Photoresponsive polymers and block copolymers by molecular recognition based on multiple hydrogen bonds

Article abstract of DOI:10.1002/pola.27373

A new methacrylate containing a 2,6-diacylaminopyridine (DAP) group was synthesized and polymerized via RAFT polymerization to prepare homopolymethacrylates (PDAP) and diblock copolymers combined with a poly(methyl methacrylate) block (PMMA-b-PDAP). These

Full text of DOI:10.1002/pola.27373

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Photoresponsive Polymers and Block Copolymers by Molecular
Recognition Based on Multiple Hydrogen Bonds
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Alberto Concellon, Eva Blasco, Milagros Pinol, Luis Oriol, Isabel Dıez, Cristina Berges,
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Carlos Sanchez-Somolinos, Rafael Alcala
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Departamento de Quımica Organica, Facultad de Ciencias, Instituto de Ciencia de Materiales de Aragon (ICMA),
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
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Departamento de Fısica de la Materia Condensada, Facultad de Ciencias, Instituto de Ciencia de Materiales de Aragon (ICMA),
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
Correspondence to: L. Oriol (E-mail: loriol@unizar.es)
Received 25 June 2014; accepted 5 August 2014; published online 00 Month 2014
DOI: 10.1002/pola.27373
ABSTRACT: A new methacrylate containing a 2,6-diacylamino-
pyridine (DAP) group was synthesized and polymerized via
RAFT polymerization to prepare homopolymethacrylates
(PDAP) and diblock copolymers combined with a poly(methyl
methacrylate) block (PMMA-b-PDAP). These polymers can be
easily complexed with azobenzene chromophores having thy-
mine (tAZO) or carboxylic groups with a dendritic structure
(dAZO), which can form either three or two hydrogen bonds
with the DAP groups, respectively. The supramolecular poly-
mers were characterized by spectroscopic techniques, optical
microscopy, TGA, and DSC. The supramolecular polymers and
block copolymers with dAZO exhibited mesomorphic proper-
ties meanwhile with tAZO are amorphous materials. The
response of the supramolecular polymers to irradiation with
linearly polarized light was also investigated founding that sta-
ble optical anisotropy can be photoinduced in all the materials
although higher values of birefringence and dichroism were
obtained in polymers containing the dendrimeric chromophore
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dAZO.
2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
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KEYWORDS: azo polymers; block copolymers; hydrogen bond-
ing; self-assembly; supramolecular polymers
optical storage media,18–20 photoinduced chiral systems,21
photoresponsive surfaces,22,23 or light responsive nanocar-
riers,24 among others. The basis of most reported applica-
tions is the reversible trans-cis photoisomerization
experienced by the azobenzene unit. Furthermore, the rod-
like structure of the azobenzene moieties is also the origin
of the liquid crystalline properties reported for many of
these azopolymers.25,26
INTRODUCTION In recent years, supramolecular chemistry
has become an exciting field of research as a versatile
approach to build up new dynamic functional materials
based on self-assembly of complementary components,1–5 in
particular because these self-assembled materials are easier
to prepare when compared to entirely covalent systems,
which usually require lengthy synthetic procedures. From
the different interactions used to hold the molecular compo-
nents together, H-bonding has been the most extensively
used as a consequence of its directionality and selectivity.6,7
In our group, different series of azobenzene containing
homopolymers and copolymers, mostly based on the cova-
lent binding of chromophores to the main polymeric chain,
have been described for different applications which include
block copolymers for volume holographic storage.27–32 Along
with the covalent one, supramolecular approaches based on
the molecular recognition between chromophores and com-
plementary side groups distributed along a polymeric chain
have been less often considered.33–37 Very recently, we
reported the preparation of a series of photoaddressable
supramolecular polymers by mixing carboxy-terminated
Among functional polymers, pioneering work on supramolec-
8
ꢀ
ular liquid crystal polymers by Kato and Frechet and Lehn
and coworkers,9 has been followed by an increasing interest
in these systems10,11 where a very common approach is
based on macromolecules in which side chain mesogenic
moieties are connected or built up by H-bonding.12–15
The design of azobenzene photoaddressable polymers has
traditionally focused on side-chain structures. These materi-
als have been investigated as photomechanical actuators,16,17
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 Chemical structures and schematic representations of (a) PDAP and PMMA-b-PDAP polymers, (b) dAZO and tAZO azo-
benzene chromophores, and (c) dAZO and (d) tAZO supramolecular complexes. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
azobenzene derivatives and commercially available poly(4-
vinylpyridine)s and polystyrene-b-poly(4-vinylpyridine) block
copolymers, in which the promesogenic azobenzene chromo-
phore was complexed to the polymer via H-bonding.38,39 Low
complexation ratios (containing ꢀ 50 molar % of the acid azo-
derivatives), provided homogeneous liquid crystalline materi-
als where interactions are formed through single-site H-
bonding of the ACOOH groups of azobenzene moieties and the
pyridyl groups of poly(4-vinylpyridine) chains. However, mix-
tures with higher molar ratios of the acids gave heterogeneous
materials with evident signs of macrophase separation38 which
suggests that stronger H-bonding interactions are necessary if
higher functionalized homogeneous materials are required.
