EPR spectroscopic characterization of local nanoscopic heterogeneities during the thermal collapse of thermoresponsive dendronized polymers

Article abstract of DOI:10.1002/anie.201001469

(Figure Presented) The collapse transition of thermoresponsive dendronized polymers was characterized on a molecular scale by CW EPR spectroscopy. Aggregation of the polymer is triggered by dynamic structural inhomogeneities of a few nanometers, and the dehydration of the polymer chains proceeds, despite the sharp phase transition, over a temperature interval of at least 30°C (see picture).

Full text of DOI:10.1002/anie.201001469

Angewandte
Chemie
DOI: 10.1002/anie.201001469
Heterogeneous Collapse
EPR Spectroscopic Characterization of Local Nanoscopic
Heterogeneities during the Thermal Collapse of Thermoresponsive
Dendronized Polymers**
Matthias J. N. Junk, Wen Li, A. Dieter Schlꢀter, Gerhard Wegner, Hans W. Spiess, Afang Zhang,*
and Dariush Hinderberger*
Thermoresponsive polymeric materials are of great interest
owing to their potential use in fields such as actuation, drug
delivery, and surface modification.[1] Ever since the discovery
by Wu and co-workers of the coil–globule transition of single
poly(N-isopropylacrylamide) (PNiPAAm) chains near the
lower critical solution temperature (LCST),[2] the collapse
mechanism including the formation of stable mesoglobules
have been intense topics of research.[1,3] Despite these efforts,
a molecular-scale picture of what happens when thermores-
ponsive polymers start to dehydrate at a certain temperature,
subsequently collapse, and then assemble to mesoglobules,
does not exist. This absence severely hampers rational
materials design.
In an exploratory research effort aimed at detecting
unusual properties of dendronized polymers,[4] we recently
discovered that such systems based on oligoethyleneglycol
(OEG) units exhibit fast and fully reversible phase transitions
with a sharpness that is amongst the most extreme ever
observed.[5] These dendronized polymers with terminal
ethoxy groups are soluble in water and their lower critical
solution temperature (LCST) is found in a physiologically
interesting temperature regime between 30 and 368C. The
LCST of these OEG dendronized polymers is as low as for
poly(ethylene oxide) and long-chain ethylene oxide oligo-
mers. For the latter, the influence of hydrophobic end groups
on the LCST has been thoroughly investigated, both exper-
imentally and theoretically.[6] Given this extraordinary behav-
ior, these polymers should be particularly suited to gaining a
deeper understanding of the processes involved. Such materi-
als also bear great potential as hosts for small molecules for
targeted release, as they have encapsulation properties, which
can be controlled by thermoresponsivity.[7]
There are indications that such thermal responses proceed
by the formation of structural inhomogeneities of variable
lifetimes on the nanometer scale that are still poorly under-
stood. Indeed, this topic has been identified as one of the
major challenges of research in the macromolecular sciences
in the coming years.[8] Herein, the focus is on a clearer
understanding of the formation, structure, and lifetimes of
these local inhomogeneities, the effect of the individual
chemical structures on the physical processes, and the
influence of the local heterogeneities on the aspired function
(for example, drug delivery).
