Tetra-sensitive graft copolymer gels with high volume changes

Article abstract of DOI:10.1039/c6ra01857b

For the preparation of multi-responsive graft copolymer gels for hydrogel-based microsystem technologies, a poly(4-vinylbenzoic acid) macromonomer was prepared in a three-step synthesis. At first, a chain transfer agent was functionalized with an azide group, followed by a reversible addition fragmentation chain transfer polymerization to give an azide bearing poly(4-vinylbenzoic acid) macromonomer. Subsequently, an acrylamide functionalized macromonomer was prepared by Cu-catalyzed alkyne and azide 1,3-dipolar cycloaddition. With this acrylamide functionalized poly(4-vinylbenzoic acid) macromonomer, various graft copolymer gels composed of a temperature-sensitive poly(N-isopropylacrylamide) backbone and pH-sensitive poly(4-vinylbenzoic acid) graft chains were prepared by radical polymerization. Moreover, the swelling behavior of the hydrogels was investigated with respect to pH, temperature, solvent and salt. It was shown that the graft copolymer gels respond to these stimuli independently with a sharp transition and a high volume change. Additionally, the swelling behavior was reversible over various cycles. This behavior is fundamental for applications of smart gels in microfluidic platforms.

Full text of DOI:10.1039/c6ra01857b

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Tetra-sensitive graft copolymer gels with high
volume changes†
Cite this: RSC Adv., 2016, 6, 34809
a
a
abc
D. Grafe,abc S. Zschoche, D. Appelhans and B. Voit
*
*
¨
For the preparation of multi-responsive graft copolymer gels for hydrogel-based microsystem
technologies, a poly(4-vinylbenzoic acid) macromonomer was prepared in a three-step synthesis. At
first, a chain transfer agent was functionalized with an azide group, followed by a reversible addition
fragmentation chain transfer polymerization to give an azide bearing poly(4-vinylbenzoic acid)
macromonomer. Subsequently, an acrylamide functionalized macromonomer was prepared by Cu-
catalyzed alkyne and azide 1,3-dipolar cycloaddition. With this acrylamide functionalized poly(4-
vinylbenzoic acid) macromonomer, various graft copolymer gels composed of a temperature-sensitive
poly(N-isopropylacrylamide) backbone and pH-sensitive poly(4-vinylbenzoic acid) graft chains were
prepared by radical polymerization. Moreover, the swelling behavior of the hydrogels was investigated
with respect to pH, temperature, solvent and salt. It was shown that the graft copolymer gels respond to
these stimuli independently with a sharp transition and a high volume change. Additionally, the swelling
behavior was reversible over various cycles. This behavior is fundamental for applications of smart gels in
microfluidic platforms.
Received 21st January 2016
Accepted 27th March 2016
DOI: 10.1039/c6ra01857b
www.rsc.org/advances
source is needed and complexity of microuidics platforms can
be signicantly reduced.16,19,20 In addition to that, it would be
Introduction
benecial and ideal if the hydrogel could respond to multiple
stimuli. Optimally, these stimuli should be addressable inde-
pendently with a sharp transition and a high volume change. In
this regard, a combination of the stimuli temperature and pH is
preferred since the regulation function of the gel can be carried
out by an external control unit (temperature) as well as by
a chemical information in the substrate ow (pH change).
It has been shown that temperature-responsive poly(N-iso-
propylacrylamide) (PNIPAAm) hydrogels are capable for ow
control.14,21 Control over the hydrogel function is caused by the
coil-to-globule transition of PNIPAAm above the lower critical
Hydrogels are three-dimensional, cross-linked networks of
hydrophilic polymers able to absorb and retain large amounts
of water. It is possible to prepare “smart” hydrogels by using
responsive monomers, which change their volume in response
to alterations of certain environmental stimuli. These stimuli
can be classied into three categories:1 physical (e.g. tempera-
ture, ionic strength, electric elds), chemical (e.g. pH, specic
ions) or biomedical (e.g. enzyme substrates, affinity ligands).
Due to the ability to control and tailor the properties of these
materials, hydrogels are very attractive for a wide range of
applications such as in biomedicine (drug delivery system),2–5
catalysis,6–8 membranes,9–11 cell culture support,12,13 and as
actuator/sensor materials in microuidics.14–16
Microuidics is a relatively new eld of interest with high
potential in multiplexing, automation and high-throughput
screening.17,18 Here, the incorporation of stimuli-responsive
hydrogels as chemomechanical valve is highly promising
because hydrogels combine sensor and actuator abilities in one
element. With these key characteristics, no additional power
ꢀ
solution temperature (LCST) around 32 C in water accompa-
nied by a macroscopic phase separation.22–25 Consequently,
PNIPAAm hydrogels exhibit a similar transition near 32 ꢀC
associated with high shrinking in volume. Note that the critical
temperature for hydrogels is called volume phase transition
temperature (VPTT). The most common approach to achieve
a multi-responsive material is the copolymerization. It is well
known that small amounts of hydrophilic comonomers
increase the LCST of PNIPAAm, while hydrophobic comono-
mers decrease the latter value.24,26 However, a limitation of
PNIPAAm hydrogel is its poor tolerance of comonomers.23,27
a
¨
Leibniz-Institut fur Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden,
Germany. E-mail: applhans@ipfdd.de; voit@ipfdd.de
Above
a critical comonomer content (5–10%), PNIPAAm
b
¨
Technische Universitat Dresden, Chair of Organic Chemistry of Polymers, 01062
hydrogels exhibited a declining temperature sensitivity caused
by the increasingly interruption of the PNIPAAm segments by
comonomer units. In order to overcome this drawback different
approaches have been investigated such as semi-
Dresden, Germany
cTechnische Universitat Dresden, Cluster of Excellence ‘Center for Advancing
¨
Electronics Dresden’, 01062 Dresden, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra01857b
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interpenetrating networks,28 interpenetrating networks,29–31 or amounts of both solutions were mixed and spotted onto the
gra copolymer gels.27,32–37 Gra copolymer gels by free radical MALDI plate.
