low coefficients of thermal expansion (CTE of 0.1 ppm K−1),12 which is comparable to that of quartz.13 These unique properties make nanocellulosic materials promising candidates for future © 2012 American Chemical Society
Received: December 16, 2011 Revised: March 26, 2012 Published: April 5, 2012 7834
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
15% available chlorine), 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), sodium phosphate dibasic and monobasic, sodium hydroxide (NaOH) standard solutions (0.5 N), and hydrogen chloride (HCl) standard solution (0.1 and 0.5 M)were purchased from Sigma-Aldrich, hydrogen peroxide (H2O2, 30%) was purchased from Fisher, and sodium chloride (NaCl) was purchased from ACP Chemistry. Chemicals for cross-linkingadipic dihydrazide (ADH) and 1-ethyl-3-[3-(dimethylaminopropyl)]carbodiimide (EDC)were purchased from Sigma-Aldrich. The chemical for hydrophobization trichloromethylsilane (TCMS)was purchased from Sigma-Aldrich. All chemicals were used as received. Milli-Q water (resistivity = 18.2 MΩ·cm) and deionized water were used throughout the experiments. Preparation of ENCC. Periodate Oxidation Process. Pulp sheets (5 g) were torn into small pieces of around 2 × 2 cm2 and soaked in distilled water for at least 3 days. The wet pulp was easily and thoroughly dispersed by a disintegrator and then filtered to remove extra water from the pulp. Next, 3.33 g of NaIO4 and 19.5 g of NaCl were dissolved in Milli-Q water, and the wet pulp was added to this solution. The total volume of the water was 333 mL, including the moisture from the wet pulp. The oxidation reaction was performed at room temperature and stirred at a speed of 105 rpm. The reaction beaker was wrapped with several layers of aluminum foil to prevent the entry of any light. After 36 h of reaction, ethylene glycol was added to this mixture to end the reaction by quenching the residual periodate. The oxidized pulp was washed thoroughly with deionized water by filtration. Chlorite Oxidation Process. The oxidized pulp was suspended in water (250 mL, including the moisture of oxidized pulp); 3.56 g of NaClO2, 14.6 g of NaCl, and 3.3 g of H2O2 were added to this mixture. The mixture was stirred at a speed of 105 rpm at room temperature, and the pH was maintained at 5 by adding 0.5 M NaOH. The mixture turned into a translucent suspension after 24 h of reaction. Next, this suspension was poured into a large amount of ethanol, and a white precipitate was obtained and thoroughly washed with a water−ethanol solution, followed by acetone. The chlorite-oxidized pulp was dried in a fume hood. TEMPO Oxidation Process. The amounts of chemicals used in this step was as follows: 1 g of chlorite oxidized pulp, 0.0016 g of TEMPO, 1.13 g of NaClO2, and 90 mL of phosphate buffer (pH 6.8) were added to a three-necked flask in one step. This mixture was stirred at a speed of 250 rpm and heated. When the temperature reached 50 °C, 0.25 mL of NaClO, diluted with 10 mL of phosphate buffer, was added to the flask. Then the speed of stirring was increased to 500 rpm. A yellow suspension was formed after 48 h of reaction at 60 °C. After the large particles were removed by filtering though a nylon cloth (maximum pore size 20 μm), the suspension was precipitated and washed with a water−ethanol solution, followed by acetone, and air dried. Thus, a white ENCC powder was prepared. The final yield of ENCC is 63%. Modification of ENCC. Periodate Oxidation Modification. Aldehyde groups were introduced into ENCC by periodate oxidation. Three grams of ENCC powder was added to 200 mL water containing 1.98 g of NaIO4 at each designed reaction time (24, 48, 96, 120, and 144 h), and one-fifth of the suspension was taken out of the beaker and precipitated by ethanol. Acid-Type Ion-Exchange Resin Treatment. A 0.5% w/w ENCC suspension was treated with a surplus amount of strong acid ionexchange resin (Amberlite IR 120 H) for 2.5 h,35 and then the ionexchange resin was separated by filtration. The pH of the filtrate was around 3. A homogeneous acid-type ENCC was obtained and ready for FTIR examination. TCMS Solution-Immersion Treatment. The silanization of ENCC films with TCMS was performed by a solution-immersion process. ENCC films were directly immersed in TCMS liquid for 5 min. The immersed films were thoroughly rinsed with ethanol and then dried in air. Fabrication of ENCC Films. Fabrication of Original and Modified ENCC Films. ENCC powder was well dispersed in Milli-Q water by magnetic stirring to obtain a 0.5% (w/w) suspension. Twenty milliliters of an ENCC suspension was poured into a Millipore vacuum