Viable option is to promote multiple H-bonding interactions as
in DNA-like complexes. DNA has been an inspiration in polymer
science and, as a consequence, the preparation of synthetic
polymers that mimic DNA has been a challenge. During the last
decades several studies reported the preparation of synthetic
polymers having bases of nucleic acids as side groups or chain-
ends.40–42 Following the approach, Rotello and coworkers
reported nucleobase analogues containing polymers by incor-
poration of side-chain functionalities such as diaminotriazine
and diacylaminopyridine.43–46
ular recognition, we report the synthesis of a series of
polymers obtained from a methacrylic monomer containing a
2,6-diacylaminopyridine unit (monomer coded as DAP). The
corresponding homopolymers (coded a PDAP), and diblock
copolymers with poly (methyl methacrylate) (PMMA; coded
as PMMA-b-PDAP) were complexed with azobenzene deriva-
tives having a complementary thymine (tAZO) or carboxylic
acid group (dAZO) to produce new azopolymers by H-bond
self-assembly (Fig. 1). Compound tAZO contains
a 4-
cyanoazobenzene unit while dAZO has a dendritic structure
with three 4-cyanoazobenzene units. By using a 1:1 molar
ratio of the azocompound to the DAP repeating unit, fully
complexed polymers were prepared to test if homogeneous
materials can be obtained. In the case of the block copoly-
mers, the PDAP block can selectively encapsulate the azo
units whereas the PMMA block confers processability and
transparency at the isomerization wavelength of the azoben-
zene chromophores. Thermal properties and microstructure,
as well as the photoinduced anisotropy induced with linearly
polarized light, were studied.
EXPERIMENTAL
Materials
In an attempt to get homogeneous photoaddressable materi-
als with high degrees of azobenzene complexation via molec-
Methyl methacrylate (Sigma-Aldrich, 99%) was passed
through a basic alumina column, stored over CaH2, and
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vacuum-distilled before use. 2,6-Diaminopyridine (Sigma-
Aldrich, 98%) was recrystallized in ethanol before use. All
other commercially available starting materials were purchased
from Sigma-Aldrich and used as received without further puri-
fication. 3,4,5-Tris[12-(4-(40-cyanophenyldiazo)phenoxy) dode-
cyloxy)]benzoic acid (dAZO) was synthesized according to a
previously described method.39
16.12 mmol), potassium carbonate (3.71 g, 26.86 mmol), 18-
crown-6 (0.28 g, 1.07 mmol) in acetone (10 mL). Then it
was stirred and heated to reflux for 12 h. The reaction was
allowed to cool down to RT and the solids filtered off and
washed with acetone. The solvent was evaporated under
reduced pressure and the residue was purified by flash col-
umn chromatography using hexane/dichloromethane (1:1)
as eluent. Yield: 67%. IR (KBr, m, cm21): 2223, 1600, 1498,
1466, 1257, 1138, 840, 560. 1H NMR (CDCl3, 400 MHz, d,
ppm): 7.9927.90 (m, 4H), 7.8527.75 (m, 2H), 7.0526.98
(m, 2H), 4.06 (t, 2H, J 5 6.5 Hz), 3.41 (t, 2H, J 5 6.9 Hz),
1.9221.77 (m, 4H), 1.5221.21 (m, 16H). 13C NMR (CDCl3,
100 MHz, d, ppm): 162.9, 155.0, 146.9, 133.3, 125.6, 123.2,
118.8, 115.0, 113.3, 68.6, 34.2, 33.0, 29.7, 28.8, 28.3, 26.1.
Synthesis of Monomer DAP
2-Amino-6-propionylamidopyridine (1)
2,6-Diaminopyridine (12.5 g, 114.50 mmol) was dissolved in
dry tetrahydrofuran (THF; 70 mL). A solution of propionyl
chloride (5 mL, 57.60 mmol) in dry THF (10 mL) was added
dropwise over more than 1 h to the solution of 2,6-diamino-
pyridine. The mixture was stirred for 3 h at RT. The precipitate
was filtered off, and the solvent was evaporated. The crude
product was recrystallized in ethanol/toluene (1:6). Yield:
77%. IR (KBr, m, cm21): 3478, 3370, 3226, 1674, 1629, 1541,
1466, 793. 1H NMR (CDCl3, 400 MHz, d, ppm): 7.86 (s, 1H),
7.57–7.48 (m, 1H), 7.42 (dd, 1H, J 5 7.9 Hz, J 5 7.9 Hz), 6.23
(dd, 1H, J 5 7.9 Hz, J 5 0.7 Hz), 4.34 (s, 2H), 2.35 (q, 2H,
J 5 7.5 Hz), 1.19 (t, 3H, J 5 7.5 Hz). 13C NMR (CDCl3, 100 MHz,
d, ppm): 172.3, 157.2, 150.0, 140.3, 104.3, 103.4, 30.8, 9.5.
N(1)-[12-(4-(40-cyanophenyldiazo)phenoxy)
dodecyloxy)]thymine (tAZO)
4-Cyano-40-(1200-bromododecyloxy)azobenzene (3 g, 6.38
mmol), thymine (2.41 g, 19.13 mmol), and potassium carbon-
ate (2.64 g, 19.13 mmol) iꢁn dimethyl sulfoxide (250 mL) were
stirred and heated at 60 C for 12 h. Then, the reaction was
poured into 150 mL of ethyl acetate, washed twice with water
and twice with brine. Finally, the organic layer was dried over
anhydrous magnesium sulfate. The solvent was distilled off
giving an orange solid that was purified by flash column chro-
matography on silica gel using hexane/ethyl acetate (2:1) as
eluent. Yield: 27%. IR (KBr, m, cm21): 3162, 3041, 2229, 1696,
1601, 1582, 1501, 1470, 1250, 1142, 853. 1H NMR (CDCl3,
400 MHz, d, ppm): 8.45 (s, 1H), 7.99–7.88 (m, 4H), 7.82–7.75
(m, 2H), 7.05–6.92 (m, 3H), 4.05 (t, 2H, J 5 6.5 Hz), 3.73–3.63
(m, 2H), 1.92 (t, 3H, J 5 1.8 Hz), 1.87–1.77 (m, 2H), 1.72–1.57
(m, 2H), 1.54–1.21 (m, 16H). 13C NMR (CDCl3, 100 MHz, d,
ppm): 164.2, 162.9, 155.0, 150.8, 146.8, 140.5, 133.3, 125.6,
123.2, 118.8, 115.0, 113.3, 110.6, 68.6, 48.7, 29.7229.1, 26.6,
26.1, 12.5. MS (MALDI1, dithranol, m/z): calcd for
2-(Propionylamino)26-[4-(2-methacryloyloxy ethoxy)24-
oxobutanoylamino]pyridine (DAP)
4-(Dimethylamino) pyridinium p-toluene sulfonate (4.53 g,
14.53 mmol), 2-amino-6-propionylamidopyridine (1) (2.00 g,
12.11 mmol), and 4-[2-(methacryloyloxy)ethoxy]-4-oxobuta-
noic acid (3.35 g, 14.53 mmol) were dissolved in dry
dichloromethane (15 mL). The reaction flask was cooled in
an ice bath and flushed with argon, then N,N0-diisopropylcar-
bodiimide (1.83 g, 14.53 mmol) was added. The mixture was
stirred at RT for 24 h under argon atmosphere. Then, the
reaction was washed twice with water and once with brine.
