3.1 Molecular structures of poly(ionic liquid)s
Fig. 1 showstheFTIRspectrum of poly-IL-0.45, poly-IL-0.35, and poly-IL-0.30, respectively. It can be noticed that three poly(ionic liquid)s showed a similarFTIR spectrum, which illustrated that the mole fraction of pendant enes showed negligible influence on the molecular structure of poly(ionic liquid)s. Take the spectrum of poly-IL-0.35 for example, the absorption peak around 2883cm−1 was ascribed to the stretching vibration of C-H, the peak situated at 1728cm-1 were attributed to the stretching vibration of C=C from the PEDGA and ionic liquid, and the absorption peaks located at 838cm-1 was the characteristic adsorption peak of PF-6. Besides, no typical characteristic peak of S-H (about 2560 cm-1) was detected in the FTIR spectrum. These results indicated that then enes on AMIMPF6 and PEGDA successfully reacted with the sulfydryl on PETMP. In addition, these poly(ionic liquid)s were also characterized using 1HNMR and 31PNMR. As shown in the Fig.2., the 1HNMR further confirmed the formation of the designed polymers. With the increasing of the mole fraction of pendant enes, the response of 31P on NMR increased gradually (Fig.3), and this was mainly due to the increment of AMIMPF6 dosage in the formulation.
3.2 Morphology of poly(ionic liquid)s
The surface morphology of poly(ionic liquid)s with different mole fraction of pendant enes (0.45/0.35/0.30)were shown in Fig. 4. In addition, poly(ionic liquid)s with mole fraction of pendant enes of 0.25, which was presented in our previous work was referred as comparison9. It can be noticed that the mole fraction of pendant enes had a significant difference on the morphology of each poly(ionic liquid), and when this value was under 0.30, porous poly(ionic liquid)s with various beading sizes could be formed. However, when the mole fraction of pendant enes was equal or greater than 0.30, only various irregularly shaped apertures showed on the surface of poly(ionic liquid)s, and with the decreasing value of the mole fraction of pendant enes in formulation, the average pore diameter diminished gradually. From the appearance photos, it can be detected that poly(ionic liquid)s which were more like white sponges were obtained with decreasing value of the mole fraction of pendant enes. These results indicated that the increasing amount of cross-linking agent (PEGDA) in the formulation of thiol-ene “click” reaction could reduce the interfacial tension of polymers, and hence made the poly(ionic liquid)s with various beading sizes easier to form.
3.3 Thermo-analysis of poly(ionic liquid)s
Figure 5. shows thermogravimetric analysis of poly-IL-0.45, poly-IL-0.35, poly-IL-0.30, and the thermogravimetric curve of neat AMIMPF6 was presented for comparison. In the range of 0°C–10°C, the as-prepared poly(ionic liquid)s showed a slight mass loss due to the evaporation of the absorbed water. From around 315°C, three poly(ionic liquid)s began the second stage of mass loss, and their maximum degradation rate all showed around 375°C, which was due to the decomposition of the ethyl group on poly(ionic liquid)s. However, these maximum degradation rates were all less than that of AMIMPF6, especially for poly-IL-0.30, and this result could be connected to the polymerization of PEGDA onto the polymers19.
It can be noticed that poly-IL-0.45 and poly-IL-0.35 showed a similar T10% (10 wt% loss temperature) values with neat AMIMPF6. However, the T10% value of poly-IL-0.30 was less than that of AMIMPF6, and it was closer to that of poly-IL-0.25 in our previous work9. Similar trends on decomposition rates of each poly(ionic liquid)s could also be observed. These results should be corresponded to the fact that the with the increment of the mole fraction of pendant enes, the average pore diameter of poly(ionic liquid)s decreased, which give the membrane a higher density. And this was in accordance with the results of morphological characterization.
Figure 6 shows that the glass transition temperature (Tg) of poly-IL-0.45, poly-IL-0.35 and poly-IL-0.30. With the increasing mole fraction of pendant enes from 0.30 to 0.45, the Tg of three poly(ionic liquid)s showed a decrease from − 12.8°C to − 25.1°C. This result could be ascribed to a loss of flexibility of the polymeric chains with less cross-linking agent20.
