Eco-friendly nano-hybrid superabsorbent composite from hydroxyethyl cellulose and diatomite

Abstract

The simultaneous introduction of green polysaccharides and low-cost inorganic clay into superabsorbent (SAP) polymeric networks has witnessed growing interest for developing eco-friendly and low-cost SAPs since the last decade. In this study, a high performance nano-hybrid SAP composite (HEC-g-PAA/diatomite) was prepared through the graft polymerization of acrylic acid into hydroxyethyl cellulose backbone chains under aqueous conditions in the presence of diatomite clay as an inorganic nano-filler, N,Nʹ-methylenebisacrylamide as the cross-linker and ammonium persulfate as the initiator. FTIR, XRD, and SEM investigations confirmed the successful synthesis of this HEC-g-PAA/diatomite nano-hybrid SAP composite with high porosity on the surface. The composite prepared under optimized conditions presented a significantly enhanced thermal stability according to the TGA and DSC analysis and remarkably improved water-retention properties at various temperatures compared with the clay-free counterpart. Our SAP showed the eminent maximum swelling ratio of 1174.85 g g⁻¹ in distilled H₂O and 99.55 g g⁻¹ in 0.9 wt% NaCl salt solutions. In addition, factors influencing various amounts of water absorbency of the prepared SAP were extensively determined and discussed. Therefore, this SAP, with a high content of biodegradable and low-cost material, could be a good candidate for hygienic products, waste-water treatment, agriculture, and horticulture uses.

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1. Introduction

Since the introduction of the first superabsorbent polymer (SAP) by the U.S. Department of Agriculture in 1976, these materials have proven their robustness and versatility in numerous applications including agriculture, horticulture, hygienic products, cosmetics, waste water treatment, controlled release delivery, and so forth^1,2 due to their unique properties such as swell-ability, insolubility, hydrophilicity, stimuli-responsiveness to external parameters. Typically, SAPs are weakly crosslinking polymeric network materials that have the ability to absorb a huge volume of water or aqueous solutions from tens to thousands of times their own weight in a relatively short time, and retain a swollen state even under slight external pressure.³ Usually, most of these hydrogels are synthesized from synthetic polymers through the copolymerization of vinyl monomers such as acrylic acid (AA), acrylamide (AM), methacrylic acid (MAA) and their sodium or potassium salt derivatives, which are the most frequently used in SAP industrial production due to their hydrophilic characteristics, as well as other important properties such as high mechanical strength in the hydrated state, high sorption capacity, etc.^4,5 Thus, it was realized that a great volume of commercially available SAPs on the worldwide market are exceedingly dominated with SAPs manufactured from fully synthetic polymers.^6,7 Besides the unquestionable performance of these SAPs prepared from synthetic petroleum-based polymers, most of them are apparently expensive, poorly degradable, and unfriendly to the environment.

Moreover, about 90% of the produced SAPs were used as disposable products and disposed of in landfills or by incineration and poses serious environment impact.⁸ Given the increasing public concern toward human health, environmental protection, and natural resource sustainability, of late, there has been a great deal of interest in the use of natural, biodegradable, and non-toxic polymers, such as polysaccharides, in the replacement of these fully synthetic polymers of petrochemical origin in SAP production.⁹ However, regardless of the cost and environmental benefits of natural polysaccharide-based SAPs, their low swelling capacity and inadequate mechanical strength are still the major limitations to meet the minimum requirement for various applications in this field.^10,11 Therefore, this realization has recently led to intense research to improve the gel strength and stiffness of these natural polysaccharide-based SAPs along with the minimization of production cost.^12–14 Subsequently, several approaches have been introduced, among them we can mention the incorporation of plenty available, low-cost, annually renewable inorganic nanoparticles (for example, silica, carbon nanotubes, silsesquioxane, titania, and clay minerals), in the polymeric matrix network of polysaccharides-based SAPs through physical or chemical intercalation due to their relatively small particle size and intercalation properties.^15,16 Therefore, we hypothesized that the application of both natural polysaccharides and inorganic clay nanoparticles into the synthetic polymeric network through the synthesis of nano-hybrid organic–inorganic SAP composites could not only offer commercial and environmental advantages, but also enable the production of SAPs with markedly improved performance properties.

Hydroxyethyl cellulose (HEC) is a water-soluble polymer, and nonionic in nature, derived from cellulose through a series of chemical and physical processes. HEC is a white to light yellowish, odorless and tasteless powder, readily soluble in hot or cold water to form a viscous gel. It has excellent properties such as thickening, binding, emulsifying, suspending, dispersing, and stabilizing.¹⁷ HEC has the ability to be graft polymerized with hydrophilic vinyl monomers to derive new materials with improved properties due to the abundance of reactive –OH groups on the HEC chains.¹⁸

Diatomite (SiO₂·nH₂O), also known as diatomaceous earth, kieselgur, or Celite, is a kind of naturally occurring siliceous sedimentary rock formed over centuries by siliceous skeletons (so-called “frustule”) of aquatic single-cell photosynthetic microalgae and diatoms deposited on the bottom of lakes or present in marine environments.¹⁹ Diatomite consists of a wide variety of diatoms of shapes and sizes, typically from 0.4 nm to 200 μm, in a structure containing up to 80–90% voids.²⁰ Since the diatomite is mined from geological deposits, it may contains several impurities such as organic matters and metallic oxides viz., Al₂O₃, Fe₂O₃, CaO, MgO, K₂O, Na₂O, and P₂O₅, which may drive negative effects toward its application properties. Thus, a variety of purification approaches, such as thermal pre-calcination and HCl washing, were generally applied to improve the diatomite pore-size distribution by reduction of impurities from frustules.²¹ Diatomite has a unique combination of physical and chemical properties i.e., high permeability, high porosity, small particle size, large surface area, low thermal conductivity, and chemical inertness. This siliceous sedimentary rock has received a great amount of attention and has been used in various applications such as filtration media for various inorganic and organic chemicals, as absorbents, catalyst carriers, fillers, as well as in the pharmaceutical field. Therefore, owing to its hydrophilic property, large availability in many areas of the world, chemical stability, extremely low cost and non-toxicity, this fossil material could be an ideal added component to an SAP network to improve the swelling properties.²² Moreover, the diatomite silica surface presents high amounts of reactive silanol (Si-OH) groups that can chemically bond to the polymeric network through hybrid organic–inorganic SAP nanocomposite synthesis, thereby improving the mechanical properties of the obtained SAP.