Recently, another pretreatment with a catalytic amount of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) inducing the oxidation of cellulose with NaClO and NaClO2 at neutral pH was developed by Saito et al.24 The primary alcohol groups of cellulose were selectively oxidized into carboxyl groups, and then the TEMPO-oxidized cellulose was separated into individual fibers with 5 nm width and about 2 μm length by mechanical and ultrasonic disintegration. Our group has developed a new method25 with which it is possible to introduce large numbers of anionic charge groups onto cellulose fibers and easily separate them into nanosized fibers by purely chemical reactions, without the necessity of intensive treatments such as mechanical shear or ultrasound. This method contains the following three steps: (i) periodate oxidation selectively oxidizing a fraction of C2 and C3 hydroxyl groups to 2,3-dialdehyde units on the cellulose chain;26,27 (ii) the conversion of the dialdehyde groups to dicarboxyl groups by chlorite oxidation,28 typically up to 2.5 mmol/g of charge groups; (iii) converting primary hydroxyl groups on C6 to carboxyl groups by TEMPO-mediated oxidation with a total content of charge groups of up to 3.5 mmol/g. After these reactions, cellulose fibers can be easily separated into nanosized structures. It was found that at high charge densities the cellulose nanofibers broke up into nanocrystalline cellulose (NCC) and water-soluble di- and tricarboxylated cellulose. The NCC produced in this way has similar dimensions to NCC made by acid hydrolysis but has a very large charge density, exceeding the theoretical maximum (Supporting Information), implying that a large number of di- and tricarboxylated cellulose chains are protruding from their surface. We refer to this form of NCC as electrosterically stabilized NCC (ENCC).29 Films of cellulose nanofibers containing low-molar-mass polymer molecules were first studied by Nakagaito and Yano.30 These films have high strength and high moduli but are very brittle. These films have potential applications in strengthening biomedical materials and as transparent materials in some hightechnology areas.14,31 Porous networks consisting of entire wood cellulose nanofibers were investigated by Henriksson,32 and this cellulose nanopaper has remarkably high toughness and tunable porosity. Chinga-Carrasco and Syverud developed a new method of characterizing nanofibrillar cellulose films by SEM without a conductive metallic layer and quantified their structural and mechanical properties upon moisture,33 and these films have very low oxygen transmission rates.34 Fukuzumi15 prepared strong transparent films from TEMPOmediated oxidized cellulose nanofibers (TOCN). Very low oxygen permeability of a thin TOCN layer on a polylactic acid film was achieved, and hydrophobization of the TOCN films was obtained by treating them with alkylketene dimer, which is a common sizing agent used in paper chemistry. In this work, we prepared ENCC by a three-step periodate, chlorite, and TEMPO oxidation and fabricated films from ENCC, as well as various modified ENCCs, by vacuum filtration. Various characterizations of these films show them to be promising candidates for flexible packaging and some hightechnology products such as flexible display panels and acoustic membranes.
■
EXPERIMENTAL SECTION
Materials. Q-90 softwood Kraft pulp sheets (made from spruce, Domtar, Canada) were used as cellulose material. Chemicals for oxidationsodium (meta) periodate (NaIO4), sodium chlorite (NaClO2, 80% purity), sodium hypochlorite solution (NaClO, 10− 7835
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
4000 cm−1 with a resolution of 4 cm−1. The spectra were normalized before plotting. Solid Carbon-13 NMR Measurements. Solid C-13 NMR spectra were obtained on a Varian/Agilent VNMRS-400 instrument operating at 100.5 MHz. Samples were packed in 7.5 mm zirconia rotors and spun at 5500 Hz. Spinning side bands were suppressed using the TOSS sequence. Spectra were acquired using a contact time of 2 ms and a recycle delay of 2 s. Typically, 6000 transients were acquired. Contents of Aldehyde Groups: Conductometric Titration Analysis. The aldehyde contents of periodate-oxidized ENCC were measured by conductometric titration. The aldehyde groups of ENCC were first oxidized to carboxyl groups by chlorite oxidation, and then the number of total carboxyl groups was determined by conductometric titration. The aldehyde content was obtained as the difference between the total and original carboxyl groups of ENCC. Conductometric titrations were performed on a Metrohm 836 Titrando instrument. Samples were first purified by dialysis (Spectra/ Por; MWCO 1000) for 24 h. The concentration of samples was determined by weighing the samples after drying at 105 °C in an oven. The content of carboxyl groups was determined according to Araki.36 A certain amount of sample (with a solid content around 0.02 g depending on its concentration) and 2 mL of a NaCl solution (20 mmol/L) were added to 140 mL of Milli-Q water, and the mixture was stirred to obtain a very well dispersed suspension. The pH of the mixture was adjusted to 3.5 by adding 0.1 M HCl. Then a 10 mmol/L NaOH solution was added at a rate of 0.1 mL/min to the mixture up to a pH of around 11. The part of the curve that represents weak acid on the titration graph gives the carboxyl content. Optical Properties: UV−Visible Spectroscopy Analysis. The optical transmittance of films was measured on a Varian UV−visible spectrophotometer with a xenon lamp. A background spectrum was acquired from the empty sample holder; then a film was attached to the sample holder to be measured. The spectra were acquired from 200 to 900 nm with a data interval of 1 nm. Mechanical Properties: Tensile Test. The tensile strength and