polymerization were rst reported in 1995 by Okano et al.36 It
IR analysis were recorded using a Vertex 80v (Bruker) with
has been shown that net-PNIPAAm-g-PNIPAAm hydrogels a DTGS-Detector. Samples were mixed with KBr (2 mg sample
exhibit enhanced deswelling kinetics compared to net-PNI- per 400 mg KBr), milled, and prepared as pellets. The spectra
PAAm hydrogels. Since then, several studies have conrmed the were recorded in the middle infrared spectroscopic range 4000–
improved swelling kinetics of graed hydrogels.27,37 This ability 400 cmꢁ1 with 2 cmꢁ1 resolution. Thirty-two scans were added
is highly demanded from the application point of view because to every spectrum.
it accelerates the potential response time of the hydrogel as
Differential scanning calorimetry (DSC) was performed on
a chemomechanical micro-valve. Another strategy for preparing a DSC Q2000 (TA-Instruments) under N2 in the range 5–50 ꢀC at
gra copolymer gels is to use controlled/living radical poly- a heating rate of 1 ꢀC minꢁ1 as cycles consisting of 1st heating–
merization (CRP). However, the synthesis of gels by CRP tech- cooling-2nd heating scans. The VPTT of the hydrogels was
niques results in
a different gelation process including determined by averaging the maximum of the endothermic
a retarded gelation kinetic and higher swelling degrees of gels.38 transition peak in the DSC plot.
These higher swelling degrees are desired for applications in
Size-exclusion chromatography (SEC) measurements were
microuidics where a pronounced stimuli response is needed. carried out on a PL-GPC 50 Plus (Polymer Laboratories, USA)
Additionally, it has been reported for CRP using RAFT agents normal-temperature size exclusion chromatograph, equipped
that low ratios of chain transfer agent (CTA) to crosslinker are with refractive index detector and one column ResiPore (Agilent
needed to obtain graed gels instead of hyperbranched Technologies, USA) and on a second system GPC Series 1200
polymers.39,40
(Agilent Technologies, USA) with refractive index detector and
Here in this study we are interested in the preparation of one column PL MIXED-C (Agilent Technologies, USA). THF was
temperature and pH-responsive hydrogels for application in used as eluent and the ow rate was 1 ml minꢁ1. Dispersity (Đ ¼
microuidics. The chosen approach allows to control the gels Mw/Mn) of obtained polymers was determined based on cali-
architecture and the segment length of two types of polymers bration with polystyrene standards obtained from Polymer
incorporated into the gels and therefore, it provides the Standards Service (PSS, Germany). We converted the poly(4-
following advantages: (1) preparation of gels based on NIPAAm vinylbenzoic acid) to poly(methyl-4-vinylbenzoate) via methyla-
and a second responsive monomer, 4-vinylbenzoic acid (VBA), tion by MeOH, dicyclohexylcarbodiimide (DCC) and 4-
without losing temperature response while pH response is dimethylamino-pyridine (DMAP) before conducting the SEC
introduced, (2) improved swelling kinetics and (3) measurement.
a pronounced stimuli response associated with high swelling
ratio and thus high volume change. Due to the choice of graed
hydrogels, not only pH but also salt and solvent responsivity has
Materials
been introduced. To our knowledge, this is the rst report of All chemicals were obtained from Sigma-Aldrich Chemical Co.,
a temperature-, pH-, solvent- and salt-sensitive gra copolymer NIPAAm was puried by recrystallization from n-hexane, and
gel. This gel is composed of a PNIPAAm backbone and PVBA vacuum dried. All other chemicals were used as received.
gra chains. In order to obtain gra copolymer gels, an acryl-
N-(Prop-2-yn-1-yl)acrylamide (3). To a 100 ml Schlenk-style,
amide functionalized macromonomer is synthesized using long-neck round-bottom ask equipped with a magnetic
reversible addition fragmentation chain transfer (RAFT) poly- stirrer prop-2-yne-1-amine (1) (1.28 ml, 20 mmol) and 20 ml of
merization and Cu-catalysed alkyne and azide 1,3-dipolar dichloromethane (DCM) were added. The solution was sealed
cycloaddition. Furthermore, we report on the preparation of net- under argon atmosphere and submerged in an ice bath. In
PNIPAAm-g-PVBA hydrogels with different graing density, a separate vessel under argon atmosphere, acryloyl chloride (2)
their detailed study with regard to swelling behaviour with (2.5 ml, 27 mmol) was dissolved in 10 ml DCM, and the
respect to pH, temperature, solvent and salt concentration and resulting solution was slowly added to the long-neck round-
we compare this behaviour with pure net-PNIPAAm hydrogel.
bottom ask. Thus the reaction was carried out at r.t. for 3 d.
