Thermostable birefringent copolyimide films based on azobenzene-containing pyrimidine diamines

Article abstract of DOI:10.1039/c7tc02958f

Azobenzene (azo)-containing polymers have attracted continuous attention due to their excellent photosensitivity. However, practical applications of their photo-induced birefringence have been hampered by low thermal stability. In this work, we report a s

Full text of DOI:10.1039/c7tc02958f

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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PAPER
Thermostable birefringent copolyimide films based on
azobenzene-containing pyrimidine diamines†
Received 00th January 20xx,
Accepted 00th January 20xx
Faqin Tong, Zhao Chen, Xuemin Lu*, and Qinghua Lu*
Azobenzene (azo)-containing polymers have attracted continuous attention due to their excellent photosensitivity.
However, practical applications of their photo-induced birefringence have been hampered by low thermal stability. In this
work, we report a series of polyimides synthesized by a low-temperature co-polycondensation of azo-containing pyrimidyl
diamine, 4,4'-diaminodiphenyl ether, and 4,4'-oxydiphthalic anhydride. The obtained co-polyimides (co-PIs) show good
DOI: 10.1039/x0xx00000x
www.rsc.org/
thermal stability, with high glass transition temperatures (Tg) of more than 200 °C and 5% weight loss temperatures (T5%
)
as high as about 400 °C in nitrogen. Their photo-induced birefringence (ꢀn) has been determined as 0.013 at room
temperature and up to 0.017 at 100 °C. The photo-induced reorientation property of the as-prepared highly thermostable
azo-PIs is expected to have wide applications in image recording and electro-optical devices.
that a PI system doped with methyl orange dye displayed
photosensitivity, although the photo-induced birefringence,
1. Introduction
ꢀ
n, was only ca. 0.00038 owing to the steric hindrance of the
The azobenzene (azo) unit is extremely well-known and widely
investigated because of its unique photoresponse feature.
Under light irradiation, trans-to-cis isomerization of the azo
group leads to changes in geometrical structure, dipole
moment, and spatial volume at the molecular level, and
furthermore in refractive index and colors at the macroscopic
level.1 Azo-containing polymers have been extensively studied
due to their excellent photosensitivity and corresponding
diverse promising applications in fields such as high-density
optical data storage, liquid-crystal devices, optical waveguides,
diffractive elements, and so forth.2–6 However, for most azo-
containing polymers, low stability of their photo-induced
anisotropy under high-temperature conditions is their main
drawback, which results from the poor thermal stability of
PI. Schab-Balcerzak et al.16, 17, 18 reported a series of poly(ester
imide)s and poly(ether imide)s with azo units as side chains,
which showed high birefringence up to 0.02. However,
incorporation of long alkoxy spacers reduced the Tg values to
20
140–170
°
poly(amideimide)s with two azochromophores per structural
C.
Konieczkowska
et
al.19,
reported
unit, which showed a high birefringence up to 0.075 and a high
Tg values of 280 °C. However, the thermal stability of the
photo-induced birefringence of the polymers was not
investigated. Thus, from a practical point of view, there is still
great demand for the design and construction of azo-PIs with
higher thermal stability and photo-induced birefringence.
As reported, the introduction of heterocyclic aromatic
structures into the main chains of PIs is considered to be an
effective approach for improving thermal resistance. Various
novel PI materials with heterocyclic structures in their main
chains, such as pyridine,21 triazine,22 naphthalene,23 furan,24 or
pyrimidine,25 have been reported. For example, Chen et al.
systematically studied single electro-spun nanofibers and
aligned nanofiber belts from co-PIs with pyrimidine units,26
and confirmed that incorporation of rigid pyrimidine units
improved the thermal performance of PI. Meanwhile,
pyrimidine rings and sulfur atoms with high molar refractions
have been incorporated to improve the optical properties of
PIs.27, 28, 29 High optical transparency is the principal concern in
designing optical polymers for high-performance components
for advanced display devices.
most polymer matrices, such as polyacrylate,7, 8 poly(methyl
10
methacrylate) (PMMA),9,
and polyethylene terephthalate
(PET),11 which generally have low Tg values of 80–150
°C.
