2.1 Chemical reagents
Polyvinyl alcohol (molecular weight 89,000-98,000, 99+% hydrolyzed) was from purchased from Sigma Aldrich Co., Ltd. Sodium alginate was purchased from Sigma Aldrich Co., Ltd. All chemical reagents were used as received, without further purification.
2.2 Methods
2.2.1 Development of sodium alginate and poly vinyl alcohol-based hydrogel composite
Polyvinyl alcohol and sodium alginate-based hydrogel composite by casted onto petri dish. The procedure was conducted based on guideline of previous work reported by Kim et al [16]. Briefly, polyvinyl alcohol (10%w/v) was dissolved in deionized water and gently stirred for 1 h at 80°C. In parallel, sodium alginate (2%w/v) was dissolved in deionized water and gently stirred for 1 h at room temperature. To prepare hydrogel, polyvinyl alcohol solution was poured into sodium alginate solution with a mixed ratio of 5:5, 6:4 and 7:3 v/v. The mixture was casted onto a petri dish, followed by freezing at -20°C for 18 h ang thawing at room temperature for 6 h, for 3, 5 and 7 consecutive cycles. After that the composite hydrogel was freeze dried and stored in desiccator to prevent moisture adsorption.
2.2.2 Swelling behavior
The swelling characteristics of the samples were investigated using a gravimetric technique. Sample was cut into 1cm x 1cm squares with the same weight (Wd) and immersed in deionized water (DI) water for 6 h. Samples were removed from the solution, dried using a filter paper to remove excess water and weighed (Ww). Three samples were investigated following equation, and data were reported as statistical average and standard deviation.
Swelling ratio (g/g) =\(\frac{\text{W} \text{w} - \text{W} \text{d}}{\text{W}\text{d}}\times 100.\)
where Ww is the weight of the swollen hydrogel at submersion time and Wd is the initial weight of the dry hydrogel.
2.2.3 Degradation behavior
The sample was cut into 1 x 1 cm2. Then, it was immersed into 10 mL of deionized water at the time intervals of 1, 3, 7, 14, 21 and 28 days. Sample were removed and dried at 80°C at each time point. The percentage degradation was calculated using following equation. The data was reported as a statistical average and standard deviation.
Degradation (%) = \(\frac{W0-Wt}{Wt}\) x 100
where W0 and Wt are the weights of the sample before and after degradation, respectively.
2.3 Instruments
2.3.1 Fourier transform infrared (FTIR)
The chemical structure was determined using a Fourier transform infrared spectrometer (SPECTRUM ONE, Perkin Elmer, USA). The samples were scanned from 400 cm−1to 4000 cm−1 at room temperature in attenuated total reflectance mode at a resolution of 4 cm−1.
2.3.2 Scanning electron microscope (SEM)
The structural features of the hydrogels were monitored by scanning electron microscope (SEM, Quanta 250 microscope, Japan). The specimens were coated with gold using a sputtering device (Jeol, JFC 1200, Japan) prior to the SEM observation. A magnification of 500\(\times\) was used, and micrographs of the samples were recorded.
2.3.3 Differential scanning calorimetry (DSC)
The thermal behavior was investigated using DSC (NETZSCH DSC 204 F1 Phoenix, Germany). The sample were placed in aluminum pans and purged with nitrogen gas at a flow rate of 40 mL/min. The temperature was set to \(20 ℃-200 ℃\), with a flow rate of \(10 ℃\)/min. Characteristic temperature were defined as the glass transition temperature, melting temperature, and specific heat capacity.
2.3.4 Tensile testing
The mechanical properties were performed using a universal tensile tester. The hydrogel sheets of 7 cm-length and 1 mm-width were stretched at a tension speed of 30 mm min−1 to calculate the tensile strength and elongation at break. For each sample, 5 specimens were tested. The statistical average and standard deviation were then reported.