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Research Article
Fabrication of PES/PVP Water Filtration Membranes Using
CyreneD, a Safer Bio-Based Polar Aprotic Solvent
Roxana A. Milescu,1C. Robert McElroy ,1Thomas J. Farmer ,1Paul M. Williams,2
Matthew J. Walters,2and James H. Clark 1
1Green Chemistry Centre of Excellence, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
2Department Chemical Engineering, Swansea University, Sketty, Swansea SA2 8PP, UK
Correspondence should be addressed to James H. Clark; james.clark@york.ac.uk
Received 13 March 2019; Revised 29 May 2019; Accepted 11 June 2019; Published 7 July 2019
Guest Editor: Zongli Xie
Copyright © Roxana A. Milescu et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
A more sustainable dialysis and water ltration membrane has been developed, by using the new, safer, bio-based solvent Cyrene
in place of N-methyl pyrrolidinone (NMP). e eects of solvent choice, solvent evaporation time, the temperature of casting gel,
and coagulation bath together with the additive concentration on porosity and pore size distribution were studied. e results,
combined with infrared spectra, SEM images, porosity results, water contact angle (WCA), and water permeation, conrm that
Cyreneis better media to produce polyethersulfone (PES) membranes. New methods, Mercury Intrusion Porosimetry (MIP) and
NMR-based pore structure model, were applied to estimate the porosity and pore size distribution of the new membranes produced
for the rst time with Cyreneand PVP as additive. Hansen Solubility Parameters in Practice (HSPiP) was used to predict polymer-
solvent interactions. e use of Cyreneresulted in reduced polyvinylpyrrolidone (PVP) loading than required when using NMP
and gave materials with larger pores and overall porosity. Two dierent conditions of casting gel were applied in this study: a hot
(∘C) and cold gel (∘C) were cast to obtain membranes with dierent morphologies and water ltration behaviours.
1. Introduction
e growing use of ltration membranes is the result of
increasing attention paid to environmental problems linked
to the availability of and growing demand for clean water
[–]. Recent work has highlighted the need for new sol-
vents in the membrane manufacturing process, in order
to minimise the problems of toxic solvent release to the
environment, with Polarclean,DMSOEvol, and ionic liq-
uids being successfully applied in this process [–]. Porous
polymeric membranes for water and wastewater treatment
have traditionally been produced from cellulose acetate [],
polysulfone [], polyvinylidene uoride (PVDF) [, ],
polyacrylonitrile (PAN) [], and polyvinyl alcohol (PVA)
[]. Polyethersulfone (PES) is used in separation due to its
excellent performance and high thermal, chemical, hydraulic,
and mechanical stability. e polymer generates membranes
ranging from nanoltration (NF) and Reverse Osmosis (RO)
used for desalinisation processes, to ultraltration (UF) used
for food, metal, and textile industry and microltration (MF)
for purication of beverages, and separation of oil and water
emulsions. Fouling of the membrane is as a result of the
hydrophobicity of PES, making separation unpredictable and
shortening its lifetime due to a higher energy demand to
push the water through the pores [, ]. To reduce the
fouling eect, the PES membranes are modied via bulk
modication, surface modication, and blending (a type of
surface modication), with the latter being the most widely
used method [].
Filtration membranes have previously been fabricated
using a casting solution of polyethersulfone (PES) in the
repro-toxic solvents NMP [–], DMF [], DMAc [],
or mixture of solvents [, ] along with a hydrophilic
homopolymer, such as polyvinylpyrrolidone (PVP) [, ]
or poly(ethylene glycol) (PEG) [, ], resulting in mem-
branes less prone to fouling [, ]. DMSO has been utilised
Hindawi
Advances in Polymer Technology
Volume 2019, Article ID 9692859, 15 pages
https://doi.org/10.1155/2019/9692859
Advances in Polymer Technology
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20
15
10
5
0
510
15
20
2525
22.5
20
17.5 15
12.5
P
H
D
F : Recommended solvents mapped in Hansen space with solubility sphere for PES. Hansen solubility parameters are given here
in units of MPa1/2.
as the casting solvent [, , ], which is not in itself toxic, but
it acts as a permeability enhancer and transfer compounds
through the skin barrier. It is used in medicine as a skin
adsorption enhancer for dierent drugs [, ]. Recently
NMP was added to REACH’s restricted substances list [],
due to the reproductive eects of this solvent. NMP and
dichloromethane (DCM) were linked to foetal and adult
deaths and their use has been banned or limited from paint
stripping products by the big retailers. [] DMF and DMAc
are also on the Substances of Very High Concern (SVHC) list
as toxic for reproduction (may damage the unborn child). e
list of available and safe polar aprotic solvents is thus getting
smaller.
e pore forming and structure controlling agents PVP
and PEG are incorporated into PES matrix, similar to other
polymer additives reported in the literature [, , ], so
as to increase the hydrophilicity, diusion properties (due to
pore size distribution on the membrane surface) antifouling,
and hemocompatibility properties of PES membranes [–
]. ese asymmetric membranes can be modied by
adjusting any of the following factors: the composition of the
casting solution (additives; solvent), the solvent evaporation
temperature/time, and the method utilised. In this study we
aim to substitute the traditional solvent used (NMP) with a
greener, safer alternative. Cyrene(dihydrolevoglucosenone)
is a new commercially available, nontoxic solvent obtained
from renewable waste and nonfood cellulosic source feed-
stock (e.g., straw, bagasse, and sawdust) (see Scheme ). It is
quickly proving to be a viable alternative polar aprotic solvent
to both NMP and DMF [–].