Herein, we provide our effort to synthesize a high performance nano-hybrid organic–inorganic SAP composite through free radical graft polymerization of partially neutralized acrylic acid (AA) into HEC polymeric chains under aqueous conditions and in the presence of N,N′ methylenebisacrylamide (MBA) as a cross-linker, ammonium persulfate (APS) as the radical initiator, and micro-powder diatomite as an inorganic nano-filler. The determination of the optimized conditions to prepare SAP with superior swelling capacities and mechanical properties is still one of the major challenges in this field as a small change in one of the variables may greatly affect the corresponding SAP properties. In this study, the various factors (i.e., the content of diatomite clay, HEC, MBA, APS, and AA neutralization degree (DN)) influencing swelling affinity of the synthesized SAP were determined and discussed. Moreover, the water retention property and clay content effect on water retention capacity of the SAP composite was evaluated.

2. Experimental setup

2.1. Materials

Hydroxyethyl cellulose (practical grade, viscosity of 25 000 mPa s in distilled H₂O at 25 °C) was purchased from Hubei Xiangtai Cellulose Co., Ltd (China). The diatomite raw material obtained from Linjiang City Tianyuan Catalyst Co., Ltd (China) was purified before use through calcination and hot acid leaching. The acrylic acid (CP grade), and NaOH (AR grade) were both supplied by Tianjin Fuchen Chemical Reagents Factory (China). The ammonium persulfate (AR grade), N,N′ methylenebisacrylamide (CP grade), NaCl (AR grade), hydrochloric acid (AR grade), and tetrahydrofuran (AR grade) were all purchased from Sinopharm Chemical Reagent Co., Ltd (China). All other chemicals used were commercially available analytical grade reagents unless otherwise noted.

2.2. Diatomite purification

The raw diatomite was pre-treated before use according to our previously reported purification method.²³ Briefly, hot acid leaching using HCl and calcination purification methods were applied to purify the raw diatomite by removing impurities such as alumina, alkaline earth, and alkali metal compounds. Ab initio, the 60 g screened raw diatomite (45 μm sieve) was immersed into a beaker containing 300 mL of 5 M HCl solvent and leached under a thermostated oil bath heated at 100 °C and equipped with a magnetic stirrer rotating at 500 rpm for 1 h. After, the solid product was filtered and washed 4 times with 200 mL distilled water each time. Then, the filtrate was calcined in a muffle furnace (G WL-80, Hengtai HSBC Branch, China). The instrument was programmed such that it provided heating from 25 °C to 500 °C by 20 °C min⁻¹ rate. Once the desired temperature was reached, it was held there for 2 h. Thereafter, the sample was cooled down to room temperature, ground with a parallel ball mill grinder and then screened through a 45 μm aperture.

2.3. Synthesis of HEC-g-PAA/diatomite nano-hybrid SAP composite

A series of HEC-g-PAA/diatomite nano-hybrid SAP composites were prepared as follows: first, an appropriate amount of HEC (4–12 wt% relative to AA monomers) was weighed and dispersed into 100 mL distilled water in a 250 mL four-necked flask equipped with a stirrer, a reflux condenser, and a thermometer. The reactor was immersed in a thermostated oil bath preset to 50 °C for 30 min with constant stirring to completely dissolve the HEC. Second, a mixture of partially neutralized AA (AA weight = 15 g) and MBA (0.03–0.07 wt%), and a certain amount of diatomite (0–12 wt%) were gradually introduced into the four-necked flask. Thereafter, the temperature was raised to 70 °C and nitrogen (N₂) gas was bubbled into this solution to displace the dissolved oxygen. Third, a weighed amount of APS (0.2–0.7 wt%) dissolved into 5 mL of distilled water was fed to the mixture drop-wise to generate free radicals and initiate the reaction. The system was kept under stirring and allowed to react for 5 h at 70 °C to ensure the completion of the graft copolymerization. Finally, the formed product was sandwiched between two glass plates (∼10 × 10 cm) and dried in an oven at 90 °C to a constant weight. The dried product in a brittle state was mechanically ground, screened (particle size 40–80 mesh) and kept free from moisture and light.

2.4. Method of characterization

2.4.1. FTIR. The sample functional groups were characterized via a FTIR spectrometer (Spectrum-2000, PerkinElmer, USA). All samples were analyzed in the solid state as a powder mixed with potassium bromide and then pressed into a 1 mm pellet. The spectra of the samples were recorded in the range of 4000 to 500 cm⁻¹.

2.4.2. TGA and DSC. The thermal stability behaviors of the synthesized superabsorbent nanocomposites were investigated using Thermogravimetric Analysis (TGA) and Differential Scattering Calorimeter (DSC). The measurements were both performed on a NETZSCH STA 409 PC (Germany) analyzer, under a heating range of 30–600 °C and at a rate of 10 °C min⁻¹ in a nitrogen environment.

2.4.3. SEM. The surface morphologies of the sample fracture were obtained on a Quanta FEG 450 (USA) operated at 5 kV. All samples were freeze dried in high vacuum and sputter coated with platinum film before examination under the microscope.

2.4.4. XRD. The sample X-ray diffraction patterns in the scanning range of 8° ≤ 2θ ≤ 80° were obtained using an X'Pert PRO MRD PAnalytical (Netherlands) equipped with a Cu Ka radiation source, operating at 40 kV and 40 mA.

2.4.5. Specific surface area and porosity. The specific surface area of diatomite and the nano-hydrogel SAP composite prepared with 0, 6, and 12 wt% clay content was determined by BET (Brunauer, Emmett, Teller) and Langmuir measurements through the N₂ adsorption–desorption isotherm at 77 K. The sample's surface cumulative pore volume and average diameter size were determined using the Barrett–Joyner–Halenda (BJH) method.²⁴ All the measurements were obtained on a V-Sorb 2800P Specific Surface Area and Pore Size Analyzer (Beijing, China). Prior the measurement, the samples were heated at 393 K for 4 h under a vacuum to remove all adsorbed materials from the surface.

