Temperature-induced phase-transitions of methoxyoligo(oxyethylene) styrene-based block copolymers in aqueous solution

Article abstract of DOI:10.1039/c3sm51208h

Novel well-defined diblock copolymers based upon styrene substituted with pendant methoxyoligo(oxyethylene) groups (4-CH₃O(EO)₄St, TrEGS) were prepared by sequential atom transfer radical polymerization. Polystyrene (PS) was synthesi

Full text of DOI:10.1039/c3sm51208h

Soft Matter
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Temperature-induced phase-transitions of
methoxyoligo(oxyethylene) styrene-based block
copolymers in aqueous solution
Cite this: Soft Matter, 2013, 9, 8897
Fengjun Hua,a Weizhong Yuan,abc Phillip F. Britt,d Jimmy W. Maysde
a
*
and Kunlun Hong
Novel well-defined diblock copolymers based upon styrene substituted with pendant
methoxyoligo(oxyethylene) groups (4-CH3O(EO)4St, TrEGS) were prepared by sequential atom transfer
radical polymerization. Polystyrene (PS) was synthesized as the first block in the copolymer using (1-
bromoethyl)benzene as the initiator. This was followed by growth of a second block composed of
PTrEGS. Each copolymer exhibits a lower critical solution temperature (LCST) in aqueous solution, and
the LCST depends on the length of the PTrEGS block and the copolymer composition. Longer PTrEGS
chains result in higher LCST values in aqueous solutions. The cloud points (Tcs) of PS-b-PTrEGS
copolymers are lower than that of PTrEGS homopolymer as determined by UV-visible spectroscopy.
Variable-temperature NMR spectra show that the intensity of proton signals from oxyethylene groups
dramatically decreases at temperatures above the cloud point, indicating phase transition due to
hydrogen bonding changes. Dynamic light scattering (DLS) measurements indicate that micelles are
formed, where the PS backbone serves as the core at 25 ꢀC. Each of these block copolymer solutions
transformed from transparent bluish solutions to white gels above certain concentrations ($10 wt%)
and temperatures (lower critical gelation temperature, LCGT). The gelation process is thermally
reversible, implying physical crosslinking through hydrophobic interactions.
Received 30th April 2013
Accepted 8th August 2013
DOI: 10.1039/c3sm51208h
www.rsc.org/softmatter
studied of the thermal-sensitive polymers.15 Another example of a
polymeric system showing thermal-responsive behavior is the
Introduction
Designed materials with controllable structure and desirable
properties hold the key to success for future science and tech-
nologies. Mimicking biopolymers is an important approach
towards such materials. One of the ubiquitous characteristics of
biopolymers is their response to external stimuli, such as light,
temperature, or pH.1–8 Synthetic stimuli-responsive polymers
(SRPs), especially thermal-responsive polymers, have been
studied intensively over the past decade because of their poten-
tial applications in drug delivery, smart surfaces, microuidics,
catalysis, and environmentally responsive emulsions.9–14 Poly-
(N-isopropyl acrylamide) (PNIPAM), whose aqueous solutions
exhibit a lower critical solution temperature (LCST), is the most
aqueous solution of commercially available poly(oxyethylene)
and poly(oxypropylene) (POE–POP) block copolymers (Pluronics),
which form micelles in water at low concentrations and exhibit
reversible sol-to-gel transitions at higher concentrations.16,17
Armes and his group has developed many thermoresponsive
block copolymers.18–20 An A–B–C triblock copolymer composed of
PPO, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)
and PNIPAM, synthesized from a PPO-based macroinitiator using
ATRP, rst formed micelles with PPO in the core at a concen-
tration of 20 wt% when temperature was raised from 15 to 31 ꢀC
and then formed a gel above 32 ꢀC.18
Besides PNIPAM and Pluronics based polymers, other water-
soluble polymers which respond to thermal-stimuli by incorpo-
rating both hydrophilic moieties and hydrophobic backbones
have been reported.21–28 Aoshima and coworkers developed water-
soluble vinyl ether-based polymers by introducing short EO
segments into the poly(vinyl ether) main chain.21,22 These poly-
mers exhibited multistage phase transitions dependent on their
composition and molecular weight.21 Nonionic water-soluble
acrylate polymers and copolymers bearing oligo(ethylene oxide)
side chains were synthesized and their solution behavior was
studied by Lutz et al.23,24 The cloud point temperatures for these
polymers were tunable by changing the copolymer composition
aCenter for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,
TN 37831, USA. E-mail: hongkq@ornl.gov; Fax: +1-865-574-4974; Tel: +1-865-574-
1753
bInstitute of Nano and Bio-polymeric Materials, School of Materials Science and
Engineering, Tongji University, Shanghai 201804, China. E-mail: yuanwz@tongji.
