Star-like poly (n-butyl acrylate)-b-poly (α-methylene-γ-butyrolactone) block copolymers for high temperature thermoplastic elastomers applications

Article abstract of DOI:10.1016/j.polymer.2010.08.017

Thermoplastic elastomers based on well-defined 10- and 20 arm star-like block copolymers containing middle soft poly(n-butyl acrylate) (PBA) block and outer hard poly(α-methylene-γ-butyrolactone) (PMBL) block were synthesized by atom transfer radical polymerization (ATRP). Phase separated cylindrical or lamellar morphologies, depending on the copolymers composition and the annealing temperature of the films, were observed by atomic force microcopy and small-angle X-ray scattering. The mechanical and thermal properties of the copolymers were thoroughly characterized. The prepared copolymers retained their phase separated morphology even at temperatures exceeding 300 °C. Both tensile strength and elongation values for the star-like copolymers were considerably higher than for linear copolymers with similar composition.

Full text of DOI:10.1016/j.polymer.2010.08.017

Polymer 51 (2010) 4806e4813
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Star-like poly(n-butyl acrylate)-b-poly(
a-methylene-g-butyrolactone) block
copolymers for high temperature thermoplastic elastomers applications
a
b
,
c
b
b
a
Azhar Juhari , Jaroslav Mosná ꢀc ek , Jeong Ae Yoon , Alper Nese , Kaloian Koynov ,
b
b,
*
Tomasz Kowalewski , Krzysztof Matyjaszewski
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA, 15213, USA
Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 36 Bratislava, Slovakia
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2010
Received in revised form
Thermoplastic elastomers based on well-defined 10- and 20 arm star-like block copolymers containing
middle soft poly(n-butyl acrylate) (PBA) block and outer hard poly(
a-methylene-g-butyrolactone)
(
PMBL) block were synthesized by atom transfer radical polymerization (ATRP). Phase separated cylin-
5
August 2010
drical or lamellar morphologies, depending on the copolymers composition and the annealing
temperature of the films, were observed by atomic force microcopy and small-angle X-ray scattering. The
mechanical and thermal properties of the copolymers were thoroughly characterized. The prepared
Accepted 8 August 2010
Available online 14 August 2010
ꢀ
copolymers retained their phase separated morphology even at temperatures exceeding 300 C. Both
Keywords:
ATRP
Block copolymers
Thermoplastic elastomers
tensile strength and elongation values for the star-like copolymers were considerably higher than for
linear copolymers with similar composition.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
not sensitive to UV and oxidation. Moreover the combination of
various (meth) acrylic monomers offer materials with a wide
ꢀ
Thermoplastic elastomers (TPEs) are materials combining
elastomeric behavior under the service conditions with thermo-
plastic properties above the orderedisorder transition (ODT)
temperatures [1e3]. The most common conventional TPEs are
polystyrene-b-polybutadiene-b-polystyrene (SBS) or polystyrene-
b-polyisoprene-b-polystyrene (SIS) block copolymers. However,
these TPEs have some drawbacks such as poor chemical, heat, and
UV resistance due to the unsaturated soft blocks as well as low
range of T
g
from ca. - 50 to þ 200 C.
Recently, atom transfer radical polymerization (ATRP) [16e20]
was used for the preparation of well-defined block copolymers
a-methylene-g-butyrolactone (MBL) [15,21]. MBL, found in
tulips, is the simplest member of butyrolactones isolated from
various plants [22e24]. PMBL possesses interesting properties
such as good durability, a high refractive index (1.540) and high T
(195 C) [25,26]. Various copolymers and blends containing MBL
of
g
ꢀ
service temperature limited by glass transition temperature (T
g
) of
units have good optical properties and resistances to heat,
weathering, scratches, and solvents [27]. These properties can be
beneficial for the use of PMBL as a hard component in thermo-
plastic properties. Preparation and study of properties of linear
triblock PMBL copolymers with soft poly (n-butyl acrylate) (PBA)
control segments were recently published [15]. These copolymers
ꢀ
the polystyrene block (ca. 100 C) [4]. Therefore there have been
continuous efforts to overcome these drawbacks by substitution of
polybutadiene or polyisoprene with more stable soft blocks and
polystyrene with polymers having higher T
service temperature, various hard blocks such as poly (4-tert-
butylstyrene) [5], poly -methylstyrene) [6], poly(4-(1-ada-
g
. In order to increase
ꢀ
(a
showed elastic properties up to 200 C, however their mechanical
mantyl)styrene) [7] or polyindene [8] were employed to substitute
polystyrene blocks of SBS or SIS copolymers. In addition, various
fully acrylic TPEs based on combination of polyacrylates as soft
blocks with polymethacrylates as hard blocks were prepared by
controlled polymerization techniques [9e15]. These polymers are
properties were relatively poor due to brittleness of PMBL block
[27,28].