Cyreneis a polar aprotic solvent similar to NMP, but
renewable, biodegradable, and nontoxic []. Cyrenehas
been only given a hazard warning for being an eye irritant
(E) receiving an ECHA level certication and has
recently received permission to be sold in large quantities
[, ]. Herein Cyrenehas been assessed for its suitability
in substituting for NMP in the fabrication of PES membranes.
2. Materials and Methods
2.1. Materials. e akes of UltrasonE P Polyar ylether-
sulfone (Figure ()) of , Da and a powder of PVP
LuvitekK- Pulver (Figure ()) with ,, Da were
obtained from INGE.BASF, Germany. e solvent Cyrene
was supplied by Circa Sustainable Chemicals Ltd., UK. All
the other solvents used in this study were of reagent grade
and purchased from Merck Co., UK, and VWR Chemicals ,
UK. Deionised water (DI) was provided in-house by the lab
using an ELGA CENTRAsystem. All chemicals were used
without any further purication.
2.2. HSPiP’s Predictive Power. Hansen Solubility Parameters
(HSP) [] were chosen to predict solubility of PES in
dierent solvents by mapping the three values (dispersion
interactions D, dipolarity P, and hydrogen bonding ability
H) in a three-dimensional ‘’Hansen space”, using 5th edition
.. of HSPiP (Hansen Solubility Parameters in Practice).
When determining whether a solvent will dissolve a
sphere, it is useful to calculate the relative energy dierence,
RED:
=
()
where Ro is the radius of interaction of a Hansen solubility
parameter sphere determined experimentally (Figure ) and
Ra is the solubility parameter distance between polymer and
solvent and can be calculated from
2=4
𝑠𝑜𝑙V𝑒𝑛𝑡 −
𝑠𝑜𝑙𝑢𝑡𝑒2+
𝑠𝑜𝑙V𝑒𝑛𝑡 −
𝑠𝑜𝑙𝑢𝑡𝑒2
+
𝑠𝑜𝑙V𝑒𝑛𝑡 −
𝑠𝑜𝑙𝑢𝑡𝑒2()
A D representation (Figure ) shows the distance of
the solvents which dissolved the polymer and PES. A
nonsolvent will have a Ra larger than the sphere’s Ro,making
Advances in Polymer Technology
O
O
O
O
O
O
O
O
O
OO
OH
OH
OH
OH
OH
cellulose levoglucosenone
Heat/ Pd
n
H
HH
H
HH
H
H
H
H
H
H
H
H
H
-（2O
（3P／4
（2
Cyren？Ⓡ
S : Scheme of Cyreneproduction from cellulose via levoglucosenone (LGO,).
S
O
n
PES PVP
O
O
NO
H
H
n
F : Chemical structure of PES and PVP.
RED >, while solvents likely to aect the material will have
aRED<. A solvent with a RED < and closer to the
polymer’s centre (with D=., P=., and H=.) is
predicted a better solvent. Cyrenehas the smallest distance
from polymer in the Hansen space, suggesting the greatest
anity to PES (Table ). A full list of the solvents used
can be found in the Supplementary Material accompanying
the manuscript (SM Table ).
As seen in Table , comparing HSP of Cyreneand the
other polar aprotic solvents, it was found that Cyrenehas a
dispersion close to DMSO (. vs. .), a polarity parameter
close to NMP (. vs. .), and a hydrogen bonding close to
NMP (. vs. .). e similarity of Cyreneto the other polar
aprotic solvents has been studied previously [].
2.3. Membrane Synthesis
2.3.1. Preparation of Casting Solutions. In this study, the
polyethersulfone membranes (PES/C and PES/N) with highly
asymmetrical pore structure were fabricated from hot (∘C)
andcold(
∘C)castinggelsofPES,PVP,andsolvent(Cyrene
or NMP), using a nonsolvent phase inversion technique
(NIPS).
Table shows the formulations of PES ultraltration
at sheet membranes using two dierent solvents and with/
without PVP.
In this study, some of the characteristics of Cyrene
and NMP are presented in Table , showing the dierence
between the two solvents.
2.3.2. Membranes Fabrication. e casting solution was pre-
pared by dissolving wt% of PES pellets (as compared to
the mass of solvent) into Cyreneor NMP at a temperature
of ∘C for h. Dierent concentrations of PVP were added
under continuous stirring (Table ). e casting solution was
degassedandthenplacedonanacrylicplateforPES/Norona
Polyester (PET) nonwoven fabric CraneMat CU (Neenah
Technical Materials, USA) for PES/C. e casting gel was
cast using a RK Print K bench casting machine (RK Print,
UK) at a speed of cm s−1 for the membranes produced in
NMP and cm s−1 for membranes produced in Cyrene.e
speedofthecastingmachinewasfoundtohaveasignicant
inuence over membrane ltration and a slower speed was
considered for PES/C due to its higher viscosity. e thickness
of all membranes was controlled at m. e casting lm
was submerged in a coagulation bath containing deionised
water at RT, causing the PES to precipitate. Membranes were
then washed three times in distilled water for minutes
while under sonication in order to wash out residual solvent
the fabricated membranes were then stored in deionised
water until further use. e membranes were prepared at RT
of ∘C and a humidity of -%. e temperature of water
bath was ∘C and the time from casting the gel to placing it
in a water bath was limited to a maximum of seconds.
