Enhanced morphology and hydrophilicity of PVDF flat membrane with modified CaCO₃@SMA additive via thermally induced phase separation method

Highlights

• A novel modified CaCO₃@SMA nanoparticles as additive were fabricated.

• The dispersion of modified CaCO₃ nanoparticles in PVDF membrane were improved.

• The PVDF membrane morphology and tensile properties were improved by modified additive.

• After pickling, the PVDF membrane water flux and hydrophilicity were enhanced.

Abstract

Modified CaCO₃@SMA nanoparticles obtained by coordination reaction between poly(styrene-co-maleic anhydride) (SMA) and calcium carbonate (CaCO₃) nanoparticles were adopted as additive to prepare polyvinylidene fluoride (PVDF) flat membrane via thermally induced phase separation (TIPS) method with dioctyl phthalate (DOP) and dibutyl phthalate (DBP) as mixed diluent. The CaCO₃ nanoparticles modified by SMA effectively reduced the adverse agglomeration of nanoparticles and made the additive disperse evenly in PVDF matrix. The presence of modified CaCO₃@SMA changed the membrane morphology from dispersed spherulites with large pores to fuzzy dendrite structures with uniform pore sizes. The membrane pore sizes, pore size distribution and tensile strength were significantly improved compared to both virgin membranes and those containing unmodified CaCO₃ nanoparticles. After the CaCO₃ was pickled, the porosity and the connectivity between membrane pores were greatly enhanced, resulting in a significant increase in pure water flux. At the same time, the amphiphilic SMA fixed in and on the membrane surface improved the hydrophilic and anti-fouling properties demonstrated in a three-cycle test. The present study provided a potential TIPS method for the fabrication of PVDF membrane combined with a simple strategy of modified inorganic particle additive.

Introduction

Membrane technology is now widely used in various separation industries. It has achieved remarkable achievements in addressing the challenges of resource and energy shortages, due to its high efficiency, energy-saving, high modularity, ease to operate and integration with other processes [1], [2], [3]. In the rapid expansion application of membrane technology, microporous membrane accounts for more than 50%, mainly including microfiltration membranes (MF), ultrafiltration membranes (UF) and membrane bioreactor (MBR), with market size of over $14 billion in 2018 [4], [5]. In order to prepare microporous membranes with excellent properties, the determination of preparation method and the selection of membrane materials are the first issues to be considered.

Phase inversion plays a dominant role in the methods of preparing microporous membranes, which mainly includes non-solvent induced phase separation (NIPS) and thermally induced phase separation(TIPS) [6], [7]. Unlike NIPS based on mass transfer, the membrane prepared via TIPS, which relies on heat transfer, has the advantages of high mechanical strength, narrow pore size distribution and less controlling factors of microstructure [7], [8]. Therefore, as an advanced membrane preparation method, TIPS has attracted more and more researchers' attention [3], [9], [10], [11]. It is recognized that the TIPS method generates three kinds of pore structures according to the compatibility difference between the polymer and diluent: spherulite structure via solid–liquid (S-L) phase separation, partially closed cellular pore structure via liquid–liquid (L-L) phase separation, and bi-continuous structure via spinodal decomposition (SD) [3], [12]. The bi-continuous structure is considered optimal for water treatment due to its high connectivity of pores and strong interactions between polymer molecules. However, the formation of membrane pores in the phase separation process is usually the synergism of multiple mechanisms. In other words, even though the bi-continuous membrane pores are mainly formed, the cellular and spherulite structures are also inevitably produced, resulting in the decrease of effective porosity and mechanical strength [12], [13], [14]. Some efforts have addressed these problems by adding inorganic particles during membrane preparation and removing them in post-treatment. N. Awanis Hashim et al. prepared PVDF hollow fiber membrane with silicon dioxide (SiO₂) particles as additive and then dissolved out with lye solution, which significantly increased the connectivity between membrane pores and membrane permeability [15]. The U. S. patent has also obtained a three-dimensional network structure PVDF hollow fiber membrane by TIPS through a similar approach [16]. B. J, Pan et al., prepared hollow fiber membrane with high porosity and permeability by adding CaCO₃, which was removed by acid in post-treatment via TIPS method [17]. Since PVDF is a semi-crystalline polymer, the nanoparticles also act as heterogeneous nucleating agents in the phase separation process. However, the severe disadvantage of adding inorganic nanoparticles without modification is the agglomeration of particles, limiting the amount of inorganic particles added and leading to the formation of defect pores and the reduction of mechanical properties. When inorganic particles are directly added to the organic membrane matrix, the surface energy of the particles is high and in a thermodynamically unstable state, which makes it easy to agglomerate. In addition, the ions on the surface of inorganic particles can be replaced by chemisorbed water to form a hydrophilic surface containing hydroxyl groups with strong polarity, which makes it difficult for inorganic particles to disperse evenly in organic matrix. The lack of binding force between inorganic particles and the organic matrix leads to interface defects [18], [19]. In addition, removing the additive silicon dioxide with lye reduces the membrane mechanical strength [20].

