Abstract
Superabsorbent hydrogels are macromolecular networks able to absorb and retain large amounts of water solutions within their fine mesh-like structure. More importantly, they are capable of multiple swelling/shrinking transitions in response to specific environmental cues (e.g., pH, ionic strength, temperature, presence of given electrolytes), thus exhibiting a stimuli-sensitive behavior, which makes them appealing for the design of smart devices in a number of technological fields. In particular, in the last two decades, cellulose-based superabsorbent hydrogels have proven to be an environmentally friendly and cost-effective alternative to acrylamide-based products. This chapter reviews the relationship between the molecular structure of cellulose-based hydrogels and their physicochemical properties. First, the network formation through the use of different cellulose derivatives and chemical or physical crosslinking agents is presented. Successively, the smart swelling capability of the hydrogels as a function of composition and structure is thoroughly discussed. Finally, several approaches to the hydrogel characterization are reviewed, with focus on the assessment of key mechanical, thermal and morphological properties.
Keywords
- smart hydrogels
- cellulose
- characterization
1. Introduction
A superabsorbent hydrogel is defined as a three-dimensional (3D) matrix formed by hydrophilic polymers in linear or branched configuration and showing the ability to absorb large quantities of water or biological fluids (usually more than 100 grams of water per gram of dry polymer) [1].
The main property of superabsorbent hydrogels is the capability to preserve the stability of their network structure, even in the swollen state and in different media and environments. This feature is the result of the presence of crosslinking nodes [2], which can be induced through two main pathways, chemical and physical crosslinking. The former allows obtaining irreversible covalent bonds among the polymeric chains,
The method adopted for crosslinking, either chemical or physical, also influences some key network properties (
Recently, particular focus has been placed on the production of novel superabsorbent hydrogels based on natural polysaccharides such as cellulose, starch and chitosan [8, 9], due to the low cost, biodegradability, availability and renewability of these raw materials. Compared to synthetic polymers (
Being susceptible to degradation by microorganisms [10] and by chemical or physical stimuli [11], polysaccharide-based superabsorbent hydrogels are particularly suitable for use in soils as fully biodegradable systems for the controlled release of nutrients. However, although biodegradability permits to avoid the contamination of soils by chemicals, it may still represent a drawback in case of too early degradation,
The most abundant polysaccharide in nature is cellulose, which has been the subject of academic and industrial studies for many years [13, 14, 15]. Although plant cellulose requires several purification steps to eliminate or reduce contaminants (
In general, the strong hydrogen bonding (both intermolecular and intramolecular) among the hydroxyl groups along the cellulose backbone not only limits the water solubility, but also leads to the poor reactivity of cellulose. For this reason, great interest has been directed to the use of cellulose derivatives, also termed cellulosics, such as ethyl cellulose, propyl cellulose and carboxymethylcellulose (CMC) [16]. In particular, sodium carboxymethylcellulose (CMCNa) is one of the most important water-soluble derivatives currently used, produced by chemical reaction between cellulose and monochloroacetic acid (MCA) in the presence of sodium hydroxide. CMC is widely applied as an additive in a variety of industrial sectors. Examples of products where CMC is used are detergents, oil drilling muds and wall paper glues, while high purity CMC grades are found in pharmaceuticals, tooth paste, cosmetics, food,
With specific regard to agricultural applications and the need to control the hydrogel degradation in the soil, some studies reported the possibility to use binary systems based on two cellulose derivatives,
In this chapter, after a brief overview of the inherent features of cellulose and cellulosics, the synthesis and characterization of cellulose-based hydrogels are discussed, with specific regard to CMCNa/HEC hydrogels, which represent a successful example of ‘green’ and biocompatible superabsorbents. The focus of the chapter is on the modulation of the physicochemical properties of the hydrogels in relation to their intended use, via changes to the molecular structure.
