Structure and thermodynamics of weakly segregated rod-coil block copolymers

Article abstract of DOI:10.1021/ma051468v

The self-assembly of a series of monodisperse rod-coil block copolymers is studied in the weak segregation limit. This unusual weakly segregated system consists of polyisoprene (PI) coil blocks and poly(alkoxyphenylene vinylene) (PPV) rod blocks solubiliz

Full text of DOI:10.1021/ma051468v

Macromolecules 2005, 38, 10127-10137
10127
Structure and Thermodynamics of Weakly Segregated Rod-Coil Block
Copolymers
Bradley D. Olsen and Rachel A. Segalman*
Department of Chemical Engineering, University of California, Berkeley, California 94720, and
Materials Science Division, Lawrence Berkeley Laboratory
Received July 6, 2005; Revised Manuscript Received August 29, 2005
ABSTRACT: The self-assembly of a series of monodisperse rod-coil block copolymers is studied in the
weak segregation limit. This unusual weakly segregated system consists of polyisoprene (PI) coil blocks
and poly(alkoxyphenylene vinylene) (PPV) rod blocks solubilized with alkoxy side groups. The order to
microphase disorder transition (ODT) and nematic isotropic (NI) transition are experimentally investigated
to produce a rod-coil block copolymer phase diagram in a system that follows polymeric scaling
relationships. Small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), polarized
optical microscopy, depolarized light scattering, and wide-angle X-ray scattering (WAXS) are used to
map the phase diagram. As the symmetric diblock copolymer is heated, a series of transitions from lamellar
to nematic to isotropic phases are observed. The NI transition temperature decreases with increasing
coil fraction, and at high coil fractions only an isotropic phase is observable. The phase behavior is in
qualitative agreement with weak segregation calculations based on Landau expansions reported by other
groups. Transmission electron microscopy (TEM) reveals unusual grain structures in the low-temperature
lamellar phase. The high bending energy of the rod microdomains results in lamellae with long persistence
lengths and grain boundaries defined by broken lamellae. Changes in domain spacing with temperature
suggest rod rearrangements within the lamellar phase.
Introduction
copolymers are promising for control on this length
scale, and these materials are being explored as a
possible route to influence the morphology of multicom-
ponent active layers in LEDs19-21 and photovoltaics.22
Block copolymers have received a great deal of atten-
tion for their ability to self-assemble into a variety of
nanophase structures;1,2 this nanometer scale structure
creates many useful properties in rubbers, adhesives,
thin films,3 and nanopatterned surfaces.4 Much of the
work to date has focused on coil-coil block copolymers
where the molecular shape closely matches Gaussian
coil models. Nanoscale control and patterning of func-
tional block copolymers presents a new challenge due
to nonidealities in molecular conformation and mixing
interactions that are present in these materials. In a
large class of polymers the primary or secondary bond-
ing structure reduces the ability of the chain to undergo
conformational rearrangement such that the polymer
exhibits rigid chain behavior; typical rodlike polymers
include helical proteins and semiconducting polymers
with rigid π-conjugated backbones. As a consequence
of the highly functional nature of many rodlike poly-
mers, rod-coil block copolymers containing a rod block
and a model coil block are interesting for several
applications. Rod-coil block copolymers with amino
acid-based rod blocks have been suggested as models
for membrane structural proteins or DNA gels5-9 and
for use as artificial membranes.10 The structural and
rheological properties of these rod-coil block copolypep-
tides11 and their gels12 are also being studied. There is
also great interest in rod-coil block copolymers for use
in organic electronics, where the importance of the
interface between two materials with different work
functions is becoming increasingly apparent.13,14 Con-
trolling the active layer morphology and interfacial
structure in multicomponent devices on the 10 nm
length scale of an exciton diffusion length is critical to
optimizing device performance.15-18 Rod-coil block
Although self-assembly of rod-coil block copolymers
has potential as an elegant path to achieve the same
type of nanoscale structural control exercised in coil-
coil systems, the necessary predictive polymer thermo-
dynamics of these materials remains largely unknown.
In classical block copolymers, the conformational prop-
erties of both blocks are characterized by Gaussian
chain statistics, and the microphase structure is a com-
promise between minimizing the interaction energy be-
tween unlike blocks and the stretching of the Gaussian
coils.1,2 In rod-coil block copolymers the rod block has
an effectively infinite persistence length, and a single
orientational vector defines its conformation. The self-
assembly of rod-coil systems is further complicated by
the anisotropic interactions and liquid crystalline be-
havior of the rod block. The chain stretching, isotropic
Flory-Huggins interaction, and anisotropic rod interac-
tions all impact the free energy and equilibrium micro-
phase structure, and the interplay between these effects
creates equilibrium structure and thermodynamics that
are distinct from coil-coil block copolymer systems.
Theoretical studies have predicted a diversity of
structures in rod-coil diblock copolymers. The first
theoretical investigations of these materials used ana-
lytical free energy calculations and scaling relationships
to understand the microphase behavior and predicted
transitions between nematic, smectic A and C, bilayer,
and “puck” phases.23-27 Alternately, several works have
treated rod-coil block copolymers in the weak segrega-
tion limit through the use of a Landau expansion for
the free energy in terms of both compositional and
orientational order parameters.28-30 A transition from
isotropic to nematic was predicted at low coil fractions,
* Corresponding author: e-mail segalman@berkeley.edu.
10.1021/ma051468v CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/02/2005

10128 Olsen and Segalman
Macromolecules, Vol. 38, No. 24, 2005
whereas the block copolymer was predicted to transition
directly to a microphase-separated structure at high coil
fractions. Self-consistent-field theory (SCFT) has also
been applied to rod-coil block copolymers.31-35 These
studies predicted complex phase diagrams including
isotropic, nematic, and lamellar structures for a wide
range of compositions across the center of the phase
diagram. Arrowhead and bilayer microphases were also
predicted for rod-rich systems, while strip or puck
morphologies were favored in coil-rich systems.
Experimentally, rod-coil diblock copolymer systems
have been studied far less than their coil-coil counter-
parts. While these prior studies have demonstrated an
intriguing structural diversity in rod-coil block copoly-
mer systems, experimental phase diagrams are incom-
plete, and a solid understanding of thermodynamics in
these systems is lacking. A major difficulty in address-
ing these problems is the preparation of a suitable model
material. To make a comparison with simulations, a
model rod-coil block copolymer must be mapped onto
a coarse-grained theory that ignores molecular detail
on a scale smaller than the statistical segment length.