Although many polymers can be used to prepare microporous membranes, PVDF was the most widely used material due to its high thermal and chemical stability, considerable mechanical strength, and good processing properties [21], [22], [23], [24]. However, the inherent low surface tension, high hydrophobicity and crystallinity of PVDF materials make the membrane easily contaminated by organic matters (e.g., natural organic matters, proteins etc.) in the treated wastewater, resulting in blockage of membrane pores, increased operating pressure and decreased water flux [7], [25], [26]. It is generally believed that improving membrane hydrophilicity can reduce membrane fouling, because a pure water layer is easy to form on a hydrophilic surface to prevent the absorption and deposition of hydrophobic pollutants [27]. Therefore, except for membrane contactor processes (e.g., membrane distillation, acid gases absorption, etc.) that utilize the hydrophobicity of PVDF membranes [21], most wastewater treatment processes have to improve the hydrophilicity of PVDF membranes to reduce membrane fouling and extend membrane life-span [28], [29]. Various hydrophilic modification methods have been extensively studied, which can be mainly classified into two categories: blending modification and surface modification. The hydrophilic modification methods of PVDF membrane prepared by TIPS method were summarized in Table 1. Surface modification is mainly achieved by coating hydrophilic compounds or grafting hydrophilic groups (hydroxyl, sulfonic acid group, etc.) on the surface of the membrane [7]. However, the surface coating is easy to fall off in long-term operation, especially when the operating conditions are harsh or significantly changed (such as backwashing, pH change etc.) [30]. Surface grafting is difficult to be applied in membrane industrial production because of the complicated process which requires chemical or irradiation treatment [31]. Blend modification is mainly achieved by blending hydrophilic compounds or inorganic nanoparticles. Compatibility with PVDF material and uniformity of dispersion in membrane matrix are the significant challenges for hydrophilic compounds, while for inorganic nanoparticles, agglomeration and the resulting non-uniform distribution severely reduce the effect of hydrophilic modification [32].

Poly (styrene-co-maleic anhydride) (SMA) is an amphiphilic copolymer with hydrophobic styrene group and hydrophilic reactive anhydride group. The proper interaction between the styrene and the CO segment in SMA and fluorine in PVDF makes SMA have good compatibility with PVDF [8], [55]. At the same time, the hydrophilic anhydride group in SMA can improve the hydrophilic and anti-fouling properties of PVDF membrane. Nano-calcium carbonate is a kind of joint functional filler that is low price, non-toxic, and easy to obtain and remove by acid [17], [41]. CaCO₃ nanoparticles are expected to increase the connectivity of membrane pores and porosity of PVDF membrane after being pickled out, and so as to improve the water flux of the membrane. Moreover, calcium ion is an alkali earth metal element with an outer eight electron structure and is easy to coordinate with the oxygen atom of anhydride group in SMA to form a stable coordinate bond [56], [57]. The surface modification of CaCO₃ nanoparticles by SMA reduced the cohesion between particles, improved the dispersion and surface activity of CaCO₃ nanoparticles, and also the compatibility with the PVDF matrix.

In this work, modified CaCO₃@SMA nanoparticles coated with an SMA layer on CaCO₃ were fabricated successfully and employed to prepare PVDF/ CaCO₃@SMA composite membranes by TIPS method. The effects of modified CaCO₃@SMA nanoparticles contents on the phase separation process, anti-fouling property and mechanical strength of the composite membrane were investigated and compared with both virgin membrane and those containing unmodified CaCO₃ nanoparticles. The SMA in modified additive acted as both hydrophilic modifier for PVDF membrane and dispersant for CaCO₃ nanoparticles, while CaCO₃ nanoparticles as nucleating agent and pore former after being washed out by acid. After being modified by SMA, CaCO₃ nanoparticles changed from hydrophilic to hydrophobic, which resulted in a good dispersion in the PVDF matrix. At the same time, the porosity and connectivity of the PVDF membrane were improved after pickling. With the increase of CaCO₃@SMA contents, the hydrophilicity of the PVDF composite membrane was greatly enhanced, which was helpful to reduce the protein adsorption of the PVDF membrane significantly. To the best of our knowledge, no efforts have been reported on the modified CaCO₃ nanoparticles with SMA used as additive in the preparation of PVDF membranes by TIPS. This study aimed to explore a strategic and straightforward method to tailor the pore structure and improve the connectivity of membrane pores, thereby increasing the overall porosity and hydrophilicity of PVDF flat membranes prepared by TIPS method.