Phase Behavior and Temperature-Responsive Molecular Filters Based
on Self-Assembly of
Polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene
Antti Nyka1nen,
†
Markus Nuopponen,
‡
Antti Laukkanen,
‡,#
Sami-Pekka Hirvonen,
‡
Marjaana Rytela1,
§
Ossi Turunen,
§
Heikki Tenhu,
‡
Raffaele Mezzenga
⊥
,
|,
Olli Ikkala,
†
and Janne Ruokolainen*
,†
Department of Engineering Physics and Mathematics and Center for New Materials, Helsinki
UniVersity of Technology, P.O Box 2200, FI-02015 TKK, Finland; Department of Chemistry, UniVersity
of Helsinki, P.O. Box 55, 00014 Helsinki, Finland; Department of Chemical Technology, Helsinki
UniVersity of Technology, P.O. Box 6100, FI-02015 TKK, Finland; Department of Physics ad Fribourg
Center for Nanomaterials, UniVersity of Fribourg, Perolles Fribourg, CH-1700 Switzerland; and Nestle´
Research Center, Vers-Chez-les-Blanc, 1000 Lausanne 26, Switzerland
ABSTRACT: This work describes the synthesis of temperature-responsive polystyrene-block-poly(N-isopropy-
lacrylamide)-block-polystyrene triblock copolymers, i.e., PS-b-PNIPAM-b-PS, their self-assembly and phase
behavior in bulk, and demonstration of aqueous thermoresponsive membranes. A series of PS-b-PNIPAM-b-PS
triblock copolymers were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymer-
ization. The hydrophobic PS end blocks were selected to form the minority component, whereas the temperature-
responsive PNIPAM midblock accounted for the majority component. The self-assembly and phase behavior in
bulk of PS-b-PNIPAM-b-PS as well as selected blends with low molecular weight PNIPAM homopolymers
were studied using transmission electron microscopy (TEM). Classical lamellar, cylindrical, spherical, and
bicontinuous double gyroid morphologies were observed in the dried state. In aqueous solutions, the glassy PS
domains act as physical cross-links, and hydrogels were therefore formed. The bulk block copolymer morphology
had a strong effect on the degree of swelling in aqueous solutions upon cooling below the coil-globule transition
temperature of the PNIPAM midblock. Bulk compositions with spherical PS domains and PNIPAM continuous
phase swelled in water up to 58 times by weight, whereas composition having cylindrical PS domains or bicontinous
gyroid structure in bulk swelled 20 or 10 times by weight, respectively. Finally, lamellar compositions did not
show any swelling. Composite membranes for separation studies were prepared by spin-coating thin films of
PS-b-PNIPAM-b-PS on top of meso/macroporous polyacrylonitrile (PAN) support membrane. The permeability
was measured as a function of temperature using aqueous mixture of poly(ethylene glycol) (PEG) with several
well-defined molecular weights. The permeability showed a temperature switchable on/off behavior, where higher
permeability is obtained below transition temperature of PNIPAM, and the molecular cutoff limits for the PEG
molecules are surprisingly lowsbetween 108 and 660 g/mol. The results encourage to further develop and optimize
these materials for responsive nanofiltration applications.
Introduction
Stimuli-responsive polymers are attractive materials in which
biological materials can be a considerable source for inspiration,
as the latter ones often respond to conditions and stimuli.
Depending on the specific behavior and application, polymers
can be designed to sense and respond to changes in the
environmental conditions such as temperature,
1-4
pH,
5-8
mag-
netic or electric fields,
9-11
ionic strength,
12,13
added saccha-
rides,
14,15
antigen binding,
16
or light.
17,18
Even multiple func-
tionalities can be combined in order to respond, e.g., to both
pH and temperature due to incorporation of two stimuli-
responsive polymers.
19-22
Poly(N-isopropylacrylamide) (PNIPAM) is one of the most
extensively investigated synthetic temperature-responsive poly-
mers. In aqueous solutions it undergoes a coil-globule transi-
tion
23
at a temperature of ca. 32 °C, and this transition
temperature can be tuned by copolymerizing hydrophobic or
hydrophilic comonomers,
24
for example acrylic acid comono-
mers (PNIPAM-rnd-PAA).
