European Polymer Journal 36 (2000) 215±219
Short communication
Evidence for a thermally reversible order±order transition
between lamellar and perforated lamellar microphases in a
triblock copolymer
Sudhir Mani a, R.A. Weiss a,*, M.E. Cantino b, L.H. Khairallah b, S.F. Hahn c,
C.E. Williams d
a
Department of Chemical Engineering and Polymer Science Program, University of Connecticut, Storrs, CT 06269, USA
Electron Microscopy Laboratory, Department of Physiology and Neurobiology, University of Connecticut, Storrs, CT 06269, USA
c
Central Research-Advanced Polymeric Systems Laboratory, The Dow Chemical Company, Midland, MI 48674, USA
d
Laboratoire des Fluides Organises, CNRS URA 792, College de France, 11 Place Marcelin-Berthelot, 75231 Paris Cedex 05, France
b
Received 7 December 1998; received in revised form 26 January 1999; accepted 1 February 1999
Abstract
The microstructure and microphase behavior of a poly(styrene-b-(ethylene-alt-propylene)-b-styrene) (PS-PEP-PS)
triblock copolymer (Mn ˆ 50,000 g/mol and 50.8 wt% PS) were characterized by transmission electron microscopy
(TEM) and dynamic rheology. The microstructure texture at room temperature and up to about 1258C consisted of
alternating PS and PEP lamellae (LAM). Oscillatory shear experiments revealed thermally-reversible post-Tg
transitions at 125±1308C and 2758C that are attributed to an order±order transition (OOT) of the mesophase
texture and an order±disorder transition (ODT), respectively. Evidence for a perforated lamellar (PL) microstructure
of alternating PS and PEP lamellae with hexagonal-packed PS connectors perforating the PEP phase above the
ODT and a thermally-reversible LAM $ PL OOT was obtained from TEM of quenched and slowly cooled
samples. Unlike previous reports of reversible OOT transitions in block copolymers, the reversibility of the OOT for
these block copolymers does not require shearing the sample. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
Block copolymers self-assemble into various wellordered microphase textures [1] that depend on composition, the segment±segment interaction parameter, the
degree of polymerization and the number of blocks.
One particular microstructure that has received attention recently is the perforated lamellae or lamellar-cate-
* Corresponding author. Tel.: +1-860-486-4698; fax: +1860-486-4745.
E-mail address: [email protected] (R.A. Weiss)
noid texture [2±9] in which channels of one component
are oriented normal to alternating lamellar microdomains of the two component blocks.
Although thermal reversibility between lamellar and
perforated lamellar microstructures has been reported
for polyole®n diblock copolymers [3,4] and poly(isoprene-b-styrene) [6], reversibility only occurred if the
specimen was subjected to a large-amplitude oscillatory
shear deformation [3,4] or was annealed at high temperatures for long times [6]. In this communication, we
report our observation of a thermally-reversible transition between a conventional lamellar microstructure
(LAM) and a perforated lamellar (PL) microstructure
0014-3057/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 0 5 2 - X
216
S. Mani et al. / European Polymer Journal 36 (2000) 215±219
for a nearly symmetrical triblock copolymer, poly(styrene-b-(ethylene-alt-propylene)-b-styrene),
SEPS.
This observation may be distinguished from prior
reports of LAM $ PL order±order transitions (OOT)
in that shearing of the sample was not required.
2. Experimental
The SEPS triblock copolymer was prepared by
hydrogenation of an anionically-polymerized poly(styrene-b-isoprene-b-styrene) copolymer (SIS); the
details are described elsewhere in Ref. [10]. The microstructure of the polyisoprene midblock of the parent
SIS contained 90% 1,4- and 10% 3,4-linkages (1 HNMR). Therefore, the PEP block of the hydrogenated
block copolymer was essentially an alternating ethylene±propylene copolymer. Mn and Mw (by GPC) of
the SEPS were 50,000 and 56,000 g/mol, respectively,
and the weight fraction of polystyrene (PS) determined
by 1 H-NMR was wPS ˆ 0:508. If one assumes complete
phase separation of the blocks and densities of PS and
PEP of 1.06 and 0.854 g/cm3, respectively, [11,12] the
volume fraction of the PS phase was fPS ˆ 0:454.
Films were prepared by slowly evaporating a 10%
(w/v) solution of the block copolymer in toluene at
room temperature over a period of one week, followed
by drying the cast-®lm at 508C under vacuum for 3
days. Because toluene is a non-selective solvent for PS
and PEP, the microstructure of the slowly dried cast®lms should be close to equilibrium. Compressionmolded specimens for rheological measurements were
prepared with a Carver laboratory press using a molding temperature of 2008C and a pressure of ca. 4.4
MPa. The compression-molded samples were slowly
cooled in the press at a rate of ca. 18C/min under a
constant load. All samples, were annealed at 1008C,
which was above the glass transition temperatures of
both blocks, for 24 h prior to testing to ensure that an
equilibrium microstructure was achieved.
