Thermo- and pH-Responsive Nanogel Particles Bearing Secondary Amine Functionalities for Reversible Carbon Dioxide Capture and Release

Article abstract of DOI:10.1021/acs.macromol.5b01367

We report the synthesis of temperature- and pH-responsive nanogel particles (NPs) consisting of N-isopropylacrylamide (NIPAM) and N-(2,2,6,6-tetramethylpiperidin-4-yl)methacrylamide (TMPMA). NPs can reversibly capture and release carbon dioxide via temperature-induced volume phase transition and changes in pH. These stimuli-responsive particles contain sterically hindered secondary amine functionalities and exhibit a volume phase transition temperature (VPTT) in aqueous solution. The fully reversible VPTT behavior involves a precise shrinkage to 40% of the initial particle size along with a large change in pH from 10.25 to 7.65 upon increasing temperature. We could reversibly release 35 mL (1.4 mmol) of CO₂ per gram of polymer in very short heating times, thereby significantly increasing the amount of CO₂ with respect to the regeneration time. This behavior could be repeated for various cycles at moderate temperatures (85 °C).

Full text of DOI:10.1021/acs.macromol.5b01367

Article
pubs.acs.org/Macromolecules
Thermo- and pH-Responsive Nanogel Particles Bearing Secondary
Amine Functionalities for Reversible Carbon Dioxide Capture and
Release
Patrick D. L. Werz, Johannes Kainz, and Bernhard Rieger*
WACKER-Lehrstuhl fur Makromolekulare Chemie, Technische Universitat Munchen, Lichtenbergstraße 4, 85748 Garching bei
̈
̈
̈
Munchen, Germany
̈
S
* Supporting Information
ABSTRACT: We report the synthesis of temperature- and
pH-responsive nanogel particles (NPs) consisting of N-
isopropylacrylamide (NIPAM) and N-(2,2,6,6-tetramethyl-
piperidin-4-yl)methacrylamide (TMPMA). NPs can reversibly
capture and release carbon dioxide via temperature-induced
volume phase transition and changes in pH. These stimuli-
responsive particles contain sterically hindered secondary
amine functionalities and exhibit a volume phase transition
temperature (VPTT) in aqueous solution. The fully reversible
VPTT behavior involves a precise shrinkage to 40% of the
initial particle size along with a large change in pH from 10.25 to 7.65 upon increasing temperature. We could reversibly release
35 mL (1.4 mmol) of CO2 per gram of polymer in very short heating times, thereby significantly increasing the amount of CO2
with respect to the regeneration time. This behavior could be repeated for various cycles at moderate temperatures (85 °C).
et al. discovered that LCST of poly(N,N-dimethylaminoethyl
methacrylate) could be reversibly tuned by CO2 and argon.26
Furthermore, they demonstrated that hydrogels can undergo a
reversible volume transition upon treatment with CO2.27
Additional reports using volume transition as a reversible
target capture-and-release mechanism have been published.29,30
Lyon and co-workers reported the thermally regulated uptake
and release of the chemotherapeutic drug doxorubicin from
microgel thin films by copolymerizing N-isopropylacrylamide
(NIPAM) with acrylic acid (AAc) into microgel structures.31
Yu Hoshino adapted this approach to synthesize microgel
particles (GPs) with NIPAM and the tertiary amine monomer
N-[3-(dimethylamino)propyl]methacrylamide (DMAPM) and
reported the reversible capture and release of CO2 through a
temperature-induced phase transition.20 Hoshino’s group then
refined their system to form thermoresponsive microgel films
to improve the capacity of CO2 capture in wet environments.21
Huang combined the thermo- and CO2-responsive function-
ality in one monomer and demonstrated the ability to tune
LCST by adjusting pH.32 Recently, we reported temperature-
and CO2-responsive polyethylenimine polymers that can
reversibly absorb and desorb CO2.14 Herein, branched
polyethylenimine was acylated with butyric anhydride to
introduce thermoresponsive behavior (LCST), and a significant
increase in the desorption CO2 flow was observed through the
phase separation of the acylated polyethylenimine.
