Hydrogen-bonded aggregates of oligoaramide-poly(ethylene glycol) block copolymers

Article abstract of DOI:10.1021/ma100501j

Rod-coil copolymers with an oligomeric rod aggregate on a nanometer length scale, which is important for many applications like e.g. organic photovoltaics. However, this aggregation behavior and the driving forces such as hydrogen bonding and π-π interact

Full text of DOI:10.1021/ma100501j

4978 Macromolecules 2010, 43, 4978–4985
DOI: 10.1021/ma100501j
Hydrogen-Bonded Aggregates of Oligoaramide-Poly(ethylene glycol)
Block Copolymers
Anne Bohle,† Gunther Brunklaus,† Michael R. Hansen,† Tobias W. Schleuss,‡
Andreas F. M. Kilbinger,‡ Jens Seltmann,§ and Hans W. Spiess*,†
†Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany,
‡
€
Johannes Gutenberg-Universitat Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany, and
§Technische Universita€t Chemnitz, Strasse der Nation 62, D-09111 Chemnitz, Germany
Received March 5, 2010; Revised Manuscript Received April 20, 2010
ABSTRACT: Rod-coil copolymers with an oligomeric rod aggregate on a nanometer length scale, which
is important for many applications like e.g. organic photovoltaics. However, this aggregation behavior and
the driving forces such as hydrogen bonding and π-π interactions, as well as the role of side groups, are
not yet fully understood. Here, we investigated these noncovalent interactions in oligo(p-benzamide)-poly-
(ethylene glycol) (OPBA-PEG) copolymers using solid-state NMR supported by wide-angle X-ray scatter-
ing (WAXS), differential scanning calorimetry (DSC), and polarization optical microscopy (POM). It was
found that longer OPBAs form layered β-sheet-like aggregates and that these are stabilized by amide
hydrogen bonds in both unsubstituted OPBAs and OPBA-PEG rod-coil copolymers. The binding of the
PEG also introduces a liquid crystalline phase. As a consequence, the local structural order is improved in the
copolymer. Thus, by combining different methods of structural investigation, we were able to develop a
model of local aggregation and packing in both the liquid crystalline and the solid state.
I. Introduction
Rod-coil block copolymers represent an interesting class
of diblock copolymers due to their particular aggregation beha-
vior. The interactions between various segments and geometric
effects are responsible for nanoscaled phase separation and liquid
crystallinity.1-4 In particular, the ability to self-assemble on the
nanometer length scale has promoted these systems for possible
applications such as light-emitting diodes (LEDs) and organic
photovoltaics5-7 but also for the preparation of nanometer scaled
architectures.8,9
The functionality of rod-coil block copolymers is mainly
determined by well-defined structures and aggregation.7 Under-
standing the driving force for their self-assembly is essential in
order to control the tailored design of nanometer scaled materi-
als. Therefore, rod-coil blockcopolymers with anoligomeric rod
segment are of particular interest because their aggregation might
depend on the respective length of the building blocks.
Here, we investigate copolymers built of oligomeric rod blocks of
oligo(p-benzamides) (OPBA) and a poly(ethylene glycol) (PEG)
coil (see Figure 1). An interesting feature of solid OPBAs is their
rigidity resulting from packing due to hydrogen-bonding and π-π
interactions. Both are noncovalent interactions, and their influence
on the structure of the studied system is not yet fully understood.
Although the synthesis and analytical characterization of OPBAs
are rather complex due to poor solubility and crystallization
behavior, the synthesis of well-defined rod segments is feasible.10-13
Insight into aggregation and local packing of both OPBAs
and OPBA-based rod-coil copolymers can be obtained from
advanced high-resolution solid-state NMR, which is a powerful
and versatile tool for the characterization of such materials.14
In particular, solid-state NMR facilitates site-selective and
noninvasive investigation of noncovalent interactions such as
Figure 1. (a) Schematic overview of unsubstituted OPBAs with n =
0-4 repeat units named as OPBA-2 to OPBA-6 and (b) the investigated
rod-coil copolymers with variation of rod and coil length (OPBA-7-
PEG110, OPBA-11-PEG110, and OPBA-4-PEG45).
