Self-assembly and photoinduced optical anisotropy in dendronized supramolecular azopolymers

Article abstract of DOI:10.1021/ma402369p

Herein we report the preparation and characterization of dendronized supramolecular polymers composed of a carboxy-Terminated azodendron, dAZO, and two different vinylpyridine-containing polymers: poly(4-vinylpyridine) (P4VP) and polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer. P4VP can selectively complex dAZO through hydrogen-bonding interactions, thus resulting in liquid crystalline materials. Additionally, this strategy is also applicable to the preparation of dendronized supramolecular block copolymers (BCs). Lamellar, cylindrical, and spherical morphologies are observed for the BC complexes depending on the dAZO to vinylpyridine repeating unit ratio. Photoinduced orientation of the azobenzene moieties is obtained in films of the H-bonded materials - both P4VP and PS-b-P4VP based complexes - by using 488 nm linearly polarized light and characterized through birefringence and dichroism measurements. High and stable values of birefringence are obtained for polymers with azobenzene content as low as 2.7 wt %, thus demonstrating the benefits of preorganization in photoactive dendritic moieties in side-chain H-bonded materials.

Full text of DOI:10.1021/ma402369p

Article
pubs.acs.org/Macromolecules
Self-Assembly and Photoinduced Optical Anisotropy in Dendronized
Supramolecular Azopolymers
Jesus del Barrio,*,† Eva Blasco,‡ Chris Toprakcioglu,§ Alexandros Koutsioubas,∥ Oren A. Scherman,†
́
Luis Oriol,*,‡ and Carlos Sanchez-Somolinos*,⊥
́
†Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United
Kingdom
‡Departamento de Química Organ
́ ́
ica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragon, Universidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain
§Department of Physics, University of Patras, Patras 26500, Greece
∥Julich Centre for Neutron Science, Forschungszentrum Julich GmbH, Außenstelle am FRM-II, c/o TU Munchen, Lichtenbergstraße
̈
̈
̈
1, 85747 Garching, Germany
⊥Departamento de Física de la Materia Condensada, Facultad de Ciencias, Instituto de Ciencia de Materiales de Aragon
́
(ICMA),
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: Herein we report the preparation and characterization of
dendronized supramolecular polymers composed of a carboxy-terminated
azodendron, dAZO, and two different vinylpyridine-containing polymers:
poly(4-vinylpyridine) (P4VP) and polystyrene-b-poly(4-vinylpyridine) (PS-
b-P4VP) block copolymer. P4VP can selectively complex dAZO through
hydrogen-bonding interactions, thus resulting in liquid crystalline materials.
Additionally, this strategy is also applicable to the preparation of
dendronized supramolecular block copolymers (BCs). Lamellar, cylindrical,
and spherical morphologies are observed for the BC complexes depending
on the dAZO to vinylpyridine repeating unit ratio. Photoinduced
orientation of the azobenzene moieties is obtained in films of the H-
bonded materialsboth P4VP and PS-b-P4VP based complexesby using 488 nm linearly polarized light and characterized
through birefringence and dichroism measurements. High and stable values of birefringence are obtained for polymers with
azobenzene content as low as 2.7 wt %, thus demonstrating the benefits of preorganization in photoactive dendritic moieties in
side-chain H-bonded materials.
does not absorb at the recording wavelength. High and stable
values of photoinduced anisotropy have been found for some of
these systems with low chromophore content. However, the
synthesis of well-defined functional BCs is, in general, a
demanding, expensive and time-consuming process. In contrast,
a two-component mixture consisting of a small molecule
azobenzene moiety and a BC-based scaffold is a plausable,
facile, and inexpensive approach to prepare photochromic BCs.
In this regard, the groups of Ikkala and ten Brinke have
pioneered research on supramolecular comb coil hydrogen-
bonded polymers based on H-acceptor polymers, including
commercially available poly(styrene-b-4-vinylpyridine) (PS-b-
P4VP), and a variety of H-donor surfactant-type mole-
cules.14−16 The concept of attaching small molecules to
vinylpyridine group-containing polymers has been exploited
by Priimagi and co-workers on poly(4-vinylpyridine) (P4VP)
1. INTRODUCTION
The photochromic properties of azobenzene derivatives have
been widely investigated for many different applications in
recent decades, and their use has enormously grown from
colorants, through to optical materials, and most recently in a
variety of biologically relevant systems.1−5 In the field of optical
applications specifically, much effort has been focused on the
development of azobenzene-containing polymeric materials
showing high and stable values of photoinduced anisotropy
after irradiation with polarized light (normally in the blue-green
region of visible light).6,7 Some particular applications such as
volume holography require thick films (hundreds of micro-
meters in thickness) with sufficient light penetration, thus
photosensitive materials with good optical photoresponse, low
levels of scattering, and low optical absorption at the recording
wavelength are required.8,9 In the past few years several groups
including our own have investigated azobenzene-containing
block copolymers (BCs) for this purpose.10−13 These
copolymers are generally composed of two different blocks
one containing the azobenzene units and a second block which
Received: November 15, 2013
Revised: January 8, 2014
© XXXX American Chemical Society
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Figure 1. Chemical structures and schematic representations of (a) P4VP and PS-b-P4VP polymers and dAZO azodendron together with (b) the
schematic representations of P4VP(dAZO)x and PS-b-P4VP(dAZO)x.
complexed with a variety of azobenzene compounds.17−20 The
Table 1. Composition of the Supramolecular Complexes
same group has investigated the photoinduction of optical
a
P4VP(dAZO)
(wt %)
dAZO
chromophore
(wt %)
anisotropy and the inscription of surface relief grating in thin
complex
x
(wt %)
films of hydrogen-bonded P4VP homopolymers.17,19,20
A
P4VP(dAZO)x
0.06
0.12
0.25
0.50
44.3
61.4
76.1
86.4
22.1
30.6
37.9
43.0
similar strategy can be expanded to BC systems to attain
good control of the microsegregated morphology through
azobenzene doping.21 We have recently investigated a series of
H-bonded materials comprised of 6-[4-(4′-cyanophenylazo)-
phenyloxy]hexanoic acid and PS-b-P4VP, which show well-
segregated morphologies and good photoresponse at higher
degrees of chromophore loading.22 These materials, however,
are limited as their optical response drastically worsens at low
chromophore content.