25
PNIPAM-based polymers are
attractive in a number of applications, and in particular in the
biomedical field, owing to the temperature-responsive behavior
in water solutions near the body temperature.
Among the several ways to exploit the conformational
transition of PNIPAM to achieve functional behavior, a widely
studied concept is based on responsive polymer networks in
water solutions to form hydrogels and gelators. The networks
can be formed by permanent covalent cross-links, but the
possibility to melt, reprocess, or redissolve the material is lost.
On the other hand, physical cross-links maintain the process-
ability, as they can successively be broken and re-formed by
heating or by dissolving. In the latter case, a common concept
is based on ABA triblock copolymers. In the bulk state, they
self-assemble into spherical, cylindrical, lamellar, or double
gyroid morphologies.
26,27
The ABA block copolymers can be
swollen by midblock selective solvents, leading to the physically
cross-linked gels, where the end-block domains form the
physical cross-links. Either of the blocks can be stimuli-
* Corresponding author. E-mail: janne.ruokolainen@tkk.fi.
†
Department of Engineering Physics and Mathematics and Center for
New Materials, Helsinki University of Technology.
‡
University of Helsinki.
§
Department of Chemical Technology, Helsinki University of Technol-
ogy.
|
University of Fribourg.
⊥
Nestle´ Research Center.
#
Current address: Drug Discovery and Development Technology Center,
Faculty of Pharmacy, University of Helsinki, Helsinki, Finland.
1
http://doc.rero.ch
Published in "Macromolecules 40(16): 5827 -5834, 2007"
which should be cited to refer to this work.
responsive. In order to achieve stimuli-responsive hydrogels,
the midblock is water-soluble and stimuli-responsive which can
lead to control of gel swelling. By contrast, to achieve stimuli-
responsive gelators, the solubility of the end groups is controlled
by conditions and stimuli while the midblock remains soluble.
The recent literature extensively deals with polymeric gelators,
where the temperature response is realized by ABA triblock
copolymers,
28-30
ABC triblock copolymers,
31
or random and
gradient copolymers.
32
Protein-based materials have been used
to design responsive gelators,
33,34
where recombinant DNA
methods were used to prepare triblock-like proteins undergoing
reversible gelation in response to both pH and temperature due
to the coil-to-helix conformation transition.
33,35
Hydrogels and gelators are relevant in the field biomaterials,
since they can be used for targeted drug delivery, sensors,
membranes capable of releasing or separating selectively specific
substances, or actuators.
36-44
Even autonomous self-beating
systems have been demonstrated using random copolymers,
based on swelling/deswelling oscillations due to changes of the
ion concentration in the gel, as a consequence of reversible
enzymatic reactions.
13
Stimuli-responsive hydrogels have also
been used as components of composite materials. Thermor-
eversible color changes or light modulation is reported by
encapsulating colored responsive PNIPAM hydrogels within a
nonresponsive transparent gel matrix;
45
composite gels are
prepared based on poly(methacrylic acid) gel particles dispersed
in poly(dimethylsiloxane) (PDMS) rubber, capable of releasing
vitamin B12 in a pH-controllable manner;
36
and solvent-induced
gel swelling is used to change the spacing of PS beads dispersed
in the PDMS rubber matrix, thus tuning the photonic band gap
and optical properties of the material.
46
Yet, the most extensive application for stimuli-responsive
polymer composite materials is probably their use as active
components in porous membranes to control permeability and
molecular filtration. Composite membranes are commonly
realized by coating the surface of an existing porous membrane
by a responsive polymeric active layer.
7,47-53
Upon external
stimuli, the chains of the responsive polymer film stretch or
collapse, leading to a controllable opening and closing of the
membrane pores. Responsive membranes have also been
prepared by using chemically cross-linked bulk gels of stimuli-
responsive polymer
37
or by encapsulating the stimuli-responsive
polymer in the membrane during the membrane preparation.
54-57
Such membranes efficiently filter poly(ethylene glycol) (PEG)
molecules of size larger than 5000 Da
56,57
or dextran molecules
of size 100 kDa,
53
separate water-isopropanol mixtures,
37
and
control the permeability of fluorescein isothiocyanate-dextran
phosphate buffer solution.
50
A highly demanded improvement of membranes based on
hydrogels is to obtain a quicker response to the external stimuli.
