Polyamide nanofilms synthesized: Via controlled interfacial polymerization on a "jelly" surface

Article abstract of DOI:10.1039/d0cc02555k

A thermal-sensitive "jelly"was used to control the diffusion of a diamine monomer for synthesizing polyamide free-standing nanofilms with an adjustable thickness of 5-35 nm. The reduced reaction rate of the interfacial polymerization at the hexane-"jelly"

Full text of DOI:10.1039/d0cc02555k

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Polyamide Nanofilms Synthesized via Controlled Interfacial
Polymerization on the “Jelly” Surface
Zhao-Yu Ma,a Xi Zhang,a Chang Liu,a Shun-Ni Dong,a Jing Yang*a, Guang-Peng Wua
Received 00th January 20xx,
Accepted 00th January 20xx
and Zhi-Kang Xu *ab
DOI: 10.1039/x0xx00000x
A thermal-sensitive “jelly” was used to control the diffusion of the applications of sacrificial layers from cadmium hydroxide
diamine monomer for synthesizing polyamide free-standing nanostrands,17 the addition of water-soluble polymer in the
nanofilms with an adjustable thickness of 5-35 nm. The reduced aqueous solution,18 and the usage of nanostructured
reaction rate of the interfacial polymerization at the hexane- interlayers from carbon nanotubes19 and cellulose
“jelly” interface made the synthesized nanofilms show high water nanocrystals20 as well as mussel-inspired coatings.21 PA films,
permeation flux and suitable salt rejection, which also have highly thicker than 100 nm in most of the cases and thus without
negative surface charges and fairly smooth surfaces.
desalination properties, were synthesized on the hydrogel
surfaces of poly(2-hydroxyethyl methacrylate) and/or
poly(acrylamide), which were facile to modify the surfaces of
(or encapsulate) hydrogels as suggested by Ding et al.22 The
original intention of all these methods is controlling the
release of diamine monomer for the interfacial polymerization.
Interfacial polymerization has been indispensably used for
synthesizing polyamide (PA) nanofilms, which have been
widely used as the selective layers of membranes in forward
2
osmosis,1,
reverse osmosis,3-5 nanofiltration6-8 and other
separation processes.9, 10 Up to now, the commercialized PA-
based membranes show great advantages in the fields of
solute concentration,11 seawater desalination,12, 13 and sewage
treatment.14, 15 However, it should be noted that synthesizing
PA nanofilms with consistent morphologies and adjustable
performances is still a tough task. One of the main reasons is
the challenge to control the diffusion of monomers and to
capture the dynamics of the rapid polymerization occurring at
the immiscible organic-aqueous interface.16
In the cases of nanofiltration, PA nanofilms are commonly
synthesized by the interfacial polymerization of diamine and
trimesoyl chloride (TMC) at the organic-aqueous interface. The
synthesis process is composed of diffusion of diamine
monomer (piperazine, in the most case), polycondensation at
the interface, and termination of the interfacial
polymerization.3 It is obvious that controlling the reaction
dynamics is the key issue to tune the structures and the
desalination properties of the PA nanofilms. In recent years,
several methods have been reported to remarkably improve
the separation performance of PA nanofilms, which include
However, it is still
a great challenge to deepen the
understanding of the interfacial polymerization by controlling
the diamine monomer release, because the porous substrates
being used as the storage matrices of the monomers limit the
direct characterization of diffusion process.23 On the other
hand, interfacial polymerization at hexane-water interface is
too quick to control and to evaluate the diffusion of diamine
monomers.24
Herein, we present a new method to effectively control the
diffusion of piperazine for synthesizing PA free standing
nanofilms. Inspired by jelly, agar hydrogel, a typical thermal-
sensitive physical gel, was used as the storage matrix of
piperazine.25 Self-standing PA nanofilms have been
conveniently synthesized by the interfacial polymerization of
TMC and piperazine at the hexane-“jelly” interface combining
with the sol-gel and gel-sol conversions. Besides, the diffusion
of piperazine can be controlled by means of the non-
flowability of “jelly”, which has been demonstrated by NMR.