Young’s modulus of films were measured on an Instron Mini 44 tensile tester with a 500 N load cell in a standard conditioned room (temperature 22.2 ± 0.6 °C, relative humidity 50 ± 2%). Films were cut into strips that were 5 mm in width and 20 mm in length. Strips were stretched at a cross-head speed of 1 mm/min with a specimen gauge length of 10 mm. Each result is based on at least three measurements. Thermal Properties: Thermogravimetric Analysis (TGA). Thermogravimetric analysis was performed on a TA Instruments Q500 TG analyzer. Samples (about 6 mg) were heated in a pure nitrogen atmosphere (flow rate 60 mL/min) from room temperature to 550 °C at a rate of 20 °C/min. Contact Angle Measurements. Contact angle measurements were performed on an OCA20 contact angle system (Dataphysics, Germany) at room temperature. A 4 μL water droplet from a microsyringe (Hamilton-Bonaduz) was placed on the surface of a flat film and then pictures were taken each minute with a CCD camera for up to six minutes. Each measurement was performed on a different new spot on the film, and the results were based on the average of at least three measurements. Contact angles were calculated on the basis of the Young−Laplace equation using the software provided by the instrument. Water Permeability Properties: Water Vapor Transmission Rates (WVTR). The rates of water vapor transmission through the prepared films were measured in a standard conditioned room (temperature 22.2 ± 0.6 °C, relative humidity 50 ± 2%). An Erlenmeyer-flaskshaped glass vial was specially made with a threaded neck (diameter ∼15 mm) and a flat rim (diameter ∼2.5 mm) at its mouth. A film (with a diameter of 15 mm) was sandwiched between two ring-shaped rubber washers (outer diameter ∼15 mm and inner diameter ∼10 mm) and then placed on the mouth of this glass vial that was filled with anhydrous calcium chloride (Sigma-Aldrich) as a desiccant; the film was held about 25 mm above the desiccant.37 A plastic cap was tightly screwed on top of this vial to acquire a good seal between the film and the mouth of the vial, ensuring that water vapor can travel
filtration glass holder with a polyester membrane filter (Sterlitech, pore size 0.2 μm, diameter 47 mm) and filtered to complete dryness. The vacuum was provided by a laboratory water aspirator. A transparent
Figure 1. Photographs of (a) a transparent film made from ENCC, (b) a cross-linked ENCC gel, (c) a film made from a cross-linked ENCC gel, (d) a cross-linked film by immersion of a non-cross-linked ENCC film in ADH and EDC solution.
film (Figure 1a) with an approximate thickness of 30 μm and a diameter of 25 mm was formed on the membrane. Another smaller film with a diameter of 10 mm was prepared in the same way by using an Advantec vacuum filtration glass holder with a polyester membrane filter (Sterlitech, pore size 0.2 μm, diameter 25 mm). After these films were peeled off of the membrane, they were dried in an oven at 50 °C for 24 h and then at 105 °C for 2 h. The films were stored in a standard conditioned room (temperature 22.2 ± 0.6 °C, relative humidity 50 ± 2%) for further characterization. Fabrication of Cross-Linked ENCC Films. Cross-Linking of Suspended ENCC. ADH (0.186 g) was added to 20 mL of a 0.5% w/w ENCC suspension, and the pH was adjusted to 4.8 by adding 0.1 M HCl. Then 0.204 g of EDC was added to this suspension. The suspension was stirred at room temperature for 8 h, and the pH was maintained at 4.8 by adding 0.1 M HCl during the reaction. The reaction was stopped by raising the pH to 6.8. The cross-linked ENCC was precipitated in ethanol and collected by a nylon cloth; then the precipitate was redispersed in water. This purification process was repeated three times in order to remove all chemicals. Finally, a gel-like cross-linked ENCC (Figure 1b) was obtained, and a film (Figure 1c) was formed from a 0.5% (w/w) cross-linked ENCC suspension by filtration. Cross-Linking of ENCC in Films. Original ENCC films were first prepared by filtration and kept on the filtration glass holder. Then 20 mL of a solution containing 0.186 g of ADH and 0.204 g of EDC (pH kept at 4.8) was poured into the glass holder. After immersion for 8 h, this solution was removed from the glass holder and the surface water was removed by filtration. The obtained film was presented in Figure 1d. Characterization. Fourier Transform Infrared Spectroscopy (FTIR). Samples were characterized with an FTIR spectrometer from Perkin-Elmer with single-bounce diamond ATR (attenuated total reflectance) accessory. Solid samples were placed directly on the ATR crystal, and maximum pressure was applied by lowering the tip of the pressure clamp using a rachet-type clutch mechanism. All of the spectra of measured samples were averaged from 32 scans from 550 to 7836
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
only though the film (Supporting Information, Figure S3). A domestic electric fan was placed before the vial to ensure air circulation around this vial during the experiments. Water vapor absorbed by the desiccant through the films leads to an increase in the weight of the whole vial. The weight changes were recorded every hour for three days (except at night) to calculate the WVTR.