Polyimides (PIs), especially aromatic PIs, which have
excellent thermal, mechanical, and electrical properties, as
well as good chemical resistance and low susceptibility to laser
damage, represent good polymer matrix candidates for
photoresponsive materials with high thermal stability. Azo-
containing PIs (azo-PIs) have been reported as photosensitive
materials, for example, as photoalignment layers for liquid
crystals,12 photomechanical response materials,13 and photo-
induced surface relief gratings.14 Zlatanova et al.15 reported
Polymers containing azo moieties in their side chains
generally require a spacer between the rigid polymer main
chain and the azo groups to impart the necessary flexibility for
the required movement.30 Stumpe and co-workers studied the
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effect of spacer length of the azo unit on the reorientation pore size: 10 μm, column size: 300 × 7.5 mm) and a refractive
behavior of liquid-crystalline polymers.31 The orientational index detector. DMF (0.03 mol L−1 LiBr and 0.03 mol L–1 H3PO4)
order increased or decreased, depending on the enthalpic and polystyrene were used as the eluent (elution rate:
stability of the mesophase, which changed with spacer length. 1.0 mL min−1) and the calibration standard, respectively.
However, to the best of our knowledge, photo-induced Thermogravimetric analysis (TGA) was performed using a TA
birefringence in azo-PIs containing azo side chains with TGA Q5000IR instrument (TA Instruments, Inc., New Castle,
different spacer lengths has not hitherto been studied.
At present, there is scarce report on the photo-induced room temperature to 700
birefringence in the polyimides containing pyrimidine aromatic calorimetry (DSC) was performed on
structures. In this work, we have designed and synthesized a instrument at a heating rate of 10
DE, USA). Samples were heated at a rate of 20
C in nitrogen. Differential scanning
TA DSC Q2000
C min−1 in nitrogen.
°
C min−1 from
°
a
°
series of novel photosensitive azo-PIs by polycondensation of Absorption spectra of PI films were measured on a Perkin-
azo-containing pyrimidyl diamine, 4,4'-diaminodiphenyl ether Elmer Lambda 750 UV/Vis spectrophotometer. Tensile tests
(ODA), and 4,4'-oxydiphthalic anhydride (ODPA). Different were
performed
on
samples
of
dimensions
alkyl spacer lengths between the main chain and the azo unit 30 mm×3 mm×20 μm at 25
°
C using an Instron 4465 test
were introduced in the side chains with thioether linkages to machine (Instron Corp., Canton, MA, USA).
optimize the photo-induced birefringence. The level and 2.3. Monomer Synthesis
dynamics of the generated birefringence, as well as its thermal
stability, were further investigated
.
2. Experimental
2.1. Materials
Sodium nitrite, phenol, potassium carbonate, 4-
methoxyaniline, sodium hydroxide, and hydrochloric acid were
purchased from Sinopharm Chemical Reagent Co. (Shanghai,
China). 4,6-Diamino-2-mercapto-pyrimidine was purchased
from Aladdin (Shanghai, China). 1,2-Dibromoethane, 1,4-
dibromobutane, 1,6-dibromohexane, and 1,9-dibromononane
were obtained from J&K Reagent Co. (Beijing, China). 4,4'-
Diaminodiphenyl ether (ODA) was purchased from Alfa Aesar
(Ward Hill, MA, USA) and purified by recrystallization from
ethanol. 4,4'-Oxydiphthalic anhydride (ODPA; >99%, Suzhou
Yacoo Chemical Reagent Corporation, Jiangsu, China) was
purified by recrystallization from acetic anhydride and dried
under vacuum prior to use. N,N'-Dimethylformamide (DMF)
and N,N'-dimethylacetamide (DMAc) were distilled under
reduced pressure after stirring over calcium hydride for 24 h
and stored over 4 Å molecular sieves before use.
Scheme 1. Synthetic route to the diamine monomers.
4-Hydroxy-4′-methoxyazobenzene (1) was synthesized
according to the reported method.32
1-Bromo-2-(4-methoxyazobenzene-4'-oxy)ethane (A1)33: A
mixture of
1 (4.56 g, 0.02 mol), 1,2-dibromoethane (7.44 g,
0.04 mol), potassium carbonate (4.2 g, 0.03 mol), and acetone
was stirred under reflux for 24 h. The reaction mixture was
2.2. Characterization
Proton and carbon nuclear magnetic resonance (1H NMR and filtered when hot, and the residue was washed with acetone.