INTRODUCTION
■
The carbon dioxide concentration in the atmosphere is steadily
increasing because of industrial processes and has reached over
400 ppm.1,2 The growing CO2 content in the atmosphere is
widely accepted to be a major contributor to climate change
and a huge challenge to mankind in the future.1 Carbon capture
and storage (CCS) is considered as one of the main options to
reduce the CO2 concentration in the athmosphere.3 Within
CCS, postcombustion CO2 separation is one of the most
promising techniques because it can be applied to already
existing fossil fuel power plants.4,5 Amine solutions such as
monoethanolamine are mostly used in these wet-scrubbing
processes; however, because of carbamate formation, the
regeneration of the solvents is energy-consuming and decreases
the overall power plant efficiency by at least 10%.6 Improve-
ments could be made using mixtures of primary and tertiary
amines or hindered amines.4,7 To reduce cost and design this
process more economically, novel CO2 capture materials are
required to decrease the temperature during sorbent regener-
ation. Various materials such as metal−organic frameworks,8,9
solid and porous polymer amines,10,11 ionic liquids,12,13 and
stimuli-responsive polymers14 have been investigated for the
low-temperature solvent regeneration of CO2.
In particular, stimuli-responsive polymers have gained
considerable interest in recent years,15−17 with CO2-responsive
polymers18 and hydrogels leading the way. Herein, CO2 can act
as a “green” trigger that switches polymer properties19 or
capture CO2 directly from air or exhaust gas.14,20,21 CO2-
responsive functionalities in these polymers can be categorized
into amidine,22−24 amine,14,25,26 and carboxyl27,28 groups. Zhao
Received: June 23, 2015
Revised: August 20, 2015
© XXXX American Chemical Society
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Figure 1. (A) Reaction scheme for the synthesis of TMPMA. (B) Synthesis of nanogel latex particles via precipitation polymerization in H2O at 70
°C for 3 h using monomers, cross-linker (BIS), and surfactant (CTABr). (C) SEM and (D) TEM images of dried NPs from an aqueous solution
showing NP sizes based on DLS measurements along with a homogeneous size distribution. (E) Catalytic cycle of carbon dioxide capture and release
through temperature-induced volume transition for P(NIPAM-co-TMPMA) nanogel particles (NPs).
Scheme 1. Reaction Pathways for CO2 in Water and in the
Presence of Primary, Secondary, or Tertiary Amines
(Reproduced with Permission from ref 34)
RESULTS AND DISCUSSION
■
Here we report the synthesis of novel secondary amine-
containing nanogel particles (NPs) for the capture and release
of CO2. We provide detailed information on the mechanism of
the volume phase transition-induced release of CO2 and
present high desorption amounts in relation to regeneration
times.
We synthesized the monomer N-(2,2,6,6-tetramethyl-
piperidin-4-yl)methacrylamide (TMPMA), which contains a
sterically hindered secondary amine. This monomer was
prepared following an adapted literature procedure33 in which
methacryloyl chloride was reacted with 4-amino-2,2,6,6-
tetramethylpiperidine, forming the corresponding methacryla-
mide (Figure 1A). This amine was chosen because of its higher
basicity than the respective tertiary amines because tertiary
amines are known to have significantly lower uptake kinetics for
CO2 than secondary or even primary amines.34,35
Whereas primary and secondary amines react to form
carbamates in the presence of CO2, tertiary amines follow a
bicarbonate mechanism (Scheme 1).34 To combine the positive
effects of the higher basicity and the formation of bicarbonate
upon reaction with CO2, TMPMA was synthesized because
sterically hindered amines (e.g., 2-amino-2-methyl-1-propanol,
AMP) are known to form bicarbonate instead of carbamates
with CO2.36 In our approach, following the bicarbonate route
to absorb CO2 into the liquid phase is crucial because
bicarbonate decomposes to water and CO2 upon the addition
of acid, whereas carbamates do not. To lower the temperature
for regeneration, which is the most energy-consuming part of
the process, temperature- and pH-responsive NPs containing
different contents of TMPMA (0%, 5%, 10%, 20%, and 30%)
and N-isopropylacrylamide (NIPAM) were synthesized. With
the addition of 2% of N,N′-methylenebis(acrylamide) (BIS) as
a cross-linker along with a surfactant, the stimuli-responsive
NPs were synthesized via precipitation polymerization in water
(Figure 1B). The pH responsiveness of NPs enables them to
reversibly produce protons upon mild increases in temperature.