2. Cellulose backbone properties
The chemical and physical properties of cellulose and cellulosics can be properly interpreted only after acquiring a deep knowledge of the cellulose molecule, in terms of chemical description, structure and morphology [21]. Generally, when approaching the study of macromolecules especially for the creation of crosslinked structures, three structural levels must be identified. The first one is the molecular level, which regards the single polymer chain and is characterized by the following structural parameters: chemical constitution, molecular mass and molecular mass distribution, reactive sites and intramolecular interactions. The second structural level to be identified is the supramolecular level, which accounts for the interactions (
As highlighted in the following, the molecular and supramolecular levels are those of primary interest for the understanding of the structure-related properties of cellulose and its derivatives.
2.1 Molecular structure
Cellulose macromolecules are linear and rigid homopolymers consisting of D-anhydroglucopyranose units (AGU), linked together by β-(1 → 4) glycosidic bonds formed between C-1 and C-4 positions of adjacent glucose moieties (Figure 1). The degree of polymerization (DP) of cellulose depends on both the cellulose source and the method of isolation/extraction. In general, plant cellulose in its native state has a DP higher than 10,000, which then drops down to 800–3000 following the extraction process [21].
The peculiarity of the glycosidic bond (Figure 1) is that the two AGU units joined together to form the repetitive unit of cellobiose are rotated by 180° with respect to each other [21]. The relative stiffness of the cellulose chain is partly attributable to the steric constraints of the β-glycosidic linkage, as well as to the chair conformation of the pyranose ring. With focus on the terminal groups at either side of the cellobiose unit, an aldehyde hydroxyl (OH) group with reducing activity is found in C-1 position (Figure 1, right), whereas an alcohol-borne, non-reducing OH group is in C-4 position (Figure 1, left). Furthermore, both AGU units have three OH groups located at C-2, C-3 and C-6 positions (one primary and two secondary groups), which are mainly responsible for the poor reactivity and solubility of cellulose. Cellulose is indeed insoluble in water (although the β-glycosidic linkage is susceptible to hydrolytic attack), as well as in common organic solvents. This is due to the formation of an extensive intra- and inter-molecular hydrogen-bonding network among the abundant hydroxyl groups on the cellulose backbone. In particular, intramolecular bonding is ascribable to two different interactions, namely the one between the OH group at C-3 and the oxygen of the pyranose ring (first described in the 1960s by Marchessault and Liang), and the one between the C-6 and the C-2 OH groups of neighboring AGUs (reported by Blackwell et al. in 1977 [22, 23]). Intermolecular hydrogen bonding also takes place via the interaction between C-3 and C-6 groups of adjacent cellulose chains. Intramolecular bonds are those that mainly contribute to the stiffness of the cellulose molecule, which results in the high viscosity of cellulose solutions and the strong tendency of cellulose to crystallize and form fibrillar strands.
2.2 Functionalization
The hydroxyl groups located on the cellulose backbone (Figure 1) can be exploited for chemical modification to introduce various functionalities, with the aim of improving the solubility and the reactivity of cellulose. As aforementioned, a number of cellulose derivatives, which display a wide range of solubility in water and water solutions (in presence of alcohol or strong acids or bases), are commercially available in various industrial fields and used for a number of applications, including coatings, laminations, optical films, absorbents, additives in building materials and also in pharmaceutical, food and cosmetic products [24, 25, 26]. Among these derivatives, the most important ones are esters and ethers (
With specific regard to carboxymethyl cellulose (CMC), which is widely used for the synthesis of superabsorbent hydrogels, the average degree of substitution (DS) and degree of polymerization (DP) of available CMC may vary considerably, although DS is generally between 0.5 and 1.5 [28]. In order to control the distribution of the functional groups in CMC, several synthetic pathways have been developed. In this regard, one pathway, developed by Fink et al. [29], includes stepwise etherification using aqueous NaOH solutions at low concentrations. Carboxymethylation is, thereby, primarily achieved in the non-crystalline regions of the cellulose structure [30].
In addition to esterification and etherification, periodate oxidation of the glycol group of glucopyranoside results in the formation of two aldehyde groups (dialdehyde cellulose) [31, 32], which are useful for introducing a variety of substituents such as carboxylic acid [33], hydroxyls [34], or imines [35, 36].