The remarkable macroscopic behavior of such materials
results from the systems being far from classical macroscopic
equilibrium. This situation can be viewed as an example of
“molecularly controlled non-equilibrium”. Such macromole-
cule-based processes far from equilibrium are extensively
found in nature, for example in DNA replication, to obtain
high specificity in the noisy environment of a cell. Inves-
tigations into similar concepts in synthetic macromolecular
systems are still rare.[9,10]
Magnetic resonance techniques as intrinsically local
methods meet the conditions required to solve questions
involved with structural inhomogeneities of functional mac-
romolecules[11] and dynamic heterogeneities in polymer melts
in the vicinity of the glass transition.[12] Advanced NMR and
pulse EPR techniques have now been established for the
[*] M. J. N. Junk,[+] Prof. Dr. G. Wegner, Prof. Dr. H. W. Spiess,
Dr. D. Hinderberger
Max-Planck-Institut fꢀr Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-100
Dr. W. Li,[+] Prof. Dr. A. D. Schlꢀter, Prof. Dr. A. Zhang[#]
Institut fꢀr Polymere, Department of Materials, ETH Zurich
Wolfgang-Pauli-Straße 10, HCI G525, 8093 Zꢀrich (Switzerland)
Fax: (+41)44-6331390
study of biological and synthetic macromolecules.[10–15]
A
particularly simple way of studying the molecular environ-
ment of systems undergoing a thermal transition utilizes
conventional continuous wave (CW) EPR spectroscopy on
nitroxide radicals as paramagnetic tracer molecules. Such spin
probes are sensitive to the local viscosity, which gives rise to
changes in the rotational correlation time, and to the local
polarity/hydrophilicity.[11c,14,15] The polarity affects the elec-
tronic structure of the radical and changes the spectral
parameters, specifically the g factor and the hyperfine cou-
pling constant to 14N. The amphiphilic radical 2,2,6,6-tetra-
methylpiperidine-1-oxyl (TEMPO) is especially suited to
sample both hydrophobic and hydrophilic regions. It has been
successfully applied to observe structural nanoinhomogene-
ities formed in thermoresponsive poly(N-isopropylacryla-
mide)-based hydrogels upon thermally induced macrosopic
E-mail: azhang@shu.edu.cn
[#] Current address: Department of Polymer Materials
Shanghai University, Chengzhong Street 20
Shanghai 201800 (China)
[+] These authors contributed equally to this work.
[**] We thank Christian Bauer and Susan Pinnells for technical support.
M.J.N.J. gratefully acknowledges financial support from the Fonds
der chemischen Industrie (FCI) and from the Graduate School of
Excellence “Materials Science in Mainz” (MAINZ).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001469.
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
collapse. These nanoinhomogeneities were static over a
Scheme 1). Furthermore, a second-generation dendronized
polymethacrylate was investigated in which the triethylene-
oxide core was replaced by a hydrophobic octane unit
(PG2(ETalkyl); Scheme 1). The turbidity measurements
reflect a phase separation process of a seemingly classical
nature, in which droplets of a concentrated solution of the
polymer separate from the dilute solution of the polymer
(binodal decomposition). The droplets of the concentrated
phase were identified by light microscopy.[5a] The EPR-
derived TC values were found from plots of the spectral
fraction of nitroxide species D1 as a function of temperature;
TC is defined as the temperature at which a virtually constant
spectral fraction of nitroxide species D1 starts to decrease
markedly (Supporting Information, Figure S3). Indeed, the
critical temperatures determined for PG1(ET) (TC from
EPR: 328C, from turbidity: 338C), PG2(ET) (348C, 368C),
PG3(ET) (348C, 348C), and PG2(ETalkyl) (308C, 318C) are
almost identical, regardless of whether they were obtained
from EPR or turbidity measurements. The slightly lower
EPR-derived values are due to higher concentrations of the
polymer solutions. The macroscopic phase separation identi-
fied by turbidimetry is due to the fact that water becomes a
thermodynamically poor solvent for the oxyethylene seg-
ments as the temperature increases. The gel phase formed in
equilibrium with the dilute phase is however still highly
swollen by water. The objective of our studies presented
timescale of at least two hours.[15] Based on previous
experience and tests with TEMPO and the more hydrophilic
TEMPOL (see Supporting Information), TEMPO was iden-
tified as the spin probe of choice. Thus, CW EPR spectros-
copy on TEMPO was applied to aqueous solutions of the
dendronized polymers, thus allowing insights to be gained
into the molecular processes associated with the thermal
transition.
Representative CW EPR spectra of TEMPO in an
aqueous solution of 10 wt% PG1(ET) (Scheme 1) well
above and below the critical temperature (TC) of 338C are
shown in Figure 1a. Whilst the low-field and center peaks
remain almost unaffected, the high-field line, which is most
sensitive to structural and dynamic effects, changes consid-
erably, and it is shown in Figure 1b at various temperatures.
The apparent splitting of this line at elevated temperatures
originates from two nitroxide species D1 and D2 that are
placed in local environments with different polarities. This
gives rise to considerable differences of the isotropic hyper-
fine coupling constants aiso (and the g values giso).