Both the rod and coil block must be chemically homo-
geneous on the length scale of a statistical segment, and
the coil must be long enough to have Gaussian chain
statistics. The rod should be nearly perfectly rigid: free
of structural defects or the possibility of folding and
conformational changes in the bulk. Both blocks should
have low polydispersity for the material to be easily
modeled, and it is also useful to have polymers with a
low enough Flory-Huggins parameter and weak enough
liquid crystalline interactions such that phase transi-
tions can be experimentally accessed.
A variety of approaches have been employed for the
preparation and study of rod-coil systems. Lamellar
and hexagonal morphologies were observed in oligo-
meric polypeptides,36,37 whereas only lamellar structures
were observed in higher molecular weight polymers.5-9
While the complex secondary and tertiary protein
structures provide interesting structural behavior, block
copolypeptide materials add extra complication beyond
a fundamental rod-coil system. Rod-coil oligomers
have received more attention as model materials since
small-molecule chemistry can be used to produce mono-
disperse samples of oligomeric molecules, and the small
size of the molecules may lead to weaker segregation
in these systems. However, rod-coil oligomers may not
self-assemble according to the same scaling behavior as
their longer polymeric analogues; in particular, the coil
block may be too short or fully extended and thus not
follow Gaussian chain statistics. Radzilowski et al.
observed strip and puck phases38-40 and that annealing
led to a disruption of ordering.38,39 Li and co-workers
observed a disordering transition with increasing tem-
perature in oligomeric rod-coil materials.41 Other re-
searchers have also observed smectic, hexagonal, ne-
matic, and isotropic phases.42-46 However, they found
domain sizes on the order of the contour length of the
polymer, suggesting the coils were fully extended.42,43
type liquid crystals.48 Because of the strongly segregated
nature of this system and the kinetic structural trapping
inherent in solvent casting, the equilibrium nature of
these structures is unclear. Work on protein-based,
oligomeric, and polymeric rod-coil systems has been the
subject of several reviews,8,52-55 and while these previ-
ous experimental studies have illustrated the rich
potential of rod-coil block copolymers for assembly into
a variety of morphologies, the underlying thermody-
namics remain largely unknown.
We report here a study of the structure and thermo-
dynamics of a weakly segregated rod-coil block copoly-
mer system with polymeric scaling properties. A model
system of monodisperse poly(alkoxyphenylene vinylene)
(PPV) rods and polyisoprene (PI) coils is prepared, and
side-chain functionalization of the PPV is used to control
the Flory-Huggins interaction parameter, liquid crys-
talline interactions, and rod-to-coil aspect ratio. The
structure and thermodynamics of the weakly segregated
rod-coil system are studied to produce an experimental
phase diagram showing transitions between lamellar,
nematic, and isotropic phases.
Experimental Methods
Materials. DMF was dried over 3 Å molecular sieves.
Benzene was purified by sparging with dried nitrogen followed
by passage through an activated alumina column (UOP A2
Alumina, 12-32 mesh). Isoprene was purified over CaH2
followed by Bu2Mg. Butanol was dried over CaH2. All other
reagents and solvents were used as received. For column
chromatography, Merck grade 9385 silica gel (230-400 mesh,
60 Å) was used. NMR spectra were acquired on a Bruker AVB-
300 magnet in CDCl3 as solvent.
Synthesis of 2,5-Di(2′-ethylhexyloxy)toluene, 1. PPV
monomer and polymer were synthesized according to Scheme
1. Etherification56 of methylhydroquinone was achieved by
dissolving 43.2 g of methylhydroquinone (0.341 mol) and 56 g
of KOH (0.998 mol) in 500 mL of absolute ethanol and
refluxing at 80 °C for 1 h. 177.5 mL of ethylhexyl bromide
(192.8 g, 0.998 mol) was added dropwise, and the mixture was
refluxed at 80 °C overnight. The reaction mixture was poured
out onto water, and the product was extracted with ether. The
product was cleaned using column chromatography. 1H NMR
δ: 0.89 (m, 12H, -CH3), 1.30 (m, 16H, -CH2-), 1.68 (m, 2H,
-CH-), 2.20 (s, 3H, Ar-CH3), 3.77 (t, 4H, -O-CH2-), 6.70
(m, 3H, ArH).
Synthesis of 2,5-Di(2′-ethylhexyloxy)-4-methylbenzal-
dehyde, 2. Formylation, as described in a previously published
procedure;57 was performed by mixing 124.39 g of molecule 1
(0.357 mol) with 178.5 mL of anhydrous CHCl3 (266.3 g, 2.231
mol) and 101.2 mL of dry DMF (95.5 g, 1.306 mol). Using a
dropping funnel, 142.8 mL of POCl3 (234.9 g, 1.532 mol) was
added dropwise while keeping the mixture below 40 °C. The
reaction was stirred for 1 h at ambient temperature and then
refluxed at 80 °C for 48 h. The product was poured out onto
ice, extracted into dichloromethane, and neutralized. The
product was purified by column chromatography. Overall yield
for the previous two steps was 79%, and no meta or ortho
formylation products were observed. 1H NMR δ: 0.88 (m, 12H,
-CH3), 1.32 (m, 16H, -CH2-), 1.73 (m, 2H, -CH-), 2.27 (s,
3H, Ar-CH3), 3.84 (d, 2H, -O-CH2- meta to aldehyde), 3.91
(d, 2H, -O-CH2- ortho to aldehyde), 6.81 (s, H, Ar-H meta
to aldehyde), 7.23 (s, H, Ar-H ortho to aldehyde), 10.41 (s, H,
-CHdO).
