Shearinduced networktonetwork transition in a block copolymer melt IRG 1.

1
University of Minnesota MRSEC Nuggets
2005Lodge T. P. DMR-0212302 (IRG
1) Shear-induced network-to-network transition in
a block copolymer melt.
Flow induced alignment of one- or two-dimensional
block copolymer microdomains, such as cylinders
and lamellae, has been known for more than three
decades. This can produce materials with inherent
anisotropic physical properties, including
thermal expansivity, elasticity, fracture
toughness, and various transport properties
including diffusivity. In contrast,
three-dimensional periodic block copolymer
structures, such as spheres organized on a cubic
lattice or the bicontinuous cubic gyroid
morphology, are isotropic and do not accommodate
large shear strains through domain alignment. The
UMN Microstructured Polymers group previously
discovered a new class of ABC triblock copolymers
with triply periodic and triply continuous
network morphologies, including an anisotropic
orthorhombic phase denoted O70 that represented
the first example of a noncubic network structure
in soft self-assembled materials (which include
surfactants, soaps, lipids, and block
copolymers). Building on this discovery, MRSEC
investigators Eric Cochran and Frank Bates have
now demonstrated that the O70 phase (space group
Fddd) exhibits anisotropy in all physical
properties, including response to flow and
deformation, which produced an unprecedented
long-range topological restructuring to a
different orthorhombic symmetry. The MRSEC
investigators synthesized a poly(cyclohexylethylen
e-b-ethylethylene-bethylene) (CEEE) triblock
copolymer, which formed the O70 phase. When this
material was subjected to dynamic reciprocating
shear and cooled to room temperature, the
poly(cyclohexylethylene) blocks vitrified as a
glass, which fixed the multicontinuous morphology
of the triblock copolymer. Small angle x-ray
scattering (SAXS), performed in the UMN Institute
of Technology Characterization Facility, revealed
a remarkable single crystal state of order, as
evinced by the scattering patterns acquired with
the beam directed along the three principal
coordinates established by the shearing geometry.
Notably, scattering peak positions from all three
patterns revealed that the dynamic shearing
transformed the O70 phase into another
(metastable) orthorhombic network structure with
Pnna (No. 52) space group symmetry, denoted O52.
This kind of field-induced network-to-network
phase transition in soft condensed matter is
exceptional. In addition to the aforementioned
impact of anisotropic structures on mechanical
properties, these experiments are significant
because they demonstrate a remarkable and
fundamentally significant tendency for network
forming linear block copolymer melts to maintain
local threefold connector symmetry, even when
subjected to massive deformation. Furthermore,
orthorhombic crystals are birefringent,
suggesting that O70 and O52 materials may be
useful as photonic materials, particularly given
their high degree of single crystal order.
Perhaps most importantly, these findings prove
that network formation in ABC triblock copolymers
is universal and not restricted to a single model
system. Cochran E.W. Bates, F.S. Phys. Rev.
Lett. 2004, 93, 087802
2
University of Minnesota MRSEC Nuggets
2005Lodge T. P. DMR-0212302 (IRG
1)Shear-induced network-to-network transition in
a block copolymer melt.
Figure 1. Upper panel Ball-and-stick
representations of the (10, 3)c and (10, 3)d
network topologies (2 unit cells, the wire-frame
box demarks the unit cell boundary). Here, both
structures are built from geometrically
equivalent planar trivalent connectors, with the
longest strut lenth L 18.5 nm for both (note
that this geometry is not unique, and that
symmetry does allow the O52 and O70 connectors to
differ). Lower panel Simulated space filling
morphologies for a unit cell of the O70 and O52
phases in CEEE-3. The shaded oval illustrates the
orientation of a polymer chain with respect to
the surfaces.
Figure 2. Small-angle x-ray scattering patterns
obtained from dynamically sheared CEEE triblock
copolymer with the beam directed along three
orthogonal axes. These single crystal data reveal
a network morphology with Pnna space group
symmetry.