13C NMR) spectra were recorded on a Varian Mercury Plus The acetone was removed under reduced pressure and
400 MHz spectrometer (Varian, Palo Alto, CA, USA) using petroleum ether was added to the concentrated organic
deuterated dimethylsulfoxide ([D6]DMSO) or chloroform extract. The resulting precipitate was collected and dried. The
(CDCl3) as solvent at room temperature, and the chemical crude product was recrystallized from ethanol. Yield: 5.2 g,
1
shifts were referenced relative to tetramethylsilane (TMS). 78%. H NMR (400 MHz, CDCl3, δ (ppm)): 7.93 (ArH, 4H), 7.01
Attenuated total reflection–Fourier-transform infrared (ATR- (ArH, 4H), 4.38 (OCH2, 2H), 3.90 (CH3O, 3H), 3.70 (CH2Br, 2H).
FTIR) spectra were recorded on a Perkin-Elmer Spectrum 100
1-Bromo-4-(4-methoxyazobenzene-4'-oxy)butane (A2): A2
FTIR spectrophotometer (Perkin Elmer, Norwalk, CT, USA). was prepared by a procedure analogous to that described for
High-resolution mass spectrometric (HRMS) analysis was A1. The product was obtained by recrystallization from ethanol
performed on
cyclotron resonance mass spectrometer (MS) (Bruker 4H), 7.01 (ArH, 4H), 4.10 (OCH2, 2H), 3.91 (CH3O, 3H), 3.52
Daltonics, Karlsruhe, Germany). Elemental analyses were (CH2Br, 2H), 2.10–1.98 (C2
4CH2Br, 4H).
performed using Vario EL Cube. Molecular weights and
1-Bromo-6-(4-methoxyazobenzene-4'-oxy)hexane (A3)
polydispersities were estimated using a Perkin-Elmer Series was prepared by a procedure analogous to that described for
200 gel permeation chromatograph (GPC) at 40 C equipped A1. After recrystallization from ethanol, the final A3 was
with two linear mixed-B columns (Polymer Lab Corporation; obtained as a yellow solid in 80% yield. 1H NMR (400 MHz,
a
SolariX-70FT-MS Fourier-transform ion in 72% yield. 1H NMR (400 MHz, CDCl3, δ (ppm)): 7.90 (ArH,
H
:
A3
°
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CDCl3, δ (ppm)): 7.95 (ArH, 4H), 7.02 (ArH, 4H), 4.07 (OCH2, 6.06 (NH2, 4H), 5.14 (ArH, 1H), 4.07 (OCH2, 2H), 3.86 (OCH3,
2H), 3.91 (CH3O, 3H), 3.46 (CH2Br, 2H), 1.94 (C
1.86 (OCH2C 2, 2H), 1.66–1.46 (OCH2CH2C2H4CH2CH2Br, 4H).
1-Bromo-9-(4-methoxyazobenzene-4'-oxy)nonane (A4.5)
A4.5 was prepared by a procedure analogous to that described
H2CH2Br, 2H), 3H), 2.94 (SCH2, 2H), 1.73–1.35 (OCH2C7H14CH2S, 14H). High-
H
resolution MS (ESI, m/z): [M+] calcd for C26H34N6O2S: 494.25;
found: 494.25.
:
The chemical structures of the diamines were confirmed by
for A1. The product was obtained by recrystallization from melting point analysis (data in the Experimental part) and by
1
ethanol in 68% yield. H NMR (400 MHz, CDCl3, δ (ppm)): 7.91 1H and 13C NMR. All of the spectroscopic data obtained were
1
(ArH, 4H), 7.02 (ArH, 4H), 4.07 (OCH2, 2H), 3.91 (CH3O, 3H), consistent with the proposed structures. The H NMR peaks of
3.44 (CH2Br, 2H), 1.95–1.34 (C7
2-((2-(4-Methoxyazobenzene-4'-oxy)ethyl)thio)-pyrimidine- shown in Figure 1. Their 13C NMR spectra are provided in
4,6-diamine (S2)34
In flask, 4,6-diamino-2-mercapto-
H
14CH2Br, 14H).