PS-b-
P4VP(dAZO)x
0.06
0.12
0.25
0.50
12.4
17.0
24.9
36.8
5.5
10.5
18.9
31.8
2.7
5.2
9.4
15.9
a
Herein we document the hydrogen-bonding-assisted self-
assembly of dAZO, an H-donor azodendron, and two H-
acceptor polymers containing vinylpyridine repeating units:
P4VP and PS-b-P4VP BCs (Figure 1). Azodendron dAZO
consists of three promesogenic and photochromic azobenzene
moieties and a single carboxylic acid group at the focal point.
The dendritic structure of dAZO favors the interaction between
pendant azobenzene units, thus enhancing the photoresponse
of the complexes in comparison to previously described
hydrogen-bonded azopolymers.22 The series of homopoly-
mer-type complexes P4VP(dAZO)x consists of dAZO and
P4VP (Figure 1). We have also investigated the BC-type series
of complexes PS-b-P4VP(dAZO)x composed of dAZO and a
particular PS-b-P4VP (Figure 1). For both series of complexes,
we have systematically altered their degree of complexation x,
i.e., azodendron to vinylpyridine repeating unit ratio (Table 1),
in order to investigate its effect on the thermal, mesomorphic,
morphological, and photoinduced optical properties of the self-
assembled materials. The P4VP block of PS-b-P4VP(dAZO)x
can selectively encapsulate the dAZO moieties whereas the PS
block confers processability and transparency to the system
both attractive qualities in optical applications. Photoinduced
orientation of the azobenzene units was obtained in films of the
supramolecular complexes by using 488 nm linearly polarized
light. The achieved photoinduced anisotropy was characterized
through birefringence and dichroism measurements.
Weight percentage of chromophoric azo units (4-oxy-4′-cyanoazo-
benzene).
Mw/Mn = 1.10) were purchased from Polymer Source, Inc., and used
without further purification. The preparation of azodendron dAZO
was carried out according to the synthetic pathway shown in Scheme
1. 4-Cyano-4′-hydroxyazobenzene was synthesized according to
previously described methods.23 The rest of the starting materials
were purchased from Sigma-Aldrich and used without further
purification.
4-Cyano-4′-(12″-bromododecyloxy)azobenzene (1). A mixture of
4-cyano-4′-hydroxyazobenzene (2.40 g, 10.8 mmol), 1,12-dibromodo-
decane (17.6 g, 53.8 mmol), finely ground potassium carbonate (3.00
g, 21.7 mmol), potassium iodide (0.25 g, 1.5 mmol), and butan-2-one
(50 mL) was stirred and heated to reflux for 16 h. The mixture was
then allowed to cool down to room temperature (RT), and the solids
were filtered off and washed with acetone. The combined organic
solvents were evaporated under reduced pressure to dryness. The
residue was purified by flash chromatography using hexane/ethyl
acetate (4:1) to yield the required product as an orange solid. Yield:
1
3.50 g (69%). H NMR (400 MHz, CDCl3, δ): 7.97−7.90 (m, 4H),
7.79 (d, J = 8.3 Hz, 2H), 7.02 (d, J = 8.8 Hz, 2H), 4.06 (t, J = 6.5 Hz,
2H), 3.41 (t, J = 6.8 Hz, 2H), 1.90−1.77 (m, 4H), 1.53−1.23 (m,
16H). 13C NMR (100 MHz, CDCl3, δ): 162.74, 154.79, 146.67,
133.14, 125.45, 123.05, 118.66, 114.86, 113.10, 68.47, 34.05, 32.81,
29.50−29.33, 29.12, 28.74, 28.15, 25.97. IR (KBr, cm−1): 2227, 1600,
1468, 1256, 1140, 860, 842.
Methyl 3,4,5-Tris[12-(4-(4′-cyanophenyldiazo)phenoxy)-
dodecyloxy)]benzoate (2). A mixture of 4-cyano-4′-(12″-
bromododecyloxy)azobenzene (2.00 g, 4.25 mmol), methyl gallate
(0.23 g, 1.2 mmol), finely ground potassium carbonate (0.90 g, 6.5
mmol), potassium iodide (0.11 g, 0.66 mmol), and butan-2-one (30
mL) was stirred and heated to reflux for 24 h. The mixture was allowed
2. EXPERIMENTAL SECTION
Materials. P4VP homopolymer (Mn = 19 kDa, Mw/Mn = 1.15) and
PS-b-P4VP diblock copolymer (Mn,PS = 24 kDa, Mn,P4VP = 1.9 kDa,
B
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concentrated hydrochloric acid. Then, the organic solvents were
evaporated under reduced pressure, and the crude product was filtered
off and washed with water. The product was purified by flash
chromatography on silica gel and eluted with dichloromethane,
gradually increasing the polarity of the eluent with methanol. Yield:
Scheme 1. Synthesis of Azodendron dAZO
1
0.85 g (43%). H NMR (400 MHz, CDCl3, δ): 7.95−7.89 (m, 12H),
7.80−7.75 (m, 6H), 7.31 (s, 2H), 7.03−6.96 (m, 6H), 4.07−3.97 (m,
12H), 1.87−1.69 (m, 12H), 1.53−1.24 (m, 48H). 13C NMR (100
MHz, CDCl3, δ): 170. 85, 162.70, 154.71, 152.84, 146.63, 133.12,
128.40, 125.18, 123.03, 122.66, 118.60, 114.77, 113.07, 108.42, 73.47,
69.12, 68.41, 30.28, 29.91−28.99, 26.14−25.73. IR (KBr, cm−1): 3240,
3070, 2640, 2580, 2226, 1682, 1250, 1139, 842. MALDI-TOF MS
calcd for C82H100N9O8 [M + H]+: 1338.8; found: 1338.8; calcd for
C82H99N9NaO8 [M + Na]+: 1360.8; found: 1360.8. Anal. Calcd for
C82H99N9O8: C, 73.57; H, 7.45; N, 9.42; found: C, 73.46; H, 7.46; N,
9.18.