This response is controlled by diffusion-activated mass transport
of the solvent through the gel. Several strategies have been
explored for increasing the response dynamics, such as intro-
ducing porosity
58
or by tailoring the hydrogel architecture at
the molecular level by grafting either hydrophobic
59
or hydro-
philic
60
side chains aiding the expulsion of water from the
network during the collapse. Since the times needed for swelling
and deswelling of the gels are directly proportional to the square
of the distance that water has to diffuse, it is, in principle,
possible to reduce the response time by decreasing the gel
dimensions.
61
It has been demonstrated that whilea1mmthick
gel of photo-cross-linked microgels has a characteristic response
time scale of several hours, the same gel responds to external
stimuli within a few second if its thickness is reduced to
300 nm.
62
In this paper the self-assembly and aqueous swelling behavior
of physically cross-linked hydrogels are studied. The system is
based on a triblock copolymer, PS-b-PNIPAM-b-PS, in which
the central PNIPAM block acts as the stimuli-responsive block.
The weight fraction of PS is varied in order to investigate the
phase behavior in bulk and to design novel aqueous hydrogels
based on lamellar, gyroid, cylindrical, and spherical block
copolymer morphologies. Figure 1 demonstrates the concept to
design the stimuli-responsive hydrogels using the spherical bulk
morphology. The gel swelling behavior is investigated, and
membranes are constructed by spin-coating triblock solutions
onto porous support membrane, whereas permeation studies are
carried out using a mixture of well-defined low molecular and
high molecular weight PEG polymers.
Experimental Methods
Materials. N-Isopropylacrylamide (NIPAM, 99%, Acros Organ-
ics, Belgium) was purified by recrystallization from toluene.
2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich Chemicals, Germany)
was purified by recrystallization from methanol. Dioxane (Lab Scan,
Ireland) and styrene were distilled prior to use. Homopolymeric
poly(N-isopropylacrylamide) (PNIPAM), which was used for blend
samples, was received from Polymer Source Inc. and was used
without further purification (M
n
) 6100 g/mol, M
w
/M
n
) 1.13).
Tetrahydrofuran (THF, 99%) was received from Fluka.
Synthesis of the S,S′-Bis(R,R′-dimethyl-R′′-acetic acid) Trithio-
carbonate (BDAT). The target compound was prepared by the
method described elsewhere.
63
Synthesis of Polystyrene (PS). Styrene, BDAT, and AIBN were
typically dissolved in 1,4-dioxane. Solutions were degassed by three
freeze-pump-thaw cycles. Vessel were sealed under vacuum and
placed in a thermostatically controlled oil bath (70 °C) to allow
polymerization during a well-defined predetermined time. After the
polymerization, the polymer was precipitated in cold methanol and
purified by repeated precipitations. The final product was dried in
vacuum to yield yellow powders. Polymerization details are listed
in Table 1.
Synthesis of the PS-b-PNIPAM-b-PS Triblock Copolymers.
A series of eight different PS-b-PNIPAM-b-PS triblock copolymers
Figure 1. Chemical structure of polystyrene-block-poly(N-isopropy-
lacrylamide)-block-polystyrene triblock copolymer. On the right, a
schematic illustration of temperature-induced conformation transition
of aqueous hydrogel having self-assembled morphology with spherical
PS domains. The latter domains act as physical cross-links for the
hydrogel, and as the temperature is raised above the coil-globule
transition temperature the PNIPAM chains become hydrophobic and
the gel collapses.
2
http://doc.rero.ch
were synthesized via reversible addition-fragmentation chain
transfer polymerization of NIPAM using well-characterized PS
precursors as macro RAFT-agents.
64,65
BDAT is a bifunctional
RAFT agent. Thus, synthesized block copolymers are A-B-A
triblock polymers with PS as an A block and PNIPAM as a B block.