the respective diamines could be unequivocally assigned, as
:
a
pyrimidine (1.42 g, 10 mmol), aqueous NaOH (0.25 m, 42 mL),
and methanol (40 mL) were combined. The reaction mixture
was stirred with a magnetic stirrer at room temperature for
2 h to form the corresponding sodium thiolate salt. The solvent
was then evaporated under vacuum. The dry sodium thiolate
salt was dissolved in dry DMF (60 mL), and A1 (3.67 g,
11 mmol, 1.1 equiv) was added. The resulting yellow solution
was stirred at room temperature for 12 h. The progress of the
reaction was monitored by thin-layer chromatography (TLC)
(CH2Cl2/MeOH, 19:1). After completion, the reaction was
quenched with water, and the mixture was extracted with
CH2Cl2. The organic layer was collected and concentrated. The
crude material was purified by flash chromatography
(CH2Cl2/MeOH, 9:1) to give the desired product. The product
was obtained as a yellow solid in 78.3% yield (3.1 g), m.p. 215–
Figure 1. 1H NMR spectra of the diamines in [D6] DMSO.
216 °
C. 1H NMR (400 MHz, [D6] DMSO, δ(ppm)): 7.85 (ArH, 4H),
7.13 (ArH, 4H), 6.13 (NH2, 4H), 5.13 (ArH, 1H), 4.29 (OCH2, 2H),
3.86 (OCH3, 3H), 3.39 (SCH2, 2H). High-resolution MS (ESI, m/z):
[M+] calcd for C19H20N6O2S: 396.14; found: 396.14.
Figure S1.
2.4. Preparation of azo-PIs
2-((4-(4-Methoxyazobenzene-4'-oxy)butyl)thio)pyrimidine- In this work, Sn/ODA/ODPA with a molar ratio of 0–0.3:1–0.7:1
4,6-diamine (S4): Diamine S4 was prepared according to an was applied to prepare PIx-Sn (Scheme 2). Here, “x” in PIx-Sn
denotes the molar ratio of the azo-containing pyrimidyl
experimental procedure analogous to that described for S2
,
except that the reaction time was 16 h in DMF. After diamine to the ODA in the co-polymer, and “n” denotes the
recrystallization from ethanol, the final S4 was obtained as a spacer length of the alkyl chain. The synthesis of azo-PI is
yellow solid in 80% yield, m.p. 196–197 °
C. 1H NMR (400 MHz, exemplified by the case of PI1-S6
. S6 (0.4526 g, 1 mmol) and
[D6] DMSO, δ(ppm)): 7.84 (ArH, 4H), 7.11 (ArH, 4H), 6.06 (NH2, ODA (1.8022 g, 9 mmol) were dissolved in DMAc (16.12 g) in a
4H), 5.15 (ArH, 1H), 4.09 (OCH2, 2H), 3.86 (OCH3, 3H), 3.04 100 mL three-necked flask fitted with a mechanical stirrer and
(SCH2, 2H), 1.86 (C
resolution MS (ESI, m/z): [M+] calcd for C21H24N6O2S: 424.17; flask was cooled to 5
found: 424.17.
(3.1022 g, 10 mmol) in DMAc (5.31 g) was added, and the
H
2CH2O, 2H), 1.77 (SCH2C
H
2, 2H). High- a nitrogen inlet. After complete dissolution of the diamine, the
°C in an ice bath. A solution of ODPA
2-((6-(4-Methoxyazobenzene-4'-oxy)hexyl)thio)pyrimidine- resultant mixture was stirred under a gentle flow of dry
4,6-diamine (S6): Diamine S6 was prepared according to an nitrogen. After 10 h, a viscous, homogeneous solution of
experimental procedure analogous to that described for S2
After recrystallization from ethanol, the final S6 was obtained
as a yellow solid in 85% yield, m.p. 192–193
C. 1H NMR
.
°
(400 MHz, [D6]DMSO, δ(ppm)): 7.84 (ArH, 4H), 7.11 (ArH, 4H),
6.07 (NH2, 4H), 5.13 (ArH, 1H), 4.07 (OCH2, 2H), 3.86 (OCH3,
3H), 2.98 (SCH2, 2H), 1.77 (OCH2CH2, 2H), 1.62 (SCH2CH2, 2H),
1.41–1.44 (OCH2CH2C2H4CH2CH2S, 4H). High-resolution MS
(ESI, m/z): [M+] calcd for C23H28N6O2S: 452.57; found: 452.19.
2-((9-(4-Methoxyazobenzene-4'-oxy)nonyl)thio)pyrimidine-
4,6-diamine (S9): Diamine S9 was prepared according to an
experimental procedure analogous to that described for S2
After recrystallization from ethanol, the final S9 was obtained
as a yellow solid in 80% yield, m.p. 181–183
C. 1H NMR
.