Due to the higher density of Cyreneand the amount
of PVP used, both of which add to the total viscosity of the
system; the gel was cast when hot. e viscosity of PES/C
was tested using a Brookeld R/S plus Rheometer and shows
a dramatic drop during the heating to ∘C(SMFigure),
where its behaviour tends towards a Newtonian uid.
2.4. Membrane Characterisation. To characterise the pre-
pared membranes, they were rst washed with deionised
wateranddriedinvacuumovenat
∘Cforhours.e
eects of the more viscous Cyreneon the pore size, porosity,
surface morphology, and mechanical properties of PES/PVP
membranes were studied by scanning electron microscopy
(SEM), pure water permeability, ATR-FTIR, and thermal
analyses. FTIR and TGA reected the physical or chemical
Advances in Polymer Technology
T : Hansen Solubility Parameters [MPa0.5] of dierent solvents and RED calculated for PES polymer.
Solvent DPHScoreRED
Cyrene. . . .
N-Methyl--Pyrrolidone . . .
N,N-Dimethyl Acetamide . . . .
DimethylSulfoxide ... .
Dimethyl Formamide . . . .
,-Dichloroethane . . .
T : Composition of casting solutions (g).
Membrane type Solution A Solution B
PES PVP NMP PES PVP Cyrene
PES/ .% % .% .% % .%
PES/. .% .% .% .% .% .%
PES/. .% .% .% .% .% .%
PES/ .% .% .% .% .% .%
PES/ % % % % % %
PES/ .% .% .% .% .% .%
T : C y reneand NMP safety data sheet.
Solvent Empirical formula Molecular weight (g mol−1) Relative density (g mL−1 at ∘C) B.P. (∘C) F.P. (∘C) M.P./F.P. (∘C)
CyreneC6H8O3. . -.
NMP C5H9NO . . -
changes on the surface or in the bulk of PES membranes aer
modication.
ATR-FTIR spectra were recorded on a PerkinElmer Spec-
tr um FT-IR/ FT-NIR Spec trometer with transmittance
peaks in - cm−1 region, with rapid scanning ( scans)
and resolution cm−1 at room temperature.
ermal stability of PES membranes was studied by TGA,
giving weight loss of the produced membranes as a function
of temperature. e membranes were heated from RT to
∘Catarateof
∘Cmin
−1 under a ow of nitrogen.
Scanning Electron Microscopy (SEM) images were taken
on a JEOL JSM-LV, at kV from Bioscience Technology
Facility, Biology Department, University of York, and were
used to determine the morphology and structure of the
membranes aer they were frozen and fractured in liquid
nitrogen followed by Au/Pd coating.
Porosity and pore size distribution test were deter-
mined using Mercury Intrusion Porosimetry (MIP) using
a Micromeritics Autopore IV instrument located in the
University of Leeds. is method is based on the behaviour
of “nonwetting” liquids in capil lary which cannot be absorbed
by the pores of a solid itself, but requires an external pressure
to be applied. By measuring the volume of mercury that
intrudes into the sample material with each pressure change,
the volume of pores in the corresponding size class can be
obtained. MIP only shows accessible interconnected pores (if
the closed pores are incompressible). e applied mercury
pressure is inversely proportional to the size of the pores.
A lower pressure is needed to penetrate large pores, while
a greater pressure is needed to access smaller pores. From
the pressure versus intrusion data, the instrument generates
volume and size distributions using the Washburn equation:
=−4 cos
()
where Dstands for pore diameter (m), Pis the applied
pressure (psi), represents Hg-air surface tension ( mN
m−1), and is Hg-air-porous material contact angle (∘).
NMR spectroscopy was used as supporting evidence
of the pore size distributions and was considered a more
sustainable and less toxic method compared to the MIP
method, which can distort the skeletal porous structure of
a sample []. However, this method was used previously
to characterise porosity, pore geometry, connectivity, and
permeability of sandstones and carbonates [, ]. In this
project, the NMR was used to conrm the volume of uid
lling the pore space and a T distribution (equivalent to a
pore size distribution, PSD) is obtained aer deconvolution
of magnetization relaxation (Figure ).
As a measure of membrane’s hydrophilicity, the water
contact angle (WCA) was measured via the sessile drop
method [] using a eta Lite optical tensiometer at a room
temperature of ∘C. A range of .–. Ldropletsizesof
water were placed on the membrane surface and the images
were recorded using the automated OneAttension soware.
e static contact angles were measured at a minimum of
three random locations and the mean values reported to
minimise experimental error.
Advances in Polymer Technology
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.001 0.01 0.1 1 10 100
Pore size distribution
Normalized signal
radius (microns)
NMR
Mercur y
F : MIP and NMR methods of porosity using a PES/C.
1
2
3
4
5
6
78
F : A schematic diagram of the frontal ltration equipment.
() Nitrogen cylinder, () valve, () pressure sensor, () water bath,
() membrane cell, () magnetic stirrer, () electronic balance, and
() PC.