Furthermore, for the first time, the apparent activation
energies have been measured, which are indispensable for
deepening the understanding of the interfacial polymerization.
The PA nanofilms synthesized in this way show reduced
thickness, smooth surface, and improved water permeability.
Fig. 1 illustrates the fabrication process of the free-standing
PA nanofilms (See detailed experiments in ESI†). Firstly, the
feasibility is verified for the formation and transformation of
agar hydrogel with different concentrations of piperazine. The
results show that the addition of piperazine does not affect
a. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Key
Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang
Province, Department of Polymer Science & Engineering, Zhejiang University,
Hangzhou 310027, China. E-mail: xuzk@zju.edu.cn, jing_yang@zju.edu.cn
b. College of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou
310027, China.
† Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0CC02555K
Fig. 1 Schematic illustration for using “jelly” (thermal-sensitive agar hydro-gel) as the
storage matrix of piperazine to synthesize PA nanofilms.
the sol-gel process of “jelly”. The concentration of piperazine
can be adjusted in a wide range from 0.05 g/L to 10.0 g/L (Fig.
S1, ESI †), which is suitable for the interfacial polymerization.
Typically, a certain amount of agar and piperazine were
Fig.2 Schematic illustration showing the diffusion process of piperazine in quantitative
NMR experiments. Piperazine was stored in a) water and c) “jelly” respectively,
reacting with benzoyl chloride in chloroform-d to form amide as shown. b) and d) 1H
NMR spectra of the products in chloroform-d from a) and c), respectively. A precisely
quantified 1,3,5-trioxane was added into the nuclear magnetic tube as the internal
standard (chemical shift: ca. 5.1 ppm).
o
dissolved in water at 90 C to form a sol (Fig. 1a). It became
like a “jelly” after cooling to room temperature. Then, the
hexane solution of TMC was added and the interfacial
polymerization was carried out (Fig. 1b, 1c). After that, the
“jelly” was heated to 90 oC to melt after removing the residual
TMC solution on it. A free-standing PA nanofilm was then
obtained after repeated washing with ultrapure water at 90 oC.
The PA nanofilm can be easily transferred to different
substrates for characterization and performance measurement
(Fig. 1d).
polymerization.27, 28 The fluctuant situations of the interfaces
are quite different when n-hexane solution was added to the
water surface and the “jelly” surface, respectively (Fig. S3, ESI†
). There are less ripples on the solid-like “jelly” surface
compared with the water surface in the case of continuously
dropping of n-hexane solution at a fixed height (Movie S1, ESI†
). The additional fluctuations of the interface during the
reaction could result in rougher PA nanofilms. Unlike the free
hexane-water interface, the hexane-“jelly” interface is steadier
under external disturbance (Movie S2, ESI†), which means that
it is facile for us to synthesize large-area PA nanofilms with
uniform thickness and roughness. PA nanofilms with a
diameter of 15 cm have been successfully synthesized and
loaded on a rounded metal mesh (Fig. S4, ESI†). On the other
hand, the thermosensitive nature of “jelly” allows us to easily
explore the effects of temperature on the synthesis of PA
nanofilms. The gelation of agar is caused by the existence of
hydrogen bonds, and all factors that can destroy the hydrogen
bonds can cause the destruction of gelation. Among them,
temperature is one of the most important factors. The
hydrogen bond network tends to gradually collapse with the
increasing temperature, which means that the molecules
originally bound in the gel will have greater activity in higher
temperature.
