Hierarchically nanostructured polyisobutylene-based ionic liquids

Article abstract of DOI:10.1021/ma202736g

A new type of highly temperature stable ionic liquid (IL) with strongly temperature dependent nanostructures is reported. The molecular design relies on the use of a liquid polymer with an ionic liquid headgroup, introducing liquid properties by both the polymeric and the ionic liquid (IL) headgroup. The IL polymers (poly(isobutylene)s) 3a-3c (PIB-ILs) were prepared by a combination of living carbocationic polymerization (LCCP) and subsequent "click" chemistry for attachment of methylimidazolium (3a), pyrrolidinium (3b), and triethylammonium cations (3c). All three investigated PIB-ILs exhibited pronounced nanostructural organization at room temperature depending strongly on the nature of the anchored cation. Whereas the morphology of the imidazolium-based PIB-IL 3a shows high thermal stability up to the decomposition temperature, order-order (OOT) and lattice disorder-order transitions (LDOT) characteristic for common ionomers could be observed in the case of pyrrolidinium 3b and ammonium-based 3c PIB-ILs. Control of flow behavior as well as adjustable relaxation times from the liquid to the viscoelastic regime can be adjusted by choice of the appropriate IL headgroup.

Full text of DOI:10.1021/ma202736g

Article
pubs.acs.org/Macromolecules
Hierarchically Nanostructured Polyisobutylene-Based Ionic Liquids
Parvin Zare,†,§ Anja Stojanovic,†,§ Florian Herbst,† Johanna Akbarzadeh,‡ Herwig Peterlik,‡
and Wolfgang H. Binder*,†
†Institute of Chemistry, Chair of Macromolecular Chemistry, Faculty of Natural Sciences II (Chemistry, Physics and Mathematics),
Martin-Luther-University Halle-Wittenberg, Von-Danckelmann-Platz 4, Halle 06120, Germany
‡Faculty of Physics, Dynamics of Condensed Systems, University of Vienna, Strudlhofgasse 4, 1090 Vienna, Austria
S
* Supporting Information
ABSTRACT: A new type of highly temperature stable ionic liquid (IL)
with strongly temperature dependent nanostructures is reported. The
molecular design relies on the use of a liquid polymer with an ionic
liquid headgroup, introducing liquid properties by both the polymeric
and the ionic liquid (IL) headgroup. The IL polymers (poly-
(isobutylene)s) 3a−3c (PIB-ILs) were prepared by a combination of
living carbocationic polymerization (LCCP) and subsequent “click”
chemistry for attachment of methylimidazolium (3a), pyrrolidinium
(3b), and triethylammonium cations (3c). All three investigated PIB-
ILs exhibited pronounced nanostructural organization at room
temperature depending strongly on the nature of the anchored cation.
Whereas the morphology of the imidazolium-based PIB-IL 3a shows
high thermal stability up to the decomposition temperature, order−
order (OOT) and lattice disorder−order transitions (LDOT) character-
istic for common ionomers could be observed in the case of pyrrolidinium 3b and ammonium-based 3c PIB-ILs. Control of flow
behavior as well as adjustable relaxation times from the liquid to the viscoelastic regime can be adjusted by choice of the
appropriate IL headgroup.
Because of their self-assembling behavior,25 both monomeric
INTRODUCTION
■
Recently, polymeric ionic liquids (POILs),1−6 carrying con-
ILs as well as POILs display a subtle micro- and nanostructural
organization resulting from the separate aggregation of apolar
strained cationic and anionic components of ionic liquids
and ionic regions.26,27 Since the pioneering work of Canongia
(ILs)7−10 within the monomer units, have been developed as a
new class of polymeric materials with widespread importance in
science and technology. In contrast to (low molecular weight)
ionic liquids (ILs), which are salts with arbitrary defined
melting points below 100 °C, POILs are usually solids at room
temperature, combining the unique properties of ILs with
increased mechanical stability,5 improved processability,6,11 and
more complex self-assembling ability of polymeric materi-
als.4,12,13 Similar to ILs, POILs generally consist of bulky
nonsymmetrical organic cations (such as an imidazolium,14
pyridinium,15 ammonium,16 or phosphonium cation17) togeth-
er with various organic or inorganic anions, displaying unique
properties such as extremely low vapor pressure, nonflamm-
ability, high polarity, high thermal stability, favorable electro-
chemical properties, and unorthodox and tunable miscibility
behavior.7,9 Therefore, POILs find widespread use, e.g., as
polymer electrolytes for electrochemical devices,5,18,19 sensors,2
supports for catalysts,20 polymeric dispersants,21 CO2 absorbing
resins,22 microwave absorbing materials,23 or self-healing
materials.24 The latter application specifically requires detailed
control over molecular relaxation and reorganization within
POILs, as both their structure and dynamics can be controlled
by the corresponding internal nanostructure.
Lopes and Pad
́
ua26 numerous studies have confirmed the
nanostructural organization of monomeric ILs.27−29 Inves-
tigated ILs show first sharp diffraction peaks (FSDPs)30 in X-
ray25,29 or neutron diffraction structure functions,27a,31 which
indicates the presence of intermediate range order (∼5−10 Å).