Polymeric rod-coil systems are more likely to scale
as the theoretical predictions, and studies of high
molecular weight systems have revealed novel and
intriguing phase behavior.47-51 For solution self-as-
sembled block copolymers with poly(hexyl isocyanate)
(PHIC) rods, wavy lamellar, zigzag, and arrowhead
morphologies were observed, and the molecular packing
was identified as similar to smectic C and smectic O
Synthesis of 2′,5′-Di(2′′-ethylhexyloxy)-4′-methyl-N-
benzylideneaniline, 3. To produce the imine,58 7.23 g of
aniline (77.6 mmol) was combined with 13.4 g of product 2
(35.6 mmol). The mixture was reacted at 60 °C under ∼10 Torr
vacuum for 2 h. This product was used without further
1
purification. H NMR δ: 0.80 (m, 12H, -CH3), 1.23 (m, 16H,
-CH2-), 1.62 (m, 2H, -CH-), 2.17 (s, 3H, Ar-CH3), 3.81 (m,
4H, -O-CH2-), 6.69 (s, H, -NdCH-Ar-H meta), 7.09 (m,

Macromolecules, Vol. 38, No. 24, 2005
Rod-Coil Block Copolymers 10129
Scheme 1. Synthesis of DEH-PPV by Seigrist Polycondensation
Scheme 2. Synthesis of PPV-b-PI Copolymer by Coupling of Anionically Grown PI with Aldehyde-Terminated
PPV
3H, dN-Ar-H ortho and para), 7.25 (m, 2H, dN-Ar-H meta)
7.47 (s, H, -NdCH-Ar-H ortho), 8.78 (s, H, Ar-CHdN).
ular weight and polydispersity. PPV (0.4 equiv) was dried
under vacuum at room temperature overnight and then
dissolved in benzene and injected to terminate the living
polyisoprene.41 The polymers were allowed to react for 30 min,
and then the remaining polyisoprene was terminated with
butanol. The polymer was precipitated in methanol, and excess
homopolymer was removed using column chromatography.
Synthesis of Poly(2,5-di(2′-ethylhexyloxy)-1,4-phe-
nylene vinylene) (DEH-PPV), 4. PPV was synthesized by
Seigrist polycondensation, as previously described.22,58 This
reaction allows for the preparation of PPV with a narrow
molecular weight distribution. The reaction temperature
determines the molecular weight of the polymer, and the
product polymer has a well-defined chemical structure con-
taining only trans-linked units. 3 equiv (14.25 g) of potassium
tert-butoxide was added to 1.8 L of dry DMF and heated to 30
°C. Compound 3 was added dropwise and allowed to react for
30 min. The reaction was poured out onto acidified water and
stirred for 48 h to hydrolyze the unreacted imines. Product
was collected, neutralized, and fractionated with a chroma-
tography column. The fractionated product was precipitated
into methanol at -20 °C. The overall yield was 4.97 g (39.0%).
Polarized optical microscopy and differential scanning calo-
rimetry demonstrated that the homopolymer was solid below
59 °C, liquid crystalline until 225 °C, and isotropic above that
temperature. 1H NMR δ: 0.89 (m, 12(n + 1)H, -CH3), 1.45
(m, 16(n + 1)H, -CH2-), 1.81 (m, 2(n + 1)H, -CH-), 2.24 (s,
3H, Ar-CH3), 3.96 (m, 4(n + 1)H, -O-CH2-), 7.20 (s, 2nH,
-CHd), 7.53 (s, 2(n + 1)H, Ar-H), 10.43 (s, H, -CHdO).
Synthesis of Poly(2,5-di(2′-ethylhexyloxy)-1,4-phe-
nylene vinylene)-block-poly(1,4-isoprene) (PPV-b-PI), 5.
PPV-b-PI block copolymers were synthesized according to
Scheme 2. Anionic polymerization of isoprene was performed
in benzene using sec-butyllithium (1.4 M in cyclohexane) as
initiator to produce predominantly 1,4 addition (∼93%) of
isoprene. The reaction was allowed to proceed at room tem-
perature, and an aliquot of polyisoprene homopolymer was
collected and terminated with dry butanol to measure molec-
Gel Permeation Chromatography (GPC). The molecular
weights of polyisoprene homopolymers were measured on a
Waters 2690 GPC with a Viscotek refractive index detector
and a low-angle light scattering detector using a 670 nm laser.
Light scattering was used for absolute molecular weight
determination without the use of calibration standards. The
polystyrene equivalent molecular weights of the PPV ho-
mopolymer and PPV-b-PI block copolymers were measured on
a Waters 150-CV GPC with a M486 tunable absorbance
detector at 450 nm (near the absorbance peak of PPV) and a
refractive index detector. GPC of one of the polymers and its
precursor homopolymers is shown in Figure 1. The chromato-
graph of the polymer as-synthesized (not shown) has two
peaks: one for excess polyisoprene homopolymer and one for
the block copolymer. This homopolymer was removed in the
following purification step. Standard-independent molecular
weights and polydispersities for the PI block were calculated
using refractive index and light scattering detectors, and the
polydispersity of all PI samples was less than 1.05. The
number-average molecular weight of the PPV block was
determined using NMR end-group analysis by comparison of
the aldehyde proton peak with the OCH2 peak. Molecular
weights for each block are shown in Table 1.
Because of the complex hydrodynamics of rodlike mol-
ecules59,60 and the lack of a suitable GPC standard, determi-
nation of the absolute molecular weight and polydispersity of

10130 Olsen and Segalman
Macromolecules, Vol. 38, No. 24, 2005
Polarized Optical Microscopy (POM) and Depolarized
Light Scattering (DPLS). An Olympus BX51 microscope
with crossed polarizers, and an Instec HCS302 heat stage was
used to image samples. Samples were pressed between two
glass slides and placed on an in situ argon-purged heat stage
for optical imaging. Samples were annealed into the isotropic
phase, cooled, and then heated at a rate of 0.2 °C/min to
determine the temperature at which birefringence disap-
peared. Samples for DPLS were prepared by annealing a
sample of polymer at 120 °C overnight in a vacuum oven to
form a 0.3 mm thick disk. The disk of polymer was then sealed
between two quartz windows and annealed for an additional
15 min at 120 °C under a nitrogen atmosphere. Depolarized
light scattering was measured using a previously described
instrument.62 The sample was heated and cooled three times,
and data to determine transitions were taken from the first
cooling and second heating pass. Reported values of I/I0 were
normalized by the transmission of the sample.
Differential Scanning Calorimetry (DSC). Samples
were also characterized using a TA Instruments 2920 DSC to
investigate thermal transitions. Samples were heated to 220
°C at 10 °C/min to erase any thermal history, then cooled to
20 °C at 10 °C/min, and reheated to 270 °C at 5 °C/min.