hydrogen-bonding and π-π interactions. Signals originating
from hydrogen-bonded protons are well separated in the 1H
magic-angle spinning (MAS) NMR spectrum, typically resonat-
ing between 8 and 20 ppm.15-17 The 1H chemical shift includes
semiquantitative information about the strength of the hydrogen
bonds.18-20 In addition, the 1H chemical shift is also a sensitive
probe with respect to ring currents associated with aromatic
moieties.21,22 This is observed as a low field shift of the chemical
shift compared to the corresponding liquid state signal and may
thereby serve as a direct hint for π-π interactions and can
likewise be simply related to the packing via so-called nucleus
independent chemical shift (NICS) maps.14,23,24
*To whom correspondence should be addressed.
pubs.acs.org/Macromolecules
Published on Web 05/03/2010
r 2010 American Chemical Society

Article
Detailed insight about the local structure can be obtained from
Macromolecules, Vol. 43, No. 11, 2010 4979
HCl(conc) was addedto the residue, and the white precipitate was
recovered by filtration. The solid was washed with water until
the pH of the filtrate was neutral and then dried under high
vacuum to yield the title compound in 86% (8 g, 0.21 mol). 1H
NMR (DMSO-d6): δ (ppm) = 5.83 (bs, -NH2), 6.62 (d, 6.64 Hz,
2 H, ar), 7.75 (d, 6.64 Hz, 2 H), 7.90 (m, 8 H), 10.05 (s, 1 H, -NH),
10.40 (s, 1 H, -NH). MS (FD): m/z (%): 375.1 (100), 376.1
(21.99), 377.1 (2.99); calcd 375.12 (100), 376.13 (23.8), 377.13
(3.5), 376.12 (1.1).
OPBA-5. 4-(4-Aminobenzamido)benzoicacid (10 g, 37mmol)
was dissolved in NMP (80 mL) and added to a stirred solution
of imidoyl chloride trimer precursor (17 g, 37 mmol) and N,N0-
dimethyl aniline (37 mmol) in NMP (80 mL) at rt. After stirring
at rt for 12 h the mixture was poured into HCl (0.1 N), and the
precipitated solid was removed by filtration. The solid was
washed with water until the filtrated was at neutral pH. The
solid was dried under vacuum and subsequently washed with
diethyl ether. The solid was then extracted with NMP in a
Soxhlet extractor for 12 h to remove all soluble impurities of
the starting materials. The solid was dried under high vacuum,
yielding 23 g of a light yellow solid.
OPBA-6. 4-(4-(4-Aminobenzamido)benzamido)benzoic acid
(2 g, 5.3 mmol) was dissolved in NMP (10 mL) and added to a
stirred solution of imidoyl chloride trimer precursor (2.5 g,
5.4 mmol) and N,N0-dimethylaniline (5.4 mmol) in NMP
(10 mL) at rt. After stirring at rt for 12 h the mixture was poured
into HCl (0.1 N), and the precipitated solid was removed by
filtration. The solid was washed with water until the filtrate was
at neutral pH. The solid was dried under vacuum and subse-
quently washed with diethyl ether. The solid was then extracted
with NMP in a Soxhlet extractor for 12 h to remove all soluble
impurities of the starting materials. The solid was dried under
high vacuum, yielding 4 g of a light yellow solid.
1H-1H double-quantum single-quantum (DQ-SQ) correlation
spectrum.14,22 Such a 2D experiment correlates DQ coherence
due to pairs of dipolar coupled protons with SQ coherence in
characteristic correlation peaks. Diagonal peaks result from so-
called like spins with the same chemical shift and cross peaks reflect
couplings among unlike spins with different chemical shifts.