SEC shows that the PS block is fully incorporated into the triblock
copolymer, and monomodal SEC traces prove pure triblock
copolymers. PS was dissolved in 1,4-dioxane before adding NIPAM
and AIBN. The mixture was stirred for 30 min at room temperature
to dissolve all the components. The solution was degassed by three
successive freeze-pump-thaw cycles (<1 mbar). The polymeri-
zation reaction was started by placing the mixture in an oil bath at
70 °C. The reaction was stopped by cooling the solution to ambient
temperature. The product was purified by two reprecipitations from
THF into diethyl ether and cold water. The product was freed from
the homopolymer PNIPAM by centrifugation at 29 °C (45 min,
5000 rpm). When the length of PS block was only 7200 g/mol, the
precipitation into cold water was not possible, however. In those
cases, the product was freed from the homopolymer PNIPAM by
dissolving the mixture in cold water and precipitating the block
copolymer by centrifugation at 29 °C (45 min, 5000 rpm). This
procedure was repeated three times. Purified polymers were freeze-
dried. The structure and purity of the polymers were ascertained
by
1
H NMR spectroscopy with a 200 MHz Varian Gemini 2000
spectrometer using deuterated chloroform as a solvent: δ (ppm,
CDCl
3
): 6.3-7.2 (5H, Ar-H), 4.0 (1H, -NCH-), 0.8-2.4 (3H,
PS backbone and 9H, PNIPAM -CH
3
and the backbone protons).
Molecular weight of PNIPAM block was calculated by comparing
the integral of 6.3-7.2 (5H, Ar-H) to 4.0 (1H, -NCH-).
Polymerization details are listed in Table 2.
Bulk Sample Preparation. Polymers were dissolved in THF to
yield 1.0 wt % solutions. To prepare the PS-b-PNIPAM-b-PS/
PNIPAM blends, the separate THF solutions of PS-b-PNIPAM-b-
PS and PNIPAM were combined according to the desired blend
composition, and the solutions were stirred for at least 4 h. The
solvent was evaporated at room temperature, and the samples were
dried in vacuum at room temperature for 4-6 h and annealed at
ca. 180 °C under a high vacuum (10
-8
mbar) for 3-4 days.
Swelling Experiments. Annealed and dried samples were
weighted and immersed in Millipore water. The water temperature
was stepwise risen from 4 °C to 15, 25, 35, 45, and 55 °C. At each
step the temperature was let to stabilize for at least 1 h, after which
the sample was taken out from the water container, and the gel
surface was gently dried with a filter paper (Shleicher and
Schuell 5892). The surface dried sample was weighted and photo-
graphed.
Filtration Experiments. Thin films were spin-coated on top of
a porous support membrane consisting of a polyacrylonitrile (PAN)
layer on a polyester fiber support fleece, as received from GKSS
Research Centre Geesthacht GmbH, Geesthacht, Germany (Figure
2). The support membrane has a diameter of 25 mm. The PAN
layer had thickness of ca. 10 μm, and pores were in the range 10-
70 nm with irregular shapes and size distribution (see Figure 2B).
The polyester fiber support fleece below the PAN layer consisted
of randomly oriented fibers with diameter of ca. 10 μm. The
thickness of the support membrane was ∼0.2 mm. For spin-casting,
60 μL of 7.5 wt % THF solution of PS-b-PNIPAM-b-PS was
dropped on the center of the support membrane, which was spinning
at speed 1000 rpm. Thin films were dried in the same manner as
the bulk samples. In addition for some of the samples, annealing
in THF vapor for several hours at room temperature was used in
Table 1. Details of the Polymerization of Polystyrene Homopolymers
polymer monomer A (concn, solvent, temp) BDAT
a
(mM) AIBN (mM) time (h) M
n
b
(g mol
-1
) M
w
/M
n
b
PS-7200 styrene (4.7 M, THF, 60 °C) 2.9 0.6 20 7 200 1.21
PS-17700 styrene (2.2 M, dioxane, 70 °C) 0.8 0.8 48 17 700 1.57
PS-27600 styrene (2.2 M, dioxane, 70 °C) 1.3 0.8 48 27 600 1.50
PS-37000 styrene (2.9 M, dioxane, 70 °C) 0.9 0.9 72 37 000 1.55
PS-41000 styrene (2.2 M, dioxane, 70 °C) 0.7 0.8 48 41 000 1.38
PS-41200 styrene (2.2 M, dioxane, 70 °C) 0.7 0.8 48 41 200 1.31
a
Chain transfer agent S,S′-bis(R,R′-dimethyl-R′′-acetic acid) trithiocarbonate.
b
Determined by SEC using calibration with PS standards.