2.3 Degradation features
2.3.1 Effect of enzymes
It is well known that cellulose undergoes enzymatic degradation by specific enzymes, like cellulase and β-glucosidase. Direct physical contact between the enzyme and the surface of the cellulose molecules is a preliminary requirement to enzymatic hydrolysis [37, 38, 39, 40]. Since cellulose is an insoluble and structurally complex substrate, this contact can be achieved only by diffusion of the enzymes into the cellulose structure [41]. Therefore, the ability of cellulolytic microorganisms (
With focus on cellulosics, the nature, concentration and distribution of substituted groups clearly influence the process of enzymatic degradation, since the chemical substitution may impair the recognition of the substrate by the enzymes [42]. However, it is worth pointing out that the effect of chemical modification on enzymatic resistance is not simply proportional to the number of substituted units. Indeed, neighboring units can also be involved in the enzymatic attack. This is due to the fact that the recognition of polymeric substrates by an enzyme generally implicates the simultaneous involvement of multiple moieties in the polymer chain [32, 45]. In an interesting study by Kim et al. [32], it has been shown that the chemical oxidation of cellulose not only makes the oxidized glucopyranosides enzyme-resistant (towards both cellulase and β-glucosidase), but also renders the adjacent unmodified glucopyranoside moieties inaccessible to the enzyme. Therefore, the enzyme-resistant portion of cellulose is much greater than the degree of oxidation when the latter is low, while becoming closer to the degree of oxidation when this is increased. The partial oxidation of cellulose can thus be a useful strategy for the design of cellulose-based materials, such as hydrogels, with predetermined degradation rates [32].
2.3.2 Effect of temperature
It is known that native cellulose exists as a mixture of two crystalline forms I
Thermal degradation of cellulose is quite a complex phenomenon, involving several chemical reactions whose understanding still appears controversial [49]. At high temperatures (approximately above 250°C), thermal degradation occurs as a first-order reaction. Pyrolysis is thought to take place rapidly via transglycosylation reactions, which lead to the formation of anhydro-sugars or ‘anhydrocellulose’ [50].
Furthermore, while at high temperature the effect of oxygen on thermal degradation is practically negligible, at lower temperatures oxygen plays a predominant role, so that oxidative degradation in air proceeds faster than pyrolysis in nitrogen. The oxidative degradation process involves the production of free-radical initiators, which then interact with oxygen to lead to autocatalytic oxidative reactions, characteristic of the thermal oxidation of polymers [50].
Along with furanic compounds and gases (
2.3.3 Effect of alkaline environment
Cellulose degradation strongly depends on the alkaline condition of environment. At temperatures <170°C the glycosidic linkages between the glucose units are stable in alkaline conditions [53]. However, a dramatic decrease of molecular weight is observed when cellulose is boiled in presence of a diluted sodium hydroxide solution, even with the careful exclusion of oxygen.
The principal mechanisms of degradation in alkaline media have been described by several investigators [53, 54], and they include endwise degradation (or peeling) at temperatures <170°C and alkaline hydrolysis of glycosidic bonds at higher temperatures. Endwise degradation is due to a number of isomerizations of the reducing end group of the cellulose molecule, which result in the migration of the carbonyl group along the carbon chain. The ketose or aldose end groups that are produced are then subjected to β-elimination [55]. If β-elimination occurs at the C-4 position, one monomer unit is released from the cellulose molecule, and the next glucose end group can take part in the reaction. In this way, the glucose units are gradually released from the macromolecule, resulting in a depolymerization process commonly known as
When the temperature is >170°C, random alkaline scission of glycosidic linkages occurs, resulting in considerable weight loss and marked decrease in degree of polymerization. It is reported that the reaction does not depend on the presence of molecular oxygen and is followed by peeling from any new reducing end group produced by the scission process, thereby resulting in much greater weight losses than alkaline degradation at lower temperatures [57]. Although alkaline scission is normally only associated with alkaline degradation at higher temperatures, it has been also observed in the alkaline degradation of amorphous hydrocellulose at temperatures <100°C.
