**4. Results and Discussion**

FTIR measurement was employed to investigate the structure of PVA-AA-t, PAA-t, and PAA, as shown in Figure 2. In the three curves, the strong absorption bands at 3000–3500 cm−<sup>1</sup> are assigned to -OH of H2O or VAc after alcoholysis (only in PVA-AA-t). The absorption bands at 2922 cm−<sup>1</sup> should be assigned to C-H. The strong absorption bands 1557 cm−<sup>1</sup> and 1406 cm−<sup>1</sup> can be attributed to the symmetric and antisymmetric stretching vibration peaks of COO-, respectively. However, the weak absorption at 1717 cm−<sup>1</sup> in the PVA-AA-t curve should also be noted, which represents the stretching vibration of carbonyl (-C=O) in the acetate group, indicating that the ester bond in the polymer molecule is incompletely alcoholized [24,25]; the C-O-C peaks of the ester bond at 1251 cm−<sup>1</sup> and 1024 cm−<sup>1</sup> also confirm this.

**Figure 2.** FTIR spectra of PVA-AA-t, PAA-t, and PAA (KBr).

Figure 3 shows the TG curve of PVA-AA-t. Under a nitrogen atmosphere, the mass decreases for the first time at 50–100 ◦C, and the weight loss rate is 13%. This is mainly because the polymer absorbs moisture at room temperature. The weight loss rate of the polymer is 10% at 180–215 ◦C, because the carboxyl group in the polymer molecular chain is removed. The weight loss rate at 270–340 ◦C is 20%, which is caused by the breaking and decomposition of the ester or hydroxyl bonds in the molecular chain. When the temperature is higher than 430 ◦C, the mass of the polymer decreases rapidly, due to the fracture and decomposition of C-C in the main chain of the polymer. In general, the structure of the polymer is damaged when the temperature is higher than 180 ◦C, so it can be used in below 150 ◦C without damaging the properties of the polymer.

**Figure 3.** TG curve of PVA-AA-t under N2.

We dissolved the PVA-AA-t in water and tested the viscosity, as shown in Figure 4. With the increase in the polymer's concentration, the viscosity of the aqueous solution increases exponentially. When the concentration is less than 0.2%, the viscosity is less than 1 Pa·s (when the solution concentration is only 0.1%, there is almost no thickening effect). When the concentration increases to 0.3%, the viscosity increases rapidly to 4 Pa·s. The viscosity of 0.5% aqueous solution can reach 11 Pa·s, showing a clear thickening effect similar to that of other thickeners. Under shear force, with the increase in shear rate, the viscosity of the solution decreases rapidly, showing the characteristics of a pseudoplastic fluid.

**Figure 4.** Viscosity curves of PVA-AA-t aqueous solutions with different concentrations.

Figure 5 shows the variation in the viscosity of PVA-AA-t solutions with different monomer ratios. It can be seen from the figure that the viscosity is the highest when VAc:AA = 7.5:2, reaching 3.6 Pa·s. With the increase in the AA content of the copolymer, the initial viscosity decreases gradually. When the ratio of VAc:AA is 6.5:3, the initial viscosity drops to 2.14 Pa·s. When the ratio of VAc:AA is 2:7.5, the whole system has no thickening effect, showing Newtonian fluid characteristics. As can be seen from previous literature [16,26], sodium polyacrylate has been used as a thickener for many years; however, under these experimental conditions, we found that increasing the content of acrylic acid did not further improve the thickening effect—possibly due to the simultaneous

copolymerization of three monomers in the system, affecting their respective reaction rates. Moreover, at high temperatures, the reaction rate of AA is faster than that of VAc, and the formed molecular chain is shorter, which leads to a poor thickening effect. On the other hand, VAc with a relatively slow reaction rate can form long-chain molecules at this temperature (65 ◦C).

**Figure 5.** Effects of different monomer ratios on the viscosity of PVA-AA-t aqueous solution (concentration: 0.3%; TAC = 0.2 phr).

In this experiment, triallyl cyanate (TAC), which contains three ethylene groups, was used as a crosslinking agent. The crosslinking structure can be formed in the reaction process, and the more TAC is used, the higher the crosslinking density. From the Figure 6, we found that the viscosity (1.79 Pa·s) was relatively low when the dosage of TAC was 0.1 phr, which may have been due to the low dosage of the crosslinking agent, low crosslinking density of the molecular chain, and the polymer stretching and dissolving in water, resulting in the decrease in viscosity. Further increasing the amount of TAC, there was little change in viscosity, because TAC acts as a connecting point in the molecular structure [27], and the carboxyl and hydroxyl groups in the molecular chain play a thickening role. Therefore, from the perspective of application economy, the crosslinking agent is expensive; the dosage of TAC in the synthesis is 0.2 phr.

