Abstract layer via interfacial polymerization (IP) of m-phenylenediamine (MPD)

Abstract

Polyamide (PA) thin
film composite (TFC) reverse osmosis (RO) membranes with high permselectivity
and excellent mechanical/chemical durability were prepared using porous
polyethylene (PE) supports. Although the uniform pore structure and high
surface porosity of the PE support are beneficial for enhancing membrane
permselectivity, its intrinsic hydrophobicity makes the formation of a PA selective
layer challenging. The oxygen plasma pretreatment on the PE support, combined
with the use of a sodium dodecyl sulfate surfactant during interfacial
polymerization, enabled the production of a PA layer on top of the support by
improving its wettability. The systematic optimization of the membrane
fabrication parameters (e.g., plasma
pretreatment, monomer and SDS compositions and post-heat treatment) led to the
formation of a highly permselective PA layer. The fabricated PE-supported TFC (PE-TFC)
membrane showed ~30% higher water flux with ~0.4% enhancement in NaCl rejection
compared to a commercial RO membrane. In addition, the PE-TFC membrane
exhibited mechanical properties and organic solvent resistance superior to the commercial
membrane, which is attributed to the excellent mechanical and chemical stability
of the PE material. The proposed strategy could expand the application of RO
membranes to mechanically and chemically harsh operating environments.

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Keywords: reverse
osmosis; thin film composite membrane; polyamide; interfacial polymerization; polyethylene
support

 

 

1. Introduction

Over
the past few decades, polyamide (PA) thin film composite (TFC) membranes,
consisting of a topmost PA selective layer on a porous polymer support (~100 mm
in thickness), have been extensively used as commercialized reverse osmosis
(RO) membranes for desalination and water treatment owing to their high
permselectivities 1-3. PA TFC membranes are fabricated by
forming a PA selective layer via
interfacial polymerization (IP) of m-phenylenediamine
(MPD) and trimesoyl chloride (TMC) dissolved in two immiscible solvents on a
porous support 4,5. For the TFC membrane, the support determines its chemical
and mechanical stability while the PA selective layer mainly controls its separation
performance 6. Although each membrane component can be independently chosen, the
structure and chemistry of the support have a critical influence on the structure
and thus separation performance of the formed PA layer, thus posing constraints
on the selection of the support material 7,8. For example, it is known that a
support having moderate hydrophobicity is favorable for fabricating a highly permselective
PA layer 9, which is consistent with the fact that commercial RO membranes
are fabricated mostly using polysulfone (PSF) or polyethersulfone (PES) as a support
material 10. However, PSF or PES-supported TFC membranes have a technical
limitation when applied to harsh environments due to their relatively low
mechanical strength and poor chemical stability (or organic solvent resistance)
11,12. To improve the mechanical and chemical durability of TFC membranes, various polymer materials
including polypropylene (PP) 13-15,
polyacrylonitrile 16,17, polyvinylidene fluoride 12,18, poly(tetrafluoroethylene)
19, polyimide 20 and sulfonated polyphenylsulfone 21 have been explored
as supports.

Among
them, PP, one of common polyolefins, has been considered a strong candidate for
such supports owing to its high mechanical strength and excellent chemical
durability. Although its strong hydrophobicity, one of the obstacles in
fabricating TFC membranes, was resolved by adopting hydrophilic modifications
such as plasma or chemical pretreatments 13-15,
its irregular slit-like pore shape
(an aspect ratio of ~2.8), unevenly
distributed surface pores and low surface porosity (~11%) stood in the way of attempts
to fabricate a defect-less and permeable PA layer. In fact, it has been
reported that the fabricated PP-supported TFC membrane had a very low
water flux and NaCl rejection (~87%) far below RO performance level due to its
unfavorable support pore structure 13.

Polyethylene
(PE) is another class of polyolefins and exhibits remarkably high mechanical and chemical durability,
like PP. PE has been manufactured into a porous membrane by sequential
processes consisting of melt extrusion, mechanical stretching and diluent
extraction, unlike PP membranes whose pore structures are formed by the simple
mechanical stretching of an extruded film 22. As a result, the porous PE membrane has higher porosity
with a more regular pore shape than the PP membrane and thus has been successfully
commercialized as a lithium ion battery
separator 23,24. This PE membrane also has beneficial structural features as
a TFC membrane support together with its inherently strong mechanical and
chemical stability. Its relatively uniform pore structure and high surface
porosity could facilitate the formation of a uniform and highly cross-linked PA
selective layer via the IP process 25.
In particular, its high surface
porosity could improve the mass transport at the PA layer-support interface, thereby
enhancing the membrane water flux 8. In addition, its highly interconnected, open
pore structure could further improve membrane permeation 26-28.