edu.cn; Fax: +86 21 69584723; Tel: +86 21 69580234
cDepartment of Materials Sciences and Engineering, The University of Tennessee,
Knoxville, TN 37996, USA
dChemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831,
USA
eDepartment of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA
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or block length while keeping the concentration constant. Block columns in series. The data were processed using Empower
copolymers containing one hydrophilic block formed aggregates soware. THF was eluent at a ow rate of 1.0 mL minꢁ1. Poly-
in aqueous solution.23 The thermally induced aggregation styrene standards were used to calibrate the columns. The 1H and
behavior of these materials depended on the block sequence and 13C NMR spectra were recorded on a Bruker Advance400 NMR
kinetic effects as well.24 Polystyrene-block-poly(ethylene oxide) spectrometer. Variable temperature 1H NMR spectra were
(PS-b-PEO) copolymers have been studied intensely due to the collected in D2O (99.9 D atom%) with a concentration of 10 mg
intrinsic properties of the two blocks and their wide applica- mLꢁ1 aer the solution equilibrated for 20 min at each temper-
tions.29–32 Both di- and triblock copolymers of PEO and PS can ature. Cloud points of thermo-sensitive homopolymers and
self-assemble into micelles in water with PS core and PEO corona. diblock copolymers in aqueous solutions were measured by
PS graed with PEO copolymers has been synthesized success- placing the solutions in the water bath (Neslab RTE 7 Refriger-
fully through “graing onto” or “graing from” approaches to ated Circulator). Temperature was increased in steps of 1 ꢀC and
generate controlled degrees of branching and low compositional the polymer solutions were thermostated for 3 minutes at each
heterogeneity. The properties of these gra copolymers in temperature. The phase transitions can be visually observed. The
aqueous solutions have also been investigated.32 Research has optical absorbencies of aqueous polymer solutions at various
focused on micellization, gelation, and surface/interfacial temperatures were recorded at 490 nm with a UV spectrometer
behaviors of these polymers.
(Cary UV-Vis-Nir spectrophotometer 5000, Varian Inc.). The
Recently, PS copolymers densely-graed with oligo(oxy- sample quartz cell was thermostated in a jacked sample holder
ethylene) side groups (EO units #5) were reported.33–35 This with an external water bath of a refrigerated circulator (Ther-
group successfully synthesized water-soluble styrenic polymers moelectric Corporation, Neslab RTE7 circulator). The solution
with pendent methoxyoligo(oxyethylene) groups (EO number ¼ was equilibrated for 10 min at each temperature. Dynamic light
3, 4 or 5) using nitroxide mediated radical polymerization scattering (DLS) measurements were conducted with a Broo-
(NMRP), and they synthesized methoxyoligo(oxythylene)-graf- khaven Instruments (BI-200SM goniometer) equipped with a PCI
ted polymethacrylates by atom transfer radical polymerization BI-9000AT digital correlator, a temperature controller, and a
(ATRP).36 These polymers exhibited LCSTs in water and tended solid-state laser (model 25-LHP-928-249, l ¼ 633.2 nm) at a
to form aggregates due to structural factors, such as end groups scattering angle of 90ꢀ. The diblock copolymers were dissolved in
effects from initiator fragments, leading to a variety of phase deionized water in an ice/water bath and the clear solutions were
transition temperatures.
