Membrane Materials Design Trends: Nanoadditives

INTRODUCTION

This chapter is concerned with membrane materials design trends with emphasis on nanoadditives. Generally, dope design incorporates key polymeric substrates such as PS. PES. PAN, PVC, and cellulosic esters. Alloying polymers are generally dissolved in appropriate solvents and their blends including, but not limited to, NMP, DMF, DMAC, THE, Acetone, and DMSO. Of course, dope formulation has witnessed numerous interesting advances. Three noteworthy advances achieve remarkable progress in membrane technology.

• • Functionalization of membrane surface enables chelation of desirable functionalities to achieve new targeted performance.
• • Balancing solvent and nonsolvent ratios in the dope enables control of permeability of modules and AFM patterns.
• • Membrane additives have been proven to introduce functional advantages and desirable morphological features. Of course, each additive is associated with its benefits and limitations, especially while consuming consortium of additives that need to be rationally adjusted in the context of intensive experimental work guided by appropriate modeling and simulation techniques. Perhaps the revolutionary role of nanotechnology has been particularly proven with the use of carbon nanotubes, graphene oxide, nano-inorganic oxides, and nanopolymers.

The main characteristics affected by the nanoadditives either positively or negatively include hydrophilicity, fouling, porosity, permeability, roughness, and certain mechanical properties.

Hydrophilicity

Water treatment by membranes necessitates a hydrophilic surface, which is achieved by adding nanoadditives directly or after functionalization of the nanoparticle’s surface before being added to the polymer solution.

Fouling

Researchers have reported that the antifouling properties of membranes increase significantly by the incorporation of nanoadditives.

Porosity

Most of the Nanoadditives tend to increase the porosity of the membranes.

Permeability

It has been reported that addition of the nanoparticles at low concentration causes pore plugging with a decrease in flux. However, as the particle concentration increases, surface hydrophilicity and mean pore size become dominant, leading to maximum flux, which is then reduced by further increasing the concentration as pore-plugging assumes the main role again (Rahimpour et al., 2012).

Rejection

Most researchers have reported that rejection does not significantly change on adding nanoparticles.

Roughness

It has been generally reported that nanomaterials cause a rougher surface, but this is not detected if the quantity of additive is small.

Mechanical Properties

The nanoparticles interact with the polymer matrix, leading to a substantial increase in rigidity and the energy required to break the polymer chain.

In the following sections, addition of specific nanoparticles is addressed. For each category, the preparation, effect on morphology and performance are presented for specific applications.

GRAPHENE OXIDE NANOADDITIVES

Graphene and its derivatives have been the focus of a huge number of studies, owing to their outstanding mechanical, physical, thermal, and optical properties, as well as their unique structures (Potts et al., 2011). Graphene oxide (GO) is one of the most important graphene derivatives and it is prepared by various methods, such as the Brodie (Brodie, 1859) and Hummers-Ofiferman methods (Hummers and Offeman, 1958). GO has a layered structure with a high aspect ratio and a negatively charged hydrophilic surface, making it an attractive solution to overcome many limitations in various applications (Wang et al., 2018).

GO has found huge potential in many applications, especially in membrane separation applications in which its incorporation successfully achieved favorable results in overcoming polymeric membrane limitations, as well as in introducing much-needed enhancements to their performance (Sears et ah, 2010).

There are different types of GO membranes, such as freestanding GO membranes (Cotet et ah, 2017), GO-modified composite membranes (Zahid et ah, 2018), and supported GO membranes (Ma et ah, 2017). Freestanding GO membranes are composed of layers of GO, only as the selective layer, while GO modified composite membranes are composite membranes where GO is incorporated in the membrane casting blend during the fabrication process (Jiang et ah, 2016). Finally. GO-supported membranes are membranes where GO is deposited on the surface of a substrate (Zhang et ah, 2017).

GO is characterized by SEM. EDX, AFM, FTIR. Zeta potential, and many other tests to identify surface morphology, elemental composition, surface topography, and surface charge (Ma et ah, 2017).

Preparation

The fabrication process of GO membranes is crucial to determining the efficiency and uniformity of the prepared membrane with good nanomaterial dispersion or deposition (Johnson et ah, 2015). GO membranes are prepared by various techniques, depending on the membrane type, mentioned previously. The most common techniques are layer by layer assembly (LBL) (Zhang et ah,

2016), casting/coating (spinning, drop casting, dip coating, spray coating) (Zahri et ah, 2016), filtration-assisted (vacuum and pressure filtration), and the evaporation-assembled method (Ma et ah, 2017).

