3.2.1. Preparation of NaCMC–HPMC hydrogel films
Hydrogel films were prepared by adding NaCMC and HPMC (3:1 wt. ratio) in 100 ml of water with citric acid (CA) as a crosslinker. In a typical experiment, to formulate a polymer of 2 wt.% concentration (NaCMC-HPMC at a ratio of 3:1), 0.5 g of HPMC
until the clear solution was observed. Then, the solution was allowed to cool at room temperature. The presence of HPMC is essential to encourage intermolecular crosslinking as well as to reduce the crosslinking between NaCMC macromolecules alone. An amount of 1.5 g NaCMC was slowly added into the above solution under vigorous magnetic stirring and was left at 900 rpm for 12 h to obtain a homogeneous mixture. Finally, various concentrations of crosslinking agent such as 5%, 10% and 20% was added into NaCMC-HPMC (w/w of 2 wt. % polymer concentrations) aqueous solutions. Air bubbles were removed completely in all the samples under vacuum and 100 ml of each sample was cast on a Petri dish of 18 cm diameter. All the samples were kept for 5 h at 80°C for CA crosslinking reaction. Neat NaCMC (2 wt.%) crosslinked with CA (5%) hydrogel films were also prepared using a similar procedure for comparison. Finally, the hydrogel films were stored in a vacuum desiccator for further analysis.
3.2.2. Preparation of zinc oxide nanoparticles
Zinc oxide nanoparticles (ZnO NPs) were prepared by precipitation method according to Akhtar et al. (2017) with slight modifications. Briefly, aqueous solutions of 0.2 M zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and 0.4 M potassium hydroxide (KOH) were initially prepared. The KOH solution from the burette was added dropwise to the beaker containing zinc nitrate solution with vigorous stirring at room temperature. The white product formed in the suspension upon addition of KOH was centrifuged at 5000 rpm for 10 min. The pellets were washed with water for three times followed by ethanol.
Finally, the white precipitate was calcined at 500°C for 3 h to obtain ZnO NPs.
3.2.3. Preparation of NaCMC–HPMC–ZnO nanocomposite hydrogel films
The nanocomposite hydrogel films were prepared from ZnO NPs, NaCMC, HPMC and citric acid. Initially, 0.5 g of HPMC was dissolved in 100 ml of water at 90°C with
continuous stirring to obtain a clear solution. The solution was left to cool at room temperature. The addition of HPMC promotes intermolecular crosslinking as well as intermolecular interactions. Then, 50 mg of ZnO NPs were added into the above solution and sonicated (33 kHz, 50 W) for 30 min to disperse the nanoparticles. An amount of 1.5 g of NaCMC was gradually added into ZnO-HPMC solution under vigorous magnetic stirring (900 rpm) for 8 h to obtain a homogeneous mixture. Finally, different concentrations of citric acid (CA) such as 5%, 10% and 20% (w/w of 2 wt.%
polymer concentrations) were added into the solution containing NaCMC-HPMC-ZnO.
All the samples were kept in a vacuum to remove air bubbles. A volume of 100 ml of each sample was cast into a Petri dish of 18 cm diameter and was kept at 80°C for 5 h for CA crosslinking reaction. The control nanocomposite films were also prepared using a similar procedure without the addition of CA. The nanocomposite films prepared were stored in a vacuum desiccator for characterizations.
In order to compare the influence of CA on ZnO NPs, 1:1, 1:2, 1:4 and 1:8 weight ratios of ZnO NPs and citric acid were mixed in 100 ml water without addition of polymers and left at room temperature for 24 h.
3.2.4. Preparation of copper oxide nanoflakes
Copper oxide (CuO) nanoflakes were synthesized using the precursors of copper chloride dihydrate (CuCl2.2H2O) and sodium hydroxide (NaOH) according to Anandan and Yang (2007) with minor modifications. Briefly, aqueous solutions of 0.5 M CuCl2.2H2O and 1 M NaOH were initially prepared. Then, the NaOH solution was added dropwise to the beaker containing CuCl2.2H2O solution with constant stirring of 500 rpm at room temperature. While stirring, the black precipitate formed upon addition of NaOH. The solution was centrifuged at 7000 rpm for 15 min. The black pellets were washed with ample amount of water and followed by ethanol to remove NaCl (by-
product) and other unreacted precursors completely. Finally, the black precipitate was dried at 80°C for 12 h to obtain CuO nanoflakes.
3.2.5. Preparation of NaCMC–HPMC–CuO nanocomposite hydrogel films
The cellulose-based nanocomposite hydrogel films were fabricated from CuO nanoflakes, HPMC, NaCMC and CA (chemical crosslinker). Initially, in order to enhance intermolecular crosslinking and intermolecular interactions, 0.5g HPMC was added into 100 ml of water under magnetic stirring (700 rpm) at 90°C. After a few minutes, the HPMC polymer was dissolved completely and the clear solution was left to attain room temperature. Then, 50 mg of CuO nanoflakes were added into the polymer solution and left for 30 min. The above solution was fed with an amount of 1.5 g NaCMC at 900 rpm for 8 h. Finally, citric acid to the extent of 5%, 10% and 20%
(w/w of total polymer) was added into the homogeneous mixture. Before pouring the samples into a Petri dish of 18 cm diameter, air bubbles in all the samples were carefully removed using a vacuum pump. All the Petri dishes were kept at 80°C for 5 h. The control films were made using a similar method without incorporation of citric acid.
The fabricated films were kept in a vacuum desiccator for further characterizations.
3.2.6. Preparation of NaCMC–HPMC–ZnO/GFSE nanocomposite hydrogel films The precursors of nanocomposite hydrogel films were NaCMC, HPMC, CA, ZnO NPs and GFSE. Initially, 0.5 g of HPMC polymer was slowly dissolved in 100 ml of water at 90°C and the mixture was continuously stirred at 700 rpm for an hour to obtain a clear solution. Then, 1.5 g of NaCMC was gradually added to the HPMC solution at 900 rpm and left for 8 h to get homogeneous mixture. An amount of 0.4 g of CA was incorporated into the mixture. After complete mixing of CA, 50 mg of ZnO NPs were supplemented to the above solution, which was left for 30 mins under vigorous stirring.
Finally, various amounts of liquid GFSE (0%, 0.25%, 0.5% and 1% v/v) was fed into
the solution containing NaCMC, HPMC, CA and ZnO. The composition of the control film was without addition of GFSE. Air bubbles were completely removed from all the samples using vacuum desiccator followed by solution casting into a Petri dish (18 cm diameter) which was kept at 80°C for CA crosslinking reaction. After complete drying, the films were peeled off from the Petri dish and preserved in a vacuum desiccator for further characterizations.