Cellulose nanocrystals (NCC) and cellulose nanofibrils (CNF) were obtained by a single step process, with synergy between 64% sulfuric acid hydrolysis and high shear from ultra-turrax stirring, which is an advantageous process for disintegrating cellulose microcrystalline and also may improve the hydrolysis process. The surface modification on the cellulose was performed by the sol-gel process, in which the sulfate groups from hydrolysis were replaced by nanoparticles of zinc oxide, which led to the increase of up to 54 °C Tonset, according to thermogravimetric analysis (TGA) results. The morphology and crystallinity degree were characterized by Helium Ion Microscopy (HIM), atomic force microscopy (AFM) and X-ray diffraction. In addition, the ZnO band was observed in Fourier transform infrared spectroscopy, furthermore, the change in the zeta potential confirmed the cellulose modification. The changes in the values of proton spin-spin relaxation time for the systems showing the confined hydrogen in the rigid domains, confirmed the results observed with the aforementioned techniques, for both cellulose after hydrolysis and ZnO modified cellulose, suggesting that ZnO disrupted crystal formation in cellulose.
Cellulose has applications in several areas due to its unique properties that are compatible with the growing search for materials from renewable sources. It is a biopolymer that is the main constituent of the cell wall of plants, and its extraction process follows the top-down strategy and can be mechanically or chemically extracted. Cellulose nanocrystals (NCC) are one of the materials obtained from this polymer, which can be used for several studies due to its versatility and characteristics. So, many studies report the obtaining cellulose nanocrystals (NCC) by hydrolysis with strong mineral acids as the most commonly used methodology, such as formic acid, acetic acid, sulfuric acid, hydrochloric acid, phosphoric acid or acid mixtures [
Some issues are attributed to acid hydrolysis process, such as the recovery of the used acid, large amounts of base employed for neutralization, large amount of water, equipment corrosion and environmental pollution [
Many works in the literature describe that the hydrolysis of cellulose with sulfuric acid can be found; where concentration, time and temperature of the reaction are cited as fundamental factors for the successful of the process. The break of glycosidic bonds in cellulose occurs preferentially in the amorphous region, while the crystalline part is less affected [
Sulfuric acid concentration and temperature were crucial in reducing the degree of polymerization (DP) of cellulose and yield of NCC, since with 16 wt% acid, the DP decreases from 700 to 200 along temperature increase from 45˚C to 85˚C, maintaining the yield; by increasing the acid concentration to 64% the DP was not altered, but the yield decreased due to competition between depolymerization and degradation reactions [
NCCs and cellulose nanofibrils (CNFs) have interesting properties beyond their mechanical reinforcement to nanocomposites; those properties are related to the numerous possibilities of chemical modifications over hydroxyls located at cellulose surface, which can improve the polymer-cellulose interaction, decreasing the tendency for agglomeration and can add new functionality to cellulose for different applications such as: biomedical engineering, electronics and energy and water treatment sectors [
Much research is oriented to increase the yield of NCCs, reduce cellulose decomposition by monitoring the production of NCC and cellulosic solid residues (CSR) obtained from acid hydrolysis processes [
Microcrystalline cellulose (MCC) was purchased from Synth (density 0.26 - 0.32 g/cm3, polymerization grade 350), nanocrystalline cellulose was purchased from Celluforce (density 0.7 g/cm3) (NCC_C), sulfuric acid, sodium hydroxide, ethyl alcohol and di-hydrated zinc acetate were obtained from Sigma-Aldrich. The reagents were used as received.
Cellulose nanocrystals were obtained from acid hydrolisis of microcrystalline cellulose. First, a suspension of MCC (10%w/v) with 32%v/v sulfuric acid was produced and homogenized on a mechanical stirrer (IKA RW20) for 30 min at 17,000 rpm. Then the acid concentration was increased up to 64%v/v, and hydrolysis continued for a further 2 h under reflux at 50˚C. During the addition of sulfuric acid, the system was cooled with a cold ice bath.