We suggest that the non-fluidity nature of “jelly” can slow
down the monomer diffusion of piperazine for the interfacial
polymerization. However, attempts to dynamically measure
the synthesized nanofilms are difficult because they show very
slight weight and thin thickness. Thus, a monofunctional
benzoyl chloride was used to substitute TMC in the interfacial
reaction to demonstrate the role of “jelly” (Fig. 2a, 2c). From
the comparison of Fig. 2b and 2d with the standard spectra
(Fig. S2, ESI†), it can be seen that only the characteristic peak
of 1,4-dibenzoylpiperazine (green diamond mark in Fig. 2) is
presented in the chloroform-d solution after 5 minutes of
reaction in both the aqueous solution and the gel system due
to the relatively excess of benzoyl chloride in the organic
phase. Thus, the mass of the produced substance can be
quantitatively determined by comparing the ratio of the
characteristic peak area between products and the internal
standard substance (red round mark) under different reaction
We can even estimate the apparent activation energy of the
interfacial polymerization after the correspondence between
the reaction temperature and the thickness of PA nanofilms
1
conditions.26 The quantitative H NMR spectra demonstrate a
less production of amides by using the “jelly” matrix under the
same conditions. In detail, the diamide produced by the was obtained, which are indispensable for deepening the
understanding of the whole process of interfacial
polymerization. The reaction process and the resulting PA
nanofilms were simplified, and then we successfully obtained
the activation energy (See detailed derivation in ESI†).Ea≈34
kJ·mol-1 is obtained from the slope of the fitted line (Fig. 3a).
Similar result (Ea≈32 kJ·mol-1) is also drawn from the changes
of the surface roughness of the PA nanofilms synthesized at
different temperatures (Fig. 3b). These activation energies
indicate that the reaction process is a fairly fast transition
between two immiscible phases although the gel is already
applied for reducing the diffusion of piperazine. When the two
phases containing two reactive monomers respectively are in
interfacial synthesis using “jelly” is reduced by 10% compared
with the original water system according to the quantitative
calculation (See detailed experiments in ESI † ). We propose
that the “jelly” could also significantly slow down the
interfacial polymerization process by controlling the release of
diamine monomer because TMC has lower activity than
benzoyl chloride. It is worth noting that the thickness and
roughness of the free-standing PA nanofilms are able to be
effectively reduced by using “jelly” to store the diamine
monomer. On the one hand, the “jelly” surface is quite
smooth, which provides an ideal interface for the interfacial
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34 Lm-2h-1bar-1 when the reaction temperature changes from
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o
45 oC to 0 C (Fig. 4b). The same trDeOndI: 10is.10a3l9s/oD0fCoCu0n2d555inK
nanofilms synthesized at free hexane-“jelly” interface (Fig. S13,
ESI † ). This phenomenon is mainly due to the suppressed
diffusivity of piperazine at lower temperature and the resulting
thinner PA nanofilms.5, 29 It is worth noting that although the
thickness of the PA nanofilm is sub-10 nm, it still retains a high
rejection to Na2SO4. Also, with the increasing reaction
temperature, the surface of the PA nanofilms remain flat (Fig.
S14, ESI†).
The PA nanofilms synthesized on the “jelly” surface show
the salt rejection sequence as: Na2SO4 (97.7%) > MgSO4
(90.5%) > NaCl (15.1%) > MgCl2 (11.1%). This phenomenon
result from the Donnan effect and the steric effect.34 Taking
the hydrolysis of TMC groups promoting the formation of
Fig. 3 Estimation of the apparent activation energies for the interfacial polymerization
according to a) the thickness and b) the surface roughness of the free-standing PA
nanofilms.
contact, the diffusion of diamine occurs immediately, along
with the rapid reaction between TMC and piperazine, leading
to an incipient PA nanofilm within few seconds.29 It is also the
reason why the traditional interfacial polymerization process is negatively charged surface into consideration, controlling the
difficult to control, and the surface of the PA nanofilms
fabricated by ordinary technique are usually crinkly.
diffusion of piperazine makes a smaller amount of piperazine
diffusing into the organic phase, which is immediately reacted
by abundant TMC once it entered, leading to more negatively
surface charge of PA nanofilms. When applying the PA
nanofilms to nanofiltration process, the lower surface charge
allows it exhibit superior divalent anion rejection (~98%), while
maintaining good permeability to monovalent anions (Fig. 4c).
