**3. Characterization**

#### **3.1 Analytical methods**

The potential applicability [23] of smart polymers are gauged based on their chemical structure, the extent of chemical and physical crosslinking, crosslink density, mechanical properties, degrees of swelling (hydrophilicity), release characteristics, hydrophobicity, surface morphology, biodegradability, biocompatibility, glasstransition temperature, thermal stability, photo-stability, bio-resorbability,

#### **Table 2.**

*Typical crosslinkers used for hydrogel synthesis.*

interaction with biological fluids, environmental sensitivity, dielectric properties, toxicity, the toxicity of the degraded products, etc. For instance, the nature of functional groups, crystallization deformation of polymers, biodegradation, moisture uptake properties, nature of interactions between components are evaluated using Fourier Transform Infrared Analysis (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy. The modification after polymerization such as chemical composition, grain size, the extent of crosslinking, pore size, pore volume are evaluated using Atomic Force Microscopy (AFM) or Scanning Electron Microscopy (SEM) and X-ray diffraction analysis. The oxidative thermal degradation, glass transition temperature, lifetime prediction, melting point, etc. are assessed through Thermal Analysis (TGA and DSC). The mechanical characteristics such as tensile strength and elastic moduli and strain are evaluated using a tensile-compressive tester.

#### **3.2 Swelling measurements**

The swelling ability of hydrogel is a significant characteristic for field application. The absorption capacity of the hydrogel can be evaluated [23] gravimetrically at successive time intervals using tea-bag, sieves, centrifugal, volumetric, microwave, gravimetric, NMR, DSC methods based on the required precision. The extent of swelling (DS) was measured using Eq. (1) by performing triplicate measurements.

$$\text{DS} = \frac{\mathbf{W}\_t - \mathbf{W}\_0}{\mathbf{W}\_0} \tag{1}$$

*Smart Polymer Hydrogels as Matrices for the Controlled Release Applications… DOI: http://dx.doi.org/10.5772/intechopen.102904*

#### **Table 3.**

*The representative natural pre polymers used for hydrogel synthesis.*

The weight of dried (W0) and swollen polymers Wð Þ*<sup>t</sup>* at a particular time are measured gravimetrically.

#### **3.3 Absorption under load (AUL)**

The extent of water absorption under load is determined by performing AUL of hydrogel samples [24] using the Eq. (2). The AUL test will display the absorption capacity of smart polymer hydrogel under stressed conditions (load) and ionic strength.

$$AUL\left(\mathbf{g}/\mathbf{g}\right) = \frac{W\_2 - W\_1}{W\_1} \tag{2}$$

*W*<sup>1</sup> and *W*<sup>2</sup> represents weight of dry and swollen hydrogel respectively.

#### **3.4 Fertilizer uptake and release studies**

The quantum of fertilizer absorption and release characteristics of smart hydrogels are measured based on Eq. (3). The percentage release of fertilizer from the loaded

hydrogels are measured gravimetrically [12]. This procedure was followed for every two-day interval to ensure maximum fertilizer release. The percentage of urea/potash release was calculated [12] using Eq. (3).

$$\text{Percentage of fertilizer released} = \frac{(\Delta \mathbf{W})\mathbf{n} \times [100 - (\mathbf{n} - \mathbf{1}) \times \mathbf{2}]/2 + \sum\_{i=1}^{\mathbf{n}-1} (\Delta \mathbf{W})\mathbf{i}}{\mathbf{W}\mathbf{o}} \tag{3}$$

The amount of fertilizer released from the hydrogel in 2 and ith ml are represented by W0 and (ΔW)i respectively. The number of nutrient releases at different time intervals for the single experiment is denoted by the term "n".

#### **3.5 Transport kinetics**

The rate of nutrient absorption by the plants depends on various parameters such as plant age, nature of fertilizers, and the concentration of fertilizers. However, the micronutrients are supplied as chelates or complexes (using synthetic complexing agents such as salicylic, lactic, formic, citric, succinic, propionic, ascorbic, tartaric and gluconic acids and their sodium, potassium and ammonium salts. Amino acids such as glutamine cysteine, glycine, and lignosulfonates can also be used as complexing agents [25]. The water, nutrients uptake and release behavior of hydrogels are regulated by their chemical constituents namely sulfonic acid, amide, hydroxyl, amine, carboxylic acid, carboxylate groups, etc.