In summary, CO2 is absorbed by the sterically hindered
secondary amines in NPs, forming bicarbonate and the
corresponding ammonium salt. Heating the system above its
volume phase transition temperature (VPTT) an entropically
induced contraction occurs in which NPs decrease in size. The
shrinkage leaves NPs in a less-hydrated state and releases
protons from the ammonium salt, forming the corresponding
amine. The released protons conclusively react with bicar-
bonate to form carbonic acid, which then decomposes to water
and CO2 (Figure 1E).
To demonstrate that the absorption of CO2 follows the
bicarbonate mechanism, a sample of NPs was charged with
CO2, and 13C NMR spectra of this sample, NPs without CO2,
and a sodium bicarbonate solution were measured. The NMR
B
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spectra provide clear evidence of bicarbonate, and the lack of
any shift in the carbamate signals confirms the suggested
mechanism (Figure S3).
To determine VPTT of NPs, the diameter of a sample was
measured with increasing temperature until no further particle
contraction could be observed. The inflection point of the
curve marks VPTT. The particles continued to shrink until the
temperature reached 85 °C; therefore, this temperature was
chosen for regeneration experiments because higher temper-
atures have no effect on the volume phase transition of NPs.
Figure 3. Heating of a nanogel solution (NP30s) induces a volume
phase transition that reversibly alters NP diameter to 40% of the
original hydrodynamic diameter.
and pH-responsive properties (Figure 4), which are most likely
due to an ion-imprinted polymerization process.37
Figure 2. Determination of the volume phase transition temperature
(VPTT) of NP30s.
The sizes of NPs in aqueous solution were determined via
dynamic light scattering (DLS). The particle diameter increases
from 134 nm for pure PNIPAM particles (NP0s) to 212 nm for
particles containing 5% of TMPMA (NP5s). The diameter then
reaches a relatively constant size distribution of around 400−
425 nm for the particles containing 10%−30% TMPMA
(NP10s, NP20s, and NP30s). The size distribution and
diameter of NP30s were additionally measured using SEM/
TEM. These values were in good accordance with those
determined by DLS and indicate a homogeneous particle
distribution (Figures S4−S6).
VPTT was studied over four cycles using DLS measurements
(Figure 3). The hydrodynamic diameter was continuously
recorded over the heating (85 °C) and cooling (30 °C) loops.
The initial hydrodynamic diameter of NP30s was 425 nm. The
diameter then shrinks steadily to a size of around 180 nm upon
heating to 85 °C. In cycles 2−4, the NP diameter oscillates
between 340 and 135 nm during the heating/cooling loops and
corresponds over all cycles to a value of 40% in comparison
with the expanded state. Upon leaving the sample for a few
hours after measurement, the diameter returns to 425 nm. The
measurement results demonstrate a fully reversible process of
NP swelling and shrinking upon temperature changes and
demonstrate the extremely precise NP behavior over various
cycles (Figure 3). Bergbreiter et al. previously synthesized and
reported the thermo- and pH-responsive behavior of a
TMPMA analogue monomer copolymerized to statistically
linear polymers with NIPAM.33 We synthesized novel
TMPMA-containing NPs that exhibit even stronger thermo-
Figure 4. pH (blue circles) of an NP30s solution reversibly changes
between 10.25 and 7.65 upon heating and cooling.