**Figure 6.** Effects of different amounts of crosslinking agent on the viscosity of polymer solution (concentration: 0.3%; VAc:AA = 7.5:2).

In this experiment, we compared the particle size distribution (PSD) of PVA-AA-t before and after swelling, as shown in Figure 7. The PSD of solid powder was in the range of 0.5–150 μm, the median diameter (D50) was 17.90 μm, and the average particle size was 24.74 μm (note: the solid polymer powder can be continuously ground by a ball mill, and the polymer powder within the fixed particle size range can be selected by standard sample separation). After swelling, the PSD of the polymer was in the range of 20–600 μm, D50 was 156.4 μm, and the average particle size was 187.2 μm. Comparing the two cases, the particle size increased nearly ninefold, showing an obvious tackifying effect. The swelling principle is the combination of non-associative thickening and associative thickening [28]; that is, -COO- in the polymer molecular chain combines with H2O to form hydrated ions, hindering the flow of molecules in the system, so as to achieve the purpose of thickening. On the other hand, the polymer molecular chain contains a small amount of acrylate chain, forming a comb-like structure. These hydrophobic short chains associate with one another to form a network structure, which can enhance the interaction between polymer particles and further increase the viscosity in water.

**Figure 7.** Particle size distribution of PVA-AA-t before and after swelling.

Figure 8 shows the effects of different electrolytes on the viscosity of the polymer solution. Under the same conditions, the viscosity decreases significantly (the original viscosity is nearly 4 Pa·s) after adding electrolytes. This is because the electrolyte can partially shield the carboxyl anion on the polymer molecular chain, resulting in the curling of the stretched macromolecular chain formed by the repulsion from the anions, reducing the friction between the molecular chains, and causing the viscosity to decrease rapidly with the addition of the electrolyte. The influence of different electrolytes on the viscosity follows the sequence CaCl2 > KCl > NaCl > CH3COONa. The greater the charge number of divalent ions at the same mole number, the stronger the shielding effect on the polymer molecules [29]. Compared with K+ and Na+, the larger the ion radius, the better the shielding effect on the charge of the molecular chain, making the molecular chain curl further and the viscosity decrease simultaneously. CH3COONa is a strong base and a weak acid salt. In aqueous solution, CH3COO- hydrolyzes to form CH3COOH, which weakens the shielding effect on the polymer molecular chain compared with the other three electrolytes. In general, the addition of an electrolyte has a great influence on the thickening effect of PVA-AA-t.

**Figure 8.** Effects of different electrolytes on the viscosity of polymer aqueous solution (polymer concentration: 0.3%, electrolyte concentration: 0.001 mol/L).

There is a carboxylic sodium salt on the polymer molecules, the pH value of which has a great influence on the viscosity of the solution. Therefore, we compared the effects of different pH values on solution viscosity. As shown in Figure 9, the viscosity of the polymer solution (about 4.0 Pa·s) is the highest in the range of pH = 5–7; with the increase in the pH value, the viscosity begins to decrease gradually. When pH = 9, the viscosity decreases to 1.2 Pa·s; further increases in the pH value cause the viscosity to further decrease. When the pH is ~13, the whole solution has no thickening effect—this is mainly because under strong alkaline conditions, the system contains a large amount of ions, which weaken the hydration of the polymer molecular chain to H2O, making the molecular chain return to a curled state, reducing the intermolecular friction, and causing the decrease in viscosity.