Thermoplastic elastomers based on star block copolymers may
possess improved properties in comparison with their linear coun-
terparts (linear ABA triblock copolymers), in respect to mechanical
properties, sensitivity to diblock contamination as well as lower melt
viscosities and thus better processibility [8,29e34]. Hereinwe report
preparation of star-like block copolymers consisting of inner soft PBA
blocks and outerhard PMBL blocks. We have studied the morphology
*
Corresponding author. Tel.: þ1 412 268 3209; fax: þ1 412 268 6897.
E-mail address: km3b@andrew.cmu.edu (K. Matyjaszewski).
0
032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.08.017

A. Juhari et al. / Polymer 51 (2010) 4806e4813
4807
and the thermal and mechanical properties of the prepared copoly-
mers and compared their thermoplastic elastomer characteristics
with those of the corresponding linear triblock ABA copolymers.
containing MBL, PBiBEA-g-PBA macroinitiator, CuCl
2
, bpy, and DMF
was degassed by three freeze-pump-thaw cycles. Then, CuCl was
added to the frozen reaction solution under nitrogen flow. The flask
was closed, evacuated, back-filled with nitrogen, and immersed in
ꢀ
2
. Experimental
an oil bath thermostated at 50 C. Molar ratios of monomer: initi-
2
ator: CuCl: CuCl : ligand are described in Table 1 (Entries C1eC10).
2
.1. Materials
After the polymerization was stopped, DMF was added and the
polymer solution was purified by passing through a neutral
alumina column. The final polymer was precipitated twice from
DMF into methanol.
All monomers were purified by passing through a basic alumina
column to remove the inhibitor. CuCl was purified according to the
literature procedures [35]. CuCl
0
2
and ligand 2,2 -bipyridine (bpy)
[16] (all from Aldrich) were used as received. All other reagents and
2.3. General analysis
solvents were purchased from Aldrich and used as received without
further purification. Poly (2-bromoisobutyryloxyethyl acrylate-
graft-butyl acrylate)s (PBiBEA-g-PBA) star-like macroinitiators were
synthesized as described previously (see Scheme 1) [34].
Monomer conversions and molecular weights of the copolymers
were determined by H NMR on a 300 MHz Bruker NMR spectrometer
1
using deuterated DMFas a solvent. The molecular weight distributions
of the polymers were measured using a GPC (Polymer Standards
5
3
2
ꢁ
2
.2. Synthesis of poly (2-bromoisobutyryloxyethyl acrylate-graft-
butyl acrylate-block- -methylene- -butyrolactone))s,
PBiBEA-g-(PBA-b-PMBL)
Services (PSS) columns (guard, 10 , 10 , and 10 A) with DMF (con-
ꢀ
(
a
g
taining 5 mM LiBr) as an eluent at 50 C, flow rate 1.00 mL/min, and
differential refractive index (RI) detector (Waters, 2410)). Polystyrene
(PS) was used as a calibration standard employing WinGPC software
The general procedure for the chain extension of the PBiBEA-
from Polymer Standards Service. Toluene was used as the internal
g-PBA macroinitiator was as follows: a round bottom flask
standard to correct for any fluctuation in DMF flow rate.
Br Br
Br BrBr
Br
Br Br BrBr
PHEA
PBA
PMBL
Scheme 1. Synthesis of graft copolymers.

4808
A. Juhari et al. / Polymer 51 (2010) 4806e4813
Table 1
Experimental conditions of ATRP of MBL during arms extension of PBiBEA-g-PBA star-like polymers, and characterization of prepared polymers.
ꢀ
f
f
M
w n
/M g
Entry
AL
I
CuCl
CuCl
2
L
DMF vol%
T ( C)
Time (h)
Conv. (%)
M
n, exp.
a
a
b
b
1
2
3
4
5
6
7
8
110
110
240
240
110
110
150
980
1 (10B115)
1 (10B115)
1 (10B240)
1 (10B240)
1.5
1.5
1.5
1.5
1.8
1.8
1.8
2.0
0.2
0.2
0.2
0.2
0.15
0.15
0.15
0.2
3.4
3.4
3.4
3.4
3.9
3.9
3.9
4.4
62
62
62
62
62
62
57
62
50
50
50
50
50
50
60
60
7.5
10.5
15
28
5
6.5
29
140
25
44
21
40
40
53
55
21
179 000
196 000
355 000
394 000
391 000
411 000
782 000
1.20
1.24
1.21
1.25
1.37
1.46
1.49
NA
1 (20B115)c
1 (20B115)c
d
1 (10B230)
1 (20B750)
e
2 230 000
AL, I and L stand for a-methylene-g-butyrolactone (MBL), PBiBEA-g-PBA macroinitiator and bpy, respectively.
a
PBiBEA10-g-PBA115 macroinitiator with M
PBiBEA10-g-PBA240 macroinitiator with M
PBiBEA20-g-PBA115 macroinitiator with M
PBiBEA20-g-PBA230 macroinitiator with M
PBiBEA20-g-PBA750 macroinitiator with M
n
n
n
n
n
¼ 147 000 g/mol, M
¼ 308 000 g/mol, M
¼ 294 000 g/mol, M
¼ 590 000 g/mol, M
w
w
w
w
/M
/M
/M
/M
n
n
n
n
¼ 1.15.