2.5. Equilibrium swelling ratio measurement

The composite swelling measurements were determined in both distilled water and 0.9 wt% NaCl salt solution at room temperature. An accurately weighed sample (W₀ = 0.1 ± 0.001 g) of the powdered composite with average particle sizes between 40 and 80 mesh were immersed in a beaker filled with 400 mL and 150 mL of distilled water and salt solution, respectively for 9 h. The fully swollen hydrogel was filtered through a 100-mesh screen to remove the surface water and weighed (W₁). The equilibrium swelling (ES) expressed in g (H₂O)/g (dried sample) was calculated according to the eqn (1):

2.6. Water retention measurement

The water retention behavior of the synthesized composite was determined according to the method used by Ma and co-workers²⁵ and Feng et.al.,²⁶ with slight modifications. 10 g of fully swollen SAPs in distilled water were gently placed in the bottom of 100 mL beakers and put into ovens set at different temperatures i.e., 45 °C, 60 °C and 100 °C. Water evaporation was monitored after every 2 h and 1 h for samples subjected to 45 °C and 60 °C as well as 100 °C, respectively. The composite retained water percentage (W_r (%)) was studied as a function of time by gravity and calculated as follows:where, W₀ stands for the mass of fully swollen SAP hydrogel initially, and W_i is the mass of SAP hydrogel after the water loss at a certain interval of time.

2.7. Gel content measurement

The synthesized nano-hydrogel SAP composite gel and sol content were measured according to the method published elsewhere.²⁷ Typically, the dry powdered sample was extracted with tetrahydrofuran (THF) by a Soxhlet extractor. From the beginning, the thimble filter was dried in the oven heated at 60 °C overnight and weighed (W₁). Then, the sample was loaded into the thimble and the weight of thimble filled with sample was recorded (W₂) before it was placed into the thimble holder. The sample was extracted by Soxhlet extractor equipped with reflux condenser and still pot containing THF under an oil bath maintained at 100 °C for 8 h. Thereafter, the filter with leftover sample was removed and subjected to a hot air oven at 90 °C overnight for drying to a constant weight. Finally, the mass of filter with sample after the extraction process was weighed (W₃) and the composite gel content percentage was determined based on the following eqn (3).

3. Results and discussions

3.1. Characterization results

3.1.1. FTIR analysis. Fig. 1 shows the spectra of diatomite, HEC, HEC-g-PAA, and HEC-g-PAA/diatomite samples. From the diatomite spectrum, Fig. 1a, the absorption bands at 3430 cm⁻¹ and 1633 cm⁻¹ were assigned to the –OH stretching and bending vibration of adsorbed water on the silicate surface. The bands at 1102 cm⁻¹ and 799 cm⁻¹ describe the asymmetric and symmetric stretching modes of the siloxane group (–Si–O–Si–) bonds, respectively.²³ From Fig. 1b, a very intense characteristic absorption band at 1065 cm⁻¹ was ascribed to the stretching vibrations of the C-OH function groups of HEC, and was strongly weakened in the FTIR spectrum of HEC-g-PAA (Fig. 1c) and HEC-g-PAA/diatomite (Fig. 1d), which can be related to the coupling reaction of –OH groups on the HEC backbone to neutralized AA.

Fig. 1 The FTIR spectra of (a) diatomite, (b) HEC, (c) HEC-g-PAA, (d) HEC-g-PAA/diatomite (6 wt%).

In addition, compared to the spectrum of HEC (Fig. 1b), the new absorption bands at 1729 cm⁻¹ (attributed to the stretching vibrations of C=O groups) and 1454 cm⁻¹ (ascribed to symmetric stretching vibrations of –COO⁻ groups) appeared in the spectra of HEC-g-PAA and HEC-g-PAA/diatomite. Moreover, the sharp absorption band at 1577 cm⁻¹ in Fig. 1b, shifted to broad bands at 1562 cm⁻¹ and 1570 cm⁻¹ (reflecting asymmetric stretching vibrations of –COO⁻ groups) in Fig. 1c and d.²⁸ Taken altogether, the absorption bands of the HEC spectra were closer to the spectra of the SAP hydrogel prepared with and without the diatomite. These evidences confirm that the partially neutralized AA was grafted into the HEC backbone. For the spectrum of the nano-hybrid SAP composite prepared with diatomite (Fig. 1d), the absorption bands at 1633 cm⁻¹ (bending vibration –OH) and 1102 cm⁻¹ (asymmetric stretching of –Si–O–Si–) in the spectrum of diatomite (Fig. 1a) were totally absent in the spectra of HEC-g-PAA/diatomite (Fig. 1d); also, the diatomite absorption band at 799 cm⁻¹ (symmetric stretching of –Si–O–Si–) was obviously weakened and shifted to 789 cm⁻¹ in the spectrum of HEC-g-PAA/diatomite. The absence of these peaks indicated that the diatomite participated in the reaction with its reactive silanol (Si-OH) groups.^29,30 Therefore, the FTIR analysis suggested the successful graft polymerization of PAA into HEC backbone chains and effective incorporation of diatomite nanoclay into the polymeric networks of the synthesized nano-hybrid SAP composite.

3.1.2. XRD analysis. The X-ray diffraction patterns of diatomite, HEC, HEC-g-PAA, and HEC-g-PAA/diatomite are displayed in Fig. 2. From Fig. 2b, the HEC XRD pattern shows main intense reflection peaks at 2θ = 11.58°, 2θ = 20.27°, and 2θ = 29.88°. After the graft polymerization of AA into HEC backbone chains, all of these peaks almost disappeared in the XRD patterns of HEC-g-PAA (Fig. 2c) and HEC-g-PAA/diatomite (Fig. 2d). However, from Fig. 2c and d, it is obviously clear that the patterns of HEC-g-PAA and HEC-g-PAA/diatomite SAP presented the same characteristics of the HEC pattern but with less intense crystallinity. This suggested that all of the HEC participated in the graft polymerization as the AA graft polymerization into the HEC backbone chains would destroy the original semi-crystallinity of HEC by leading to an amorphous pattern.^31,32

Fig. 2 The XRD patterns of (a) diatomite, (b) HEC, (c) HEC-g-PAA, (d) HEC-g-PAA/diatomite (6 wt%).