The solution was then added to 20 ml water, ltered, and
extracted with diethyl ether (4 ꢂ 40 ml). The organic layer was
dried with potassium sulfate, ltered, and then stripped of
solvent under reduced pressure. The crude product was eluted
through a silica gel column using n-hexane/ethyl acetate (40 : 60
Experimental
Characterization techniques
1H and 13C NMR spectra were recorded using Bruker Avance III v/v) to yield 3 as a white-yellow solid, yield 1.5 g (67%). 1H NMR
500 spectrometer operating at 500.13 MHz (1H) and 125.77 MHz (500 MHz, CDCl3, d): 2.2 (t, J ¼ 2.8 Hz, 1H, CH), 4.1 (dd, J ¼ 5.4,
(13C), with CDCl3 or DMSO-d6 as solvent at room temperature. 2.5 Hz, 2H; CH2), 5.7 (dd, J ¼ 10.5, 1.6 Hz, 1H; CH2), 6.0 (br. s.,
MALDI-TOF mass spectra were recorded on an Autoex 1H; NH), 6.1 (dd, J ¼ 16.8, 10.3 Hz, 1H; CH), 6.3 (dd, J ¼ 16.9, 1.6
Speed MALDI-TOF-/TOF-TOF-System (Bruker, USA) in reector Hz, 1H; CH2). 13C NMR (126 MHz, CDCl3, d): 29.18 (CH2), 71.58
mode with a smart beam laser (modied Nd:YAG laser). Solu- (CH), 79.31 (C), 127.08 (CH2), 130.18 (CH), 165.28 (C]O).
tions of the analyte were prepared in THF (1 mg mlꢁ1) and
dithranol was used as the matrix (10 mg mlꢁ1 in THF). Equal bottom equipped with a magnetic stirrer and reux condenser
3-Azido-1-propylamine (5). To a 250 ml one-neck round-
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3-chloropropyl-1-amine hydrochloride (4) (8.2 g, 63.1 mmol), Da) in water for 3 days and freeze-dried to yield PVBA-AAm as
sodium azide (12.6 g, 193.8 mmol) and 50 ml of water were a yellow solid, yield 175 mg (86%). 1H NMR (500 MHz, DMSO-d6,
added. The reaction was carried out for 24 h at 80 ꢀC. The d): 0.6–0.8 (t, 3H); 1.05–2.3 (m, 3H); 4.15 (m, 2H); 4.36 (m, 2H);
resulting solution was basied with sodium hydroxide and 2.28 (s, 6H); 5.58 (d, 1H); 6.10 (d, 1H); 6.24 (dd, 1H); 6.3–7.3 (m,
extracted with diethyl ether (4 ꢂ 40 ml). The organic layer was 2H); 7.3–8.0 (m, 2H); 12.0–13.5 (m, 1H).
dried with potassium sulfate, ltered, and then stripped of
Hydrogel synthesis. To a 5 ml one-neck round-bottom ask
solvent under a reduced pressure. Compound 5 was obtained as equipped with a magnetic stirrer NIPAAm (141.5 mg; 1.25
volatile, colourless oil, yield 4.1 g (65%). 1H NMR (500 MHz, mmol), N,N0-methylenebisacrylamide (BIS) (2.89 mg, 0.0186
CDCl3, d): 3.39 (t, J ¼ 6.7 Hz, 2H; CH2), 2.82 (t, J ¼ 6.8 Hz, 2H; mmol), azobisisobutyronitrile (AIBN) (2.05 mg, 0.0125 mmol)
CH2), 1.74 (m, 2H; CH2), 1.26 (br. s., 2H; NH2).
and 1 ml of pyridine were added. The PVBA-AAm content was
DTP-N3. To a 50 ml one-neck round-bottom ask equipped varied as follows: (1) 0 mg (0 mmol); (2) 15.5 mg (0.0031 mmol);
with a magnetic stirrer 2-(dodecylthiocarbonothioylthio)-2- (3) 31 mg (0.0063 mmol); (4) 62 mg (0.0125 mmol). The resulting
methylpropionic acid (DTP) (915 mg, 2.51 mmol), solution was subjected to three freeze–pump–thaw cycles,
(benzotriazol-1-yloxy)tripyrrolidinophosphonium hexauoro- transferred into a glass tube of 3 mm diameter, sealed under
phosphate (PyBOP) (1470 mg, 2.83 mmol) and 8 ml of DCM argon atmosphere, and submerged in an oil bath maintained at
were added. The solution was stirred at r.t. for 2 h. In a separate 70 ꢀC. The reaction was carried out for 24 h. Subsequently, the
reaction vessel 5 (250 mg, 2.5 mmol) and N,N'-diisopropyle- gel was separated from the glass tube and washed several times
thylamine (DIPEA) (797 mg, 6.17 mmol) was added to 2 ml of with water.