¼ 1.15.
¼ 1.14.
¼ 1.13.
b
c
d
e
f
¼ 1 920 000 g/mol, M
w
/M
n
¼ 1.21.
1
Based on H NMR spectra.
Based on GPC using PS standards.
g
2.4. Tapping Mode Atomic Force Microscopy (TMAFM)
2.7. Dynamic Mechanical Analysis (DMA)
For a phase separated film morphology investigation, the block
DMA was performed on a Rheometrics RMS 800 mechanical
spectrometer. Shear deformation was applied under conditions of
controlled deformation amplitude, which was kept in the range of
the linear viscoelastic response of studied samples. Plateeplate
geometry was used with plate diameters of 6 mm. Experiments
have been performed under a dry nitrogen atmosphere. Results
copolymers were dissolved in DMF (1 mg/mL) and were deposited
by drop casting onto silicon wafer surface (1 cm ꢁ 1 cm) cleaned by
rinsing with acetone and isopropanol followed by oxygen plasma
treatment. The samples were dried in a vacuum oven at room
temperature overnight. Tapping mode AFM experiments were
carried out using a Dimension V scanning probe microscope with
a NanoScope V controller (Veeco). The measurements were per-
formed in air using commercial Si cantilevers with a nominal spring
constant and resonance frequency of 42 N/m and 330 kHz,
respectively. The height and phase images were acquired simulta-
0
are presented as temperature dependencies of the storage (G ) and
0
0
loss (G ) shear moduli measured at a constant deformation
ꢀ
frequency of 10 rad/s. The results were obtained with a 2 C/min
heating rate.
neously at a set-point ratio A/A
respectively, to the “tapping” and “free” cantilever amplitude. For
0
¼ 0.7e0.8, where A and A
0
refer,
investigations of morphology changes by annealing, samples used
for AFM observations were annealed at 230 C for 30 min under
2.8. Tensile tests
ꢀ
nitrogen flow in an oven. After annealing, the oven was turned off
and cooled down naturally while maintaining nitrogen flow. The
annealed samples were observed by TMAFM under the similar
conditions described earlier and figured out the morphology
changes.
Tensile tests were performed using a mechanical testing
machine Instron 6000. Samples with thickness in the order of
.2e0.5 mm were drawn with the rate of 5 mm/min at room
temperature. Dependencies of stress vs. draw ratio were
recorded. Elastic modulus, elongation at break and stress at
break were determined by averaging of 3e5 independent
drawing experiments performed at the same conditions.
0
2.5. Power spectral analysis of TMAFM images
Phase mode AFM images showing contrast periodicities were
analyzed by a 2-D Fourier transform (FT). Subsequently, the 2-D FT
maps were azimuthally averaged to produce magnitude plots
analogous to the scattering patterns. After recalculating the spatial
frequency scale to scattering vector units (q, 2p/d), the domain
spacing (d) was calculated from the q value that showed the peak
maximum of the power spectrum.
a
CH2
b
Br
CH 115
C
O
f
O
c
2.6. Small-Angle X-ray Scattering analysis (SAXS)
d
CH2
H C
2
e
SAXS measurements were conducted using a rotating anode
f
CH2
a + d
(
Rigaku RA-Micro 7) X-ray beam with a pinhole collimation and
a two-dimensional detector (Bruker Highstar) with 1024 ꢁ 1024
pixels. A double graphite monochromator for the Cu K radia-
tion (
¼ 0.154 nm) was used. The beam diameter was about
.8 mm, and the sample to detector distance was 1.8 m. The
H C
3
e
c
a
l
b
0
a
recorded scattered intensity distributions were integrated over
the azimuthal angle and are presented as functions of the
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5 ppm
scattering vector (s ¼ 2sin(
q)/l, where 2q is the scattering
Fig. 1. 1H NMR spectra of PBiBEA10-g-PBA115 in CDCl3.
angle).