Sample Preparation. Supramolecular complexes P4VP(dAZO)x
and PS-b-P4VP(dAZO)x consist of mixtures of dAZO and the
polymers P4VP and PS-b-P4VP, respectively. In the complex names, x
denotes the number of dAZO moieties per vinylpyridine repeating
unit. The compositions of all the complexes are listed in Table 1.
P4VP(dAZO)x complexes were prepared as follows. The required
amounts of P4VP and dAZO were first weighted and dissolved
separately in chloroform to form 1−2% (w/v) stock solutions. Then,
the P4VP solution was added dropwise to the dAZO solution followed
by stirring overnight at RT. Finally, the solvent was slowly evaporated,
and the mixtures were dried under vacuum at 40 °C for 2 days. PS-b-
P4VP(dAZO)x complexes were prepared in an analogous manner
from tetrahydrofuran solutions of dAZO and the BC.
Characterization Techniques. IR spectra were obtained on a
Nicolet Avatar 360-FT-IR spectrophotometer using KBr pellets.
Thermogravimetric analysis was performed using a TA Q5000IR
instrument at a heating rate of 10 °C min−1 under a nitrogen
atmosphere. Mesogenic behavior was evaluated by POM using an
Olympus BH-2 polarizing microscope fitted with a Linkam THMS600
hot stage. Thermal transitions were determined by DSC using a TA
DSC Q-2000 instrument under a nitrogen atmosphere with powdered
samples (about 3 mg) sealed in aluminum pans. Glass transition
temperatures were determined at the midpoint of the baseline jump.
Other phase transition temperatures different from Tg were read at the
maximum of the corresponding peaks. XRD measurements were
performed with an evacuated Pinhole camera (Anton−Paar) operating
with a point-focused Ni-filtered Cu K beam. The patterns were
collected on flat photographic films perpendicular to the X-ray beam.
Powdered samples of the supramolecular complexes were placed into
quartz Lindemann capillaries (1 mm diameter). SAXS experiments
were performed at the SWING beamline of the French synchrotron
facility SOLEIL. The X-ray wavelength of λ = 1.24 Å and sample−
detector distance of D = 2000 mm corresponded to a scattering
wavevector range of 0.005 Å−1 < Q < 0.5 Å−1. The beam path was held
under vacuum conditions, except for a path length of about 1 cm,
around the sample. Then scattering data (40 frames of 1 s duration for
each sample) were collected with a 17 × 17 cm2 low-noise AVIEX
CCD detector. The 2D scattering images were radially averaged,
divided by the transmitted intensity, and finally averaged for each
sample.
to cool down to RT, and the solids were filtered off and washed with
acetone. The combined organic solvents were evaporated under
reduced pressure to dryness. The residue was purified by flash
chromatography on silica gel and eluted with dichloromethane,
gradually changing the composition of the eluent to 1:20, ethyl
1
acetate:dichloromethane. Yield: 3.10 g (54%). H NMR (400 MHz,
Transmission Electron Microscopy Observations. P4VP-
(dAZO)x samples for TEM analyses were prepared as follows. The
complexes were dissolved in chloroform to obtain solutions with a
concentration of 1 mg/mL. Thin films of the complexes were then
prepared by drop-casting the polymer solutions onto carbon-coated
copper grids (previously treated in a UV-ozone chamber for 1 min)
followed by thermal annealing. In order to enhance the contrast, the
films were exposed to ruthenium oxide (RuO4) vapor for 1 h before
the measurement. The staining agent was purchased from Poly-
Sciences and used without further purification. In the case of PS-b-
P4VP(dAZO)x, the samples were prepared as follows. A few
milligrams of each material was heated at 200 °C for 1 min and
simultaneously molded to form a pellet with a rectangular prism shape.
After this process, the sample was rapidly quenched to RT. Ultrathin
CDCl3, δ): 7.99−7.89 (m, 12H), 7.80−7.75 (m, 6H), 7.27 (s, 2H),
7.05−6.98 (m, 6H), 4.09−3.97 (m, 12H), 3.89 (s, 3H), 1.88−1.70 (m,
12H), 1.54−1.25 (m, 48H). 13C NMR (100 MHz, CDCl3, δ): 166.89,
162.72, 154.80, 152.78, 146.66, 133.13, 125.44, 124.69, 123.04, 118.69,
118.58, 114.84, 113.15, 108.01, 73.39, 69.12, 68.46, 52.14, 30.36−
29.06, 26.04−25.96. IR (KBr, cm−1): 2228, 1717, 1601, 1472, 1252,
1139, 849. MALDI-TOF MS Calcd for C83H101N9NaO8 [M + Na]+:
1374.8; found: 1375.9.
3,4,5-Tris[12-(4-(4′-cyanophenyldiazo)phenoxy)dodecyloxy)]-
benzoic Acid (dAZO). A mixture of compound 2 (2.00 g, 1.48 mmol),
methanol (25 mL), and aqueous potassium hydroxide (10 mL, 4M)
was heated under reflux, and the evolution of the reaction was
followed by thin layer chromatography. When all the starting material
had been consumed, the reaction mixture was neutralized with
C
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sections of about 50 nm were obtained from the thermally annealed
samples using a Leica ultramicrotome (model EM UC6) equipped
with a diamond knife (Diatome ultra 35°) and picked up on carbon-
coated copper grids. The sections were then exposed to RuO4 vapor
for 1 h at RT to enhance contrast. The staining agent RuO4 was
purchased from PolySciences and used without further purification. A
JEOL-2000 FXIII microscope operating at 200 kV was employed for
all the observations.
investigate the influence of dAZO weight fraction on the
thermal and photoinduced optical properties of the complexes,
the degree of complexation was varied systematically between x
= 0.50 and x = 0.06. Complexes with these x’s are fully miscible
supramolecular blends with also relatively high loading of
azobenzene chromophore (Table 1). For homogenization
purposes, all the mixtures were heated for 1 min at 200 °C, a
temperature well above both the melting point of dAZO and
the glass transition (Tg) of P4VP (see the thermal character-
ization section), and then rapidly quenched to RT. The samples
obtained in this way appeared as homogeneous materials, and
this provided the first evidence for the complete miscibility of
the individual components (vide infra). The formation of
hydrogen-bonding interactions was first investigated by FT-IR.