By the end of the reaction time, 300 ml of cold water was added to stop the hydrolysis reaction, after which, the suspension (identified as NCC_H) was taken to the mechanical stirrer for a further 20 min at 17,000 rpm. Finally, dialysis (molecular weight cut-off 14,000) was performed to neutralize the acid suspension, and the suspension stored in a refrigerator for approximately 24 h.
The previously obtained cellulose suspension (200 ml) was submitted to reaction with the same volume of ethanol, adjusting the pH to 8 with sodium hydroxide solution (5 M).
The metal precursor used was zinc acetate dihydrate, which, being previously dissolved in ethanol for 24 h, reacted with cellulose in a 1:1 ratio. The solution containing the precursor was dropped into the cellulose suspension, while pH was monitored and corrected to pH 8. After the addition was complete, the reaction was heated at 80˚C under reflux for 2 h. Then, part of the solvent was removed by evaporation, the precipitate washed with ethanol and centrifuged (4x at 3500 rpm for 10 min). At each washing step the cellulose was re-suspended with the aid of tip ultrasonic homogenizer (Ultronique QR500, frequency of 20 KHz). The sample was oven dried at 120˚C for 1 h [
The Zeta potential of the samples was measured on the NICOMP DLS/ZLS, model Nano Z3000, by the Electrophoretic light scattering technique. Concentration suspensions of 0.001 wt% were prepared, which before being analyzed were dispersed for 1 min in Ultrasonic QR500 ultrasound, then tripled analyzed at room temperature.
Helium beam assisted electron microscopy images were acquired on the Zeiss Orion nanofab model equipped with tungsten filament, operating at a constant voltage of 30 kV. The sample preparation involved suspension of 0.5 mg cellulose in 1 ml acetone, which was then diluted 50× and dispersed in ultrasonic bath for 15 min. Then a drop was deposited on the silicon substrate, and the samples analyzed. As these are insulating samples, floodgun was used to neutralize the surface charge.
Cellulose samples were prepared in aqueous suspensions (1%w/v), these suspensions were diluted 100× and sonicated for 15 min. A drop of the diluted suspension was deposited on the surface of the freshly cleaved mica, then dried under N2 flux. Images were acquired on a Dimension Icon® (Bruker, Santa Barbara-CA) AFM instrument using Peak Force Tapping QNM scan mode, cantilever model HQ-300-AU (Oxford Instruments®, UK), spring constant 40 N/m, average radius of 10 nm and 300 kHz frequency.
A Rigaku Ultima IV X-ray diffractometer, operating with voltage and current of 40 Kv and 30 mA, respectively, was used for X-ray diffraction characterization. X-rays were obtained from a CuKα source (ʎ = 1.5418 Å). The analysis conditions for the MCC, NCC_C and NCC_H samples were made at 0.01˚/s, 2θ between 10˚ and 40˚, with 2θ for the NCC-Zn sample between 10˚ and 80˚. The analyses were carried out at room temperature.
The size of the crystallite, Ʈ, perpendicular to the crystalline (2 0 0) plane, peak 2θ at 22.5˚, was calculated by the Scherrer equation:
Ʈ = K λ β cos θ (1)
where K is the correction value 0.9, λ is the wavelength of the X-rays (1.54 Å), β is the FWHM of the diffractogram peak (radians), and θ is the peak diffraction angle.
The crystallinity index, CrI, was calculated by the Segal Method whose equation is:
CrI = I t − I a I t × 100 (2)
where It is the total intensity of peak 200 (2θ at 22.5˚), Ia is the intensity of the amorphous contribution at 2θ = 18.4˚.
Analyses were performed in a Nicolet 6700-FT-IR equipment, using KBr pellet, 4 cm−1 spectral resolution and 16. Sample was dried for 48 h before analysis.