A. Juhari et al. / Polymer 51 (2010) 4806e4813
PBiBEA10- g-PBA115
4809
PBiBEA10- g-PBA240
10B240-AL46
1
1
0B115-AL30
0B115-AL47
10B240-AL86
5
6
5
6
10
10
10
10
Molecular weight (g/mol)
Molecular weight (g/mol)
PBiBEA20- g-PBA115
PBiBEA20- g-PBA230
20B230-AL96
2
2
0B115-AL47
0B115-AL57
5
6
5
6
7
10
10
10
10
10
Molecular weight (g/mol)
Molecular weight (g/mol)
Fig. 2. GPC traces of PBiBEA-g-(PBA-b-PMBL) star-like block copolymers prepared by arms extension from PBiBEA-g-PBA star-like macroinitiators.
3
. Results and discussion
2-hydroxyethyl acrylate (HEA) was protected with trimethylsilyl
group (TMS) giving the monomer trimethylsilyloxyethyl acrylate
(HEATMS). HEATMS was polymerized by ATRP technique using
3.1. Synthesis
0
00 00
ethyl 2-bromoisobutyrate (EBiB) initiator, N,N,N ,N ,N -pentam-
ethyldiethylenetriamine (PMDETA)/CuBr catalyst system and ani-
sole as a solvent. Degree of polymerization of the oligomers
PBiBEA-g-PBA star-like macroinitiators were prepared in four
steps like described in our previous paper (see Scheme 1) [34].
Briefly, as a first step of the synthesis, an alcohol group of
1
determined from H NMR spectra was equal to 10 and 20. In the
f
a + d
e
c
b +g
+
h
a + g
i
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
ppm
Fig. 3. 1H NMR spectra of 10B115-AL47 (PBiBEA10-g-(PBA115-b-PMBL47)) in DMF-d
.
7

4810
A. Juhari et al. / Polymer 51 (2010) 4806e4813
Table 2
Composition of the prepared PBiBEA-g-(PBA-b-PMBL) star-like block copolymers and their phase separation spacing values in thin films.
Entry
Label
Triblock composition
PMBL (mol %)a
PMBL
Spacing (nm)
AFMb
Type of morphology form SAXS
a
(wt %)
SAXS
1
2
3
4
5
6
7
8
10B115-AL30
10B115-AL47
10B240-AL46
10B240-AL86
20B115-AL47
20B115-AL57
20B230-AL96
20B750-AL156
PBiBEA10-(PBA115ePMBL30
PBiBEA10-(PBA115ePMBL47
PBiBEA10-(PBA240ePMBL46
PBiBEA10-(PBA240ePMBL86
PBiBEA20-(PBA115ePMBL47
PBiBEA20-(PBA115ePMBL57
PBiBEA20-(PBA230ePMBL96
)
)
)
)
)
)
)
10
10
10
10
20
20
20
20.7
29.0
16.1
26.4
29.0
33.1
29.4
17.2
16.5
24.2
12.7
21.6
24.2
27.6
24.2
13.7
34
41
45
53
41
44
53
103
34
44
51
55
43
52
60
79
25.3
28.5
36.7
42.1
28.5
35.3
49.6
73.3
cylinders
cylinders
cylinders
lamellar
cylinders
lamellar
no sec. peaks
no sec. peaks
PBiBEA20-(PBA750ePMBL156
)
20
a
Based on 1H NMR spectra.
b
Analyzed by a 2-D Fourier transform of phase mode images taken before annealing (left) and after annealing (right).
next step, PHEATMS oligomers were esterified with 2-bromoiso-
butyryl bromide in the presence of KF to give the multibrominated
ATRP macroinitiator PBiBEA. For PBA arms growth from the PBiBEA
macroinitiators, PMDETA/CuBr catalyst system was used with ani-
10B115-AL30 (16.5 wt %), 10B240-AL46 (12.7 wt %) and 20B750-
AL156 (13.7 wt %) clearly showed PMBL cylinders dispersed in the
continuous PBA phase and oriented either parallel or perpendicular
to the substrate. For the samples with higher PMBL contents
(21e28 wt %), mixed morphologies of cylinders and lamellae were
observed. In particular, the areas of no periodicity in 10B240-AL86
and 20B115-AL57 were assigned to lamellar layers parallel to the
substrates. This assignment was further confirmed by the results of
the SAXS experiments performed on thick films as discussed below.
The power spectral analysis of the AFM images was performed in
order to evaluate the domain spacing (d). The results summarized in
Table 2 clearly show the effect of the copolymer composition. The
increase of PMBL block length leads to the increase in the d-spacing
for the same length of the PBA segment. In all cases, the spacing
values were larger than that determined by SAXS, presumably due to
the more stretched polymers in the very thin films used for the AFM
studies. Interestingly, after annealing of these thin films for 30 min at
ꢀ
sole as a solvent. Reaction temperature was 70 C for all the PBA
polymerizations. Three various monomer: initiator ratios, 300, 700
and 2100, were used for the preparation of stars with various arms
length. Polymerizations were stopped at low monomer conversion,
between 30 and 40%, in order to minimize the cross-linking of the
stars and loss of the end groups of the growing PBA chains.
