The production of cellulose/chitosan blends in alkyl imidazolium ionic liquids (ILs) was studied in this work. Selected organic solvents, such as dimethyl sulfoxide, ethyl acetate, and diethyl ether, were used as cosolvents. The addition of cosolvents decreased the viscosity of cellulose/chitosan solutions in ILs and facilitated the dissolution of polysaccharides, thereby decreasing the
Since 2002 ILs have been established as direct solvents for cellulose dissolution [
The similarity in the chemical structures of chitosan and cellulose predicts their compatibility and the possibility of their homogeneous blending. Cellulose, as most of the naturally occurring polysaccharides, is acidic in water media [
Xie et al. reported in 2006 [
It is known that the addition of cosolvent may facilitate dissolution of polysaccharide in ILs [
The blended material could be obtained in both solid and liquid states. Solid-phase blending is provided under high pressure and shear deformation [
The aim of this work is to study the direct dissolution of chitosan and cellulose/chitosan blends in ILs and to determine the effect of selected cosolvents on the dissolution process in order to facilitate the process of producing blended materials for different purposes, including medical or textile applications, and to make this process more “green.” We believe that the use of ILs as a solvent, replacing the previously used and more cumbersome solvents, will greatly benefit the technology. It was found that aprotic cosolvents allow for the dissolution of cellulose in ILs much faster and easier [
High-purity wood-derived chemical cellulose pulp Alicell-Super (degree of polymerization, DP, 599), produced from western hemlock (Western Pulp Inc., Canada), was mechanically dispersed with Corner mill (calculated alpha content 93.5%, moisture content 8%). Chitosan “A” “Polymar Brazil” (
The blends of cellulose and chitosan were obtained by 2 methods: simultaneous mixing of cellulose, chitosan, and ILs or by separate preparation of solution of each polymer in the same IL following subsequent combination. To separate preparations of the solutions of each polysaccharide, microwave irradiation 90 Wat during 5–120 s, acid media such as 10 wt% IL/water solution, and different temperatures (25, 50, 75, and 110°C) over a period of 4 h with stirring were applied to dissolve chitosan. To the solution of chitosan in IL, the cellulose/IL solution was added in calculated proportions. The two solutions in a common solvent were stirred at different temperatures (50–110°C) to obtain a homogeneous blend solution. In the second method, simultaneous dissolution of blended polysaccharides was carried out at different temperatures (room temperature: 50, 75, and 110°C) over 4 hours with stirring. Cosolvents were added simultaneously with other components at the preparation step in amount of 5 wt%. Turbidity, measured as an indicator of the progress of dissolution, was determined with an optical microscopy method.
pH measurements of IL/water solution with IL content 10, 20, and 50 wt% were recorded with a pH meter, Metler Toledo, Germany.
Viscosity measurements were carried out with a Rheometer (Rheological Instruments Inc.) using the plate-plate system. Viscosity measurements were carried out with a shear rate in the range of gradient 5–120 s−1 (at 90°C) and temperature heating from 0 to 100°C at constant shear rate.
Cellulose and mixtures of cellulose/chitosan were dissolved in ILs and were casted onto a warm glass plate (40°C), smoothed with a glass stick to obtain a thickness of 0.5 mm, and rinsed with distilled water to remove solvent. The solvents utilized are miscible with water. Solvent removal was carried out in water in a tub, with at least 30 minutes of rinsing after each of 3 washes.
The tensile strength and elongation at break of the obtained films were measured with an Instron-5544 apparatus according to ISO 527-3:1998.
TG analysis of samples cellulose, chitosan, and blended films was carried out by an NETZSCH STA 409 PC/PG differential scanning calorimeter at a heating rate of 10 K/min under a continuous nitrogen flow in a temperature range between 20 and 500°C.
Fourier transform infrared spectroscopy (FTIR) was made with a Genesis Series FTIRTM (Unicam Ltd.) spectrometer. The measurements were provided in the midinfrared region, approximately 4000–500 cm−1, using a KBr tablet.
GPC was provided with a set of three PLgel Mixed A columns (
Zetasizer Nano (Malvern Instruments Ltd., UK) was used for determining the size of polymers aggregates in solution at 90°C. The Zetasizer system determined the size by first measuring the Brownian motion of the particles in the sample using dynamic light scattering (DLS) and then interpreting a size from this using established theories. The size range measured by this apparatus is 2 nm–3
A selection of different types of chitosan, including microcrystalline chitosan, were used in this work. Chitosan degrades at high temperatures; therefore, to dissolve chitosan in the ILs, the samples were treated with microwave treatment first. Even at the lowest power setting available with our equipment, the appearance of smoke was observed after few seconds of treatment, indicating the degradation of the chitosan. All types of chitosan degraded under microwave irradiation in ILs. Thus, an alternative dissolution procedure was adopted.
