*2.4. Performance Test of SAPC*

#### 2.4.1. Swelling Ratios

To determine the swelling ratio, dry composites (0.1 ± 0.001 g) were immersed in 500 mL of solutions (distilled water, various salt solutions and pH solutions) for 4 h at room temperature to achieve equilibrium. The swollen product was filtered through a 100-mesh screen to remove excess moisture and then weighed. Each specimen was tested thrice, and the average value was used. The water absorbent capacity was calculated using the following Equation (1):

$$Q\_{eq} = \frac{(m\_1 - m\_0)}{m\_0} \tag{1}$$

where *Qeq* is the water absorbent capacity and *m*<sup>0</sup> and *m*<sup>1</sup> are the mass of the dried and swollen products, respectively.

#### 2.4.2. Swelling Kinetics

The pre-weighed dry product (0.1 ± 0.001 g) was added into distilled water (500 mL), and swelled for a period of time (5, 10, 15, 30, 45, 60, 120, 180 and 240 min). The water absorption at time *t* was measured by the method described in Section 2.4.1.

#### 2.4.3. Swelling in Salt Solutions

NaCl and FeCl3 were used to prepare salt solutions with different concentrations (0.02–0.1 mol/L). The pre-weighed dry product (0.1 ± 0.001 g) was dispersed in various salt solutions (500 mL), and water absorption was measured by the method described in Section 2.4.1.

#### 2.4.4. Water Retention Measurement

The fully swollen product (80 g) was placed in petri dishes at various temperatures (25, 45 and 60 ◦C), and then weighted every 2 h. According to Equation (2), water retention of the product was calculated:

$$\mathcal{W}\_r = \frac{(M\_t - M\_0)}{(M\_1 - M\_0)} \times 100\% \tag{2}$$

where *Wr* is the water retention rate of product, *Mt* is the mass of the swollen product at time *t*, *M*<sup>0</sup> is the mass of the dry product and *M*<sup>1</sup> is the initial weight of the fully swollen products.

#### 2.4.5. Swelling Measurement in Buffer Solutions

The buffer solutions (hydrochloric acid and sodium hydroxide solutions) with pH values from 2 to 12 were prepared to test the pH sensitivity of the SAPC. A pH-meter (PHS-3C, accuracy is ± 0.01) provided by Shanghai INESA Scientific Instrument Co., Ltd. (Shanghai, China) was used to detect the pH value of the solution. Then, the dry product (0.1 ± 0.001 g) was dispersed in various buffer solutions (500 mL), and water absorption was calculated according to Equation (1).

#### **3. Results and Discussion**