Molecular weight and molecular weight distribution of the PBiBEA-
g-PBA star-like macroinitiators are reported as footnotes in Table 1.
1
A typical H NMR spectrum of the macroinitiator is shown in Fig. 1.
To prepare star-like thermoplastic elastomers, the ATRP tech-
nique was used for chain extension of the PBiBEA-g-PBA macro-
initiators with PMBL blocks. As the ATRP equilibrium constants for
tertiary alkyl halides derived from MBL are much higher than those
for n-BA [36], extension of arms of PBiBEA-g-PBA involved the
halogen exchange strategy to provide for higher, or at least equal,
rate of cross-propagation, in comparison with rate of propagation
ꢀ
230 C, the phase separated morphologies transformed to cylinders
perpendicular to the substrate (Fig. 4 b) for all studied copolymers.
Furthermore the values for the domain spacing increased compared
to that measured before the annealing.
[18,37]. Experimental conditions for preparation of all studied
copolymers are given in Table 1. An evolution of GPC traces with
conversion for extension from PBiBEA-g-PBA polymers with MBL
segments is shown in Fig. 2. A smooth shift of the molecular weight
was observed for all samples. Unlike the copolymers having 10
arms, a shoulder formation towards the higher molecular weight
region was observed for the copolymers having 20 arms, and
assigned to intermolecular radical coupling reactions between the
growing arms of the stars. The fraction of coupled stars increased
with an arm length (increased with a monomer conversion). During
extension of PBiBEA-g-PBA stars with the DP of PBA arms equal to
SAXS analysis of thick films (w0.3 mm) of the PBA-b-PMBL star-
like copolymers were performed in order to get a better insight on
the effect of phase separation and the resulting morphologies. The
scattering measurements were first made without thermal treat-
ment of the copolymer films and then repeated after annealing
ꢀ
them for 60 min at 150 C. While in all cases the annealed films
exhibited better ordering than the untreated ones, displaying
clearer secondary peaks, the type of morphology (e.g. cylindrical or
lamellae) did not change due to this thermal treatment.
Typical SAXS spectra are shown in Fig. 5. From the relative
positions of the scattering peaks the type of the phase separated
morphology was evaluated and is summarized in Table 2. In good
agreement with the AFM results, most of the block copolymers
7
50 (Table 1, Entry 8), intermolecular coupling reactions between
PBiBEA-g-(PBA-b-PMBL) stars formed so big macromolecules that
cannot be filtered and injected to GPC column. Majority of the
polymer sample was captured in 0.2
m
l filter, used before GPC
revealed morphologies consistent with hexagonally packed cylin-
pffiffiffi
analysis. Degree of polymerization of the PMBL blocks was calcu-
ders (two scattering peaks at relative positionss; 3s) as shown in
Fig. 5 for 10B240-AL46 and 20115-AL47. Only the higher PMBL
content copolymers 10B240-AL86 and 20B115-AL57 showed clear
lamellar structure with scattering peaks occurring at relative
positions s, 2s. However after further annealing the thick films of
1
lated from H NMR spectra based on the ratio of signals from PBA to
signals from PMBL (Fig. 3, signal c to signal i). The compositions of
the prepared star-like block copolymers determined by NMR are
given in Table 2. Entries 1e8 in Table 2 correspond to entries 1e8 in
Table 1.
ꢀ
these copolymers at 230 C, i.e. above the glass transition
temperature of the PMBL, their SAXS spectra changed and showed
scattering peak consistent with hexagonal morphology. This result
is in a good agreement with the change of morphology observed by
AFM for very thin films of the same copolymers.
3
.2. Morphology of the PBiBEA-g-(PBA-b-PMBL) star-like block
copolymers
AFM imaging was performed for all star-like block copolymer
films, in order to directly visualize their morphology. Average film
thicknesses were around 400 nm, based on the substrate surface area
and the deposited polymer weights. All the samples displayed strong
phase separation (Fig. 4 a), implying the immiscibility of PBA and
PMBL. The block copolymers with relatively low PMBL contents
3.3. Thermo-mechanical properties of the PBiBEA-g-(PBA-b-PMBL)
star-like block copolymers
Characterization of dynamic mechanical properties of the multi-
arm stars PBAePMBL block copolymers was done by measuring the

A. Juhari et al. / Polymer 51 (2010) 4806e4813
4811
ꢀ
Fig. 4. Phase mode AFM images of film samples. (a) Images taken before annealing. (b) Images taken after annealing. Annealing conditions: 230 C, 30 min, under N
2
flow, slow
2
cooling down to room T. Image size ¼ 1.0 ꢁ 1.0
mm . Brighter areas correspond to PMBL phase.