2.5. Pure Water Permeability Test. Membrane permeability
was determined by measuring the pure water uxes using a
stirred cell (Sterlitech HP). e ltration solutions were
stirred magnetically at rpm and a constant temperature
of ±.∘C. Rates of ltration were determined by continu-
ously weighing theltrate on an electronic balance connected
to a data logger. A digital electronic balance from Ohaus
(Scout Pro Range), with an accuracy of . g was used to
continuously measure the weight of the permeate. Recording
the weight at certain time intervals also allows the calculation
ofthepermeateux:
= (+)−()
()
where 𝑤is the water ux (L m−2 h−1 (LMH)), mis the mass
at a given time, is the density of water, 𝑚is the area of the
membrane (m2), and tis the time (s).
Figure shows schematically the arrangement of the
membrane ltration and ux measurement equipment. e
ltration cell was pressurised using nitrogen gas (oxygen
free) with pressure controlled by means of a regulator on the
cylinder. e mm diameter membranes were rst subject
to an initial pressure of bar until a stable ux was evident,
then the pressure increased to and bar.
2.6. Gravimetric Method of Water Adsorption. is method
is repeatedly used in literature to evaluate the porosity of
the membranes[, ] via determining take up of water.
e membranes were vacuum dried at ∘C, before being
weighed and their size and thickness measured. ey were
then immersed in distilled water at RT for h, before being
weighed again, and the increase in mass registered. e
%water retained in pores was calculated using the following
equation:
=Ww −Wd
Axlxx100 ()
where Ww and Wd represent wet and dry masses (g) of the
membranes, respectively, Aisthemembranesurfacearea
(m2), lis thickness of membrane (m), and is density of water
at RT ( Kg m−3).
3. Results and Discussion
3.1. Porosity and Pore Size Distribution. e membranes are
coded based on the solvent used (PES/C is the membrane
produced with Cyrenewhile PES/N represents the one with
NMP) and the concentration of PVP used, e.g., “” means
a membrane produced with no PVP, while “.” through to
‘’” means a membrane produced with .% through to %
PVP, respectively (Table ). For example, a PES membrane
produced with Cyreneand .% PVP will be referred to
as “PES/C.” while the same membrane produced in NMP
would be denoted “PES/N.”.
As seen in Figure , the Cyrene-based membranes pre-
sented higher porosity in comparison to NMP in all cases,
with a maximum of %and .% for PES/C. and PES/C,
respectively. e membrane prepared with Cyrene developed
porosity even when not using pore forming PVP (PES/CO
with .% total porosity), which demonstrates superior
porosity than a NMP-based membrane using pore forming
additive (PES/N.) with .% porosity. e membrane
with % PVP has the lowest total porosity (.%), and
at this point the membrane exhibits a dierent morphology
(phenomenon seen in SEM images). On the other hand, the
Advances in Polymer Technology
0
10
20
30
40
50
60
70
80
90
0 0.1 0.5 1 5 10
Porosity (%)
PVP (%)
Cyrene
NMP
F : Overall porosity of the PES membranes prepared with Cyrene(blue) and NMP (orange).
−0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.1 0.5 1 5 10
Pore diameter (m)
PVP (%)
Cyrene
NMP
F : Overall pore diameters (m) of PES/C (blue) and PES/N (orange).
highest porosity (.%), for the membranes produced with
NMP is obtained with a PVP concentration of .%. e
lowest porosity for the membranes produced using NMP
occurs for PES/N (.%).
It is also signicant that .% PVP is enough to produce
good quality PES/C membranes, lowering the amount of sac-
ricial polymer represents an important starting point for the
sustainable membrane fabrication together with substitution
of a toxic petroleum-derived solvent with a safe, bio-based
alternative.
Both Cyrene- and NMP-based membranes present a
slightly decreasing porosity when .% PVP is used (PES/
C. and PES/N.) and clearly decrease when using a
higher concentration of PVP (PES/C and PES/N). is
decrease in porosity is more visible in the case of NMP-based
membranes. However, this means that % of additive is clearly
sucient to give the maximum porosity in both types of
membranes, with Cyrene-based membranes more superior
than the NMP equivalents.
Figure shows the pore diameter of membranes pro-
duced with Cyreneand NMP. PES/C membranes developed
bigger pores than PES/N, except for PES/C, which has a
dierent morphology. Interestingly, the pore size of a Cyrene-
based membrane with no additive (PES/C) has a similar
pore diameter to that obtained in NMP-based membrane
using at % PVP (PES/N). is means that no additive
is necessary to form pores when using Cyreneas solvent.
Similarly a membrane produced using Cyreneand % PVP
(PES/C) shows the same pore diameter as a NMP-based
membrane when using % (PES/N) of the same pore
forming agent. at means that less sacricial pore forming
agent is required when using Cyreneas solvent compared to
NMP-based systems.
Membranes prepared with Cyrenehave pore diameters
from . m(PES/C)tom(PES/CandPES/C),while
the membranes prepared with NMP range between . m
(PES/N, ., ., m) to m(PES/N).Figureclearly
shows similarity between PES/C and PES/C to PES/N in
pore size and pore distribution (SM Figure and SM Figure
). e membranes made using Cyreneneed % PVP to
develop the biggest pores (PES/C), while the membranes
produced with NMP need twice as much PVP to develop the
same pore diameter.