As mentioned for enzymatic degradation, the cellulose supramolecular structure plays a key role in the degradation process [58]. In general, a high supramolecular order of the polymer chains prevents or delays degradation [58, 59], with amorphous regions being much more sensitive to degradation than crystalline ones. It has been reported [53] that the rate-limiting step for slower chemical attack depends on the rate of mid-chain scission or the reaction of ‘inaccessible’ end groups. Peeling and chemical stopping are inhibited in fibrous hydrocellulose, which shows an ordered physical structure, and the majority of partially degraded molecules terminate with inaccessible reducing end groups,
3. Synthesis of cellulose-based hydrogels
Cellulosics are interesting precursors for the synthesis of superabsorbent hydrogels, due to the low cost, large availability, biocompatibility and biodegradability of cellulose, along with the responsiveness of some cellulosics (
In general, cellulose hydrogels can be stabilized via either physical (reversible) or chemical (irreversible) bonding of the cellulose chains, starting from dilute aqueous solutions of single or composite cellulosic precursors [15]. Furthermore, natural and/or synthetic derived polymers might be combined with cellulose to obtain composite hydrogels with controlled properties [61, 62]. The number of crosslinking sites per unit volume of the network, called crosslinking degree, is a parameter that affects multiple properties of the hydrogel, including diffusive, mechanical and degradation properties. The control of the crosslinking degree, through adjustment of the synthesis protocol, thus represents a powerful tool to produce tunable hydrogels for the specific application at hand.
Commonly used physical hydrogels are those prepared from aqueous solutions of methylcellulose (MC) and/or hydroxypropyl methylcellulose (HPMC) [63]. The gelation process involves hydrophobic associations among the macromolecules via the methoxy group. While at low temperatures the polymer chains are well hydrated, at higher temperatures they start losing their bound water, so that polymer-polymer hydrophobic associations take place to form a 3D network. The sol-gel transition temperature clearly depends on the degree of substitution (DS) of the cellulose ethers and the presence of salts. A higher DS implies a more hydrophobic character of the cellulose chains, thus lowering the sol-gel transition temperature. Similarly, the addition of salts reduces the hydration level of the macromolecules. Both the DS and the salt concentration can be adjusted to obtain cellulose-based aqueous formulations able to gel at 37°C, for potential biomedical applications [64, 65, 66]. Several studies have addressed the use of low viscosity, injectable solutions, which may directly crosslink
Unlike physical crosslinking, the formation of covalent bonding among the polymer chains creates a stable 3D network with given stiffness. Both physical treatments (
Radiation crosslinking of cellulose, using electron beams or gamma radiation, appears also suitable for the production of biocompatible hydrogels, especially for biomedical applications. Although irradiation can lead to scission of the polymer backbone, as demonstrated also for cellulosics [70], mild radiation conditions have been successfully adopted for the crosslinking and simultaneous sterilization of cellulose-based devices [71, 72, 73].
Finally, it is also important to highlight that cellulose backbone can be specifically functionalized before crosslinking, with the double aim of producing cellulose-based hydrogels free of potentially toxic contaminants as well as providing a higher control of the crosslinking process. For instance, several cellulose derivatives have been added with acrylate moieties to enable the photo- or redox- crosslinking of aqueous solutions [74, 75]. Silylated HPMC, which crosslinks in solution by means of pH-driven condensation reactions, is another example of modified cellulose, proposed for the
4. Smart swelling capability
4.1 Swelling ratio
The amount of water retained in the hydrogel network is a parameter of crucial importance, since it affects all the properties of the material that are relevant for the selected application (
In Eq. (1),
The volume swelling ratio (
where
In general, the hydrogel absorption capacity depends on both internal parameters (related to the structure of the polymer network) and external parameters (related to the solution bathing the hydrogel). Superabsorbent hydrogels, in particular, are those that display intrinsic large sorption capabilities (with
4.2 Swelling mechanism
Superabsorbent hydrogels can be obtained by crosslinking hydrophilic polyelectrolyte species. Indeed, the presence of ions or fixed charges on the polymer network greatly improves its swelling behavior [80, 81].
