Isolation of Nanocellulose from Cotton Cellulose and Computer Modeling of Its Structure

Nanocellulose is a new class of derivatives of cellulose, which is characterized by high crystallinity, surface area, degree of dispersion, ability to decomposition by microorganisms and etc. There is high attention solving problems of obtaining nanocellulose and its application as high quality filler for polymers, biodegradable materials, additives for papers, clotting dispersion and etc. Obtaining of particles of nanosized nanostructure on the base cellulose, studying of processes of their formation, properties and creation nanotechnology on this basis give the chance to obtain materials with unique properties. In this work nanocellulose was obtained from cotton cellulose by hydrolysis with sulfuric acid, ultrasonic dispersion and microwave irradiation. The properties and structure of nanocellulose are investigated by AFM, IR-spectroscopic, X-ray methods. Nanocellulose has rod-like shape with sizes 50 300 nm in length and 10 40 nm in diameters and spherical shape with sizes 50 300 nm depending on the synthesis conditions of obtaining. Quantum-chemical methods have been used to calculate the electronic characteristics of nanocellulose; the change in the energy difference between HOMO and LUMO is shown, showing the change in reactivity and the manifestation of specific properties.


Introduction
Nanocellulose (NC) is a new class of derivatives of cellulose, which is characterized by high crystallinity and surface area, and ability to degradation by micro-organisms [1]. NC can be used as a high quality filler for polymers, additives for papers, clotting dispersion etc. [2] [3] [4]. In addition, their reaction activity increases due to the decrease of cellulose particles size, which allows the production of NC derivatives with new properties [5].
Nanocellulose and its derivatives present a new brand of nanotechnology that appears to have very wide applications in a variety of materials related domains where physical-chemical characteristics such as strength, weight, rheology and optical properties can be affected in a very positive manner [6].
Nanocellulose can be classified according to various factors such as shape, size and structure. For the shape characteristic, there are basically three types: sheet, spherical and whiskers. It is obvious that micro fibrillated nanocellulose (MFC) shape is sheet-like while microcrystalline cellulose exists in two forms: spherical and whisker. It should be noted that within each structure of nanocellulose there is a distribution of length, width, and percent crystallinity. These structures have several varieties such as (particle-like, rod-like, web-like and fiber-like). MFC has two forms: web-like and fiber-like, while NFC due to its structure has the form of fiber-like only [7] [8] [9].
A wide range of cellulose from plant raw materials: wood, bamboo, ambary, cellulose etc. are used as an initial material for obtaining NC, and its geometrical characteristics, such as shape, particle size and composition (aldehyde and carboxyl groups) differ significantly depending on the source of raw materials and the obtaining conditions [10] [11] [12]. Decrease of the cellulose particle size until nanoscale is possible to achieve via various methods: mechanical treatment, acid hydrolysis, enzymatic hydrolysis and etc. [13] [14] [15].
Mechanism of obtaining NC usually is explained by acid hydrolysis of amorphous domains of cellulose, while the more persistent nanocrystallite remains intact and can be isolated in the form of rod-like particles. However, this mechanism does not explain why nanoparticles cannot be isolated even after intensive mechanical disintegration of cellulose hydrolyzing diluted acid solutions to limit the degree of polymerization [16]. The probable reason for this phenomenon is that the crystallites of cellulose are linked by strong intermolecular bonds.
Sufficient concentrated solution of acid can break these strong bonds and separate nanocrystallites. However, when cellulose hydrolyses with diluted solutions of acids, these bonds between the crystallites remain intact and nanoparticles of cellulose are formed instead of nanoparticles.
According to literature, the main properties of nanocellulose materials are: biodegradability, biocompatibility, high strength, high surface area, moisture absorption, dimensional stability and chemical functionality [14].

Preparation of Cellulose and Nanocellulose
Cotton cellulose with degree of polymerization (DP) 1200 was used for the preparations of NC.