DCM. Subsequently, the resulting mixture was added to the
one-neck round-bottom ask and stirred at r.t. for 3 d. The
solution was then stripped of solvent under reduced pressure
and the crude product was then eluted through a silica gel
Results and discussion
Synthesis
column using n-hexane/ethyl acetate (80 : 20 v/v) to yield DTP-
1
The aim of this study is to prepare multi-responsive hydrogels
N3 as a yellow solid, yield 870 mg (78%). H NMR (500 MHz,
based on gra copolymer gels. For this reason, an acrylamide
DMSO, d): 0.86 (t, J ¼ 6.8 Hz, 3H; CH3), 1.2–1.4 (m, 18H; C9H18),
end-functionalized poly(4-vinylbenzoic acid) macromonomer,
PVBA-AAm, was synthesized. This was done by a combination of
RAFT and click chemistry, a well-known approach for preparing
polymers with high end-group functionalization (Scheme 1).41
1.5–1.7 (m, 10H; C2H4 and 2CH3), 3.09 (q, 2H; CH2), 1.8 (quin, J
¼ 6.5 Hz, 2H; CH2), 3.2–3.4 (m, 4H; 2 ꢂ CH2), 7.89 (t, 1H; NH).
13C NMR (126 MHz, CDCl3, d): 14.07 (CH3), 22.65 (CH2), 25.85
(2CH3), 27.74 (CH2), 28.92 (CH2), 29.07 (CH2), 29.30 (CH2), 29.41
The synthetic approach of the macromonomer designed for
(CH2), 29.51 (CH2), 29.59 (CH2), 29.60 (2CH2), 31.88 (CH2), 37.12
the fabrication of gra copolymer gels is shown in Scheme 1.
(CH2), 37.93 (CH2), 49.53 (CH2), 57.16 (C), 172.64 (C]O), 220.31
Azide bearing RAFT agents have been widely studied in the
context of post-polymerization functionalization. For example,
(C]S).
PVBA-N3. To a 10 ml Schlenk-style, long-neck round-bottom
ask equipped with a magnetic stirrer DTP-N3 (150 mg, 0.34
mmol), 4-vinylbenzoic acid (2.43 g, 16.4 mmol), 4,40-azobis(4-
cyanovaleric acid) (ACP) (4.2 mg, 0.015 mmol) and 5.5 ml of
DMF were added. The solution was then subjected to three
freeze–pump–thaw cycles, sealed under argon atmosphere, and
submerged in an oil bath maintained at 80 ꢀC. Aer 4 h the
reaction was exposed to oxygen and quenched in liquid
nitrogen. The solution was stripped of solvent under reduced
pressure; the crude product was then dissolved in THF/MeOH
(50 : 50 v/v) and precipitated in toluene to yield PVBA-N3 as
a yellow solid. Mn ¼ 4500 g molꢁ1 (54%), Đ ¼ 1.4, 1H NMR (500
MHz, DMSO-d6, d): 0.6–0.8 (t, 3H), 1.05–2.3 (m, 3H), 6.3–7.3 (m,
2H), 7.3–8.0 (m, 2H), 12.0–13.5 (m, 1H).
PVBA-AAm. To a 10 ml one-neck round-bottom ask equip-
ped with a magnetic stirrer PVBA-N3 (200 mg, 0.044 mmol), 3 (7
mg, 0.064 mmol) and 1.5 ml of DMF were added. In a second
reaction vessel, CuSO4$5H2O (1.1 mg, 0.0044 mmol) and
sodium ascorbate (0.88 mg, 0.0044 mmol) were dissolved in 150
ml water, respectively, and the resulting mixtures were added to
the one-neck round-bottom ask. The reaction was carried out
at r.t. for 3 d. Subsequently, the solution was stripped of solvent
under reduced pressure; the crude product was then dissolved
Scheme 1 Preparation of N-(prop-2-yn-1-yl)acrylamide (3), PVBA-
AAm and net-PNIPAAm-g-PVBA hydrogels. Reagents and conditions:
(i): NEt3, DCM, 0 ꢀC to r.t., 3 d, 67%. (ii) NaN3, H2O, 80 ꢀC, 1 d, 65%. (iii)
PyBOP, DIPEA, DCM, r.t., 78%. (iv) VBA, ACP, DMF, 80 ꢀC, 4 h, Ar, 54%.
in THF/MeOH (50 : 50 v/v) and precipitated in n-hexane/ethyl
acetate (60 : 40 v/v). The polymer was dialyzed (MWCO ¼ 1000 AIBN, pyridine, 70 ꢀC, Ar, 1 d.
(v) CuSO4$5H2O, sodium ascorbate, DMF/H2O (5 : 1), r.t., 3 d, 86%. (vi)
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Chen et al. and Sumerlin et al. reported the functionalization of
DTP with 3-azidopropan-1-ol.22,42–44 For our purposes, we
selected to use 3-azidopropan-1-amine (5) (Scheme 1) due to the
fact that amide bonds appear to be generally more stable to
hydrolysis than ester bonds.45 This is an important requirement
for long-term stability of pH sensitive hydrogels in microuidic
platforms. The precursor 5 was prepared according to Pike et al.