Finally, the organic layer was dried over anhydrous magne-
sium sulfate and the solvent was evaporated. The crude
product was purified by flash column chromatography on
silica gel using hexane/ethyl acetate (1:1) as eluent. Then
the product was recrystallized in ethanol. Yield: 64%. IR
(KBr, m, cm21): 3336, 1724, 1690, 1639, 1586, 1512, 1453,
1316, 1152, 805. 1H NMR (CDCl3, 400 MHz, d, ppm): 8.10–
7.60 (m, 5H), 6.14–6.09 (m, 1H), 5.60–5.53 (m, 1H), 4.44–
4.30 (m, 4H), 2.85–2.74 (m, 2H), 2.72–2.63 (m, 2H), 2.43 (q,
2H, J 5 7.5 Hz), 1.98–1.88 (m, 3H), 1.24 (t, 3H, J 5 7.5 Hz).
13C NMR (CDCl3, 100 MHz, d, ppm): 172.6, 172.5, 167.4,
149.8, 149.3, 140.9, 136.0, 126.4, 109.6, 109.5, 62.7, 62.5,
32.3, 30.8, 29.4, 18.4, 9.5. MS (MALDI1, dithranol, m/z):
calcd for C18H23N3O6, 377.16; found, 378.17 [M 1 H]1,
400.15 [M 1 Na]1. Anal. calcd for C18H23N3O6: C, 57.29%; H,
6.14%; N, 11.13%. Found: C, 57.32%; H, 6.31%; N, 11.20%.
C
30H37N5O3, 515.29; found, 516.29 [M 1 H]1, 538.29
[M 1 Na]1. Anal. calcd for C30H37N5O3: C, 69.88%; H, 7.23%;
N, 13.58%. Found: C, 69.66%; H, 7.10%; N, 13.59%.
Synthesis of Polymers
Homopolymers PDAPA and PDAPB
The monomer DAP (1.0 g, 2.65 mmol), 4-cyano-4-(phenylcar-
bonothioylthio)pentanoic acid (for PDAPA: 14.0 mg, 0.05
mmol; for PDAPB: 7.0 mg, 0.025 mmol), 2,20-azobisisobutyro-
nitrile (AIBN; for PDAPA: 2.0 mg, 0.012 mmol; for PDAPB:
1.0 mg, 0.006 mmol), and DMF as solvent (3 mL) were
added to a Schlenk flask closed with a rubber septum. The
flask was deoxygenated by three freeze-pump-thaw cycles
and flushed with argon. The reaction mixture was stirred at
80 ꢁC. After 48 h, the mixture was quenched with liquid
nitrogen and diluted with THF, and then it was carefully pre-
cipitated using coldꢁmethanol. The polymer was dried in a
vacuum oven at 40 C for 24 h. Yield: 71% (for PDAPA) and
88% (for PDAPB).
Synthesis of Azocompounds
4-Cyano-40-(1200-bromododecyloxy) azobenzene (see
Scheme S1 in Supporting Information, Compound 2)
A solution of 4-cyano-40-hydroxyazobenzene (3.00 g, 13.43
mmol) in acetone (50 mL) was added dropwise over more
than 1 h to a mixture of 1,12-dibromododecane (5.30 g,
Characterization Data of PDAPA
IR (KBr, m, cm21): 3333, 1736, 1700, 1586, 1516, 1450,
1293, 1152, 803. 1H NMR (CDCl3, 400 MHz, d, ppm):
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9.0828.30 (m, 2H), 7.9827.48 (m,3H), 4.5023.86 (m, 4H),
2.9822.52 (m, 4H), 2.4922.25 (m, 2H), 2.0221.60 (m,2H),
1.4720.64 (m, 6H), 1.4720.64 (m, 6H). Anal. calcd for
2.9322.54 (m, 4H), 2.4622.22 (m, 2H), 2.1420.61 (m, 43H).
Anal. calcd for C4262H6332N252O1604: 59.00%; H, 7.36%; N,
4.07%. Found: C, 58.52%; H, 7.64%; N, 4.14%; S, 0.31%.
C18H23N3O6: 57.29%; H, 6.14%; N, 11.13%. Found: C,
56.16%; H, 6.38%; N, 10.88%; S, 0.34%.
Supramolecular Polymers Preparation
The required amounts of polymer and azobenzene chromo-
phores were weighted and dissolved in THF followed by stir-
ring overnight at room temperature. Finally, the solvent was
slowly evaporated and the mixtures were dried under vac-
Characterization Data of PDAPB
IR (KBr) m (cm21): 3331, 1737, 1700, 1586, 1512, 1449,
1293, 1151 803. 1H NMR (CDCl3, 400 MHz) d (ppm):
9.0528.32 (m, 2H), 8.0027.46 (m,3H), 4.5223.84 (m, 4H),
3.0622.58 (m, 4H), 2.5422.25 (m, 2H), 2.0221.60 (m, 2H),
1.4620.63 (m, 6H). Anal. calcd for C18H23N3O6: 57.29%; H,
6.14%; N, 11.13%. Found: C, 56.16%; H, 6.48%; N, 11.14%;
S, 0.23%.
