Two-dimensional correlation ATR-FTIR studies on PEO–PPO–PEO tri-block copolymer and its phosphorylcholine derivate as thermal sensitive hydrogel systems

Abstract

In this work, both 1D and 2D ATR-FTIR analyses were applied to study the behaviors of the PEO–PPO–PEO tri-block copolymer (Pluronic-F88) aqueous solution during the thermal gelling process. Furthermore, a novelly synthesized phosphorylcholine-modified F88 was also studied by the same technique, for the explanation to its different performance in the rheological study, as compared to F88. It was mainly focused on the changes in the water structure between sol and gel states, and the interactions between chemical bonds in the polymer chains with water molecules at a molecular level.

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Two-dimensional correlation ATR-FTIR studies on PEO–PPO–PEO tri-block
copolymer and its phosphorylcholine derivate as thermal sensitive
hydrogel systems
Sheng Meng, BingJie Sun, Zhang Guo, Wei Zhong*, QiangGuo Du, PeiYi Wu*
The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, 220 Handan Road,
Shanghai 200433, China
a r t i c l e i n f o
Article history:
Received 17 December 2007
Received in revised form 26 March 2008
Accepted 7 April 2008
Available online 10 April 2008
Keywords:
2D-IR
Hydrogel
Thermal sensitive
a b s t r a c t
In this work, both 1D and 2D ATR-FTIR analyses were applied to study the behaviors of the PEO–PPO–PEO
tri-block copolymer (Pluronic-F88) aqueous solution during the thermal gelling process. Furthermore,
a novelly synthesized phosphorylcholine-modified F88 was also studied by the same technique, for the
explanation to its different performance in the rheological study, as compared to F88. It was mainly
focused on the changes in the water structure between sol and gel states, and the interactions between
chemical bonds in the polymer chains with water molecules at a molecular level.
? 2008 Elsevier Ltd. All rights reserved.
1. Introduction
‘‘Smart’’ hydrogels which are sensitive to the temperature, ionic
strength or pH value [1–7] have been extensively studied for their
potential applications in site-specific controlled drug delivery
and bio-separation [8–10]. Among these, the commercialized PEO–
PPO–PEO tri-block copolymer named Pluronic or Poloxamer was
already applied in some cases [11,12] for its well studied thermal
reversible gelation properties.
The PEO–PPO–PEO block copolymers can be dissolved in water
in the form of unimer at low temperature. With the increase of
concentration or temperature, critical micellization concentration
or critical micellization temperature of its aqueous solution can be
observed,whiletheaggregationofPEO–PPO–PEOblockcopolymers
occurs with the micellization followed by a close packing process
[13–18]. Though the general micellization and gelation behavior of
PEO–PPO–PEOblockcopolymers have beenextensivelystudied, the
‘‘molecular level’’ mechanism of micellization and gelation remains
a controversial issue [1].
Mechanism studies on the characterization and sol–gel phase
transitions of Pluronics using various instrumental techniques
[13,18,19]. Among those, Liu et al. have achieved great success in
studying the effect of temperature, concentration, ionic strength,
etc. on the phase transition of Pluronic hydrogels using ATR-FTIR
[20–25]. Also FTIR was employed in studying other thermal sensi-
tive hydrogel systems [26].
Recently, two-dimensional (2D) correlated ATR-FTIR technique,
which had been reported to be useful in investigating the dynamic
variations, was employed in addition to the normal 1D ATR-FTIR
studies. The generalized 2D correlation spectroscopy proposed
by Noda [27–29], which is an extension of the original 2D corre-
lation spectroscopy, has recently been proved as a powerful tool in
studying the molecule–molecule interactions in some particular
systems [30–32]. In this kind of 2D analysis, two kinds of correla-
tion maps, synchronous and asynchronous, are generated based
upon a set of dynamic spectra calculated from the fluctuations of
the spectroscopic signals during the process under examination.
The generalized 2D method can handle signal fluctuating as an
arbitrary function of time or any other physical variables such as
temperature, pressure, or concentration [29,33–36].
In the present study, Pluronic F88 modified with the phos-
phorylcholine (PC) moieties was synthesized, which had the ‘‘bio-
inspired’’ structure of the membranes of the erythrocyte cells. Such
modification was reported to be able to considerably improve the
anti-thrombus properties and anti-protein adhesion of biomedical
polymers [37,38] as well as introduce some special interactions
between the polymer chains [39,40]. The thermally induced gela-
tion process of the aqueous solutions of F88 and phosphorylcholine
end-capped F88 (PC88), including the water structure changes
between sol and gel states, and the interactions between chemical
bonds in the polymer chains with water molecules were analyzed
* Corresponding authors. Tel.: þ86 21 65642392; fax: þ86 21 65640293.