On the one hand, such intermediate range order can originate
from either charge ordering (similar to the classical molten
salts30) or, on the other hand, from the interdigitation of
“bilayered” alkyl chains. Because of their nanostructural
organization and hence their extraordinary miscibility behavior,
ILs can be used as entropic drivers (the “IL effect”) for the
preparation of well-defined nanoscale structures with extended
order, either in the bulk phase or at the gas/vacuum interface.32
Because of their structural similarity to classical ionomers,
POILs may form ionic aggregates, so-called “multiplets”. Such
ionic clustering and self-aggregation of POILs can lead to an
increase of ionic conductivity of POILs, making them suitable
for applications as valuable polyelectrolytes.5 Furthermore,
Received: December 19, 2011
Revised: January 21, 2012
Published: February 6, 2012
© 2012 American Chemical Society
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several investigators have reported that POILs containing
monomers with mesogen character tend to self-assemble into
various liquid-crystalline phases with different organiza-
tion.4−6,33 Thus, a pronounced case of liquid-crystalline order
was achieved by Ohno et al.,34 who synthesized mesogen POIL
containing tris(alkoxy)phenyl groups attached to acryloyl
groups. Structural changes of IL monomer led to the formation
of columnar phases which produced oriented one-dimensional
ion conductive channels; thus, subsequent photopolymerization
of ionic liquid film resulted in a well-ordered, highly conductive,
liquid-crystalline POILs. The same working group further
increased the conductivity of POIL materials via design of
thermotropic bicontinuous cubic liquid-crystalline POILs based
on a polymerizable ammonium moiety complexed with a
lithium salt and thus obtained lithium ion-conductive solid
ray scattering (SAXS) investigations as well as rheology studies
have been conducted to reveal structure and relaxation behavior
of the prepared POILs.
EXPERIMENTAL SECTION
■
Materials. All materials were obtained from Sigma-Aldrich and
used without further purification if not mentioned otherwise. 1-
Methylimidazole, 1-methylpyrrolidine, and N,N,N-triethylamine was
distilled over CaH2 prior to use. Dichloromethane and toluene were
predried over CaCl2 and freshly distilled over CaH2, sodium, and
benzophenone.
1
Instrumentation. H NMR and 13C NMR spectra were recorded
on a Varian Gemini 2000 FT-NMR spectrometer (400 MHz) and a
Varian Unity Inova 500 (500 MHz). MestRec-C software (version
4.9.9.6) was used for data interpretation. Deuterated chloroform
(CDCl3) and dimethyl sulfoxide (DMSO-d6) were used as solvents.
All chemical shifts (δ) are reported in parts per million (ppm) relative
to tetramethylsilane (TMS) and referenced to the significant solvent
signals. Coupling constants (J) are given in hertz (Hz).
Gel permeation chromatography (GPC) analysis was performed on a
Viscotek GPCmax VE2001 system combined with a Viscotek TDA302
(triple detector array) using polyisobutylene standards and THF as
solvent. PIB standards with a molecular weight of 340, 1650, 7970, 26
300, 61 800, and 87 600 g/mol were used for calibration. Data were
analyzed with the OmniSec (4.5.6) software. The polystyrene−
divinylbenzene-based column set consists of a HHR-HGuard-17,369
precolumn followed by a GMHHR-N-Mixed Bed 18055 (1000 to 4 ×
105 Da) and a G2500HHR-17,354 (100 to 2 × 104 Da) column. Both
the detector and the column temperature was set to 35 °C, with flow
rate of 1 mL min−1, and injection volume of 100 μL.
Matrix-assisted laser desorption/ionization time-of-flight mass spectrome-
try (MALDI-TOF-MS) experiments were performed on a Bruker
Autoflex III system operating in reflection and linear modes. The data
evaluation was carried out on flexAnalysis software (version 3.0). Ions
were formed by laser desorption (smart beam laser at 355, 532, 808,
and 1064 6.5 nm; 3 ns pulse width; up to 2500 Hz repetition rate),
accelerated by a voltage of 20 kV, and detected as positive ions. The
matrix solution was prepared by dissolving 1,8,9-anthracenetriol
(dithranol) in THF at a concentration of 20 mg mL−1. Polymers
were dissolved in THF at a concentration of 10 mg mL−1; salts sodium
trifluoroacetate (NaTFA) and potassium chloride (KCl) were
dissolved at a concentration of 10 mg mL−1 in THF. Solutions of
the matrix, the polymer, and the salt were mixed in a volume ratio of
100:10:1, and 1 mL of each mixture was spotted on the MALDI-target
plate. Baseline subtraction and smoothing of the recorded spectra were
performed using a three point Savitzky−Golay algorithm. The
instrument was externally calibrated with poly(ethylene glycol)
standards (Mw = 2000 and 4200 g mol−1) applying a quadratic
calibration method with an error of 1−2 ppm.
Thermogravimetric analysis (TGA) was conducted on a Mettler Toledo
(DSC-H22) instrument. The sample was heated in a Pt pan, under a
nitrogen atmosphere, over a temperature range 25−800 °C, with a
heating rate of 10 °C min−1.
Differential scanning calorimetry (DSC) was conducted on Perkin-
Elmer Pyris Diamond instrument. The glass transition temperatures
were determined by cooling the samples at −90 °C and then heating
up to 150 °C, both at rate 10 °C min−1. The glass transition
temperature is taken as a midpoint of a small heat capacity change
upon heating from amorphous glass state to a liquid state.