1H NMR study of 5 outlined the requested patterns as found in
literature.46 Then, the functionalization of DTP with 5 was
examined to obtain the azide bearing DPT-N3. It should be
noted that trithiocarbonates rapidly undergo aminolysis with
primary or secondary amines to form thiols and subsequently
dithiocarbamates.47,48 In fact, the functionalization of DPT was
attempted by using a standard Steglich approach with DCC and
DMAP.49 The corresponding results indicated a high degrada-
tion of the DPT and low yields (>25%) of the desired DPT-N3
(Scheme 1). Aiming for an increase of conversion, a more
reactive coupling agent, PyBOP with DIPEA, was used
(Scheme 1). Indeed, high yields up to 75% were obtained with
1
this coupling agent. The H NMR spectrum of DPT-N3 clearly
shows the propyl protons of the azide group at 3.30–3.40 ppm
(Fig. 1-top (A)). Further FTIR analysis of DPT-N3 conrmed the
presence of an additional IR band corresponding to the azide
stretch at 2100 cmꢁ1 (Fig. 1-bottom (A)).
A homopolymer of 4-vinylbenzoic acid (VBA) PVBA-N3 was
subsequently prepared under standard RAFT conditions
employing the prepared DTP-N3 in conjunction with ACP as
ꢀ
initiator in DMF at 80 C (Scheme 1). With this a quantitative
conversion of 6500 g molꢁ1 was attempted (conversion 54%).
Initially, the polymerization initiated by DPT-N3 was carried out
with acrylic acid (AA) as monomer, well-known for its good pH-
responsive in hydrogels. However, the polymerization of AA was
associated by a degradation of the azide end-group. It has been
reported that azides undergo 1,3-cycloaddition with electron-
poor olens at high temperatures and long reaction time as
side reaction.50,51 In order to circumvent this latter side reaction
and to preserve high end group functionalization of DPT-N3,
VBA was used. FTIR analysis showed no side reaction of VBA
with the azide group at the given polymerization conditions (for
detailed information see ESI†). The PVBA-N3 homopolymer was
further characterized by SEC, FTIR spectroscopy, 1H NMR
spectroscopy and MALDI TOF mass spectrometry (MALDI TOF
MS). The SEC measurements revealed a moderate dispersity
Fig. 1 1H NMR (top) and FTIR (bottom) spectrum of the functionalized
DPT-N3 (A), the azide bearing polymer PVBA-N3 (B) and the acrylamide
containing macromonomer PVBA-AAm (C).
To prepare the nal PVBA-AAm macromonomer, an acryl-
amide, N-(prop-2-yn-1-yl)acrylamide 3, was synthesized (Scheme
1). Aer testing the efficiency of the click reaction with a variety of
Cu sources (CuBr, CuI and CuSO4/sodium ascorbate), ligands
(DIPEA and PMEDTA), and solvents (DMF, DMF/H2O and THF),
1H NMR studies indicated that the catalyst CuSO4 in conjunction
with sodium ascorbate in DMF/H2O gives the best results
obtaining reaction yields close to completion. In the resulting 1H
NMR spectrum (Fig. 3, ESI,† magnied spectrum), the appear-
ance of the proton signals at 5.58 and 6.11 ppm, attributed to CH2
of the acrylamide unit, revealed the effective conversion of PVBA-
N3 with 3 (Scheme 1). This indicates the formation of the triazole
ring on the polymer PVBA-N3 upon click reaction (Fig. 1-top (C)).
Additionally, IR experiments showed the complete disappearance
of the azide stretch at 2100 cmꢁ1 (Fig. 1-bottom (C)).
indicating that the polymerization was controlled (Mw/Mn
¼
1.4). Note that for the SEC measurement the carboxylic acid
group of PVBA-N3 was methylated to suppress interactions with
the column material. Retention of the terminal azide groups
was conrmed by the presence of the azide stretch at 2100 cmꢁ1
(Fig. 1-bottom (B)). The corresponding peaks of PVBA-N3 were
identied by the structure assignment of PVBA-N3 in the 1H
NMR spectrum (Fig. 1-top (B)). Furthermore, the absolute molar
mass was calculated to be 4500 g molꢁ1 by end-group analysis.
This value is in good consistence of an average molar mass of
4400 g molꢁ1 determined by MALDI TOF MS (Fig. 2, ESI†).
Hydrogel synthesis and characterization
Moreover, the difference in mass between the major peaks was Finally, a series of graed PNIPAAm gels containing 0.25 to 1
calculated to be 148 g molꢁ1. This conrms that VBA is the mol% PVBA-AAm were synthesized in pyridine with 1.5 mol%
repeating unit in PVBA-N3 (repeat unit mass ¼ 148.05 g molꢁ1). BIS as crosslinker and 1 mol% AIBN as initiator (Entry 2–4 in
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Table 1 Composition and VPTT of the prepared net-PNIPAAm (Entry 1) and net-PNIPAAm-g-PVBA (Entry 2–4) hydrogels
Molar
Feed composition
NIPAAm [mmol]
compositiona
Entry
Sample name
PVBA-AAm [mmol]
BIS [mmol]
AIBN [mmol]
NIPAAm
VBA
VPTTb [ꢀC]
1
2
3
4
0.00% PVBA
0.25% PVBA
0.50% PVBA
1.00% PVBA
1.25
1.25
1.25
1.25
0.00
3.13
6.25
12.5
18.6
18.6
18.6
18.6
12.5
12.5
12.5
12.5
100
96
89
0
4
11
18
32.4
33.0
33.2
33.3
82
a
b
Composition in the prepared hydrogel determined by FTIR, for detailed information see ESI. Determined in water by DSC.