ltered into borosilicate glass tubes with an inner diameter of
Herein, in this work, we report the successful ATRP of styr- 4 mm by use of Millipore Teon lters (800 nm pore size). The
enic monomers carrying pendant methoxyoligo(oxyethylene) tubes were then sealed and were thermostated in a sample vat
side groups. In addition, using sequential ATRP techniques, we connected to a Fisher Scientic refrigerated circulator. The data
have synthesized several diblock copolymers of styrene with were recorded at each temperature that the solutions were
these monomers that have well-dened structures and narrow equilibrated at over 20 min. The second-order correlation func-
polydispersities. These copolymers were found to be thermally tion of the scattered light intensity, g (2)(t), was analyzed by the
responsive in aqueous solution, and some of the diblock CONTIN analysis method. g (2)(t) can be expressed by the
copolymers were observed to exhibit specic sol–gel transitions following equation:
in solutions with high concentration. Their solution properties
g
(2)(t) ¼ A(1 + b|g(1)(t)|2)
(LCST, micellization) in aqueous solutions were studied by
various techniques including NMR spectroscopy, dynamic light
scattering (DLS), and UV-Vis spectroscopy.
where A is the baseline, b is the coherence factor determined by
the geometry of the detection, g (1)(t) is the normalized electric
eld correlation function. Generally, g (1)(t) is expressed by the
distribution function G(G) of the decay time s as
Experimental section
Materials
Ð
|g(1)(t)| ¼ G(G)exp(ꢁGt)dG
Copper bromide (98%) was puried was puried by stirring in
glacial acetic acid and rinsing with ethanol and diethyl ether.
Dioxane (99%), styrene (99%), (1-bromoethyl)benzene (98%),
pentamethyldiethyl triamine (99%), anisole (99.7%) were
distilled over CaH2 (95%), 2,20-bipridyl (bpy, 99%) and sodium
hydride (NaH, 60 wt%), and tetraethylene glycol monomethyl
ether (TrEG) were obtained from Aldrich and used as received.
All other chemical reagents were used without further
purication.
For a translational diffusion mode, G can be used to deter-
mine the diffusion coefficient D through G ¼ Dq2, where q is the
scattering vector. The hydrodynamic radius Rh is related to the
diffusion coefficient D by the Stokes–Einstein equation:
D ¼ kBT/(6phRh)
where kB is the Boltzmann constant, T is the absolute temper-
ature, and h is the viscosity of the solvent. The polydispersity
index of the micellar aggregates was estimated from the m2/G2
Characterization
ꢀ
Gel permeation chromatography (GPC) was carried out at 35 C ratio, in which m2 was determined by analysis of the rst-order
using Waters GPC (2695) equipped with 4 standard Styragel GPC correlation function g (1)(t) by the method of the cumulants.
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(1)(t) ¼ exp[ꢁGt + (m2/2!)t2 ꢁ (m3/3!)t3 + .]
(3.26 g, 10.06 mmol), dioxane (3.26 g), and CuBr (14.7 mg, 0.103
mmol) were added into a 50 mL two-necked ask. The mixture
was then degassed by three freeze–pump–thaw cycles. PMDETA
(16.4 mg, 0.0948 mmol) was injected into the reaction ask
before warming up to room temperature, aer which the ask
was placed in an 85 ꢀC oil bath. The concentrations were: [PS]0 ¼
14.1 mM; [TrEGSt]0 ¼ 2.87 M; [CuBr]0 ¼ 29.0 mM; [PMDETA]0 ¼
g
Synthesis of 4-vinylbenzyl methoxytetrakis(oxyethylene) ether
(TrEGSt)
TrEGSt was synthesized according to the literature.37 Typically,
TrEG (41.23 g, 0.198 mol) was added to a 500 mL three-necked
ask containing dry tetrahydrofuran (THF) (110 mL) and the
mixture was cooled in a water-ice bath. NaH (12.65 g, 0.316 mol)
was added in 5 portions over one hour with vigorous stirring.
Aer warming to room temperature, a solution of 4-vinylbenzyl
chloride (33.24 g, 0.196 mol) in dry THF (30 mL) was added
drop-wise over one hour with stirring. The reaction mixture was
reuxed for 20 h and then poured into a 500 mL beaker. Aer
neutralizing with HCl solution (1 M), the mixture was extracted
with 300 mL of dichloromethane (DCM) and the water phase
was separated and extracted with DCM two times (50 mL each).