ꢁ
uum at 40 C for at least 2 days.
Characterization Techniques
IR spectra were obtained on a Bruker Vertex 70 FT-IR spec-
trophotometer. NMR spectra were recorded on a Bruker AV-
400 spectrometer. Elemental analysis was performed using a
Perkin-Elmer 2400 microanalyzer. MALDI-TOF MS was per-
formed on an Autoflex mass spectrometer (Bruker Daltonics)
using dithranol as matrix. Size exclusion chromatography
(SEC) was carried out on a Waters e2695 Alliance liquid
chromatography system equipped with a Waters 2424 evap-
oration light scattering detector and a Waters 2998 PDA
detector using two Styragel columns, HR4 and HR1 from
Waters. Measurements were performed in THF with a flow
of 1 mL min21 using PMMA narrow molecular weight stand-
ards. SEC measurements for PDAP homopolymers were car-
ried out in DMF with LiBr (50 mM) with a flow of 0.5 mL
min21 to avoid aggregation effects observed in THF; in this
case polystyrene (PS) narrow molecular weight standards
were used with the UV-PDA detector. Thermogravimetric
analysis (TGA) was performed using a TA Q5000IR instru-
PMMA-CTA for RAFT Polymerization
Methyl methacrylate (7.5 mL, 70.12 mmol), 4-cyano-4-(phe-
nylcarbonothioylthio)pentanoic acid (32.7 mg, 0.234 mmol),
AIBN (3.85 mg, 0.047 mmol), and toluene (2.5 mL) were
added to a Schlenk flask closed with a rubber septum. The
flask was deoxygenated by three freeze-pump-thaw cycles
and flushed with argon. The reaction mixture was stirred at
60 ꢁC. After 12 h the mixture was quenched with liquid
nitrogen and diluted with THF, and then it was carefully pre-
cipitated using cold methanol. The polymer was dried in a
vacuum oven at 40 ꢁC for 24 h. Yield: 82%. IR (KBr) m
(cm21): 1731, 1244, 1149. 1H NMR (CDCl3, 400 MHz) d
(ppm): 3.71–3.37 (s, 3H), 2.10–0.62 (m, 5H). Anal. calcd for
C5H8O2: C, 59.98%; H, 8.05%. Found: C, 59.81%; H, 8.79%;
N, 0.53%; S, 0.50%.
21
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ment at heating rate of 10 C min under a nitrogen atmos-
phere. Mesogenic behavior was investigated by polarized-
light optical microscopy (POM) using a Olympus BH-2 polar-
izing microscope fitted with a Linkam THMS600 hot stage.
Thermal transitions were determined by differential scan-
ning calorimetry (DSC) using a TA DSC Q-2000 instrument
under nitrogen atmosphere with powdered samples (2–
5 mg) sealed in aluminum pans. Glass transition tempera-
tures (Tg) were determined at the half height of the baseline
jump, and the mesophase-to-isotropic phase transition tem-
peratures were read at the maximum of the corresponding
peaks. X-ray diffraction (XRD) were performed with an evac-
Block Copolymers PMMA-b-PDAPC and PMMA-b-PDAPD
DAP (0.5 g, 1.33 mmol), PMMA-CTA (for PMMA-b-PDAPC:
1.37 g, 0.025 mmol; for PMMA-b-PDAPD: 0.68 g, 0.012
mmol), AIBN (for PMMA-b-PDAPC: 1.0 mg, 0.006 mmol; for
PMMA-b-PDAPD: 0.5 mg, 0.003 mmol) and DMF (for PMMA-
b-PDAPC: 5 mL; for PMMA-b-PDAPD: 3 mL) were added to a
Schlenk flask closed with a rubber septum. The flask was
deoxygenated by three freeze–pump–thaw cycles and flushed
ꢁ
with argon. The reaction mixture was stirred at 80 C. After
48 h the mixture was quenched with liquid nitrogen, and
then it was carefully precipitated using cold methanol. The
polymer was dried in a vacuum oven at 40 ꢁC for 24 h.
Yield: 82% (for PMMA-b-PDAPC) and 68% (for PMMA-b-
PDAPD).
uated Pinhole camera (Anton-Paar) operating
a point-
focused Ni-filtered Cu-Ka 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).
Characterization Data of PMMA-b-PDAPC
IR (KBr) m (cm21): 3340, 1734, 1588, 1452, 1244, 1147. 1H
NMR (CDCl3, 400 MHz)
d (ppm): 8.8628.44 (m,2H),
Samples for transmission electron microscopy (TEM) meas-
urements were prepared as follows. A few milligrams of
each material were molded to form a pellet, the samples
7.9327.50 (m, 3H), 4.3923.92 (m, 4H), 3.68–3.43 (m, 47H),
2.8922.61 (m, 4H), 2.4622.31 (m, 2H), 2.1520.69 (m,
115H). Anal. calcd for C3398H5228N108O1316: 59.45%; H,
7.68%; N, 2.20%. Found: C, 59.00%; H, 8.04%; N, 2.47%; S,
0.28%.
ꢁ
were then heated at 150 C and quenched to room tempera-
ture. After this process, ultrathin sections of about 100 nm
were obtained using a Leica ultramicrotome equipped with a
diamond knife, and picked up on carbon-coated copper grids.
To enhance the contrast, the grids were exposed to ruthe-
nium oxide (RuO4) vapor for 1 h. The staining agent was
purchased from PolySciences and used without further
Characterization Data of PMMA-b-PDAPD
IR (KBr) m (cm21): 3337, 1733, 1588, 1452, 1243, 1149. 1H
NMR (CDCl3, 400 MHz)
7.9927.44 (m, 3H), 4.4623.86 (m, 4H), 3.66–3.49 (m, 20H),
d (ppm): 8.9028.43 (m,2H),
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SCHEME 1 Synthesis of DAP monomer, PDAP homopolymers, and PMMA-b-PDAP block polymers.
purification. TEM was measured using a Tecnai T20 electron
microscope operating at 200 kV.
using polymer standards (MnSEC) are gathered in Table 1.