The FT-IR spectrum of dAZO together with those of
P4VP(AZO)0.50 and P4VP are shown in Figure 2. In the case
Optical Measurements in Films. Thin films of P4VP(dAZO)x
for optical measurements were prepared by casting from chloroform
solutions of the supramolecular complexes onto clean glass substrates.
In the case of PS-b-P4VP(dAZO)x tetrahydrofuran (good solvent of
the material) was used instead of chloroform. The films were heated
up to 200 °C for 1 min and rapidly quenched to RT (by putting the
films onto a cold aluminum plate) before performing the optical
measurements. The thickness of the thermally treated films was
measured using a DEKTAK profilometer, with values in the range of
0.1−4.0 μm. A Varian Cary 500 UV−vis−IR spectrophotometer was
used for optical absorption and dichroism measurements. For the
latter measurements a linear polarizer was introduced in front of the
film. The dichroic ratio is defined as the ratio of the maximum
absorption measured using linearly polarized light perpendicular and
parallel to the polarization of the 488 nm beam, respectively.
Birefringence (Δn) measurements were performed using a standard
setup reported elsewhere.12 The sample was placed between crossed
polarizers with their polarization directions at 45° with the vertical
axis and irradiated with vertically polarized 488 nm light from an Ar+
laser. The light from a 633 nm He−Ne laser (30 mW) transmitted
through the polarizer−sample−polarizer system was measured with a
Si photodetector. It was confirmed that 633 nm light at this power did
not introduce any significant change in the optical properties of the
film. The transmitted intensity I is given by eq 1:
I = I0 sin2(π|Δn|d/λ)
(1)
Figure 2. IR spectra of azodendron dAZO (black dashed line),
P4VP(dAZO)0.50 (black full line), PS-b-P4VP(dAZO)0.50 (gray full
line), and P4VP (gray dashed line).
where I0 is the intensity transmitted by the “as quenched” films
between parallel polarizers, d the film thickness, and λ the wavelength
of the measuring light (633 nm).
of the azodendron, the stretching modes and Fermi resonances
of the hydroxyl groups involved in hydrogen-bonded dimers are
detected as three bands at 3070, 2640, and 2580 cm−1. The
appearance of two new bands around 2500 cm−1 (broad band)
and 1940 cm−1 in the spectra of the complexes are attributed to
the O−H stretching band and its Fermi resonance of nonionic
OH groups bound to pyridine groups. Also, nonassociated
pyridine groups of P4VP contribute to the absorption at 992
cm−1. However, the intensity of this band decreases in
P4VP(AZO)x, and a new absorption peak is detected at 1014
cm−1, which is related to pyridine rings involved in hydrogen
bonding.24 Additionally, the carbonyl band of dAZO shifts from
1682 to 1700 cm−1, indicating the formation of hydrogen-
bonding interactions between the carboxylic acid group of
dAZO and the pyridine groups of P4VP.25
3.2. Thermal Characterization of P4VP(dAZO)x Supra-
molecular Complexes. The thermal stability of the P4VP
homopolymer and azodendron dAZO was studied by
thermogravimetry (TGA) using powdered samples. The
polymer and the azodendron showed good thermal stability
up to 250 °C, with onsets of decomposition at temperatures
higher than 300 °C (Figure S2). The thermal and
thermodynamic properties of the supramolecular complexes
P4VP(dAZO)x together with those corresponding to their
individual components, P4VP and dAZO, were studied by
polarizing optical microscopy (POM) and differential scanning
calorimetry (DSC). While P4VP is an amorphous polymer (Tg
∼ 140 °C), dAZO exhibits monotropic liquid crystalline
behavior, with a melting transition at 151 °C and an isotropic-
3. RESULTS AND DISCUSSION
3.1. Preparation and Characterization of dAZO
Azodendron and P4VP(dAZO)x Supramolecular Com-
plexes. Azodendron dAZO was obtained following the
convergent synthetic approach depicted in Scheme 1. In the
first step, 4-cyano-4′-hydroxyazobenzene is alkylated with and
excess of 1,12-dibromododecane under standard Williamson
etherifications conditions to yield 1. Subsequent etherification
with methyl gallate under similar conditions yielded 2. Target
dAZO was obtained by alkaline hydrolysis of the methyl ester
group of 2. The evolution of the saponification reaction was
monitored by FT-IR spectroscopythe shift of the carbonyl
stretching absorbance toward longer wavelengths (from 1717
to 1682 cm−1), together with the appearance of the band
associated with the O−H stretching of the aromatic carboxylic
acid group of dAZO at 3420 cm−1, indicated the successful
hydrolysis of the methyl ester group of 2. The azodendron was
purified using standard column chromatography techniques and
1
fully characterized. The H NMR spectrum of the azodendron
is consistent with the proposed chemical structure and
confirmed the purity of this compound (Figure S1). In
addition, mass spectrometric analysis of dAZO showed the
expected peak, and no other signals corresponding to dendrons
with a partial functionalization or traces of nonhydrolyzed ester
2 were detected. With dAZO in hand, the supramolecular
complexes P4VP(dAZO)x were prepared by mixing chloroform
solutions of the corresponding amount of polymer and
azodendron followed by slow evaporation at RT. In order to
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to-mesophase transition at 140 °C (detected on the DSC by
using a sweep rate of 10 °C/min; see Figure S3). The liquid
crystalline phase was classified as SmA on account of the fan-
shaped textures observed by POM (Figure S4). A latent heat
value at the I−SmA transition of ΔHI−SmA ∼ 13 kJ mol−1 was
obtained. The combination of dAZO and P4VP leads to
supramolecular complexes with different thermal properties
than those exhibited by the individual components. It has been
previously shown that the noncovalent grafting of low
molecular weight pro-mesogenic or mesogenic compounds to
a non-mesomorphic polymer chain can lead to liquid crystalline
polymeric materials.26 Mesomorphic textures were indeed
observed by POM for P4VP(dAZO)x, which showed
enantiotropic mesomorphism (contrarily to the monotropic
behavior of dAZO). The DSC traces for P4VP(dAZO)x
collected on heating at 10 °C/min are shown in Figure 3,
to the polymer.17 Additionally to the Tg, an endothermic peak
is also detected on heating at around 120−130 °C.27 This
transition was identified as the mesophase-to-isotropic
transition due to the disappearance of the liquid crystal textures
observed under the POM. Mesomorphic textures were detected
again when P4VP(dAZO)x samples were cooled down from the
isotropic state. The nature of the liquid crystalline phase could,
however, not be unambiguously determined by POM
observations due to the poorly defined textures (Figure S5).