The combined organic phase was dried over anhydrous Na2SO4
overnight. A light yellow liquid was obtained aer removing the
solvent by rotary evaporation. Further purication by column
chromatography (1 : 2 ethyl acetate–hexanes) afforded 25.95 g
of nearly colorless liquid (yields: 67%). 1H NMR (CDCl3): d
(ppm) ¼ 7.38 (d, 2H, aromatic), 7.27 (d, 2H, aromatic), 6.65 (dd,
1H, CH2]CH–), 5.71 (dd, 1H, CHH]CH–), 5.20 (dd, 1H, CHH]
CH–), 4.57 (s, 2H, (CH)2C–CH2–), 3.69–3.40 (m, 16H, –OCH2-
CH2O–), 3.27 (s, 3H, OCH3) and 13C NMR (CDCl3): d (ppm) ¼
137.33 ((CH)2CCH2O), 136.54 (CH2CHC(CH)2), 136.58
(CH2CHC(CH)2), 127.98 ((CH)2CCH2O), 126.54 (CH2CHC(CH)2),
113.59 (CH2CHC(CH)2), 72.76 ((CH)2CCH2O), 71.98 (CH2OCH3),
70.63, 70.58, 70.48, 69.39 (CH2O), 58.91 (CH3OCH2); IR (cmꢁ1):
2877 (CH2 and CH3, ether), 1632 (CH]CH2), 1519 (aromatic
C]C), 1450 (CH2), 1100 (C–O–C); MS (ES) m/z 347.2 ([M + Na]+).
1
27.1 mM. Samples for H NMR spectroscopy and GPC analysis
were taken at regular intervals. Aer polymerizing for 45 h, the
mixture was cooled to room temperature and then diluted with
THF (40 mL), passed through a silica column and precipitated
into hexanes (200 mL). The polymer was re-dissolved in THF and
precipitated into hexanes to remove possible soluble low
molecular weight species. 1.44 g (38.0% yield) of almost colorless
liquid polymer was obtained (Mn,GPC ¼ 27 400, PDI ¼ 1.16) aer
drying. The nal monomer conversion of TrEGSt was 38.6%.
Results and discussion
Synthesis and characterization of diblock copolymers of
polystyrenics with methoxyoligo(oxyethylene) by sequential
ATRP
Styrene is one of the most studied monomers polymerized
through controlled radical polymerizations, including ATRP.38
For these experiments, PS with bromide at one chain end,
obtained from the PEBr/bpy/CuBr initiating system, was used as
macroinitiator in subsequent block copolymer formation by
addition of a second monomer. The degree of polymerization
(DP) was calculated from nal conversion (43.1%) and mono-
mer-to-initiator ratio (95). Aer chain extension with various
block lengths of 4-methoxyoligo (oxyethylene) styrene as indi-
cated in Scheme 1, different block copolymers were obtained
(Table 1, the polymerization kinetics and GPC traces shown in
Fig. 1). Fig. 1 shows a typical example of the relationship of
ln([M]0/[M]) versus reaction time and molecular weight versus
monomer conversion during copolymerization. The linearity
PS–Br macroinitiators by ATRP
A typical procedure is described below. Styrene (12.55 g, 120
mmol), bpy (0.53 g, 3.42 mmol), CuBr (0.17 g, 1.20 mmol),
(1-bromoethyl)benzene (0.22 g, 1.20 mmol) and anisole
(12.5 mL) were put into a three-neck ask. The mixture was
degassed using three freeze–thaw cycles. Aer warming to room
temperature, the ask was placed in a 105 ꢀC oil bath. Samples
were taken using an argon-exchanged syringe at regular inter-
indicated the block copolymerization proceeded via
a
controlled process. The 1H NMR spectra of PS41-b-PTrEGS77
diblock copolymer in DMSO-d6 and D2O are shown in Fig. 2.