carboxyl vibration in the acid form40 indicate that all of the sodium forms of carboxyl groups were converted to the acid form under the acid-type ion-exchange resin treatment. In curve c, cross-linking resulted in the generation of amide bonds as shown by the presence of CO stretching via the 1650 cm−1 peak.41 Film Optical Transmittance. The optical transmittance of the films was obtained using a UV−visible spectrophotometer. Transmittance (%T = I/I0, where I and I0 are the intensities of emergent and incident radiation, respectively) is used to define the transparency of a measured film. The transmittance versus wavelength curve (Figure 3) shows that the transmittance of the films increases sharply at about 400 nm, which is the lower limit of the visible light region. To compare the effects of different modifications on the transparency of the films, optical transmittance values were taken at 600 nm, which is the approximate average wavelength of the visible-light region. From Figure 3a, it can be seen that the transmittance of films prepared from ENCC with carboxyl groups in the sodium form is about 87%. Introducing various numbers of aldehyde groups does not significantly influence the transparency of these films. (Figure 3b shows the transmittance for the film containing the highest content of aldehyde groups. Results for other contents of aldehyde are similar and not shown.) When the sodium form of the carboxyl groups on cellulose nanofibers was changed to the acid form, the transmittance of the films decreased to about 75%. According to Figure 4, the transmittance of films from crosslinked ENCC decreases to about 55%, which is only 60% of the unmodified films. When the diameters of elements forming a film are less than 1/10 of the visible-light wavelength, the film does not scatter any appreciable amount of light.14 When ENCC was cross-linked by ADH in solution, the ENCC aggregated together and formed larger units, increasing the light scattering and leading to an apparent decrease in transparency (Figure 5c). The transmittance of films in which ENCC was cross-linked by immersion of the film in an ADH solution was around 70%, which is smaller than for the original ENCC films but higher than that of films made from cross-linked ENCC in suspension. Under this condition, cross-linking occurs in an adjacent crystalline part of the ENCC crossings, which would not affect the transparency; the cross-linking between ENCC and protruding DCC (dicarboxylcellulose) chains or among protruding DCC chains can form additional structures that would slightly increase the light scattering (Figure 5d). Mechanical Properties. The tensile strength and Young’s modulus of films are presented in Table 2. After introducing different numbers of aldehyde groups for films with carboxyl groups in the sodium form, the tensile strength is in the range of 61 to 138 MPa and the Young’s modulus is in the range of 3.31 to 4.68 GPa; for films with carboxyl groups in the acid form, the tensile strength is in the range of 80 to 103 MPa and Young’s modulus is in the range of 2.48 to 4.07 GPa. As shown in Figure 6, without aldehyde groups (reaction time 0 h), the tensile strength of films with carboxyl groups in the acid form is higher than that in the sodium form. This is due to the fact that carboxyl groups in the acid form can form ester bonds with hydroxyl groups, thus increasing the strength of the films. When different numbers of aldehyde groups are introduced, the tensile strength of films with carboxyl groups in the sodium
■
RESULTS AND DISCUSSION Effects of Modification on ENCC. Different numbers of aldehyde groups were generated in ENCC with periodate oxidation. After 24, 48, 96, 120, and 144 h of reaction, the aldehyde contents were 3.4, 5.2, 6.0, 6.5, and 7.2 mmol/g, respectively. The details of chemical groups after periodate oxidation are presented in Table 1. Table 1. Contents of Chemical Groups after Periodate Oxidation of ENCC Containing 3.5 mmol/g −COONa Groupsa −OH reaction time (h)
−CHO C2, C3
C6
C2, C3
0 24 48 96 120 144
0 3.4 5.2 6.0 6.5 7.2
5.2 5.2 5.2 5.2 5.2 5.2
9.9 6.5 4.7 3.9 3.4 2.7
a
C2, C3, and C6 indicate the location of these chemical groups on the cellulose chains. Units for the quantity of chemical groups are mmol/g.
The evidence of converting the sodium form of carboxyl groups on ENCC to the acid form was provided by FTIR transmittance spectra (Figure 2). The broad peak at 3330 cm−1
Figure 2. FITR transmittance spectra of (a) ENCC with carboxyl groups in the sodium form, (b) ENCC with carboxyl groups in the acid form, and (c) ENCC after cross-linking.
is due to the stretching of −OH groups, the peak at 1315 cm−1 is for the −OH bending vibration,38 and the peaks at 2900, 1425, and 1030 cm−1 are assigned to C−H stretching, −CH2 scissoring, and CH2−O−CH2 stretching, respectively.39 Curve a is for the original ENCC, which has sodium carboxyl groups, and the peak at 1605 cm−1 is for the carboxyl vibration in the sodium form. In curve b, the lack of a peak at 1605 cm−1 and the appearance of a new peak at 1740 cm−1 that is due to 7837
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
Figure 3. Optical transmittance of (a) ENCC films with carboxyl groups in the sodium and acid forms and (b) ENCC films after the introduction of aldehyde groups by periodate oxidation for 144 h (aldehyde content 7.2 mmol/g).