Small-angle X-ray scattering (SAXS) experiments were carried out
under vacuum with a rotating copper-anode X-ray generator
(Nanostar, Bruker AXS), Cu Kα radiation (wavelength 0.1542 nm)
monochromatized and collimated from crossed Goebel mirrors, and a
2-D position sensitive detector (Vantec 2000). For the in situ SAXS
measurement a specially designed X-ray transparent furnace was
developed, which allows stepwise heating of the samples from room
temperature to 550 °C with an accuracy of 0.5 °C. The samples were
placed either in a quartz glass capillary with a diameter of 1.5 mm and
a wall thickness of 10 μm (from Hilgenberg, Germany) or between
polymeric films having 3D interconnected ionic channels.35
A
different approach for increasing the conductivity of POIL
electrolytes is the specific design of ionic liquid block
copolymers.12,36,37 Thus, the use of microphase separating
block copolymers displays numerous advantages such as a
higher level of ionic conductivity by lowering the glass
transition temperatures (Tg) or the increase of mechanical
properties by selective cross-linking of ionic liquid monomers
in one phase of an POIL.12 Weber et al.37 have shown that
microphase separation of ionic liquid block copolymers
strongly influences their conductivity. Well-structured lamellar
microphase morphology of styrene−ionic liquid copolymers led
to a 10-fold increase in conductivity as compared to copolymers
with poorly organized hexagonal morphologies. Thus, control
of the nanostructural organization of POILs is the key point in
their application in many of the aforementioned uses,
particularly when relaxation and flow behavior dominate
major structural properties.
In this paper we for the first time report on a nanostructured
IL with new features based on the high fluidity of a polymeric
backbone in addition to the presence of ionic moiety similar to
ionic liquids (ILs). The basic idea is the use of a polymer with
an extremely low glass transition temperature (Tg) so as to
introduce liquid properties already into the native polymer of
the POIL. Additionally, the extremely large difference between
the hydrophobic polymer and the attached IL end groups
(affixed to one end group of the polymer) can induce
significant nanostructural order within the then polymeric
ionic liquid. Until now predominantly ionic liquid cations have
been incorporated into polymer backbones obtained from
vinyl,1,38 styrenic,22a methacrylic,39 ethylene glycol,40 vinyl
ether,41 and norbornene monomers.22c,39,42 As only few
polymers display such liquid properties in themselves we
have chosen poly(isobutylene) as polymer with such highly
liquid properties. The use of poly(isobutylene) as biocompat-
ible polymer with high hydrophobicity and chain mobility (as
exemplified by its low glass transition temperature, Tg = −80
°C) has found increased use in fields of supramolecular
polymer science,43,44 amphiphilic conetworks,14,45 and self-
healing materials.46 In contrast to the previously predominantly
used radical polymerization, we have applied living carboca-
tionic polymerization (LCCP) for the preparation of POILs,
thus allowing for chain-length control and therefore control of
the nanostructure’s dimension.47 The projected PIB-ILs were
prepared by combining LCCP with a microwave-assisted azide/
alkyne “click” reaction between an azido telechelic PIB and the
corresponding alkyne containing IL headgroup. As significant
phase separation effects were expected, detailed small-angle X-
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a
Scheme 1. Synthetic Route toward Nonsymmetric Ionic Liquid-Functionalized PIBs 3a−3c
a
(a) Living carbocationic polymerization (LCCP):47c (i) synthesis of allyl-functionalized PIB; (ii) synthesis of hydroxyl-functionalized PIB; (iii)
synthesis of bromide functionalized PIB; (iv) synthesis of azide functionalized PIB (1).48 (b) Azide/alkyne “click” reaction. Reaction conditions are
given in the Experimental Section.
commercial aluminum foils. The samples were heated up to the
desired temperature, kept at this temperature for 5 min to ensure
thermal equilibrium, and measured for 15−30 min depending on the
scattering intensity of the respective samples. With a sample-to-
detector distance of 108 cm an accessible q-range from 0.1 to 2.8 nm−1
was obtained. The SAXS patterns were radially averaged in order to
obtain the scattering intensities I(q), where q = (4π/λ) sin θ is the
scattering vector and 2θ the scattering angle. The d100 peak (the
strongest Bragg reflection) was fitted with a Lorentzian function. This
results in numerical values for the peak position (corresponding to the
distance of the crystalline units in real space) and the peak breadth
(proportional to the domain size).
Rheological measurements were performed on an Anton Paar MCR 101-
DSO rheometer using parallel plates (diameter 8 mm). The sample
temperature was regulated by thermoelectric cooling/heating in a
Peltier chamber under a dry oxygen atmosphere. Frequency sweep
measurements were performed within the LVE (if not mentioned
otherwise). Temperature sweep measurements were performed with a
heating rate of 1 °C/min at ω = 10 rad/s. Before each measurement
the sample was annealed for 30 min.
purified by washing with ethyl acetate. Yield of (2a): 84% (1.74 g, 8.70
mmol), as a brownish powder. H NMR (400 MHz, DMSO-d6) δ
1
(ppm): 9.29 (s, 1H), 7.81 (t, J = 1.8, 1H), 7.78 (t, J = 1.8, 1H), 5.24 (d,
J = 2.56, 2H), 3.89 (s, 3H), 3.84 (t, J = 2.6, 1H). 13C NMR (100.6
MHz, CDCl3) δ (ppm): 136.2, 123.7, 121.8, 78.8, 75.9, 38.4, 35.8.
1-Propargyl-1-methylpyrrolidinium Bromide (2b). After addition
of 1-methylpyrrolidine (1.06 g, 12.43 mmol) to a solution of propargyl
bromide (1.23 g, 10.36 mmol) in dry toluene (25 mL), the reaction
mixture was stirred for 24 h. After purification the product was
obtained as slightly yellow solid with yield of 88% (1.69 g, 8.28 mmol).