Despite many potential advantages, the PE
membrane has not been explored as a support material for the RO membrane. In
this work, for the first time, we demonstrate that the commercialized, porous
PE membrane can be used as a support to fabricate highly performing,
mechanically strong and chemically durable RO membranes via conventional IP. The fabrication parameters, including the support
pretreatment, monomer and additive compositions and post-heat treatment, were systematically
controlled to achieve high separation performance of the PE-supported TFC (PE-TFC)
membrane. The fabricated PE-TFC membrane exhibited ~30% higher water flux with ~0.4%
enhancement in NaCl rejection compared to a commercial RO membrane (SWC4+). In
addition, it was demonstrated that the PE-TFC membrane has superior mechanical
strength and organic solvent resistance compared to the commercial counterpart
by characterizing the mechanical and chemical stability of the membranes. The
structures and physicochemical properties of the prepared TFC membranes were
comprehensively characterized using various analysis tools including scanning
electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform
infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and
contact angle measurement and correlated to their separation performance to
understand the membrane structure-property-performance relationship.

 

2. Materials and
methods

2.1. Materials

Trimesoyl chloride
(TMC, 98.0%, Tokyo Chemical), m-phenylenediamine
(MPD, 99.0%, Tokyo Chemical), sodium dodecyl sulfate surfactant (SDS, 99.0%, Sigma-Aldrich), n-hexane (95.0%, Daejung Chemical), sodium chloride (NaCl, 99.0%, Samchun Chemical),
dodecane (99.0%, Sigma-Aldrich), tetrahydrofuran (THF, 99.0%, Daejung Chemical)
and toluene (99.5%, Daejung Chemical) were purchased and used without purification. De-ionized
(DI) water (18.2 M? cm) was supplied
from a Millipore Milli-Q purification system. A PE support (~20 mm
in thickness) and a commercial RO membrane (SWC4+) were received from SK
Innovation Co., Ltd. and Hydranautics/Nitto Denko, respectively.

 

2.2. Membrane
fabrication

The fabrication
process of a PE-TFC membrane is depicted in Fig. 1. High hydrophobicity of the pristine PE
support whose water
contact angle is ~120° prevents
the support pores from being impregnated with an MPD aqueous solution 25. In
addition, the intrinsically weak chemical interaction between the PA layer and
the support due to the absence of polar functional groups on the pristine PE hampers
the formation of a TFC membrane 13,14. Thus, the PE support was pretreated
with O2 plasma prior to IP to improve support hydrophilicity (water
wettability) and to enhance the PA-support interfacial adhesion by generating oxygen-containing
polar functional groups on the support 13,14. It has been reported that the O2
plasma process can effectively modify porous polyolefin (PP and PE) membranes
to increase their hydrophilicity 29,30. The support hydrophilicity was systematically
controlled by adjusting the plasma exposure time (0 ~ 300 s) at the fixed
plasma power of 20 W and operating pressure of 0.09 kPa using an oxygen plasma
treatment system (UVFAB systems, CUTE-MPR) to find the optimum treatment condition
that can result in the best separation performance of the prepared TFC
membrane. We selected the plasma power of 20 W because the power higher than 20
W tended to significantly deform the PE support structure. Unfortunately, the plasma
treatment alone was found unable to completely wet the support pores with the
MPD solution, failing to form a uniform PA layer, presumably because the support pores cannot be uniformly
modified with a one-sided plasma dose. Hence, a SDS surfactant was used during
the IP process to ensure the complete wetting of the support with the MPD
solution by reducing the support-water interfacial tension. This was evidenced
by the observation that the
plasma-treated PE support can be completely soaked into the MPD solution only
with the use of SDS (see Supplementary Material S1) and has a lower contact
angle with a SDS/water mixture than that with pure water (see Supplementary
Material S2). The plasma-treated PE
support was mounted on a glass plate with a silicone frame. Next, an MPD (0.5 ~
7.0 wt.%) aqueous solution containing SDS (0.03 ~ 0.2 wt.%) was poured on the plasma-treated PE support.
After 10 min, the MPD solution was decanted, and the excess MPD solution was carefully
removed with a rubber roller. Subsequently, a TMC (0.05 ~ 0.4 wt.%) solution in n-hexane was immediately poured on the MPD-impregnated
support and allowed to react at room temperature for 60 s, which is sufficient
to ensure the complete reaction and thus the maximized NaCl rejection (see
Supplementary Material S3). Then, the membrane was rinsed with pure n-hexane to terminate the IP reaction and
remove the unreacted TMC. The prepared PE-supported TFC (PE-TFC) membrane was
dried at 70 °C for a certain time period (0 ~ 15
min) and stored in DI
water prior to the use.
  