A similar observation was noticed while comparing the diatomite XRD pattern (Fig. 2a) and the HEC-g-PAA/diatomite pattern (Fig. 2d). It is clear that all diatomite minor and major crystalline characteristic intensities are no longer detectable from the HEC-g-PAA/diatomite XRD curve, which seems to be more amorphous in nature. This ultimately evidenced that the diatomite was not intercalated but fully exfoliated and dispersed into the HEC-g-PAA/diatomite nano-hybrid SAP composite polymeric network.^30,33

3.1.3. SEM analysis. The surface morphology micrographs of diatomite and the SAP composites prepared with various clay content i.e., SAP hydrogel containing 0, 12, and 6 wt% diatomite nanoclay, were taken and presented in Fig. 3a–d, respectively. The porous diatom and disc shaped frustule structures, and some other fine particles, are evidently observed as dispersed in the diatomite nanoclay (Fig. 3a–aʹ). In comparison with the dense and smooth morphological surface of the SAP hydrogel prepared without diatomite (Fig. 3b), the SAP hydrogel composite prepared with 6 wt% diatomite SEM micrograph (Fig. 3d–dʹ) shows an obvious rough and super porous surface. Moreover, several diatomite frustule porosities can be easily identified as distributed into the HEC-g-PAA/diatomite nano-hybrid SAP composite surface (Fig. 3d–dʹ), which can be an added advantage to enhance the water absorbance proficiency of the HEC-g-PAA/diatomite.

Fig. 3 SEM micrographs of (a–aʹ) diatomite, (b) HEC-g-PAA, (c) HEC-g-PAA/diatomite (12 wt% diatomite), and (d–dʹ) HEC-g-PAA/diatomite (6 wt% diatomite).

Interestingly, the morphology of the nano-hybrid SAP composite prepared with the highest amount of clay content, i.e., with 12 wt% diatomite demonstrates the roughest surface; however, with a limited and diminished number of pores on its surface (Fig. 3c) compared with the micrograph of the nano-hybrid SAP hydrogel composite synthesized under the optimized condition viz., with 6 wt% diatomite (Fig. 3d–dʹ). Actually, since the reactive groups of the diatomite are proposed to have chemically reacted with the rest of the components of the synthesized SAP, as the clay content rose from 0 wt% to 6 wt% (optimized value), the clay nanoparticles are suggested to be perfectly distributed into the polymeric networks of the HEC-g-PAA/diatomite nano-hybrid SAP composite; and thus, an improved surface roughness was observed. However, the relatively higher amount of inorganic clay in the SAP composite preparation, i.e. beyond the equilibrium value, should result in an increased percentage of the physically filled clay nanoparticles, which could affect the graft copolymerization reaction and negatively influence the surface structure roughness of the prepared SAP. Therefore, the incorporation of the proper amount of diatomite into the prepared SAP was beneficial to the improvement of the surface structure of the prepared nano-hybrid SAP composite, as evidenced by the SEM observation.³⁴ Typically, these fractured and porous structures found on surface morphology of SAPs are convenient and necessity to facilitate water penetration into the SAP polymeric network. Therefore, our synthesized SAP hydrogel composite surface morphology investigations are accordingly in good agreement with the swelling ratio behaviors of SAP prepared with or without the clay.

3.1.4. TGA and DSC analysis. The thermal stability behaviors of the synthesized nano-hybrid SAP composite was studied with TGA and the results were presented in Fig. 4a. Apart from the earlier weight loss of 11.5% up to 197 °C for the TGA curve of HEC, which was due to the evaporation of absorbed and bounded water, HEC thermal degradation occurred in one stage in the temperature range from 197 °C to 597 °C with 76% weight loss attributed to the decomposition of HEC backbone. Whereas, both the TGA curves of HEC-g-PAA and HEC-g-PAA/diatomite exhibited three thermal decomposition stages: the initial weight loss about 12.57% and 10.23% observed at 236 °C and 252 °C, respectively, was attributed to the moisture loss in the sample. The second decomposition stage occurred in the range of 236–408 °C for HEC-g-PAA and 252–412 °C for HEC-g-PAA/diatomite, with the weight loss around 28% and 24%, respectively, which are attributed to the degradation of the crosslinking network of the SAP composite. The third breakdown stage was observed at 408–597 °C for HEC-g-PAA and 412–597 °C, with a weight loss of 55.92% and 52.05%, respectively, which are assigned to the SAP polymeric chain decomposition. Expectedly, the diatomite nanoclay thermogram presented a much higher thermal stability up to 600 °C with only less than 6% of its weight lost. This clearly demonstrates the significantly improved thermal stability of HEC-g-PAA/diatomite with a delayed weight loss compared with HEC-g-PAA due to the incorporation of diatomite nanoclay, which can act as a heat barrier material into the polymeric network of the HEC-g-PAA/diatomite.

Fig. 4 (a) TGA curves of diatomite, HEC-g-PAA/diatomite (6 wt% diatomite), HEC-g-PAA and HEC, (b) DSC curves of HEC-g-PAA/diatomite (6 wt% diatomite), and HEC-g-PAA.

Fig. 4b illustrates the DSC curves of HEC-g-PAA/diatomite and HEC-g-PAA. The initial peak at 85 °C and 96 °C for the DSC curves of HEC-g-PAA and HEC-g-PAA/diatomite, respectively, corresponds to the removal of water content from the sample. It was obviously observed that the additional of diatomite clay reduces the intensity of the peak temperature for the endothermic peak. Consequently, in comparison with the sharp endothermic peaks of the HEC-g-PAA DSC curve, the peaks of HEC-g-PAA/diatomite become broader and weakened. Therefore, this shift suggested that there was a reduction in crystalline size and the presence of crystal imperfections due to the improved compatibility between the SAP polymeric network and the diatomite nanoclay, as also proved by the composite XRD analysis.