The uptake and release mechanisms are clearly understood by analyzing the transport kinetics. The movement of solvent and solute either into or out of hydrogel is also regulated by the shrinking and swelling of hydrogels. The second-order kinetic model [Eq. (4)] was used to explain the swelling of hydrogel [23].

$$\frac{d\mathbf{M}}{dt} = \mathbf{k}\_s \left(\mathbf{M}\_\infty \ -\mathbf{M}\right)^2\tag{4}$$

where, M: uptake at time t, M∞: uptake at equilibrium condition, and ks: kinetic rate constant.

The swelling rate (*SR*), and swellability (S*t*) andð*St*þΔ*<sup>t</sup>*) at time '*t*' and '*t*+Δ*t*' respectively are measured using the Eq. (5).

$$\mathcal{S}\_{\mathcal{R}} = \frac{\mathcal{S}\_{t + \Delta t} - \mathcal{S}\_t}{\Delta t} \tag{5}$$

#### **3.6 Diffusion**

A random molecular process causes the movements of solvent or solute molecules from one part to another part of hydrogels. Further, this movement is also influenced by temperature, pressure, solute size and viscosity. Generally, in hydrogel water molecules diffusion is connected to the extent of polymer-solvent interactions. Based on hydrogel relaxation rate, the diffusion is categorized as non-Fickian and Fickian [26], and the power-law Eq. (6) is used to evaluate the penetration characteristics of solvent into the hydrogel [26].

*Smart Polymer Hydrogels as Matrices for the Controlled Release Applications… DOI: http://dx.doi.org/10.5772/intechopen.102904*

$$\mathbf{M}\_{\mathbf{t}} = \mathbf{k} \mathbf{t}^{\mathbf{n}} \tag{6}$$

The value of diffusion exponent (n) is ranged from 0.5 to 1 and the parameter k represents the rate constant.

#### **3.7 Fickian and non-Fickian**

The diffusion mechanism [26] of solution in the hydrogel during network collapse or swelling was analyzed using Fick's law. Fickian diffusion was noticed when the operating temperature of the system was greater than the glass transition temperature (*T*g) of the hydrogel. Fickian type diffusion was also predicted if the solvent diffusion rate (Rdiff) was slower than hydrogel relaxation rate (Rrelax) i.e., (Rdiff < <Rrelax). Besides, the diffusion distance and the square root of time were found to have a direct relationship [Eq. (7)]

$$\mathbf{M}\_{\mathbf{t}} = \mathbf{k} \mathbf{t}^{1/2} \tag{7}$$

The value of 'n' provides the diffusion characteristics, for instance, if n = 0.5 in Eq. (6) Fickian diffusion is followed, and the 'n' values lie between 1 and 0.5 non-Fickian (anomalous) transport mechanism is followed. Further, non-Fickian model was noticed below glass transition of the hydrogel.

## **4. Application of hydrogel in agriculture field**

The substantial foodstuff production requires an adequate amount of primary and secondary nutrients [6] along with water during cultivation. To achieve expected yield farmers used to feed an additional amount of fertilizers than the required quantity [6] during each amendment. However, 90% of the applied fertilizers are going as waste due to different climatic conditions and the application method [6]. An excess dose of fertilizers leads to economic losses, toxicity problems and effects on aquatic organisms [6] which cause uninvited effects such as water and soil pollution. Hence, there is a necessity to adopt the method, which facilitates the controlled release of fertilizers without affecting efficacy. An execution of controlled release using polymer based matrix is being used for a long time [6]. The loaded fertilizers have been released through chemical cleavage of the polymer-active agents or by depolymerization reaction (originated other factors) [6]. However, the implementation of a controlled release technique for the particular application depends on the factors namely release rate, cost, effectiveness and properties of synthetic fertilizers.

#### **4.1 Advantages**

In agricultural field, smart hydrogels have discharged numerous applications [27] and the notable merits are minimum use of fertilizers and water through controlled a release mechanism. The list of noteworthy advantages of hydrogel amendment in the soil is displayed in **Figure 1**. However, hydrogels used for the controlled release of fertilizers and water in the field must have

**Figure 1.** *Advantages of hydrogel in field.*


The use of smart hydrogels in agricultural sector have attracted great attention as water management material in soil and matrices for the controlled release of primary and secondary fertilizers. The release rates of hydrogels [23, 27] are depends on the functional groups that are present in the polymer, functionality of crosslinker, pH, temperature, ionic strength of the medium, etc. Besides, the incorporation of natural pre-polymers in synthetic polymer hydrogel will bring down the operation cost, since they are readily available at a low cost and highly biodegradable. Nevertheless, natural polymer incorporation may induce a few limitations such as the lack of solubility of monomers in aqueous

#### *Smart Polymer Hydrogels as Matrices for the Controlled Release Applications… DOI: http://dx.doi.org/10.5772/intechopen.102904*

and non-aqueous solvents during hydrogel synthesis [16]. This characteristic behavior will result in excess utilization of pre-polymers to enhance agricultural yield.