The enthalpy change per azobenzene unit at the isotropic-to-
mesophase transition (ΔHi/au in Table 2) for P4VP(dAZO)x is
ca. 4 kJ mol−1, a value which is comparable to that of neat
dAZO. Therefore, the interaction strength between the
azobenzene units is similar for these complexes regardless of
x, and it is not affected after blending dAZO with a non-
mesomorphic material such as P4VP. This is also supported by
the fact that the complexes show similar mesophase temper-
ature range.
In order to gain further insight into the organization of the
liquid crystalline phases of P4VP(dAZO)x, transmission
electron microscopy (TEM) observations were performed on
stained samples of the complexes. The samples were prepared
by casting solutions of the complexes onto copper grids
followed by thermal annealing (the samples were heated to 200
°C for 1 min and then rapidly quenched to RT) and
subsequent staining with RuO4. A clear layered structure with
a periodicity of ca. 40 Å is observed, and this could be indicative
of smectic-type liquid crystalline behavior (a TEM image of the
complex with x = 0.50 is shown in Figure S6 as a representative
example).28 The smectic nature of the mesophase was further
evidenced by XRD studies. The results for a P4VP(dAZO)0.25
fiber sample drawn from the isotropic phase are discussed here
as a representative example. Specimens of the complex were
obtained by heating a powdered sample at 200 °C for 1 min
immediately followed by drawing from the melt (thus resulting
in a fast quenching of the drawn fiber). The diffractogram of
these fibers consists of a pair of arcs perpendicular to the fiber
axis (drawing direction) that corresponds to a layer spacing of
ca. 39 Å (Figure S7). The orientation of the arcs indicates that
the smectic layers orient parallel to the flow direction upon
drawing. In addition, a diffuse halo is also detected in the 4−5 Å
range, which is typically associated with the distance between
mesogenic units. The molecular length of dAZO and a pyridine
repeating unit estimated with Dreiding stereomodels for their
fully extended conformations are 36 and 5 Å, respectively. Since
the calculated length of a dendron bound to a pyridine
repeating unit is comparable to the experimentally obtained
smectic spacing, a monolayer structure is a reasonable model
for the SmA mesophase of the complexes. No significant
differences of the smectic spacing were detected for the rest of
the complexes by XRD analysis.
Figure 3. DSC thermograms of (a) P4VP(dAZO)0.50, (b) P4VP-
(dAZO)0.25, (c) P4VP(dAZO)0.12, and (d) P4VP(dAZO)0.06 corre-
sponding to the second heating cycle at a sweep rate of 10 °C/min
(endotherms are shown upward).
and the corresponding thermal transitions are gathered in Table
2. A single Tg was detected at 34−37 °C on the heating process
which is considerably reduced when compared to the Tg of
pure P4VP. A similar feature has already been described for
other noncovalent azobenzene-containing polymers and
attributed to a plasticization effect of dAZO hydrogen-bonded
Table 2. Transition Temperatures of dAZO and
P4VP(dAZO)x Supramolecular Complexes
a
a
b
supramolecular
complex
Tg
Ti (°C)
ΔHi/au
(°C) [ΔHi (kJ mol−1)]
(kJ mol−1
)
dAZO
2nd cool
2nd heat
2nd cool
2nd heat
2nd cool
2nd heat
2nd cool
2nd heat
2nd cool
140 [13]
4.3
2.9
4.0
3.0
4.1
2.9
4.2
2.8
4.1
P4VP(dAZO)0.50
36
26
37
23
37
22
34
21
132 [790]
130 [1080]
130 [410]
128 [560]
131 [200]
127 [288]
121 [95]
P4VP(dAZO)0.25
P4VP(dAZO)0.12
P4VP(dAZO)0.06
3.3. Preparation and Characterization of PS-b-P4VP-
(dAZO)x Supramolecular Complexes. The confinement of
azobenzene chromophores in one of the blocks of azo-BCs
allows achieving further reduction of the chromophore fraction
compared to homopolymer systems while preserving favorable
interactions between photoactive moieties within the azo
domains.6 Therefore, we combined dAZO with a PS-b-P4VP
BC composed of a PS block (nonabsorbing polymer at the
recording wavelength) and a P4VP block which can be
complexed with H-donor moieties. The general structure of
these complexes is schematically represented in Figure 1, and
their compositions are gathered in Table 1. We have prepared
115 [138]
a
Transition temperatures were determined by DSC from the second
heating/cooling scan (10 °C min−1). Tg is the glass transition
temperature determined at the midpoint of the heat capacity increase.
Ti is the mesophase-to-isotropic transition temperature read at the
maximum of the peak, and ΔHi is the corresponding enthalpy. ΔHi/
au is the enthalpy of the mesophase-to-isotropic transition per
azobenzene unit.
b
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complexes with similar degrees of complexation to those of
P4VP(dAZO)x, for the sake of comparison. PS-b-P4VP-
(dAZO)x was prepared by dissolving the neat BC in
tetrahydrofuran (1−2% w/v solution) and then adding the
desired amount of dAZO dissolved in the same solvent.