The signals at 7.0 ppm and 4.5 ppm in DMSO-d6 are attributed
to the protons on phenyl groups in both blocks and of oxy-
methyl in the PTrEGS block, respectively. The length of the
1
vals for H NMR spectroscopy and gel permeation chromatog-
PTrEGS block was calculated using the equation: 102.5/(A7.3–6.3
/
raphy (GPC) analysis (for GPC measurements the ligand was
removed using a silica column). Aer polymerization proceeded
for 260 min, the ask was removed from the oil bath. The
mixture was cooled to room temperature, diluted with THF
(50 mL), passed through a silica column, and precipitated in
methanol (500 mL). 5.27 g of PS–Br macroinitiator was collected
aer drying in vacuum at room temperature overnight (42.0%
yield, Mn,GPC ¼ 4200, PDI ¼ 1.10). The nal monomer conver-
sion of styrene was 43.1%.
(A4.7–4.2)ꢁ2), where A is the integration of the respective peak
1
areas, and the results are listed in Table 1. H NMR spectra of
the polymer in D2O indicate that phenyl peaks disappeared due
to micellization. All of the block copolymers had relative low
polydispersities (PDI < 1.29).
Polymerization of 4-vinylbenzyl methoxytetrakis(oxyethylene)
ether (PTrEGS) using PS–Br macroinitiator
A typical procedure for the polymerization of TrEGS is described
below. Dry PS–Br (0.21 g, 0.049 mmol, Mn,GPC ¼ 4200), TrEGSt Scheme 1 Synthesis of PS-b-PTrEGS copolymers by sequential ATRP.
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Table 1 Characteristics of PS–Br macroinitiator and PS-b-PTrEGS copolymers
synthesized via ATRP
Tc (ꢀC) at
Tgel (ꢀC)
Polymersa
Mn
PDIb
0.5c wt%
at 20d wt%
b
PS41–Br
4200
10 400
21 900
27 400
32 600
6900
1.10
1.20
1.16
1.16
1.22
1.18
Not soluble
Not soluble
28
31
36
43
PS41-b-PTrEGS26
PS41-b-PTrEGS51
PS41-b-PTrEGS77
PS41-b-PTrEGS99
PTrEGS34
29
38
39
e
—
a
Subscript is degree of polymerization (DP). The DPs of macroinitiator
PS–Br were calculated from monomer conversions determined by 1H
NMR spectroscopy and the monomer-to-initiator ratios. The DPs of
PTrEGS block in the diblock copolymers were obtained from 1H NMR
spectra of copolymers in DMSO-d6 using the peaks located at 4.45 ppm,
from –CH2– of the benzyl group of PTrEGS, and 7.0 ppm, from phenyl
b
rings of PS and PTrEGS. Number-average molecular weights (Mn) and
polydispersity index (PDIs) were obtained from GPC analysis relative to
Fig. 2 1H NMR spectra of PS41-b-PTrEGS77 in DMSO-d6 and D2O, respectively.
The degree of polymerization (DP) of PTrEGS blocks was calculated from the
integral values ratio between peaks a (5H) plus a0 (4H) and b (2H). The DP of
PTrEGS block was 77 based on a calculated DP ¼ 41 for PS–Br macroinitiator.
c
polystyrene standards. The cloud point in aqueous solution was
d
determined using turbidity. The gelation point in concentrated
e
aqueous solution was determined using the inverse tube method. No
gel was formed.