Information about the presence of polymorphs, local confor-
mations, secondary structure motifs, and moieties comprising the
asymmetric unit of powdered samples can be obtained by 13C
cross-polarization (CP) MAS measurements. Moreover, the 13C
line shapes include hints of structural order: broad lines indicate
an ill-defined or possibly amorphous structure whereas narrow
lines indicate crystallinity or high local order.25-27
To link the above-described methods, 2D 13C/1H correlation
can be done which provide additional tools for studying local
molecular assemblies and dynamics. Often applied NMR tech-
niques are based on rotational echo double resonance (REDOR)
for reintroducing the otherwise averaged heteronuclear dipo-
lar coupling.28 Specifically, the recoupled polarization transfer
(REPT) technique allows recording of 13C/1H chemical shift
correlation spectra and the determination of 13C/1H dipolar
coupling constants by means of spinning sideband analysis in
the indirect dimension of the 2D experiment.29 In a recoupled
polarization-transfer heteronuclear single quantum correlation
(REPT-HSQC) experiment, the 13C/1H resonances are correlated
in a two-dimensional fashion where the recoupling condition can
be chosen such that either short- or long-range 13C-1H interac-
tions are observed.30 In contrast, the recoupled polarization-
transfer heteronuclear dipolar order (REPT-HDOR) experiment
1
does not correlate the chemical shifts of H and 13C, but the
13C-1H dipolar coupling.31 Thus, REPT-HDOR is particular
promising for the study of molecular dynamics where the reduc-
tion of heteronuclear dipolar coupling due to fast motional
averaging can be investigated in a quantitative fashion.32,33
This paper is organized as follows. First, the aggregation
behavior of unsubstituted OPBAs of different length is clarified
by solid-state NMR, augmented by wide-angle X-ray scattering
(WAXS). Second, the aggregation and site-specific local dynamics
is investigated in rod-coil copolymers with varying rod length as
well as different molar mass of the PEG block. Finally, the phase
behavior of the rod-coil copolymers is clarified by differential
scanning calorimetry (DSC), solid-state NMR, and optical micro-
scopy, and the impact of the liquid crystalline phase on the local
order of the aggregate in the solid is considered.
OPBA-11-PEG110. Imidoyl chloride trimer precursor
(580 mg, 1.3 mmol) was added to a solution of NH2-OPBA-7-
PEG110 (1.5 g, 0.25 mmol) and N,N0-dimethylaniline (160 μL) in
DMF (10 mL). After stirring at rt for 24 h the polymer was
precipitated into a 10-fold excess of diethyl ether, and the solid
was recovered by filtration. Drying under high vacuum yielded
the title compound.
1
Solid-State NMR Spectroscopy. All H MAS NMR spectra
were measured at a Bruker Avance 700 MHz spectrometer using
30 kHz MAS and a π/2 pulse of 2.5 μs. The recycle delay was 60 s
1
for unsubstituted OPBAs and 5 s for the copolymers. H-1H
DQ-SQ MAS NMR were also performed at 700 MHz at 30 kHz
MAS and a π/2 pulse of 2.5 μs. The back-to-back (BABA)
recoupling sequence was used applying States-TPPI for phase
sensitive detection.34 The recycle delay was 2 s, and for each
of the 128 t1-slices 32 transients were recorded. 13C CP/MAS
measurements were carried out at 125.77 MHz (Bruker Avance
500 machine) using 25 kHz MAS and a π/2 pulse of 2.5 μs.