Afterward, the solvent was allowed to evaporate at RT, and the
complexes were dried in a vacuum oven at 40 °C. The samples
were heated to 200 °C for 1 min and then rapidly quenched to
RT in order to erase any thermal history of the material. FT-IR
studies verified the formation of hydrogen-bonding interactions
between the pyridine groups and dAZO [a partial FT-IR
spectra of PS-b-P4VP(dAZO)0.50 is shown in Figure 2, as an
example]. The appearance of new bands around 2500 and 1940
cm−1 in the spectra of the complexes was attributed to nonionic
O−H···N bonds, similarly to the homopolymer-based com-
plexes. Also, the characteristic peaks of self-associated
carboxylic groups at 2640 and 2580 cm−1 are not detected in
the complex, thus confirming the absence of dAZO dimers.
3.4. Thermal Characterization and Morphological
study of PS-b-P4VP(dAZO)x Supramolecular Complexes.
Two distinct levels of hierarchical self-assembly can take place
in PS-b-P4VP(dAZO)x complexes: (a) the liquid crystalline
organization within the P4VP(dAZO) domains and (b) the
phase-separated structure due to segregation between PS and
P4VP(dAZO) blocks.15 The hierarchical self-assembly of the
complexes together with their thermodynamic and liquid
crystalline properties were characterized by a combination of
techniques including DSC, POM, XRD, and TEM. The neat
BC is an amorphous material, and as the P4VP weight fraction
is relatively small, only the Tg corresponding to the PS block
(Tg ∼ 95 °C) was detected. As it can be anticipated from
previous results, the efficient incorporation of dAZO into the
P4VP segment significantly modifies the thermal properties of
the individual building blocks, both the BC and the H-donor
dendron. Similarly to P4VP complexes, mesomorphic proper-
ties were detected for PS-b-P4VP(dAZO)x (Table 1).
Thermally treated samples of the complexes were studied by
POM and XRD in order to determine their liquid crystalline
phases. Textures observed under the polarized-light optical
microscope were poorly defined, as expected for complexes
with low dAZO weight fraction (Figures S8 and S9). A
progressive fading and eventual disappearance of the textures
was observed between 175 and 195 °C. We have previously
shown that P4VP(dAZO)x self-assembles into a smectic-type
liquid crystalline organization. The same type of arrangement is
also present in PS-b-P4VP(dAZO)x which was verified by XRD
studies. All PS-b-P4VP(dAZO)x complexes show a diffuse halo
in the wide angle region corresponding to a distance of 4.5 Å,
which is associated with the average distance between
mesogenic moieties (Figure S10a shows, as a representative
example, an X-ray diffraction pattern of a powder sample of PS-
b-P4VP(dAZO)0.25 heated at 200 °C for 1 min followed by
quenching to RT). Also, the absence of sharp Bragg reflections
in the WAXS profiles of the samples confirms that the
formation of crystallized species is fully avoided, at least on the
time scale of the measurements. The scattering peaks related to
the smectic liquid crystalline domains of PS-b-P4VP(dAZO)x, q
and the second order 2q, are present in the SAXS intensity
profiles for all x’s and correspond to a layer spacing of 4.4 nm
(peaks at q > 0.1 Å−1 in Figure 4), which is similar to the
smectic spacing of P4VP(dAZO)x.
Figure 4. SAXS intensity profiles of samples of PS-b-P4VP(dAZO)0.50
,
PS-b-P4VP(dAZO)0.25, PS-b-P4VP(dAZO)0.12, and PS-b-P4VP-
(dAZO)0.06 (from bottom to top) heated at 200 °C and fast quenched
to RT.
(dAZO)x collected on heating at 10 °C/min are depicted in
Figure 5, and the transitions are gathered in Table 3. Two clear
Figure 5. DSC thermograms of (a) PS-b-P4VP(dAZO)0.50, (b) PS-b-
P4VP(dAZO)0.25, (c) PS-b-P4VP(dAZO)0.12, and (d) PS-b-P4VP-
(dAZO)0.06 corresponding to the second heating cycle at a sweep rate
of 10 °C/min (endotherms are shown upward).
Table 3. Transition Temperatures of PS-b-P4VP(dAZO)x
Supramolecular Complexes
a
a
a
supramolecular
complex
Tg1
Tg2
Ti
TODT
(°C)
(°C)
(°C)
(°C)
PS-b-
P4VP(dAZO)0.50
2nd heat
2nd cool
36
24
80
67
112
126
173
PS-b-
P4VP(dAZO)0.25
2nd heat
2nd cool
38
27
82
69
117
189
178
PS-b-
P4VP(dAZO)0.12
2nd heat
2nd cool
38
40
90
80
PS-b-
P4VP(dAZO)0.06
2nd heat
2nd cool
28
22
76
68
189
176
a
Transition temperatures were determined by DSC from the second
heating scan (10 °C min−1). Tg is the glass transition temperature
determined at the midpoint of the heat capacity increase. Ti is the
mesophase-to-isotropic transition temperature read at the maximum of
the peak.
The main thermal transitions of PS-b-P4VP(dAZO)x were
also investigated by DSC. The DSC curves of PS-b-P4VP-
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Tgs were detected for all the complexes. The transition at lower
temperature corresponds to the P4VP(dAZO) block while the
one at higher temperature is associated with the PS block. The
P4VP(dAZO) block Tg range is similar to that of the
homopolymer-based series. On the other hand, the PS Tg is
slightly reduced when compared to the neat BC, a feature that
might be related to the plasticization effect of a minor fraction
of nonbound dAZO dispersed in the PS domains. Additionally
to the Tgs, two endotermic peaks were identified on the heating
scans of each of the two complexes with higher degree of
complexation. While no significant changes of the mesomor-
phic textures were observed at the lower temperature transition,
the disappearance of the textures (at 175−195 °C) for all PS-b-
P4VP(dAZO)x is related to the transition at higher temper-
ature. We have preliminary assigned the lower temperature
transition to the isotropization of the liquid crystalline ordering
(Ti in Table 3) of the P4VP(dAZO) domains as it is relatively
close to the smectic-to-isotropic transition temperature of
P4VP(dAZO)x complexes. This feature was confirmed by the
disappearance of the smectic halo in the XRD patterns of PS-b-
P4VP(dAZO)0.50 and PS-b-P4VP(dAZO)0.25 at temperatures
higher than Ti (Figure S10). Since the isotropization of the
smectic ordering is likely to take place at around 110−120 °C
but the birefringent textures persist at higher temperatures, we
have tentatively assigned the second endothermic peak to the
order-to-disorder phase transition of the supramolecular BCs
(order-to-disorder transition, TODT in Table 3).
