The demand for substitution of fossil-based materials by renewable bio-based materials is increasing with the fossil resources reduction and its negative impacts on the environment. In this study, environmentally friendly regenerated cellulose films were successfully prepared using 1-allyl-3-methylimidazolium chloride (AmimCl), 1-butyl-3-methylimidazolium chloride (BmimCl), 1-ethyl-3-methylimidazolium chloride (EmimCl), and 1-ethyl-3-methylimidazolium acetate (EmimAc) as solvents, respectively. The results of morphology from scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that all the cellulose films possessed smooth, highly uniform, and dense surface. The solid-state cross-polarization/magic angle spinning (CP/MAS) 13C NMR spectra and X-ray diffraction (XRD) corroborated that the transition from cellulose I to II had occurred after preparation. Moreover, it was shown that the ionic liquid EmimAc possessed much stronger dissolubility for cellulose as compared with other ionic liquids and the cellulose film regenerated from EmimCl exhibited the most excellent tensile strength (119 Mpa). The notable properties of regenerated cellulose films are promising for applications in transparent biodegradable packaging and agricultural purpose as a substitute for PP and PE.
With the development of modern society and industry, there is growing demand for development of renewable and biodegradable materials as substitutes for petroleum-derived synthetic polymers [
Over the past decades, several cellulose solvent systems have been available for dissolving cellulose, such as viscose process (CS2) [
Cotton linter, supplied by Silver Hawk Fiber Corporation (Shandong province, China), was used as cellulose sample with the degree of polymerization (DP) 920. The degree of polymerization was measured by TAPPI test method using cupriethylenediamine (CED) as a solvent and a capillary viscometer, to give an indication of the average degree of polymerization of the cellulose materials. The viscosities determined as centipoises (cP) were converted to degree of polymerization (DP) based on the following formula:
The cellulose sample used was dried in vacuum at 105°C for 24 h before using. The ILs 1-ethyl-3-methylimidazolium acetate (EmimAc), 1-allyl-3-methylimidazolium chloride (AmimCl), 1-ethyl-3-methylimidazolium chloride (EmimCl), and 1-butyl-3-methylimidazolium chloride (BmimCl), used as solvents for dissolution of the cellulose, were purchased from Chengjie Chemistry Corporation.
In a typical procedure for preparation of regenerated cellulose film, 5 g cotton linter sample was dispersed in 100 g ILs of EmimAc, AmimCl, EmimCl, and BmimCl, respectively. The mixture was heated at different temperatures in the range of 80°C–120°C under magnetic stirring until cellulose dissolved in ILs completely. After dissolution, the resulting transparent solution was cast on a glass plate and then immediately coagulated in the water to obtain transparent regenerated cellulose gel. The regenerated cellulose gel was washed with running distilled water and then air dried. All films were kept in a conditioning cabinet at 50% relative humidity (RH) and 25°C to ensure the stabilization of their water content before characterization.
A micrometer (Lorentzen & Wettre, precision 1
Scanning electron micrographs (SEM) were taken on a HITACHI S-3400N scanning electron microscope with 10 kV acceleration voltage and at a magnification of 2000. The free surface and the fracture surface of the films were sputtered with gold-palladium on a HITACHI E-1010 and then observed and photographed.
Atomic force microscopy (AFM) (SPM-9600, Shimadzu) was used to study the morphology of film surface. In AFM scanning, two to four interest locations on each sample were tested. Small pieces of films were glued onto metal disks and attached to a magnetic sample holder located on the top of the scanner tube. Phase images were recorded under ambient air conditions. All of the images were recorded in contact mode using silicon cantilevers.
Crystallinity of the regenerated cellulose film samples was determined by X-ray diffraction patterns using XRD-6000 instrument (Shimidzu, Japan). The method was the same as that at literature [
Thermal analysis was determined by using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) on a simultaneous thermal analyzer (SDT Q600 TGA/DSC, TA Instrument). The samples weighed around 10 mg were heated from room temperature to 600°C at a heating rate of 10°C/min under an inert atmosphere of N2.
Infrared spectra of cellulose film samples were recorded with a Fourier transform IR spectrometer (FT-IR TENSOR27, Germany) in ATR mode. The specimens were measured directly with a scan range from 400 cm−1 to 4000 cm−1.