tensile strength, which may be due to nanofiber aggregation prior to film formation. However, the tensile strength of crosslinked films from method 2 was improved by almost 65%. This improvement shows that the amide bonds formed between ENCC can strengthen the films. This phenomenon is similar to the cross-linking of pulp fibers: cross-linking after the formation of a paper sheet can increase the tensile strength whereas crosslinking prior to the preparation of a paper sheet reduces the strength.45 Thermal Stability and Decomposition Properties. Derivative weight loss (DWL) curves for all measured samples are shown in Figure 8. In Figure 8a, the DWL curve of ENCC films in the sodium carboxyl form (without aldehyde groups) shows a wide irregular peak, which has the highest initial decomposition temperature (220 °C) of all of the samples. The films with various numbers of aldehyde groups exhibit multiple peaks. The initial decomposition temperature increased from 175 to 185 to 190 °C with increasing aldehyde groups after 24, 96, and 144 h of periodate oxidation. The first transition peak is relatively weak and shifts to higher temperature (190, 200, and 212 °C, respectively) with an increase in aldehyde content; the second and third transition peaks of these three samples are around 260 and 320 °C, respectively, and also slightly increased with the increase in aldehyde content. In Figure 8b, the initial decomposition temperature for ENCC films in the acid carboxyl form increased from 170 to 175, 185, and 190 °C with increasing aldehyde groups after 24, 96, and 144 h of periodate oxidation; however, the dominant transition peaks of all samples are at the same temperature (320 °C). By comparing films with the same content of carboxyl groups but in the sodium and acid forms, one can observe that they have almost the same transition peak at around 320 °C, but the one in the acid form shows a higher initial decomposition temperature than the one in the sodium form. As shown in Figure 8c, an obvious improvement in the thermal stability of ENCC was achieved after cross-linking, and the dominant transition temperature was shifted from 250 °C (before cross-linking treatment) to ∼320 °C, although the cross-linked film made from cross-liked ENCC in suspension shows a little higher initial and dominant decomposition temperature. Water Contact Angle of ENCC Films. One intrinsic property of cellulose is its hydrophilic character. However, ENCC films also show certain hydrophobic properties. It was noticed that after the removal of the capillary tip from the water drop the initial contact angle for the ENCC film with carboxyl
Figure 4. Optical transmittance of ENCC films prepared with nonmodified ENCC and with cross-linked ENCC by cross-linking ENCC in suspension (method 1) and cross-linking ENCC by immersion of the non-cross-linked ENCC film in ADH and EDC solution (method 2).
form increased with an increase in the aldehyde content, and these films were much stronger than the films with carboxyl groups in the acid form. This phenomenon is likely due to the hemiacetal linkage that forms between aldehyde groups and hydroxyl groups. As shown in the solid C-13 NMR spectrum (Figure 7), the peak at 175 ppm indicates that part of the primary alcohol groups were oxidized to carboxyl groups; the peak at 105 ppm is assigned to C1, the peaks at 89 and 66 ppm correspond to C4 and C6, and the strong doublet peak is for C2, C3, and C5.42,43 The wide peaks between C1 and C4 are for hemiacetal linkages.44 A higher content of hemiacetal linkages in a film results in a higher tensile strength. Films with carboxyl groups in the acid form did not show an obvious improvement in tensile strength with increasing aldehyde content. The reason is not very clear but could be related to the possibility that hydroxyl groups are inclined to form ester bonds with carboxyl groups first or they form ester bonds more easily than hemiacetal linkages. Therefore, after ester bonds are formed, there are not enough hydroxyl groups left to form hemiacetal linkages. The tensile strength and Young’s modulus were also investigated for cross-linked films (Table 3). The cross-linked films made by method 1 did not show any improvement in 7838
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
Figure 5. Schematic representation of (a) ENCC in suspension (1, crystalline part of ENCC; 2, protruding DCC chains providing electrosteric stability).30 (b) Network of ENCC after forming dry films, where DCC chains were collapsed during the drying process. (c) Aggregated ENCC in a film prepared from cross-linked ENCC in suspension. (d) New structures formed in the cross-linked film by immersion of the non-cross-linked film in EDC and ADH solution (1, cross-linking in an adjacent crystalline part of the ENCC crossing; 2, cross-linking between ENCC and protruding DCC chains; 3, cross-linking among protruding DCC chains).