Table 2. Details of RAFT Polymerizations and the Resulting PS-b-PNIPAM-b-PS Triblock Copolymers
sample code
g
PNIPAM block
(concn, temp, time)
a
PS block
b
(concn)
c
M
n
d
(g mol
-1
) M
w
/M
n
e
wt %
f
PNIPAM morphology
PN82.40K 1.0 M, 70 °C, 24 h PS-7200 (0.5 mM) 40 500 1.44 82 spherical
PN79.35K 1.0 M, 70 °C, 24 h PS-7200 (0.8 mM) 34 900 1.26 79 spherical
PN77.118K 1.1 M, 70 °C, 20 h PS-27600 (1.2 mM) 118 300 1.51 77 spherical
PN72.25K 1.0 M, 70 °C, 24 h PS-7200 (1.6 mM) 25 400 1.23 72 spherical
PN72.63K 2.2 M, 70 °C, 18 h PS-17700 (2.8 mM) 63 200 1.41 72 cylindrical
PN61.106K 0.9 M, 70 °C, 18 h PS-41200 (1.2 mM) 106 000 1.52 61 gyroid
PN55.91K 1.0 M, 70 °C, 24 h PS-41000 (1.6 mM) 90 500 1.26 55 lamellar
PN43.65K 0.7 M, 70 °C, 18 h PS-37000 (1.3 mM) 64 600 1.27 43 lamellar
a
Polymerizations were conducted in 1,4-dioxane.
b
RAFT macro-transfer agent.
c
Chain transfer agent S,S′-bis(R,R′-dimethyl-R′′-acetic acid) trithiocarbonate.
d
M
n
of the A-B block copolymer determined with
1
H NMR spectroscopy.
e
Determined by SEC using calibration with PS standards.
f
Determined with
1
H
NMR spectroscopy.
g
Sample notation is as follows: the value after PN label (PN ) poly(N-isopropylacrylamide)) is the total weight fraction of the PNIPAM
in the PS-b-PNIPAM-b-PS sample. The second number is the total molecular weight of the sample (g mol
-1
).
Figure 2. (A) Schematic view of the multilayer composite filter. (B)
SEM micrograph of the top surface demonstrating the meso/
macroporous structure of the porous support membrane consisting of
a polyacrylonitrile (PAN) layer on top of the polyester fiber support
fleece.
3
http://doc.rero.ch
order to improve the thin film morphology. Figure 2 illustrates the
layered structure of the membranes described in this work.
The filtration experiments were performed below and above the
transition temperature of PNIPAM, i.e., at 4 and at 60 °C,
respectively, to study the effect of temperature on the filtration.
The membranes were immersed in water at the given temperature
for at least 1 h before the experiment to stabilize the swelling of
the PNIPAM block. Ultrafiltration cell model 8003 from Millipore
was used. The filtration cell was connected to nitrogen gas line,
and by applying 4 bar pressure the solution was pushed through
the membrane. To probe the permeability, an aqueous mixture of
six different poly(ethylene glycol) (PEG) molecules of molecular
weights of 108, 660, 1130, 5400, 30 800, and 187 000 g/mol was
used where the total PEG concentration was 0.2 wt %. The filtered
solution was collected for size exclusion chromatography (SEC)
analysis. The relative concentration of each PEG component in the
filtered solution was determined by comparing the peak areas of
each PEG component in the original and filtered solutions.
Size Exclusion Chromatography (SEC) for the Synthesis
Analysis. Measurements were performed with a Waters liquid
chromatography system equipped with a Waters 2410 differential
refractometer and a Waters 2487 UV as detectors. Three Styragel
columns (HR2, HR4, HR6) were used in series. HPLC-grade THF
was used as an eluent with a flow rate of 0.8 mL/min at 35 °C.
Size Exclusion Chromatography for the Filtration Analysis.
Measurements were performed with a Waters Alliance 2690 liquid
chromatography system equipped with a RI Waters 2414 detector.
Three Shodex columns (OH pak SB 802 HQ, OH pak SB 802,5
HQ, OH pak SB 803 HQ) from Showa Denko were used in series.
Water was used as an eluent with a flow rate of 0.7 mL/min at
50 °C.