The phase segregation of diblock copolymers, A-b-B, is
typically dictated by the Flory−Huggins interaction parameter,
χ, the degree of polymerization, N, and the volume fraction of
the blocks, fA and fB. In general, diblock copolymers with a low
value for χN show a disordered state disregarding the volume
fraction of the blocks. For volume fractions of the minority
block and χN values above certain limits, diblock copolymers
can show different microphase-separated states depending on
the volume fraction of the blocks. Matsen and Bates have
predicted the phase diagram of diclock copolymers in between
the weak (χN ≤ 10) and the strong segregation regime (χN ≥
10) where spherical, cylindrical, and lamellar morphologies are
described for increasing volume fractions of the minor
block.29,30 Assuming that PS and P4VP(AZO) are the two
distinct blocks and also that the phase behavior of PS-b-
P4VP(dAZO)x will be analogous to a regular diblock
copolymer, the selective incorporation of dAZO into the
P4VP domains of the parent BC can alter its original
morphology on account of a change in its composition (i.e.,
volume fraction of the blocks) as well as the interaction
parameter. Also, the induction of liquid crystalline properties
may affect the phase separation behavior.31,32 We have studied
the microdomain morphology of the parent diblock copolymer
as well as those of the supramolecular BC complexes by TEM.
As described in the Experimental Section, the samples were
heated at 200 °C for 1 min and rapidly quenched to RT.
Ultrathin sections were stained with RuO4 and studied by
TEM. No clear phase separation was observed for the parent
PS-b-P4VP (Figure S11). Typical TEM bright-field images
corresponding to the supramolecular complexes are shown in
Figure 6. A clear lamellar morphology was found for PS-b-
P4VP(dAZO)0.50. In the cases of PS-b-P4VP(dAZO)0.25, PS-b-
P4VP(dAZO)0.12, and P4VP(dAZO)0.06, the segregated mor-
phologies could be consistent with both cylindrical and
spherical morphologies. SAXS measurements performed on
thermally treated samples (annealing at 200 °C for 1 min and
Figure 6. TEM bright-field micrographs of (a) PS-b-P4VP(dAZO)0.06
(b) PS-b-P4VP(dAZO)0.25, and (c) PS-b-P4VP(dAZO)0.50 (the white
bar is 100 nm).
,
subsequent quenching to RT) allowed us to clarify which are
the types of segregated morphologies of the complexes as well
as the domain spacings. The SAXS intensity profiles of PS-b-
P4VP(dAZO)x are shown in Figure 4. The scattering peaks in
the q < 0.1 Å−1 region of PS-b-P4VP(dAZO)0.50 are at a ratio of
1:2, which is characteristic for a lamellar morphology. The
domain spacing is calculated to be about 26 nm. Two scattering
signals are also observed in the q < 0.1 Å−1 region of the
scattering profiles of PS-b-P4VP(dAZO)0.25 and PS-b-P4VP-
(dAZO)0.12one intense peak at lower q and another one at
higher q, which appears as a shoulder of the main peak. These
two signals are at a ratio of 1:√3 and indicate a hexagonally
packed cylinder structure with domain spacing between the
(100) planes of 18 and 15 nm for x = 0.25 and x = 0.12,
respectively. The reflections in the SAXS profile of PS-b-
P4VP(dAZO)0.06 are not as well resolved as in the cases above;
however, in comparison to PS-b-P4VP(dAZO)0.25 and PS-b-
P4VP(dAZO)0.12, the main peak at lower q is shifted toward
lower q values, which may indicate a body-centered cubic
sphere structure. In general, these results are in agreement with
the previously presented diagram for BCs. The different
morphologies of the complexes are related to the changes in
their composition. The parent BC did not show clear phase
separation. However, an increase in the weight fraction of the
P4VP block after complexationfrom a P4VP weight fraction
of 0.07 of the parent PS-b-P4VP to a P4VP(dAZO) weight
fraction of 0.12 for PS-b-P4VP(dAZO)0.06induces the
formation of segregated microstructures, probably with a
body-centered cubic sphere structure. Several groups including
our own have observed spherical morphologies for H-bonded
complexes based on sphere-forming PS-b-P4VP with similar
22,33
weight fraction to PS-b-P4VP(dAZO)0.06
.
As x increases,
the evolution of morphologies, from spherical (x = 0.06), to
cylindrical (x = 0.12, 0.25), and eventually to lamellar phases (x
= 0.50), is ascribed to the increase in the weight fraction of the
P4VP(dAZO) block and qualitatively follows the typical phase
diagram predicted for BCs. Also, the presence of anisotropic
liquid crystalline phases probably favors the stabilization of
lamellar and cylindrical morphologies at relatively low P4VP-
(dAZO)x weight fractions. It has been suggested that the
addition of H-donor additives to PS-b-P4VP might decrease the
χ parameter between PS and P4VP (thus changing the value for
χN accordingly);34 however, it may still remain in the strong
segregation regime as microphase-separated structures have
been observed even at low x.
3.5. Photoinduced Optical Response. This section
includes a discussion on the optical properties of the
supramolecular complexes both in solution and in the solid
state and also a study on the photoinduced optical anisotropy
in films of the title compounds. The spectrum corresponding to
dAZO dissolved in chloroform is characterized by a strong and
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quite symmetric band with a maximum at 367 nm,
corresponding to the π−π* transition of the trans-cyanoazo-
benzene moiety, and a weak band in the 450 nm region
associated with the n−π* transition of the same configurational
isomer (Figure S12). No significant changes were found in the
absorption spectra of the supramolecular complexes in solution
(Figure 7). Films for optical measurements were obtained by
Figure 8. Evolution of the photoinduced birefringence in thermally
treated films of complexes (a) P4VP(dAZO)0.25 and (b) PS-b-
P4VP(dAZO)0.25 under irradiation with 488 nm linearly polarized
(films were irradiated between t = 0 and t = 60 min).
a
Table 4. Values of |Δn|norm after the Irradiation and
Relaxation Periods and Equilibrium Dichroic Ratio
Figure 7. UV−vis absorption spectra of P4VP(dAZO)0.50 (a) and PS-
b-P4VP(dAZO)0.50 (b) in thin film (dashed lines) and in chloroform
and tetrahydrofuran solutions, respectively (full lines).
complex
x
|Δn|norm × 103
dichroic ratio
P4VP(dAZO)x
0.06
0.12
0.25
0.50
0.06
0.12
0.25
0.50
1.5
4.9
4.9
5.3
5.5
3.2
4.0
3.8
3.7
1.6
1.6
casting solutions of the complexes onto clean glass substrates
[chloroform and tetrahydrofuran were used as solvents for
P4VP(dAZO)x and PS-b-P4VP(dAZO)x respectively]. The
films were heated at 200 °C for 1 min in order to erase any
previous thermal history and then rapidly cooled down to RT.