Before proceeding further, we checked whether the
critical temperatures from turbidity measurements and EPR
spectroscopy coincided. The polymers in this study are
polymethacrylate derivatives with first-, second-, and third-
generation triethyleneoxide dendrons (PG1—3(ET);
Scheme 1. a) Thermoresponsive dendronized polymers PG1(ET) (A), PG2(ET) (B, R=Et), PG3(ET) (B, R=(H(CH2CH2O)4)3Ph), PG2(ETalkyl) (C),
and the spin probe TEMPO (D). b) Synthetic route to the PG2(ETalkyl) monomer. Reagents and conditions: a) NaOH, TsCl, THF, H2O, 0–258C,
3 h (56%); b) DHP, PPTS, ꢀ5–258C, 4.5 h (86%); c) KI, [15]crown-5, NaH, THF, RT, 12 h (96%); d) PTSA, MeOH, RT, 2 h (90%); e) TsCl, TEA,
DMAP, DCM, ꢀ5–258C, 3 h (89%); f) methyl gallate, K2CO3, KI, DMF, 808C, 24 h (83%); g) LAH, THF, ꢀ58C–258C, 2.5 h (95%); h) MAC,
DMAP, TEA, DCM, ꢀ5–258C, 3 h (84%). DHP=3,4-dihydro-2H-pyran, PPTS=pyridinium toluenesulfonate; PTSA=p-toluenesulfonic acid;
LAH=lithium aluminum hydride; MAC=methacryloyl chloride; DMAP=N,N-dimethylaminopyridine; TEA=triethylamine.
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The exchange detected by the spin probes can be caused
by two effects: hopping of the spin probe between collapsed
and hydrated polymer aggregate regions, or fluctuations of
the aggregates themselves. The latter can be viewed as a fast
opening and closing of hydrophobic cavities or a fast swelling
and de-swelling of regions surrounding the spin probe. The
size of the inhomogeneities can be estimated by the transla-
tional displacement of TEMPO in the polymer matrix, given
by hx2i = 6DT tT. At T= 348C, a maximum translational
1/2
displacement hx2i ꢂ 5.1 nm of the spin probes due to
diffusion is obtained; this diffusion is assisted by fluctuations
of the polymer undergoing the thermal transition (for details,
see the Supporting Information).[17,18] Thus we can conclude
that slightly above TC, the few hydrophobic cavities formed
are still small, that is, in the range of a few nanometers. Spin
probe movement and/or local polymer fluctuations then lead
to an exchange of the probe molecules on the EPR timescale
between the hydrophobic and large hydrophilic regions. The
hydrophilic regions are still overwhelmingly more abundant
and the fraction of species D1 in these regions is larger than
60% (see Figure 2b). The spin probes thus mainly sample the
interface between the two fundamentally different regions.
Note that a few local dynamic heterogeneities on a nanometer
scale are sufficient to induce a macroscopically observable (by
turbidity measurements) transition in the sample. Remark-
ably, the transition is detected at the same temperature by two
methods probing length scales which differ by at least two
orders of magnitude. This suggests that the small hydrophobic
regions detected by EPR might be visualized as cross-links
affecting the organization of the dendronized macromole-
cules on much larger scales. The sharp macroscopic transition
can then be viewed as the onset of a complex de-swelling
process that is broad rather than a sharp transition on the
molecular scale. The existence of clusters in oligoethylene
oxides as a function of temperature and concentration has
long been observed, and has been attributed to the oxy-
ethylene segments becoming increasingly hydrophobic with
increasing temperature.[19]
An increase in temperature leads not only to an increase
in the fraction of the hydrophobic regions, but also their size
grows, and exchange of probe molecules between hydro-
phobic and hydrophilic sites becomes unlikely. The spin
probes now sample the bulk hydrophobic (and remaining
hydrophilic) regions rather than their interface. Together with
the increase in size, the dynamics of the polymer fluctuations
slow down, as both effects are coupled. In combination, a final
state of distinct hydrophobic and hydrophilic regions is
observed that are static on the EPR timescale.