1H NMR (400 MHz, DMSO-d6) δ (ppm): 4.50 (d, J = 2.5, 1H), 4.01
(dt, J = 2.45, 0.74, 2H), 3.63−3.49 (m, 4H), 3.15 (s, 3H), 2.18−2.05
(m, 4H). 13C NMR (100.6 MHz, CDCl3) δ (ppm): 81.6, 72.9, 63.0,
52.3, 48.7, 21.4.
N-Propargyl-N,N,N-triethylammonium Bromide (2c). According
to the general procedure N,N,N-triethylamine (1.25 g, 12.72 mmol)
was added to a solution of propargyl bromide (1.23 g, 10.36 mmol) in
dry toluene (20 mL), and it was stirred for 16 h. The purification of
the product resulted in a white solid with 82% (1.87 g, 8.49 mmol)
1
yield. H NMR (400 MHz, CDCl3) δ (ppm): 4.34 (d, J = 2.56, 2H),
4.01 (t, J = 2.53, 1H), 3.30 (q, J = 7.2, 6H), 1.23 (t, J = 7.2, 9H). 13C
NMR (100.6 MHz, CDCl3) δ (ppm): 82.1, 71.9, 52.9, 30.5, 7.3.
General Procedure for the Synthesis of PIBs Containing an
Ionic Moiety via “Click” Reaction (3a−3c). Azido telechelic PIB
(1, 1 equiv, Mn = 2920 g/mol; Mw/Mn = 1.14) and the corresponding
ionic liquid containing alkyne (2 equiv) were dissolved in a solvent
mixture of toluene/water/isopropanol (2:1:1) and placed in a
microwave vial. After addition of N,N-diisopropylethylamine
(DIPEA) (10 equiv), the vial was closed with a septum and the
solution was purged with nitrogen for 30 min. Then copper(I) iodide
(CuI) (0.2 equiv) was added to the mixture, and the solution was
again purged with nitrogen for more than 30 min. Subsequently, the
vial was sealed, placed in a microwave reactor, and irradiated under
70−80 W for several hours. After termination of the irradiation, the
organic phase was separated and washed with water (3 times). The
crude product which was obtained after removal of the solvents was
purified by column chromatography (stationary phase: SiO2; eluent:
chloroform) to eliminate the unreacted azido-telechelic PIB. Then the
eluent was changed to chloroform/methanol (15:1), and the fraction
with Rf = 0.2 was collected. After evaporating of the solvent, the
residue was dissolved in a small amount of chloroform and precipitated
into methanol. The precipitate was collected and dried under high
vacuum.
Synthesis. Allyl telechelic PIB was synthesized via living
carbocationic polymerization (LCCP) based on procedure known
from the literature.47c Isobutylene was polymerized via LCCP in the
presence of 2-chloro-2,4,4-trimethylpentane (TMPCl) and TiCl4 as
initiator and co-initiator (for details see Supporting Information S2).
Quenching of the polymerization with allyltrimethylsilane (ATMS)
resulted in allyl-functionalized PIB. Azido telechelic PIB (Mn = 2920
g/mol; Mw/Mn= 1.14) was synthesized via further modification of allyl
telechelic PIB following procedure described by Binder et al.48 The
allyl moiety was converted into the hydroxyl group by hydroboration
of the double bond using 9-borabicyclo[3.3.1]nonane (9-BBN) and
subsequent oxidation with m-chloroperoxybenzoic acid (m-CPBA).
Bromination of the obtained PIB was accomplished in the presence of
carbon tetrabromide and triphenylphosphine with a yield of 99%.
Bromo-functionalized PIB was converted to the azide using
tetrabutylammonium fluoride (TBAF) and azidotrimethylsilane
(TMSA) with a yield of 89%. The pure product PIB (1) was
characterized using NMR and GPC (see Supporting Information, S2−
S4).
General Procedure for the Synthesis of Compounds 2a−2c.
To a solution of propargyl bromide (1 equiv) in dry toluene
corresponding amine (1.2 equiv) was added dropwise. The reaction
mixture was stirred at 50 °C. Then, the solvent was removed from
reaction mixture under reduced pressure, and the obtained crude
product was washed with ethyl acetate.
1-Propargyl-3-methylimidazolium Bromide (2a). 1-Methylimida-
zole (1.02 g, 12.43 mmol) was added dropwise to a solution of
propargyl bromide (1.23 g, 10.36 mmol) in dry toluene (25 mL). After
stirring the reaction mixture for 20 h, the obtained product was
3-Methyl-1-imidazolium Telechelic PIB (3a). 3-Propargyl-1-meth-
ylimidazolium bromide (2a, 12.09 mg, 0.06 mmol), azido-telechelic
PIB (100 mg, 0.03 mmol), CuI (10 mg); irradiation conditions: 80 W,
1
85−90 °C, 17 h. Yield of 3a: 58% (67 mg). H NMR (400 MHz,
CDCl3) δ (ppm): 10.70 (s, 1H) 8.47 (s, 1H), 7.63 (s, 1H), 7.09 (s,
1H), 5.86 (s, 2H), 4.29 (t, 2H, J = 7.5), 3.98 (s, 3H), 1.94−1.83 (m,
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Table 1. Reaction Conditions, Molecular Weight Data, Decomposition (Td,onset), and Glass Transition Temperature (Tg) of the
PIB Ionic Liquids 3a−3c
a
b
entry POIL
catalyst
reaction condition
conv (%) yield (%) Mn(NMR) (g/mol) Mn(GPC) (g/mol) PDI Td,onset (°C) Tg (°C)
1
2
3
3a
3b
3c
CuI (0.03 mmol)
CuI (0.03 mmol)
CuI (0.03 mmol)
75 W/75 °C/16 h
75 W/75 °C/16 h
70 W/75 °C/16 h
86
81
74
58
52
48
3256
3259
3334
3241
3207
4700
1.2
1.2
1.2
310
398
352
−70.6
−71.4
−70.9
a
b
Reaction condition is reported based on irradiation power (W), reaction temperature (°C), and duration (h). Conversions were determined by 1H
NMR, comparing integration ratio of cationic moiety signals to the rest of the polymer chain, especially unreacted −CH2−N3.