Acidic media were used in order to study the possibility of dissolution of chitosan under mild conditions (without heating). Solutions of water/IL have different pH values dependent on the water content of the mixture. After measuring the pH of the IL/water solutions, the most acid solution was chosen for chitosan dissolution. Table
The pH values of the studied IL/water solutions.
pH values of IL/water solutions ILs content at | |||
---|---|---|---|
IL | 20°C, wt% | ||
10 | 20 | 50 | |
BMIMCl | 6.85 | 6.75 | 6.27 |
BMIMAc | 5.30 | 5.48 | 6.54 |
EMIMAc | 6.17 | 6.36 | 7.73 |
At room temperature chitosan could be more effectively dissolved in 10 wt% BMIMAc/water solution, because of its greater acidity compared with the other studied solutions. An amount of 1 wt% of chitosan A and B could be easily dissolved in BMIMAc/water solution, containing 10 wt% of IL, over 1.5 h with magnetic stirring. Due the fact that chitosan C has a larger
As it was discussed earlier, chitosan degraded rapidly at high temperatures; therefore, water was removed at low temperatures under vacuum conditions. The residual chitosan/IL system after the removal of water appeared to be a gelatinous material. The appropriate amount of cellulose/IL solution was added to chitosan/IL system after calculation of the component ratio in the obtained chitosan/IL system. The sample was heated to 50°C to blend the solutions of the two polysaccharides avoiding the degradation of chitosan. However, after heating over the course of 24 hours at 50°C, and even at 110°C, the blending of the components was ineffective; the system formed a heterophase gelatinous material. The formation of such a system is likely to occur due to the formation of a strong interaction between chitosan and IL on the removal of water from the system. The newly formed interaction between the two phases was so strong that the obtained chitosan/IL material was not miscible with the cellulose/IL solution, and it was insoluble in pure IL.
Among the ILs studied only EMIMAc and BMIMAc were able to dissolve at least 1 wt% of each of the analyzed chitosans; clear solutions were obtained after heating at 110°C for 4 hours. The unsuccessful experiments of the dissolution of chitosan in EMIMAc in previous studies [
Direct dissolution of chitosan in IL is very challenging; therefore, the progress of the dissolution was only observed in solutions with little chitosan content. The maximum ratio of the components of the cellulose/chitosan/IL systems achieved was 4.5/0.5/95, independent of the chitosan type. The ratio of polysaccharides was 90 wt% cellulose to 10 wt% chitosan.
Aprotic solvents are efficient cosolvents for cellulose dissolution in ILs [
The influence of the shear rate on the viscosity of cellulose/chitosan C solution in BMIMAc with and without cosolvents at 90°C. The ratio of components cellulose : chitosan : BMIMAc : cosolvent is 4.5 : 0.5 : 90 : 5.
The temperature dependence of viscosity was studied for cellulose/IL and cellulose/chitosan/IL solutions over a temperature range of 273–373 K. Figure
The temperature dependence of viscosity of 5 wt% cellulose solutions in EMIMAc with and without cosolvents (solid lines) and 4.5 wt% cellulose/0.5 wt% chitosan A solutions in EMIMAc with and without cosolvents (dashed lines) on temperature at constant shear rate 10 s−1. The concentration of cosolvents was 5 wt%.
The Arrhenius plot for calculating the activation energy (
Arrhenius plot for cellulose/chitosan A blend solution in EMIMAc with cosolvents. Trend lines correspond to first Arrhenius approximation.
By taking the log of both sides and rearranging we can get the equation of a straight line (
The sizes of polymer aggregates (hydrodynamic radius) and
IL | Cosolvent | Concentrations of components in the solutions, wt% |
|
Aggregates size, nm | |||
---|---|---|---|---|---|---|---|
Cellulose | Chitosan | IL | Co-solvent | ||||
EMIMAc | — | 5 | 0 | 95 | 0 | 23 | 419 |
— | 4.5 | 0.5 | 95 | 0 | 24 | 600 | |
DMSO | 4.5 | 0.5 | 90 | 5 | 21 | 552 | |
DEE | 4.5 | 0.5 | 90 | 5 | 21 | 203 | |
EAc | 4.5 | 0.5 | 90 | 5 | 19 | 368 | |
| |||||||
BMIMAc | — | 5 | 0 | 95 | 0 | 24 | 197 |
— | 4.5 | 0.5 | 95 | 0 | 27 | 208 | |
DMSO | 4.5 | 0.5 | 90 | 5 | 25 | 199 | |
DEE | 4.5 | 0.5 | 90 | 5 | 21 | 196 | |
EAc | 4.5 | 0.5 | 90 | 5 | 20 | 175 |
The larger the value of
Thus, the dissolution of polysaccharides starts easily in the presence of cosolvents due to penetration of cosolvents into the polysaccharide’s structure and subsequent replacement of cosolvent by IL. This mechanism facilitates the dissolution of polysaccharides in ILs. In Table
The mechanical characteristics of cellulose films were compared with those of the blended films obtained with and without the use of cosolvents. The blended film had a relatively similar maximum force and elongation at break in comparison with cellulose film, but the blended film had better tensile strength (Figure
Comparison of mechanical properties of pure cellulose film, obtained from 5 wt% cellulose/EMIMAc solution, cellulose/chitosan B film obtained without cosolvents from 4.5 wt% cellulose/0.5 wt% chitosan/90 wt% EMIMAc solution, and cellulose/chitosan B films, obtained from 4.5 wt% cellulose/0.5 wt% chitosan/90 wt% EMIMAc/5 wt% cosolvent solutions.