3.1.5. Specific surface area and porosity analysis. Nitrogen adsorption/desorption is considered as a sophisticated technique to study the specific surface area of the particles and investigate the presence of pores down to the meso- and micro-pore ranges.³⁵ From Table 1, it is obviously clear that the diatomite showed the largest BET specific surface area of 24.604291 m² g⁻¹ with a huge BJH adsorption cumulative pore volume of 0.087259 cm³ g⁻¹ compared to the other samples tested, and the pores identified in this type of clay through this test were characterized as meso-pores since their diameter size ranged from 2–50 nm.²⁴ In this test, the SAP composite prepared without diatomite (HEC-g-PAA) showed a slightly larger BET surface area of 0.142309 m² g⁻¹, but with a smaller total pore volume (0.000342 cm³ g⁻¹) and average diameter size (7.805122 nm) compared with the nano-hybrid SAP synthesized with 6 and 12 wt% diatomite. The nano-hybrid SAP composite prepared under the optimized conditions (i.e. HEC-g-PAA/diatomite (6 wt%)) exhibited a moderate BET surface area of 0.137989 m² g⁻¹ but with a cumulative pore volume (0.000442 cm³ g⁻¹) and average pore diameter (9.741815 nm) that were higher than the SAP prepared without any diatomite. This fact also suggested that the incorporation of a certain amount of diatomite clay into the SAP composite can improve its surface porosity.

Table 1 Specific surface area and porosity results

Sample code	Surface area		BJH adsorption
	BET (m^2 g^−1)	Langmuir (m^2 g^−1)	Cumulative pore volume (cm^3 g^−1)	Average pore diameter (nm)
Diatomite	24.604291	33.074209	0.087259	17.337891
HEC-g-PAA	0.142309	0.416479	0.000342	7.805122
HEC-g-PAA/diatomite (6 wt%)	0.137989	0.288014	0.000442	9.741815
HEC-g-PAA/diatomite (12 wt%)	0.121238	0.222280	0.000759	20.651212

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On the other hand, the SAP nano-hybrid composite prepared with the highest amount of clay (i.e. with 12 wt% diatomite) was found to possess the smallest BET surface area of 0.121238 m² g⁻¹; however, with an improved BNJ adsorption cumulative pore volume (0.000759 cm³ g⁻¹) and remarkably, the highest average pore diameter of 20.651212 nm. One possible way to explain this phenomena is that the highest pore size observed in the HEC-g-PAA/diatomite (12 wt%) sample, which is closer to the determined pore size of the diatomite nanoclay, could be related to the increased amount of physically intercalated clay into the HEC-g-PAA/diatomite (12 wt%). Thus, the clay surface porosity could be easily detected instead. In overall, the specific surface and porosity study for the synthesized products indicated that the incorporation of the proper amount of clay into the SAP preparation could be beneficial to surface porosity improvement.

3.2. The composite gel content results

Swelling of SAP can be influenced by the gel and sol content in composites. The prepared nano-hybrid SAP composite gel content was investigated and the results are summarized in Table 2. It was noticed that the gel content of the SAP composite prepared with 0 wt%, 6 wt% and 12 wt% diatomite was 95.79%, 94.40%, and 94.57%, respectively. The results of this experiment revealed that difference in the gel content percentage of the SAP prepared with, or without, diatomite was not significant and thus, the incorporation of a certain amount of diatomite nanoclay did not show any remarkable influence on the gel content of the obtained nano-hybrid SAP composite. Additionally, the fact that the nano-hybrid SAP composite synthesized with 6 and 12 wt% diatomite was higher than 94% indicated that the sol content in the composite and the effects of homopolymers on the monomer reactivity ratio could be low and ignored.²⁷

Table 2 Composite gel content results

Sample code	Clay content	Swelling ratio in distilled water	Gel content
HEC-g-PAA	0 wt%	602.75 g g^−1	95.79%
HEC-g-PAA/diatomite	6 wt%	1174.85 g g^−1	94.40%
HEC-g-PAA/diatomite	12 wt%	369.35 g g^−1	94.57%

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3.3. Factor contents effect on water absorbency of the HEC-g-PAA/diatomite nano-hybrid SAP composite

3.3.1. Diatomite content effect. The diatomite content effect on the swelling ratio of the prepared SAP composite in distilled water and 0.9 wt% NaCl salt solutions were investigated and the results presented in Fig. 5. This study was done by preparing a series of SAP with various amounts of diatomite content in the feed over a range of 0 to 12 wt% while keeping the other variables constant (AA = 15 g, HEC = 8 wt%, MBA = 0.05 wt%, APS = 0.5 wt%, reaction temperature 70 °C, neutralization degree = 70%). From Fig. 5, it was observed that the SAP water absorbance capacity increased from 0 to 6 wt% (which was the optimized condition) in both distilled water and salt solutions. Thereafter, a drastic decrease of SAP water absorbency was noticed when the amount of diatomite increased in the composite polymeric network feed. This can be explained by the availability of highly reactive –OH groups in diatomite binding water molecules, thereby improving the SAP water absorbance capacity before equilibrium. Actually, the diatomite silanol groups can also chemically participate in the reaction by generating additional network points to enhance the SAP composite crosslinking density.²⁹ Therefore, the high amount of diatomite in the feed i.e., beyond the equilibrium, may cause a hindrance to the nano-hybrid SAP composite polymeric network elasticity, which may strictly reduce the water absorption efficiency of the HEC-g-PAA/diatomite nano-hybrid SAP composite due the high cross-linking density generated by diatomite. Moreover, the high amount of diatomite in the feed above the equilibrium value may interrupt the graft copolymerization of AA into HEC chains and could result in the formation of SAP with a low molecular weight copolymer and a high amount of physically filled diatomite nanoclay content, thus resulting in reduced water absorbency.³³ The present experimental results are also in good agreement with our previous study.²³ Therefore, a suitable amount of inorganic clay is needed to prepare an ideal SAP with both improved swelling and mechanical properties.

Fig. 5 Diatomite content effect on the water absorbency. (Data given as the mean ± SD (n = 3).) AA = 15 g, HEC = 8 wt%, MBA = 0.05 wt%, APS = 0.5 wt%, reaction temperature 70 °C, neutralization degree = 70%.