Effect on Morphology

As shown in Table 2.1, (Huang and Feng, 2018) prepared PI/GO HF МММ via the phase inversion technique for pervaporation. They found that the morphological structure of the PI/GO membrane is similar to the pristine PI membrane, but with several differences, such as introduction of lateral pores that enhanced the membrane's performance, as well as much less denser pores and a uniform smooth surface. They confirmed the ability of GO to form porous membranes, which may be attributed to the hydrophilic nature of GO, and accordingly accelerates the rate of solvent and nonsolvent exchange during phase inversion. (Fahmi et al., 2018) prepared PES/GO flat sheet МММ for hemodialysis via phase inversion. GO incorporation once again yielded a smoother surface with the same cross-sectional morphological structure. (Alam and Ali, 2018) studied PES/GO HF membranes with different GO-loading content and found that the

Effect of GO nanoadditives on membrane morphology and performance.

morphological structure changed, and closely packed fingerlike structures were formed upon adding GO. but changed little upon increasing GO content; surface roughness of the PES/GO modified membranes decreased with closer ridges and valleys, however. (Shukla et ah, 2017) found that the incorporation of GO into PPSU membranes made the pores in the subporous layer of the membrane thinner, and also reduced the skin layer and rendered it reduced with denser pores. Conversely. AFM results confirmed the increase of surface roughness. (Zahri et ah, 2016) investigated the effect of GO incorporation on PSf HF membranes for C0₂ separation. Results showed that the morphological structure was affected as the development of fingerlike structure near the outer surface of the membrane was suppressed, while less open pores were observed on the membrane’s surface. (Goh et ah, 2015) fabricated PA1/PEI modified membranes via phase inversion, and GO was electrically deposited on the surface via dip coating. SEM results revealed GO nanosheets wrinkling on the membrane surface after GO deposition, as well as a rougher outer surface after PEI cross-linking. (Xia and Muzi, 2014) fabricated PVDF/GO membranes via phase inversion for natural organic matter (NOM) removal. Results showed that upon GO incorporation, the macrovoid layer increased, and a smoother surface with higher pore density was observed, which can be attributed to the formation of larger pores, owing to the increased mass transfer between solvent and nonsolvent resulting from GO hydrophilicity. Also, surface roughness decreased from 7.48 nm to 6.7 nm at 0.5 wt. % GO, leading to better antifouling properties. (Zinadini et ah, 2014) prepared PES/GO МММ via phase inversion. SEM images revealed that PES/GO have wider fingerlike pores, as well as lateral pore formation, while AFM results demonstrated a decreased surface roughness on GO loading, but as GO loading increases, roughness increases as well.

Effect on Performance

As shown in Table 2.1, (Huang and Feng, 2018) confirmed enhancement of PI/GO МММ desalination performance as a result of facilitated water diffusion in the membrane owing to increased hydrophilic sites in the modified membrane. (Fahmi et ah. 2018) studies of PES/GO also showed noticeable improvement in the membrane’s solute flux and rejection. (Alam and Ali, 2018) revealed that the water flux increased 36% on incorporating 0.5 wt. % GO into a PES membrane blend, along with increases in the MWCO and porosity, while these decreased upon increasing the GO content, which may be attributed to agglomeration of GO and bad dispersion. (Shukla et ah, 2017) found that best performance was achieved at 0.5 wt. % GO loading in a PPSU membrane matrix, with highest porosity as well as MWCO. resulting in increased flux and rejection. Also, fouling resistance improved by 58%. (Zahri et ah, 2016) observed an increase in C0₂ permeance by 14%, which is mainly a result of GO's high absorption properties toward C02 gas, as well as enhancement in CO2/N2 and CO2/CH4 selectivity by 158% and 74%, respectively for Psf/GO HF MMMs. (Goh et ah, 2015) found that GO deposition on PAI/PE1 modified membranes increased water permeability by 86%, while salt permeation decreased and salt rejection increased by 15%. These results confirm the effective action of GO nanosheets on the surface as a barrier restricting water flow. (Xia and Muzi, 2014) results demonstrated an increase of pure water permeability upon 0.5 wt. % GO incorporation in PVDF membrane, from 47 L/m2.h to 97 L/m2.h, which mainly results from the increased porosity and decreased contact angle caused by the hydrophilic groups in GO. (Zinadini et al., 2014) results demonstrated the increase of Water flux of PES/GO МММ as well as protein rejection was maintained above 98% for all GO incorporation schemes. This may be explained by the decreased contact angle from 65° to 53°, increased hydrophilicity as well as 0.5 wt. % GO loading had highest mean pore radius of 4.5 nm and porosity of 83.1% . Also 0.5 wt. % GO loading showed highest fouling resistance owing to decreased surface roughness.

Effect on Mechanical Properties

As shown in Table 2.1, (Huang and Feng, 2018) tested the Pl/GO HF МММ tensile strength and found that it decreased upon GO incorporation, which may be attributed to the formation of lateral pores, while the tensile modulus almost doubled owing, to good GO dispersion in the PI matrix. (Fahmi et al., 2018) studied PES/GO flat sheet МММ mechanical properties and found that the tensile strength and tensile modulus were enhanced, while the tensile strain was not significantly affected. (Alam and Ali. 2018) demonstrated the enhanced tensile strength of the PES/GO modified HF membrane upon adding 0.5 wt. % GO while the elongation-at-break decreased. (Shukla et al., 2017) achieved best mechanical enhancement 1 wt. % GO loading, owing to good interaction between GO and PPSU matrices. (Zahri et al., 2016) reported the increase of tensile strength by 4.36%, as well as increased elongation-at-break by 8.79% for PSF/GO HF MMMs, while the enhancement is not of true significance because mechanical properties were not sacrificed upon modification. (Goh et al., 2015) found that tensile strength decreased upon GO nanosheet deposition, while elongation-at-break increased.