Characterization
Structural and phase analyses of the samples were implemented using an X-ray diffractometer XRD-6100 (Shimadzu) with Cu Kα radiation, generated at 60 kV and 80 mA and step-scan mode (2θ range: 0.1˚C -50˚C). The crystalline index of cellulose was calculated using the Segal's empirical method [17].
Atomic force microscopy observations were carried out using Agilent 5500.
After a short sonication to prevent the formation of aggregates, drops of dilute cellulose suspensions were deposited onto freshly cleaved mica. After some time, the excess liquid was removed and the remaining film allowed to dry, then it was measured.
The FTIR spectra of nanocellulose were measured in the range from 4000 to 400 cm −1 (IRTracer-100, Shimadzu, Japan). The average DP was measured by the viscosity method using diluted solutions of cellulose powders in Schweizer's Reagent.
Quantum-chemical and semi-empirical methods in the framework of Hyper-Open Journal of Polymer Chemistry chem programs, as well as ChemDraw and Chem3DPro, included in the package ChemBioOffice 2010 (CambridgeSoft) were used in studies

Results and Discussion
The study of the influence of temperature and hydrolysis time on the DC showed that the dependence is directly proportional, i.e. with increasing temperature (25˚С -45˚С) and duration of hydrolysis (0.5 -2 h) the DC of the obtained samples are increased (79% -84%) ( Table 1).
However, it should be noted that the strong conditions of hydrolysis not only lead to a decrease in particle size, but also affects negatively the appearance quality of the final product. Therefore, the concentration of sulfuric acid of 61% at temperature 45˚C for an hour formed a white suspension and the yield of which amounts up to 58%. Color of the suspension becomes yellow with yield of 35% under the temperature 45˚C. With increasing temperature up to 60˚C in the first 10 minutes there is a yellowing of the suspension, then after 1 h suspension becomes black, i.e., charring of the cellulose happens under these conditions. It was found that the optimum temperature for acid hydrolysis is 40˚C -45˚C and the optimal ratio for cellulose:acid solution = 1:10.
IR-spectroscopic studies show that the peaks of the typical cotton cellulose characterize NC; however, the intensity of peaks is higher (See Figure 1).
Research of influence of conditions for obtaining NC also showed that with the tightening of the conditions of hydrolysis process of removal of amorphous parts and crystalline parts occur due to dissolution of low molecular weight  The results showed the possibility of obtaining nanosized particles of cellulose (<500 nm) at a concentration of hydrolyzing agent more than 60%. However, with increasing acid concentration, the average particle size decreases and the loss is increased due to the transition of cellulose into the solution. With further increase in acid concentration more than 66% there is a continued particle size reduction to the nano range and that product finally leads to the transition of cellulose into the solution and the solution becomes transparent.
According to XRD investigations, in the range of H 2 SO 4 concentration from 50% to 61% the crystallinity of NC increases, further increase in acid concentration leads to a sharp decrease in the degree of crystallinity (DC) up to 25% ( Figure 2 Table 2). On the base of XRD diffraction width (10 -40 nm) and length (50 -300 nm) elementary crystallites of NC were detected.
At increasing acid concentration process is accompanied by reduction of the sizes of particles with simultaneous depolymerization macromolecules of cellulose, at that intermolecular bonds between fibrils of cellulose start rupturing and      along the entire length of the chain, but for each chain its change depends on the geometry of the structure and the number of formed intra-chain hydrogen bonds.
A quantum-chemical calculation of the models of possible end groups of nanocellulose obtained during hydrolysis was carried out ( Figure 6). Since the presence of such groups changes the electronic characteristics, as a result, the reactivity changes and the specific properties of cellulose nanoparticles appear.

Conclusions
The possibility and optimal conditions for NC preparation and its stabilization were shown. The structure of the NC is proved by physical methods. Nanocellulose has rod-like shape with length of 50 -300 nm and width of 10 -40 nm with high crystallinity and spherical shape with low degree of crystallites and sizes of 50 -300 nm is obtained depending on the synthesis conditions.
Thus, influence of condition of obtained NC on its properties and structures were investigated.
It was found that at mild conditions with using of physical factors (USD, MW) obtained NC has rod-like shape (whiskers) with sizes (50 -300) × (10 -40) nm with high degree of crystallinity and yield. Severe hydrolysis conditions lead to obtain NC like spherical shape (balls) with sizes by (50 -300) nm low degree of crystallinity and less yield.
Quantum-chemical methods are used to calculate the electronic characteristics of cellulose and its hydrolysis products; the change in the difference between the energy of HOMO and LUMO is shown, which indicates a change in reactivity and the manifestation of specific properties.