A green strategy to endow superabsorbents with stretchability and self-healability
Graphical abstract
Introduction
Hydrogels are hydrophilic polymeric networks, capable of absorbing and retaining large amount of aqueous solutions [1]. Various hygienic and non-hygienic applications for hydrogels have been developed, including tissue engineering [2], drug delivery [3], water treatment [4], [5], agricultural [1], structural [1], etc. Although hydrogels offer spectacular features, still their poor mechanical strength and low deformation is a challenge [6]. The employment of the conventional crosslinkers, such as methylene bisacrylamide and polyethylene glycol diacrylate (PEGDA), results in an inhomogeneous crosslinking, and subsequently the stress and the hydrostatic pressure during swelling cannot be distributed uniformly through the system [7]. Thus, these soft materials can be easily ruptured at low deformations (or during swelling).
The common strategy to overcome this deficiency is the incorporation of nanoparticles [2], [8]. The highly-stretchable (physically and covalently crosslinked) nanocomposite hydrogels have been prepared employing silicate and hydroxyapatite nanoparticles [9], [10], [11]. Physical interaction between nanoparticles and polymer chains at high deformations would result in “self-healing” and viscoelastic behavior, whereas covalent crosslinking of polymer chains can provide almost elastic network [12]. Rose et al. have invented a method to produce “glue hydrogels” by silica nanoparticles with strong and rapid adhesions [13]. The reversible interactions between the repeating units of the polymer chains and the surface of the silica nanoparticles confer this adhesion to the system [13]. The other concepts employed for developing the mechanical properties of hydrogels were interpenetrating polymer network (IPN) and double network (DN); however, these methods still suffer from inhomogeneity [14]. The employment of dendritic structures in the hydrogels is also another way to increase the gel toughness [6], [15]. Moreover, the reversible and covalent crosslinkers are attractive to design tough hydrogels. The reversible crosslinking could be based on ionic forces, hydrogen bonding, crystalline domain, hydrophobic associations, etc [6].
Changing the architecture of a polymer alters its inherent characteristics, and is a common practice to tailor its chemical, diffusional, and physical–mechanical properties [16]. Molecular weight, rigidity of the core molecule, length of branches, and the degree of conversions are factors often engineered to achieve specific features in the star-shaped systems [16]. Although star-shaped systems are particularly interesting, the research on star-shaped systems for hydrogel applications seem rather limited so far. Keys et al. introduced polyethylene glycol (PEG) star polymer hydrogels which were promising to be used as drug carriers [17]. Since then, the star-shaped structures were tailored for different characteristics [18]. In a very recent work, we have improved the mechanical properties of conventional superabsorbent hydrogels via surface modification of superabsorbent by star-shaped lactic acid based oligomers. In that study, star-shaped oligomers were successfully employed as an external crosslinkers [19]. The arms lengths and the functionalities would affect the network properties, the swelling, and the elasticity of the hydrogels [20]. In fact, the star-shaped architecture is believed to improve the swelling in some cases. Furthermore, the rate of drug release in star-shaped hydrogels can be engineered by changing the dimension of the hydrophobic core, the arm number and the arm length [21]. Homogeneous crosslinking increase the hydrogels network homogeneity, and develops uniform stress distribution [7]. Therefore, the architecture of the crosslinker significantly influence the network performance [6], [7], [15].
