## 3 Results
### Polar Acid-Base Surface Energies of S-DVB Copolymer Modified by HMU  
Using Equations (11) to (13), the polar acid \(\gamma_{s}^{+}\) and base \(\gamma_{s}^{-}\) surface energies of S-DVB copolymer modified by different HMU percentages were determined with the help of the values of the free polar energy \(-\Delta G_{u}^{SP}(T)\) of dichloromethane and ethyl acetate polar solvents given in Table 5.  
The obtained results were plotted in Figure 7.  
\begin{table}
\begin{tabular}{c c c c c} \hline \multicolumn{5}{c}{**Dichloromethane**} \\ \hline \(T\)(K) & Dowex L-285 & 3.5\% HMU/Dowex L-285 & 1\% HMU/Dowex L-285 & 10\% HMU/Dowex L-285 \\ \hline
453.15 & 15.274 & 15.609 & 24.349 & 9.737 \\ \hline
458.15 & 15.239 & 15.559 & 24.294 & 9.352 \\ \hline
463.15 & 15.204 & 15.509 & 24.239 & 8.967 \\ \hline
468.15 & 15.169 & 15.459 & 24.184 & 8.582 \\ \hline \multicolumn{5}{c}{**Ethyl Acetate**} \\ \hline \(T\)(K) & Dowex L-285 & 3.5\% HMU/Dowex L-285 & 1\% HMU/Dowex L-285 & 10\% HMU/Dowex L-285 \\ \hline
453.15 & 10.106 & 8.503 & 9.667 & 10.567 \\ \hline
458.15 & 9.593 & 8.378 & 9.507 & 10.314 \\ \hline
463.15 & 9.079 & 8.253 & 9.347 & 10.06 \\ \hline
468.15 & 8.566 & 8.128 & 9.187 & 9.807 \\ \hline \end{tabular}
\end{table}
Table 5: Values of \(-\Delta G_{u}^{p}(T)\) (in kJ/mol) for dichloromethane and ethyl acetate adsorbed on Dowex L-285 modified with different HMU percentages at different temperatures.  
Figure 6: Curves of the Lewis acid–base parameters \(K_{A}\), \(K_{D}\), \(K_{D}/K_{A}\), \(\omega_{D}\)/\(\omega_{A}\), and the parameters \(S_{K}=K_{A}+K_{D}\) as a function of HMU percentage.  
Figure 7: Variations in acid and base surface energy components, and total energy (in mJ/m\({}^{2}\)) of S-DVB copolymer modified by different HMU percentages as a function of temperature: S-DVB copolymer **(a)**, 1% HMU **(b)**, 3.5% HMU **(c)**, 10% HMU **(d)**, base surface energy **(e)**, acid surface energy **(f)**, acid–base surface energy **(g)**, and total surface energy **(h)**.  
The values of \(\gamma_{s}^{p}\) were obtained using Equation (11), while the total surface energy \(\gamma_{s}^{tot}\) was determined from Equation (13) as a function of temperature. The surface polar parameters such as \(\gamma_{s}^{+}(T)\), \(\gamma_{s}^{-}(T)\), \(\gamma_{s}^{AB}(T)\), \(\gamma_{s}^{d}(T)\), and \(\gamma_{s}^{tot}(T)\) of the modified copolymer were given in Table S2. The variations of the different surface polar parameters were plotted in Figure 7 against the temperature. Linear variations of the different polar surface energies versus temperature are shown in Figure 7. The results show that the highest polar surface energy values were obtained for the highest HMU percentage, again confirming the important role of the modifier in increasing the polarity of the copolymer and consequently its use in many industrial applications such as catalysis and selective adsorption of several organic molecules.  
The results given in Table S2 and Figure 7 allowed the drawing of the polar surface energy of the adsorbed polar probes against temperature in Figure 8, using the Fowkes relation and the Hamieh thermal model. It seems that the adsorbed ethanol and i-propanol polar solvents exhibit the highest values of polar surface energy. In contrast, the lowest values were obtained for the adsorption of toluene and benzene. Identical results were obtained when the S-DVB copolymer was modified by melamine in a previous study [56].