# Styrene-Based Copolymer for Polymer Membrane Modifications
## 3 Results and Discussion
### Study of Thermal Behavior of Blend Membranes  
Figure 4 shows the DSC thermograms for 100% PVDF, 10% styrene-acrylonitrile, 60% styrene-acrylonitrile blends, and 100% styrene-acrylonitrile membranes. PVDF has a glass transition temperature range of \(-40\) to \(-25\)\({}^{\circ}\)C [20]. The melting point (Tm) increased from 171.4 \({}^{\circ}\)C for the 100% PVDF membrane to 172.5 for the 10% styrene-acrylonitrile blend membrane as shown in Table 3. However, it then decreased to 169 \({}^{\circ}\)C. The 100% styrene-acrylonitrile has the melting point of 220 \({}^{\circ}\)C [24], which is 28% higher than the melting point of 100% PVDF. The loss of crystalline nature in PVDF membranes modified with higher styrene-acrylonitrile concentrations affects its melting behavior and the melting point of the 60% styrene-acrylonitrile blend membrane decreases to 169 \({}^{\circ}\)C. It possesses a high melting point and good thermal properties due to the presence of the phenyl ring in the structure [24]. All membranes exhibited a marginal weight loss in a 50 to 200 \({}^{\circ}\)C temperature range, which is attributed to the evaporation of moisture and residual solvents present in the sample. The 100% PVDF membrane showed the minimum weight loss and the highest degradation temperature (Td). It starts to degrade at 417.5 \({}^{\circ}\)C and degradation becomes significant at 477.5 \({}^{\circ}\)C as shown in Figure 5a.  
Figure 4: Melting behavior of styrene-acrylonitrile-blended PVDF membranes. (**a**) 100% PVDF; (**b**) 10% styrene-acrylonitrile blend; (**c**) 60% styrene-acrylonitrile blend; (**d**) 100% styrene-acrylonitrile membranes.  
Figure 5: (**a**) Degradation behavior; (**b**) Degradation rate of styrene-acrylonitrile-blended PVDF membranes.  
Similarly, the degradation of the 10% styrene-acrylonitrile blend membrane starts at 368 \({}^{\circ}\)C as mentioned in Table 4. They also exhibit a lower degradation temperature than the 100% PVDF membrane. The decrease in degradation temperature of the blend PVDF membrane was observed with an increase in its styrene-acrylonitrile content. The 100% styrene-acrylonitrile membrane starts degrading at 340 \({}^{\circ}\)C and degrades almost 92% at 430 \({}^{\circ}\)C. Its degradation curve, presented in Figure 5b, shows two peaks, which may be due to the presence of two polymers in the structure: polystyrene and polyacrylamide. The first peak corresponds to the acrylonitrile content of the styrene-acrylonitrile polymer, which is present in a lesser proportion and degrades at a lower temperature. However, it has a higher melting temperature than styrene; hence, styrene-acrylonitrile has a higher Tm than the pure polystyrene polymer. This decrease in degradation temperature of blend PVDF polymer is the result of different bond energies for the C-F, C=N, and C-phenyl bonds present in the blends. The bond energy of a C-F bond is 20% higher than that of a C-phenyl bond, while the bond energy of a C=N bond is even lower than a C-phenyl bond. The 100% PVDF membrane has the highest degradation rate, which reduces with increasing styrene-acrylonitrile content of the blend membranes. The styrene-acrylonitrile blend PVDF polymer membranes displayed two degradation temperature peaks, which were clearly observed for the 60% styrene-acrylonitrile blend membrane. However, thermodynamically compatible responses were observed in the thermograms of the blend membranes and only one Tm was observed. Analysis of DSC and TGA also confirmed the appropriate styrene-acrylonitrile blending with the PVDF polymer in blend membranes.