Membrane characterization was performed to establish the morphology of all membrane types using scanning electron microscopy (SEM, Hitachi Co, S-800, Japan) with voltage acceleration at 20 kV, with a convergence angle of -30°, beam current x. 6.09 mm; y.5.79 mm, and diameter of the beam of 0.006 nm. Before the scanning electron microscopy (SEM) test, the membrane was soaked in liquid nitrogen for around five minutes, after the membrane turned rigid. This treatment was performed to prevent any change in the membrane structure when placing the sample on the SEM plate. The membrane was dried in freeze-drier overnight. One piece of the membrane was taken randomly and mounted on the sampling tubs (Hitachi, 10-004139-1, stubs Ø25 x 16mm x M4, 2 x 90°) wrapped with carbon tape, and then coated with osmium coater (Neoc-STB, Meiwafosis Co., Ltd., Japan) sputtering. The sample was placed on the SEM panel for the picture-capturing process at 100 and 3000 magnification for whole and enlarged cross-section, respectively. Captured images were processed using Irfan View v.3.45

The ultrafiltration module was designed using a single membrane module as previously described ¹⁰ . Ultrafiltration module consisted of a peristaltic pump (Watson Marlow, SciQ 323), a circuit of hose for feed flow, two pressure gauges (WIKA 213.53), valve, and hollow fibre membrane installed in the circuit. It was also equipped with feed, permeate, and retentate container.

The filtration of pure water and BSA solution was conducted by flowing the feed solution to the membrane using a peristaltic pump (Watson Marlow, SciQ 323). For pure water flux measurements, filtration was carried out at an operating pressure of 0.3; 0.5; 1.0; and 1.5 atm. Three pieces of the membrane were tested at every operating pressure by repeating three times. The permeate was collected every 10-minute filtration, and its weight was measured by an auto digital balance (Shimadzu, Japan; ATY224). The received data of permeate weight were then converted in a volume unit. The average of flux at each operating pressure is obtained from 9 times the filtration process. BSA solution filtration process were carried out for 120 minutes at a constant operating pressure of 0.5 atm. BSA solution was made at a concentration of 500 ppm at constant pH of 4.5. The permeate volume of BSA filtration was also collected every 10 minutes. The average permeability data were obtained from 3 measurements of each membrane.

Pure water flux and permeability of albumin solution were measured with Equation 1) and Equation 2) ¹¹ . The filtration process is carried out at several operating pressures, which are 0, 5; 1.00; and 1.5 Atm.

Flux = V A . t ( 1 )

Permeability = V A . t . P ( 2 )

V is a permeate volume, A is the membrane surface area, t is filtration time, and P is operational pressure. BSA concentration in permeate and retentate was then analyzed using a spectrophotometer. Rejection capability of the membrane can be determined using the equations as follows ¹¹ :

Rejection = C F - C p C F × 100 % ( 3 )

C _F and Cp are protein concentration in sample and permeate analyzed by a spectrophotometer UV-Vis (Shimadzu UV-1800) at a wavelength of 278.0.

SEM images of the two membrane are shown in Figure 1 (see underlying data ¹² ). The morphological structure of membrane of the inner and outer surface were recorded with SEM at 3000x magnification as shown in Figure 1b and Figure 1c. Differences in pore structure and macrovoid pattern are clearly visible. The UFT304 membrane had larger pores with greater quantity of longer macrovoids. This likely effected ultrafiltration process of BSA protein as discussed below. Figure 1b and Figure 1c also shows a large sponge structure in the middle of UF0 membrane which is absent in the UFT304 membrane. The increase in number and size of macrovoid was caused by the addition of polymeric surfactant. During the solidification process of the membrane in the coagulation batch, part of the additive leached out of the polymer system. Therefore, a large macrovoid structure was formed.