Several factors governing the sorption mechanism can be identified. First of all, the polymer hydrophilicity promotes the polymer-solvent mixing,
where
In case of polyelectrolyte networks, two additional beneficial contributions to swelling occur: (a) an osmotic mechanism called ‘Donnan effect’, which is proportional to the number of ionic fixed charges on the hydrogel network and induces the penetration of water into the network to dilute its high charge concentration; and (b) the electrostatic repulsion between charges of the same sign on the polymer backbone, which tends to expand the network, thereby promoting the swelling.
The Donnan effect (also known as the Gibbs-Donnan effect) is related to the behavior of free charged particles in the presence of a semipermeable membrane separating two different solutions. Being the membrane semipermeable, only some charged species are able to pass through it in order to reach the equilibrium between the two solutions. A typical Donnan-type mechanism takes place when a 3D polyelectrolyte network is placed in contact with a water solution, since electrical charges are tethered on the polymer backbone, which thus acts as a semipermeable membrane. The equilibrium of the whole system (composed by the swelling solution and the polymer network itself) is attainable only if a passage of water is established, going from the external solution to the polymer network, thus diluting the concentration of the charges inside the network.
The polyelectrolyte nature of CMCNa explains why this cellulose derivative is widely used for the synthesis of superabsorbent cellulose-based hydrogels, especially in conjunction with HEC [13, 15, 18, 19, 20]. The simultaneous presence of HEC has been shown to be fundamental to achieve intermolecular (rather than intramolecular) crosslinking reactions, thus enabling the network stabilization [15]. The hydrogels crosslinked in presence of CMCNa clearly exhibit a much higher sorption capacity if compared with those crosslinked with HEC only, due to the Donnan-type swelling mechanism. The swelling capability of CMCNa/HEC hydrogels has been further increased by the use of difunctional molecules (
4.2.1 Effect of temperature
The hydrogel state can be considered as a peculiar solution composed of water and hydrophilic polymer chains. Because of the presence of crosslinks, which impede the polymer dissolution, the hydrogel solution is characterized by an elastic (rather than viscous) behavior [79].
The polymer-solvent interaction can be described by the thermodynamic theory of polymer solutions. In 1953, Flory [81] showed that the free energy change associated with the mixing process between the solvent and the polymer network can be calculated as follows:
where
where χa, χb, etc. are function of the temperature.
This means that the polymer-solvent interaction parameter, which in turn affects the polymer hydrophilicity and the hydrogel swelling, depends on temperature (and on polymer concentration). The effect of the temperature on χ1,2 allows to design thermosensitive hydrogels. Most polymers increase their water solubility as the temperature increases (
4.2.2 Effect of ionic strength (constant pH)
Polyelectrolyte networks (
4.2.3 Effect of pH (constant ionic strength)
In general, the degree of ionization
The dissociation of the carboxylic groups fixed on the cellulose chains in CMCNa/HEC hydrogels is strongly affected by the pH of the external solution [18, 80]. A reduction in the number of dissociated carboxylic acid groups in the polymer network is evident at low pH. This mechanism reduces the swelling of the material, in accordance with the reduction of the polyelectrolyte property of the network. At very low pH values, the majority of the carboxylic acid groups are in a non-dissociated state, and the hydrogel seems to be composed of non-polyelectrolyte chains. On the other hand, when the pH of the swelling solution increases, there is a growth of the number of dissociated carboxylic group with a consequent increment in swelling.