A high degree of variation in the pore system is seen
when PVP concentration is increased. is is observed
by the greater decrease in total porosity and an increase
of pore diameters in the range between . and mfor
PES/C-, followed by a dramatic decrease of pore diameter
for PES/C. e Cyrene-based membranes pore diameters
follow a trend from a minimum with PES/C to a maximum
Advances in Polymer Technology
with PES/C. Above a % PVP loading, this type of mem-
branes drops drastically in pore diameter. is is as a result
of the morphology of the membrane changing signicantly,
with large cavities containing very small pores on the walls
(as clearly seen in Figure ).
is morphology is clearly seen in PES/C where a
‘diusion membrane’ has been formed, at which point it
is believed that the pores of . m are incorporated in
the walls of cavities, replacing the classical macrovoids. e
asymmetric membranes have variable pore diameters, from
. to m while the symmetric one (PES/C) present
more uniform pores, with majority of pores of mdiameter.
PES/C presents a small quantity of pores of . m; the
quantity of this type of pores increases with increasing of
PVP in casting solution and their presence is exclusively in
PES/C.
Membranes made using NMP increase their pore diam-
eters with increasing PVP concentration from PES/N-.
e NMP-based membranes present the same small pore
diameter until a concentration of % of pore forming additive
is added to the casting gel, leading to a bigger pore diameter
with a maximum for PES/N.
e results from NMR spectroscopy are consistent with
the previously reported MICP results. e largest pores
(around m) can be seen in the membranes produced with
Cyrene, increasing in line with the concentration of PVP
used, with a maximum in PES/C. For PES/N the number
of larger pores increases with increasing PVP concentration,
while smaller pores are no longer seen (e.g., pores in the
range . to . microns are only observed for PES/N.).
Aer deconvolution of magnetization relaxation of Nuclear
Magnetic Resonance (see Figure ), the obtained T curves
conrm the volume of uid lling the pore space and
indirectly the pore size distribution, described above by
MICP method.
3.2. Membrane Characterisation
3.2.1. Membrane Functional Groups via FTIR Spectroscopy.
e membranes present functional groups specic to PES,
S=O symmetric stretch at cm−1,C-SO
2-C asymmetric
stretch at cm−1, C-O asymmetric stretch at cm−1,
C6H6ring stretch at and cm−1,C-Hstretchat
- cm−1, in addition to the residual PVP as indicated
by a C=O stretch at - cm−1, pyrrolidinyl radical at
and cm−1,C-Nvibrationatcm
−1,andC-H
asymmetric stretch at cm−1.
During the membrane precipitation step, some of the PES
fromthesurfaceleavesthemembranetogetherwithmostof
the water-soluble polymer, but enough PVP remains (with its
presence at cm−1 in Figure ), providing the polymer
surface with a more hydrophilic nature than the surface of
PES alone.
InthecaseofPES/Cyrene, coloration due to release of
PES into the gelation media was les s apparent than in the PES/
NMP systems. is indicates a stronger PES/PVP interaction
in Cyrene(SM Figure ).
3.2.2. Scanning Electron Microscopy (SEM) Analysis. All
membranes produced herein (except for PES/C) present a
typical structure with a thin top layer supported on a sponge-
like substructure (important for the mechanical resistance of
the membranes), and macrovoids, due to the instantaneous
demixing during the phase inversion [], where PVP behaves
as an antisolvent agent in the demixing step due to its high
solubility in water. e casting solution has a high anity for
water which can cause changes in morphology and perfor-
mance []. In this study the gel was cast and the glass plate
with the casting gel was quickly immersed in the coagulation
bath meaning the exposure to the atmosphere was less than
seconds. Finger-like structures can be seen near the inner
(connecting the top layer to the sponge-like structure) and
outer surface, while sponge structure can be seen in the
centre of membranes. It has been generally accepted that
instantaneous demixing leads to macrovoid structure and
delayed demixing leads to a sponge-like structure [, ].
While the sponge-like structure looks the same in both
Cyreneand NMP producing membranes, Figure shows
the dierence between them in a macrovoid layer at the same
magnication.
When pure PES is used, the phase separation of polymer
solution occurs immediately, due to low viscosity of the
casting gel. e higher viscosity of Cyrene(. m Pa s
at ∘C) compared to NMP (. m Pa s at ∘C) adds
to the casting gel’s viscosity leading to a slower demixing
process and hence to a more visible sponge-like structure
(Figure ). e modication of polymer solution (by adding
PVP) changes the pore structure of the membrane []. Also,
dierent temperatures of the casting gel lead to dierences in
their morphology (Figure ):
e membranes cast from the cold gels present more
nger-like layers than the plain PES/C (which is more
viscous), especially in the middle of the membrane. e
membranes cast from Cyrenehot gels showed more sponge-
like structure, with macrovoids all the way through the
membrane. A greater dierence in morphology starts from a
concentration of % PVP, with the greatest at a concentration
of % PVP. PES/C from a hot gel (Figure (l)) developed
a completely dierent surface morphology, a symmetrical-
like structure, looking like a porous web or sponge. is
could be caused by the large amount of PVP which slowed
precipitation. When cast from a cold gel, the membrane
shows asymmetrical layers with bigger space (Figure (i)).