The pH of an NP30s solution was continuously recorded
(blue circles) and plotted against temperature. Upon heating
and cooling the solution for 12 min each, pH was reversibly
changed from 10.25 to 7.65. At room temperature, NPs are in
their expanded state, and the secondary amines act as a base.
Heating the system above its volume phase transition
temperature induces an entropically driven contraction of the
PNIPAM particles.38,39 For cross-linked gels such as NPs in this
study, this leads to swelling and shrinking behavior. In this
collapsed state, NPs are in less-hydrated, and amphoteric
groups such as the ammonium salts undergo a significant
change in basicity and release their protons to form the
corresponding amines. This leads to a general neutralization of
the NP solution, and it reverts to its basic state after cooling
(Figure 4).
Even more interesting is the shapes of the titration curves.
While the curves of both TMPMA and NPs at room
temperature exhibit shapes of a standard weak base/strong
acid, the shape of the titration curve of NPs at elevated
C
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Figure 5. (left) CO2 release measured through pH recovery of a CO2-saturated solution of NP30s via temperature swing. pH could be raised to 10.0
after six heating cycles. (right) CO2 release measured through pH recovery of NP30s via temperature swing and pressure swing (bubbling argon, red
boxes, 2 × 10 min). pH could be raised to value over 10.0 after only two heating cycles.
temperatures is clearly different. Although there is no slight
decrease upon added HCl, all acid is buffered at neutral pH
until the equivalence point is reached (Figure 6, ○). This is
because of the fact that at elevated temperatures, nearly all of
the amines are in nonprotonated states being in their less-
hydrated, contracted state. By adding acid, the amines are then
protonated, and pH remains constant at around 7. All amines in
NPs are being protonated until the equivalence point is
reached.
Both TMPMA and NPs have a significant ΔpKA, although it is
notably higher for the nanogel particles (ΔΔpKA = 0.54).
All titration curves show only one equivalence point, and no
difference on accessibility of amine functionality. Based on the
amount of titrated NP30s and the equivalence point, the amine
content was determined to be 22.5% (compared with 30% in
TMPMA) during the polymerization process. This could
indicate that the polymerization only led to 22.5% TMPMA
and an increased incorporation of NIPAM. However, a more
probable explanation is that polymerization led to the desired
ratio of 30/70, but not all amines were accessible on the inside
of the nanogel particles.
After CO2 saturation, pH of the solution drops to around
5.70 (Figure 5). The release of CO2 was indirectly measured by
recording pH. The solution was regenerated following several
heating/cooling cycles, while pH increased in a linear fashion
(Figure 5, left) to approximately 10. When we performed the
same heating/cooling cycles while also bubbling argon during
heating (Figure 5, right, red boxes), pH rose above 10 in just
two cycles. However, bubbling argon into the CO2-saturated
NPs solution at room temperature for 100 min resulted in a pH
value of just 8.5 (Figure S7). Thus, even purging the solution
over a long time with argon at room temperature could not
release all of the CO2; however, CO2 rapidly desorbs at elevated
temperature.
We then constructed titration curves to determine the pKA
values for the TMPMA monomer and the corresponding NPs
at room temperature as well as at 85 °C. The pKA values were
then estimated from the half volumes of the equivalence points
(Figures S8−S15). The pKA values of monomer TMPMA were
determined to be 9.72 at room temperature and 8.37 at 85 °C,
corresponding to ΔpKA of 1.35. The calculated pKA values of
NP30s at room temperature and 85 °C were 8.81 and 6.72,
respectively (Figure 6), corresponding to a large ΔpKA of 1.89.
●
Figure 6. ( ) Titration curve of NP30s at room temperature, resulting
○
in pKA of 8.61. ( ) Titration curve of NP30s at 85 °C, resulting in
pKA of 6.72. ( ) Titration curve of TMPMA monomer at room
temperature, resulting in pKA of 9.72. ( ) Titration curve of TMPMA
at 85 °C, resulting in pKA of 8.37.