angle was still greater than 90° (Figure 9). The higher hydrophobicity of ENCC films with carboxyl groups in the acid form than in the sodium form is possibly due to the formation of ester bonds between carboxyl groups in the acid form and
Table 2. Tensile Strength and Young’s Modulus for ENCC Films with Carboxyl Groups in the Sodium and Acid Forms after Periodate Oxidation Treatment for Various Reaction Times sodium form reaction time/h 0 24 48 96 120 144
tensile strength/ MPa 63 80 115 129 134 126
± ± ± ± ± ±
2 6 8 6 4 5
acid form
Young’s modulus/GPa 3.99 3.45 4.66 3.96 4.05 4.03
± ± ± ± ± ±
0.15 0.14 0.02 0.05 0.31 0.03
tensile strength/ MPa 81 85 91 86 92 96
± ± ± ± ± ±
1 3 5 10 4 7
Young’s modulus/GPa 3.93 3.90 3.38 3.77 2.96 2.82
± ± ± ± ± ±
0.14 0.03 0.07 0.13 0.57 0.34
groups in the sodium form was around 104° (Figure 9a). However, this contact angle was not stable; it dropped to around 50° at the end of the measurement (6 min later), which was still higher than the initial contact angle of TEMPOoxidized nanocellulose films (TOCN films) reported by Fukuzumi.15 In their experiment, the initial contact angle is as low as 47°; even after treatment with a 0.05% alkyl ketene dimer dispersion, the contact angle increased to 94°, but this value could be maintained only for 10 s. For the ENCC film with carboxyl groups in the acid form, the initial contact angle was around 110° (Figure 9b), and even at 6 min, this contact
Figure 6. Tensile strength of films from ENCC treated by periodate oxidation as a function of reaction time or equivalently as a function of aldehyde content: (a) ENCC with carboxyl groups in the sodium form; (b) ENCC with carboxyl groups in the acid form. 7839
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
Figure 7. Solid C-13 NMR spectra for ENCC films with carboxyl groups in the sodium form and various aldehyde contents (various reaction times).
Table 3. Tensile Strength and Young’s Modulus for ENCC Films after Cross-Linkinga films
tensile strength (MPa)
Young’s modulus (GPa)
original ENCC cross-linking (method 1) cross-linking (method 2)
63 ± 2.0 65 ± 2.5 104 ± 5.5
3.93 ± 0.14 2.90 ± 0.30 3.37 ± 0.12
a Method 1: films made from a cross-linked ENCC suspension. Method 2: a cross-linking treatment was applied after the ENCC films were fabricated.
hydroxyl groups, thus greatly decreasing the content of hydrophilic groups on cellulose fibers. After treating the film with TCMS, it is very hard for the water drops to touch the films at first contact during the measurements. The contact angle of these treated films is around 160° (Supporting Information, Figure S4). Water Vapor Transmission Rates. Water vapor transmission rates (WVTR) were determined for a variety of samples, including office paper, microfibrillated cellulose (MFC) films (preparation process, see Supporting Information), and ENCC films and without any barriers (as a control). Without any barriers, the WVTR was as high as 5000 g m−2 day−1, and office copy paper gave a WVTR of 1000 g m−2 day−1.37 The WVTR for MFC films was around 800 g m−2 day−1, which is still relatively high. This is due to the high water affinity of cellulose fibers and the high porosity of MFC films (Figure 10). ENCC films were found to reduce the WVTR greatly to ∼385 g m−2 day−1. The WVTR was decreased to 160 g m−2 day−1 after cross-linking by the immersion of a noncross-linked film in EDC and ADH solution (method 2, Figure 10, inset graph), which is only 40% of the WVTR of the noncross-linked ENCC films. The film from cross-linked ENCC in suspension (method 1, Figure 10, inset graph) does not show a large decrease in WVTR, which is as high as 84% of that of the non-cross-linked ENCC film; the higher WVTR is possibly due to the large pores formed by the large aggregates in this film (Figure 5c). The superhydrophobic film prepared from TCMS treatment shows a low WVTR of 175 g m−2 day−1 (Figure 10, inset graph). All of the WVTR data were obtained under 50%
Figure 8. Derivative weight loss (DWL) curves for various ENCC films: (a) sodium form of carboxyl films with different numbers of aldehyde groups, (b) acid form of carboxyl films with different numbers of aldehyde groups, and (c) original ENCC film and crosslinked ENCC films by method 1 (c1) and method 2 (c2).
humidity at 22 °C. It should be noticed that in real applications other humidities and temperatures may affect the water permeability of films differently.
■
CONCLUSIONS Transparent films from various modified ENCCs were fabricated by vacuum filtration. The tensile strength of these films can be increased by introducing a certain number of aldehyde groups onto the ENCC with carboxyl groups in the sodium form or by cross-linking ENCC in films. The initial decomposition temperature of films could be improved by converting the sodium form of carboxyl groups on ENCC to 7840
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
vials for WVTR measurements. Figure of the initial water contact angle on ENCC films treated with TCMS. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We gratefully acknowledge funding from the Industrial Research Chair by the Natural Science and Engineering Research Council of Canada (NSERC) and FPInnovations.