1
Figure 1. H NMR spectra of the PIB-ILs (3a−3c) and the starting material (azido-telechelic PIB (1)).
2H), 1.47−1.36 (m, 104H), 1.16−1.05 (m, 312H), 1.00−0.97 (m,
15H). 13C NMR (100.6 MHz, CDCl3) δ (ppm): 139.8, 137.8, 125.2,
122.7, 122.5, 59.5, 58.8, 58.2, 55.9, 51.5, 44.4, 42.2, 38.1, 36.6, 34.8,
32.6, 32.5, 31.2, 30.8, 29.1, 25.5.
102H), 1.15−1.05 (m, 306H), 0.99−0.95 (m, 15H). 13C NMR (100.6
MHz, CDCl3) δ (ppm): 135.1, 128.4, 59.5, 58.8, 58.2, 55.9, 53.5, 51.9,
51.5, 42.1, 38.2, 37.9, 37.8, 34.8, 32.6, 32.4, 31.2, 30.8, 29.1, 25.4, 7.8.
1-Methylpyrrolidinum Telechelic PIB (3b). 1-Propargyl-1-methyl-
pyrrolidinum bromide (2b, 12.24 mg, 0.06), azido-telechelic PIB (100
mg, 0.03 mmol), CuI (8 mg); irradiation conditions: 80 W, 85−90 °C,
RESULTS AND DISCUSSION
■
Synthesis of Polyisobutylene-Based Ionic Liquids
(PIB-ILs). In general, POILs can be prepared either via direct
polymerization of IL monomers or via chemical modification of
existing polymers. Both strategies often involve polymerization
techniques such as conventional6 and living radical polymer-
ization (ATRP49 and RAFT41,50), ring-opening metathesis
polymerization,51 or chemical and electrochemical oxidative
polymerization.52 Our synthetic route for the preparation of
polyisobutylene-based ionic liquids (PIB-ILs) follows the use of
living carbocationic polymerization (LCCP) as presented in
Scheme 1. It should be noted that for end-group modification
of polyisobutylene with strongly polar moieties (as the IL
groups are), only a few qualitatively useful reactions are
1
17 h. Yield of 3b: 52% (56 mg). H NMR (400 MHz, CDCl3) δ
(ppm): 8.52 (s, 1H), 5.02 (s, 2H), 4.34 (t, 2H, J = 7.5), 4.11−4.00 (m,
2H), 3.60- 3.50 (m, 2H), 3.24 (s, 3H), 2.40−2.18 (m, 4H), 1.98−1.81
(m, 2H), 1.47−1.36 (m, 104H), 1.16−1.05 (m, 312H), 1.00−0.97 (m,
15H). 13C NMR (100.6 MHz, CDCl3) δ (ppm): 128.4, 63.9, 59.5,
58.8, 58.2, 57.6, 55.9, 51.6, 49.4, 42.2, 38.2, 37.8, 34.8, 32.6, 32.4, 31.2,
30.8, 29.2, 25.5, 21.9.
N,N,N-Triethylammonium Telechelic PIB (3c). N-Propargyl-N,N,N-
triethylammonium (2c, 13.20, 0.06 mmol), azido-telechelic PIB (100
mg, 0.03 mmol), CuI (10 mg); irradiation conditions: 80 W, 85−90
1
°C, 17 h. Yield of 3c: 48% (51 mg). H NMR (400 MHz, CDCl3) δ
(ppm): 8.81 (s, 1H), 5.01 (s, 2H), 4.36 (t, 2H, J = 7.5), 3.48 (q, 6H, J
= 7.3), 1.98−1.88 (m, 2H), 1.52 (t, 9H, J = 7.2), 1.45−1.36 (m,
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Figure 2. MALDI-TOF mass spectra of the PIB-ILs (3a−3c): (a) Compound 3a, inset: simulation for [C186H368N5]+ with 43 units of isobutylene.
(b) Compound 3b, inset: simulation for [C187H373N4]+ with 43 units of isobutylene. (c) Compound 3c, inset: simulation for [C188H377N4]+ with 43
units of isobutylene.
known.47c,e,f,53 The azide/alkyne “click” reaction to attach these
highly polar moieties therefore represents the best method to
achieve this goal.