■
□
As expected from the previous measurements, NPs exhibited
a large difference in pKA of nearly two units, and the difference
in basicity is stronger in the contracted NP state than in the
monomer. The titration curves show only one equivalence
point, whereas Hoshino et al. reported two-stage titration
curves indicative of different amine environments in their
GPs.20 Interestingly, this effect seems to increase with
increasing temperature.
To quantify the amounts of released CO2, we used a
microGC setup. The CO2-saturated sample was placed in a
calibrated ∼200 mL Schlenk tube, which was then closed with a
rubber septum. Samples (50 μL) were drawn from the gaseous
D
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volume above the solution at 0 min and after each heating
cycle.
Table 2. Desorption Amounts, Amine Contents, and
Regeneration Temperatures
After only two 15 min heating cycles, the CO2 content in the
gaseous phase was nearly constant (32 mL/g), and it only
increased in small amounts during the next three cycles (35
mL/g; Figure 7, blue). In comparison, a sample of NIPAM and
desorbed CO2
[mL/g]
desorbed CO2
[mmol/g]
amine
reg temp
absorb
content [%]
[°C]
NP30s
∼35
19
∼1.4
0.8
23
30
85
95
75
b-PEI14
GP30s21
∼38
∼1.7
CONCLUSION
■
In summary, we present an approach to synthesize nanogel
particles via the precipitation polymerization of NIPAM and
TMPMA. Titration experiments show that accessibility ranges
from 75% to 92% depending on the amine content. The
sterically hindered secondary amines in TMPMA follow a
bicarbonate mechanism upon treatment with CO2, as
demonstrated by 13C NMR experiments. The sizes of the
nanogel particles were determined using DLS measurements.
The particles showed almost no swelling behavior upon CO2
treatment. We demonstrated that the volume phase transition
temperature and the resulting phase transition of the nanogel
particles (NPs) are crucial for a fast and complete regeneration
of the bicarbonate solution. Herein, protons are released
because of the phase transition of NPs, forming carbonic acid
with bicarbonate, which then decomposes to CO2. The
regeneration temperature (85 °C) was significantly decreased
in comparison with standard amine processes (>120 °C). NPs
show greater CO2 desorption (92%−99%) than the free
monomers (21%) or pure water. In terms of desorption, the
amount of desorbed CO2 reached 35 mL or 1.4 mmol per gram
of polymer.
Figure 7. (blue) Amount of CO2 released from NP30s (170 mg
polymer, 1.4 mmol g−1) as a result of temperature change and
measured via microGC in the gas volume above the solution. (black)
Measured CO2 volume from an equivalent mixture of TMPMA/
NIPAM monomers in water. (inset) Reversible capture and release of
carbon dioxide over various cycles.
EXPERIMENTAL SECTION
■
General. All reactions were conducted under an argon atmosphere
using standard Schlenk or glovebox techniques. All glassware used for
water-sensitive reactions was heat-dried under vacuum prior to use.
Unless otherwise stated, all chemicals were purchased from Sigma-
Aldrich or ABCR and used as received. Chloroform, 4-amino-2,2,6,6-
tetramethylpiperidine, and triethylamine were dried over calcium
hydride and distilled prior to use. The water used in the experiments
was purified using a Merck Millipore Direct-Q system and degassed
prior to use. 2,2′-Azobis(isobutyramidine) dihydrochloride (AAPH)
was recrystallized twice from water, and N-isopropylacrylamide
(NIPAM) was recrystallized twice from a 10:1 mixture of hexanes
and toluene. ZelluTrans dialysis tubes with MWCOs of 12−14 000
from Carl Roth were used for the purification of nanogel particles.
TMPMA with the same concentration was saturated with CO2
and identically desorbed. The released CO2 amounts were
significantly lower for the monomers (9 mL) because no VPTT
and no subsequent proton release produced CO2 (Figure 7,
black).