The main absorption band is clearly broadened and shifted
toward shorter wavelengths in the absorption spectra of the
films (Figure 7). These features are more pronounced in the
complexes with higher x and have been previously attributed to
chromophore−chromophore intermolecular interactions.19,35
Photoinduced anisotropy has been extensively reported in a
broad variety of azobenzene-containing materials.36−39 This
phenomenon is due to the preferential orientation of
azobenzene chromophores with their long axis perpendicular
to the light polarization direction through trans−cis−trans
photoinduced isomerization cycles.1 In our case, optical
anisotropy was induced in thermally treated films of the
complexes by using 488 nm linearly polarized light (2.8 W/
cm2). After a 60 min irradiation period, the samples were kept
in darkness and the birefringence was monitored for further 30
min. As representative examples, Figure 8 shows the evolution
of |Δn| for the complexes with x = 0.25. Birefringence increased
by the action of polarized light until saturation. When the 488
nm light was switched off, |Δn| either remained constant or
slightly increased until quickly reaching a final stable value (the
increase in birefringence after switching off the light was always
smaller than 5−10% of the |Δn| saturation value at the end of
the irradiation period). A similar qualitative evolution of the
|Δn| was found for the rest of the complexes. Also, the same
values of |Δn| were obtained for samples of the same material
with different thickness (film thickness values in the range of
0.1−4.0 μm). As |Δn| depends on the azobenzene content, we
discuss our results in terms of normalized photoinduced
birefringence |Δn|norm which is obtained by dividing |Δn| by the
azobenzene content of the supramolecular complex (AZO wt %
in Table 1). The final |Δn|norm values after the irradiation and
relaxation periods for all the complexes are gathered in Table 4.
Final |Δn|norm values were approximately constant for each of
the series of complexes with |Δn|norm = 0.0015−0.0016 for
1.6
PS-b-P4VP(dAZO)x
0.87
0.97
0.94
0.94
a
|Δn|norm is obtained by dividing |Δn| by the azobenzene content of the
supramolecular complex (AZO wt % in Table 1).
P4VP(dAZO)x and |Δn|norm ∼ 0.0009 for PS-b-P4VP(dAZO)x.
In all cases, the photoinduced birefringence could be
completely erased by thermal annealing, i.e., heating at 200
°C for 1 min followed by quenching to RT.
Dichroism measurements were performed on films of the
complexes to further characterize the photoinduced optical
anisotropy. The polarized optical absorption spectra corre-
sponding to films of P4VP(dAZO)0.25 and PS-b-P4VP-
(dAZO)0.25 irradiated in the same conditions as in the
photoinduced anisotropy experiments are gathered in Figure
9 as representative examples (the polarized optical absorption
spectra of all the complexes are shown in Figure S13). The
Figure 9. Stable polarized absorption spectra of (a) P4VP(dAZO)0.25
and (b) PS-b-P4VP(dAZO)0.25 measured at RT after irradiation with
488 nm linearly polarized light for 60 min and subsequent evolution in
darkness for 30 min at the same temperature.
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measuring light was polarized either perpendicular or parallel to
the polarization direction of the 488 nm light. The equilibrium
dichroic ratios obtained after the irradiation and relaxation
periods for all the studied complexes are gathered in Table 4.
High and stable dichroic ratios were obtained for the complexes
with values in the ranges 4.9−5.5 and 3.2−4.0 for P4VP-
(dAZO)x and PS-b-P4VP(dAZO)x, respectively. The slightly
lower values of |Δn|norm and dichroic ratios of PS-b-P4VP-
(dAZO)x could be attributed to the inherent phase-separated
nature of the BC-derived complexes, which can partially hinder
photo-orientation processes. Overall, the photoinduced re-
sponse of P4VP(dAZO)x and PS-b-P4VP(dAZO)x is higher
and more stable than to that of recently reported systems based
on 6-[4-(4′-cyanophenylazo)phenyloxy]hexanoic acid. This
clearly highlights the benefits of preorganizing the photo-
chromic moieties in the dendritic tecton dAZO. The final
|Δn|norm values do not substantially decrease at lower xin
contrast to other supramolecular azopolymers19,20,22thus
allowing for a larger reduction of the azobenzene content
without any detrimental effect on |Δn|norm. Although it is still
unclear why |Δn|norm does not depend on x, dendritic liquid
crystal tectons such as dAZOwith preorganized azobenzene
moietiescould induce favorable interdendron cooperative
interactions during complexation. Moreover, they are known to
enhance mesogen-to-mesogen interactions,32 thus resulting in
higher degrees of photoinduced order.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jd529@cam.ac.uk (J.d.B.).
*E-mail: loriol@unizar.es (L.O.).
*E-mail: carloss@unizar.es (C.S.S.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
a
́
The authors acknowledge Dr. M Angeles Laguna of the
́
Servicio de Microscopıa Electron
́
ica-Universidad de Zaragoza
for her help with TEM image analyses. Eva Blaso acknowledges
a JAE-predoc grant. This work was supported by the MICINN,
Spain, under the project MAT2011-27978-C02-01 and
MAT2011-27978-C02-02, FEDER and DGA funding.
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ASSOCIATED CONTENT
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* Supporting Information
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available free of charge via the Internet at http://pubs.acs.org.
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