Figure 1. GPC of block copolymer PPVbPI-42 and its precur-
sor polymers. PPV-b-PI was synthesized by linking a polyiso-
prene anion to PPV with low polydispersity. The increased
molecular weight and low polydispersity of the block copolymer
indicates the success of the coupling reaction.
Results and Analysis
Microphase Structure. Nondimensionalization of
the free energy expression in a rod-coil block copolymer
system yields four independent thermodynamic quanti-
ties that parametrize phase space:31,35 øN, the Flory-
Huggins interaction parameter; µN, the Maier-Saupe
parameter; φ, the coil volume fraction; and ν, the
relative block size. The relative coil-to-rod block size is
defined as bN1/2/Na, where b is the statistical segment
length of the coil block and N is the number of volu-
metric units. For the calculation of N, φ, and a, the
volumetric unit is defined as the volume of a single
isoprene unit, making the number of volumetric units
in the coil block equal to its number-average degree of
polymerization. The rod’s segment length, a, is defined
as the length of rod that will fit into a volumetric unit,
so a is much shorter than both the contour length and
the Kuhn length of the rod. The number of volumetric
units, the coil fraction, and the relative block size are
estimated on the basis of molecular weights and ho-
mopolymer densities for each block. Using the density
of PPV, average bond lengths from the literature,63 and
assuming a cylindrical PPV shape, the effective molec-
ular diameter is calculated to be 1.09 nm. The contour
length of a PPV rod is 6.61 nm, and a is 0.135 nm. The
statistical segment length of polyisoprene, b, is 0.652
nm,64 and this value is used to calculate the radius of
gyration for the coil and ν. The contour lengths of the
block copolymers are estimated in Table 1 for a pre-
dominantly cis-PI with 93% 1,4 addition. Calculated
values of all molecular parameters are tabulated in
Table 1.
PPV by GPC is difficult. The polystyrene (PS) equivalent PDI
for the PPV block was measured to be 1.17. For rod polymers
the PS equivalent PDI is a gross overestimate of the actual
polydispersity because the hydrodynamic volume of the rod
molecules increases much more rapidly with molecular weight
than the hydrodynamic volume of coil polymers. This has an
effect of stretching the high molecular weight tail of the
distribution in rod polymers, increasing their polydispersity.
Therefore, 1.17 is an upper limit on the true polydispersity of
the rod block.
Density. The density of PPV was found to be 0.988 ( 0.001
g/cm3 using a density gradient column at 23 ( 0.1 °C with
methanol and ethylene glycol as solvents. For PI, a density of
0.90 from the literature61 was used.
Small- and Wide-Angle X-ray Scattering (SAXS and
WAXS). Samples for SAXS and WAXS were prepared by
annealing polymers in a vacuum oven at 120 °C for 24 h to
form 1 mm thick disks and then sealing the sample between
Kapton windows. SAXS and WAXS experiments were per-
formed on beamline 1-4 of the Stanford Synchrotron Radiation
Laboratory (SSRL). The beamline was configured with an
X-ray wavelength of 1.488 Å and focused to a spot size of ∼0.5
mm diameter. A single quadrant of two-dimensional scattering
patterns was collected on a CCD detector with a 100 mm
diameter. The two-dimensional profiles were radially averaged
and corrected for detector null signal, dark current, and empty
cell scattering. SAXS profiles were converted to absolute
intensities using a polyethylene standard calibrated at NIST,
and WAXS profiles were left in arbitrary units.
Transmission Electron Microscopy (TEM). Samples for
TEM were prepared by spin-coating films of ∼100 nm thick-
ness from 2% toluene solution onto silicon nitride windows.
All samples were annealed under vacuum at 80 °C for 12 h
and cooled slowly to room temperature. Polymers were stained
by exposure to the vapor from a 2% OsO4 solution for 4 h or
the vapor of a 0.5% RuO4 solution for 30 min. Bright-field
images were taken within 3 days after staining on a JEOL
200CX microscope at the National Center for Electron Micros-
copy operating at an accelerating voltage of 200 kV.
Thermodynamic repulsion between rod and coil blocks
leads to microphase separation in rod-coil copolymers
analogous to the classical phase behavior of coil-coil
block copolymers. The model rod-coil block copolymers
are investigated with X-ray scattering and electron
Table 1. PPV-b-PI Block Copolymers
PPV Mn
(g/mol)
PI Mn
(g/mol)
rod length
(nm)
coil Rg
(nm)
block copolymer contour
length (nm)
block copolymer
N
φ
ν
PPVbPI-42
PPVbPI-59
PPVbPI-72
PPVbPI-89
3545
3545
3545
3545
2367
4938
82
116
166
421
0.42
0.59
0.72
0.89
0.533
0.448
0.374
0.235
6.61
6.61
6.61
6.61
1.57
2.21
2.90
5.15
22.3
37.5
8129
59.8
25483
173.8

Macromolecules, Vol. 38, No. 24, 2005
Rod-Coil Block Copolymers 10131
coil fraction of 0.85, the polymer shows no scattering
peaks throughout the accessible q range.
TEM yields additional information about the mor-
phology of these lamellae. Samples stained with OsO4
show dark polyisoprene block microdomains and light
PPV block lamellae, as shown in Figure 3a-c. The
micrographs clearly indicate qualitative variations in
lamellar structure as the composition of the block
copolymer is changed. The approximately symmetric
block copolymer PPVbPI-42 has grains of lamellae
oriented parallel and perpendicular to the plane of view.
