Flow-related packing of biopolymeric fibers to generate microfibrous bioassemblies (paper-based products) is a sustainable commercial process (Tayeb et al., 2017), where intermolecular forces play a central role in determining process efficiency and product functionalities. It would be interesting to identify new possibilities of utilizing existing facilities and/or process basics of the well-established green paper industry to produce value-added products. Essentially, the facile strategy of assembling biopolymeric fibers, i.e., papermaking process, would be extended to any liquid systems containing liquid-dispersible particles (e.g., graphene oxide) (Fukahori et al., 2003; Dikin et al., 2007). The use of mass-producible paper-based bioassemblies as substrates for diversified functional materials would also be very promising (Sadri et al., 2018). Indeed, there is an ongoing need to identify new strategies of product design in light of inspirations from the paper industry. In this context, we herein propose a process concept of converting a dilute papermaking fiber slurry into dynamic hydrogels (Fig. 1).
Concept of using crosslinking additives to convert a dilute slurry of bio-derived, micro-sized fibers into dynamic hydrogels for diversified, value-added applications
Hypothesized features of the process concept are: (1) production of valuable dynamic hydrogels is readily implementable by utilizing existing facilities of commercial paper mills; (2) production of hydrogels instead of paper- based commodities leads to the unnecessity of dewatering and whitewater treatment; (3) commercial practices of wet-end chemistry, such as those related to the use of chemical additives, provide insights into hydrogel formation; (4) paper-grade pulp fibers function as skeletons for hydrogel formation, while still contributing to intermolecular bonding due to accessible functional groups (hydroxyls in particular); and (5) manipulation of dynamic interactions in fiber slurry by using fiber-compatible additives can lead to tailorable formation of dynamic hydrogels with diversified functionalities.
In an attempt to demonstrate the proposed concept shown in Fig. 1, we used polyvinyl alcohol and sodium tetraborate decahydrate as chemical additives to construct dynamic bonding/networking interactions in a dilute slurry of paper-grade pulp fibers. In the paper industry, polyvinyl alcohol has found widespread commercial use as a chemical additive for paper surface treatment. It is also used in the formulation of pigmented coating colors for paper. Cationic modification of this hydrosoluble synthetic polymer can facilitate its efficient use in papermaking wet-end applications (Fatehi et al., 2011). According to recently published works (Spoljaric et al., 2014; Lu et al., 2017), a combined use of nanoscale cellulosic particles, polyvinyl alcohol, and borax can generate dynamic hydrogels, and these components are likely to interact synergistically with each other. However, there remains a need to assess the possibility of using regular paper-grade pulp fibers for hydrogel formation.
The fibers used in the current study exhibited typical microsized characteristics (Fig. 2A). They are also widely known as hollow tube-like structures (Tejado et al., 2010). The combination of these fibers with polyvinyl alcohol resulted in the formation of bridged structures (Fig. 2B–C). Noticeably, the addition of sodium tetraborate decahydrate generated interlocked composite networks with significantly increased bonding sites (Fig. 2D–E). It is noted that, sodium tetraborate decahydrate can form dynamic crosslinks with both cellulose and polyvinyl alcohol, and hydrogen bonding also contributes to hydrogel formation (Han et al., 2014). The networks of the hydrogel containing no fibers, as shown in Fig. 2E–F, were somehow "finer" in comparison to fiber-based hydrogels. Nevertheless, fibers were largely embedded in matrices of hydrogels, and polyvinyl alcohol networks were efficiently anchored onto fibers.