5. Mechanical properties
The viscoelastic nature of polymers offers several analytical methods to investigate their mechanical behavior. In addition to ‘traditional’ mechanical tests (
In general, the gelation of polymer solutions can be investigated by rheological and dynamic mechanical analyses. As the crosslinking reaction takes place, the viscosity of the polymer solution starts to increase progressively, due to the increase of the average molecular weight, until the gel state, defined as the one where no flow occurs (
Since hydrogels show a rubber-like behavior, the theory on the entropic elasticity of rubbers, described by Flory [81], can be exploited to estimate the degree of crosslinking of a hydrogel by measuring its macroscopic mechanical properties. Assuming that the deformation of the polymer chains is affine and that the volume of the polymer network does not change upon uniaxial deformation, Flory derived the following relationship between the uniaxial stress and the uniaxial deformation:
where σ is the stress,
If the polymer network is swollen isotropically in a solvent [81], and the crosslinking reaction occurs in the presence of the solvent [86], as for most hydrogels, the modulus
where
Therefore, the mechanical characterization of hydrogels via uniaxial elongation or compression tests allows estimating not only the gel stiffness and strength, but also the corresponding elastically effective crosslink density. However, it is worth recalling that, due to viscoelasticity, which implies a time-dependent material’s response, the rate of gel loading is particularly important in the determination of its mechanical properties. Commonly used strain rates for hydrogel testing are in the range 1–50 μm/s [87, 88, 89, 90].
Although derived for uniaxial deformation, Eq. (8) has a general validity. DMA, which directly measures the modulus
For selected types of hydrogels, especially those used in tissue engineering and cell culture applications, the determination of the local mechanical properties of the material, on the nano/submicron scale, is also of great interest for fully understanding the multiscale material behavior, as well as the cell response to the substrate stiffness or elasticity [90, 92]. Such local measurements can be performed via atomic force microscopy (AFM) and nanoindentation techniques [93]. Apart from its use for morphological surface analysis, in recent years AFM has emerged as a well-established method to map the nano-mechanical properties of elastic and viscoelastic materials [94], and to probe the elasticity of cells and biological tissues [92]. AFM can also be adapted to perform nanoindentation tests [94]. Several soft hydrated materials, such as hydrogels, have been characterized via AFM and/or nanoindentation and their local elasticity has been correlated to the bulk one [90, 92, 93]. Notably, the stiffness of single cellulose nanofibres in bacterial cellulose hydrogels has been very recently determined via AFM [95].
As a final consideration on the evaluation of mechanical properties, it is worth noting that Eqs. (7) and (8) are referred to the analysis of non-macroporous hydrogels. The mechanical behavior of porous gels, which are briefly dealt with in a later section, is clearly affected not only by the precursor polymer(s) and the crosslink density, but also by porosity itself. The determination of the crosslink density of porous hydrogels from mechanical data should thus take into proper account the role of porosity. As for the assessment of the local mechanical properties, so far, AFM has been mostly used for probing of non-macroporous hydrogels, being the analysis of porous substrates particularly challenging. However, the mechanical characterization of porous hydroxypropyl cellulose methacrylate (HPC-MA) hydrogels by means of AFM has been successfully reported in recent literature [96]. The obtained elastic modulus, together with data from microstructural analysis, has also allowed the mechanical modeling of individual pores and the bulk scaffold [96]. These findings pave the way to a more extensive use of the AFM technique in the near future for the mechanical analysis of porous hydrogels.
6. Thermal stability
Thermal stability of hydrogels is commonly investigated by means of thermogravimetric analysis (TGA), a technique that allows identifying the temperatures at which sample weight losses occur, due to heating-induced transitions, such as evaporation of volatile substances (
6.1 TGA analysis of hydrogel precursors: CMCNa and HEC
As discussed in the previous sections, CMCNa and HEC are widely employed for the synthesis of superabsorbent hydrogels [87, 97, 98, 99, 100]. In particular, CMCNa is the polyelectrolyte species that provides the hydrogels with enhanced swelling capability and sensitivity to environmental stimuli (
When analyzing the thermal behavior of cellulose derivatives, it is worth recalling that, due to the major role of oxygen in cellulose degradation, especially at low temperatures (as discussed previously), the choice between air or nitrogen flow as an atmosphere in which performing the TGA analysis is fundamental and should always be specified.