Table 1). Low graing density of PVBA-AAm in the gels resulted (0–100%) and salt concentrations (0–1 mol lꢁ1). Thus, hydrogel
in the desired sharp VPTT of the PNIPAAm network. The disks 5 mm in diameter were placed in solution and equili-
network structure thus obtained is indicated in Scheme 1: the brated for 1 d. Samples were removed from solution, blotted
network is formed by a slightly crosslinked PNIPAAm gel with lter paper to remove excess water on the surface and
structure with a few free tangling chains made from PVBA-AAm weighed. The swelling degree Qw, was calculated for the
of about Mn ¼ 4500 g molꢁ1. This allows for retaining the hydrogels using the following equation:
physical interactions of two homopolymers.
Ws ꢁ Wd
Qw
¼
(1)
The hydrogel synthesized with PVBA-AAm are summarized
in Table 1. The networks formed were classied as net-PNI-
PAAm-g-PVBA hydrogel (Entry 2–4 in Table 1). A pure net-PNI-
PAAm hydrogel was used as a model gel (Entry 1 in Table 1). The
determined molar composition of NIPAAm and VBA calculated
by FTIR spectroscopy is in good agreement with the initial feed
compositions (for further information see ESI†). Importantly,
all prepared gels were washed several times with water to
remove unreacted monomer as well as organic solvent before
conducting stimuli response studies. Equilibrium swelling
properties of the gra copolymer gels were studied at various
temperatures (20–50 ꢀC), pH values (2–10), solvent contents
Wd
where Ws is the weight of the swollen hydrogel and Wd is the
weight of the dried gel. Equilibrium swelling studies indicated
that the prepared gra copolymer gels are capable of respond-
ing to environmental pH, temperature, organic solvent and salt.
The basic principle of the temperature and pH sensitivity of net-
PNIPAAm-g-PVBA hydrogels is illustrated in Fig. 2A. In total, 4
different swelling states can be addressed depending on the
temperature and the pH value.
pH response. Fig. 3 shows the inuence on the swelling
degree at r.t. (A) and 50 ꢀC (C). The pH dependency was
Fig. 2 (A) Schematic illustration of the temperature and pH response of net-PNIPAAm-g-PVBA hydrogels: (I) PNIPAAm and PVBA swollen; (II)
PNIPAAm swollen, PVBA collapsed; (III) PNIPAAm collapsed, PVBA swollen; (IV) PNIPAAm and PVBA collapsed. (B) A net-PNIPAAm-g-PVBA
hydrogel containing 1 mol% PVBA-AAm at different conditions: hydrogel totally swollen (pH 9, r.t.); hydrogel partially collapsed (pH 9, r.t. and pH
9, 50 ꢀC); hydrogel completely collapsed (pH 3, 50 ꢀC). (C) A net-PNIPAAm-g-PVBA hydrogel containing 0.25 mol% PVBA-AAm at different
solvent mixtures at r.t.: hydrogel totally swollen (H2O/EtOH 100%/0%); hydrogel completely collapsed (H2O/EtOH 60%/40%).
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the rst cycle. This behavior is more pronounced for hydrogels
with higher graing density. For the net-PNIPAAm-g-PVBA
hydrogel containing 1 mol% PVBA-AAm a drop of the swelling
degree aer the rst cycle at r.t. from 171 to 141 is observed,
whereas the net-PNIPAAm-g-PVBA hydrogel containing 0.25
mol% PVBA-AAm shows a drop from 67 to 61. These swelling
degrees stabilize over the remaining 3 cycles. We assume that
this phenomenon is caused by the buffer changing. This leads
to higher ionic strength over various cycles resulting in lower
swelling degrees. Regardless, the net-PNIPAAm-g-PVBA hydro-
gels show a reliable pH response with high volume different
upon multiple pH changes.
Temperature response. As done for the pH response, the
temperature behavior was investigated for two cases: at a pH
above (pKa < pH, PVBA swollen) and below the pKa of VBA (pKa >
pH, PVBA collapsed). The temperature dependence at pH 9 of
net-PNIPAAm and net-PNIPAAm-g-PVBA hydrogels is shown in
Fig. 4A. The net-PNIPAAm hydrogel shows the typically VPTT
between 30 and 35 ꢀC. This value is in good agreement with 32.4
ꢀC determined by differential scanning calorimetry DSC (Fig. 5-
le). The VPTT of the gra copolymer gels is in the same range
from 30 to 35 ꢀC. DSC measurements further conrm these data
and display a slight shi of the VPTT to higher values from 32.4
to 33.3 ꢀC (Table 1). The degree of deswelling is signicantly
affected by the PVBA content. Gra copolymer gels with high
PVBA content undergo higher shrinkage compared to gels with
lower PVBA content. For example, pure net-PNIPAAm shows
a change in swelling from 51 to 1, whereas net-PNIPAAm-g-PVBA
containing 1 mol% PVBA-AAm exhibits a change in swelling
from 204 to 19. Additionally, all net-PNIPAAm-g-PVBA hydrogels
show a sharp volume phase transition indicated by a high drop
of swelling between 30 and 35 ꢀC. Turbidity experiments of net-
Fig. 3 Equilibrium swelling behaviour as a function of pH at r.t. (A) and
50 ꢀC (C) in pH buffer solution and oscillatory swelling behaviour as
a function of pH at r.t. (B) and 50 ꢀC (D) for pure net-PNIPAAm and net-
PNIPAAm-g-PVBA hydrogels.