Morphologies, FTIR spectra, TG-DTG profiles, and a colored hydrogel*s photograph. (A) Freeze-dried fiber slurry containing no additives. (B) Freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 1 g/g). (C) Freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 0.67 g/g). (D) Freeze-dried, fiber-based hydrogel (polymer dosage: 1 g/g). (E) Freeze-dried, fiber-based hydrogel (polymer dosage: 0.67 g/g). (F) Freeze-dried polyvinyl alcohol hydrogel containing no fibers. (G-H) TG-DTG diagrams of freeze-dried fiber slurry containing no additives (F), freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 0.67 g/g) (F-H), and freeze-dried polyvinyl alcohol hydrogel containing no fibers (polymer content: 5%) (P-H). (I) FT-IR spectra of freeze-dried fiber slurry containing no additives (F), freeze-dried fiber slurry containing polyvinyl alcohol (polymer dosage: 3.33 g/g) (F-H), and freeze-dried polyvinyl alcohol hydrogel containing no fibers (polymer content: 3.33%) (P-H). (J) Photograph of a fiber-based hydrogel (polymer dosage: 0.67 g/g)
Structural bonding plays a governing role in thermal decomposition. TG-DTG profiles (Fig. 2G-H) show notable differences among freeze-dried samples (biopolymeric fibers and hydrogels) as regards pyrolysis characteristics. Evaporation of water molecules is responsible for initial weight losses (e.g., at a temperature of less than 100 ℃). For the tested samples, fast removal of water molecules was achieved at around 70 ℃. Clearly, as indicated from TG-DTG profiles, the water content of freeze-dried polyvinyl alcohol hydrogel was much higher than that of freeze-dried, fiber-based hydrogel or freeze-dried fibers, largely due to high hydrophilicity of polyvinyl alcohol. The use of fibers in hydrogel formation shifted the peak of weight loss rate to a higher temperature, indicating their role as a skeleton in hurdling thermal decomposition. On the other hand, fiber-induced increase of the yield of final residue (after the end of heating) can be linkable to the networking and structural bonding role of fibers in hydrogel formation.
FTIR spectra of selected samples are shown in Fig. 2I. Freeze-dried fiber slurry without any additive shows characteristic peaks for cellulose, particularly 1030 cm–1 and 1053 cm–1 associated with C—OH bonds (primary and secondary alcohols) (Bian et al., 2018). For this sample, O—H stretching vibrational bonds at 3332 cm–1 can be attributed to hydrogen bonding in cellulose I (Liu et al., 2011). The peak at 2895 cm–1 pertains to C—H stretching vibration (Yang et al., 2007). Conversion of the fiber slurry into a hydrogel (polymer dosage, 3.33 g/g) resulted in noticeable change of FT-IR spectrum, and the characteristic peaks at 1417 cm–1 and 1338 cm–1 are indicative of the formation of boron-based dynamic crosslinks (Spoljaric et al., 2014). These crosslinks can bind polyvinyl alcohol to fibers, forming 3D networks. In such a reaction medium, hydrogen bonds can also be generated, contributing to complex network formation. As seen in Fig. 2I, polyvinyl alcohol hydrogel and fiber-based hydrogel had similar FT-IR spectra, presumably due to the efficient embedment of fibers in complex networks. Thus, these FT-IR results provide useful evidence for the role of fibers as structural skeleton in hydrogel formation. The photograph shown in Fig. 2J further confirms that a dilute slurry of hollow, micro-sized fibers can be facilely converted into hydrogel-based networks with the aid of chemical additives.
Degree of swelling is a critical parameter for the design of hydrogels with tailorable functionalities (Rauner et al., 2017; Nojoomi et al., 2018). For hydrogels generated from a dilute slurry of hollow tube-like, micro-sized fibers, degree of swelling was governed by polymer dosage (Fig. 3A–C). Very interestingly, after about 3 d, a degree of swelling of as high as about 800 was achieved at a polymer dosage of 3.33%. In other words, 1 g of freeze-dried sample can absorb about 800 g of water. Reducing polymer dosage led to lowered degree of swelling. It is noteworthy that swelling behavior has a strong correlation with the nature of crosslinkers. A high degree of swelling translates to the possibility of using only a small amount of starting materials to produce hydrogels. These products are usable as a replacement for commercially available petroleum-based plastics, and in such circumstances huge environmental benefits can be expected (Wang et al., 2010).
Degree of swelling (DS), pH-directed phase-reversibility, and stretchability. (A–C) Swelling of fiber-based hydrogels with polymer dosages of 3.33, 1.00, and 0.67 g/g, respectively. (D) pH-responsiveness of a representative fiber-based hydrogel with polymer dosage of 3.33 g/g. Addition of hydrochloric acid solution (1 mol/L) is responsible for gel-sol transformation, and post-addition of sodium hydroxide solution (1 mol/L) can lead to sol-gel transformation. (E) Stretchability of a fiber-based hydrogel (polymer dosage, 3.33 g/g)
Hydrogels that can easily undergo phase transition when subjected to stimuli would have interesting smart applications (Piest et al., 2011; Li et al., 2018). As shown in Fig. 3D, the selected fiber-based hydrogel was pH-sensitive, and the addition of hydrogen chloride solution (1 mol/L) resulted in gel-sol transition. Upon further addition of sodium hydroxide solution (1 mol/L), the sol can be converted into a gel. This pH-induced phase-reversibility agrees well with previous studies involving the use of boron-based crosslinking additives for hydrogel formation (Piest et al., 2011; Lu et al., 2017; Hong et al., 2018). Such a phase-reversible nature is due to the reversible crosslinking interactions.