Transmission Electron Microscopy (TEM). The bulk samples
were glued on top of microtome sample holder tip and sectioned
using Leica Ultracut UCT ultramicrotome with a Diatome diamond
knife at temperature -100 °C. Sections thickness of ca. 70 nm were
collected on 600-mesh size copper grids. For spin-coated mem-
branes the sample preparation was done as follows. Pieces of the
membranes were embedded into epoxy (Ebonate 12, Electron
Microscopy Science). Before the embedding, membranes were
carbon-coated and the epoxy was precured at 60 °Cfor3hto
minimize the epoxy diffusion into the sample. Thereafter, curing
of the epoxy was continued overnight. Cross-sectional thin sections
of the membrane were cryo-cut at -37 °C using 50/50 solution of
DMSO (Fluka)/water in which PNIPAM is insoluble.
Results and Discussion
PS-b-PNIPAM-b-PS Structures in Bulk. The self-assembled
morphologies in bulk were characterized using TEM. Figure 3
shows the lamellar, gyroid, cylindrical/wormlike, and spherical
structures for PS-b-PNIPAM-b-PS with 43, 61, 72, and 77 wt
% PNIPAM, respectively, i.e., samples PN43.65K, PN61.106K,
PN72.63K, and PN77.118K. Compositions with a minor fraction
of PNIPAM were not investigated as they were not expected
to swell efficiently in aqueous solutions.
In order to additionally tune the compositions for phase
diagram studies of block copolymers, a well-known method was
also employed to blend relatively low molecular weight ho-
mopolymers in the block copolymer.
66
Therefore, low molecular
weight PNIPAM homopolymer (M
n
) 6100 g/mol) was blended
with PN55.91K and PN43.65K, which had PNIPAM block
lengths M
n
PNIPAM
) 49 500 g/mol and M
n
PNIPAM
) 27 600 g/mol,
respectively, and had lamellar structures without blending. The
short length of the homopolymer chain length allowed sufficient
difference in chemical potential with respect to PNIPAM block
and thus sufficient penetration into the PNIPAM blocks, leading
to conditions of “wet brush”.
67,68
Blending PNIPAM homopoly-
mer leads to new structures: for example, pure PN55.91K
(55 wt % PNIPAM, M
n
total
) 90 500 g/mol) triblock copolymer
is lamellar in bulk but becomes cylindrical and then spherical
upon addition of PNIPAM homopolymer (Figure 4), as the total
weight fraction of PNIPAM is increased from 55 wt % to 66
and 76 wt %, respectively.
The observed morphologies are quite expected: At the strong-
segregation limit, the ABA triblock copolymer phase diagram
can be considered to be similar to that of AB diblock copolymers
with half of the molecular weight of the corresponding ABA.
26,69
In this work, the observed structures were plotted into molecular
weight/composition diagram in order to illustrate the phase
behavior for the PNIPAM-rich compositions (see Figure 5). At
72 wt % PNIPAM, spherical and cylindrical morphologies are
observed for both low and high molecular weights, respectively,
as may be expected by a change from the weakly to strongly
segregated regimes. However, the structure remains spherical
beyond 77 wt % PNIPAM in all studied cases, irrespective of
the PS-b-PNIPAM-b-PS molecular weight.
Aqueous Swelling. The purpose of the aqueous swelling
experiments was to study permeation through the gels as a
function of the temperature. Upon immersing PS-b-PNIPAM-
b-PS in water, the PNIPAM middle block is expected to swell
at temperatures below its coil-globule transition temperature
of ca. 32 °C, thus forming network structures, i.e., gels, due to
the physical cross-links by the hydrophobic PS domains. The
self-assembled bulk structure is expected to strongly affect the
extent of swelling due to the shapes of the glassy PS domains.
As temperature is increased beyond the transition temperature,
also PNIPAM becomes hydrophobic, causing a tendency toward
conformational collapse. Results of the aqueous swelling
experiments are shown in Figure 6 as a function of temperature
for the different bulk morphologies of PS-b-PNIPAM-b-PS. The
vertical axis depicts the weight ratio between the wet and dry
PS-b-PNIPAM-b-PS, while the horizontal axis gives the tem-
perature at which the water uptake is measured. Above ca.
35 °C, no swelling is observed for any of the morphologies, as
both PS and PNIPAM are hydrophobic. As expected, PS-b-
PNIPAM-b-PS with spherical bulk morphology swells most as
the glassy PS spheres do not restrict expansion in any direction.