Spectra were measured with a CP contact time of 3 ms, 4096
scans, a recycle delay of 15 s, and TPPM decoupling.35 REPT-
HSQC experiments30 were carried out using a Bruker Avance-
III 850 spectrometer with a 13C resonance of 213 MHz and a π/2
pulse of 2.5 μs. The measurement was acquired with 2 rotor
periods recoupling, 72 increments in the indirect dimension,
1024 transients per increment, and a recycle delay of 2 s. REPT-
HDOR measurements31 were performed using a Bruker Avance
500 spectrometer at 25 kHz MAS and a π/2 pulse of 2.5 μs. The
experiment was measured employing 4 rotor periods recoupling,
20 increments in the indirect dimension, 1120 transients per
increment, and a recycle delay of 2 s. For analyzing the data a
home written Matlab routine was used which is based on the
II. Experimental Methods
Samples Preparation. The syntheses of OPBA-2, OPBA-3, and
OPBA-4 as well as the rod-coil copolymers OPBA-4-PEG45
and OPBA-7-PEG110 were carried out as described previously.10
4-((Z)-Chloro(4-((Z)-chloro(4-nitrophenyl)methyleneamino)-
phenyl)methyleneamino)benzoyl chloride (compound 6 in ref
10) will be referred to as imidoyl chloride trimer precursor. The
amine-terminated block copolymer corresponding to OPBA-7-
PEG110 (i.e., where the nitro group is reduced to a primary
amine) will be referred to as NH2-OPBA-7-PEG110. The re-
duction of OPBA-7-PEG110 to the amine-terminated polymer
was carried out in analogy to the previously reported procedures
for shorter OPBA-PEG block copolymers.10
4-(4-(4-Aminobenzamido)benzamido)benzoic Acid. 4-(4-(4-Ni-
trobenzamido)benzamido)benzoic acid (10 g, 25 mmol) was dis-
solved in a mixture of methanol (300 mL) and DMF (50 mL).
Ammonium formate (15 g, 250 mmol) was added to the stirred
solution. Pd/C (10%, 1 g) was subsequently added in small
portions under a nitrogen atmosphere. After stirring for 12 h at
room temperature (rt) the mixture was filtered through Celite
(Aldrich), and the solvents were removed under reduced pressure.
1
dipolar coupling of a two-spin system.29 All H and 13C MAS
NMR spectra were referenced with respect to tetramethylsilane
(TMS) using solid tetrakis(trimethylsilyl)silane (TTSS) and
solid adamantane as a secondary standard.36,37
POM observations were made with a Zeiss Axioscop 40
equipped with a Linkam THMS600 hot stage.

4980 Macromolecules, Vol. 43, No. 11, 2010
Bohle et al.
Figure 2. 13C CP/MAS spectra of OPBAs with increasing number of
repeat units. Resonance assignment is clarified by the color code. It is
shown that for OPBA-4 (n = 2) and the longer OPBAs all spectra are
rather similar, indicating an equilibrium structure.
Figure 3. 1H MAS NMR spectra of OPBA-2 to OPBA-6. The inset
illustrates the hydrogen bond formation through an acid dimer
(resonance at 14 ppm present in OPBA-2 and -3) The spectrum of
OPBA-6 (n = 4) shows additional signals due to impurities since the
solubility is rather poor and the purification difficult.10
Wide-angle X-ray scattering measurements were carried out
with a D8 Advanced Bruker on powder samples on a glass
substrate.
additional signals due to impurities since the solubility of the
longer OPBAs is rather poor and the purification difficult.10
Obviously, the rod length affects what kind of hydrogen
bonding determines the structure. On the one hand, the
hydrogen bonding of two carboxylic acid groups is known
to be rather strong compared to the much weaker amide
hydrogen bonds.41 But on the other hand, the number of
amide hydrogen bonds is increasing with increasing rod size,
whereas the formation of the carboxylic dimers is limited to
the end groups. For OPBA-2 and -3, the ratio of the two
types of hydrogen bonding is 1:1 and 1:2, respectively, and
therefore the carboxylic acid dimer formation is facilitated.
Both structures including the hydrogen-bonding network
have already been discussed in detail in a previous paper.40
With increasing number of repeat units the ratio of the
carboxylic acid hydrogen bond formation compared to the
amide hydrogen bond formation decreases. The amide hy-
drogen bonding dominates the structure formation for
the longer oligomers, starting with OPBA-4, as shown by
III. Results and Discussion
Unsubstituted OPBAs. First, the dependence of aggrega-
tion on the length of the oligomer was studied in unsubsti-
tuted OPBAs. Particularly suited for this kind of investiga-
tion is the 13C CP/MAS experiment which is sensitive to
structural changes and local degree of organization (spectra
shown in Figure 2). The resonance at 172 ppm is assigned
to the COOH group, while the CONH carbon has a chemi-
cal shift of about 164 ppm and the signals at about 150 and
142 ppm can be assigned to the quaternary carbon atoms
bearing nitro- and amide groups (see color code Figure 2).