°
(400 MHz, [D6] DMSO, δ(ppm)): 7.84 (ArH, 4H), 7.10 (ArH, 4H),
Scheme 2. Synthetic route to all of the azo-PIs.
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polyamic acid (PAA) had formed.
3. Results and discussion
1
PAA1-S6: H NMR (400 MHz, [D6] DMSO, δ(ppm)): 1.41–
1.44 (OCH2CH2C2H4CH2CH2S, 4H), 1.62 (SCH2CH2, 2H), 1.77
(OCH2CH2, 2H), 2.98 (SCH2, 2H), 3.86 (OCH3, 3H), 4.07 (OCH2,
2H), 5.13 (ArH, 1H), 7.11 (ArH, 4H), 7.24 (ArH, 4H), 7.53
(ArH,4H), 7.67 (ArH, 4H), 7.81 (ArH,4H), 7.95 (ArH, 4H), 10.37
3.1. Synthesis of azo-PIs
(CONH, 2H), 10.43 (CONH, 2H), 13.03 (COOH, 4H). (PI1-S6
:
Anal. Calcd. for C291H156N24O61S (4993): C, 69.93%; N, 6.73%; H,
3.12%. Found: C, 68.52%; N, 7.46%; H, 3.52%.)
1
The H NMR of other PAAs and the elemental analysis of
Figure 3. (a) GPC chromatogram of PAA3-S6; eluent: DMF containing LiBr and
H3PO4. (b) UV-vis absorption spectra of the PAAs of PIx-S6 in DMAc solution
(concentration 0.05 mg/mL) and S6 in DMAc solution (concentration 0.008
mg/mL).
the corresponding azo-PIs are shown in supporting
information.
2.5. Preparation of azo-PI films
The pyrimidine-containing azo-PIs were synthesized by
Synthesis of the azo-PI films is exemplified by the case of PI1-
reaction of Sx/ODA/ODPA systems in molar proportions of 0–
0.3:1–0.7:1 according to a conventional two-step procedure as
shown in Scheme 2. The molecular weights of the PAAs were
measured by GPC and are summarized in Table S1. The GPC
trace for PAA3-S6 is shown in Figure 3(a). No peaks due to
S6
solution cast onto a glass plate. This involved a preheating
program (70 C/1 h, 90 C/2 h, 110 C/1 h) followed by the
imidization procedure (200 C/2 h, 220 C/2 h, 250 C/2 h) to
. PI1-S6 film was obtained by thermal imidization of a PAA
°
°
°
°
°
°
produce a fully imidized PI film. The PI1-S6 film was exfoliated
by placing the glass plate in hot water. The other azo-PI films
were prepared analogously.
small molecules were observed in the GPC trace for PAA3-S6
,
indicating that the pyrimidyl diamine had been covalently
incorporated into the polymer chains. UV absorbance spectra
of the corresponding PAAs of the respective azo-PIs are shown
in Figure 3(b), and those of the others are shown in Figures S2–
S4. The UV absorption intensity at 360 nm, corresponding to π-
π* electronic transition of the azo chromophore, increased in
2.6. Photo-induced birefringence
The experimental set-up for measurements of photo-induced
birefringence is shown schematically in Figure 2. He-Ne laser
beam (λ = 632.8 nm) was used as a probe light beam. Any
light-imposed molecular reorientation in an azo-functionalized
the order PI0-S6
< PI1-S6 < PI2-S6 < PI3-S6. This result further
polymer film results in the creation of in-plane optical
proved the successful synthesis of azo-PAAs.
anisotropy, that is, birefringence.
A continuous linearly
ATR FTIR spectra were also recorded to confirm the
successful synthesis of azo-PIs. The ATR FTIR spectra of PIx-S6
polarized 405 nm laser was used as the pump light, and the
beam intensity was about 200 mW/cm2. The thickness for the
polymer films utilized in the photoinduced birefringence
measurement was 20 μm by means of a film thickness
measuring instruments. The absorption coefficient value of PI
films at the wavelength of 405 nm was 1.73×103 cm-1, 2.62×103
cm-1, and 2.73×103 cm-1 for PI1-S6
, PI2-S6, and PI3-S6,
respectively. The values increased with the amount of
pyrimidyl diamine increasing, which can be attributed to the
absorption of azo chromophore. The birefringence (Δn) was
calculated according to the following equation:
I = I0 sin2(πΔnd/λ)
where I is transmitted intensity of the probing light through
crossed-polarizer set-up, I0 is the intensity of the probe beam,
λ is the wavelength of the probe light, and d is the thickness of
the film.
