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Efforts have been made in thermally induced phase separation (TIPS) process to modify the morphology of polyvinylidene fluoride (PVDF) membranes in order to improve their permeation performance and mechanical properties. Nevertheless, many methods not only altered the outer surface but also impacted the overall membrane structure, resulting in a trade-off between permeability and mechanical properties. In this study, we utilized a modified TIPS process to refine the outer surface morphology without altering the bulk structure. This was achieved by introducing NaCl in the coagulation bath. The PVDF membranes were fabricated using a dope with water insoluble diluent dimethyl phthalate (DMP) as main solvent and water-soluble additives polyethylene glycol 400 (PEG400)/triethylene glycol (TEG) as pore formers. The inclusion of NaCl in the coagulation bath serves to decrease the solubility of PEG within this medium, owing to the salting-out effect. Consequently, the NaCl concentration in the coagulation bath emerges as a crucial factor in regulating the migration of PEG400 toward the membrane surface. This control mechanism facilitates the precise adjustment of the outer surface morphology in the fabrication of membranes. As the NaCl concentration increases in the coagulation bath, the outer surface of the fabricated membrane transited from a mesh-like structure to a spherulite structure. As 0.5 mol L−1 NaCl was added to the coagulation bath, the membranes displayed a pure water permeability of 1073.9 L m−2 h−1 bar−1 while maintaining a narrow pore size distribution. Compared to the membranes fabricated without NaCl addition, the increment of the PWP contributed to the slight increase in mean pore size from 65 nm to 84 nm. Meanwhile, the water-insoluble diluent DMP was not affected by the addition of NaCl, which means that the bulk structure of the membrane could be maintained. Consequently, the increase in permeability did not compromise the mechanical properties of the membranes. All the membranes fabricated in this study maintained a reasonable tensile strength of approximately 3 MPa. This study introduces a simple and environmental method to increase the permeability effectively and fine-tune the pore size of the TIPS membranes while having little effect on the bulk structure. Furthermore, the study provides valuable insights into how changes in outer surface morphology can impact the pore size and permeability of TIPS PVDF membranes. [Display omitted] • Ultrafiltration PVDF membranes was fabricated by a modified TIPS method. • NaCl coagulation bath could control the outflow of PEG toward membrane surface. • The surface of the membranes could be fine-tuned without altering the bulk structure. • The ultrafiltration membranes present a narrow pore size distribution and high permeability. [ABSTRACT FROM AUTHOR]

The use of amphiphilic pore formers and hydrophilic non-solvents has demonstrated its effectiveness in improving the surface porosity and permeability of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes through the nonsolvent induced phase separation (NIPS) process in literatures. However, in the thermally induced phase separation (TIPS) process, few reports illustrated how the amphiphilic pore-former and water-soluble non-solvent interact with water-insoluble diluents. In this study, water-insoluble dimethyl phthalate (DMP) was used as the main diluent to rule out the effect of diluent outflow. The amphiphilic pore former polyethylene glycol 400 (PEG400) failed to open pores on the outer surface of the membranes because of its amphiphilic nature. On the contrary, UF membranes with a mean pore size of 55 nm could be obtained by adding hydrophilic triethylene glycol (TEG) to the dope with a satisfactory pure water permeability (PWP) of 262.6 L m−2 h−1 bar−1. Moreover, PEG400 can serve as a stabilizer for the TEG droplets during the phase separation if both were added to the polymer dope solution. The addition of both PEG400 and TEG could significantly increase the membrane's PWP to 645.4 L m−2 h−1 bar−1 with moderate enlargement of the mean pore size of the membrane to 84 nm. Additionally, the well-connected membrane structure resulted in the tensile strength of the membranes ranging from 3.07 to 6.65 MPa. This study expands the range of additives used in the TIPS process and demonstrates the different roles of amphiphilic and hydrophilic non-solvents in the TIPS process. [Display omitted] • The effects of hydrophilic and amphiphilic non-solvent in the TIPS process are analyzed. • Hydrophilic non-solvent TEG could tailor the surface morphologies of the PVDF membranes. • Amphiphilic PEG failed to open pores on the outer surface of the PVDF membrane. • Amphiphilic PEG served as the stabilizer of TEG in the PVDF dope, increasing the porosity and pore size of the membranes. • The resulting PVDF membranes sit within the UF range, presenting narrow pore distribution and high pure water permeability. [ABSTRACT FROM AUTHOR]

