Molecules that associate to form cross-links by hydrophobic association are designed and synthesised. Hydrogels, based on cellulose nanowhiskers (CNWs), acrylamide (AM), and stearyl methacrylate (C18), were synthesised by micellar copolymerisation, using ammonium peroxydisulfate as an initiator. CNWs composite hydrogels were characterised by Fourier transform infrared spectroscopy (FTIR) and their morphologies were investigated by scanning electron microscope (SEM). The system shows the original extensibility up to about 2500%: the tensile strength and compressive strength have maximum values of 1.338 MPa and 2.835 MPa, respectively. Besides excellent mechanical properties, CNWs composite hydrogels also have the ability to self-heal and remould: this is mainly attributed to the dissociation and reassociation of the associated micelles. In contrast to conventional cellulose hydrogels, these systems, when broken or cut, can be simply repaired by bringing together fractured surfaces to self-heal at room temperature.
The preparation and application of hydrogels based on cellulose have been reviewed by some groups, because of their applications across several technologies, such as hygienic products, horticulture, gel-actuators, drug delivery systems, water blocking tapes, and coal dewatering [
An attractive method of designing multifunctional hydrogels is to use the concept of supramolecular polymers. Nanocomposite gels are a useful strategy when seeking to improve hydrogel mechanical strength. CNWs have attracted much attention, not only because of their unsurpassed quintessential physicochemical properties, but also because of their inherent renewability and sustainability in addition to their abundance. They have been the subject of a wide array of research projects as reinforcing agents in nanocomposites due to their low cost, nanoscale dimension, renewability, availability, and unique morphology [
Here, we report a new type of CNWs composite hydrogel that can overcome the aforementioned shortcomings. The reported CNWs composite hydrogel is a hydrophobic association hydrogel, which was prepared by micellar copolymerisation. AM and CNWs acted as the main monomer to form a hydrophilic backbone. C18, dodecyl 2-methylacrylate (C12), and tridecyl methacrylate (C13), respectively, acted as hydrophobic monomers to form associated micelles that are the physical cross-linking points in a network of CNWs composite hydrogels: because of the unique network structure, CNWs composite hydrogels exhibit excellent strength and rubber-like properties, the most remarkable properties being that CNWs composite hydrogels are self-healing.
Acrylamide, sodium dodecylsulfate (SDS), stearyl methacrylate, ammonium persulfate (APS), N,N,N,N′-tetramethylethylenediamine (TEMED), tridecyl methacrylate, dodecyl 2-methylacrylate, and NaCl were commercially available and used as received.
IR spectra were recorded by FTIR (Nicolet iN10 Thermo Fisher Scientific China) over the region from 4000 to 400 cm−1. The CNW structure was analysed by atomic force microscopy (Shimadzu, SPM-9600).
For morphological characterisation, the hydrogels were analysed by scanning electron microscope (SEM) (S-3400N, Hitachi, Japan) with an acceleration voltage of 40 kV. A thin layer of the sample was cast on a silica wafer and freeze-dried, overnight, in a lyophiliser. A layer of gold was sputter-coated over the sample by vacuum spray to form a conductive surface.
Images of CNWs were obtained using atomic force microscopy in intermittent contact mode. Samples for AFM were prepared by placing a drop of dilute CNW suspension on freshly cleaved mica, followed by rinsing in deionised water and drying under a gentle flow of N2.
Tensile stress-strain measurements were performed by using an Instron 3365 Universal Testing Machine (Norwood, MA, USA) with the following parameters: sampling rate, 10.000 pts/sec; beam speed, 100 mm/min; full scale load range, 0.1000 kN; humidity, 25%; and temperature, 24°C. The strip-shaped gel samples measured 100 mm × 10 mm × 3 mm and the original length between top and foot clamps was 25 mm. Each data point was measured on six samples, and the average value of five measurements was taken. Statistical analysis of data was performed by one-way analysis of variance, assuming a confidence level of 95% (
The gravimetric method was used to measure the swelling ratios of the gels. After immersion in distilled water for approximately 48 hr at 25°C to reach swelling equilibrium, the gel samples were weighed. The average value of three measurements was taken. The equilibrium swelling ratio (SR) was calculated as
The cotton fibres were firstly soaked in dimethyl sulphoxide (60 mL) for 6 h at room temperature. After this, the fibres were washed in deionised water. The CNWs were obtained through acidic hydrolysis of the cotton fibres using 45 wt.% H2SO4 (a cellulose/H2SO4 ratio of 1/20 g/mL) at 75°C for 10 h under vigorous magnetic stirring. After this process, the resulting solution was centrifuged at 10,000 rpm for 5 min and washed thoroughly with deionised water until a pH of 7 was reached. The resultant material was lyophilised at 57°C for 48 h.