The additional expected physicochemical and mechanical properties from the synthesized hydrogel for field applications are good stability during swelling (without dissolving), photostability, ability to uptake and hold maximum water with good swelling rate, particle size, maximum fertilizer uptake, porosity, odorless, neutral pH, colorless, low residual monomer content, non-toxicity, biodegradability without yielding toxic reside, and low cost [28, 29]. However, it should be remembered that the synthesis of hydrogel with all these features is difficult to achieve. However, some of its features namely porosity, stimuli responsiveness (pH and temperature), residual monomer content and swellability [30–32] are fine-tunable. The extent of hydrogel swellability, which are amended in the soil can be fine-tuned based on the requirement by making modification in the functional groups such as �NH2, �COOH, �OH, �CONH2, �CONH� and –SO3H. Besides, osmotic pressure, movable counter ions and capillary effect have also influenced swelling and release phenomena [33]. During swelling, the process of water uptake by the hydrogel will follow multiple steps that include hydration of polar hydrophilic and hydrophobic groups leading to the formation of primary and secondary bound water respectively. Meanwhile, infinite dilution of the hydrogel network will be resisted by the formation of either chemical or physical cross-links. Hence, the water molecules that are entering into the network during the initial and equilibrium stages are known as total bound and bulk water/free water respectively. During swelling these water molecules shall occupy the gaps available between chains and the midpoint of pores. The quantum of water uptake by the hydrogel networks is influenced by various parameters such as temperature, pH, nature of interactions, etc. that exist between networks and water molecules [33]. The list of representative hydrogels that are used as water-retaining agents and matrices for the controlled release of nitrate, potash, phosphate fertilizers are presented in **Tables 4**–**7**.



**Table 4.**

*Hydrogel used as soil conditioner and water retention material in soil.*


#### **Table 5.**

*Representative Hydrogel used for the controlled release of nitrogen fertilizer.*


#### **Table 6.**

*Typical hydrogels used as matrices for the controlled release of potassium.*

*Smart Polymer Hydrogels as Matrices for the Controlled Release Applications… DOI: http://dx.doi.org/10.5772/intechopen.102904*


**Table 7.**

*Representative hydrogels used for the controlled release of phosphate fertilizer.*

#### **4.2 Effects of hydrogel amendment**

Smart hydrogel amendment in the soil during cultivation process will alter the hydraulic conductivity and pore size of soil to some extent due to water absorption [85, 86]. However, it will improve residual and saturated water content, which results in the reduction of subsequent water loss and infiltration due to percolation, this will facilitate aeration in soil due to expansion and contraction of hydrogel through absorption and evaporation [85]. The suitability of hydrogel for semi-arid and arid regions was due to the release of water and fertilizers with reference to environmental temperature, which results in increased survival [85] of plants. Besides, the hydrogel amendment has reduced the uptake of toxic metals and soil salinity by plants [87, 88].

#### **4.3 Safety aspect and environmental concern**

The practical applicability of hydrogel in field applications is dependent on safety, toxicity and eco-friendly degradability under soil conditions after its service and other environmental issues. Most of the hydrogels used in agricultural sector have stable service life (5–7 years), but their degradability is suspected. Hydrogels amended in the soil will experience stress from various factors such as microbes, light, pH, temperature, etc. The degradability of hydrogels depends on their structures and other environmental factors such as intensity of light, soil microbes, heat, pH, etc. The degradability of hydrogels could be attained by incorporating favorable functional groups such as ester, amide, urethane, anhydride, glycocidic (ether), urea, orthoester, carbonate, etc. in the backbone. The degradation sequence of polymers have predicted as anhydride > ester> orthoester> carbonate> urea>urethane> ether [89].

The monomers of hydrogels are known to be toxic and carcinogenic, but the polymer derived from the same monomers are proved to be non-toxic [18]. This characteristic behavior could be attributed to low boiling point and the low molecular weight of acrylic monomers and crosslinkers, which may effortlessly enter into the human body through skin absorption and inhalation [90, 91]. The studies have also recorded that these acrylic monomers imposed wide a range of health effects such as

skin and eye irritation, allergic action, asthma, nerves problem, internal organ toxicity and impacts on fertility [90, 91]. The contentious exposures of acrylates will yield acrylic acid [90, 91] in the human body during metabolic activity. However, the crosslinked hydrogels will not cause any harmful effects on living organisms due to their insolubility and non-volatile nature [90, 91].