Despite the fact that hydrogel-related research is booming, most hydrogels are brittle (i.e., not easily stretchable), which limits their scope of applications (Sun et al., 2012). Delivering stretchability to hydrogels involves the design of functional structures such as those related to capability of "frictional" sliding of molecular tubes (Ke et al., 2019). As shown in Fig. 3E, a dilute slurry of papermaking fibers can be converted into stretchable structures (note that the hydrogels can be easily stretched to two folds of original length), favorable for the design of functional products like stretchable electronics and devices.
Endowing hydrogels or other categories of materials with self-healing characteristics has been an active research area (Taylor et al., 2016; Azevedo et al., 2017; Chakraborty et al., 2019). As shown in Fig. 4A, fiber-based hydrogels with polymer dosages of 3.33, 1.00, and 0.67 g/g, respectively, had noticeable self-healing characteristics. Self-healing was easily achievable, and the healed sites were basically intact when hydrogels were turned upside down. Fig. 4B shows that the healed sites were strong enough to support two stainless steel weights (totally 200 g). The change of storage modulus and loss modulus due to alternatively varied strains indicates the self-healability of hydrogels (Fig. 4C). Under mechanical forces with a strain of 1%, storage modulus was higher than loss modulus, indicating solid-like nature of hydrogels. When the stain switched from 1% to 20%, both moduli were lowered, and storage modulus was lower than loss modulus, indicating liquid- like nature of hydrogels. It is interesting to note that both moduli were easily recoverable in combined treatments of alternatively varied strains. Self-healing characteristics combined with phase-reversibility would facilitate the use of fiber-based hydrogels in diversified applications, e.g., toys, soft robots, 3D/4D printing materials, drug delivery systems, and medical devices.
Self-healing characteristics. (A) Self-healing characteristics of fiber-based hydrogels with polymer dosages of 3.33, 1.00, and 0.67 g/g (top, middle, and bottom), respectively. For all hydrogels, self-healing was easily achievable after 15 seconds of contact. Self-healing sites remained intact under the influence of hydrogels' gravities. (B) A self-healed hydrogel (polymer dosage, 0.67 g/g) easily supported two stainless steel weights (totally 200 g). (C-D) Storage modulus (G') and loss modulus (G") as a function of alternatively varied strains [1% (180 s)—20%(120 s)—1% (180 s)—20%(120 s)—1% (180 s)]. C and D represent a fiber-based hydrogel (polymer dosage, 0.67 g/g) and a polyvinyl alcohol hydrogel (polymer dosage, 3.33 g/g), respectively
To further elucidate the dynamically crosslinked structures of hydrogels, viscoelastic characteristics were examined by strain sweeps, frequency sweeps, and time sweeps. The linear viscoelastic region was determined by strain sweeps (0.01%–10%) (Fig. 5A-B). In this region, storage modulus was constantly higher than loss modulus, indicating "solid-like" behaviors of polyvinyl alcohol hydrogels and fiber-based hydrogels. On the other hand, at a high strain (e.g., around 50%), noticeable reduction of storage modulus and loss modulus were identified, which is due to significant disruption of internal bonding interactions; in such circumstances, storage modulus was lower than loss modulus, representing "liquid-like" behaviors. For polyvinyl alcohol hydrogels, an increase of polymer content during hydrogel formation resulted in the increase of both storage modulus and loss modulus (Fig. 5C) due to enhanced crosslinking. As shown in Fig. 5C–E, the "solid-like" nature of all hydrogels was enhanced with the increase of angular frequency. Time sweep profiles (Fig. 5F–5G) show good structural stabilities of tested hydrogels.
Viscoelastic characteristics. (A–B) Dynamic strain sweeps of a polyvinyl alcohol hydrogel (polymer content: 5%) (left) and a fiber-based hydrogel (polymer dosage: 0.67 g/g) (right). (C–E) Storage modulus (G'), loss modulus (G"), and loss tangent (tan δ) as a function of angular frequency. (F–G) Dynamic time sweeps (time ramps) of different hydrogels. P-H-a and P-H-b represent polyvinyl alcohol hydrogels with polymer contents of 3.33% and 5%, respectively. F-H-a and F-H-b represent fiber-based hydrogels with polymer dosages of 1 g/g and 0.67 g/g, respectively