One important parameter for the membrane performance is its capability to filter solutions, referred to as flux or permeability. Another important parameter is the capability of membrane to remove components or particles from the solution, commonly known as solute rejection. In this study the ultrafiltration capability of the membrane was tested using a single membrane of hollow fibre (single module) designed with pressure driven inside (PDI) flow. The filtration profile of the albumin protein solution using UF0 and UFT304 membranes at various operational pressures is shown in Figure 2a (underlying data ¹³ ). The flux of UFT304 membrane at the beginning of this experiment with an operating pressure of 0.3 atm was 63.60 L/m ².h. Flux increased significantly with the increase of the operating pressure which reached 161.00 L/m ².hour at a pressure of 1.5 atm. The flux of UF0 membrane at the starting point (operating pressure 0.3 atm) was much lower, 32.0 L/m ².h. At the highest operating pressure (1.5 atm), the flux of UF0 membrane was about 53.0 L/m ².h.

The difference of flux between the two membranes is likely due to the differences in porosity. Tetronic 304 is a polymeric additive which acts as a pore-forming media. This additive is usually called the membrane-modifying agent (MMA). The increase of membrane pore size with blending MMA into the polymer solution is one of the easiest methods to improve membrane performance fabricated by the phase inversion method ¹⁴ .

The rejection capability of albumin solution of the two membranes is shown in Figure 2b (underlying data ¹⁵ ). Albumin rejection by UF0 membrane >90% was achieved at 0.5 atm. Increasing the operating pressure resulted more albumin particles slipping into the permeate section. For all of the tested operating pressures the rejection of albumin solution by the two membranes remained above 80%.

Filtration stability is the capability of a membrane to filter solute per time unit and operating pressure. This test corresponds to the durability of the membrane (lifetime). The result of stability test of the membrane on BSA filtration using UF0 and UFT304 membranes at 0.5 atm are shown in Figure 3 (underlying data ¹⁶ ).

The permeability of the UFT304 membrane at the initial operating pressure of 0.3 atm was 90.75 L/m ².h.atm. Meanwhile, the permeability of UF0 membrane at 0.3 atm much lower, 14.38 L/m ².h.atm. The performance of filtration in these two membrane types was closely related to the morphological structure of the membrane as shown in Figure 1. Compared with the UF0 membrane, the UFT304 membrane had a larger pore structure with a higher porosity. The structure of finger-like macrovoid was also longer, and there was no sponge structure in the middle of membrane section. This difference of pore structure likely resulted in the UFT304 membrane’s higher filtration capability.

Along with longer filtration time, the permeability of these two membrane types decreased due to the fouling in membrane pore surface. However, the permeability of UFT304 membrane was still at 5.60 L/m ².h.atm after 120 minutes of filtration. In contrast the permeability of the UF0 membrane which was almost 0. The filtration profile of UFT304 membrane is more stable than UF0 membrane. The stability of filtration for UFT304 membrane was closely related to the hydrophilicity property of this membrane. UFT304 membrane was more hydrophilic with water contact angle of 62.0°, compared with UF0 membrane with water contact angle of 74.0°. The hydrophilic membrane has the capability to hold the fouling better than hydrophobic membrane ¹⁷ .

The capability to reduce fouling in this hydrophilic membrane is related to the interaction force between membrane surface and solution particle (protein). The content of polyethylene oxide (PEO) chain in the Tetronic additive becomes a block (steric repulsion) for protein molecules, preventing them reaching the membrane surface ¹⁸ . Thus, the protein molecule unlikely to attach the membrane surface. Therefore, the fouling process can be minimized.

Figshare: Supplementary data 1. https://doi.org/10.6084/m9.figshare.8109335.v1 ¹³

This project contains the following underlying data:

Figshare: Supplementary data 2. https://doi.org/10.6084/m9.figshare.8109332.v3 ¹⁵

This project contains the following underlying data:

Figshare: Supplementary data 3. https://doi.org/10.6084/m9.figshare.8109338.v2 ¹⁶

This project contains the following underlying data:

Figshare: SEM images. https://doi.org/10.6084/m9.figshare.8215862.v2 ¹²

This project contains the following underlying data:

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).