Increasing the PVP content in the casting solution led to
slower demixing, especially towards the bottom layer of
membrane (closest to the glass slide). When the exchange
from solvent to antisolvent occurs, it starts at the surface,
where nger voids are observed and slows as it approaches
the bottom layer, when more sponge-like structures are seen.
is structure presents pores which increase in diameter
from the top to the bottom surface of the membrane. e
rheology of the casting gel of PES/C was studied, with
a decrease in viscosity with increasing temperature clearly
visible (SM Figure ). is had a great impact on the
membrane morphology and dierences in the permeability
data are expected. A clear dierence between the membranes
cast from hot/cold gels is seen in NMP’s case too (red circle
Advances in Polymer Technology
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Normalized signal
radius (microns)
PES/C0.1
PES/C1
PES/C5
(a)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Normalized signal
radius (microns)
PES/N0.1
PES/N1
PES/N10
(b)
F : Pore size distribution of PES produced with Cyrene(a) and NMP (b) using NMR.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
650115016502150265031503650
Absorbance
PES/C0
Wave Number (cＧ-1)
F : Fourier transform infrared spectrum of PES/C.
in SM Figure ) where the one cast from a hot gel presents a
middlespongelayer,whiletheonecastfromacoldgelclearly
shows the nger layer all the way through the membrane.
3.2.3. ermal Stability of Membranes. e results of ther-
mogravimetric analysis on the membranes prepared here are
shown in Figure and Table . PVP has a decomposition
temperature around ∘C, while PES decomposes at
-∘C, which means that PVP would lose more weight
before PES began to decompose.
e rst degradation took place below ∘Cdueto
moisture loss, followed by Cyrenevolatilisation at around
∘Cand
∘C for NMP, followed by PVP decomposition
at ca. ∘C and PES decomposition at ca. -∘C. e
dierential thermogravimetric data shows a greater amount
of residual PVP from PES/C membranes.
PES/C membranes consistently show a higher thermal
stability than PES/N equivalents. PES/C has a thermal
decomposition of .∘C, while PES/N decomposes at
.∘C, which means that a greater energy is required to
break the bonds of PES/C membrane. e peak of the 1st
derivative (inection point) indicates the point of greatest
rate of change on the weight loss curve, as seen in Figures
(b) and (d).
e addition of PVP to the membranes decreases the
thermal stability, due to a lower decomposition temperature
of PVP than of PES, but the miscibility of both polymers
was conrmed from the thermal analysis. e dierences
between the concentration of PVP and TGA residue may
be due to PVP remaining in the membranes aer washing
with water, depending on the membranes porosity and pore
size.ItwasfoundthatPES/NmembraneslosesomePES
fromthemembranesurfacewhencastingthelm.esame
membranes lost more PES and PVP than PES/C, which
means that Cyrenemakes a better media for this type of
ltrationaswellasbeingmorehydrophilic.
3.3. Surface Wetting Property of the New Membranes Produced
with Cyrene.AsshowninFigure(a),thestaticwater
contact angle of the new PES lms produced with Cyrene
decreased from .% for PES/C to .% for PES/C, with
a hydrophilic character in all cases. is decreasing of the
contact angle indicates increasing in hydrophilicity of these
membranes with increasing residual PVP content.
e wetting changes were recorded over time by means
of the same methodology used for static contact angle, but
with values taken over a second range. It can be seen
in Figure (b) how the droplets change in a reproducible
Advances in Polymer Technology
(a) (b)
F : SEM images of PES/C. (a) and PES/N. (b) in detail.
T : ermogravimetric (TGA) analysis measurements of PES/C and PES/N membranes.
C/Sample Residue% Tm (∘C) N/Sample Residue Tm (∘C)
PES .
PVP K . .
PES/C . . PES/N . .
PES/C. . . PES/N. . .
PES/C. . . PES/N. . .
PES/C . . PES/N . .
PES/C . . PES/N . .
PES/C . . PES/N . .
manner and reduce their contact angle. A larger initial drop
occurs as water rapidly lls pores at the surface of the
membrane, until locally saturated. is phenomenon was
found to be due to a result of the advancing angle (higher
anglewhichreliesonthehydrophobicsurfacecomponent)
changing into a receding angle (low contact angle of the
hydrophilic component) upon a decrease in droplet volume
and is due to imbibition and evaporation. []
e contact angle of PES/C recorded the smallest change
during the test time, due to a less hydrophilic character
and the tendency of the droplet to be repulsed by the
membrane surface at least at the beginning. Over the time,
when the membrane is locally saturated, the droplet moves
into the porous membrane and the contact angle changes.
On the other side, PES/C membrane was found to have the
most dramatic change in its contact angle, due to the most
hydrophilic character and its porosity. e SEM images of
the PES/C demonstrated a very porous membrane which
would allow the droplet of water to be completely absorbed
in under seconds during the WCA experiment.
3.4. Pure Water Permeability. Pure water permeability (PWP)
testing was evaluated based on water ux. Evidence in the
literature indicates that the sponge layer lters are slower than
the nger layer of a membrane [].