Lamellae oriented perpendicular to the surface appear
as alternating light and dark stripes whereas lamellae
oriented parallel to the surface appear as solid gray
regions. Imaging with a large tilt angle reveals lamellar
edges near the boundary of the parallel regions, and
examination of several grains of parallel lamellae shows
many of them have visible defect structures where a
single lamellar layer is oriented perpendicular to the
surface in the middle of the grain. These structures both
indicate the lamellar nature of the gray regions. The
orientation of perpendicular lamellae is correlated
across 3-10 rod microphases, and rod microphases have
a very long persistence length. In contrast, the lamellae
in coil-coil block copolymers show a great deal of
curvature.1,4 Because of the preferential alignment of
the rod block directors, grains of rodlike mesogens have
a large elastic energy of bending. In rod-coil block
copolymers this elasticity will impart rigidity to the rod
microphase, resulting in a free energy penalty for
bending a lamellae and creating structures with a long
persistence length. TEM also illustrates that defects and
grain boundaries are characterized by discontinuities
in the rod microphase. Lamellar structures break at
Figure 2. SAXS of all block copolymers at 40 °C. PPVbPI-89
is microphase disordered, showing no peaks. The three block
copolymers with smaller coil volume fractions are microphase
separated with peaks at integer multiples of their respective
q*, indicating lamellar structure. In the case of PPVbPI-42,
higher order peaks are outside of the experimentally accessible
q range, but the lamellar structure is confirmed by electron
microscopy.
microscopy to establish the microphase structure and
structural transitions. SAXS curves for each polymer
at 40 °C are shown in Figure 2; the polymers organize
into lamellae as indicated by peaks occurring at integer
multiples of q*. Peaks remain at integer multiples of
q* upon heating, demonstrating that the lamellar
structure persists throughout the microphase-separated
region. The lamellar region is surprisingly wide, extend-
ing to a coil volume fraction approaching 0.85. Above a
Figure 3. TEM of block copolymer films on silicon nitride windows. Staining with OsO4 reveals the lamellar structure of PPVbPI-
42 (a), PPVbPI-59 (b), and PPVbPI-72 (c). Regions with little or no phase contrast are a result of lamellar orientation parallel or
nearly parallel to the substrate. The lamellar structures have remarkably high persistence lengths and orientational correlations,
and grain boundaries are characterized by the breaking of lamellar structures. PPVbPI-72 stained with RuO4 (d) provides a
complementary picture of the lamellar structures in this polymer.

10132 Olsen and Segalman
Macromolecules, Vol. 38, No. 24, 2005
grain boundaries, and Y-shaped dislocations are only
observed in the coil-rich regions. Most defects in coil-
coil copolymer lamellar structure involve curvature or
junctions between individual microphases, and these
types of defects carry a large penalty due to bending
elasticity in rod-coil block copolymers. Breaking a
lamellae and generating additional interface is favored
in a rod-coil system over the curvature imposed by a
defect, resulting in broken rod microphases at grain
boundaries. It should be noted that the domain spacing
in this polymer was too small to allow for detection of
higher order SAXS peaks within the experimental q
range (Figure 2), but TEM confirms the lamellar
morphology.
As the coil fraction is increased, the grain structure
qualitatively changes. A slightly coil-rich sample (PPVb-
PI-59) is characterized by smaller grains of lamellae
that appear intertwined with one another. Regions of
lower and higher contrast indicate lamellae oriented at
different angles to the surface. Tilting the sample
accentuates some structures while blurring others as
lamellae are rotated through a variety of angles with
respect to the plane of view. Lamellar orientation is
correlated across groups of 3-5 lamellae, and intersec-
tions between lamellae are again characterized by
breaks in the PPV phase. With a further increase in coil
fraction (PPVbPI-72), the lamellar orientation becomes
more strongly correlated and grain size grows, produc-
ing grains as wide as 15-20 parallel lamellae. The
lamellae show very little bending over several hundred
nanometers and always break at grain boundaries,
demonstrating the large bending modulus associated
with liquid crystalline order in the rod microphase.
RuO4 staining provides a complementary image of
PPVbPI-72 by preferentially staining the PPV phase.
RuO4 has a complicated staining chemistry,65 but test
samples of blended PPV and PI suggest that the PPV
phase is stained more rapidly, appearing as the dark
phase in Figure 3d. Under these staining conditions
contrast is much lower and the overlap of lamellae
becomes visible. The pattern of overlapping lamellae
indicates that the width of the lamellae is less than the
sample thickness and that the same breaking of rod
microphases at grain boundaries observed in the plane
of view also occurs throughout the depth of the sample.
As the block copolymers are heated, they undergo a
transition from an ordered lamellar phase into a mi-
crophase disordered regime, as shown in SAXS intensity
plots for a representative polymer in Figure 4. The
order-disorder microphase transition (ODT) is charac-
terized by a drop in intensity of the primary scattering
peak and the disappearance of higher order peaks with
increasing temperature. This transition temperature
can be further quantified through a discontinuity in the
plot of inverse primary peak intensity vs inverse tem-
perature, as shown in Figure 5. This discontinuity
occurs at the same temperature as the disappearance
of higher order peaks and is shown in Table 2. The
chemical link between blocks should generate weak
concentration correlations, and the polymer should show
a weak primary peak due to this correlation even in the
disordered regime.66 In this case the PPV and PI blocks
have low contrast, so the peak intensity decreases to
such an extent that it is lost in the baseline noise at
high temperatures. As a result, the number of intensity
measurements that can be extracted above the ODT is
limited.
Figure 4. Small-angle X-ray scattering curves for PPVbPI-
72 in the vicinity of the order-disorder transition. Curves are
offset along the vertical axis for clarity. The spacing of peaks
at integer multiples of q* indicates a lamellar structure.
Higher order peaks disappear above 120 °C, indicating the
order-disorder transition.
In the case of a rod-coil block copolymer, the nature
of the ODT is different than in a coil-coil copolymer.
In a coil-coil material the disordered phase is spatially
homogeneous and liquidlike, with no possibility of
orientational order.67 Although density fluctuations or
micelles may persist, the ensemble average density is
spatially invariant. In a rod-coil block copolymer, the
ODT is still a transition from an ordered to a spatially
homogeneous phase, but orientational order may persist
in the disordered state as driven by rod-rod interac-
tions, implying that there may be multiple disordered
phases. More succinctly, in coil-coil materials the ODT
breaks both orientational and positional symmetry, but
in rod-coil materials the ODT need only break posi-
tional symmetry.
Liquid Crystalline Order. Above the ODT, it is
possible for the rod-coil block copolymer to be in
multiple disordered states: a nematic (oriented) or an
isotropic (unoriented) phase, as illustrated in Figure 6.