The signals in the region of 120-140 ppm can be assigned to
the aromatic carbon atoms.38
Comparison of the 13C CP/MAS spectra show that the
spectra of OPBA-2 and -3 differ as compared to those of
OPBA-4 to -6. This indicates that the structure for the short
OPBAs with n = 0 and 1 is different from that of the longer
oligomers. In particular, the well-resolved peak at 172 ppm
of the COOH group is only present in OPBA-2 and -3.
Remarkably, longer OPBAs with four and more repeat units
(n g 2) have rather similar 13C spectra, suggesting an equi-
librium structure (see Figure 2). The only observable differ-
ence for the longer OPBAs is a noticeable degradation of the
spectral resolution, probably due to decreasing crystallinity
of the samples.
1
the absence of the peaks at 14 ppm in the H MAS NMR
spectrum and 172 ppm in the 13C CP/MAS NMR spectrum.
To further reveal the impact of hydrogen bonding on the
structure, the influence of a hydrogen bond breaking solvent
was probed upon crystallization of OPBA-3 with dimethyl-
formamide (yielding OPBA-3-DMF). The carboxylic acid
end groups are blocked by DMF, and almost no dimer
formation is observed as for OPBAs with n g 2. Comparison
of the 13C CP/MAS NMR spectra of OPBA-3-DMF and
OPBA-5 (shown in Figure 4) indicates that both spectra
match, apart from the presence of a minority component
with carboxylic dimers as indicated by the 13C signal at
172 ppm. All signals in the 13C CP/MAS NMR spectrum
have rather similar chemical shifts. An important difference
however is the spectral resolution due to the fact that OPBA-
3-DMF forms a regular crystal lattice whereas OPBA-5 does
not. Therefore, the 13C CP/MAS NMR spectrum of OPBA-
3-DMF was also processed with an additional line broad-
ening of 150 Hz (see Figure 4b), obscuring fine splitting. This
underlines the similarity of both spectra and, hence, indicates
a comparable solid-state packing.
The 1H MAS NMR spectra (shown in Figure 3) also
reflect this behavior. For OPBA-2 and -3 a strong peak at
about 14 ppm is observed that can be assigned to the
hydrogen bonding of two carboxylic acid groups forming a
dimer (see inset Figure 3).39 The absence of this signal for
longer OPBAs indicates that these are not able to form
hydrogen bonds between carboxylic acid groups, which also
explains the absence of the peak at 172 ppm in the 13C CP/
MAS spectra. Because of the lack of hydrogen bonding the
chemical shift of the carboxylic acid carbon atom is shifted
toward higher field and thus overlaps with the chemical shift
of the CONH peak at 164 ppm. All 1H MAS NMR spectra
include a broad peak at about 7 ppm which can be assigned
to the aromatic protons. The resonance for the amide pro-
tons of the peptide bond is located at about 9 ppm and
hidden below the broad aromatic signal, as demonstrated
by deuterium exchange and subsequent 2H MAS NMR
measurements.40 The spectrum of OPBA-6 (n = 4) shows
As an independent probe, wide-angle X-ray scattering
(WAXS) of both OPBA-3-DMF and OPBA-5 was per-
formed (shown in Figure 5). The wide angle range (2θ >
15°) reflects the local structure comparable to the data
available by solid-state NMR. There are three main reflexes
at 2θ = 20.3 (h, k, l: -2, 1, 3), 2θ = 23.7 (h, k, l: -4, 0, 4), and
2θ = 27.8 (h, k, l: -3, 1, 5) which match for OPBA-3-DMF

Article
Macromolecules, Vol. 43, No. 11, 2010 4981
Figure 4. 13C CP/MAS NMR spectrum of (a) OPBA-3-DMF and
(b) OPBA-3-DMF processed with an additional line broadening of
150 Hz and (c) OPBA-5. Especially the comparison of (b) and (c) shows
that both spectra match almost perfectly.
