## 3 Results and Discussion
### Morphology and Characterization of the CuBTC@PET Membranes
#### 3.1.2 Post-Processing PET Fiber Morphology Study  
In this experiment, non-solvent-induced phase separation of PET recrystallization in the choice of solvent is the key to the success of the experiment. There are mainly the following requirements: one is to ensure that the experimental process of PET fiber swelling rather than dissolution by controlling the solvent system on the PET solubility and immersion time to ensure the solvent system into the interior of the fiber, without dissolving the fiber structure, and the other requirement is to select a good solvent that is denser than the poor solvent, such that the poor solvent is relatively hydrophilic and the good solvent is relatively hydrophilic. In this experiment, the solvent system of NMP and ethanol was chosen as the solvent system for non-solvent-induced phase separation solvation and recrystallization, taking into account the Hildebrand solubility parameter, density, volatility, and hydrophilicity. The Hildebrand solubility parameter is a widely recognized theoretical model for assessing the solubility between polymers and solvents, and its value, \(\delta\), approximates the dispersion (dD), polarity (dP), and hydrogen bonding (dH) vector sums in the Hansen solubility parameter [42]. As the \(\delta\) values of the polymer and the solvent are closer, it is easier for the polymer to dissolve. Table 2 shows the physicochemical parameters of the solutions used in the experiment [43]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Solvent** & \begin{tabular}{c} **Boiling** \\ \({}^{\circ}\)**C** \\ \end{tabular} & **Hydrohilicity** & \begin{tabular}{c} **Density** \\ **g/mL** \\ \end{tabular} & \begin{tabular}{c} **Solubility** \\ **Parameters \(\delta\)** \\ \end{tabular} & \begin{tabular}{c} **Vapor Pressure** \\ **mmHg. 20 \({}^{\circ}\)**C** \\ \end{tabular} \\ \hline NMP & 203 & \
\begin{tabular}{c} Relatively \\ hydrophobic \\ Relatively \\ hydrophilic \\ \end{tabular} & 1.028 & 11.4 & 0.3 \\ EtOH & 78.3 &
\begin{tabular}{c} Relatively \\ hydrophilic \\ \end{tabular} & 0.729 & 13.2 & 44.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of physicochemical properties between NMP and EtOH. Figure 2: SEM images of PET fiber membranes with different parameters: (**A**) better spinning parameters, (**B**,**C**) TFA/DCM ratio control, (**D**,**E**) PET mass fraction control, (**F**,**G**) voltage control, (**H**,**I**) flow rate control. The solvent ratio is an important parameter that directly affects the degree of swelling and the phase separation process. Figure 3 shows the SEM control images under different solvent ratios, and Figure 3a,b demonstrates the SEM images when the EtOH ratio is high, which show that the fiber is slightly rougher compared with the original PET fiber, but there are no voids or pores, which may be due to the low ratio of NMP as a good solvent; the solvent system has a low solubility for PET. This may be due to the low solubility of NMP as a good solvent; in the solvent system with PET, the solution did not penetrate the fiber. When the proportion of NMP increases, as shown in Figure 3c,d the fiber has a completely different surface morphology, and the surface roughness increases dramatically, with voids and pores and other structures and with a superior surface morphology. However, with the further increase in the proportion of NMP, the fiber surface morphology changes more drastically, but on top of the fiber fracture, this seriously affects the mechanical properties of the fiber: due to the increase in the proportion of NMP and the increase in the solvent system solubility of PET, the PET fiber swelling degree is too high and nearly dissolved, and the remaining PET is not enough to support the complete fiber.