Dynamic mechanical behavior was measured with a
Rheometrics System 4 mechanical spectrometer using
25-mm-diameter parallel-plates. Measurements were
made using a frequency of 0.1 Hz and strains of 1±
2%. Isothermal measurements were made in ca. 28C
increments, ®rst heating and then cooling and reheating the sample between 80 and 1708C. The sample was
held at each temperature for ca. 7±8 min before collecting data in order to equilibrate the sample temperature and microstructure.
Transmission electron micrographs were obtained
with either a Zeiss EM910 or Philips EM300 electron
microscopes operated at accelerating voltages of 100
and 80 kV, respectively. The Zeiss EM910 microscope
was ®tted with a goniometer that allowed observation
while tilting the specimen. Ultrathin sections were pre-
Fig. 1. (a) TEM micrograph of a solution-cast sample
annealed at 1008C and cooled slowly to room temperature
prior to staining. In this and the other micrographs shown in
this paper, the dark and light regions correspond to the
stained PS and unstained PEP phases, respectively. (b) TEM
micrograph for a sample annealed at 2008C and then
quenched in liquid nitrogen. The scale bar of 200 nm applies
to both micrographs.
pared by cryomicrotomy at temperatures below
ÿ1008C using a MT7 ultramicrotome equipped with a
CR21 cryokit and a glass knife. The sections were collected on Formvar and carbon-coated copper grids
and the PS microdomains were preferentially stained
by exposing the sample to RuO4 vapors at 258C for 5
min.
3. Results and discussion
We previously reported [13] observing a heterogeneous microstructure for a compression-molded
sample of the particular block copolymer which is the
subject of the present paper. Although most of that
sample showed a microstructure of alternating PS and
PEP lamellae, a PL texture was also observed. In contrast, a solution-cast sample showed only an LAM texture, which was presumed to be the equilibrium
microstructure between room temperature and the
annealing temperature of 1008C. The latter temperature was above the Tg of the PS block. The presence
of the PL texture in the compression-molded sample
was thought to be a remnant of a PL microstructure
that was produced at the high temperatures used for
compression molding. Furthermore, dynamic mechanical measurements obtained while heating the solutioncast sample (0.58C/min and 1% strain) revealed a postTg transition at ca. 1258C that we tentatively assigned
to an OOT of the block copolymer microstructure
from LAM to PL. An ODT was also observed at
2758C. Both dynamic mechanical transitions were also
observed for the compression-molded sample during a
heating scan, which indicated that the origin of the
transitions was independent of the sample preparation.
That result led additional credence to the hypothesis
that the lower temperature transition may represent an
OOT with a thermodynamic origin.
S. Mani et al. / European Polymer Journal 36 (2000) 215±219
Fig. 2. Tilt-series images from TEM on quenched polymer for
tilt angles of (a) 08, (b) 108, (c) 158, (d) 308. Regions 1 and 2
on micrograph (a) correspond to di€erent projections from a
PL microphase while region 3 corresponds to a LAM microphase. The scale bar of 200 nm applies to all four micrographs. (e) A schematic representation of the PL and LAM
microstructures.
In the present study, evidence for a stable PL microphase at elevated temperature was obtained from TEM
of samples cooled from 2008C, Fig. 1; the dark areas
on the micrograph correspond to the stained PS microphase and the lighter areas are the PEP microphase.
Fig. 1(a) shows an LAM microstructure that resulted
from cooling the sample slowly to ca. 228C after preannealing it at 1008C. Because the staining process
hardened the sample and ®xed the morphology, the
same TEM specimen could not be reused and instead,
fresh samples were used for studying the e€ect of temperature history on the block copolymer microstructure.
A second specimen was heated from 22 to 2008C,
annealed at 2008C under vacuum for 24 h, and then
cooled slowly, nominally 18C/min, back to 228C. The
resulting texture was identical to that shown in Fig.
1(a), which indicates that either no transition of the
microstructure occurred during heating to 2008C or
217
that any OOT that occurred was completely thermally
reversible.
A third sample was heated from 22 to 2008C,
annealed for 24 h, but then cooled rapidly by immersing it in liquid nitrogen. Upon removal from the
liquid nitrogen, it was immediately exposed to the
RuO4 vapors. Although, the specimen warmed to
room temperature during the staining process, the
stain hardened the polymer, which prevented any reorganization of the morphology. The resulting morphology is shown in Fig. 1(b). In this case, a `mesh'
texture was produced. The mesh structure was not
observed throughout the sample; a LAM texture was
also seen and was predominant in many areas of the
quenched sample. All quenched specimens showed
LAM and some PL texture, while only an LAM
microstructure was observed in the slowly cooled
samples. Although caution must be exercised when
drawing conclusions based on the small areas sampled
by TEM, the consistent picture that emerged from our
experiments was that an LAM microstructure was
stable below 1008C, a PL microstructure existed at
2008C and re-establishment of the LAM microstructure occurred when a sample at 2008C was cooled to
room temperature. These results suggest that a thermally reversible OOT occurs between 100 and 2008C
for this SEPS sample without having to apply a shear
deformation.