micelles with PS in the core and hydrophilic oxyethylene
segments as corona (Fig. 2, PS41-b-PTrEGS77). The bluish and
clear aqueous solutions of PS41-b-PTrEGS77 (0.5 wt%) became
ꢀ
turbid upon heating above 31 C. These temperature-induced
phase transitions were sharp and were further conrmed by UV-
Vis spectroscopy as shown in Fig. 3. The absorbance (at 490 nm)
was almost constant as the temperature increased fromꢀ12 ꢀC to
ꢀ
near 30 C. It then increased dramatically around 31 C, indi-
cating a phase transition at the LCST. This process was thermo-
reversible (Fig. 3). The LCST for PS41-b-PTrEGS51 and PS41-b-
PTrEGS99 block copolymers were obtained in the same way. The
LCST increased with the PTrEGS block length as shown in Table 1
(also Fig. 4) but was lower than that for PTrEGS homopolymer
(0.5 wt%, 43 ꢀC). The concentration dependence of the cloud
points for these polymers is demonstrated in Fig. 5. Clearly,
introducing the PS block shied LCSTs to lower temperatures
due to the hydrophobicity of PS. Variable temperature NMR
spectroscopy was used to monitor phase transitions of 0.5 wt%
of PS41-b-PTrEGS77 in D2O solution (Fig. 6) and indicated that
Fig. 1 (a) Relationship between ln([M]0/[M]) and time for the polymerization of
styrene in anisole (67 w/w%, relative to St) at 105 ꢀC. [M]0 ¼ 6.23 M, [M]0/[I]0
¼
95.0, [CuBr] ¼ 0.063 M, [bpy] ¼ 0.172 M. (b) Number-average molecular weight
(Mn,GPC, -) and polydispersity index (PDI, ;) versus monomer conversion. The DP
of macroinitiator PS was obtained from the monomer-to-initiator ratio (95.0) and
final monomer conversion (43.1%) in the synthesis of PS macroinitiator and found
DP ¼ 41; (c) gel permeation chromatography analysis of macroinitiator poly-
styrene and four diblock copolymers (PS-b-PTrEGSs). The DPs of PTrEGS blocks
calculated from 1H NMR spectra (DMSO-d6 as solvent) are 26, 51, 77 and 99.
Micellization and temperature-induced phase transition of
diblock copolymer in aqueous solution
The block copolymers of PS41-b-PTrEGSn were not water soluble
at room temperature when n ¼ 26. However, when n $ 51
(PS41-b-PTrEGS51,77,
99), the polymers were water soluble
or
Fig.
PTrEGS77 in water during heating (-) and cooling (C) cycles (recorded at
wavelength of 490 nm); (b) optical pictures for 0.5 wt% aqueous solution of PS41
3
(a) Temperature-dependent absorbencies of a 0.5 wt% of PS41-b-
(room temperature, 0.5 wt%) and exhibited a bluish color
indicative of micellization. The 1H NMR signals from phenyl
-
groups were screened in D2O indicating the formation of b-PTrEGS77 at 25 ꢀC (bluish) and 37 ꢀC (white). The color changed reversibly.
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Fig. 4 Temperature-dependent optical absorbencies for PTrEGS34 homopolymer
and PS-b-PTrEGS diblock copolymers in 0.5 wt% aqueous solutions.
Fig.
6
1H NMR spectra of PS41-b-PTrEGS77 in D2O (1.0 wt%) at various
temperatures.
Fig. 7 (a) Temperature dependence of hydrodynamic diameters (Dh) for PS41-b-
PTrEGS77 in water (0.5 wt%) during heating or cooling; (b) Dh distributions at 25
ꢀC and 34 ꢀC.
Fig. 5 LCST curves of PTrEGS34, PS41-b-PTrEGS51, PS41-b-PTrEGS77, and PS41-b-
PTrEGS99 in water.
the peak at 3.3 ppm assigned to methoxyl groups in a PTrEGS
ꢀ
block was suppressed and broad upon heating above 35 ꢀC, scattering (DLS). Below 30 C, a unimodal Dh distribution was
which was close to its LCST (38 ꢀC). Throughout the investigated observed with mean size ꢂ68.4 nm (Fig. 7a), indicating micelle
ꢀ
temperature range (5 to 40 C), the signals from phenyl rings formation. The size of the micelles depended on the PTrEGS
and alkyl backbone were nearly invisible, conrming the block length (52.1 nm for PS41-b-PTrEGS51 and 108.9 nm for
formation of micelles with PS as core. The solubility of PTrEGS PS41-b-PTrEGS99, Fig. 8), since the hydrophilicity of the block
block in water decreased due to the dehydration of methoxy- copolymers increased as the oxyethylene content increased.17 Dh
oligo(oxyethylene) and associations among phenyl or ethylene increased signicantly and multiple peaks appeared at 34 ꢀC
moieties in PS and PTrEGS backbones at high temperature. which is above cloud point for this polymer (Fig. 7b). The
Upon heating above the cloud point, aggregation among increase in Dh started at 28 ꢀC, which is very close to the cloud
micelles was dramatically enhanced due to hydrophobic inter- poiꢀnt obtained from turbidity measurements (the LCST is
ꢀ
ꢀ
actions between PS backbones and/or PS block, causing the 31 C). The change in Dh distribution at 25 C and 34 C indi-
phase transition.