Solid-state cross-polarization/magic angle spinning (CP/MAS) 13C NMR spectra of the regenerated cellulose film samples were recorded on a Bruker AV-III 400 M spectrometer (Germany) operated at a 13C frequency of 100.6 MHz. A 4 mm zirconia (ZrO2) rotor was used to pack cellulose films, and the measurement was performed using a CP pulse program with a 2 ms contact time and a 2 s delay between transitions. The spinning speed was set at 5 kHz for all samples.
Contact angles of the cellulose films were measured to calculate the surface free energies of the cellulose films. The measurements were performed at room temperature by the sessile drop method using a goniometer equipped with a high-speed camera (OCA 20, Data physics Ltd., Germany).
The tensile strength of the regenerated cellulose films was measured by using a tensile testing machine (Zwick Universal testing machine Z005) at a speed of 5 mm/min. The samples were cut in the rectangular specimens with a width of 20 mm and length of 60 mm, and eight replicate specimens were tested from each film type. A grip distance of 20 mm was maintained. The measurements were performed at 25°C and relative humidity of 50%.
The topography of produced films was analyzed by SEM and AFM. SEM and AFM micrographs of the regenerated films are shown in Figures
SEM images of regenerated films, (a) AmimCl, (b) BmimCl, (c) EmimCl, and (d) EmimAc.
AFM images of regenerated films, (a) AmimCl, (b) BmimCl, (c) EmimCl, and (d) EmimAc.
The X-ray diffraction patterns of cotton linter and cellulose films regenerated from AmimCl, BmimCl, EmimCl, and EmimAc are shown in Figure
Crystallinity, peak positions in X-ray diffractograms, and contact angle of cellulose films.
Sample | Crystallinity (%) | Diffraction angle |
Contact angle/ |
|||
---|---|---|---|---|---|---|
(−110) | (110) | (200) | (020) | |||
Cotton linter | 54.01 | 15.0 | 16.4 | 22.6 | — | — |
AmimCl | 38.41 | 12.8 | 20.2 | — | 21.2 | 38.48 |
BmimCl | 39.25 | 12.6 | 20.2 | — | 21.2 | 40.08 |
EmimCl | 39.24 | 12.8 | 20.6 | — | 21.2 | 54.12 |
EmimAc | 34.04 | 12.8 | 20.2 | — | 21.2 | 34.18 |
XRD images of films, (a) cotton linter, (b) AmimCl, (c) BmimICl, (d) EmimCl, and (e) EmimAc.
FT-IR spectroscopy was used to obtain direct information on chemical changes in cellulose during dissolution and regeneration. Figure
FT-IR images of films, (a) AmimCl, (b) BmimCl, (c) EmimCl, and (d) EmimAc.
Thermal degradability is affected by the chemical composition of the material. The typical TGA and DSC curves of the regenerated cellulose films are shown in Figures
TGA of films, (a) virgin fibers, (b) AmimCl, (c) BmimCl, (d) EmimCl, and (e) EmimAc.
DSC of films, (a) virgin fibers, (b) AmimCl, (c) BmimCl, (d) EmimCl, and (e) EmimAc.
The 13C CP/MAS NMR spectra of cotton linter and different film samples are shown in Figure
13C CP-MAS spectra of films, (a) virgin fibers, (b) AmimCl, (c) BmimCl, (d) EmimCl, and (e) EmimAc.
To examine the suitability of the cellulose films for industry applications, the mechanical properties of the regenerated cellulose films were determined. The typical stress-strain curves of regenerated films prepared in AmimCl (a), BmimCl (b), EmimCl (c), and EmimAc (d) are shown in Figure
Strength of films, (a) AmimCl, (b) BmimCl, (c) EmimCl, and (d) EmimAc.
The regenerated cellulose films were successfully prepared using different ionic liquids including AmimCl, BmimCl, EmimCl, and EmimAc as solvents. It was shown that the regenerated cellulose films displayed smooth, highly uniform, and dense morphology properties. 13C CP/MAS NMR spectra and XRD corroborated that the transition from cellulose I to II had occurred after preparation. In comparison, the ionic liquid EmimAc possessed much stronger dissolubility for cellulose and the cellulose film regenerated from ionic liquid EmimCl exhibited the most excellent tensile strength. The notable properties of the regenerated cellulose films are promising for applications in transparent, biodegradable packaging and agricultural purpose as a substitute for PP and PE.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful for the financial support of this research from the Beijing Higher Education Young Elite Teacher Project (YETP0766), Program for New Century Excellent Talents in University (NCET-12-0782), Natural Science Foundation of China (no. 31170557), Major State Basic Research Projects of China (973-2010CB732203), and China Ministry of Education (no. 111).