To quantify aggregation and collapse associated with the
thermally induced transition, effective hyperfine coupling
constants of those TEMPO molecules in hydrophobic envi-
ronments aD2 and the fraction of TEMPO in hydrophilic
environments yD1 were determined as a function of temper-
ature. EPR lineshapes were fitted to these parameters (see
the Experimental Section in the Supporting Information).[20]
By plotting these parameters against the reduced temperature
(TꢀTC)/TC it is then possible to check whether the collapse
results from a well-behaved phase transition. As can be seen
in Figure 2, both parameters do not follow one straight line, as
Figure 1. a) CW EPR spectra (X-band, microwave frequency ca.
9.3 GHz) for 0.2 mm TEMPO in an aqueous solution of 10 wt%
PG1(ET) recorded at 158C and 658C. b) Detailed plot of the high-field
transition line (mI =ꢀ1, marked by a rectangle in (a)) at selected
temperatures. The contribution to the high field peak at 335.4 mT,
denoted D2, originates from TEMPO molecules in a hydrophobic
environment, whilst nitroxides in a hydrophilic surrounding give rise to
the contribution D1 at 335.65 mT. The broken lines at the outer
extrema of the peak serve as guides to the eye.
below was to analyze the properties of this gel phase as seen
by the spin probe.
The spectroscopic parameters for component D1 coincide
with those of TEMPO in pure water (aD1 ꢁ 48.3 MHz); that is,
this spin probe is located in a strongly hydrated, hydrophilic
environment. The observed decrease of aiso by 3.7 MHz for
species D2 at 658C is indicative of much more hydrophobic
and less hydrated surroundings for these spin probes (com-
parable to chloroform or tert-butanol).[16] At temperatures
below TC, only the hydrophilic component D1 is observed as
all dendritic units are water-swollen. Above the critical
temperature of 338C, an increasing fraction of hydrophobic
species D2 is observed with increasing temperature. The
dehydration of the dendritic units thus leads to a local phase
separation with the formation of hydrophobic cavities.
More strikingly, the peak position of the component D2 is
not fixed, but approaches its final value only at temperatures
well above TC. This indicates a dynamic exchange of the spin
probes between hydrophilic and hydrophobic regions. This
exchange leads to an intermediate hyperfine coupling con-
stant that is an effective weighted average between the two
extreme values of the hydrophilic and the (static) hydro-
phobic regions at 658C. Thus, the inhomogeneities formed
upon the phase separation are not static but dynamic, and
they strongly influence the EPR spectral shape.
Angew. Chem. Int. Ed. 2010, 49, 5683 –5687
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Communications
expected for a simple phase transition, but instead strongly
ature results in only smaller changes of aD2, which is a sign of
the expulsion of smaller amounts of residual water from the
collapsed polymeric regions.
deviate from linearity.[21] Figure 2a shows that static, non-
exchanging hyperfine coupling values aD2 are not reached
until around 308C above the critical temperature. Thus, in this
wide temperature range, a complex dehydration takes place
that cannot be described as a classical phase transition based
on a single de-swelling process. For all the polymers, at least
two dehydration processes are found, as indicated by the
different straight lines. The first process takes place at
temperatures slightly above the critical temperature
((TꢀTC)/TC < 0.02), the second process in a temperature
regime far above TC. Extrapolations of the two linear fits meet
at around (TꢀTC)/TC = 0.02 (ca. 78C above TC) for all the
polymers, thus indicating that in all cases the collapse
processes are equivalent. These results suggest that in this
narrow temperature range, the major part of the dehydration
takes place, as aD2 in this interval is reduced to values already
close to the static final values. A further increase in temper-
Straight-line best fits to two processes are obtained for
those polymers with no or a hydrophobic dendritic core
(PG1(ET) and PG2(ETalkyl)). For those materials possessing
a hydrophilic dendritic core (PG2(ET) and PG3(ET)), a
significant deviation from the simple two-process fit is
observed, indicating that the collapse is not fully described
by two well-defined processes. Moreover, the first process
turns out to be most effective when the dehydration is
supported by a hydrophobic core, as in the case of PG2-
(ETalkyl). It deteriorates when the core contains oxyethylene
groups, which can trap more water.