Optical techniques provide complementary information
to SAXS that allows this shorter scale order to be
investigated. Optically anisotropic phases, such as the
nematic and lamellar phases, exhibit birefringence
whereas the isotropic phase does not, and these tech-
niques have been widely used to provide information
regarding liquid crystalline phases and phase transi-

Macromolecules, Vol. 38, No. 24, 2005
Rod-Coil Block Copolymers 10133
of pure PPV. Around a coil fraction of 0.72 the NI
transition and ODT converge, suggesting the possibility
of two regions in the phase diagram: one region where
the isotropic phase transitions to the nematic phase and
a second region where the isotropic phase transitions
directly to an ordered phase. Very coil-rich block co-
polymers are isotropic throughout the accessible tem-
perature range.
Depolarized light scattering is used to further quan-
tify these transitions, and a representative curve, shown
in Figure 8, exhibits two temperature-dependent transi-
tions. At low temperatures, the sample is characterized
by extremely low transmission due to strong scattering
(opacity) but strong birefringence. Although the DPLS
signal is still strong in this region, the error in the
quantitative value is large due to errors inherent in
accurately normalizing a small signal to account for the
transmission through a strongly scattering sample. The
strong birefringence, polarized optical microscopy tex-
ture, and SAXS structure all confirm a smectic-like
morphology at low temperatures. As the sample is
heated, the polymer reaches a lower temperature tran-
sition at approximately the same temperature as the
ODT observed in SAXS, as shown in Table 2. At the
lower temperature transition, transmission increases
rapidly to a high level. There is still strong birefringence
above this transition, and the samples show all the
optical characteristics of a nematic phase.68 The DPLS
signal drops with further increasing temperature until
a higher temperature transition point is reached at
which the birefringence disappears entirely. This higher
temperature transition occurs at the same temperature
as the NI transition identified from polarized optical
microscopy. Above the second transition, the sample is
characterized by high transmission and low birefrin-
gence, indicative of an isotropic disordered phase. For
coil volume fractions greater than 0.85, the sample
shows isotropic characteristics throughout the entire
experimentally accessible range. The observed sequence
of transitions from smectic-like to nematic to isotropic
represents characteristic behavior for a mesogenic
system, and hysteresis observed in the DPLS curve
suggests that both transitions are weakly first order,
consistent with results on other polymeric1 and me-
sogenic68 systems. DSC was also used to probe transi-
tions in the block copolymers. Unfortunately, the changes
in enthalpy and heat capacity associated with these
transitions are relatively subtle. As a result, the DSC
data are far less illustrative than the combination of
scattering and microscopy.
Figure 5. Inverse intensity of peak heights in the vicinity of
the order-disorder transition. The discontinuity in slope of
the inverse I vs inverse T curve indicates the order-disorder
transition (ODT). Above the ODT the curve is linear, and
extrapolation to zero inverse intensity gives the spinodal
temperature.
Table 2. Phase Transition Temperatures (°C) in PPV-b-PI
Copolymers
optical clarity
(DPLS)
NI transition
(microscopy)
block copolymer
ODT
PPVbPI-42
PPVbPI-59
PPVbPI-72
131
149
123
225
195
134
110
120
120
tions68 as well as block copolymer microphase and grain
structure.62
Below the ODT the rod-coil block copolymers exhibit
a smectic-like pattern through polarized optical micros-
copy, while slightly above the ODT the polymers display
a nematic texture, as shown in Figure 7. Further
heating results in complete loss of birefringence, indi-
cating the transition to the optically inactive isotropic
phase. The nematic-isotropic (NI) transition tempera-
ture, given in Table 2, depends strongly on coil volume
fraction. For symmetric block copolymers, the nematic
region is wide. As coil fraction decreases toward rod-
rich copolymers, the NI transition temperature in-
creases rapidly, approaching the transition temperature
WAXS probes the molecular spacing between rods
within a lamellar layer. Representative WAXS profiles
as a function of temperature are shown in Figure 9. The
Figure 6. Illustration of molecular packing in possible rod-coil block copolymer microphases. From left to right the phases are
bilayer, smectic A-like monolayer, smectic C-like monolayer, nematic, and isotropic. While regions of the lamellar, nematic, and
isotropic phases can be clearly demarcated on the phase diagram, discerning between the lamellar structures is more difficult.
On the basis of domain spacing, the two monolayer phases are the most probable lamellar morphologies, with transitions in rod
tilt and chain stretching occurring as the polymers are heated through the order-disorder transition.

10134 Olsen and Segalman
Macromolecules, Vol. 38, No. 24, 2005
Figure 8. Depolarized light scattering and transmission of
PPVbPI-72. Calculated values of I/I0 are normalized by trans-
mission. The polymer exhibits three phases: a birefringent
phase with low transmission at low temperatures, a birefrin-
gent phase with moderate transmission from 123 to 137 °C,
and a nonbirefringent phase at high temperatures. The low-
temperature phase corresponds to the lamellar region identi-
fied by SAXS and TEM, the middle phase is nematic, and the
high-temperature phase is isotropic.
Figure 7. Optical micrographs of liquid crystalline textures
in PPVbPI-42. At 120 °C (top) a nematic phase is observed,
while at 80 °C (bottom) the material exhibits a smectic phase.
Heating above the nematic region results in the disappearance
of all optical texture, indicating the isotropic region.
large peak at 6 nm-1 corresponds to lateral rod-rod
spacing of 1.05 nm, in reasonable agreement with the
value of 1.09 nm calculated on the basis of bond length
and density arguments. The slight decrease in q from
6.13 to 5.81 nm-1 over a range of 160 °C corresponds to
a change in intermolecular spacing from 1.02 to 1.08
nm, and this small change is attributed to thermal
expansion. Unlike the other three materials, PPVbPI-
89 shows only a weak peak at ambient temperatures
that decreases in intensity upon heating, consistent with
its disordered and isotropic nature throughout the
experimentally accessible temperature region.