Allyl-functionalized telechelic PIB was synthesized via living
carbocationic polymerization (LCCP) using 2-chloro-2,4,4-
trimethylpentane (TMPCl) as described elsewere.47c Trans-
formation of the allyl-functionalized PIB to the azido-telechelic
PIB (1) was conducted according to the method developed
earlier by us,48 yielding PIBn-N3 (Mn = 2920 g/mol; Mw/Mn =
1.14) in 90% overall yield. Subsequent microwave-assisted
azide/alkyne “click” reaction between PIBn-N3 (1) and the
alkyne containing ILs (2a−2c) with a copper(I) catalyst
resulted in the desired ionic-liquid-functionalized PIB. Upon
testing different Cu(I) catalysts (Cu(I) iodide, Cu(I) bromide
and tris(triphenylphosphine)copper(I) bromide, [Cu-
(PPh3)3Br]), only [Cu(PPh3)3Br] and CuI proved conversions
between 30 and 86% (see Table 1 and Supporting Information,
Table S1). Best results were achieved using copper(I) iodide
under microwave irradiation conditions, with a strong influence
of the microwave-irradiation power on the obtained yields.
Although the increase of irradiation power often leads to higher
conversions, the formation of side products via Hofmann
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elimination could be observed. Lowering of the irradiation
power also resulted in a decline of conversion. Therefore,
optimization of microwave irradiation conditions for all three
compounds led to a good conversion of the azide (conversions
74−86%, see Table 1).
Characterization of the PIB-ILs. The structure of the
prepared PIB- ionic liquids (3a−3c) was confirmed via 1H and
13C NMR spectroscopy (see Figure 1 and Supporting
Information, S8−S10) as well as MALDI-TOF-MS measure-
ments (see Figure 2). Chromatographic and NMR spectro-
scopic data are shown in Table 1, revealing a good agreement
between the Mn(GPC) and Mn(NMR) data and hence indicating a
complete end-group functionalization. After purification by
column chromatography it was possible to obtain pure
compounds 3a−3c, thus removing eventually present impur-
ities or incompletely functionalized polymers.
All compounds showed the expected resonances in NMR
spectroscopy as well as the expected absolute molecular weights
in the respective mass spectra. The successful “click” reaction
was confirmed by the proton shift of the terminal CH2− group
of poly(isobutylene) chain (triplet d in Figure 1) from 3.21
ppm for azido telechelic PIB (1) to 4.29−4.36 ppm in the case
of PIB-ILs 3a−3c and the appearance of a resonance of the
triazole ring in the range between 8.47 and 8.81 ppm (signal f
in Figure 1). Furthermore, no signals originating from the
Figure 3. Thermogravimetric analysis (TGA) of the PIB-ILs (3a−3c)
at a heating rate of 10 °C min−1 under a nitrogen atmosphere.
description of morphology of random ionomers postulated by
Eisenberg et al.,55,56 ion pairs aggregate to structures named
“multiplets”. Electrostatic interactions between “multiplets”
favor their agglomeration to form phase-separated regions
(clusters), whereas the elastic forces of free polymer chains are
opposing cluster formation. Additionally, the mobility of the
polymer chain in the immediate vicinity of the “multiplets” is
strongly restricted, resulting in a region of restricted mobility
surrounding each multiplet. As the ion content increases the
average distance between “multiplets” decreases, resulting in
overlap of regions with restricted mobility of polymer chains.
After a certain critical ion concentration is reached cluster
formation becomes more energetically favorable, therefore
resulting in the formation of aggregates. As a consequence, the
presence of clusters leads to the existence of large regions of
material with restricted mobility. Since the stability of
microphase separation is strongly temperature dependent, a
loss of the internal structure at higher temperature due to the
weakening of electrostatic interactions is expected. However,
after cooling and distinct relaxation time re-establishment of the
internal structure can be observed, indicative of the self-healing
properties of such ionomers.24c,56,57
1
unreacted azido-telechelic PIB could be observed in H NMR
spectra of the PIB-ILs 3a−3c (see Figure 1). MALDI-TOF-MS
spectra of synthesized polymeric ionic liquids are shown in
Figure 2. All three spectra show one main series with 56 Da
differences which corresponds to the mass of one repeating
unit. The observed signals in all three cases can be simply
assigned to PIB containing cationic moiety. According to Figure
2a, the signal appearing at 2672.626 Da can be assigned to PIB
containing imidazolium cation with chemical formula of
[C186H368N5]+ with n = 43 units of isobutylene. In Figure 2b,
the signal at 2675.695 Da can be represented by [C187H373N4]+,
and the PIB-containing ammonium moiety can be identified by
its signal at 2691.692 Da, which is assigned for [C188H377N4]+
(Figure 2c). The simulated isotopic pattern of the synthesized
polymeric ionic liquids for repeating unit of n = 43 shows an
excellent match with the pattern of the observed signals in
MALDI spectra which confirms the formation of expected
structures.
In order to investigate the morphologies of prepared PIB-ILs,
SAXS measurements were conducted (see Figures 4−6 and
Supporting Information, S11−S12).
All three investigated PIB-ILs exhibit glass transition
temperatures in the range −70.6 to −71.4 °C (see Table 1),
indicating the increase of Tg compared to the PIB (−80 °C)
due to the presence of the ionic moieties.
The observed Bragg reflections in the SAXS patterns can be
interpreted as the formation of ordered clusters in POILs,
predominantly representing intracluster spacings. Both 3a and
3b show five reflections, which could be attributed to a simple
cubic lattice with a d-spacing (the distance of the units) of 7.6
and 6.9 nm for 3a and 3b, respectively. The ammonium-
containing compound (3c) exhibits four reflections, clearly
indicating an arrangement in the form of 2-D hexagonally
packed cylinders with a d-spacing of 7.95 nm (see also
Supporting Information, S11−S12). Thus, it can be assumed
that PIB-ILs 3a and 3b form “multiplets” clustered in a micellar
fashion at room temperature, while 3c assembles as a cylindrical
phase (see Figure 4).