Abstract The combination of nonsolvent and thermally induced phase separation (N-TIPS) method has shown promising ability in harvesting the features from the nonsolvent induced phase separation (NIPS) and thermally induced phase separation (TIPS) processes for developing membranes with a tailorable surface pore structure. However, previous approaches have been subjected to either the formation of macrovoids or dense layer due to the dominant NIPS effect, or required sophisticated instruments and operation skills. In this work, a facile attempt was carried out to fabricate novel polyvinylidene fluoride (PVDF) hollow fiber membranes with tunable surface pore structure while maintaining the narrow pore size distribution and mechanical strength. A modified N-TIPS method was developed by using mixed diluents: dimethyl phthalate (DMP) as a water-immiscible poor solvent for TIPS process, and triethyl phosphate (TEP) as a water-miscible neutral solvent to bridge the TIPS and NIPS processes. To further control the membrane formation especially near the membrane surface, an amphiphilic additive Pluronic F127 was also added as a potential pore-former and surface hydrophilicity modifier. PVDF hollow fiber membranes with a highly porous structure and a narrow pore size distribution were successfully synthesized by using TEP and Pluronic F127 in the N-TIPS process. The mechanism of N-TIPS process was thoroughly discussed. The water permeability of the membrane increased significantly from 389 ± 30–922 ± 36 L m–2 h–1 bar–1, with overall porosity improved from 50 ± 2.2–69 ± 2.9%, and a mean pore size of ~ 0.18 µm. The membranes produced by N-TIPS method also exhibited a good tensile strength ranging from 5.6 ± 0.1–6.5 ± 0.2 MPa, showing a great potential for a broad range of water applications after further modifications. Besides, the formation of piezoelectric β-phase crystals of the PVDF membrane was observed when the mixed diluent was used, which sheds light on the possible applications of resultant membranes in electrochemical separation process. Highlights • PVDF hollow fiber membranes were developed via a modified N-TIPS method using mixed diluents together with Pluronic F127. • The NIPS effect could be induced at the outer surface with the co-occurrence of TIPS process along the bulk of membrane. • The surface pore structure of membrane could be tailored without the formation of mechanically weak macrovoids. • The prepared membranes possess a hydrophobic surface with good porosity, permeability and mechanical strength. [ABSTRACT FROM AUTHOR]

The use of amphiphilic copolymer Pluronic F127 as an additive has shown effectiveness in fabricating polyethersulfone (PES) membranes with excellent antifouling properties due to its roles in enhancing pore structure and surface hydrophilicity. However, F127 was found to be unstable in polyvinylidene fluoride (PVDF) membranes as its hydrophilic modifying function was deactivated over time. In present work, we developed a novel approach to immobilize F127 in PVDF hollow fiber membranes using polytetrafluoroethylene (PTFE) particles as a binding agent through the combined nonsolvent and thermally induced phase separation (N-TIPS) method. The results suggest that the hydrophobic segment of F127 could adsorb firmly onto PTFE with the hydrophilic segments protruding outwards. The dual-functions of F127 were observed in pore formation and surface hydrophilization for PVDF membranes. The water contact angle of PVDF/PTFE/F127 membranes decreased from 102 ± 4° to 76 ± 3° compared with membranes without additives. The resultant membranes possess a pure water permeability (PWP) of 869 ± 39 L m−2h−1bar−1 with a mean pore size of 0.09 ± 0.01 μm and an outstanding tensile strength of 7.0 ± 0.3 MPa, suggesting the potential of N-TIPS method for tuning the membrane pore structure and hydrophilicity by using multifunctional additives. Image 1 • PTFE or Pluronic-F127 could effectively help tailor bulk and surface properties of PVDF membranes through N-TIPS method. • The instability of F127 in PVDF membranes was significantly relieved in the presence of PTFE. • The mediating effect of PTFE allowed F127 to be a pore-former and surface hydrophilic modifier for PVDF membranes. • The prepared PVDF/PTFE/F127 membranes possess a narrower pore size distribution. • The membrane has improved porosity, tensile strength, hydrophilicity and antifouling property. [ABSTRACT FROM AUTHOR]