E-mail addresses: weizhong@fudan.edu.cn (W. Zhong), peiyiwu@fudan.edu.cn
(PeiYi Wu).
Contents lists available at ScienceDirectContents lists available at ScienceDirect
PolymerPolymer
journal homepage: www.elsevier.com/locate/polymerjournal homepage: www.elsevier.com/locate/polymer
Polymer 49 (2008) 2738–2744
0032-3861/$ – see front matter ? 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2008.04.007

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by 2D ATR-FTIR technique at a molecular level. A preliminary
rheological study was also performed in the present work for the
further investigation of the difference between PC88 and F88 in
their gelation behaviors.
2. Experiments
2.1. Materials
The PEO–PPO–PEO tri-block copolymers were purchased from
BASF. In the present study, Pluronic?F88 with the structure of
EO97PO39EO97 was used. 2-Chloro-2-oxo-1,3,2-dioxaphospholane
(COP) was synthesized in our lab as previously reported [38], which
was characterized using the GC–MS technique (Voyager), with the
purity of 97%. All the other raw materials and solvents were dried
and purified according to standard methods.
2.2. Synthesis of phosphorylcholine-modified F88
The phosphorylcholine end-capped F88 denominated as PC88
was prepared by the similar method we have used for the prepa-
ration of phosphorylcholine-modified poly-3-caprolactone and
chitosan in our previous reports [38]. The synthesis process was
described briefly as follows.
Pluronic?of 0.01 mol either F88 or together with 0.02 mol
triethylamine (TEA) was dissolved in 100 mL dried THF in a flask
purged with nitrogen. The solution of 0.02 mol COP and 50 mL
THF was added on gentle agitation within 3 h through a pre-dried
dropping funnel. All operations were performed at 0?C. Then the
system was slowly warmed to room temperature and stirred for
another 2 h before removing the solvent from the intermediate
COP–Pluronic on a rotary evaporator under reduced pressure. Then
COP–Pluronic was moved to another round-bottom flask and 40 mL
dry acetonitrile saturated with trimethylamine (TMA) was added.
The solution was heated at 65?C and then maintained on gentle
agitation for 48 h. Residual TMA was removed on careful heating
and then precipitated in cold petroleum ether. The precipitate of
phosphorylcholine end-capped Pluronic was collected and after
subsequent dissolution/precipitation cycle with THF/petroleum
ether, it was dried in vacuo to constant weight.
F88 and PC88 were resolved in distilled water at a concentration
of 20 wt%, respectively, and restored at 4?C for 3 days before the
measurement to have a more sufficient dissolution.
2.3.
1H NMR study
1H NMR spectra of F108, F127, PC108 and PC127 were recorded
in CDCl3 with a Bruker model AVANCE DMX-500 spectrometer
using tetramethylsilane (TMS) as an internal standard.
2.4. ATR-FTIR characterization
The 20% aqueous solutions of F88 and PC88, respectively, were
cast into an ATR cell with a ZnSe reflection element, which was
attached to a Nicolet Nexus Smart ARK FTIR spectrometer equipped
with a DTGS detector. About 4 mL of the polymer solutions was
used to fill the ATR cell and covered with a plastic plate during
the measurement for the limitation of the water evaporation.
The temperature-resolved ATR-FTIR test began at 25?C, when the
polymer solutions appeared in a sol state. The spectrum of the
solution at this temperaturewas collected at a spectral resolution of
4 cm?1by accumulating 128 scans. Then the temperature of the
ATR cell was raised by 1?C and equalized for 10 min before another
spectrum was collected with the same parameters. This process
was repeated until the cell temperature reached 45?C when the
polymer solutions had already undergone a complete sol–gel
transition to the state of hydrogels. Thus, there were totally 21
spectra collected for F88 and PC88. The measurements were carried
out within the wavenumber range of 4000–650 cm?1. All of the
original spectra were smoothed and baseline corrected using
Omnic 6.0 software.
2.5. 2D-IR calculation
For the generalized 2D correlation analysis, in both the systemof
F88 and PC88, all the 21 spectra collected at every temperature
point were selected and subjected to the 2D software named 2D
Pocha (developed by Daisuke Adachi, Kwansei Gakuin University,
Nishinomiya, Japan). Three domains in the spectra located at 1800–
1500, 1400–1300 and 1200–1000 cm?1, respectively, were focused
for the information of the variations occurred in the systems
throughout the thermal sensitive gelation process at a molecular
level.