cates that large aggregates appeared at the higher temperature
Fig. 7 shows the hydrodynamic diameters (Dh) and their with a corresponding decrease in the percentage of smaller
distributions at various temperatures for PS41-b-PTrEGS77 in sized micelles. It is possible that these aggregates are formed
aqueous solution (0.5 wt%) as obtained using dynamic light from groups of micelles and are not very stable seeing that the
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Fig. 8 Hydrodynamic diameter for different block copolymers (concentration
Fig. 9 Gelation temperature curve for concentrated aqueous solutions of the
block copolymer PS41-b-PTrEGS77 (insert optical pictures for 20 wt% aqueous
solution of PS41-b-PTrEGS77 heated from 25 ꢀC (point a) to 40 ꢀC (point b), and to
65 ꢀC (point c)).
0.5 wt% in water).
sizes changed during different heating/cooling cycles. For
PS41-b-PTrEGS99, the aggregates are formed with a broad size
distribution (m2/G2 ¼ 0.154) because groups of micelles come
together when the phenyl moieties of the PTrEGS blocks in a
micelle enter the PS core of another micelle, driven by strong
hydrophobic interactions (Scheme 2).
Sol–gel transition of diblock copolymer in concentrated
solution
Three block copolymers prepared from PS macroinitiator formed
gels upon heating over a certain temperature (LCGT) in concen-
trated aqueous solutions, as shown in Table 1. The gelation
temperatures decreased with increasing concentration (Fig. 9
and 10), like Pluronic and PEG-PLGA-PEG.39 As shown in Fig. 9,
the bluish and ow-able sol (20 wt%) became a white gel when
ꢀ
ꢀ
heated from 25 C (point a) to 40 C (point b). Upon heating to
65 ꢀC (above upper gelation temperature), the gel precipitated
(point c). As discussed above, the polymers assembled into
micelles with PS as core and formed larger aggregates with
increasing concentration, through partial inter-connection Fig. 10 Gelation temperature curves for concentrated aqueous solutions of
PS41-b-PTrEGS51 and PS41-b-PTrEGS99
.
between micelles (bridging effect). This effect was enhanced by
increasing either concentration or temperature. Consequently,
the solvent (water molecules) could be trapped into these phys-
ical networks and gelation could take place above the lower
critical gelation temperature (Scheme 2). Each solution started to
turn into gel at a certain temperature below which polymer
solution stays only in sol form. Lower concentrations and longer
PTrEGS chain lengths lead to form gels at higher temperatures.
The critical concentration at which PS41-b-PTrEGS51 formed a gel
was found to be around 6.3 wt%, while the gelation concentra-
tion for PS41-b-PTrEGS77 and PS41-b-PTrEGS99 were 13.5 and
18.5 wt%, respectively. This trend is probably due to the
increased hydrophilicity, as reported in literature.40
Scheme 2 Temperature-induced micellization of diblock copolymer in water
and aggregation mechanism.
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16 R. Yoshida, K. Uchida and Y. Kaneko, Nature, 1995, 374, 240–
241.
Conclusions
Various well-dened diblock copolymers composed of poly-
styrenes with short pendant methoxyoligo(oxyethylene) groups
(4-CH3O(EO)4) (PS-b-PTrEGSn) were successfully synthesized by
sequential ATRP. Their solution properties were studied by
UV-Vis spectroscopy, dynamic light scattering, NMR spectros-
copy, and turbidity measurements. The block copolymers were
water soluble to a degree corresponding to their compositions.
These copolymers exhibited different lower critical solution
temperatures (LCST) depending on the PTrEGS block length.
Below the LCST, these copolymers formed micelles with PS as
core and PTrEGS as corona. As the concentration increased, large
aggregates were formed by a bridging interaction among micelles
and eventually underwent thermoreversible sol–gel transitions.
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Acknowledgements
This research was conducted at the Center for Nanophase
Materials Sciences, which is sponsored at ORNL by the Division
of Scientic User Facilities, U.S. Department of Energy. We also
acknowledge the partial funding of this work through the
support of Department of Energy NA-22 program.
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