 

Fig.
1. Schematic illustration of the fabrication of the PE-TFC
membrane via the IP process.

 

2.3. Membrane characterization

The surface and
cross-section images of the PE supports and the PE-TFC membranes were collected
with scanning
electron microscopy (SEM, FEI Inspect F50) under an accelerating voltage of 5 kV. The surface
pore size and porosity of the PE support were estimated from its SEM surface
image. To quantify the surface porosity, the obtained SEM surface images of the
support were converted to black-and-white formats using an imageJ program. The
average surface porosity of the support was calculated from the percentage of
the area occupied by black dots corresponding to the pores (see Supplementary
Material S4). The overall porosity of the PE support was measured using the gravimetric method proposed by others
25,31,32. The PE support was soaked into a bath containing a wetting solvent
(dodecane) for 12 h
and then removed from
the bath. The excess solvent on the PE support was immediately removed using
tissue paper. The average overall porosity (?,
%) of the PE support was calculated by measuring the weights of the dry (mdry) and wet (mwet) PE support with the densities of the PE (?m = of 0.97 g cm-3)
and wetting solvent (?w = 0.75
g cm-3) as given by,

The arithmetic
average (Ra) roughness of the prepared
PA selective layers was estimated from the topographical surface images (5 mm
´
5 mm)
obtained using atomic force microscopy (AFM, NanoScope 5, Veeco) in a tapping
mode. At least three different regions were scanned to obtain the average roughness
value for each sample. The chemical properties of the PE supports and PE-TFC membranes were characterized using
Fourier transform infrared spectroscopy (FT-IR, Spectrum
Two). The chemical structure near the PE support surface was analyzed using
X-ray photoelectron spectroscopy (XPS). XPS spectra were collected using a PHI
X-tool system with monochromatized Al-Ka
radiation at 1.49 keV. The water contact angles of the PE supports and PE-TFC
membranes were measured by the sessile drop method using a contact angle
measurement system (Phoenix-300, SEO Corporation).

The membrane
mechanical properties were characterized using a universal tensile testing
machine (UTM, H5KT, Tinius Olsen). A membrane sample (length ´
width = 50 mm ´ 10 mm) was clamped at both ends with an initial gauge length of 30
mm and then stretched with a constant
cross head speed of 20 mm min-1 to obtain the strain-stress curve. The
key mechanical properties of the membrane, tensile strength, elongation at
break and Young’s modulus, were determined from the strain-stress curve. At
least five measurements were carried out to obtain the average values.

 

2.4. Membrane performance
evaluation

Water flux and NaCl
rejection of the prepared PE-TFC membranes were evaluated using a cross-flow
filtration apparatus, where the circular flat sheet membrane coupon of 4 cm in
diameter (an effective membrane area of 14.5 cm2 (A))
was mounted (see
Supplementary Material S5). Separation
performance
was evaluated
using a NaCl
aqueous solution (2,000 mg L-1) at an operating pressure of 15.5 bar,
an operating temperature of 25.0 ± 0.5 °C
and a flow rate of 1 L min-1
(a corresponding cross-flow velocity of
52.7 cm s-1). Performance data
were collected after 12
h, when
the membrane water flux reached
a steady state. The water flux (Jw,
L m-2 h-1) was calculated by measuring the total
volume
of the permeate (?V)
through the predetermined
membrane area for a certain time period
(?t),
as given by,

NaCl rejection (R, %) was calculated
by measuring the NaCl salt concentrations
of the feed (Cf) and
permeate (Cp) solutions
using a conductivity meter (Cond 730P,
INOLAB), as given by,

 

2.5. Organic solvent
resistance assessment

Organic
solvent resistance?r each sampleerty ne btained from the strain-stress
curve.  of the membranes was assessed by monitoring performance
changes before and after exposure to two organic?lvent resistance assessmentrations of MPD and
SDS  structure solution systems, an THF (20
wt.%) aqueous solution and pure toluene, according to the previous reported method
33. THF and toluene were selected as representative water-miscible and
immiscible organic solvents, respectively. The membranes were immersed in each
organic solution system at 25 °C for
10 min, and then thoroughly rinsed with pure n-hexane and water. The separation performance of the membrane before
and after the solvent treatment was measured and compared to assess its organic
solvent resistance. 

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