Figure 6. Projections of the OPBA-3-DMF single crystal X-ray struc-
˚
˚
˚
ture with the unit cell dimension of a = 24.5 A, b = 5.4 A, c = 17.1 A,
and β = 99.05°.8 (a) Amide hydrogen bonding within a layer and the
formation of a β-sheet-like structure; (b) stacking between the layers.
Figure 5. WAXS pattern of (a) OPBA-3-DMF and (b) OPBA-5 which
underlines the structural similarity.
and OPBA-5 rather well. Similar to the NMR data, the match
of the main reflexes underlines the structural similarity. The
single-crystal X-ray structure of OPBA-3-DMF exhibits a
hydrogen-bonded stack of rods.8 On the basis of that observa-
tion, the structural features must also be present in the longer
OPBAs. Thus, it is expected that longer OPBAs form a
regularly layered β-sheet-like hydrogen-bonded structure
with no linkage of the head groups (see Figure 6a). Moreover
π-π stacking interactions are present between the layers (see
Figure 6b).
Rod-Coil Copolymers. Bonding the rod to PEG polymers
yields rod-coil copolymers. Potential structural changes
can then be monitored in the corresponding 13C CP/MAS
NMR spectrum (shown in Figure 7). The signals attributed
to the OPBA rod within the copolymer appear in the
same region of 110-165 ppm as in unsubstituted OPBAs.
Comparison of the 13C signals shows a good match between
the unsubstituted OPBAs and the OPBA rods incorporated
in the polymer (shown in Figure 7a). The additional signal
at 70 ppm reflects the PEG coil. Since the coil length may
influence the structure, a comparison of the 13C CP/MAS
NMR spectra of OPBA-7-PEG110 and OPBA-4-PEG45 is
shown in Figure 7b. Although the length of the PEG coil has
more than doubled, the 13C CP/MASNMR spectrum has not
changed. The same consistency of the 13C signals is observed
upon changing the rod length (see in Figure 7c), indicating
that the rod structure is unchanged under these circum-
stances.
Figure 7. 13C CP/MAS NMR spectra of (a) OPBA-5 (black) and
OPBA-7-PEG110 (blue), (b) OPBA-4-PEG45 (black) and OPBA-7-
PEG110 (blue), and (c) OPBA-11-PEG110 (black) and OPBA-7-
PEG110 (blue). Concerning the consistency of the 13C signals, it can
be concluded that the structure of OPBAs remains although a PEG coil
is attached. The rod structure is also unaffected by variation of the coil
and rod length.
On the basis of the observation that the structure of
OPBAs with n g 2 is very similar to the structure of
OPBA-3-DMF and the structure of OPBA rods within the
copolymer reflects that of the unsubstituted OPBAs, we
propose a tentative packing of the rod-coil copolymer as
depicted in Figure 8. Within a layer there is a β-sheet-like
hydrogen-bonding network where all PEG chains are or-
iented in the same direction. Between the layers π-π stack-
ing interactions are present and the polymer chains are
oriented in an alternate way. The volume filled by the
attached PEG polymer chains is described by an elongated
sphere of different sizes, depending on the length of the
specific PEG.42 At first sight, the fact that the oligomers
are oriented in the same direction within one layer might

4982 Macromolecules, Vol. 43, No. 11, 2010
Bohle et al.
Figure 8. Schematic structure of the rod coil copolymer. The rod block is depicted with a blue box and the PEG coil with a gray ellipse. (a) shows the
idea of aggregation within a layer and the driving forces of hydrogen bonding and (b) the aggregation between the layers with the driving forces of π-π
interactions.