Calculation of average molar masses of PDAP homopolymers
was not possible by 1H NMR (MnNMR) as end-groups were
not detected by this technique.
Film thickness was measured a DEKTAK profilometer. For
optical absorption and dichroism experiments, a Varian Cary
500 UV–vis-NIR spectrophotometer was used. For dichroism
measurements, a linear polarizer was placed before the film.
Diblock copolymers can be obtained by two sequential RAFT
polymerizations, firstly preparing
a
thiocarbonylthio-
terminated prepolymer that is subsequently used as a
macro-RAFT agent in a second polymerization step. In the
synthesis of PMMA-b-PDAP diblock copolymers, a PMMA
block was first prepared and used as macro-RAFT agent
(PMMA-CTA) (Scheme 1). The molar mass of this PMMA
was determined by SEC using PMMA standards. Then, two
diblock copolymers were prepared by adjusting the mono-
mer DAP to PMMA-CTA molar ratios to get different degree
RESULTS AND DISCUSSION
Synthesis and Characterization of Building Blocks
The polymeric precursors of the supramolecular polymers
were prepared from the monomer DAP, which was obtained
through a nonsymmetric two-steps amidation of 2,6-diami-
nopyridine moiety (Scheme 1). The first step was a monoa-
cylation of 2,6-diaminopyridine with propionyl chloride47
and the second step involved an amidation with 4-[2-(metha-
cryloyloxy) ethoxy]-4-oxobutanoic acid to introduce the
methacrylic polymerizable group. Monomer DAP was puri-
fied by flash column chromatography and subsequent recrys-
tallization and adequately characterized by FTIR, NMR (see
Fig. S1 in Supporting Information), MALDI-TOF MS, and ele-
mental analysis.
TABLE 1 Molar Mass and Molar Mass Distributions of the
Synthesized Polymers
NMR
SEC
Polymer
Mn
Mn
ÐM
PDAPA
–
–
–
60,900a
73,400a
55,000b
67,600b
88,800b
1.19a
1.15a
1.02b
1.04b
1.09b
PDAPB
PMMA-CTA
PMMA-b-PDAPC
PMMA-b-PDAPD
Two homopolymers and two diblock copolymers were pre-
pared by reversible addition2fragmentation chain transfer
(RAFT) polymerization.48,49 PDAP homopolymers were
obtained using the RAFT agent 4-cyano-4-(phenylcarbono-
thioylthio)pentanoic acid (Scheme 1) and adjusting the
monomer to RAFT agent molar ratios to obtain two different
molar masses. The SEC curves showed monomodal molar
68,600c
86,700c
a
SEC
Mn
and ÐM were calculated by SEC using DMF (LiBr 50 mM; 0.5
mL min21) as the mobile phase relative to PS standards.
b
SEC
Mn
mobile phase relative to PMMA standards.
and ÐM were calculated by SEC using THF (1 mL min21) as the
c
NMR
Mn
were calculated by comparing NMR integrated signals of
-
mass distributions with low polydispersity values, DM < 1.2.
The relative average number molar masses estimated by SEC
repeating units of both blocks and using the molar mass of PMMA
block measured by SEC (using PMMA standards).
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FIGURE 2 1H NMR spectrum (400 MHz, CDCl3) recorded for PMMA-b-PDAPD. The degree of polymerization of the PDAP block (y)
was calculated from the relative integration of signals e and o, taking for the PMMA block x 5 550, which is the value determined
by SEC using PMMA standards (see text).
of polymerization of the PDAP block. This time, 1H NMR
hydroxyazobenzene with 1,12-dibromododecane under
standard Williamson conditions, and second by a nucleo-
philic substitution with thymine (see Scheme S1 in Support-
ing Information). Both compounds showed onsets of
decomposition at temperatures higher than 300 ꢁC in the
thermogravimetric study (Table 2). The thermal and meso-
morphic properties of the azocompounds were studied by
POM and DSC (Table 2). tAZO was a crystalline material that
melted to give an isotropic liquid phase. However, dAZO
exhibited a monotropic smectic A (SmA) liquid crystalline
phase, with a melting transitionꢁat 150 ꢁC and an isotropic-
to-mesophase transition at 138 C. The SmA mesophase was
characterized by fan-shaped textures and a latent heat value
at the I-SmA transition of DHI2SmA 5 13 kJ mol21, which is
in accordance with previously described results.39
spectroscopy was employed to calculate average molar
NMR
masses (Mn
in Table 1) of the block copolymers by rela-
tive integration of the PMMA methyl proton signals (OACH3,
coded as e in Fig. 2) and the corresponding ones to the
methylene protons of PDAP block (CH3ACH2ACO, coded as o
in Fig. 2) and taken as reference the Mn value determined by
SEC for PMMA-CTA (MnSEC). SEC curves of PMMA-b-PDAP
showed monomodal molar mass distributions with low poly-
-
dispersities (DM < 1.1) and residual block precursors were
not detected (see Fig. S5 in Supporting Information). Mn val-
ues estimated by SEC were in good agreement with those
1
calculated by H NMR.
Thermal analysis of the polymers was undertaken by TGA
and DSC, and results are collected in Table 2. In the ther-
mogravimetric study, degradation associated to weight loss
ꢁ
was observed above 230 C for all the synthesized polymeric
Preparation and Characterization of PDAP and PMMA-b-
PDAP Supramolecular Complexes
precursors. All these polymers were amorphous materials as
determined from the DSC curves where clear baseline jumps
corresponding to the glass transitions were only observed.
PDAP homopolymers exhibited a Tg around 75 ꢁC. Diblock
copolymers showed two glass transitions, which points to a
microphase separation of both amorphous blocks. The lowest
Tg corresponds to the PDAP block while the highest one is
assigned to the PMMA block.