3.4 Adsorption performances of poly(ionic liquid)s
To investigate the adsorption performance of as-prepared poly(ionic liquid)s, nonionic dye of DR was selected as target adsorbates, and the effects of pH and C0 of solution, dosage of adsorbent, and the contact time on the adsorption efficiency of poly-IL-0.45, poly-IL-35 and poly-IL-0.30 were shown in Fig. 7. It can be noticed that three samples all showed the highest adsorption efficiency for DR at pH of 5.5, and with the increment of adsorbent dosage, the removal efficiency increased gradually and tended to balance. The influence of C0 on the adsorption behaviors of three poly(ionic liquid)s is provided as Fig. 7c. It can be detected that poly(ionic liquid) with a small mole fraction of pendant enes showed a high removal efficiency for DR. Commonly, the synergistic effect of the hydrogen bonding and the hydrophobic interaction played a critical role during the adsorption of poly(ionic liquid)s toward DR, and this synergistic effect could be enhanced by cross-linked structure, which was donated by decreasing the mole fraction of pendant enes21.
The experimental adsorption kinetic data and the linearly fitted plots of poly(ionic liquid)s toward DR in accordance with the pseudo-first- and the pseudo-second-order kinetic models, which were given in our reported literature, are presented in Fig. 89, and the calculated fitting parameters are listed in Table 3. By contrast, we can see that the pseudo-second-order model could better describe the adsorption process of DR by poly-IL-0.45, poly-IL-35 and poly-IL-0.30 than the pseudo-first-order model, and that the mole fraction of pendant enes did not significantly change the valence forces between the molecule of DR and poly(ionic liquid)s. These results indicated that the chemical adsorption process was the main control step of the adsorption rate 22.
Table 2
The kinetics parameters for DR adsorption onto different adsorbents
Sample
|
Qe,exp
mg/g
|
Pseudo-first order
|
Pseudo-second order
|
Qe,cal
mg/g
|
k1
|
R2
|
Qe,cal
mg/g
|
k2×104
|
H
mg/(g min)
|
R2
|
Poly-IL-0.45
|
145.001
|
8.700
|
0.005
|
0.485
|
143.678
|
54.613
|
112.740
|
1.000
|
Poly-IL-0.35
|
145.017
|
7.543
|
0.004
|
0.408
|
143.472
|
67.473
|
138.889
|
1.000
|
Poly-IL-0.30
|
145.033
|
9.006
|
0.005
|
0.500
|
143.472
|
51.958
|
106.952
|
1.000
|
The adsorption isotherms of DR on poly(ionic liquid)s in accordance with Langmuir and the Freundlich adsorption isotherm models are presented in Fig. 89, 23, and the fitting parameters are listed in Table 4. Results showed that the Freundlich adsorption isotherm models were more suitable in describing the adsorption of poly-IL-0.45, poly-IL-35 and poly-IL-0.30 toward DR than the Langmuir adsorption isotherm models, indicating a heterogeneous adsorbent surface during the adsorption. In addition, the values of n were all more than 1 and followed the order: poly-IL-0.30 > poly-IL-35 > poly-IL-0.45, showing that the adsorption of DR onto poly-IL-0.45, poly-IL-35 and poly-IL-0.30 were all favorable, and this effect could be enhanced by decreasing the mole fraction of pendant enes24.
Table 3
Adsorption isotherm parameters for DR adsorption onto different adsorbents
Sample
|
Langmuir isotherm
|
Freundlich isotherm
|
Qm,cal
mg/g
|
kL
L/mg
|
R2
|
Kf
L/g
|
n
|
R2
|
Poly-IL-10
|
8685.079
|
0.003
|
0.373
|
32.041
|
1.086
|
0.995
|
Poly-IL-15
|
5399.626
|
0.006
|
0.512
|
37.022
|
1.120
|
0.996
|
Poly-IL-20
|
4941.850
|
0.007
|
0.340
|
44.510
|
1.161
|
0.990
|