raise the question how such a packing could lead to a proper
space filling structure. In similar rigid rod systems with alkyl
side chains of different lengths it has been observed that the
flexibility of these chains allows for a dense packing imposed
by the rigid rods.43 In the present case of OPBA with
attached PEG chains this will be even easier since such chains
are know to be more flexible.42 It is also reasonable to assume
that the arrangement of OPBA-PEG copolymers is very
similar, or identical, to micelles formed in solution and in the
solid state. The observations made here, however, are in
contrast to the previous sketch of molecular arrangement
that OPBA rods form a bilayer within the core of the
“hockey-puck micelles”.8 The molecular arrangement was
derived from topography images obtained by scanning force
microscopy (SFM). While SFM can give very accurate
height dimensions, the finite size of the scanning tip itself
limits the accuracy of objects laterally. Therefore, the tip
geometry must be considered in the interpretation of SFM
data. Considering this and analyzing the SFM phase con-
trast images, the existence of bilayer hockey-puck micelle is
very unlikely. With the current solid-state NMR data, it can
be assumed that the geometric arrangement as depicted in
Figure 8 is the most likely description of the arrangement of
the OPBA rod blocks within the micellar core. To underline
this property, the packing of OPBA-7-PEG110 has been
12.5 ppm (resonance BC 7 ppm þ 5.5 ppm) indicates contacts
among aromatic protons with and without π-π stacking.
The DQ auto-peak at 14 ppm (resonance BB 7 ppm þ 7 ppm)
indicates contacts among aromatic protons, while the DQ
cross-peak at 16.1 ppm (resonance AB 9.1 ppm þ 7 ppm)
reflects proximity of aromatic protons and amide protons.
Additional insights into the local packing is derived from
13C/1H REPT-HSQC NMR (shown in Figure 9b). The peak
assignment is pointed out by the color code shown in
Figure 9b. It can be noticed that not only the proton chemical
shift is affected by π-π stacking interactions but also the
13C chemical shift.21 Both 2D measurements underline the
aggregation behavior derived from the single-crystal X-ray
structure.
Phase Behavior of Rod-Coil Copolymers. Beside all
structural similarities discussed above, it should be noticed
that the resolution in the 13C/CP MAS spectrum of the
copolymer is significantly increased compared to the corre-
sponding unsubstituted OPBAs (cf. 13C/CP MAS spectra
of OPBA-6 and OPBA-7-PEG110 in Figure 10). In princi-
ple, there are two possibilities for an increased resolution of
a
13C CP/MAS solid-state NMR spectrum: either the mo-
bility or the local order of the OPBA rod incorporated in
the copolymer has increased. To clarify this question,
the mobility of the rod segment is analyzed via the 13C/1H
REPT-HDOR method. The aromatic 13C resonance at
123 ppm was chosen for detecting the dipolar sideband
pattern and subsequent fitting to obtain the 13C/1H dipolar
coupling constant (see fitted sideband pattern Figure 11).33
For both OPBA-7-PEG110 and OPBA-5, the measured
13C-1H dipolar coupling constant is 18.0 ( 0.5 kHz,
indicating that the aromatic rings are rather rigid (for a
completely rigid system 21.0 kHz is expected). The almost
identical 13C-1H dipolar coupling constant also shows that
the mobility of the OPBA rod is unaffected by PEG attach-
ment. Thus, the increased spectral resolution in the 13C/CP
MAS spectrum stems from a higher local order of the rod
structure.
1
analyzed via 2D H-1H DQ-SQ MAS NMR and 13C/1H
REPT-HSQC NMR.