**Figure 9.** Effect of pH value on viscosity (polymer concentration: 0.3%).

On the other hand, when the pH value is reduced, white turbidity begins to appear in the solution. With the decrease in pH value, the sediment increases. We centrifuged the solution to obtain white insoluble matter, which was analyzed by FTIR, as shown in Figure 10. The peaks at 2942 cm−<sup>1</sup> and 2864 cm−<sup>1</sup> correspond to the symmetrical stretching vibration peaks of the C-H bond, while 1721 cm−<sup>1</sup> is the carbonyl peak of the ester bond C=O, but the peak strength is significantly higher than the infrared absorption peak before acidification. It is possible that the carboxylic acid in the molecular chain reacts with the hydroxyl group to form ester bonds during acidification. The C-O-C peaks at 1242 cm−<sup>1</sup> and 1164 cm−<sup>1</sup> demonstrate the formation of ester bonds in the molecular chain. It is also possible that the peak intensity increases due to the increase in the carbonyl peak in the acidified carboxyl group, and the infrared absorption peaks of the acidified sample at 1557 cm−<sup>1</sup> and 1408 cm−<sup>1</sup> disappear, which may indicate that Na<sup>+</sup> in the carboxylate is replaced by H+. The above research shows that PVA-AA-t has excellent thickening performance, and is sensitive to electrolytes and pH.

**Figure 10.** Infrared absorption of white turbidity after acidification.

In addition, we studied the dispersion of Gr by PVA-AA-t. Gr was added to the polymer solution for high-speed stirring and dispersion, and the change in the system's viscosity was measured using a portable viscometer (model: VL7-100B-d21-TS). At the same time, the dispersibility of Gr by PAA and PAA-t was compared. As shown in Figure 11, the initial viscosity of the three thickeners (PAA, PAA-t, and PVA-AA-t) was 4.3 Pa·s, 3.6 Pa·s, and 0.8 Pa·s, respectively. When t = 0 s, 0.5% Gr was added to three solutions. With the increase in dispersion time, the viscosity of the PAA solution decreased to a certain extent, and then remained stable. After long-time dispersion and grinding, there was still a large amount of agglomerated Gr in the PAA solution. This phenomenon also appeared in the PAA-t system, the viscosity of which decreased after the addition of Gr, with a large amount of agglomerated Gr. Conversely, when Gr was added to PVA-AA-t, the viscosity increased rapidly to more than 10 Pa·s under high-speed stirring, showing paste-like characteristics without fluidity. After full grinding and dispersion, the Gr slurry appeared as a dark glossy paste. Three kinds of Gr slurry were evenly coated on silicon wafers and glass (thickness: 60 μm) with an applicator, before being transferred to a vacuum-drying oven and dried at 60 ◦C for 5 h. The dried silicon wafers were used to measure the resistivity. The dried glass sheets were placed under a microscope (YKP-700C magnification: ×600) for observation (as shown in Figure 12a,b). As shown in Figure 12, the Gr dispersed by PVA-AA-t showed good resistivity, while the Gr dispersed by the other two thickeners (PAA, PAA-t) showed poor resistivity; this is because the Gr dispersed by PVA-AA-t was evenly distributed, without obvious agglomeration. After the Gr slurry was dried, it can be seen from Figure 12b that it showed evenly distributed pores (left by the dried polymer powder particles), and the Gr particles were overlapped with one another, showing good adhesion and good conductivity. However, the Gr dispersed by PAA and PAA-t had a lot of agglomeration, and the existence of the acrylic acid on the polymer chains likely inhibited bridging between particles, causing the force between the Gr particles to weaken, yielding obvious cracking and even falling off after drying, which affected the conductivity of the Gr electrode sheet.

**Figure 11.** The changes in the viscosity of graphene dispersion slurry (polymer concentration: PAA: 1%; PAA-t: 1%; PVA-AA-t: 0.1%; Gr: 0.5%).

**Figure 12.** Resistivity of graphene electrode sheet. (**a**) PAA and PAA-t; (**b**) PVA-AA-t.

The Gr in dispersed systems is stabilized mainly by charge stabilization and steric stabilization. The Gr surface contains a small amount of hydroxyl, carboxyl, and other functional groups, and is generally shown as an inorganic powder with weak polarity (except some with special chemical treatment). Therefore, a widely used sodium polyacrylate dispersant—such as PAA or PAA-t—in which the polymer chains contain acrylic acid only, has poor compatibility with Gr. If other components, such as alkyl acrylate, are introduced into a polymer such as PVA-AA-t, the second component will influence the adsorption of the polymer and the charge density of the particles adsorbed by it, consequently increasing the ability of that polymer to disperse Gr. In addition, the introduction of TAC makes the linear polymer molecules form a crosslinking structure, causing the polymer to form gel particles. When mixing and dispersing Gr, gel particles reduce the time needed for disentanglement between the long-chain polymers, and improve the dispersion efficiency Gr. After drying, the Gr powders are connected by dehydrated gel particles (PVA-AA-t), showing better adhesion and better electrical conductivity.