The nature of the mesh morphology was determined
by progressively tilting the quenched specimen in the
TEM from 0 to 308, Fig. 2(a)±(d). As the specimen
was tilted around an axis corresponding to the north±
south directions in Fig. 2(a)±(d), the observed texture
changes are consistent with a microstructure in which
the PS lamellae are bridged by PS connectors that perforate the PEP lamellae. Idealized schematics of the
LAM and PL microstructures are given in Fig. 2(e).
The interpretation of the morphology seen in the
micrographs in Fig. 2 is complicated by the fact that
the sample was not aligned to produce a monodomain
mesophase. As a result, the direction of the lamellae
and perforations in the sample change with position
on the micrograph as opposed to the well-de®ned morphology shown in Fig. 2(e). Nevertheless, the tilting experiment appears to con®rm the PL microstructure.
At a tilt angle of 08, which corresponds to the surface normal of the specimen being parallel to the incident electron beam, Fig. 2(a), three di€erent regions
are observed in the micrograph. Region 1 shows a hexagonal-packed mesh structure corresponding to an
obtuse slice through the cube in Fig. 2(e) that bisects
the PS perforations and the lamellae. Lamellae of PS
(the dark regions) and PEP run diagonally in a northwest±southeast direction and PS connectors perforate
the lamellae in a direction somewhat north of east. In
the micrograph, the PS connectors are at an angle to
218
S. Mani et al. / European Polymer Journal 36 (2000) 215±219
data in Fig. 3, which were obtained by heating, cooling
and reheating a compression-molded specimen of the
block copolymer. Two thermal transitions are evident:
(1) the Tg of the PS block at 958C and (2) a thermallyreversible transition at 1308C that is consistent with an
LAM $ PL OOT. The thermal reversibility of the latter transition during the cooling scan is consistent with
the TEM observations that an LAM microstructure
was quickly re-established upon relatively slow cooling
from 2008C. One cannot discount that the small shear
strains involved in the dynamic mechanical experiments may have contributed to the reversibility of the
OOT shown in Fig. 3, but no shear deformation was
necessary to achieve the microstructural reversibility
represented by the micrographs in Figs. 1 and 2.
4. Conclusions
Fig. 3. Dynamic mechanical data for SEPS upon heating,
cooling and reheating a compression-molded SEPS specimen.
The data represent isothermal measurements obtained after a
thermal soak of 7±8 min (frequency = 0.1 Hz and strain =
1±2%) (a) storage modulus, G', (b) tan d. The transitions are
marked as (1) PS Tg = 958C and (2) LAM $ PL order±
order transition, TOOT = 1308C. (c) Schematic representation
of the LAM and PL microphases involved in the OOT.
the plane of the sample surface. As the sample is tilted,
the orientation of the connectors becomes closer to the
viewing direction, and region 1 appears more like hexagonally arranged holes, though these are still skewed
so that they appear ellipsoidal in the micrographs in
Fig. 2(c) and (d). The appearance of region 2 changes
in more or less the opposite manner. Region 2 in Fig.
2(a) looks similar to region 1 in Fig. 2(d), and region 2
in Fig. 2(d) looks like the mesh structure seen in region
1 of Fig. 2(a). The changes in both regions upon tilting
the sample are consistent with PS channels normal to
the lamellae, though the lack of a uniform orientation
of the microdomains produces variation of the
observed structure within the two regions.
Region 3 shows a more conventional lamellar texture that corresponds either to a surface through the
specimen that does not bisect the PS connectors or a
local LAM mesophase resulting from failure to freezein the PL microstructure. In either case, tilting the
sample has little e€ect on the appearance of that
region; it always appears lamellar.
Further evidence for a thermally reversible LAM $
PL transition is provided by the dynamic mechanical
A thermally reversible order±oreder transition
(OOT) between conventional lamellar (LAM) and perforated lamellar (PL) microphases was observed for a
nearly-symmetric
poly(styrene-b-(ethylene-alt-propylene)-b-styrene)
triblock
copolymer
(SEPS).
Reversibility occurred without the application of an
external shear-orienting ®eld. The PL microphase comprises PS and PEP lamellae that are bridged by hexagonal close-packed PS connectors that perforate the
PEP lamellae. Using a recent report by Sakurai et al.
[14] for wSEP , we estimate that wN for the SEPS varies
between 44 and 51 for temperatures between 25 and
2008C, which places all the specimens discussed in this
paper in the strong segregation limit.
Acknowledgements
RAW and SM are grateful to the Polymers Program
and the International Programs of the National
Science Foundation (Grants INT-9216859 and DMR9712194) for support of this research and CEW
acknowledges support from CNRS (Accord de cooperation CNRS/NSF LET92/MDRI). We are also
grateful to Mr. Joey Eichen for help with the cryomicrotomy and Ms. Bridget Bartos for the GPC analyses.
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