Pure water showed no relevant CO2 absorption or
desorption. The complete desorption amounts and relevant
data for the NPs can be found in Table 1. As mentioned before,
the CO2 content after two 15 min heating cycles is only
increasing in small steps, resulting in 35 mL (1.4 mmol) of CO2
per gram of polymer (Table 2). This value is significantly
higher than that we achieved previously and equivalent to that
reported in the literature on stimuli-responsive polymers for
CO2 capture in aqueous solution (Table 2).
NMR spectra were recorded on a Bruker AVIII-300. H and 13C
1
spectroscopic chemical shifts δ are reported in ppm relative to
tetramethylsilane and calibrated to the residual proton and carbon
Table 1. Polymerization Data, Volume Phase Transition Temperature (VPTT), and Hydrodynamic Diameter of NPs with and
without CO2 along with Accessible Amine Content and Reversible CO2 Capture
hydrodynamic diam
reversible CO2 capture/
b
d
f
TMPMA
BIS
yield
vol phase transition hydrodynamic
CO2 saturated
titrated amine
e
release per amine
c
absorbent
[mol %] [mol %]
[%]
temp (VPTT) [°C]
diam [nm]
[nm]
content [%]
[mol/mol]
a
a
TMPMA/NIPAM
P(NIPAM)
NP5s
30
0
0
2
2
2
2
2
30
0.21
91
88
85
76
87
34.0
47.0
66.0
72.0
72.5
134
212
5
8
136
7
0
5
308 29
405 23
460 32
464 44
4.6
8.9
16.3
22.5
∼1.0
∼1.0
0.95
0.92
NP10s
10
20
30
354 18
415 26
425 38
NP20s
NP30s
a
b
c
Identical monomer concentrations as in NP30s. After dialysis and lyophilization. Hydrodynamic diameter in degassed Millipore water.
d
e
f
Hydrodynamic diameter after CO2 saturation for 1 h. Titrated amine content in NPs compared with TMPMA content of the polymerization. CO2
capture and release in relation to titrated amine content. For all measurements, CO2 uptake from water was subtracted.
E
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signal of the deuterated solvent. Deuterated solvents were obtained
from Sigma-Aldrich and used as received. Elemental analyses were
conducted at the Laboratory for Microanalytics at the Institute of
NMR spectra of TMPMA and nanogel particles, SEM
and TEM images, control experiments, and microGC
chromatograms of released CO2 (PDF)
Inorganic Chemistry at the Technische Universitat Munchen. IR
̈
̈
spectra were recorded on a Bruker Vertex 70 spectrometer with a
Bruker Platinum ATR setup and an integrated MCT detector.
Hydrodynamic diameters were measured on a Wyatt DynaPro
NanoStar using regularization analysis for the time-dependent light
intensity fluctuations. For the long-term hydrodynamic diameter
measurements (VPTT), samples were subjected to an automatic
temperature program in which the samples were heated in 2 K
intervals from 30 to 85 °C and then cooled back to 30 °C. After
reaching each temperature, the samples were equilibrated for 3 min
before measuring 3 × 10 data points. The entire sequence was
repeated four times. pH measurements were conducted with a Mettler-
Toledo HA405-DPA-SC-S8/225 electrode, and data points were
monitored with an HI2215 pH/ORP meter from HANNA instru-
ments. High-resolution scanning electron microscopy (HR-SEM) was
performed on a JEOL JSM-7500F instrument using an accelerating
voltage between 0.5 and 1 kV for secondary electron observation.
Samples were prepared on copper tape from a dilute aqueous solution.
High-resolution transmission electron microscopy (TEM) was
performed on a JEOL JEM100CX using an accelerating voltage of
100 kV. Samples were prepared from a dilute aqueous solution using a
2% uranyl acetate solution as the negative stain. The gaseous reaction
products (CO2) were determined by GC-TCD (Varian 490 GC). Gas
samples (100 μL) were drawn from the headspace above the solution
and injected into the GC-TCD gas analyzer.