The thermal stability of the materials was analyzed on TA Instruments model TGA Q-50, under nitrogen atmosphere, heating rate of 10˚C/min in the temperature range of 30˚C - 700˚C.
Relaxation time measurements were performed on a MARAN Ultra NMR (Oxford Instruments) NMR spectrometer at 30˚C ± 2˚C with 0.54 T (23 MHz for 1H). The samples were dried for 48 h in a vacuum oven at 60˚C and placed directly into the 18 mm diameter glass tube for analysis.
The Magic Sandwich Echo pulse sequence was used to acquire the fully refocused Free Induction Decay (MSE-FID). All relaxation curves were acquired 512 points spaced by 0.5 microseconds, with a repeating time of 1 second, 128 scans and 34 dB receiver gain.
The obtained signals were adjusted according to the following equation:
F R exp [ − 1 2 ( t − μ a T 2 * F R ) 2 ] . ( sin ( 2 π t ) 2 π t ) + F M exp [ − ( t T 2 * F M ) β M ] + k (3)
where FR, and FM are the fractions of the rigid (crystalline) and mobile (amorphous) regions, respectively. T2* is the transverse relaxation time of one of these fractions, ν is a constant component of sine wave oscillation based on Van Vleck’s second and fourth moments. βM corresponds to the Weibullian function exponents ranging from close to zero to 1. μa is centroid of the Abragamian and k is the offset or baseline of the relaxation signal that compensates for the influence of background signal during nonlinear adjustment.
Considering a = 1/T2*FR and b = 2πν, the mean value of the second dipolar coupling moment can be obtained through the equation:
〈 M 2 〉 = a 2 + 1 3 b 2 . (4)
Despite being an important parameter to evaluate the stability of suspensions, this technique was employed in order to follow the influence of the surface modification influenced the cellulose surface charge. NCC_H presented a zeta potential around −44 mV and reached a value close to 0 mV when modified with zinc. The value for NCC_H is consistent with the literature, since there is a direct relationship of this value with hydrolysis times [
The morphology of the materials was examined by helium ion microscopy (HIM) and atomic force microscopy (AFM). Microcrystalline cellulose (
(commercially obtained cellulose nanocrystals), both samples showed the same agglomeration tendency in round-shaped clusters and the presence of a thin layer around the cluster. The topography image for sample NCC_C (
HIM image
The AFM image (
The X-ray diffraction pattern shown in
Gaussian deconvolution of the diffractograms allowed to better evaluate the profile of the crystalline planes before and after hydrolysis, making the changes of the crystallographic patterns more noticeable. There is a greater crystalline contribution to the MCC when compared to the NCC_H, if we consider the peak intensity at (110) and (200). The degree of crystallinity was calculated by the Segal method, which, as described by [
Samples | Ʈ200 (nm) | CrI (%) |
---|---|---|
MCC | 5.38 | 84 |
NCC_C | 4.73 | 79 |
NCC_H | 5.39 | 81 |
conditions (64%, 50˚C, 120 min) favored degradation reactions, with cellulose being reduced to sugars or soluble by-products [
The well-defined diffraction peaks at 2θ = 31.9˚ (100), 34.7˚ (002), 36.4˚ (101), 47.7˚ (102), 56.7˚ (110), 63.1˚ (103), 68.2˚ (200) and 69.3˚ (201) correspond to the hexagonal structure of the wurtzite, as shown in
No adjustments could be made for the NCC_ZnO sample due to the high zinc oxide diffraction signal.
