## 3 Results and Discussion
Figure 6: Gel particles submerged in water at elevated temperatures: (**a**) 10 \({}^{\circ}\)C; (**b**) 25 \({}^{\circ}\)C; (**c**) 40 \({}^{\circ}\)C in case A and (**d**) 10 \({}^{\circ}\)C; (**e**) 25 \({}^{\circ}\)C; (**f**) 40 \({}^{\circ}\)C in case C. The scale bar corresponds to 1 mm. Figure 7 shows the variation in gel diameter for cases A, B, C, and D in water at different temperatures. Figure 8 shows the variations in gel diameter with the PAANa solution for the same-sized gels. Both figures show that as the temperature of the aqueous solution increased, the diameter of the gels oppositely decreased. This general correlation is evident for both solution types (water and the PAANa solution). Indeed, the exact pattern of both figures was different, especially within the range of 25-35 \({}^{\circ}\)C, where the most significant volume change occurred. For case A in both figures, considerable changes in gel diameter occurred at temperatures between 25 and 35 \({}^{\circ}\)C. For the smaller-sized gels, as shown  
The plots reveal a slight decrease in gel diameter within two temperature ranges of the polymeric solution: 10-25 \({}^{\circ}\)C and 35-40 \({}^{\circ}\)C. However, in the temperature range between 25 and 35 \({}^{\circ}\)C, a sharp decrease in gel size is evident because significant changes in gel volume occurred within this range. As shown in both graphs, a significant difference in the slope of the curve is observed at a temperature range between 25 and 30 \({}^{\circ}\)C. In this range, the gels exhibited a sharper response to the increasing temperature than the other two ranges. In particular, the largest slope is evident at temperatures between 25 and 30 \({}^{\circ}\)C. This phenomenon is attributed to the particular characteristics of the NIPA gel used in the experiment. At lower temperatures, the gels were in a swollen state and were larger in size. However, at higher temperatures, the gels shrank and became smaller as water molecules detached from the polymer chain (hydrophobic state). Water-molecule detachment started on the outer layer of the gels and continued to the inner layers. The plotted points represent the average value of the three gel samples, and the curve was obtained by polynomial (quaternary) approximation. The error bars in the figure represent the deviation in measured value at each temperature. The graph reveals that the diameter of the gel particles gradually decreased as the temperature of the polymer solution increased, indicating a negative expansion characteristic. When the temperature of the solution was constantly modified at specific intervals, the gels showed reversible transformation from a swelling state to a shrinking state before reverting to their initial state. Furthermore, the polymeric gel's higher molecular weight had a longer chain length and possessed a network with a lower cross-link density, which resulted in higher swelling rates in the gel [42]. The volumetric change in the gel particles is discussed below. A gel particle was approximated to a sphere of diameter \(d_{g}\) such that its volume \(V_{g}\) was estimated as \(\pi d_{g}\)3/6. The relationship between \(V_{g}\) and the temperature of the gels in the aqueous solution \(T\) together with the relationship between the gels' volume ratio \(V_{g}\)/\(V_{shrink}\) and \(T\) can be seen in Figure 9, where \(V_{shrink}\) is the volume at 40 \({}^{\circ}\)C. The figures clearly demonstrate that case A, which is represented by the blue line, had the biggest gel volume, followed by cases B, C, and D. Figure 9b shows that the biggest change in gel volume occurred between 25 and 30 \({}^{\circ}\)C. In Figure 9a, the smallest gel (case D) showed no significant volume changes in the same range of temperature. Thus, by considering the volume ratio of gel \(V_{g}\)/\(V_{shrink}\), the temperatures at which significant volume changes occurred could be clearly observed.