With the help of reduced thickness and lower surface charge,
PA nanofilms prepared by our method show outstanding
nanofiltration performance (Fig. 4d).6-8, 18-21, 31, 35-46
In summary, we have demonstrated a novel method to
synthesize PA nanofilms via interfacial polymerizing piperazine
and trimesoyl chloride at the hexane-“jelly” interface. Intact
PA nanofilms have been fabricated with smooth surface and
quite thin thickness with the help of a more stable interface
Using the “jelly” surface to suppress the monomer diffusion
endows the prepared PA nanofilms unique properties as well.
The thickness of the PA nanofilms is obviously reduced by
o
o
lowering the reaction temperature from 45 C to 0 C (Fig. S5
and S6, ESI†).30 Moreover, the surface roughness has the same
trend and remains below 10 nm (Fig. S7, ESI†), which may help
to reduce the membrane fouling in practical applications.31 We
can even synthesize intact PA nanofilms with a thickness down
to 5 nm, which is far thinner than most literature reports.32
Besides, the surface potential of the PA nanofilms (-80 mV at
pH=7) is much lower than that reported in many other
literatures for PA-based membranes prepared using different
method (Fig. S9, Table S2, ESI†).31, 33, 38 Thinner thickness and
lower surface charge mean the PA nanofilms have potential to
increase water permeability and salt rejection simultaneously
in a pressure-driven nanofiltration process.
In practical applications, the PA nanofilm often acts as a
selective layer supported by a porous substrate to form
composite membrane. In our cases, the free-standing PA
nanofilms are facilely composited with polyether sulfone
substrate by vacuum filtration after the gel was heated to sol.
The nanofiltration performance can be tuned by controlling
the synthesis of PA nanofilms. Firstly, the content of piperazine
in “jelly” have a large impact. In general, water permeability
declines with the increases of piperazine concentration in
“jelly” (Fig. 4a). When the concentration of piperazine is less
than 0.1 g/L, the PA nanofilms are too thin to hold the
pressurized conditions, which is reflected in the sharp drop in
salt rejection and the pore profiles of the polyether sulfone
substrate beneath the nanofilms shown in the SEM images
(Fig. S10 and S11, ESI † ). As the concentration of piperazine
increases, the synthesized PA nanofilms become thicker and
the surface generates some typical nodal protrusions.
However, when come to the concentration of TMC, it only
slightly affects the separation performance of the PA
nanofilms (Fig. S12, ESI†).
Fig. 4 Nanofiltration properties of the PA nanofilms. a) Water permeability and salt
rejection to Na2SO4 of the PA nanofilms synthesized using different concentrations of
piperazine in “jelly”. The TMC in hexane is 1.0 g/L. (Na2SO4 concentration: 1000 ppm;
applied pressure: 4 bar) b) Effect of the reaction temperature on the permselectivity
of the PA nanofilms. (piperazine/TMC = 0.2/0.7 g/L; Na2SO4 concentration: 1000 ppm;
applied pressure: 4 bar) c) Mono/divalent ion selectivity for four slats of the PA
nanofilms. (salt concentration: 1000 ppm; applied pressure: 4 bar) d) Nanofiltration
performance of some state-of-the-art composite membranes reported in literatures.
A: PA nanofilm synthesized at hexane-“jelly” interface, B: PA nanofilm synthesized at
hexane-water interface (piperazine/TMC = 0.2/0.7 g/L).
Another important factor affecting the nanofiltration
performance is the temperature at which the PA nanofilms
been synthesized. The water permeability ranges from 15 to
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J. Name., 2013, 00, 1-3 | 3
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20 J.-J. Wang, H.-C. Yang, M.-B. Wu, X. Zhang and Z.-K.VXieuw,AJr.tioclfe Online
provided by the “jelly”. The apparent activation energies of the
interfacial polymerization have been estimated for the first
time by controlling the reaction temperature. The desalination
properties of the PA nanofilms can be conveniently tuned by
adjusting the concentration of piperazine and the temperature
of the interfacial polymerization. The synthesized PA nanofilms
show remarkably high water permeance while preserving salt
rejection in practical nanofiltration process. This new synthetic
system is expected to be a simple but efficient tool to help to
deepen the understanding of interfacial polymerization
between polyamines and poly(acid chloride)s, and other
interfacial reactions.
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DOI: 10.1039/D0CC02555K
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