Changes in the chemical environment and the appearance of new absorption bands after chemical modifications were observed in the FTIR spectrum (
Although negative zeta potential value was observed for NCC_H, cellulose-O-SO3 binding was not detected in the IR spectrum, therefore, the degree of sulfonation is below the FTIR detection limit [
The degradation of MCC and NCC_C presents a single degradation pattern, as
seen in
In the DTG curve, it is possible to see the changes in the cellulose degradation behavior. The degradation of the NCC_H sample occurs over a wider temperature range, and events that occur at lower temperatures are related to the most accessible sulfated region, which, according to Kargarzadesh [
Sample | T10% (˚C) | Tpeak (˚C) | % Residue |
---|---|---|---|
MCC | 316 | 340 | 4 |
NCC_C | 282 | 300 | 19 |
NCC_H | 226 | 300 | 18 |
NCC_ZnO | 336 | 335 | 68 |
to the amorphous region; also, it was attributed to the increase in sulfated chain terminals, which begin to degrade faster [
Zinc oxide surface modification (NCC_ZnO) improved cellulose thermal stability compared to NCC_H, evidenced by a Tonset increase of 54C, as shown in
The NCC_ZnO sample exhibited 5% mass loss at 100˚C, followed by a single degradation event with 30% mass loss. The residue percentage corresponds to the sulfur residue, derived from the NCC_H sulfate ester groups, and to the modification with zinc oxide. Azizi’s group [
The nuclear magnetic relaxation technique allows the morphological investigation of nanostructured systems such as cellulose, through the relaxation times of protons present in each environment or domain, where the polymer segments assume a specific conformation/arrangement. From the measurements obtained by MSE-FID (
The mean value of the second dipole coupling moment 2> is associated to
the distance between cellulose chain segments in the crystalline fraction, being proportional to the coupling force between the hydrogen nuclei in the crystals. The value of 9262 rad·ms−2, shown in
Nanocrystals and cellulose nanofibrils were produced in a single step combining acid hydrolysis mechanical stirring with ultra-turrax, which was responsible for the improvement of MCC dispersibility in acid medium, the high shear promoted the mass transfer and helped the disintegrations and isolation to NCC
Sample | T2*Fr (µs) | Xc = FR (%) | T2*Fm (µs) | Fm (%) | 2> (rad·ms −2) |
---|---|---|---|---|---|
MCC | 20 | 94 | 84 | 6 | 6311 |
NCC_C | 20 | 94 | 79 | 6 | 6311 |
NCC_H | 10 | 91 | 91 | 9 | 9262 |
NCC_ZnO | 20 | 86 | 75 | 14 | 5966 |
and NFC. Next, the nanoparticles were superficially modified with nano oxide by sol-gel methodology under low temperature, as found by HIM and AFM, FTIR, XRD, TGA and NMR analyses. The NMR technique clarified the changes in the constituent fractions of cellulose, rigid and mobile fraction, validating the observations made through the XRD technique. Furthermore, the crystalline fraction of NCC_ZnO was detected by this technique. The properties presented by nanocellulose and zinc oxide were combined into a single particle, and the improvement in thermal stability enables the preparation of nanocomposite via melting mixture. Therefore, this nanoparticle is expected to play the reinforcing role and barrier property, characteristic of nanocellulose, and antimicrobial and anti-UV activity, property of ZnO, when added to a polymer. According to all properties studied about this particle we can say that it could be used in packing.
This work was financially supported by CAPES code 001 and CNPQ. The authors thank for the Biological Physical Laboratory, located in Carlos Chagas Institute of Biophysics, special for the Professors Dr. Gilberto Weissmüller and Dr. Paulo Bish. We also thank for Inorganic Chemistry Department, Institute of Chemistry, mainly MSc. Glaucia Martins for all the support and contributions. Both Institutes are in the Federal University of Rio de Janeiro.
The authors declare no conflicts of interest regarding the publication of this paper.
Melo, A.R.A., Filho, J.C.D., Neto, R.P.C., Ferreira, W.S., Archanjo, B.S., Curti, R.V. and Tavares, M.I.B. (2020) Effect of Ultra-Turrax on Nanocellulose Produced by Acid Hydrolysis and Modified by Nano ZnO by Sol-Gel Method. Materials Sciences and Applications, 11, 150-166 https://doi.org/10.4236/msa.2020.112009