Based on these criteria, the permeate ux seen in Figure
shows dierences between the membranes depending on the
solvent used, the concentration of PVP used, and temperature
of the casting gel. e membranes produced using Cyrene
and cast from a cold gel show an increasing permeability
when PVP is added with a maximum at % PVP, but with
no permeability when using ., , and % PVP. is is
explained by the morphology of the membrane, with more
nger layers on top of the membrane a dense sponge layer
at the bottom of the membrane. is is due to the thickness
of membrane chosen in this project ( m) which results
in dierent morphologies most likely depending on speed of
exchange between the solvent and water (SM Figure ). At
the top surface, this is rapid, with the polymer precipitating
out and voids forming as a result of water displacement. At
the bottom, crystallisation has dominated, with fewer voids.
is results in low permeability as the nger regions are
not interconnected. e concentration of PVP used had a
great impact of permeability when using over .% PVP,
when the produced membranes showed no ux, due to the
presence of dead ends of the nger layers presented in their
morphologies. e new membranes PES/C and PES/C.
cast from cold gels together with PES/C from a hot gel
are believed to be suitable for a ultraltration (UF), while
a permeability is lower than LMH/bar for a nanoltra-
tion/reverse osmosis (NF/RO), while the PES/C membrane
showed greater uxes more in keeping of a microltration
membrane (MF). Instead, when the same membranes are
cast from a hot gel, the produced membranes showed a
permeability with a maximum at % PVP, but with a smaller
permeate ux than the corresponding cold gel membrane.
e membranes produced from a cold gel, using Cyreneand
a PVP concentration above .%, developed a morphology
Advances in Polymer Technology
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
(j) (k) (l)
F : SEM cross-section of membranes produced from Cyrene and dierent concentrations of PVP: % (a, d), .% (b, e), .% (c, f),
% (g, j), % (h, k), and % (i, l) in cold (rst letter)and hot gels (second letter).
Advances in Polymer Technology
0
100
200
300
400
500
600
700
0% PVP
0.1% PVP
0.5% PVP
1% PVP
5% PVP
10% PVP
PVP K90
PES3020
Temp (∘C)
0
10
20
30
40
50
60
70
80
90
100
110
WtPercent (%)
(a)
0% PVP
0.1% PVP
0.5% PVP
1% PVP
5% PVP
10% PVP
PVP K90
PES 3020
250
300
350
400
450
500
550
600
650
Temp (∘C)
−1.8
−1.6
−1.4
−1.2
−1.0
−0.8
−0.6
−0.4
−0.2
0.0
0.2
Derivative weight (%/∘C)
(b)
0% PVP
0.1% PVP
0.5% PVP
1% PVP
5% PVP
10% PVP
PVP K90
PES3020
0
100
200
300
400
500
600
700
Temp (∘C)
0
10
20
30
40
50
60
70
80
90
100
110
WtPercent (%)
(c)
0% PVP
0.1% PVP
0.5% PVP
1% PVP
5% PVP
10% PVP
PVP K90
PES 3020
250
300
350
400
450
500
550
600
650
Temp (∘C)
−20
−18
−16
−14
−12
−10
−8
−6
−4
−2
0
2
Derivative weight (%/∘C)
(d)
F : TGA and DTG spectra of PES/C (a-b) and PES/N membranes (c-d). Note: TGA, thermogravimetric analysis; DTG, dierential
thermogravimetric.
with nger layers in the middle of the membrane but are not
interconnected to the surface, thus registering no permeate
ux.
When adding PVP, the permeability also increased in
PES/N, with a maximum ux at % PVP and when the
membrane was cast from a cold gel. When cast from a hot
gel, the membrane produced from the same concentration of
PVP showed a higher ux of water due to a void in the middle
of the membrane which permitted a higher permeability.
Looking at Figures SM (b) and (e) relating to .N, the
hot cast seems to have a layer in which all the pores appear
blocked about 1/2 waythroughthemembrane(redcirclein
SM Figure (e)). However, in the cold cast, the pores appear
to traverse the membrane (red circle in SM Figure (b)). e
presence of the middle sponge layer in PES/N membranes
which were hot cast showed a lower permeability than the
ones coming from a cold gel, except for PES/N., where
the morphology shows a thicker sponge layer, which would
generally slow the ux down.
3.5. Gravimetric Analysis. Figure shows the absorption
capacity of PES/C (in blue) and PES/N (in orange) to retain
pure water in their pores. Although less precise than other
methodologies applied in this research, this is the standard
test to evaluate membrane porosity [].
Figure shows that PES/C consistently developed a
greater water retention capacity than the respective PES/N
membrane (consistent with MICP and SEM data). MICP
and gravimetric data present dierences between the samples
generated using the same solvent. For example, gravimetri-
cally PES/C presents the biggest capacity of water absorp-
tion, while for MICP data, PES/C. has the biggest total
Advances in Polymer Technology
WATER CONTACT ANGLE (∘C)
73.1
67.5
62.1
44.9
37.5
0
10
20
30
40
50
60
70
80
PES/C0 PES/C0.1 PES/C1 PES/C5 PES/C10
(a)
0
10
20
30
40
50
60
70
80
0 60 120 180 240 300
TIME (SEC)
PES/C0
PES/C0.1
PES/C1
PES/C5
PES/C10
WATER CONTACT ANGLE (∘C)
(b)
F : e static water contact angle (a) and the time-depended values (b) for a sessile drop spreading over a range of PES membranes
produced with Cyrene.