■
(1) Staudinger, H. Ü ber Polymerisation. Ber. Dtsch. Chem. Ges. 1920, 53, 1073−1085. (2) Kaplan, D. L. In Biopolymers from Renewable Resources; Kaplan, D. L., Ed.; Springer: Berlin, 1998; pp 1−29. (3) Klemn, D.; Heublein, B.; Fink., H. P.; Bohn, A. Cellulose: Fascinating Biopolymer and Sustainable Raw Material. Angew. Chem., Int. Ed. 2005, 44, 3358−3393. (4) Siegel, R. W., Hu, E., Roco; M. C., Eds. WTEC Panel Report on Nanostructure Science and Technology: R & D Status and Trends in Nanoparticles, Nanostructured Materials, and Nanodevices; Kluwer Academic Publishers: Boston, 1999. (5) Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose. Biomacromolecules 2007, 8, 2485−2491. (6) Aulin, C.; Ahola, S.; Josefsson, P.; Nishino, T.; Hirose, Y.; Ö sterberg, M.; Wågberg, L. Nanoscale Cellulose Films with Different Crystallinities and MesostructuresTheir Surface Properties and Interaction with Water. Langmuir 2009, 25, 7675−7685. (7) Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. G. Effects of Ionic Strength on the Isotropic-Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites. Langmuir 1996, 12, 2076−2082. (8) Wainwright, S. A.; Biggs, W. D.; Currey, J. D.; Gosline, J. M. Mechanical Design in Organisms; Princeton University Press: Princeton, NJ, 1982. (9) Page, D. H.; EL-Hosseny, F. The Mechanical Properties of Single Wood Pulp Fibers. Part VI. Fibril Angle and the Shape of the StressStrain Curves. J. Pulp Pap. Sci. 1983, 9, 99−100. (10) Sakurada, I.; Nukushina, Y.; Ito, T. Experimental Determination of the Elastic Modulus of Crystalline Regions in Oriented Polymers. J. Polym. Sci. 1962, 57, 651−660. (11) Nishino, T.; Takano, K.; Nakamae, K. Elastic Modulus of the Crystalline Regions of Cellulose Polymorphs. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1647−1651. (12) Nishino, T.; Matsuda, I.; Hirao, K. All-Cellulose Composite. Macromolecules 2004, 37, 7683−7687. (13) Nogi, M.; Iwamoto, S.; Nakagaito, A. N.; Yano, H. Optically Transparent Nanofiber Paper. Adv. Mater. 2009, 21, 1595−1598. (14) Yano, H.; Sugiyama, J.; Nakagaito, A. N.; Nogi, M.; Matsuura, T.; Hikita, M.; Handa, K. Optically Transparent Composites Reinforced with Networks of Bacterial Nanofibers. Adv. Mater. 2005, 17, 153−155. (15) Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Transparent and High Gas Barrier Films of Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation. Biomacromolecules 2009, 9, 162−165. (16) Syverud, K.; Stenius, P. Strength and Barrier Properties of MFC Films. Cellulose 2009, 16, 75−85. (17) Somerville, C.; Bauer, S.; Brininstool, G.; Facette, M.; Hamann, T.; Milne, J.; Osborne, E.; Paredez, A.; Persson, S.; Raab, T.; Vorwerk, S.; Youngs, H. Toward a Systems Approach to Understanding Plant Cell Walls. Science 2004, 306, 2206−2211.
Figure 9. Change in contact angle with time for a water drop on an ENCC film with carboxyl groups in the sodium and acid forms. The inset presents the profiles of initial water contact angles on (a) ENCC films with carboxyl groups in the sodium form and (b) ENCC films with carboxyl groups in the acid form.
Figure 10. WVTR for different conditions: without films, copy paper, MFC films, and ENCC films. The inset presents WVTR for ENCC films, ENCC films after cross-linking (by methods 1 and 2), and superhydrophobic ENCC films from TCMS treatment.
the acid form. The maximum decomposition temperature of films was shifted from 250 to 320 °C after cross-linking the ENCC by ADH and EDC. ENCC films have quite low water vapor transmission rates compared to MFC films; a further reduced water vapor transmission rate can be obtained by cross-linking. Cross-linked films prepared from the immersion of non-cross-linked films in EDC and ADH solutions show a much higher tensile strength and a lower WVTR than those prepared with cross-linked ENCC suspensions. Although the original cellulose was hydrophilic, the ENCC films also show a certain hydrophobic behavior. TCMS-treated films become superhydrophobic and have a low water vapor permeation. These unique properties of ENCC films are very promising for applications in biodegradable packaging and other hightechnology products.