As thermal stability is an important selection criterion for a
high-temperature use, thermogravimetric analysis of the PIB-
ILs was performed (see Figure 3), indicating only one
decomposition step with the onset decomposition temperature
in the range between 310 and 398 °C (see Table 1). As
expected, the thermal stability of the presented PIB-ILs 3a−3c
is higher compared to the classical monomeric ionic liquids54
and lies in the range of other previously investigated POILs.5
SAXS Measurements of PIB Ionic Liquids. As already
reported in the literature, the morphology of some POILs
exhibits a behavior similar to those described for ionomers.4,5,12
As the structure of prepared polyisobutylene-based POILs
shows distinct resemblance to the structure of classical
ionomers, composed of a polymeric tail and ionic groups at
the end positions, similar microphase separation may be
expected. According to the multiplet-cluster model for the
In the next step, PIB-IL samples were also subjected to
variable-temperature in situ SAXS to identify any disorder−
order or order−order transition in the range 25−300 °C and to
further understand the relaxation of structural changes. As can
be seen in Figure 5b, compound 3b shows a lattice disorder−
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broadness of the main peak. After cooling to room temperature
and waiting for a relaxation time of 132 h a clear re-
establishment of the nanostructure can be observed (see Figure
5b). The relaxation time of 3b is comparable with the results
obtained by Varley et al.24c for classical ionomers with
relaxations times up to 7 days.
Additionally, the obtained data were fitted with Lorentzian
function, and the variation of the main peak position as well as
its half-width at half-maximum (hwhm) with temperature are
plotted. The variation of the main peak position with
temperature and after relaxation is presented in Figure 6 (see
also Supporting Information, S13−S14), indicating a continu-
ous shift of the main peak to larger q values with increasing
temperature, which corresponds to the formation of smaller
structures. After a relaxation time of 132 h the original
nanostructure is almost completely restored, which is in
accordance with the observations of Eisenberg56,57d and
others57 regarding the thermal behavior of classical ionomers.
Therefore, the investigated ionic liquid 3b exhibits self-healing
characteristic for ionomers.
However, a markedly different behavior was observed in the
case of PIB-IL 3a (see Figure 5a). The internal cubic
nanostructure of the imidazolium-based IL 3a is stable even
at higher temperatures. Only a partial loss of the nanostructure
can be observed at 275 °C (loss of the smallest two peaks). The
strong electrostatic interactions between imidazolium cations,
probably combined with pronounced π−π stacking between
adjacent imidazolium rings, lead to the significantly stable
internal structure. In the range between 275 and 300 °C the
order−disorder transition can be observed, followed by the
extreme broadening of the main peak. However, this loss of
internal structure is probably due to the thermal decomposition
of the compound, rather than due to the thermal transition of
nanostructural organization. Furthermore, even after 1 week no
structural relaxation could be observed, suggesting the
irreversible thermal decomposition of the starting compound
at 300 °C, which is in accordance with the thermal stability data
obtained with TGA (see Figure 3).
Figure 4. (a) SAXS profiles of PIB-based ILs (3a−3c) at room
temperature. Each curve is shifted in the intensity axis (y-axis) for
clarity. (b) Schematic illustration of simple cubic and hexagonally
packed cylindrical nanostructure formed by PIB-ILs. The “multiplets”
are representing the inner phase of the structures.
order transition (LDOT) at 80 °C, indicated by the broadening
of the first and the gradual disappearance of higher order peaks.
This resembles the phase disordering process of micelles during
heating, which was distinguished by Han et al.58 from the
demicellation/micellation transition (DMT) at still higher
temperatures. The increase of temperature leads to a gradual
weakening of the electrostatic forces between ionic clusters.
After reaching the transition temperature, loss of the internal
structure due to the electrostatic interaction can be observed
(loss of all four higher order scattering maxima). Breaking of
the ionic clusters between “multiplets” subsequently leads to an
increase in mobility of polymer chains, resulting in a shift and
Figure 5. SAXS profiles of the prepared PIB-containing ionic liquids at different temperatures and after relaxation: (a) 3a; (b) 3b; (c) 3c. Each curve
is shifted in the intensity axis (y-axis) for clarity.
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Figure 6. Variation of the main peak position with temperature (left), after heating to 90 °C, and relaxation at room temperature (right) obtained for
3b via temperature-dependent SAXS measurements.
Figure 7. Temperature-sweep measurements for PIB-ILs: (a) for sample 3b, (b) for sample 3c, and (c) for sample 3a. The heating rate was 1 °C/
min at ω = 10 rad/s.
Figure 8. (a) Frequency-sweep measurement of 3b at 20 °C (γ = 2%). (b) Frequency-sweep measurements of 3c with different strains at 20 °C.
Whereas in the case of the compounds 3a and 3b no order−
order transition (OOT) could be observed at any temperature,
this is clearly the case for compound 3c (see Figure 5c).
Starting from the hexagonally packed cylindrical morphology at
room temperature, the first OOT can be observed at 50 °C
where the coexistence of hexagonally packed cylindrical and
simple cubic morphology is clearly visible (see also Supporting
Information, S11−S12). At 65 °C a second OOT can be
observed, indicating the loss of the hexagonally packed
cylindrical morphology. Finally, at 90 °C the loss of the cubic
morphology and hence the order−disorder transition can be
observed. Corresponding to the much more flexible structure of
the cation in compound 3c, significantly shorter relaxation
times were observed compared to PIB-IL 3b (see Figure 5c).