The reversible and dynamic crosslinking strategies, including hydrophobic interactions [22], [23], hydrogen bonding [24], [25], host–guest interactions [26], and electrostatic interactions [27], have been employed to achieve highly stretchable and self-healable hydrogels. Incorporation of multiple mechanisms, covalently and non-covalently bonding, could act as sacrificial bonds to create tough hydrogels with high stretchability [22]. In addition, various nanocomposite hydrogels, with or without conventional crosslinkers, have been developed with adequate toughness and stretchability, using mineral nanoparticles, such as montmorillonite [28], silicates [10], laponite [29], graphene oxide [30], and silica [25]. In fact, the strong physical interactions of nanoparticles with the backbone of the polymers and grafting on the nanoparticles have led to such a stretchable and tough hydrogels
It is believed that hydrophobic interactions confer stretchability to hydrogels [22], [26], [31]. The copolymer of the polymerizable surfactant (dodecyl glyceryl itaconate) and poly acrylamide (PAAm) has been employed to design super-tough and self-recoverable gels with great hysteresis and fatigue resistance [22]. Micellar polymerization is another approach to design highly stretchable hydrogels. Copolymerization of stearyl methacrylate and dodecyl acrylate, and arylamide, in the presence of sodium dodecyl sulfate (SDS) have been also employed for preparation of stretchable, self-healable hydrogels [31]. Recently, Jeon et al. have reported the preparation of the acrylamide-based hydrogels, via micellar polymerization, using SDS and NaCl. This hydrogel, which has been crosslinked by a polymerizable supramolecular, exhibits outstanding stretchability (100 m m−1), but poor strength (5–10 kPa) [26]. The utilization of polymerizable macromolecular surfactant (i.e., amphiphilic polyurethane) in micellar copolymerization of acrylamide for preparation of stretchable hydrogels, has also been reported [32]. Moreover, the stretchable and self-healable hydrogels have great potentials to be used in stretchable electronic devices [33], such as supercapacitors [34] and self-healable energy-storage devices [34], These supercapacitors can be made through prestrain-stick-release assembly [35] with the use of carbon nanotube or graphene oxide as electrodes [35], [36].
The employment of star-shaped architectures in the hydrogel field has been reported previously [6], [15], [17], [37], [38]. In this study, the feasibility of achieving simultaneous stretchability and healability in superabsorbent hydrogels via employing star-shaped oligomers was demonstrated. The structure of the oligomers engineered to confer the required features to the finished SAPs. These manipulations include the substitution of the toxic or unsuitable catalysts, employment of less hydrophobic functionalization agent (which increases the compatibility of the oligomer and SAP), change in the length of the arms (for achieving the best performance), and finally, the omission of polymerization-interfering chemicals, i.e. inhibitors. Herein, the synthesis of highly stretchable superabsorbent with self-healability characteristics is reported. In spite of many toughened gels which are reported to be colored or opaque, these super-swelling hydrogels are transparent and colorless. In the authors’ previous works, star-shaped lactic acid based oligomers have been synthesized and characterized [16], [19], [39], [40], [41]. Herein, the star-shaped oligomers are engineered to be used for the SAP modification. We expect hydrophobic interactions, associated to these crosslinkers, confer stretchability, self-healing and strength to the SAPs. In this study, we report the synthesis and employment of acrylated oligomers (AOs), as the interior crosslinker, in the acrylic acid (AA)-based SAPs. Chemical structures of the crosslinker, as well as the mechanical and rheological properties of the modified SAP films, in both as-prepared and swollen states will be evaluated. In addition, the self-healing of the modified SAPs will be studied in detail through different tests.
Section snippets
Materials
L (+)-lactic acid (LA) (≥80%; Merck), anhydrous glycerol (≥95.5; Merck), p-toluenesulfonic acid (≥99.0%; Merck), toluene (≥99.8%; Merck), and acrylic acid (AA, ≥99%; Sigma Aldrich) were employed for the synthesis of the star-shaped oligomers. For the synthesis of the acrylate-based SAP films (named modified SAPs (MS)), the following chemicals were used: sodium hydroxide (≥99%, Merck), polyethylene glycol diacrylate (PEGDA, Mw 400, Rahn, Switzerland), ammonium persulfate (≥90%; Dae Jung) and
Chemical characterization
FTIR, 1H and 13C NMR studies were conducted to confirm the chemical structures of the HTO (and the AOs; FTIR spectra of AO-4 is presented in Fig. 1, and its 1H and 13C NMR spectra are presented in Fig. 2.a, and b, respectively. Chemical shifts were assigned based on authors’ previous reports and presented in Table 1 [16], [39], [40], [41]. The peaks revealed at ∼5.9 & ∼6.5 ppm, represent the protons of H2CC (denoted by c in Fig. 2.a). The formation of carboncarbon double bonds was also
Conclusions
A set of star-shaped acrylated oligomers (AOs) were synthesized through polycondensation of glycerol and LA, followed by acrylation of the branches. The oligomers utilized as the interior crosslinker for preparation of novel superabsorbent transparent sheets, which resulted in the transparent, stretchable, flexible films, in both as-prepared and swollen states, which can be attributed to the presence of physical attractions. Various facts support the presence of physical interactions: a) the
Notes
The authors declare no competing financial interest.
Acknowledgement
The authors would like to acknowledge the financial support from Iran Polymer and Petrochemical Institute and Iran National Science Foundation.
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Both authors contributed equally to this work.