The addition of DMSO and DEE improved the mechanical characteristics of the blended films, but the addition of EAc slightly decreased its values; the cosolvents used may act as plasticizers. The dissolution of the polymer blends in the presence of EAc is very intensive, and the molecular structure of polysaccharides was greatly destroyed, which leads to inferiority in the mechanical properties of the blended films. Generally, the blended films were stronger than the pure cellulose films, produced from the ILs.
The influence of blending polysaccharides on the degradation of cellulose was studied by means of GPC. The amount of chitosan extracted from blended film was approximately 3wt%. From the analysis of the extracted cellulose component it was found that the increase in chitosan content toward cellulose leads to a decreasing of the cellulose DP in the blended films (Table
The influence of the amount of chitosan A in blended film on the
Components ratio in solutions for films producing | Analysis of cellulose component extracted from blended films | |||
Cellulose | IL | Chitosan |
|
DP |
| ||||
4.5 | 85 | 0.5 | 64,681 | 399 |
4.9 | 85 | 0.1 | 69,995 | 432 |
Chitosan |
|
---|---|
Original chitosan powder | 54,400 |
Chitosan obtained from films prepared from the following: | |
Chitosan/EMIMAc (0.5 wt% of chitosan) solution | 12,400 |
4.5 wt% cellulose/0.5 wt% chitosan/EMIMAc solution | 7,600 |
4.5 wt% cellulose/0.5 wt% chitosan/EMIMAc/5 wt% DMSO solution | 7,400 |
4.5 wt% cellulose/0.5 wt% chitosan/EMIMAc/5 wt% DEE solution | 6,900 |
We found that using a cosolvent to facilitate the dissolution of polysaccharides in ILs leads to additional swelling and deep destruction of the structure of chitosan, which resulted in lowering its
Using DTG analysis two peaks were observed for the degradation of cellulose/chitosan-blended film (Figure
DTG plot of cellulose, chitosan A, and cellulose/chitosan blended film, obtained from BMIMAc solution.
Cellulose/chitosan-blended films were produced as final products from the studied solutions. The blended films produced, with polymer concentrations of 4.5 wt% and 0.5 wt%, respectively, consist theoretically of 90 wt% of cellulose and 10 wt% of chitosan. By the FTIR analysis it is possible to confirm the presence of chitosan in the blended films. Bands of OH-stretching (3650–3200 cm−1, 1200–1000 cm−1) and C–H stretching (2900–2850 cm−1, 1500–1300 cm−1) frequencies, which could be attributed to both polymers, were found in FTIR spectra (Figure
Comparison of FTIR spectra of films produced from EMIMAc solutions and original polysaccharides.
Also, the peaks corresponded to NH-stretch of amide and carbonyl group (1597 cm−1, 1659 cm−1) of chitosan [
At more close look at the chitosan spectra (Figure
Shifting of characteristic peaks in cellulose/chitosan blend.
Among the ILs studied, EMIMAc and BMIMAc are suitable to produce cellulose/chitosan blends by heating from all types of chitosan used in this study. Microwave treatment was not sufficient for chitosan dissolution and led to its rapid degradation. Simultaneous blending of components was more efficient than mixing the separately prepared chitosan and cellulose solutions in a common solvent. The addition of cosolvents decreased the viscosity of cellulose/chitosan solutions in ILs and facilitated the dissolution of polysaccharides. The use of cosolvents facilitates polysaccharides blending in ILs. Increasing the chitosan concentration in the blends promoted the degradation of the cellulose component. The addition of DMSO and DEE improved the mechanical properties of blended films. Despite different characteristics, all of three types of chitosan showed very similar dissolution and blending properties. Chitosan is a promising polysaccharide for cellulose modification. ILs are prospective and efficient solvents for blending cellulose and chitosan, which do not require chitosan derivatives to improve the miscibility of chitosan and cellulose.
The research presented received funding from the European Community's Seventh Framework Program [FP7/2007–2013] under Grant Agreement no. PITNGA-2008-214015.