investigated in the range of pH values from 2 to 10 for two cases:
one was at a temperature below (T < VPTT, PNIPAAm swollen)
while the other was above the VPTT of PNIPAAm (T > VPTT,
PNIPAAm collapsed). For both, pure net-PNIPAAm hydrogel
exhibits a nearly constant swelling degree of about 40 at r.t. and
2 at 50 ꢀC independent on the pH value. Considerably high
stimuli response is already observed with low molar amount of
PVBA-AAm between 0.25 and 1 mol%. The swelling curves of
ꢀ
net-PNIPAAm-g-PVBA hydrogels at r.t. and 50 C show a sharp
increase in swelling from pH 6 to 8, near the pKa 7.1 of VBA
homopolymers.52 At pH values above and below the transition
from pH 6 to 8, the swelling degree is almost consistent. This
pH response is caused by the carboxylic acid groups of the PVBA
gras. They are ionized at pH values above 6, resulting in large
osmotic swelling force associated with signicantly swelling of
the hydrogel. Furthermore, the pH response increases with
increasing PVBA content. This behavior is independent whether
PNIPAAm-g-PVBA containing
1 mol% PVBA-AAm further
conrmed a sharp transition similar to pure net-PNIPAAm
(Fig. 5-right). This behavior is particularly benecial from the
ꢀ
the PNIPAAm backbone is swollen at r.t. or collapsed at 50 C.
For example, the change in swelling of net-PNIPAAm-g-PVBA at
r.t. between pH 3 and pH 9 increases from 40, over 90 to 160.
Importantly, the pꢀH response of net-PNIPAAm-g-PVBA at r.t. is
higher than at 50 C due to the collapsed state of PNIPAAm at
temperatures above the VPTT.
For technical use, for instance as a chemomechanical valve,
the swelling/deswelling behavior should be consistent and
therefore the reversibility was tested by repeated cycling
between buffers at pH 3 and pH 9. The experiment was con-
ducted at r.t. (Fig. 3B) and 50 ꢀC (Fig. 3D) to investigate the
impact of PNIPAAm backbone. Naturally, pure net-PNIPAAm
hydrogels exhibit no appreciable change in swelling across
several cycles independent on the temperature. On the contrary,
reversible swelling/deswelling response is detected for the gra
copolymer gels at r.t. and 50 ꢀC. It is notable that net-PNIPAAm-
g-PVBA shows a substantially reduced degree of swelling aer
Fig. 4 Equilibrium swelling behaviour as a function of temperature at
pH 9 (A) and pH 3 (C) and oscillatory swelling behaviour as a function of
temperature at pH 9 (B) and pH 3 (D) for pure net-PNIPAAm and net-
PNIPAAm-g-PVBA hydrogels.
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Fig. 5 VPTT determination of pure net-PNIPAAm and net-PNIPAAm-
g-PVBA hydrogels by DSC (left) and UV/Vis light transmittance (right)
determined in water.
application point of view, because small temperature changes
already lead to high stimuli response.
Fig. 6 Possible p-interactions and H-bonded complexes between the
CONH-groups of the PNIPAAm backbone and the COOH-groups of
the PVBA graft chains as well as between COOH-groups of different
PVBA graft chains at pH 3, leading to a reduced repeated swelling.
In contrast, the VPTT at pH 3 for net-PNIPAAm and net-
PNIPAAm-g-PVBA hydrogels is shied towards lower tempera-
tures below 30 ꢀC (Fig. 3C). It has been shown that the LCST of
PNIPAAm and the VPTT of net-PNIPAAm hydrogels is highly
affected by salts and their concentrations.23,53–56 For this reason,
we suspect that this shi is provoked by the salt of the buffer
solution. Furthermore, the deswelling of the gra copolymer
gels at pH 3 is less pronounced compared to the net-PNIPAAm
hydrogel. As more PVBA gras are incorporated into the gel, the
hydrophobicity of the gel increases when these gra are
protonated. The resulting hydrophobicity at pH 3 is associated
with a signicant lower swelling degree. Oscillatory swelling
experiments were also performed to investigate whether the
responses to the environmental temperature is reversible. Fig. 4
ESI†). The net-PNIPAAm and net-PNIPAAm-g-PVBA hydrogels
are totally swollen at r.t. in pure water. As expected, the swelling
degree increases with increasing PVBA-content. Furthermore,
the hydrogels exhibit a signicant higher swelling degree in
pure water than in buffer solution caused by the lower ionic
strength. At about 20 vol% acetone, all hydrogels collapse and
start to swell again at about 60 vol% acetone. At higher acetone
content, between 70 and 90 vol%, higher swelling of the net-
PNIPAAm-g-PVBA compared to pure PNIPAAm is observed. This
may be explained by a good solubility of the PVBA gras at these
water/acetone compositions. In any case, net-PNIPAAm-g-PVBA
hydrogels respond to different water/acetone mixtures almost
similar like the net-PNIPAAm hydrogel.