The gradual decrease in the intensity of the rod-rod
spacing peak is consistent with the preservation of
short-range order in the block copolymers across the
ODT and NI transitions. It is expected that nematic
liquid crystals show short-range smectic-like order, and
the arrangement of rodlike molecules in an isotropic
fluid is still governed by anisotropic short-range packing
constraints.68 As the block copolymer becomes disor-
dered, the increased mixing of the rod and coil blocks
disrupts the regularity of the rod-rod spacing. The
presence of a rod-rod spacing peak indicates the
persistence of short-range correlations between rods
above the ODT and NI transition. As the correlations
gradually decay with increasing temperature, the in-
tensity of the peak continuously decreases. At high coil
Figure 9. Wide-angle X-ray scattering of PPVbPI-72. The
primary peak around 6 nm-1 corresponds to the spacing
between rod blocks. The intensity of this peak decreases with
increasing temperature but shows no discontinuity at the
transition temperatures identified by SAXS, optical micros-
copy, or depolarized light scattering. The dip and rise at 3.6
nm-1 are a result of error in the empty cell scattering
subtraction, as the Kapton windows of the empty cell show a
large feature at that q value.
volume fractions the disordering effect of mixing rod and
coil blocks dominates to such an extent that the rod-
rod spacing is disrupted even at low temperatures.
Transitions in the Ordered Phase. As with coil-
coil block copolymers, the domain spacing of rod-coil
copolymers is expected to be a function of the polymer’s
radius of gyration and temperature, and the domain
spacing provides clues as to the relative stability of
different ordered phases. In rod-coil block copolymers,
the lamellar phase may consist of several different rod
arrangements. Rods may form bilayers or monolayers,
as shown in Figure 6. Furthermore, the rods may be

Macromolecules, Vol. 38, No. 24, 2005
Rod-Coil Block Copolymers 10135
Figure 11. Peak positions for first- and second-order scat-
tering in PSbPPV-72. Error bars reflect uncertainty in back-
ground subtraction in these weakly scattering samples. An
obvious decrease in domain spacing is seen and may be
attributed to subtle changes in liquid crystalline structure.
Figure 10. Lamellar layer spacings in ordered block copoly-
mers. The increase in domain spacing with temperature
suggests a transition in lamellar morphologies.
oriented perpendicular to the rod-coil interface or tilted
at some angle off of perpendicular.
affected by background scattering, so the error bars
reflect more uncertainty in this direction. Ideally, the
first- and second-order peaks ought to track exactly with
temperature, and as shown in Figure 11, the error bars
overlap, but complete agreement is not present. At this
early stage, we do not understand the origin of this
discrepancy. Conservative comparisons of the shifts in
the first- and second-order peaks as a function of
temperature, however, result in an estimate of domain
spacing changes of at least 5 nm. A better understand-
ing of the detailed structure of the lamellae and the
chain shape as a function of temperature is the subject
of ongoing work.
Weak Segregation Limit (WSL) Phase Diagram.
Using the aggregate data from scattering, TEM, DPLS,
optical microscopy, and accompanying analysis, a phase
diagram for weakly segregated rod-coil block copoly-
mers is assembled, as shown in Figure 12. At low
temperatures, the polymers form lamellar phases. Upon
heating, the lamellae disorder into a nematic phase. The
nematic region is widest for large rod fractions, and it
appears to pinch off as the coil fraction is increased.
After further heating the liquid crystalline clearing
temperature is reached and the polymers transition into
the isotropic phase. Polymers with the largest rod
fractions have a broad nematic region and high liquid
crystalline clearing temperature, consistent with the
behavior of the PPV homopolymer. The polymer with
the largest coil fraction is isotropic throughout the
experimentally observable range, consistent with the PI
homopolymer behavior.
The phase diagram for this PPV-b-PI system com-
pares favorably with theoretical calculations by Landau
expansions and SCFT. The Landau expansion calcula-
tions, valid in the weak segregation limit, predict
transitions from isotropic to nematic at lower coil
fractions and transitions directly to a microphase-
separated structure at higher coil fractions.28,29 The
theory also predicts a wide nematic region at low coil
fractions and a broad isotropic region at high coil
regions, in good agreement with experimental results.
The experimentally observed pinching off of the nematic
region with increasing coil fraction suggests that the
theoretically predicted direct isotropic to lamellar tran-
The relative stability of these lamellar phases is the
subject of several theoretical predictions. Bilayers mini-
mize the unfavorable interaction between the blocks and
may be preferred in highly segregated regimes.41 Se-
menov and co-workers predict, however, that since the
bilayer phase affords less interfacial area to the coil,
the chain must stretch more to adopt this configura-
tion.23,24 For this reason, the bilayer is not predicted to
occur in the weakly segregated limit or in block copoly-
mers with large coil fractions.23,31 Experimentally, it is
difficult to ascertain exact thicknesses of individual PPV
lamellae from our TEM images due to broadened
interfaces, which may result from both staining and
interpentration in this weakly segregated regime. Com-
parison of the lamellar spacing (obtained from SAXS)
to block copolymer dimensions (Table 1) indicates that
either the rods are arranged in greatly tilted bilayers
or in monolayers. In either monolayers or bilayers, the
contour length of the block copolymer is greater than
the domain spacing, so the coils are never fully ex-
tended. No transition between bilayer and monolayer
arrangements (as would be indicated by a discontinuity
in domain spacings observed in X-ray scattering or a
transition in DSC) was observed.
Within the lamellar phase, the rod blocks may also
assume tilted or perpendicular orientations. In fact,
several theoretical works suggest rods tilted relative to
the lamellar normal (smectic C) will reorient parallel
to the normal (smectic A) as temperature is in-
creased.31,35 This reorientation will result in changes in
domain spacing as the lamellar distance increases to
accommodate the rod’s length, and the coil rearranges
to accommodate a loss in interfacial area. As seen in
Figure 10, PPVbPI-42 and PPVbPI-59 domains do not
appear to change in size with temperature below the
microphase disorder temperature. From the scattering
spectra of PSbPPV-72 (Figure 4), however, the first- and
second-order peak positions appear to be temperature
dependent, as shown in Figure 11. As the microphase
disorder transition is approached, the intensity of the
first-order peak dramatically decreases, and errors in
background subtraction become substantial, as reflected
in the error bars for the data from PSbPPV-72. In
particular, the low q side of the peak appears to be more

10136 Olsen and Segalman
Macromolecules, Vol. 38, No. 24, 2005
due to the highly segregated nature of these systems
or to the effect of lyotropic as opposed to thermotropic
self-assembly.