The gels were synthesised by micellar copolymerisation. The reaction system generally consisted of CNWs, water-soluble monomers (AM), hydrophobic monomers (C12, C13, or C18), surfactants (SDS), and water: SDS (0.7 g) and NaCl (0.30 g) were dissolved in 9.9 mL of dispersion of CNWs at 35°C to obtain a transparent solution. Then, hydrophobic monomer C18 (0.09 g, 4.3 wt.% relative to the amount of solid content) was dissolved in this solution under stirring for 2 h at a temperature of 35°C. After adding and dissolving AM (0.90 g, 45.9 wt.% relative to the amount of solid content) for 30 min, TEMED (25
CNWs can be isolated from various renewable sources (wood, cotton, wheat/rice straw, etc.). Figure
Scheme of the CNWs composite hydrogel matrix.
We used CNWs, AM, and metacrylic acid ester (C12, C13, or C18) as reactor monomers and added SDS to form solubilised micelles or comicelles with the hydrophobic monomers in an aqueous solution. Copolymerisation was initiated by ammonium persulfate herein, and the hydrogels thus obtained were transparent and resilient (Figure
In this reaction system, CNWs and AM acted as the main monomer to form a hydrophilic backbone. C12, C13, and C18 acted as hydrophobic monomers to form associated micelles (Figure
The structure of hydrogels of acrylamide with different hydrophobic monomers (stearyl methacrylate (a), tridecyl methacrylate (b), or dodecyl 2-methylacrylate (c)) and CNWs were characterised by FTIR (Figure
FTIR spectra of CNWs composite hydrogels (stearyl methacrylate (a), tridecyl methacrylate (b), or dodecyl 2-methylacrylate (c)).
The effect of the concentration of cellulose, acrylamide content, and content (and types) of hydrophobic monomers on the gel mechanical properties was analysed. First, we synthesised the gel at the same concentration and experimental conditions with only CNW concentration changed (0.0007 g/mL, 0.0014 g/mL, 0.0021 g/mL, 0.0028 g/mL, and 0.0035 g/mL).
Figure
Effect of CNW concentration on CNWs composite hydrogel mechanical properties ((a) tensile strength; (b) compressive strength; (c) compression strength test images of hydrogel with a CNW concentration of 0.0014 g/mL).
We synthesised gel of C18 under the same concentration and experimental conditions with only the dosage of AM changed from 34.5 wt.% to 54.4 wt.%. Figure
Effect of acrylamide content on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressive strength).
Figure
Effect of stearyl methacrylate content on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressive strength).
Figure
Effect of hydrophobic monomer on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressive strength).
Figure
Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 0.0007 g/mL, (b) cellulose concentration of 0.0014 g/mL, (c) cellulose concentration of 0.0021 g/mL, (d) cellulose concentration of 0.0028 g/mL, and (e) cellulose concentration of 0.0035 g/mL).
Hydrogels can absorb large amounts of water and release the absorbed water in dry conditions. A decrease in the water uptake capacity of these CNWs composite hydrogels was observed (Figure
Swelling ratios of CNWs composite hydrogels.
CNWs composite hydrogels exhibit excellent strength and rubber-like properties in a unique network structure (Figure
Self-healing behaviour of CNWs composite hydrogels.
Gels with SDS exhibited a self-healing efficiency of nearly 100% after a healing time of 60 min. However, when swollen in water, no self-healing ability was observed. This was maybe because the gels swelled in water leading to the extraction of SDS micelles from the gel network: the surfactant SDS controlled the hydrophobic associations formed in the hydrogels. This suggested that the key factor leading to self-healing was the weakening of strong hydrophobic interactions due to the presence of surfactant molecules [
A simple method of making CNWs composite hydrogels that are transparent and soft materials containing a large amount of water has been proposed. In many applications, the use of hydrogels is often severely limited by their mechanical properties. Using CNWs to prepare “smart” hydrogels could not only improve their self-healing and reforming capacity but also improve their mechanical strength. CNWs composite hydrogels exhibited excellent mechanical properties; both the tensile strength and the compressive strength strongly depended on hydrophobic monomer content, hydrophobic side chain length, and AM and cellulose contents. C18 in the CNWs composite hydrogels played an important role in the domination of the physical cross-linking points formed by hydrophobic association, leading to self-healing and reforming capacity, whereas CNWs and AM mainly contributed to the increasing equilibrium between mechanical strength and swelling ratio thereof. Their unique self-repairing properties, the simplicity of their synthesis, their availability from renewable resources, and the low cost of their raw ingredients (cellulose) bode well for future applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This paper is supported by “2015 Beijing Construction Project of Scientific Research and Cultivation of Graduate Student” and “Research Program of Circulation Utilization of Plant Waste 2014HXKFCLXY034.”