0
00
temperature dependencies of the real (G ) and the imaginary (G )
parts of the complex shear modulus at constant deformation
decrease above the T
g
of PMBL but the copolymers did not flow in
ꢀ
the entire temperature range of up to 330 C. The classical SBS and
SIS thermoplastic elastomer systems exhibit phase separation that
may also persists beyond the upper (PS) glass transition temper-
ature [3]. However, above a certain temperature (ODT), the phase
separation disappears and a monophase fluid is formed. This
process known as ODT and is observed for most block copolymers
[38e41]. For example, in their extensive work on the meth-
acrylateealkyleneemethacrylate triblock copolymer thermo-
plastic elastomers [12,42e44] Jerome et al. showed that
depending on the degree of miscibility of PMMA with poly (alkyl
acrylate)s the ODT temperature in these systems may exceed
ꢀ
frequency of 10 rad/s and heating rate of 2 C/min.
Typical results for copolymers of constant PBA content and
different PMBL block lengths are shown in Fig. 6. Two distinct glass
transition temperatures corresponding to that of PBA (T
and PMBL (T w 195 C) segments were observed in the DMA
spectra for most copolymers that is a further evidence for their
microphase-separated morphology. However, the glass transition
of PMBL becomes less obvious with decreasing its content in the
copolymers and, as can be seen in the Fig. 6, is barely detectable for
ꢀ
g
w ꢂ 50 C)
ꢀ
g
10B240-AL46.
ꢀ
With respect to the mechanical properties, all samples were
glassy below the glass transition temperature of PBA, with storage
modulus G w 1 GPa. Above this glass transition, the copolymers
become elastic and show a G plateau extending up to the glass
transition temperature of PMBL (w195 C). In this elastic state the
PMBL blocks form glassy domains connecting the flexible PBA
matrix and serving as physical cross-links in a way typical for
a thermoplastic elastomer. Finally, both storage and loss moduli
200 C. In this respect the results shown in Fig. 6 demonstrate that
the star-like PBA-b-PMBL copolymers maintain their micro-
domain morphology and elastic properties up to a very high ODT
0
0
ꢀ
ꢀ
temperature of more than 330 C, i.e., w130 C above the glass
transition of the PMBL. Thus the very high glass transition
temperature and high thermal stability [26] of PMBL make the
PMBL based thermoplastic elastomers particularly suitable for
high temperature applications.
ꢀ

4812
A. Juhari et al. / Polymer 51 (2010) 4806e4813
Fig. 7. Tensile mechanical properties of star-like block copolymers with different
number of arms and compositions.
synthesis of the PBAePMBL stars such as incomplete functionali-
zation or relatively high polydispersity may decrease their tensile
properties there is also more important fundamental reason for this
difference. As discussed before [12,42e44], the molecular weigh
e
between chain entanglements, M , of the soft PBA block is much
higher than those of the central polybutadiene/polyisoprene blocks
in the SBS/SIS. Since the number of chain entanglements is limited
in the PBAePMBL systems, the deformation stress is not dissipated
by the soft PBA block, but directly transferred to the hard PMBL
microdomains. Furthermore, the intrinsic brittleness of the hard
PMBL blocks [26] may also have a negative effect on the ultimate
tensile strength and elongation at break.
It was previously reported that thermoplastic elastomers based
on star block copolymers may exhibit superior properties in
comparison with the linear ABA triblock type TPEs [8,29e33]. In
an attempt to explore these effects for PBAePMBL based materials,
we compared the newly synthesized 10 arm 10B115-AL30, 10B115-
AL47 and 10B240-AL46 copolymers with linear PMBL-b-PBA-b-
PMBL block copolymers with similar composition and arm
molecular weight and prepared previously also using ATRP [15]. As
shown in Table 3, there is more than double increase in both
ultimate tensile strength and the elongation at break with the
increase of number of arms from 2 up to 10.
Fig. 5. SAXS spectra 10 and 20 arm star-like PBAePMBL block copolymers measured
after annealing at 150 C.
ꢀ
The elasticity of the copolymers, i.e. the height of the rubbery
plateau, strongly depends on the PMBL content. As seen in Fig. 6 the
0
value of G at the rubber plateau region is approximately 0.1 MPa
for the copolymers with PMBL fraction of 12.7 wt% (10B240-AL46)
and up to approximately 10 MPa for the copolymers with PMBL
content of 21.6 wt% (10B240-AL86).