3.3.2. HEC content effect. Fig. 6a demonstrates the HEC content effect on the water absorbency of the HEC-g-PAA/diatomite nano-hybrid SAP composite in distilled water and 0.9 NaCl salt solutions. In general, the influence from the other factors (diatomite, MBA, APS, and DN) studied in this paper was greater than the extent of influence by the amount of HEC polysaccharide. One possible way to explain this is that the HEC used in our study had a high viscosity of 25 000 mPa s in distilled H₂O at 25 °C, which implies it possessed a high molecular weight and a great number of hydroxyl groups on its backbone chains. Thus, even a little concentration of HEC in the reaction system was able to accommodate a huge number of partially neutralized AA monomers through graft polymerization. However, in the HEC-g-PAA/diatomite nano-hybrid SAP composite, a slight increment of water absorbance was observed in Fig. 6a when the HEC amount was raised from 4 to 8 wt% in the reaction system, and was possibly a result from the increase of readily available –OH groups in HEC interacting with AA monomers through graft polymerization. Therefore, more HEC increased before reaching the equilibrium (from 4 to 8 wt%), and the amount of non-grafted AA monomers that may undergo homopolymerization was reduced in the reaction system, and this can effectively contribute to increased water absorbency. In contrast, the water absorbency decrease observed after the equilibrium could be attributed to the reaction aqueous medium apparently becoming too viscous to allow uniform distribution of other reactants in the reaction system as the amount of HEC increased. Consequently, both grafting efficiency and the molecular weight of the grafted copolymer chains decreased, and this cannot contribute to the water absorbency of the resulting SAP.³⁶ Thus, an appropriate amount of polysaccharide content must be extensively optimized to control the reaction medium viscosity and enhance the grafting efficiency to prepare a SAP with high molecular weight and amended equilibrium swelling.

Fig. 6 HEC (a), APS (b), MBA (c) and AA neutralization degree (d) content effect on the water absorbency. (Data given as the mean ± SD (n = 3).) AA = 15 g, reaction temperature 70 °C.

3.3.3. Cross-linker (MBA) content effect. The cross-linker content effect on water absorbency of HEC-g-PAA/diatomite SAP in deionized water and 0.9 wt% NaCl salt solutions was studied by varying the MBA amount from 0.03 to 0.07 wt%. According to Fig. 6b, the maximum swelling ratio of 1174.85 g g⁻¹ in distilled H₂O and 99.55 g g⁻¹ in 0.9 wt% NaCl salt solutions was achieved when the SAP was prepared with 0.05 wt% MBA in the feed. Then, before and after the optimized condition, a gradual increase and decrease of SAP water absorbency was observed, respectively, in distilled water and 0.9 wt% NaCl salt solutions. Particularly, the cross-linker is a very important factor in hydrogel formation as it can create the networks and loosely cross-link the adjacent copolymers formed during the reaction. This permits the gel to swell without dissolving. Consequently, the obvious increase of the composite water absorbency observed before the optimized condition in Fig. 6b should be assigned to the improvement of cross-linking density of the HEC-g-PAA/diatomite nano-hybrid SAP composite, thereby reducing the dissolvable materials in the hydrogel polymeric network. Whereas, the tremendous decrease obtained after the 0.05 wt% MBA was likely due to the decreased space between the polymeric networks induced by high crosslinking density of the HEC-g-PAA/diatomite nano-hybrid SAP composite. According to the results of this experiment, it was noticed that the relatively tiny alteration in cross-linker content in the feed while preparing the SAP exhibited a significantly huge effect on the resulting SAP water absorbance capacity. Therefore, the degree of cross-linking needs to be carefully balanced to simultaneously prepare a SAP with adequate absorption capacity and ideal mechanical properties based on the desired end-use of the synthesized SAP.

3.3.4. Initiator (APS) content effect. The initiator content influence on water absorbency of the HEC-g-PAA/diatomite nano-hybrid SAP composite was investigated by varying the APS concentration in the range of 0.2 to 0.7 wt% and the results were collected in Fig. 6c. It was evidently seen that the composite swelling ratio was considerably increased after raising the APS amount in the feed from 0.2 up to 0.5 wt% in distilled water and 0.9 wt% NaCl salt solutions; thereafter, it started to decrease gradually as the initiator concentration increased. This phenomenon is possibly due to the following explanation: before the equilibrium is reached, the number of free radicals available to initiate copolymerization in the reaction matrix, and the reaction velocity, increases to contribute to the effective formation of high molecular weight copolymers with enhanced water absorption capacities.³⁷ However, the high reaction rate caused by the overabundant APS concentration in the reaction system may induce the fast termination of the growing molecular chins, and result in the formation of a hydrogel with low molecular weights and small polymer network spaces. Consequently, swelling would reduce.

3.3.5. Acrylic acid neutralization degree effect. The ratio of available hydrophilic groups such as carboxylic group (–COOH) and carboxyl group (–COO⁻) in the SAP polymeric network could have a great influence on the hydrogel affinity toward water absorbance. Fortunately, the ratio of those hydrophilic groups can be adjusted by neutralizing the vinyl monomers with inexpensive bases such a NaOH or KOH.³⁸ In this study, the AA monomer was partially pre-neutralized with NaOH solution at various degrees (i.e., 60, 65, 70, and 75%) as shown in Fig. 6d to assess the neutralization degree effect on water absorbency of the HEC-g-PAA/diatomite nano-hybrid SAP composite. It can be clearly seen in Fig. 6d that the composite water absorbency in both distilled water and 0.9 wt% NaCl salt solutions increased with increasing AA neutralization degree at a certain range until the equilibrium swelling of 1174.85 g g⁻¹ in distilled H₂O and 99.55 g g⁻¹ in 0.9 wt% NaCl salt solutions was reached at the AA neutralization degree of 70%; thereafter, the water absorbency curve downturned. Briefly, while the AA is being neutralized by sodium hydroxide, the AA carboxylic groups change to carboxylate groups surrounded by mobile Na⁺ in the SAP polymeric 3D network, which in turn, promotes an electrostatic repulsion and increased osmotic pressure favoring enhanced water absorbance. Thus, a significant improvement in hydrophilicity of the HEC-g-PAA/diatomite nano-hybrid SAP composite is observed with increases in the AA neutralization degree within a certain range.³⁹ On the other hand, the decreased water absorbance tendency of the composite above the optimized condition is related to the overabundance of sodium ions that reduces repulsion by screening the negative charges of carboxyl groups between the polymer network of the HEC-g-PAA/diatomite nano-hybrid SAP composite, and is not worthy for water absorbency due the decreased osmotic pressure in the system.