90.2 124.7 000
2542.7
15 187.2 10.3 140.5
720.3 656.3
0
500
1000
1500
2000
2500
3000
Average permeability (LMH/bar)
Membrane name
PES/C0
PES/C0
PES/C0.5
PES/C1
PES/C5
PES/C10
PES/N0
PES/N0.1
PES/N0.5
PES/N1
PES/N5
PES/N10
PES/C0
PES/C0.1
PES/C0.5
PES/C1
PES/C5
PES/C10
PES/N0
PES/N0.1
PES/N0.5
PES/N1
PES/N5
PES/N10
71.4
2.4 4.5 2.3 23.2
898.4
65.5 3.5
260.4
103.5 157.2 210.6
0
100
200
300
400
500
600
700
800
900
1000
Average permeability (LMH/bar)
Membrane name
F : Pure water permeability of membranes cast from cold (in blue) and hot (in red) gel.
porosity. However, PES/C showed the smallest capacity to
retain water, which is entirely consistent with MICP and SEM
data.
4. Conclusions
Polyethersulfone at sheet membranes for water ltration
applications were prepared by the NIPS technique employing
Cyreneas an alternative solvent and compared to PES
produced using the traditional NMP. Membrane morphol-
ogy and performance were tailored acting on the casting
solution composition and temperature. e new membranes
produced with Cyreneare more sustainable, with less
of both polymers’ loss and tunable pore size and contact
angles, from a less hydrophilic PES/C to a more hydrophilic
PES/C when the hydrophilic additive is added to the
Advances in Polymer Technology
0
10
20
30
40
50
60
70
80
0 0.1 0.5 1 5 10
Water adsorption capacity (%)
PVP (%)
Cyrene
NMP
F : Gravimetric analysis of PES/C and PES/N.
casting solution. e produced membranes with the bio-
based solvent Cyreneshowed a greater total porosity, bigger
pore size, and higher thermal stability, compared to the
PES membranes produced with NMP. It was found that
no additive is necessary to form pores when Cyreneis
applied as solvent, PES/C having the same pore diameter as
NMP-based membranes produced with % PVP (PES/N),
meaning that no additive many not be necessary for the
role of pore forming when using Cyrene.PESmembranes
produced with Cyreneand .% PVP (PES/C.) and %
PVP (PES/C) developed the largest porosity (ca. %) for the
both Cyreneand NMP-based membranes. PES produced
with Cyreneand minimal concentration of PVP (.%
PVP) developed the same pore diameter as PES produced
with NMP at a higher concentration of PVP (%) and the
largest pores are registered in PES when using Cyrenewith
%PVP,whileforamembraneproducedwithNMP,the
largest pore diameter appears using a higher concentration
of PVP (%). e permeability of the new membranes
produced with Cyrenewas easily tailored in the range
between NF/RO to MF, by changing the temperature of the
casting gel, whereas only slightly small changes were observed
when using NMP. While the PES/N showed a trend with a
maximum ltration for PES/N cast from a cold gel when
% PVP was used, the same membranes cast from a hot
gel showed no supporting trend, probably because NMP
evaporated quicker than Cyrene at elevated temperatures.
Completely new morphologies were found in PES produced
in Cyrene, when using % PVP, regardless if cast hot or
cold. When using Cyrene, the morphology was dierent for
the two casting gels, with a symmetrical one observed when
hot cast; this is less obvious when cast from a cold gel. Both
hot and a cold cast PES/C have showed the best ltrate ux,
with the potential, for application in microltration. Utilising
cold casting, minimal addition of PVP leads to better ltra-
tion properties than the corresponding membrane produced
without any additive; this suggests PES/C. has applications
as a hydrophilic, highly biocompatible/haemodialysis ultra-
ltration membrane. e relatively high viscosity of Cyrene
adds to the overall viscosity of the PES/PVP casting gel and
this can be exploited to tailor membranes to dierent ranges
of ltration with dierent physical properties. Decreasing
viscosity with temperature is a potential factor to consider
when casting membranes containing Cyrene.
Data Availability
Data supporting this publication is available in the supporting
information while additional raw data is available via https://
doi.org/./cfa-caa--ad-fefcf.
Conflicts of Interest
e authors declare that they have no conicts of interest.
Acknowledgments
We extend our thanks to INGE.BASF for providing PES and
PVP polymers used in this project, Meg Stark from Bio-
science Technology Facility, Biology Department, University
of York for SEM images, and Dr Carlos Grattoni from School
of Earth and Environment, Leeds University for mercury
porosimetry and low-eld NMR spectroscopy. e authors
would like to acknowledge Circa Sustainable Chemicals Ltd.
for its nancial support of this work.
Supplementary Materials
Supplementary information is available and contains a table
listing all the solvents studied via HSPiP (SM Table ); a
plot for the temperature dependent viscosity of the PES/C
sample over the range of room temperature to ∘C(SM
Figure ); MICP pore diameter distributions for each sample
prepared in Cyrene(SM Figure ) or NMP (SM Figure
); a photograph showing the coagulation bath of PES/PVP
membranes in Cyreneand NMP (SM Figure ); SME cross-
section images of each membrane prepared in NMP (SM
Figure ); SEM cross-section images of PES/C at dierent
thickness (SM Figure ). (Supplementary Materials)
Advances in Polymer Technology
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