2.6. Rheological study
Storage (G0) and loss (G00) moduli of F88 and PC88 hydrogels
were recorded by the Advanced Rheometric Expansion System
(ARES, Rheometric Scientific), equipped with a Couette geometry
accessory. All the samples were dissolved in distilled water to make
20 wt%, solutions of which were stored at 4?C for 3 days before the
measurement. During a dynamic temperature scanning rheological
study, the sample solution was poured into the Couette geometry
accessory and was carefully overlaid with a lower density 50 CP
silicone oil to minimize drying and evaporation. The measurements
were conducted on shear modulus as a function of temperature. A
frequency of 10 rad/s was employed. The heating rate was 0.5?C/
min from 10 to 60?C. The strain amplitude for all the measure-
ments was set at 10% in the linear range where moduli were almost
independent of strain amplitude.
3. Results and discussion
3.1. Structural characterization of F88 and PC88
Changes in the terminal groups of the PEO–PPO–PEO tri-block
copolymers could be observed in1H NMR spectra. The terminating
hydroxyl group in original F88 tri-block copolymer (Scheme 1)
provided chemical shift at 2.51 ppm. In the spectra of PC 88 (Fig.1),
the peak of the hydroxyl group almost totally disappeared, while at
the same time three new peaks appeared. They were located at 2.57
(peak a), 3.09 (peak b) and 4.25 ppm (peak c), respectively (consult
Scheme 1 and Fig. 1). These groups of peaks corresponded to the
protons of the PC groups (Scheme 1).
Scheme 1. Chemical structures of F88 and PC88.
S. Meng et al. / Polymer 49 (2008) 2738–27442739

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3.2. Investigation of waters in the hydrogel systems during
transition
The IR spectra, especially the 2D spectra, have already been
proved to be powerful tools in the investigation of the water
structures inside polymeric materials [41]. In the present study,
changes in the spectra of F88 and PC88 in the range of 1800–
1600 cm?1were investigated to study the water structure changes
in these two hydrogel systems during their sol–gel transitions.
As shown in Fig. 2a and b, the FTIR spectra of aqueous solutions
F88 and PC88 showed no significant differences through their
gelation process with the change of the temperature. There was
an increase in 1637 cm?1with the temperature rising, while this
peak was narrowed with the shoulder located at around 1690 cm?1
disappeared. The former peak was attributed as the d(OH) band of
free water in the system, while the latter was assigned to the d(OH)
band of hydrogen bonded water (bound water). Compared with the
peak position of the freewater, the bending vibration band of water
would shift to the higher wavenumber after the formation of the
hydrogen bonds [42]. These results indicated that in the system of
both F88 and PC88, a transition from bound water to free water
happens during the thermal induced gelation process [21,22].
It could be observed that the only difference between the two
groups of the spectra in Fig. 2 was that the spectra of F88 had a
milder temperature than that of PC88. This was supposed to be the
result of the not so completed sol–gel transition of the PC88
hydrogel compared to the F88 induced by the ultra-hydrophilic
phosphorylcholine end groups.
In the 2D correlated ATR-FTIR spectra, the PC88 system also
performed a very similar result with the F88 system in the spectral
region of 1800–1500 cm?1. The 2D-IR spectra of F88 within 1800–
1500 cm?1are illustrated in Fig. 3, where panel a shows the
synchronous correlation spectrum and panel b shows the asyn-
chronous correlation spectrum. In the synchronous contour map,
one autopeak locatedat 1637 cm?1could be obviouslyobserved; as
described above, this autopeak is assigned tothed(OH) band of free
water, the high intensity of this peak shows the change of the
relatively high content of the free water during heating. One neg-
ative cross-peak at 1637/1690 cm?1could also be found, and the
band at 1690 cm?1is attributed to the d(OH) band of bound water.
This negative cross-peak indicated that the two peaks at 1690 and
1637 cm?1varied contrarily, which could help to conclude that
there existed the transition between bound water and free water
during the whole gelation process, in accordance with the 1D-IR
results mentioned above.
More information could be derived from the corresponding
asynchronous correlation spectrum in Fig. 3b. The asynchronous
correlation spectrum is anti-symmetric with respect to the diagonal
line.Inthe2Dspectrum,anasynchronouscross-peakappearsonlyif
the intensities of two spectral features change out of phase (i.e.,
delayed or accelerated) with each other. Thus, the absence of an
asynchronous cross-peak denotes that the two spectral features
changesynchronously[27,28].Twonegativecross-peaks(1641/1637,
1690/1637 cm?1) and one positive cross-peak (1637/1585 cm?1)
could be observed in Fig. 3b, indicating that the broad d(OH) water
band in the region of 1800–1500 cm?1was split to three separate
bands locating at 1690,1641,1637 cm?1, respectively.