By cooling, sample PN79.35K (79 wt % PNIPAM, M
n
total
)
Figure 3. Representative TEM micrographs of pure PS-b-PNIPAM-
b-PS triblock copolymers in bulk: (A) PN43.65K (43 wt % PNIPAM)
is lamellar, (B) PN61.106K (61 wt % PNIPAM) is gyroid, (C)
PN72.63K (72 wt % PNIPAM) is cylindrical/wormlike, and (D)
PN77.118K is spherical.
4
http://doc.rero.ch
34 900 g/mol) swells ca. 58 times of its dry weight. Not
unexpectedly, the lowest swelling is observed for PS-b-
PNIPAM-b-PS having lamellar morphology in bulk, where
swelling was not observed at all. In this case, the glassy and
lamellar PS domains hinder the diffusion of water in the
direction orthogonal to the lamellae, and water transport through
the entire sample is reduced. Second, the glassy PS lamellar
domains have fixed shapes and in principle allow swelling of
the PNIPAM domains only in the orthogonal direction vs the
lamellar plane. Finally, since the lamellar phase is not oriented,
there exists a large number of defects and grain boundaries
between the glassy PS domains, thus effectively causing mutual
interlocking between the glassy PS lamellae. In the bicontinuous
gyroid morphology, penetration and diffusion of water are not
hindered along any specific direction, since both PS and
PNIPAM phases are in principle continuous throughout the
material, thus allowing free expansion of the gel. Yet, the
presence of a three-dimensional percolating glassy PS skeleton
restricts the swelling of the PNIPAM domains, and therefore
sample PN61.106K (61 wt % PNIPAM, M
n
total
) 106 300
g/mol) swells up to 10 times. In the cylindrical morphology
where PS forms cylinders and PNIPAM the continuous phase,
the expansion in the direction radial to cylinders is unrestricted,
while in the direction parallel to the cylinders the PS the swelling
is reduced. The larger degree of swelling observed for cylinders
(up to 20) as compared to the gyroid phase is therefore due to
the reduced reinforcing effect of the glassy cylinders in
comparison to that of the gyroid skeleton. Finally, for each case,
the swelling is reversible upon heating and cooling, and the
absorbed water can be released by increasing the temperature
above the coil-globule transition temperature.
Filtration Experiments. The temperature-responsive separa-
tion was studied using thin films of PS-b-PNIPAM-b-PS spin-
cast on the porous PAN support membrane. The cutoff
molecular weight of the PAN support membrane was above
30 kg/mol; i.e., rather large molecules can permeate without
essentially hindered through it. Two PS-b-PNIPAM-b-PS block
copolymers were selected for closer filtration studies, both
having a continuous PNIPAM phase in the annealed bulk state
with either self-assembled spherical (PN77.118K, PNIPAM
77 wt %, M
n
total
) 118 300 g/mol) or gyroid (PN61.106K,
PNIPAM 61 wt %, M
n
total
) 106 300 g/mol) structure. Cross-
sectional TEM micrographs of a spin-coated polymer films on
the PAN support membrane are shown in Figure 7a,b. The
thickness of the film was of the order of 500-1000 nm, and
Figure 4. TEM micrographs of PS-b-PNIPAM-b-PS/PNIPAM blends
based on PN55.91K (55 wt % PNIPAM, M
n
total
) 90 500 g/mol). (A)
Without added homopolymer a lamellar morphology is observed. (B)
76/24 w/w blend is cylindrical. (C) Blend 53/47 w/w is spherical.
Figure 5. Observed block copolymer morphologies of PS-b-PNIPAM-
b-PS and PS-b-PNIPAM-b-PS/PNIPAM blends in bulk.
Figure 6. Swelling of PS-b-PNIPAM-b-PS hydrogels. (A) A photo-
graph of dry PN77.118K sample (77 wt % PNIPAM, M
n
total
)
118 300 g/mol). (B) The same sample at 5 °C in water solution and
(C) the same sample at 55 °C in water solution. (D) Relative gel mass
m
wet
/m
dry
plotted as a function of temperature. The morphologies as
labeled as lamellar (L), gyroid (G), cylindrical (C), and spherical (S).
5
http://doc.rero.ch