After cooling to room temperature and only 48 h relaxation
time the hexagonally packed cylindrical morphology of 3c was
almost completely re-established. Our results clearly indicate
that the self-assembly and the stability of the microphase
morphology of the presented POILs strongly depends on the
nature of the cation anchored on the polymeric chain.36
Rheology Measurements of PIB Ionic Liquids. A closer
look on the order/order and order/disorder transitions was
achieved by melt rheology,59 as in our samples the relaxation
behavior expectedly stems from two different contributions. On
the one hand, microphase separation should influence the
viscous behavior similar to conventionally microphase-sepa-
rated block copolymers above the Tg, whereas the ionic
multiplets should strongly tend to retain the (cubic) structure
due to their high stability even at high temperatures.60 As
shown in Figure 7, the samples exhibited strongly different
rheological behavior, depending on the nature of the cation
anchored onto the polymeric chain. At low temperatures only
the PIB-IL 3b shows terminal flow, as exemplified by the
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dependency of G′ ∼ ω2; G″ ∼ ω1 at ωτ ≪ 1, indicative of a
viscoelastic fluid.
at room temperature are strongly dependent on the nature of
ionic group, with the relaxation time of compound 3c being
significantly shorter as compared to the 3b with the rigid, cyclic
pyrrolidinium cation. The reported PIB-ILs represent new
types of ionic liquids, where both the polymeric part and the IL
headgroup can be used to tune both the nanostructure and the
rheological properties. Because of their additional high thermal
stability, the so-identified ILs therefore are important
candidates for strongly shear- and wear-resistant fluids in
science and technology, in particular aiming at self-healing
polymers, where the relaxation time can now be engineered by
the IL end group, together with the strong shear- and
temperature-dependent flow behavior.
This behavior can be explained by the internal cubic
structure of the PIB-IL, which consists of (spherical) ionic
multiplets. These multiplets slide over each other similar to the
model described by Antonietti et al.61 for spherical microgels.
Therefore, a small (rubbery) plateau can be observed, which fits
into the model of viscous flow via cooperative movements of
the ionic multiplets upon shear. Depending on the size of the
multiplets (3b (6.9 nm); 3c (7.24 nm); 3a (7.6 nm) as
determined via SAXS), flow is then achieved at increasing
temperatures 40, 105, and 140 °C, respectively (see frequency-
sweep measurement at ω = 10 rad/s in Figure 7). As our
polymers are below the entanglement limit, the formation of a
plateau by entangled chains can be definitely ruled out.
ASSOCIATED CONTENT
■
However, in accordance with the work of Hadjichristidis,59
a
S
* Supporting Information
deaggregation of the ionic clusters at lower frequencies cannot
be excluded. In contrast to the other PIB-ILs (3a, 3b), PIB-IL
3c shows a significant transition around T = 50 °C, which
matches with the order/order-transition from cubic to
hexagonal as observed in SAXS. Thus, a strongly shear-
dependent behavior of G′ and G″ at low strain (γ = 2−70%) was
observed (see Figure 8b and Supporting Information, S15−19),
similar to the behavior of nanotubes, dispersed in a IL matrix.62
The tubular structures can form a transient network, which can
then be broken by the applied forces with increasing strain.
However, the obtained results may also indicate the alignment
of the aggregates of the ionic groups under the applied strain.63
As those two possibilites can hardly be discriminated, and the
SAXS data point to the former behavior of cluster retainment,
we tend to the first hypothesis. In general, the observed data
clearly demonstrate an entirely different behavior than expected
for conventional block copolymers, which are introduced by the
ionic liquid head groups. Clearly fascinating is this strong
change by the presence of only one small ionic headgroup
within the comparable long polymer chain.
Optimization of reaction conditions, synthetical procedure for
the compound 1, NMR spectra of the compounds 1, 2a−2c,
and 3a−3c, GPC of the compound 1, SAXS (including fitted
Lorentzian curves), and rheology data of compounds 3a−3c.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail wolfgang.binder@chemie.uni-halle.de; Fax (+49) 345
55 27392.
■
Author Contributions
§P.Z. and A.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support from the research
training network MINILUBES (FP-7 Marie Curie Action)
(P.Z., A.S.), grants DFG BI 1337/7-1 (F.H.), DFG BI 1337/8-1
(within the SPP 1568 (“Design and Generic Principles of Self-
Healing Materials”)), DFG INST 271/249-1, INST 271/247-1,
INST 271/248-1 (W.H.B.), and the Austrian science funds
FWF proj. nr. I449 (J.A., H.P.).
CONCLUSIONS
■
A new IL with a strongly temperature-dependent nanostructure
is reported in this paper, relying on the synthesis of a liquid
polymer with a IL headgroup, thus introducing liquid
properties by both the polymer and via the IL headgroup.
Thus, the poly(isobutylene)s containing polymeric ionic liquids
3a−3c were prepared, and their temperature-dependent self-
assembly behavior was investigated by SAXS. The synthetic
approach combined living cationic polymerization with azide/
alkyne “click” chemistry and enabled a full end-group
transformation to the final polymeric ionic liquids, modified
with either imidazolium, pyrrolidinium, or triethylammonium
end groups.
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