Qualitatively, the same behavior is seen in Figure 2b for
the temperature-dependent fraction of TEMPO in a hydro-
philic environment yD1. The same intersection point of the two
linear fits is found, and the dependence of the efficiency of the
first strong dehydration process on the chemical structure
described in Figure 2a is again manifest in Figure 2b. How-
ever, the graphs show one major difference: At high temper-
atures, yD1 is only determined by the volume fraction of the
collapsed polymer in water and thus approaches 0.3 for all
polymer solutions. In contrast, the (static) isotropic hyperfine
values of TEMPO in hydrophobic regions aD2 differ depend-
ing on the structure of the dendronized polymer. PG2-
(ETalkyl), which has a hydrophobic core, provides the most
hydrophobic environment, followed by PG1(ET) bearing no
core, and PG2(ET) with a hydrophilic ethyleneoxide core; the
least hydrophobic regions are provided by PG3(ET) bearing
an extended hydrophilic core. The differences can be
explained by the hydrophilic cores entrapping more residual
water molecules. This effectively increases the hydrophilicity
of the environment of the entrapped spin probe.
The data in Figure 2 support the picture of a few small
hydrophobic patches triggering a macroscopically observable
transition to a gel phase that is still highly swollen by water
and is composed of regions differing in local water concen-
tration. Furthermore, the process in the first temperature
interval is in agreement with a growing number of uncorre-
lated hydrophobic regions up to a concentration and/or a
volume fraction that is similar to that of the remaining
hydrophilic regions (thus the kink at yD1 ꢁ 0.5). This could be
an indication that the growth of hydrophobic regions reaches
a threshold that could be interpreted as a percolation point.
When the fractions of species D1 to D2 become equal, the
likelihood of two hydrophobic regions (which up to that point
can be largely uncorrelated) becoming neighbors increases
immensely and the role of the interface becomes less
important.[22]
In conclusion, the collapse transition of thermoresponsive
dendronized polymers with different cores was characterized
on a molecular scale by CW EPR spectroscopy. When the
temperature is raised above TC, the aggregation of the
complete polymer sample is triggered by dynamic structural
inhomogeneities of a few nanometers: The employed spin
probes in this temperature regime exchange between large
hydrophilic and small hydrophobic regions. Whilst macro-
scopic turbidity measurements suggest a sharp phase tran-
Figure 2. a) The hyperfine splitting constant of the hydrophobic spec-
tral component aD2 as a function of the reduced temperature
Tr =(TꢀTC)/TC for 0.2 mm TEMPO in 10 wt% aqueous solutions of
four dendronized polymers, which differ in the dendron generation
(PG1(ET), PG2(ET), and PG3(ET)) and the structural properties of the
dendritic core (PG2(ETalkyl)). b) Variation of the fraction of TEMPO in
a hydrophilic environment yD1 with increasing temperature for the
above-specified polymers. Two linear fits of data points close to and
far from the critical temperature illustrate at least two different de-
swelling processes, clearly indicating that the temperature-induced
collapse of the polymers under investigation cannot be described by a
thermodynamic phase transition. The reduced temperature at which
the two lines meet, which is common to all the polymers under
investigation, is indicated by a dashed line.
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transition. Rather, the dehydration should be viewed as a
molecularly controlled non-equilibrium state and takes place
in two steps. Within about 78C above TC, the majority of the
dehydration is completed and percolation for the fraction and
volume of hydrophobic regions is reached. Heating the
samples even higher only leads to an additional loss of
residual water from the collapsed system. While the aggre-
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dendrons, the dehydration efficiency is strongly related to the
hydrophobicity of the core.
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Keywords: EPR spectroscopy · nanostructures ·
non-equilibrium processes · phase transitions · polymers
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[22] Note that at the chosen polymer and spin probe concentrations,
about 30% of the spin probes reside in hydrophilic regions even
at high temperatures. Therefore, the proposed percolation point
at yD1 ꢁ 0.5 appears when (0.5/0.7 ꢁ 0.7) 70% of all probes taking
part in the transition are trapped in hydrophobic regions.
Angew. Chem. Int. Ed. 2010, 49, 5683 –5687
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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