The supramolecular polymeric materials were prepared by
dissolving the corresponding amount of the polymer (PDAPA,
PDAPB, PMMA-b-PDAPC, or PMMA-b-PDAPD) and the azo
chromophore (dAZO or tAZO) in THF, followed by stirring
and slow evaporation of the solvent. The amount of chromo-
phore was calculated to complex all the DAP moieties. For
comparison purposes, all the mixtures were heated at 150
ꢁC for 5 min and then rapidly quenched to room tempera-
ture. The mixtures obtained in this way appeared as homo-
geneous materials, providing evidence for the formation of
an H-bonded complex. To confirm the existence of H-bonding
interactions between DAP derivative units and tAZO or
Two different photochromic azobenzene derivatives were
used for complexation with the described polymers, dAZO
and tAZO. The azodendrimer dAZO was obtained following a
previously described synthetic approach.39 tAZO was synthe-
sized in two steps; first by alkylation of 4-cyano-40-
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TABLE 2 Thermal Parameters Obtained for the Precursors and Supramolecular Polymers
Chromophoree
(wt %)
a
b
b
c
c
c
Tonset
(ꢁC)
Tg,1
Tg,2
Tm (ꢁC)
TI–M (ꢁC)
Tc (ꢁC)
d
Compound
(ꢁC)
(ꢁC)
[DHm/az.u.]
[DHI–M/az.u.]
[DHc/az.u.]
w
PDAP
PDAPA
245
255
320
235f
240
340
280
280
290
280
345
235
255
250
260
75
73
–
–
–
–
–
1
1
0
–
PDAPB
–
–
–
–
–
PMMA-CTA
123
118
119
–
–
–
–
–
PMMA-b-PDAPC
PMMA-b-PDAPD
tAZO
83
82
–
–
–
–
0.20
0.37
0
–
–
–
–
–
153 [13.4]
–
90 [32.3]
43
25
25
9
{PDAPA ꢂ tAZO}
{PDAPB ꢂ tAZO}
{PMMA-b-PDAPC ꢂ tAZO}
{PMMA-b-PDAPD ꢂ tAZO}
dAZO
49
51
60
54
–
–
–
–
–
1
–
–
–
–
1
113
109
–
–
–
–
0.37
0.58
0
–
–
–
15
50
39
39
20
29
150 [35.0]
146 [28.6]
145 [25.7]
146 [27.0]
145 [29.3]
138 [4.3]
125 [3.6]
124 [3.6]
127 [4.3]
125 [4.2]
89 [23.4]
74 [18.7]
69 [18.2]
75 [16.4]
70 [19.4]
{PDAPA ꢂ dAZO}
{PDAPB ꢂ dAZO}
{PMMA-b-PDAPC ꢂ dAZO}
{PMMA-b-PDAPD ꢂ dAZO}
36
38
34
34
–
1
–
1
107
108
0.53
0.72
a
Onset of the decomposition detected in the thermogravimetric
at the minimum of the peak (second cooling). DH/az.u.: enthalpy nor-
curve.
b
malized by mol of azobenzene unit in kJ mol21
.
d
Glass transition determined by DSC from the second heating scan
Mass fraction of PDAP block (complexed with dAZO or tAZO in case
of supramolecular complexes).
Weight percentage of chromophoric units (4-oxy-40-cyanoazobenzene).
(scan rate: 10 ꢁC min21): Tg,1 corresponds to PDAP and Tg,2 to PMMA.
c
e
T
m: melting temperature read at the maximum of the peak (second
I–M: isotropic-to-mesophase temperature read at the mini-
f
heating).
T
A small weight loss was detected at 165 ꢁC associated to the presence
mum of the peak (second cooling). Tc: crystallization temperature read
volatile compounds.
dAZO, FTIR spectroscopic studies were carried out using
polymer films deposited onto KBr pellets.
of DAP groups and dAZO [Fig. 4(a)]. Also the C@O st. region
[Fig. 4(b)] suffered, after thermal treatment, an absorbance
decrease of the peak at 1730 cm21 and an increase of the
peak at 1685 cm21, most probably due to H-bonding interac-
tions with the DAP core. The carbonyl stretching band in car-
boxylic acids usually appears at 1730 or 1685 cm21 for free
or associated carbonyl groups, respectively. After thermal
treatment, 1730 cm21 band did not completely disappear
because it comprises contributions from the polymer ester
groups. These changes reveal that the thermal annealing is
required for a good homogenization. In a similar way, ther-
mal treatment of dAZO containing supramolecular diblock
copolymers induced a decrease of the wavenumber of the
NAH st. vibration (see Fig. S8 in Supporting Information). In
addition, after thermal treatment the band at 1685 cm21
(C@O st. region) was clearly detected. All these changes,
which are similar for the homopolymer-based complexes,
were attributed to H-bonding interactions between comple-
mentary dAZO and DAP repeating units.
On complexing with tAZO, the IR spectra of the supramolec-
ular homopolymers or copolymers recorded for the just
evaporated and the thermal annealed samples are very simi-
lar (see Fig. S6 in Supporting Information). Because the 2,6-
diaminopyridine repeating unit can form three H-bonds with
the thymine group, preparation of the supramolecular poly-
mers by mixing in solution and evaporation probably leads
to a essentially stable and homogeneous system and, for this
reason, thermal treatment did not cause any significant
changes in the IR spectra. In these polymers, complexation
induced a decrease of the wavenumber of the NAH st. vibra-
tion band of DAP unit indicating the H-bond formation with
the photochromic units containing thymine groups [Fig.
3(a)]. In the CAO st. region, a decrease of around 5–
10 cm21 in the wavenumber of the CAO st. vibration of thy-
mine group (1696 cm21) after complexation also supported
the H-bonding interactions between complementary tAZO
and DAP repeating units [Fig. 3(b)].