The same effect is observed when studying the tensile proper-
ties of these materials. The measured stressestrain dependencies
plotted in Fig. 7 show that both the initial modulus (E) and the
Further increasing number of arms from 10 up to 20 has showed
little influence on the mechanical properties. These results are not
very surprising in view of earlier studies of polystyrene-b-poly-
isobutylene [31] and polystyrene-b-polydiene [45] star-like copol-
ymers that have shown a saturation point in the improvement of
the mechanical properties with respect to number of arms at
tensile strength (s break) increase with PMBL content that suggest
a simple way of tuning these properties by adjusting copolymer
composition. The ultimate tensile strength and elongation at break
of the PBAePMBL star block copolymers are significantly lower
than those of the classical SIS and SBS thermoplastic elastomers
with comparable molecular weights. While problems with the
N
arm w 5e10. In our case, both 10 and 20 arm star-like polymers fall
in the region where the effect of Narm is minor. This finding is also
consistent with our recent studies of star-like PBA-b-PMMA
copolymers, for which only minor effect of the number of arms on
the mechanical properties was observed for Narm ¼ 10 and 20 [34].
Table 3
Ultimate Tensile Strength and Elongation at Break of linear and star-like PBA-b-
PMBL Block Copolymers of Similar Composition.
Sample
Composition
PMBL
content tensile
wt%) strength at break
Ultimate Maximum
elongation
(
s
(MPa)
l
(%)
BA215-AL68[15] 2 arms PBA107.5-b-PMBL34 19.3
1.9
3.1
7.8
0.7
3.3
55
1
1
0B115-AL30
0B115-AL47
10 arms PBA115eb-PMBL30 16.5
10 arms PBA115eb-PMBL47 24.2
140
140
100
250
BA375-AL60[15] 2 arms PBA187.5-b-PMBL30 10.9
0B240-AL46 10 arms PBA240eb-PMBL46 12.7
Fig. 6. Temperature dependence of shear moduli of PBAePMBL star-like block
copolymers with different compositions.
1

A. Juhari et al. / Polymer 51 (2010) 4806e4813
4813
4
. Conclusions
[7] Kobayashi S, Kataoka H, Ishizone T, Kato T, Ono T, Kobukata S, et al. Macro-
molecules 2008;41:5502e8.
[
[
8] Shim JS, Kennedy JP. J Polym Sci Part A Polym Chem 2000;38:279e90.
9] Jerome R, Bayard P, Fayt R, Jacobs C, Varshney S, Teyssie P. In: Holden G,
Legge NR, Quirk RP, Schroeder HE, editors. Thermoplastic elastomers. Munich,
Germany: Hanser; 1996.
In summary, star-like 10 and 20 arm PBA macroinitiators were
extended with outer hard PMBL block in order to prepare star-like
thermoplastic elastomers. Halogen exchange strategy was applied
during the ATRP of PMBL to provide at least equal rate of cross-
propagation compared to rate of propagation. Partial star coupling
during PBA arms extension with PMBL blocks was observed, and the
coupling increased with number of arms and arm length. The
morphology of solution cast films of the star-like PBAePMBL block
copolymers was studied byAFM and SAXS. Bothtechniques revealed
a phase separated morphology of either cylindrical PMBL domains
hexagonally arranged in the PBA matrix or lamellar morphology
[
[
10] Moineau C, Minet M, Teyssie P, Jerome R. Macromolecules 1999;32:8277e82.
11] Matyjaszewski K, Shipp DA, McMurtry GP, Gaynor SG, Pakula T. J Polym Sci
Part A Polym Chem 2000;38:2023e31.
[
[
12] Tong JD, Leclere P, Doneux C, Bredas JL, Lazzaroni R, Jerome R. Polymer
2000;41:4617e24.
13] Matyjaszewski K, Spanswick J. Thermoplastic elastomers by controlled/living
radical polymerization. In: Holden G, Kricheldorf HR, Quirk RP, editors.
Thermoplastic elastomers. Munich, Germany: Hanser; 2004. p. 365.
14] Braunecker WA, Matyjaszewski K. Prog Polym Sci 2007;32:93e146.
15] Mosná ꢀc ek J, Yoon JA, Juhari A, Koynov K, Matyjaszewski K. Polymer
[
[
2009;50:2087e94.
[
16] Wang JS, Matyjaszewski K. J Amer Chem Soc 1995;117:5614e5.
[17] Kamigaito M, Ando T, Sawamoto M. Chem Rev 2001;101:3689e746.
18] Matyjaszewski K, Xia JH. Chem Rev 2001;101:2921e90.
in the samples with higher content of PMBL (21e27 wt %).
ꢀ
After annealing at 230 C (over T
g
of PMBL) the change of
[
morphology leading to PMBL cylinders oriented perpendicularly to
the surface was observed for all star-like copolymers. The mecha-
nical and thermal properties of the star-like block copolymers have
been thoroughly characterized. These materials possess typical
elastomeric behavior in a broad range of service temperatures
[19] Tsarevsky NV, Matyjaszewski K. Chem Rev 2007;107:2270e99.