TMPMA Synthesis. 4-Amino-2,2,6,6-tetramethylpiperidine (20.0
g, 0.128 mol, 1.0 equiv) and triethylamine (14.2 g, 0.140 mol, 1.1
equiv) were dissolved in 300 mL of dry chloroform and cooled to 0 °C
using an ice bath. Methacryloyl chloride (13.4 g, 0.128 mol, 1.0 equiv)
in chloroform (15 mL) was added dropwise to the solution and stirred
for 6 h at 0 °C and then allowed to warm to room temperature
overnight. The reaction mixture was washed with bicarbonate solution
(3 × 25 mL) followed by brine (2 × 25 mL) and dried over sodium
sulfate. The combined organic phases were concentrated under
reduced pressure, and the crude product was crystallized overnight.
The crystals were filtrated, washed with pentane, and recrystallized
from 2-propanol to obtain colorless crystals (22.4 g, 78%) of N-
(2,2,6,6-tetramethylpiperidin-4-yl)methacrylamide (TMPMA). 1H
NMR (298 K, 300 MHz, CDCl3): δ = 5.63 (m, 1H), 5.54 (br, 1H),
5.29 (m, 1H), 4.37−4.18 (m, 1H), 1.99−1.85 (m, 5H), 1.25 (d, J = 5.6
Hz, 6H), 1.12 (d, J = 6.2 Hz, 6H), 0.93 (t, J = 12.2 Hz, 2H). 13C NMR
(298 K, 75 MHz, CDCl3): δ = 167.7, 140.4, 119.3, 51.3, 45.3, 42.8,
35.0, 28.6, 18.8. EA: calculated: C 69.60, H 10.78, N 12.49; found: C
69.35, H 10.81, N 12.15. IR: 3282 (m), 2991 (s), 1674 (m), 1534 (s),
1221 (s), 928 (m).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail rieger@tum.de, Tel +49-89-289-13570, Fax +49-89-
289-13562 (B.R.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for financial support from BMBF
(subsidy indicator: 033RC1106A) and the Clariant AG. The
authors thank Dr. Marianne Hanzlik from the electron
microscopy chair of Prof. Weinkauf at the Technische
Universitat Munchen for measuring TEM images. We also
̈
̈
thank Peter T. Altenbuchner, Benedikt S. Soller, and Alexander
Kronast for proofreading of the manuscript.
ABBREVIATIONS
■
AAPH, 2,2′-azobis(isobutyramidine) dihydrochloride; BIS,
N,N′-methylenebis(acrylamide); CTABr, cetyltrimethyl-
ammonium bromide; DLS, dynamic light scattering; LCST,
lower critical solution temperature; NPs, nanogel particles;
TMPMA, N-(2,2,6,6-tetramethylpiperidin-4-yl)methacryl-
amide; VPTT, volume phase transition temperature.
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■
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Particle Synthesis. N-Isopropylacrylamide (2.31 g, 20.4 mmol,
0.68 equiv), TMPMA (2.02 g, 9.00 mmol, 0.30 equiv), and N,N′-
methylenebis(acrylamide) (92 mg, 0.60 mmol, 0.02 equiv) were
dissolved in 100 mL of water. Cetyltrimethylammonium bromide (74
mg, 0.20 mmol) was added as the surfactant, and the solution was
degassed for 30 min at 70 °C. 2,2′-Azobis(isobutyramidine)
dihydrochloride (AAPH; 70 mg) was dissolved in 2 mL of water,
degassed, and added to the monomer mixture. The polymerization
reaction was conducted for 3 h, and the reaction mixture was then
cooled while being exposed to air. The reaction mixture was then
dialyzed against water for 5 days, changing the water at least three
times a day. The resulting solution was lyophilized, resulting in dried
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01367.
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DOI: 10.1021/acs.macromol.5b01367
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