Thermal Characterization of Supramolecular Complexes
The thermogravimetric study of the supramolecular com-
plexes revealed that weight losses associated to the presence
of residual solvents or water were not detected and all mate-
rials showed good thermal stability up to around 250 ꢁC
(Table 2). The thermal transitions and mesomorphic proper-
ties were studied by POM and DSC and the results are
In the case of supramolecular homopolymers containing
dAZO, where the DAP unit can form two H-bonds with the
ꢁ
carboxylic acid, the thermal annealing at 150 C was accom-
panied by significant changes in different regions of the IR
spectra. The thermal treatment induced a shift to lower
wavenumber of the NAH st. vibration due to the association
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FIGURE 3 FTIR spectra of {PDAPB
• tAZO} (black), PDAPB
FIGURE 4 FTIR spectra of {PDAPB • dAZO} before (solid) and
after (dash) thermal annealing at 150 ꢁC, 5 min: (a) NAH st.
and (b) C@O st. region. (See Supporting Information for the
spectra of the corresponding block copolymers).
(gray), and tAZO (dash) as obtained (no significant variations
were observed after thermal annealing): (a) NAH st. and (b)
C@O st. region. (See Supporting Information for the spectra of
the corresponding block copolymers).
mation Figure S9 displays a typical smectic texture observed
by POM for dAZO-containing supramolecular homopolymers
when was cooled down from the isotropic state (dAZO-com-
plexed block copolymers exhibited less defined smectic tex-
tures as it is observed in Figure S10, Supporting
Information). This monotropic liquid crystal phase was
exhibited in a temperature range similar to that of the chro-
mophore precursor dAZO. Thermal transition temperatures
and enthalpy values for these complexes were investigated
gathered in Table 2. It should be mentioned that all the
supramolecular polymers appeared as homogeneous materi-
als by POM in heating-cooling cycles. In the case of dAZO
complexes, the stꢁudy of the samples was carried out after
annealing at 150 C. Segregation of the components was not
observed for any of the complexes, which seems to indicate
the formation of
interactions.
a single material through H-bonding
Supramolecular homopolymers containing tAZO were amor-
phous materials, DSC curves showed a baseline jump corre-
sponding to the glass transition (Fig. 5). Glass transition of
these supramolecular complexes was detected around 50 ꢁC
and was lower than that of the corresponding PDAP poly-
meric precursor. This fact can be attributed to a plasticiza-
tion effect of the pendant tAZO moieties hydrogen bonded to
the polymer. In the case of block copolymers containing
tAZO, DSC curves showed two glass transitions agreeing
with the constituent amorphous blocks, PMMA and the com-
plexed PDAP blocks (Fig. 5). The presence of the two glass
transitions seems to evidence the typical microphase segre-
gation of block copolymers with a confinement of the azo-
benzene chomophores in the nanodomains of PDAP blocks.
FIGURE 5 DSC traces of: (a) {PDAPB • tAZO}, (b) PDAPB, (c)
{PMMA-b-PDAPD • tAZO}, and (d) PMMA-b-PDAPD correspond-
ing to the second heating scan (10 ꢁC min21; Exo down).
In contrast to tAZO, the dendritic azobenzene derivative
dAZO gave rise to mesomorphic materials. Supporting Infor-
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Dreiding stereomodels for its fully extended conformations is
36 Å and for the DAP repeating unit is 5 Å. Since the calcu-
lated length of dAZO plus the DAP repeating unit is compa-
rable to the experimentally obtained smectic spacing, a
monolayer structure is a reasonable model for the SmA mes-
ophase of the complexed polymers. The measured smectic
spacing was similar for all polymers regardless of the molec-
ular weight and the structure (homo or block copolymer
structure) of the polymers.
The supramolecular complexes of PMMA-b-PDAP block poly-
mers complexed with dAZO chromophores can lead to mate-
rials with two hierarchical self-assembly levels: (a) the liquid
crystalline organization of the chromophores within the
PDAP domains and (b) the phase separated structures
because of segregation between the PMMA and PDAP form-
ing hydrogen bonds with dAZO blocks.50,51 In fact, the
appearance of several thermal events in DSC ascribable to
the different blocks already suggested phase segregation in
these materials. It is well known that diblock copolymers are
able to undergo microphase separation in the solid state
leading to different morphologies such as spheres, cylinders
or lamellaes. This microphase separation depends on several
parameters and in particular on the volume fraction of the
blocks.52,53 We have studied the phase segregation of the
parent diblock copolymers as well as their corresponding
supramolecular complexes by TEM. As described in the
Experimental section, the samples were annealed at 150 ꢁC
and then quenched to room temperature. PMMA-b-PDAPC
showed a weakly phase segregated morphology [Fig. 7(a)],
while phase segregation, with cylindrical morphology, was
FIGURE 6 DSC traces of: (a) {PMMA-b-PDAPD • dAZO}, (b)
{PDAPB • dAZO}, and (d) dAZO corresponding to the second
cooling scan (10 ꢁC min21; Exo down).
by DSC (Fig. 6 and Table 2). It can be observed that DH/az.u.
(enthalpies normalized per mol of azobenzene unit) were
similar to that of neat dAZO, therefore mesophase stability is
not highly affected by complexing dAZO with a nonliquid
crystal material. To characterize the structural parameters of
mesophase, XRD studies were carried out. At low angles the
first-order reflection of the smectic layers was detected at
around 42 Å for samples rapidly cooled from the isotropic
state to RT (Supporting Information Fig. S9). In addition, a
diffuse halo was also detected in the 425 Å range which is
typically associated to molecules in a short range distance
correlation. The molecular length of dAZO estimated with
FIGURE 7 TEM bright field micrographs of: (a) PMMA-b-PDAPC, (b) PMMA-b-PDAPD, (c) {PMMA-b-PDAPC • tAZO}, (d) {PMMA-b-
PDAPD • tAZO}, (e) {PMMA-b-PDAPC • dAZO}, and (f) {PMMA-b-PDAPD • dAZO} (the size of the black bar is 200 nm).