3.4. Water retention property results

It is crucial to study the water retention behavior of the SAP in view of practical applications.³³ In this work, the SAP prepared with various dosages of diatomite nanoclay (i.e., 0, 6, and 12 wt%) were applied to assess the composite water retention capacity under various temperatures of 45 °C, 60 °C, and 100 °C to evaluate the clay content effect on the water retention of the resulting SAP composite. Obviously, it is remarkable (Fig. 7a–c) that the nano-hybrid SAP composite prepared under the optimized conditions (with 6 wt% diatomite) showed an outstanding water retention capacity at 45 °C and 60 °C, compared to the SAP prepared with 0 and 12 wt% of clay content (Fig. 7a and b).

Fig. 7 Water retention behavior of SAP prepared with 0, 6, and 12 wt% of diatomite at (a) 45 °C, (b) 60 °C, and (c) 100 °C. (Data given as the mean (n = 3).)

Surprisingly, the nano-hybrid SAP composites prepared with 6 and 12 wt% diatomite showed almost the same water retention trend at 100 °C, but their water retention capacity was significantly higher than the water retention behavior of SAP composite prepared without diatomite (Fig. 7c). Meanwhile, the HEC-g-PAA/diatomite nano-hybrid SAP composite prepared under the optimized conditions exhibited excellent water retention behaviors as even only 10 g of fully swollen SAP can still retain more than 25% of the absorbed water when subjected at 45 °C for 16 h (Fig. 7a), which was far higher than ever. The composite can also retain water up to 10 and 8 h, under 60 °C and 100 °C, respectively, as shown in Fig. 7b and c. Typically, given that the SAP prepared with the proper amount of diatomite showed the highest heat resistance compared to the SAP synthesized without diatomite, as evidenced by TGA observations, this could be given as one of the reasons behind the water retention improvement upon the incorporation of diatomite filler. Moreover, according to the study conducted by Li et al.,⁴⁰ the swollen water in SAP could be classified into bound water, half-bound water, and free water. Subsequently, compared to bound water and half-bound water, the free water in a SAP has a high mobility and it can be easily lost. In the same context, the number of hydrophilic groups such as COOH and COONa are assumed to be higher in the HEC-g-PAA/diatomite nano-hybrid SAP compared to HEC-g-PAA SAP composite. This could result in a higher percentage of bound water and half-bound water in HEC-g-PAA/diatomite nano-hybrid SAP compared to free water. In this way, the incorporation of a certain amount of diatomite in the SAP polymeric network can significantly improve its water retention capacity.

3.5. Mechanism of HEC-g-PAA/diatomite nano-hybrid SAP composite formation

The proposed formation mechanism of the HEC-g-PAA/diatomite nano-hybrid SAP composite is outlined in Scheme 1. As can be seen, the ammonium persulfate used as an initiator decomposes under heating to generate persulfate ion-radicals. These radicals cleave hydrogen from the polar –OH groups of HEC and give rise to the free radicals on the HEC backbone to initiate graft polymerization. Consequently, polymer chain propagation starts when the AA monomer molecules, which are in close proximity to reaction sites, became acceptors for HEC radicals resulting in chain initiation; thereafter, they themselves became free radical donors to neighboring molecules.^41,42

Scheme 1 Possible reaction mechanism that can occur during the synthesis of biodegradable organic–inorganic HEC-g-PAA/diatomite nano-hybrid SAP composite through graft copolymerization.

In this process, the end vinyl groups of cross-linker (MBA) may participate in the reaction system by creating networks between the growing polymer chains during graft polymerization resulting in a cross-linked structure. Since diatomite was present in the system, it is presumed to have chemically entered into the reaction due to its highly reactive silanol (Si-OH) groups either through reaction with neutralized PAA carboxylate groups on the growing chains to generate additional polymeric networks points,³⁰ or by preventing the polymer chains from growing through a chain transfer mechanism.⁸ In this way, the HEC-g-PAA/diatomite nano-hybrid SAP composite was formed. The proposed mechanism in Scheme 1 is in good agreement with our synthesized product from FTIR, XRD, SEM, TGA, and DSC study analyses, as well as from the determined properties of the composite.

4. Conclusion

In summary, the organic–inorganic nano-hybrid SAP composite (HEC-g-PAA/diatomite) was successfully prepared through the introduction of diatomite nanoclay into HEC and neutralized AA copolymers obtained from free-radical graft copolymerization under aqueous conditions as evidenced by FTIR, XRD, SEM, TGA, and DSC studies. The HEC-g-PAA/diatomite nano-hybrid SAP composite prepared under the optimized conditions present a high swelling degree of 1174.85 g g⁻¹ in distilled H₂O and 99.55 g g⁻¹ in a 0.9 wt% NaCl salt solution. Moreover, the composite exhibited significantly enhanced and beneficial properties such as high thermal stability and excellent water retention capabilities compared with the clay-free counterpart or the nano-hybrid SAP composite prepared with a high amount of clay content. The synthesized SAP composite could be applied in various applications such as in agriculture, waste water treatment, hygienic products, and so on. All concentrations of the components in the HEC-g-PAA/diatomite nano-hybrid SAP composite synthesis reaction need to be sequentially balanced in order to prepare an SAP with a superior degree of swelling degree and improved mechanical strengths. Thus, the authors anticipate that the simultaneous introduction of both fully degradable polysaccharides and natural inexpensive inorganic clay into the SAP carries the promise to offer several advantages in SAP production, such as the synthesis of eco-friendly, low cost SAP with a high degree of swelling degree and improved mechanical properties as well as the reduction of the application of common partially-, or sometimes non-,degradable and costly synthetic polymers.

Acknowledgements

This research was supported by the grants from the Self-determined Research of Central China Normal University (Fundamental Research Funds for the Central Universities CCNU15A02062), China Spark Program (Ministry of Science and Technology, 2013GA740073), and Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry, 2010-1174).