[20] Matyjaszewski K, Tsarevsky NV. Nat Chem 2009;1:276e88.
[
[
21] Mosná ꢀc ek J, Matyjaszewski K. Macromolecules 2008;41:5509e11.
22] van Rossum MWPC, Alberda M, van der Plas LHW. Phytochemistry
1998;49:723e9.
[23] Wong HF, Brown GD. Phytochemistry 2002;59:99e104.
[24] Trendafilova A, Todorova M, Mikhova B, Vitkova A, Duddeck H. Phytochem-
istry 2006;67:764e70.
ꢀ
maintaining phase separated morphology even over 300 C. Both
the ultimate tensile strength andelongation at breakof the 10 and 20
arm star-like PBAePMBL copolymers are significantly higher than
those of their linear ABA type counterparts with similar composi-
tion, indicating a strong effect of the higher number of arms on the
mechanical properties. Even though the elongation at break is still
lower than for commercial SBS or SIS thermoplastic elastomers,
thanks to the very high glass transition temperature, high thermal
stability of PMBL and higher polarity of methacrylates, these mate-
rials could be suitable for some special high temperature
applications.
[
25] Akkapeddi MK. Macromolecules 1979;12:546e51.
[26] Brandenburg CJ. US6841627; 2005.
[
[
[
27] Pickett JE and Ye Q. US2007/122625; 2007.
28] Schwind H, Hauch D, Hasskerl T, Dorn K, and Hopp M. US5880235; 1999.
29] Shim JS, Asthana S, Omura N, Kennedy JP. J Polym Sci Part A Polym Chem
1998;36:2997e3012.
[30] Storey RF, Shoemake KA. J Polym Sci Part A Polym Chem 1998;36:471e83.
[31] Shim JS, Kennedy JP. J Polym Sci Part A Polym Chem 1999;37:815e24.
[32] Dufour B, Koynov K, Pakula T, Matyjaszewski K. Macromol Chem Phys
2008;209:1686e93.
[33] Dufour B, Tang CB, Koynov K, Zhang Y, Pakula T, Matyjaszewski K. Macro-
molecules 2008;41:2451e8.
[
34] Nese A, Mosná ꢀc ek J, Juhari A, Yoon JA, Koynov K, Kowalewski T, et al.
Macromolecules 2010;43:1227e35.
[
[
35] Keller RN, Wycoff HD. Inorg Syn 1946;2:1e4.
36] Tang W, Kwak Y, Braunecker W, Tsarevsky NV, Coote ML, Matyjaszewski K.
J Amer Chem Soc 2008;130:10702e13.
Acknowledgements
[
37] Matyjaszewski K, Shipp DA, Wang JL, Grimaud T, Patten TE. Macromolecules
The financial support from National Science Foundation (DMR
9-69301 and CBET 06-09087) is greatly appreciated.
1998;31:6836e40.
0
[
[
38] Helfand E, Wasserman ZR. Macromolecules 1976;9:879e88.
39] Leibler L. Macromolecules 1980;13:1602e17.
References
[40] Floudas G, Pakula T, Fischer EW, Hadjichristidis N, Pispas S. Acta Polym
994;45:176e81.
1
[
[
[
1] Fetters LJ, Morton M. Macromolecules 1969;2:453e8.
[41] Floudas G, Pakula T, Velis G, Sioula S, Hadjichristidis N.
1998;108:6498e501.
[42] Tong JD, Jerome R. Polymer 2000;41:2499e510.
[43] Tong JD, Leclere P, Rasmont A, Bredas JL, Lazzaroni R, Jerome R. Macromol
Chem Physic 2000;201:1250e8.
[44] Tong JD, Leclere P, Doneux C, Bredas JL, Lazzaroni R, Jerome R. Polymer
2001;42:3503e14.
J Chem Phys
2] Holden G, Bishop ET, Legge NR. J Polym Sci Polym Symp 1969;26:37e57.
3] Holden G, Legge NR. In: Holden G, Legge NR, Quirk RP, Schroeder HE, editors.
Thermoplastic elastomers. 2nd ed. Munich, Germany: Hanser; 1996. p. 71.
4] Jerome R, Fayt R, Teyssie P. In: Legge NR, Holden G, Schroeder HE, editors.
Thermoplastic elastomers. Munich, Germany: Hanser; 1987.
[
[
[
5] Malhotra SL, Lessard P, Blanchard LP. J Macromol Sci Chem 1981;A15:121e41.
6] Cunningham RE. J App Polym Sci 1978;22:2907e13.
[45] Bi LK, Fetters LJ. Macromolecules 1976;9:732e42.