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observed in Figure 7(b) for PMMA-b-PDAPD. In the supra-
molecular block copolymers, the incorporation of H-bonded
azoderivates into the PDAP domains of the parent diblock
copolymers alters their original morphology because of a
change in the composition (Table 2). TEM images show a
cylindrical morphology for {PMMA-b-PDAPC • tAZO} [Fig.
7(c)] while a lamellar one seems to be observed for {PMMA-
b-PDAPD • tAZO} [Fig. 7(d)]. In the case of supramolecular
block copolymers with dAZO, block copolymer {PMMA-b-
PDAPC • dAZO} did not show a clear and well-defined mor-
phology [Fig. 7(e)], although a lamellar one was expected
according to its composition, meanwhile {PMMA-b-PDAPD
dAZO} seems to be cylindrical [Fig. 7(f)].
•
Optical Properties and Photoinduced Optical Anisotropy
The optical absorption, as well as the photoinduced birefrin-
gence (Dn) and dichroism of the four supramolecular block
copolymers have been investigated. The results were differ-
ent for dAZO and tAZO containing supramolecular polymers
but similar within each family, once normalized to the same
azo content. Because of this, only the results corresponding
to {PMMA-b-PDAPC • dAZO} and {PMMA-b-PDAPC • tAZO}
will be reported as representative examples of each family.
Films for optical studies (0.5–1 mm thick) were prepared by
casting dichloromethane solutions of the supramolecular
complexes onto glass substrates. Prior to perform optical
measurements, films were annealed in air at 150 ꢁC for 5
min and rapidly quenched to RT.
FIGURE 8 Time evolution of the normalized birefringence of
UV–vis absorption spectra of the films after thermal treat-
ment are given in Supporting Information (Fig. S11). They
showed a strong band with a peak in the 355–365 nm
region, together with a shoulder at about 450 nm. These
absorptions correspond to the p-p* and n-p* transitions of
the trans azo moiety, respectively. The width of the UV band
is higher in the dAZO copolymers than in the tAZO ones.
Besides, the band position in dAZOs appears at higher
energy than in tAZOs. This can be associated with the pres-
ence of azobenzene H aggregates that are favored in the
dAZO due to its dendrimeric structure.
the block copolymers {PMMA-b-PDAPC
• dAZO} (top) and
{PMMA-b-PDAPC • tAZO} (bottom) under irradiation with line-
arly polarized 488 nm light. Exciting light was switched off at
60 min.
final value of |Dn|norm was about 0.8 3 1023 for these two
copolymers.
For complexes with dAZO, |Dn| also showed a fast increase
in the first 5 min and then it continued growing more slowly
without reaching a saturation value (in 60 min), even when
increasing the irradiation time (up to 140 min) or the energy
density (up to 2W cm22). In this case, when the 488 nm
light was switched off |Dn|norm did not decrease (even a
slow increase was observed in some of the measurements)
|Dn| values were obtained following the procedure described
elsewhere.29 Linearly polarized light (488 nm, 750 mW
cm22) was used to induce anisotropy. In order to compare
samples with different azo content we have calculated the
normalized birefringence, |Dn|norm (values given in Table 2),
by dividing |Dn| by the content of azochromophore (only
considering the 4-oxy-40-cyanoazobenzene unit) in the com-
plexes. The time evolution of |Dn|norm with irradiation time
is given in Figure 8. Films were first irradiated for 60 min
with the 488 nm light. This light was switched off for other
30 min.
reaching values up of |Dn|norm up to 2.1 3 1023
.
The different time evolution and final values of photoinduced
|Dn|norm in the two types of polymers can be understood
taking into account the liquid crystalline behavior of poly-
mers containing the dendrimeric dAZO moities in contrast
to amorphous polymers containing tAZO. It is known that
the interactions of azo moieties in liquid crystal polymers
generally improve both the photoinduced response and its
stability.
For complexes containing tAZO, |Dn| increased initially until
saturation (in about 5 min) and kept the same value for lon-
ger irradiation times (up to 60 min) and then, when the
488 nm light was switched off, |Dn| decreased. A final stable
value of about 70% of the maximum one was reached. The
Dichroism measurements have also been performed in the
block copolymers. The polarized optical absorption of
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weight and having DAP pendant groups, which were fully
complexed with azobenzene chromophores having thymine
or carboxyl complementary groups. The spectroscopic study
of these materials reveals that the supramolecular polymers
behave as homogenous materials, although a previous ther-
mal annealing at the isotropic state is needed in case of
polymers complexed with dAZO chromophores. The photoor-
ientational properties of these materials are similar to other
azobenzene covalent polymers. Thus it can be concluded that
this synthetic methodology is an effective and easy approach
to obtain photoaddressable polymers and avoids the time-
consuming procedures associated to the preparation of azo-
monomers, as well as the synthetic problems found in the
radical polymerization of azo (meth)acrylic monomers.54,55
ACKNOWLEDGMENTS
This work was supported by the MINECO, Spain, under the pro-
ject MAT2011–27978-C02-01 and 202, Fondo Europeo de
ꢀ
Desarrollo Regional (FEDER), Gobierno de Aragon and Fondo
Social Europeo. Authors would also like to acknowledge the
Advanced Microscopy Laboratory and Servicio de Microscopia
ꢀ
Electronica de Materiales of the Universidad de Zaragoza (Ser-
ꢀ
ꢀ
vicios de Apoyo a la Investigacion, SAI). I. Dıez acknowledges
funding from CSIC and Fondo Social Europeo through a JAE-
Doc fellowship.
REFERENCES AND NOTES
FIGURE 9 Polarized absorption spectra, of the {PMMA-b-PDAPC
• dAZO} (top) and {PMMA-b-PDAPC • tAZO} (bottom) copoly-
mers, after irradiation with linearly polarized 488 nm light for
60 min: parallel (dashed line) and perpendicular (continuous).
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