Figure 4. ATR FTIR spectra of PIx-S6 films.
films are shown in Figure 4 as representative cases. They were
measured after thermal imidization at 250 °C. The ATR FTIR
spectra of the other films after imidization are shown in
Figures S5–S7. The characteristic IR bands of PAA (carbonyl
=1651 cm−1 and
absorption at
absorption at
ν
a
broad carboxylic acid
ν
=3387 cm−1) had clearly disappeared in all of
the ATR FTIR spectra, indicating the successful conversion of
PAAs to PIs. Strong bands at
=1780 and 1714 cm–1,
ν
attributable to the -C=O asymmetrical and symmetrical
stretching vibrations of imide rings, were also observed for all
Figure 2. Experimental set-up for measuring photo-induced birefringence.
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of the azo-PIs. The absorption peak at
corresponded to imide ring deformation, and the bands at for the second heating scans of the azo-PIs. The length of the
=1375 and 1244–1103 cm–1 could be assigned to vibrations of side chain proved to have little effect on the Tg of azo-PIs. The
the C(Ph)–N and C–O–C units, respectively. The absorption
peak at
=1563 cm–1 could be attributed to a stretching
vibration of the pyrimidine ring. The increased intensity of this
peak at
=1563 cm–1 reflected the changes in the molar ratio
ν
=720 cm–1 data are summarized in Table S1. Figure 6 shows DSC curves
ν
ν
ν
of Sn/ODA in the respective azo-PI backbones.
3.2. Properties of azo-PIs
3.2.1.
The obtained films were yellow in color, which could be
attributed to the azo unit, as shown in Figure 5 for PIx-S6
Film quality and mechanical properties of azo-PIs
,
while the others are shown in Figures S8–S10. The average
thickness of each azo-PI film was estimated as about 20 μm by
means of
a film thickness measuring instrument. The
mechanical properties of the azo-PI films, including tensile
strength, tensile modulus, as well as elongation at break, are
summarized in Table S2, except those for PI3-S2, which proved
to be too fragile to be subjected to tensile tests. Each result
was obtained from five drawing experiments. The tensile
strengths were in the range 64–127 MPa, elongations at break
were in the range 3.1–9.6%, and tensile moduli were in the
range 1.94–3.02 GPa. With increasing amounts of rigid
pyrimidyl diamine, the molecular eights of the PIs decreased
and their mechanical properties deteriorated.35 However, the
collected tensile data demonstrated that the films prepared
from these azo-PIs were tough and could be used for further
processing as optical coating layers.
Figure 6. DSC second heating scan curves of the azo-PIs at 20 °C/min.
Tg values decreased with increasing amount of pyrimidyl
diamine, which can be attributed to the introduction of more
azo side chains. Although the introduction of heterocyclic
aromatic structure into PI led to an enhancement of thermal
stability, the appending of long side chains resulted in loose
chain packing and weak intermolecular interactions.
Therefore, the free volume in the polymer system increased,
which reduced the Tg values of the azo-PIs.36 Notably, these
azo-PIs exhibited better thermal stability, with higher Tg, than
that of previously reported azo-PI materials.17
The thermal stabilities of the azo-PIs were further estimated
from the decomposition temperatures corresponding to 5%
and 10% weight losses (Td5 and Td10), as shown in Figure 7 and
summarized in Table S1. PI0, containing no alkyl chain, was
thermally stable up to 510
°
°
C. All of the obtained azo-PIs
C and Td10 above 400 C. The
showed high Td5 above 370
°
thermal properties of the azo-PIs deteriorated with the
introduction of alkoxyl side chains. Moreover, their char yields
after pyrolysis at 700 °C were higher than 40%. Compared with
PI0, the azo-PIs showed two-step weight-loss processes.37 The
weight loss associated with the first step was equivalent to the
weight of the side chain, indicating that the side chains began
to degrade at ca. 400 °C. The second step of weight loss
Figure 5. Optical photographs of PIx-S6 films.
occurred over the same temperature range as the degradation
of PI0. This suggests that the first step of weight loss was
related to degradation of the side chains, and the second step
corresponded to degradation of the PI main chain.