The sign of the cross-peaks in an asynchronous spectrum would
provide additional information about the order of the intensity
changes in different bands. According to the rule of Noda [27,28],
and the positive sign of the cross-peak at 1641/1637 cm?1in
the slice spectrum, the band at 1690 cm?1varied prior to the
two bands at 1637 and 1641 cm?1, respectively, while the band
at 1641 cm?1
changed even after the band at 1637 cm?1
(1690 cm?1>1637 cm?1>1641 cm?1). Thus, the signs of the 2D-IR
asynchronous spectrum suggested that there occurred a transition
from bound water centered at 1690 cm?1to free water at around
1640 cm?1(1637 and 1641 cm?1). Furthermore, the bound water
varied prior to the free water. It was previously reported that the
Pluronic molecules, especially the PPO segments in this kind of
thermal sensitive hydrogel undergo a dehydration process when
heated to a certain critical temperature to form hydrophobic cored
micelles, followed by the sol–gel transition due to the close packing
of these spherical micelles [21,22,24,25]. This dehydration process
would also, as a result, free most part of the bound water which are
Fig. 1.1H NMR spectra of F88 and PC88 in CDCl3.
Fig. 2. 1D ATR-FTIR spectra of F88 (a) and PC88 (b) in the region 1800–1500 cm?1corresponding to temperature variation from 25 to 45?C.
S. Meng et al. / Polymer 49 (2008) 2738–27442740

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hydrogen bonded to the polymer chains. In the present study, the
change from bound water to free water was investigated, providing
the evidence of the dehydration process. Furthermore the variation
sequence of the water structure in the system was also confirmed
by 2D-IR study.
3.3. Study on the methyl and methylene groups in the gelation
process
For a further study of the dehydration process, the IR spectra
in the region of 1400–1300 cm?1of both F88 and PC88 were in-
vestigated in corresponding to the temperature variation (Fig. 4).
This region was reported to offer the information about the sym-
metric deformation vibration of methyl groups and CH2groups
[43,44]. It could be observed that variations of three bands were
involved in this region, locating at 1381, 1373 and 1350 cm?1, re-
spectively. In both the groups of IR spectra of F88 and PC88, band
at 1381 cm?1performed a red shift to 1373 cm?1, while the band at
1350 cm?1kept decreasing during the heating process. The first
two bands were assigned to the symmetric deformation vibration
of methyl groups in the PPO segments as reported [21,22] in which
the band at 1381 cm?1was assigned to the hydrated state of the
methyl groups surrounded bywaterand the band at 1373 cm?1was
assigned to the dehydrated state. The assignment of the band at
1349 cm?1had also been reported as the amorphous phase of PEO
segments [21,22]. Thus, the following conclusion can be drawn
from the temperature-resolved 1D ATR-FTIR spectra of the 20 wt%
aqueous solutions of F88 and PC88 in the domain 1400–1300 cm?1
as shown in Fig. 4a and b: the methyl groups in the PPO segments
undergo a dehydration process with the temperature increasing,
corresponding tothe band at 1381 cm?1shifted to 1373 cm?1when
heated; and there was a transition of the PEO segments from
amorphous phase to the crystalline phase during the gelation
process.
With the powerful tools of the 2D correlated spectra, the de-
hydrationbehaviorscouldbefurtheranalyzed.AlsothePC88system
performed very similar to F88 in 1400–1300 cm?1in the 2D-IR
analysis. The 2D-IR spectra of F88 within 1400–1300 cm?1are
shown inFig. 5,inwhichpanel ashowsthesynchronouscorrelation
spectrum and panel b shows the asynchronous correlation spec-
trum.Inthesynchronousspectrum,threeautopeakscouldbefound
at 1383, 1370 and 1350 cm?1, respectively, indicating that these
three bands varied during the thermal induced gelation process of
the F88 solution. The assignment of the bands had been described
in the temperature dependent 1D-IR spectra already [24,25].
Two negative cross-peaks at 1383/1370 and 1371/1350 cm?1, re-
spectively,andonepositivecross-peakat1383/1350 cm?1werealso
observed in the synchronous map. These cross-peaks offered the
information about the change directions of the bands involved
duringheatingthatthebandsat1383and1350 cm?1changedinthe
same direction, while the band at 1370 cm?1changed contrarily.