Most importantly, the phase behavior of our weakly
segregated polymeric system differs from previous
thermodynamic observations, all of which were con-
ducted on shorter rod-coil systems. Li et al. observed
lamellae across a wide range of coil fractions in a lower
molecular weight system based on PPV and PI. Their
material also exhibited an ODT, but this temperature
corresponded to the liquid crystalline clearing temper-
ature and a separate nematic region was not observed.41
Radzilowski and Stupp synthesized a rod-coil block
copolymer with an oligomeric rod attached by a flexible
spacer to a short coil.38-40 They found thermodynamic
transitions between an ordered and disordered state
similar to those observed here; however, they saw a wide
region of puck and strip structures across the same
range of volume fractions where lamellae and disordered
phases were observed in this study. These differences
may be due to the length or aspect ratio of the rod block.
Microphases of larger rod blocks are expected to have a
higher bending energy, making them less able to ac-
commodate curvature and reduce chain crowding in a
puck or strip phase. The impact of aspect ratio on phase
behavior can be understood on a scaling law basis25,27
by the greater interfacial area per chain allowed in the
DEH-PPV system, resulting in a reduced force for
chain contraction. Since the formation of pucks and
strips is theoretically driven by reducing the entropy of
chain stretching, a high aspect ratio system is expected
to favor pucks. In terms of molecular parameters,35
Radzilowski and Stupp’s polymers have a greater
monomer unit length in the rod block (smaller rod
diameter) than the side-chain functionalized PPV, giv-
ing them a smaller value of ν. Other authors have also
observed nonlamellar and nonsmectic phases in short
rod-coil block oligomers.42-45 A direct comparison with
the oligomer studies to determine whether puck and
strip phases are stable in polymeric materials would
require producing PPV-b-PI polymers of higher molec-
ular weight to match ν and obtain polymers that are
ordered across the entire compositional range of inter-
est; this is the subject of ongoing research.
Figure 12. Experimental phase diagram for rod-coil block
copolymers in the weak segregation limit. In the above phase
diagram, rod length is kept constant while the coil length is
changed. As a result, the geometrical factor, ν, ranges from
0.3 to 0.7 in the above plot. For coil fractions below ∼0.85,
polymers transition from an isotropic to nematic to lamellar
structure upon cooling, and with increasing coil fraction the
nematic region becomes increasingly narrow. For coil fractions
above 0.85, the isotropic phase spans the entire experimentally
accessible range. The lamellar region potentially contains a
variety of layered structures, including smectic A-like and
smectic C-like regions.
sition would be observed in a more strongly segregated
system. These results indicate that above the ODT
anisotropic interactions between rods dominate the free
energy, creating a nematic phase. Sufficiently large coil
fractions modulate this rod-rod interaction, lowering
the NI transition temperature, and for coil fractions
greater than 0.85, the isotropic phase is the only
experimentally accessible one. Considering the highly
miscible nature of this system, agreement between
experiment and the WSL calculations is not surprising.
These experimental results also agree qualitatively
with the results of one-dimensional self-consistent-field
theory. The calculations of Matsen and Barett31 and
Pryamitsyn and Ganesan35 qualitatively predict the
shape of the isotropic, nematic, and lamellar regions.
The experimental result bears less resemblance to the
2D calculations performed on systems with lower values
of ν (higher rod aspect ratio)35 in which Pryamitsyn and
Ganesan predicted strip or puck phases at high coil
fractions, lamellar structures for more symmetric poly-
mers, and bilayer or arrowhead structures for low coil
fractions. These more complex phases were not experi-
mentally observed, possibly due to the weakly segre-
gated nature of our system or the difference in ν
between the experiment and simulation.
The PPV-b-PI system showed some similar structures
to those previously observed in high molecular weight
rod-coil systems. Polymeric polypeptide-coil copoly-
mers were5-9 observed to form only lamellar morphol-
ogies across a range of coil fractions similar to this
study. In addition, our smectic-like lamellar phases are
similar to those seen in mesogen jacketed block copoly-
mers51 and solution self-assembled high molecular
weight rod-coil systems;47,48 both are characterized by
the formation of layers, rod tilting, and locally planar
interfaces between the microdomains. However, we do
not observe the more unusual phases seen in the
solution self-assembled systems. The difference may be
Conclusions
A series of monodisperse rod-coil block copolymers
were prepared that could be parametrized in terms of
polymeric theories, making it useful for studying fun-
damental phase behavior in rod-coil systems. Because
of thermodynamic compatibilization of the rods and
coils, these polymers are in the weak segregation limit,
and order-disorder and nematic-isotropic transitions
could be accessed. The polymers were characterized by
scattering and microscopy to identify the ordered struc-
ture and phase transitions. At high temperatures and
coil fractions, the polymers exhibit an isotropic phase,
with transitions to a broad nematic region followed by
lamellar (smectic-like) phases with decreasing temper-
ature. Lamellar grains are characterized by long per-
sistence lengths and broken lamellae at grain bound-
aries due to the high elastic energy of bending in the
rod microphase. At some volume fractions, there is a
change in domain spacing with temperature which
suggests a reorientation of the rods within the lamellae.
The experimental phase diagram derived from these
observations is in qualitative agreement with weak
segregation limit theories of rod-coil block copolymers

Macromolecules, Vol. 38, No. 24, 2005
Rod-Coil Block Copolymers 10137
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and one-dimensional SCFT calculations. The results of
this study are promising for the use of self-assembly in
rod-coil systems. By employing weakly segregated
materials, isotropic, nematic, and lamellar phases can
be self-assembled from functional rodlike materials such
as semiconducting polymers or biomaterials, offering a
level of structural control that will be useful in their
application.
Acknowledgment. We gratefully acknowledge sup-
port from the ACS-Petroleum Research Fund and the
Department of Energy Office of Basic Energy Sciences
through the Plastic Electronics Program at Lawrence
Berkeley National Lab (LBNL). SAXS and WAXS
experiments were performed at the Stanford Synchro-
tron Radiation Laboratory, a national user facility
operated by Stanford University, and TEM experiments
were performed at the National Center for Electron
Microscopy at LBNL, both supported by the Department
of Energy, Office of Basic Energy Sciences. We gratefully
acknowledge helpful discussions with Nitash Balsara
and Georges Hadziioannou. We also thank Enrique
Gomez, John Pople, and Jean Fre´chet for use of equip-
ment and experimental assistance. B. D. Olsen grate-
fully acknowledges the Fannie and John Hertz Foun-
dation for a graduate fellowship.
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