In addition to solvent response, the salt response of net-
PNIPAAm-g-PVBA hydrogels was also investigated. It was re-
ported that high salt concentration leads to partial deswelling of
net-PNIPAAm hydrogels.58 On the contrary, Satoh et al. reported
from high salt resistivity of net-PVBA hydrogels compared to net-
PAA hydrogels.59 They supposed a stabilization of hydrogen-
bonding hydrations of PVBA through ionic hydrations. For
this reason, we were particularly interested in our net-PNIPAAm-
g-PVBA hydrogels behavior and comparing it with net-PNIPAAm
and net-PVBA hydrogels. The effect of the salt concentration of
ꢀ
shows the repeated cycling between r.t. and 50 C at pH 9 (B)
and pH 3 (D). As it could be expected, the deswelling and
swelling cycles at pH 9 are reproducible at least for four times of
all investigated hydrogels. Here, the swelling/deswelling
response is more pronounced with increased PVBA content.
In contrast to pH 9, net-PNIPAAm-g-PVBA hydrogels with higher
PVBA content exhibit at pH 3 no repeated swelling/deswelling
behavior whereas for the net-PNIPAAm hydrogel, a reversible
swelling/deswelling response is detected. We propose that this
phenomenon is caused by the formation of intrachain
complexes by hydrogen bonds between the amide groups of the
PNIPAAm backbone and the protonated carboxylic groups of
the gra chains as well as between carboxylic groups of single
gra chains (Fig. 6). Moreover, p-interactions are also possible
due to increased spatial proximity of the collapsed gra chains.
These interactions act as diffusion barrier and decrease the
accessibility of water to the NIPAAm-units. As a consequence,
the temperature stimulus of net-PNIPAAm-g-PVBA hydrogels at
pH 3 is not suitable for multiple uses. However, further research
is needed for full understanding.
Solvent and salt response. It is known that net-PNIPAAm
hydrogels undergo swelling/deswelling at different water/
solvent compositions.14,57 For this reason, we were particularly
interested in the swelling behavior of the net-PNIPAAm-g-PVBA
hydrogels in different solvent compositions. As shown in
Fig. 7A, the model systems acetone/water was used and the
Fig.
7 Equilibrium swelling behaviour as a function of acetone
composition of the solvent mixture was varied. Note that beside concentration at r.t. (A). Change in swelling as function of various salts
at different concentrations for net-PNIPAAm-g-PVBA containing 0.5
mol% PVBA-AAm (B).
acetone/water further alcohol/water mixtures were tested (Fig. 6,
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deswelling cycles were reversible. Solely the temperature oscil-
latory swelling experiments at pH 3 were not repeatable in all
cases.
A hypothesis about the formation of intrachain
complexes by hydrogen bonds between amide groups and
carboxylic groups as well as between carboxylic groups of
different gra chains has been proposed to explain this
phenomenon. Additionally, net-PNIPAAm-g-PVBA hydrogels
exhibited a water/acetone and salt response similar to net-PNI-
PAAm hydrogels. These results show us the great potential
suitably designed tetra-sensitive net-PNIPAAm-g-PVBA hydro-
gels as potential chemomechanical valve due to their specic
ability to respond independently to four different environ-
mental alterations with a sharp transition and a high volume
change. In further studies, we will investigate the swelling
kinetic and we will incorporate our gra copolymer gels in
microuidic platforms.
Fig. 8 SEM images of the freeze-dried hydrogels with average pore
diameter. Scale bar 100 mm (top) and 30 mm (bottom).
various salts is presented in Fig. 7B. In this experiment we
detect a decreased swelling degree of net-PNIPAAm-g-PVBA
containing 0.5 mol% PVBA-AAm macromonomer with
increasing salt concentration for all studied salts except NaI.
This behavior is in good accordance with the results of
a conventional net-PNIPAAm hydrogel (Fig. 5, ESI†). A high salt
resistivity by the incorporated PVBA gras chains is not detec-
ted or dominated by the deswelling of PNIPAAm with increased
salt concentration. The incorporated molar amount of PVBA
was low and its impact is comparatively small. We belief
therefore that high salt resistivity might only be detected for
a high content of PVBA.
Acknowledgements
This work is partly supported by the German Research Foun-
dation (DFG) within the Cluster of Excellence ‘Center for
Advancing Electronics Dresden’ and was also done within the
German Research Foundation (DFG), Project: 1865/1 Hydrogel-
Based Microsystems. We also thank K. Sahre for MALDI TOF MS
measurements, M. Malanin for FTIR measurements and L.
¨
Haussler for DSC measurements.
Morphology studies. Fig. 8 shows the SEM studies of the
swollen freeze-dried hydrogels. It is important to note that the
observed honeycomb macropores are a result of the gradual
growth of microstructural ice crystals during the freeze-drying
process.60,61 However, the results showed that the gra density
of the PVBA-AAm macromonomer has a great inuence on the
morphology of the freeze-dried samples. Conventional net-
PNIPAAm hydrogels exhibited smaller pore size compared to
the gra copolymer gels. This is because the incorporation of
ionic VBA increases the water content when the hydrogels are
swollen. Thus, with increasing gra density the pore size
increases, while the number of pores per unit area of the net-
PNIPAAm-g-PVBA versus the pure net-PNIPAAm hydrogels in
turn decreases. In addition, the net-PNIPAAm-g-PVBA hydrogels
exhibit a more homogenous and dense architecture compared
to pure net-PNIPAAm.
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