■
REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Surface morphology of ENCC and ENCC films by AFM measurements. Preparation of MFC films. Figures of assembled 7841
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
Langmuir
Article
(18) Baker, A. A.; Helbert, W.; Sugiyama, J.; Miles, M. J. HighResolution Atomic Force Microscopy of Native Valonia Cellulose I Microcrystals. J. Struct. Biol. 1997, 119, 129−138. (19) Herrick, F. W.; Casebier, R. L.; Hamilton, J. K.; Sandberg, K. R. Microfibrillated Cellulose: Morphology and Accessibility. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1983, 37, 797−813. (20) Turbak, A. F.; Snyder, F. W.; Sandberg, K. R. Microfibrillated Cellulose, a New Cellulose Product: Properties, Uses, and Commercial Potential. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1983, 37, 815−827. (21) Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Liquid Crystal Systems from Fibrillar Polysaccharides. Nature 1959, 184, 632−633. (22) Päak̈ kö, M.; Ankerfors, M.; Kosonen, H.; Nykanen, A.; Ahola, S.; Osterberg, M.; Ruokolainen, J.; Laine, J.; Larsson, P. T.; Ikkala, O.; Lindström, T. Enzymatic Hydrolysis Combined with Mechanical Shearing and High-Pressure Homogenization for Nanoscale Cellulose Fibrils and Strong Gels. Biomacromolecules 2007, 8, 1934−1941. (23) Henriksson, M.; Henriksson, G.; Berglund, L. A.; Lindström, T. An Environmentally Friendly Method for Enzyme-Assisted Preparation of Microfibrillated Cellulose (MFC) Nanofibers. Eur. Polym. J. 2007, 43, 3434−3441. (24) Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A. Individualization of Nano-Sized Plant Cellulose Fibrils by Direct Surface Carboxylation Using TEMPO Catalyst under Neutral Conditions. Biomacromolecules 2009, 10, 1992−1996. (25) van de Ven, T. G. M.; Tejado, A.; Alam, M. N.; Antal, M. Novel Highly Charged Non-Water Soluble Cellulose Products, Includes All Types of Cellulose Nanostructures Especially Cellulose Nanofibers, And Method of Making them. U.S. Provisional Patent Application 3776923-v3, 2011. (26) Maekawa, E.; Koshijima, T. Properties of 2,3-Dicarboxy Cellulose Combined with Various Metallic Ions. J. Appl. Polym. Sci. 1984, 29, 2289−2297. (27) Guthrie, R. D. Advances in Carbohydrate Chemistry; Wolfram, M. L., Ed.; Academic Press: New York, 1961; p 105. (28) Pagliaro, M. Autocatalytic Oxidations of Primary Hydroxyl Groups of Cellulose in Phosphoric Acid with Halogen Oxides. Carbohydr. Res. 1998, 308, 311−317. (29) Yang, H. Investigation and Characterization of Oxidized Cellulose and Cellulose Nanofiber Films. M.Sc. Thesis, McGill University, 2011. (30) Nakagaito, A. N.; Yano, H. Novel High-Strength Biocomposites Based on Microfibrillated Cellulose Having Nano-Order-Unit Weblike Network Structure. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 155−159. (31) Czaja, W. K.; Young, D. J.; Kawecki, M.; Brown, R. M., Jr. The Future Prospects of Microbial Cellulose in Biomedical Applications. Biomacromolecules 2007, 8, 1−12. (32) Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindström, T.; Nishino, T. Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 2008, 9, 1579−1585. (33) Chinga-Carrasco, G.; Syverud, K. Computer-Assisted Quantification of the Multi-Scale Structure of Films Made of Nanofibrillated Cellulose. J. Nanopart. Res. 2010, 12, 841−851. (34) Syverud, K.; Stenius, P. Strength and Barrier Properties of MFC Films. Cellulose 2009, 16, 75−85. (35) deButts, E. H.; Hudy, J. A.; Elliott, J. H. Rheology of Sodium Carboxymethylcellulose Solutions. Ind. Eng. Chem. 1957, 49, 94−98. (36) Araki, J.; Wada, M.; Kuga, S. Steric Stabilization of a Cellulose Microcrystal Suspension by Poly(ethylene glycol) Grafting. Langmuir 2001, 17, 21−27. (37) Huang, B.; Reghan, J. H.; van de Van, T. G. M. Nanopaper: Thin Films Prepared from Polymeric Nanotubes. To be submitted for publication. (38) Yuen, S. N.; Choi, S. M.; Phillips, D. L.; Ma, C. Y. Raman and FTIR Spectroscopic Study of Carboxymethylated Non-Starch Polysaccharides. Food Chem. 2009, 114, 1091−1098. (39) Sherif, M. A.; Keshk, S. Homogenous Reactions of Cellulose from Different Natural sources. Carbohydr. Polym. 2008, 74, 942−945.
(40) Rolande, B.; Agnese, M.; Marco, C. Swelling Behavior of Carboxymethylcellulose Hydrogels in Relation to Cross-Linking, pH, and Charge Density. Macromolecules 2000, 33, 7475−7480. (41) Long, L.; Dengru, L.; Miao, W.; Guocheng, D.; Jian, C. Preparation and Characterization of Sponge-like Composites by CrossLinking Hyaluronic Acid and Carboxymethylcellulose Sodium with Adipic Dihydrazide. Eur. Polym. J. 2007, 43, 2672−2681. (42) Horri, F.; Yamamoto, H.; Kitamaru, R.; Tanahashi, M.; Higuchi, T. Transformation of Native Cellulose Crystals Induced by Saturated Steam at High Temperatures. Macromolecules 1987, 20, 2946−2949. (43) Ragnar, E. K.; Wormald, P.; Ö stelius, J.; Iversen, T.; Nystrom, C. Crystallinity Index of Microcrystalline Cellulose Particles Compressed into Tablets. Int. J. Pharm. 1995, 125, 257−264. (44) Ung., J. K.; Shigenori, K.; Masahisa, W.; Takeshi, O.; Tetsuo, K. Periodate Oxidation of Crystalline Cellulose. Biomacromolecules 2000, 1, 488−492. (45) Tejado, A.; Antal, M.; Liu, X. J.; van de Ven, T. G. M. Wet Cross-Linking of Cellulose Fibers via a Bioconjugation Reaction. Ind. Eng. Chem. Res. 2011, 50, 5907−5913.
7842
dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834−7842