For analyses in nitrogen flow, typical TGA/DTG curves for CMCNa and HEC (commonly reported in the range 25–500°C) basically show a two-step weight loss, with removal of moisture (up to 10% weight) in the temperature range 40–100°C, and rapid thermal degradation, mostly due to decarboxylation [101], in the range 230–300°C (about 40–50% weight) (Figure 2). Weight loss then slowly proceeds at higher temperatures due to further degradation mechanisms, such as those described in Section 2 (
6.2 TGA analysis of CMCNa/HEC hydrogels
As reported in the literature, CMCNa/HEC hydrogels may display two or more steps of thermal degradation, depending on the crosslinking agent(s) used [97, 98]. Additional steps of weight loss in the TGA curve, compared to the curves of cellulose precursors, may be indeed due to the degradation of the crosslinker molecules incorporated in the gel network. As an example, CMCNa/HEC (3/1) hydrogels, crosslinked by either fumaric or malic acid [98], have been recently shown to display a three-step degradation process, including: a first stage, occurring at low temperatures (up to 100°C), due to water/moisture evaporation; a second stage, in the range 200–270°C, related to decomposition of the crosslinker; a final stage, between 270 and 400°C, where thermal decomposition of cellulose backbone occurs. It is worth noting that, in this study, the hydrogels were found to possess a higher thermal stability than the corresponding cellulose precursors. In particular, increased hydrogel degradation temperatures were detected up to 298 and 317°C, when using increasing concentrations of malic and fumaric acid, respectively. This suggested the formation of highly stable cellulose networks upon crosslinking.
Clearly, such a finding is strictly related to the given crosslinking agent/reaction being investigated. For instance, in an independent investigation CMCNa/HEC hydrogels crosslinked by divinylsulfone (DVS) have been shown to be less thermally stable than native polymers [97]. The maximum degradation temperature of cellulose was indeed shifted from about 285 to 276°C, upon DVS crosslinking of a 5/1 CMCNa/HEC mixture. However, in that study the effect of different crosslinker concentrations on the hydrogel stability was not taken into account.
7. Hydrogel morphology
In general, the analysis of hydrogel morphology (
In the following, the analysis of the gel structure is briefly discussed, with particular reference to the evaluation of pores at the nano-, micro- and macroscale. Where appropriate, specific examples related to the analysis of cellulose-based hydrogels are provided.
7.1 Nanoscale: hydrogel mesh size
Hydrogel networks show a fine mesh-like structure, where the free space among crosslinked chains can host the diffusion of water and other molecules of suitable size, smaller than or comparable to the mean hydrogel mesh size (Figure 3). According to the Canal-Peppas theory for isotropically swollen hydrogels [86], the average mesh size ξ, also referred to as correlation length, can be calculated from the following:
In Eq. (9),
In spite of the fact that most real hydrogels show a broad range of mesh sizes, various studies show that the calculation of the average mesh size according to Eq. (9), starting from theoretical swelling or mechanical models for
With particular reference to cellulose-based hydrogels, it is worth mentioning thermoporosimetry as another interesting and powerful technique to measure the mesh size distribution, rather than the average mesh size, of polymer networks. The method, based on differential scanning calorimetry (DSC) and first described by Kunh et al. [115], relies on the application of the Gibbs-Thomson equation, which quantifies the freezing point depression of a liquid (
7.2 Micro- and macroscale: hydrogel porosity
For an environmentally sensitive hydrogel, the swelling rate,
The investigation of hydrogel porosity is primarily performed, with different levels of resolution, via optical methods, such as scanning electron microscopy (SEM) and CLSM, usually followed by software-based image analysis [122, 129]. As discussed above, particular caution is needed in sample preparation to avoid artifacts, especially in cases where preliminary sample dehydration is required (
8. Conclusions
Cellulose-based superabsorbent hydrogels are currently explored for a number of technological applications, which range from the traditional use of hydrogels as water absorbents in different contexts (
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