Supporting Information
Table S1. The resultant PVDF membranes and their corresponding preparation conditions
Membrane No. / Casting solution temperature / SolutionComposition (mass ratio)
(PVDF/ Tween80/ DMAc,) / H2O content (wt.%)
M-0- 60ºC / 60ºC / 18/ 3/ 79 / 0
M-0- RT / RT / 18/ 3/ 79 / 0
M-1.5- 60ºC / 60ºC / 18/ 3/ 77.5 / 1.5
M-1.5- RT / RT / 18/ 3/ 77.5 / 1.5
M-2- 60ºC / 60ºC / 18/ 3/ 77 / 2
M-2- RT / RT / 18/ 3/ 77 / 2
M-2.5- 60ºC / 60ºC / 18/ 3/ 6.5 / 2.5
M-2.5- RT / RT / 18/ 3/ 76.5 / 2.5
M-3- 60ºC / 60ºC / 18/ 3/ 76 / 3
M-3- RT / RT / 18/ 3/ 76 / 3
Note: Pure water was chosen as coagulation bath; RT: ambient temperature.
Tween 80 is a water-soluble non-ionic surfactant with a strong affinity with water. H2O molecules will be solubilized by reverse micelle when water is added into the DMAc- Tween 80 mixing medium, and to form the water pool of polar head groups of Tween 80 reverse micelle, see Figure S1. Combining with the H-bond formed between the polar head groups and water improved the stability of reverse micelle structure, the interconnection between hydrocarbon chain of reverse micelle and hydrophobic polymer could improve the thermodynamics stability of casting solution, and to lessen the aggregates of polymer (incipient precipitation) caused by non-solvent.
Figure S1. The H2O Solubilization process of Tween 80 reverse micelles in DMAc medium
The influence of different concentration non-solvent H2O on PVDF-Tween 80 solution property was revealed by dynamic light scattering (DLS), which was used to provide more direct information on the H2O solubilization process of Tween 80 reverse micelles in DMAc medium.
DLS uses the scattered light to measure the rate of diffusion of the aggregated macromolecules in solution. The technique measures the time-dependent fluctuations in the intensity of scattered light from a suspension of particles undergoing random, Brownian motion. Analysis of these intensity fluctuations allows for the determination of the diffusion coefficients, which in turn yield the particle size through the Stokes-Einstein equation. It has been widely used to measure the size of the micelle or the morphology of polymer in solution. Due to the sensibility of the equipment, only the solution with a low polymer concentration can be measured. However, it is possible to provide the information about the morphology change in casting solution caused by the addition of water.
Figure 5 showed the intensity-weighted micelle size distribution of DMAc-Tween 80 system at RT and 60ºC, respectively. The scattering intensity data was derived by the auto-measure software (Malvern Instruments, Malvern UK) to obtained the harmonic intensity weighted average hydrodynamic diameter (Dh), also referred to as the z-average diameter of the reverse micelles [1]. At room temperature with the increase of water content, the Dh of the micelle first decreased and then increased. That could be explained by the structure change of micelle after adding water. As shown in figure 5A, with small amount of water added, Tween 80 was tightly bound to water molecules, so the micelle hydrodynamic diameter decreases. With amount of water increasing, the size of the inner core of the micelle composed by water increased, so the Dh of micelle increased [2]. At 60ºC as showed in Figure 5B, the micelle hydrodynamic diameter increased with the addition of water. The enhanced Dh micelles at 60ºC were attributed to the thermal movement of molecules at high temperature, and the bounding between surfactant and water became less tight [3].
Figure S2. Intensity-weighted micelle size distribution of different composition solvents at RT (A) and 60ºC (B)
The DLS data of 1.0 wt.% PVDF solution at RT (A) and 60ºC (B) were showed in Figure 6. PVDF chains were in a lowly coiled state and tended to extend, then to be entangled with neighboring chains in pure DMAc solvent23, and its corresponding Dh at RT was 248.5 nm. However, Figure 6A showed that the Dh of micelles significantly increased due to the formation of large aggregates (incipient precipitation) when H2O was taken as single additive [4]. While the Dh of micelles decreased when Tween 80 was chosen as single additive, which contributed to the further enhancement of lowly coiled state and better entangled with neighboring chains of PVDF caused by the interconnection between reverse micelle and polymer. Besides, the Dh of micelles increased when further addition of water as mixture additive with Tween 80. Pure PVDF and PVDF-H2O species with 60ºC were detectable with Dh ranging from a few nanometers to 2 μm, indicating that the large aggregates formed at high temperature and the two systems were not in a thermodynamic equilibrium state [5]. The slow increased Dh of micelles of 76:3:3:1 system at 60ºC revealed the improved thermodynamic stability of PVDF-DMAc-Tween 80-H2O system when compared with other samples. Reason behind that was the interaction between polar head of surfactant and water, providing a balance resistance to the interconnection between PVDF and hydrocarbon chains of surfactant.
For polymer-surfactant solution in terms of micelles size to intensity signal showed the micelle structures in solution were very complex when compared with Figure 5, and both polymer aggregate and micelle structure existed in solution. Mainly two peaks can be observed in Figure 6, the narrower intensity signal of a few hundred nm scale corresponding to the micelle structure, the other corresponding to the aggregated structure of PVDF themselves or mixed with separated surfactant molecules.
Above data about the morphology solution of different composition revealed that the interaction between water and surfactant as well the interconnection between hydrocarbon chains of reverse micelle and polymer played an important role in the micro-structure adjustment of the solution. Such change surely would be embodied in the membrane formation process.
Figure S3. Intensity-weighted particle size distribution of 1.0 wt.% PVDF in different composition solvents at RT (A) and 60ºC (B)
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