The 2D 1H-1H DQ-SQ MAS NMR spectrum of OPBA-
7-PEG110 shown in Figure 9a was recorded at low tempera-
ture (250 K) in order to shrink the intense PEG signal. In the
projection, besides the signal at 3.8 ppm (resonance D)
assigned to PEG, three additional resonances at 9.1 ppm
(resonance A), 7 ppm (resonance B), and 5.5 ppm (resonance
C) are observed. The signal at 9.1 ppm can be assigned to the
amide protons which are directly observable at that tem-
perature. The remaining two peaks at 7 and 5.5 ppm origi-
nate from the aromatic protons, where the chemical shift of
5.5 ppm indicates π-π stacking present in the system
(protons are marked in red, see chemical structure in
Figure 9a).19 The correlation among protons participating
in different local packing can be identified via characteristic
cross-peaks in the 2D spectrum. The DQ cross-peak at
Apart from phase separation and aggregation, another
important characteristics of rod-coil copolymers is their
ability to form liquid crystalline phases.2,4,44 Of special
importance are lamellar and nematic phases. For model

Article
Macromolecules, Vol. 43, No. 11, 2010 4983
Figure 11. 13C-1H REPT-HDOR spinning sideband pattern of
(a) OPBA-5 and (b) OPBA-7-PEG110. The measurement was per-
formed at 25 kHz MAS and 4 rotor periods recoupling. The red spectra
correspond to the measured 13C-1H dipolar spinning sideband
pattern, and the black spectra are the fitting curves. Multispin interac-
tions affect the innermost dipolar sidebands in (a), and therefore the
fifth and seventh sidebands were chosen to evaluate the goodness of
the fit. The determined 13C-1H dipolar coupling constant is in both
cases 18 kHz.
1
Figure 9. (a) 2D H-1H DQ-SQ MAS NMR spectrum of OPBA-7-
PEG110 performed at 250 K and 30 kHz MAS with 1τR recoupling.
(b) 2D 13C-1H REPT-HSQC spectrum of OPBA-7-PEG110 measured
at 25 kHz MAS and 2τR recoupling.
Figure 12. 13C CP/MAS NMR spectra of OPBA-7-PEG110 recorded
at 250 K (black) and 340 K (blue). The OPBA signals (120-165 ppm)
are hardlyinfluencedbytemperature changes while thePEG signal at71
ppm shows a significantdecrease ofthelinewidth (from130 Hzat 250K
to 50 Hz at 340 K), indicating a higher mobility.
Indeed for OPBA-7-PEG110 the DSC trace shows an
exothermic peak at 332 K, in agreement with the recent
studies of the melting temperature of a similar rod-coil
copolymer at 329 K.11 To investigate how the nature of this
transition affects the structure, VT-13C CP/MAS measure-
ments in the temperature range up to 340 K were performed
to monitor possible changes of the structure. Since the 13C
resonances in the 13C/CP MAS spectrum attributed to the
rod (120-165 ppm) are well separated from those of the coil
(71 ppm), structural changes can be observed individually.
Figure 12 shows the 13C CP/MAS NMR spectra recorded at
specific temperatures of 250 and 340 K, i.e., well below and
slightly above the phase transition. While OPBA signals in
Figure 10. 13C CP/MAS NMR spectrum of OPBA-6 (black) and
OPBA-7-PEG110 (blue). The aromatic signals in case of the copolymer
are much better resolved, although the rod part includes more repeat
units.
systems also complete phase diagrams are reported. Depend-
ing on the asymmetry between the rod and coil also a
hexagonal packed phase was observed.45

4984 Macromolecules, Vol. 43, No. 11, 2010
Bohle et al.
Figure 13. POM of OPBA-4-PEG45 images at 105 °C. (A) and (B) show a certain position of the sample but rotated by 45°. (C) Birefringent
nonspecific texture.
the range of 120-160 ppm exhibit almost no changes, the
PEG signal at 71 ppm narrows substantially above 332 K.
Thus, the phase transition detected by DSC reflects melting
of the PEG block only. This is in accord with the statement in
the literature that the rodlike character is retained under
virtually all circumstances.2
Probing the formation of a liquid crystalline phase with
a polarization optical microscope (POM) is presented in
Figure 13, and these indeed show that above the melting
temperature a liquid crystalline phase is formed for all the
examined rod-coil copolymers. Rotation of the samples
exhibited alternately birefringent and dark textures every
45° (shown in Figure 13A,B). The nonspecific textures can be
sheered but have not been assigned to a certain kind of
mesophase. It is known that the presence of a liquid crystal-
line phase favors supramolecular organization when cooling
down to a solid phase.33,46 Thus, the better local order of the
aggregation in the rod-coil copolymer noted above is likely
to be due to the preorganization in the liquid crystalline
phase.
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The structures of both unsubstituted OPBAs and OP-
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