References

1. Y. Wang, W. Wang, X. Shi and A. Wang, Polym. Bull., 2013, 70, 1181–1193.
2. B. H. Cipriano, S. J. Banik, R. Sharma, D. Rumore, W. Hwang, R. M. Briber and S. R. Raghavan, Macromolecules, 2014, 47, 4445–4452.
3. M. Yadav and K. Y. Rhee, Carbohydr. Polym., 2012, 90, 165–173.
4. M. J. Zohuriaan-Mehr and K. Kabiri, Iran. Polym. J., 2008, 17, 451.
5. F. H. Rodrigues, C. Spagnol, A. G. Pereira, A. F. Martins, A. R. Fajardo, A. F. Rubira and E. C. Muniz, J. Appl. Polym. Sci., 2014, 131, 39725.1–39725.13.
6. K. Kabiri, H. Omidian, M. J. Zohuriaan-Mehr and S. Doroudiani, Polym. Compos., 2011, 32, 277–289.
7. K. Sharma, V. Kumar, B. S. Kaith, V. Kumar, S. Som, S. Kalia and H. C. Swart, Polym. Degrad. Stab., 2015, 111, 20–31.
8. Y. Bao, J. Ma and N. Li, Carbohydr. Polym., 2011, 84, 76–82.
9. R. L. de Oliveira, H. da Silva Barud, D. T. B. de Salvi, G. F. Perotti, S. J. L. Ribeiro and V. R. L. Constantino, Ind. Crops Prod., 2015, 69, 415–423.
10. H. Li, J. Yang, X. Hu, J. Liang, Y. Fan and X. Zhang, J. Biomed. Mater. Res., Part A, 2011, 98, 31–39.
11. A. Pourjavadi, S. Barzegar and G. Carbohyd, Polymers, 2006, 66, 386–395.
12. H. Ferfera-Harrar, N. Aiouaz, N. Dairi and A. S. Hadj-Hamou, J. Appl. Polym. Sci., 2014, 131 DOI:10.1002/app.39747.
13. G. Ma, F. Ran, Q. Yang, E. Feng and Z. Lei, RSC Adv., 2015, 5, 53819–53828.
14. Y. Mu, D. Du, R. Yang and Z. Xu, Mater. Lett., 2015, 142, 94–96.
15. K. Haraguchi, Polym. J., 2011, 43, 223–241.
16. M. S. Islam, M. S. Rahaman and J. H. Yeum, Carbohydr. Polym., 2015, 115, 69–77.
17. M. Li, X. Wang, Y. Yang, Z. Chang, Y. Wu and R. Holze, J. Membr. Sci., 2015, 476, 112–118.
18. W. Wang, J. Wang, Y. Kang and A. Wang, Composites, Part B, 2011, 42, 809–818.
19. K. W. Jung, D. Jang and K. H. Ahn, Int. J. Miner. Process., 2014, 131, 7–11.
20. Y. Al-Degs, M. Khraisheh and M. Tutunji, Water Res., 2001, 35, 3724–3728.
21. I. Ruggiero, M. Terracciano, N. Martucci, L. De Stefano, N. Migliaccio, R. Tatè, I. Rendina, P. Arcari, A. Lamberti and I. Rea, Nanoscale Res. Lett., 2014, 9, 1–7.
22. J. F. Mukerabigwi, Q. Wang, X. Ma, M. Liu, S. Lei, H. Wei, X. Huang and Y. Cao, J. Coat. Technol. Res., 2015, 12, 1085–1094.
23. J. F. Mukerabigwi, S. Lei, H. Wang, S. Luo, X. Ma, J. Qin, X. Huang and Y. Cao, RSC Adv., 2015, 5, 83732–83742.
24. H. Wu, P. Badrinarayanan and M. R. Kessler, J. Am. Ceram. Soc., 2012, 95, 3643–3650.
25. G. Ma, Q. Yang, F. Ran, Z. Dong and Z. Lei, Appl. Clay Sci., 2015, 118, 21–28.
26. E. Feng, G. Ma, Y. Wu, H. Wang and Z. Lei, Carbohydr. Polym., 2014, 111, 463–468.
27. J. Q. Jiang and S. Zhao, Iran. Polym. J., 2014, 23, 405–414.
28. W. Wang and A. Wang, Carbohydr. Polym., 2010, 82, 83–91.
29. J. Wang, W. Wang, Y. Zheng and A. Wang, J. Polym. Res., 2011, 18, 401–408.
30. P. S. Liu, L. Li, N. L. Zhou, J. Zhang, S. H. Wei and J. Shen, J. Appl. Polym. Sci., 2007, 104, 2341–2349.
31. M. Irani, H. Ismail and Z. Ahmad, Polym. Test., 2013, 32, 502–512.
32. S. K. Sharma, B. Kaith, V. Kumar, S. Kalia, V. Kumar and H. Swart, Geoderma, 2014, 232, 45–55.
33. Y. Zhang, Q. Gu, J. Yin, Z. Wang and P. He, Adv. Polym. Technol., 2014, 33 DOI:10.1002/adv.21400.
34. Y. Wang, W. Wang, X. Shi and A. A. Wang, J. Appl. Polym. Sci., 2013, 130, 161–167.
35. E. Orozco-Guareño, F. Santiago-Gutiérrez, J. L. Morán-Quiroz, S. L. Hernandez-Olmos, V. Soto, W. De la Cruz, R. Manríquez and S. Gomez-Salazar, J. Colloid Interface Sci., 2010, 349, 583–593.
36. W. Wang and A. Wang, Carbohydr. Polym., 2010, 80, 1028–1036.
37. C. Nakason, T. Wohmang, A. Kaesaman and S. Kiatkamjornwong, Carbohydr. Polym., 2010, 81, 348–357.
38. Z. Liu, Y. Miao, Z. Wang and G. Yin, Carbohydr. Polym., 2009, 77, 131–135.
39. Q. Wei, Iran. Polym. J., 2014, 23, 637–643.
40. A. Li, A. Wang and J. Chen, J. Appl. Polym. Sci., 2004, 92, 1596–1603.
41. A. Pourjavadi, S. Barzegar and F. Zeidabadi, React. Funct. Polym., 2007, 67, 644–654.
42. K. Sharma, B. Kaith, V. Kumar, V. Kumar, S. Som, S. Kalia and H. Swart, RSC Adv., 2013, 3, 25830–25839.

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