3.2.2.
Thermal properties of the azo-PIs
The thermal properties of the azo-PIs were evaluated by DSC
as well as TGA under a nitrogen atmosphere, and the thermal
3.2.3.
Photo-induced birefringence
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J. Name., 2013, 00, 1-3 | 5
Figure 7. TGA curves of azo-PIs at 20 °C /min in nitrogen.
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Journal of Materials Chemistry C
Figure 8. Photo-induced birefringence recorded for thin films of PIs with a 405 nm
linearly polarized beam at room temperature. The points at which the pumping
light was switched on and off are marked with arrows.
Figure 9. Photo-induced birefringence of PI1-S6 and PI1-S9 recorded with a 405 nm
linearly polarized beam at 25 and 100 °C.
at 100 °C. The decay for PI1-S9 was more obvious. This is
because the longer spacer is beneficial for molecular
The photo-induced birefringence of the azo-PIs was
investigated using a 405 nm laser as pump source. As shown in
Figure 8, when the pump beam was switched on, a slow
increase in the transmitted light was recorded. The
birefringence (Δn) increased with increasing irradiation time
movement of the azo group.
The samples irradiated at different temperatures were
annealed to further investigate the thermal stability of the
photo-induced birefringence. As shown in Figure 10, for PI1-S6
the birefringence decays were monitored at 25 C, 100 C, and
150 C. A characteristic feature of these decays was that none
,
and reached
a high saturation value after 280 s. The
°
°
birefringence values of PI1-S6 and PI1-S9 were found to be
higher than those of the other films, with high Δn values of
0.0134 for PI1-S6 and 0.0122 for PI1-S9 at room temperature.
These results clearly proved that the spacer length of the azo
°
of them reached the zero level after relaxation in darkness.
According to previous reports, this was because, after the
reorientation, some of the molecules remained “frozen” in
their positions, which depended on the Tg value of the
polymer.40 Szukalski et al. studied the photo-induced
birefringence of pyrazoline-doped PMMA, revealing that the
birefringence decreased to 54.7% after relaxation in darkness
for 200 s at room temperature.41 Rochon studied the optically
induced birefringence of azo-aromatic polymers. The
transmitted signal was observed to decrease to 60% of the
saturation value just 10 s after the writing beam was turned
off.40 In this work, after relaxation treatment for 1000 s, the
birefringence was observed to decrease to 59.4% of the
units had
a significant effect on the photo-induced
birefringence, whereby the trans-to-cis isomerization of the
azo groups occurred much less readily in the rigid main chain
of PIs with shorter spacer lengths. When the spacer was longer
than six methylene units, the effect on birefringence was not
obvious, similar to the phenomenon reported by Wu Yiliang.30
The authors studied the effect of spacer length of the
azobenzene unit on alignment behavior of liquid crystal
polymer was investigated by poly(methyl methacrylate). The
results proved that alignment change is more difficult to be
generated in the liquid crystal polymer having a short spacer
length of azo units. Finally, it was noted that the photo-
induced birefringence of the azo-PIs showed a slight decrease
after turning off the pump light at room temperature. The
highest photo-induced birefringence value among the PIx-S6
was that of the PI1-S6 film, which had a relatively low azo
content of 10%. It is assumed that most of the molecules were
located in larger free volumes when undergoing isomerization.
The decrease in birefringence with increasing azo content can
be attributed to the lower free volume, which limited in-plane
isomerization of the azo group, thereby reducing the efficiency
of the photo-induced birefringence process.38
saturation value at 25
molecules did not remain oriented on a long-term basis;
nevertheless, significant number remained and the
°C. This result indicated that some of the
a
birefringence was observed to be quite stable for a long
In order to reveal the influence of temperature on the
birefringence of the azo-PIs, PI1-S6 and PI1-S9 were chosen as
samples. As shown in Figure 9, when the pump beam was
switched on, a faster increase in the transmitted light was
recorded at 100 °C than at 25
was higher than that at 25
°C. The birefringence at 100 °C
C for both azo-PIs. This could be
°
attributed to certain motions of the azo groups, such as
thermally induced cis-to-trans isomerization.39 In addition,
switching off the pump beam led to a faster decay for azo-PIs
Figure 10. Changes in optical birefringence during dark relaxation processes for PI1-S6
after exposure to different temperatures.