Further information was provided with the cross-peaks in the
asynchronous 2D-IR contour map. Two main cross-peaks at 1383/
1350 cm?1and at 1370/1350 cm?1, positive and negative, re-
spectively, could be found in the asynchronous spectrum, with
another positive ‘‘shoulder’’ at 1383/1370 cm?1. The variation
Fig. 3. 2D correlated ATR-FTIR spectra of F88 in the 1800–1500 cm?1wavenumber region: (A) synchronous; (B) asynchronous contour maps. (In 2D-IR correlation spectra, the
unshaded and shaded areas in the contour maps represent positive and negative peaks, respectively, and the averaged 1D reference spectrum is represented at the left side and at
the top of the map.)
Fig. 4. 1D ATR-FTIR spectra of F88 (a) and PC88 (b) in the region 1400–1300 cm?1corresponding to temperature variation from 25 to 45?C.
S. Meng et al. / Polymer 49 (2008) 2738–2744 2741

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sequence of the bands involved could be thus confirmed in the light
of Noda’s rules [27,28]. The band at 1350 cm?1varied after both the
bands at1370and1383 cm?1,while theband at 1370 cm?1changed
prior to the band at 1383 cm?1, i.e.,1370/1383/1350 cm?1. The
dehydration process could be proved here again for the sequenced
change betweenthebands represented thehydratedmethylgroups
and the dehydrated methyl groups. Furthermore, the change of the
band at 1350 cm?1assigned to the CH2wagging vibration of the
amorphous state, occurred after the changes of the bands related to
the dehydration process, indicating that the transition of the PEO
segments to the crystalline phase was partly induced by the de-
hydration process of the PPO segment during the thermal sensitive
sol–gel transition of the Pluronic aqueous solution.
3.4. Study on the ether groups during the sol–gel transition
Spectral changes occurred in the domain of 1200–1000 cm?1
were investigated to study the variations of the C–O–C bonds
during gelation, because there was a strong band at around
1080 cm?1assigned to the conjugation of the C–O–C stretching
vibration of both PPO and PEO blocks [43,44]. It was reported that
there was also a dehydration process of the ether backbone with
the temperature increasing, causing a band shift to the higher
wavenumber and band width broadening because of the disorder
of the conformation of the C–O–C skeletons with the different
dehydration degrees of PPO and PEO segments [21]. In the present
study, in the temperature-resolved 1D ATR-FTIR spectral series, F88
performed such a behavior as reported: the band at 1085 cm?1
decreased when heated while the band at around 1120 cm?1kept
increasing; the band width broadening of 1085 cm?1was also
observed. The band at around 1120 cm?1was thus assigned to
the dehydrated C–O–C bonds as the C–O–C bands were at higher
wavenumbers when Pluronic was in solid state [21]. However, PC88
didn’t show such a clear dehydration process as F88 in its FTIR
spectra, probably because of the hindrance from its hydrophilic end
groups, though there was still a decrease of the band at 1085 cm?1
with the temperature increasing (Fig. 6).
In the 2D-IR analysis of the temperature-resolved spectra in the
region 1200–1000 cm?1, F88 and PC88 appeared to be different
in the synchronous and asynchronous contour maps. In the case
of F88 as shown in Fig. 7A and B, one broad auto peak at about
1120 cm?1could be observed in the synchronous spectrum, with
a negative cross-peak located at 1120/1085 cm?1in the asynchro-
nous contour map. Therefore, it could be concluded that bands at
1120 and 1085 cm?1, which had already been assigned to the band
of dehydrated ether bonds and hydrated ether bonds, respectively
[24,25], were involved and correlated in the whole process. The
shape of the negative cross-peak in the asynchronous contour
maps indicated that this cross-peak was formed in the correlation
of the variation of a broad band at around 1120 cm?1with the
variation of the band at 1085 cm?1. As described in the 1D FTIR
spectral study, the dehydrated C–O–C bands would split because of
the different dehydration degrees between PPO and PEO segments.
Furthermore, the 2D-IR analysis could also tell us that the band at
1085 cm?1varied earlier than the band at around 1120 cm?1.
In the 2D correlated ATR-FTIR contour maps of PC88 of 1200–
1000 cm?1wavenumber region as shown in Fig. 8, which were
much different from those of F88, two auto peaks at 1120 and
1085 cm?1could be found in the synchronous spectrum, with a
negative cross-peak at 1120/1085 cm?1; and in the asynchronous
Fig. 5. 2D correlated ATR-FTIR spectra of F88 in the 1400–1300 cm?1region: (A) synchronous; (B) asynchronous contour maps.
Fig. 6. 1D ATR-FTIR spectra of F88 (a) and PC88 (b) in the region 1200–1000 cm?1corresponding to temperature variation from 25 to 45?C.
S. Meng et al. / Polymer 49 (2008) 2738–27442742