6 | J. Name., 2012, 00, 1-3
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time37. The birefringence decay to 54.8% of the saturation
polyester containing azobenzene moieties, Macromolecules,
2005, 38, 10480-10486.
A. Bobrovsky, V. Shibaev, V. Hamplova, M. Kaspar and M.
Glogarova, Chirooptical and photooptical properties of a
novel side-chain azobenzene-containing LC polymer,
Monatsh. Chem., 2009, 140, 789-799.
N. J. Li, J. M. Lu, X. W. Xia, Q. F. Xu and L. H. Wang, Synthesis
and the third-order nonlinear optical properties of soluble
polymers with different substituted azobenzene side chains,
Polymer, 2009, 50, 428-433.
S. Furumi and K. Ichimura, Photogeneration of high pretilt
angles of nematic liquid crystals by non-polarized light
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Funct Mater, 2004, 14, 247-254.
value at 150 °C was comparable to that at room temperature.
These results proved that the photo-induced birefringent films
7
8
9
have excellent thermal stability.
Conclusions
In conclusion, this work demonstrates a series of new optically
anisotropic azo-PIs prepared by directly appending different
lengths of azo chains to pyrimidyl diamine, which show good
combined properties of both polyimide and azo. All of the azo-
PI films showed outstanding thermal stability, with high Td5
10 A. Bobrovsky, K. Mochalov, A. Chistyakov, V. Oleinikov and V.
Shibaev, AFM study of laser-induced crater formation in films
of azobenzene-containing photochromic nematic polymer
and cholesteric mixture, J. Photochem. Photobiol., A, 2014,
275, 30-36.
11 S. Z. Yang, K. Yang, A. Jain. R. Nagarajan and J. Kumar,
Patterning flexible substrates using surface relief structures
in azobenzene functionalized polymer films, J. Macromol.
Sci., 2008, 45, 938-941.
values of about 400 °C in nitrogen and Tg around 200 °C. Azo-
PI films fabricated through a casting method showed good
dimensional stability, moderate to high thermal stability, and
good mechanical performance. The trans-to-cis isomerization
of the azo group (N=N) occurred less readily in the rigid main
chains of azo-PIs that contained azo units with short spacer
lengths. When the spacer length was longer than six
methylene units, its effect on the birefringence was not
obvious. Obvious photo-induced birefringence (0.0166) was
12 K. Sakamoto, K. Usami and K. Miki, Photoalignment
efficiency enhancement of polyimide alignment layers by
alkyl-amine vapor treatment, Appl. Phys. Express, 2014, 7,
measured for PI1-S6 at 100 °C by using a continuous 405 nm
081701.
laser as the pump light. Our as-prepared azo-PI films displayed
high photo-induced birefringence as well as good
thermostability. The resultant azo-PI films are expected to
have promising potential applications in optical materials.
13 N. Hosono, M. Yoshikawa, H. Furukawa, K. Totani, K.
Yamada, T. Watanabe and K. Horie, Photoinduced
deformation of rigid azobenzene-containing polymer
networks, Macromolecules, 2013, 46, 1017-1026.
14 I. Sava, A. Burescu, I. Stoica, V. Musteata, M. Cristea, I.
Mihaila and V. Pohoatab, Properties of some azo-
copolyimide thin films used in the formation of
Conflict of interest
photoinduced surface relief gratings, RSC Adv., 2015,
10125-10133.
5,
The authors declare no competing financial interests.
15 K. Zlatanova, P. Markovsky, I. Spassova and G. Danev,
Photoinduced changes in methyl orange polyimide layers,
Opt. Mater., 1996, 5, 279-283.
Acknowledgements
16 E. Schab-Balcerzak, M. Siwy, M. Kawalec, A. Sobolewska, A.
Chamera and A. Miniewicz, Synthesis, characterization, and
study of photoinduced optical anisotropy in polyimides
containing side azobenzene units, J. Phys. Chem. A, 2009,
113, 8765-8780.
The authors are grateful for financial support from the
National Science Foundation of China (51733007 and
21574081) and 973 Projects (2014CB643605).
17 E. Schab-Balcerzak, M. Grucela-Zajac, A. Kozanecka-Szmigiel
and K. Switkowski, Poly(etherimide)s and poly(esterimide)s
containing azobenzene units: Characterization and study of
photoinduced optical anisotropy, Opt. Mater., 2012, 34, 733-
740.
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