Flexible nanocrystalline-titania/polyimide hybrids with high refractive index and excellent thermal dimensional stability

Article abstract of DOI:10.1002/pola.23914

In this study, a novel synthetic route was developed to prepare polyimide-nanocrystalllne-titania hybrid optical films with a relatively high titania content (up to 50 wt %) and thickness (20-30 μm) from soluble polyimides containing hydroxyl groups, Two series of newly soluble polyimides were synthesized from the hydroxy-substituted diamines with various commercial tetracarboxylic dianhydrides. The hydroxyl groups on the backbone of the polyimides could provide the organic-inorganic bonding and resulted in homogeneous hybrid solutions by controlling the mole ratio of titanium butoxide/hydroxyl group. AFM, SEM, TEM, and XRD results indicated the formation of well-dispersed nanocrystalline-titania. The flexible hybrid films could be successfully obtained and revealed relatively good surface planarity, thermal dimensional stability, tunable refractive index, and high optical transparency. A three-layer antireflection coating based on the hybrid films was prepared and showed a reflectance of less than 0.5% in the visible range indicated its potential optical application.

Full text of DOI:10.1002/pola.23914

Flexible Nanocrystalline-Titania/Polyimide Hybrids with High Refractive
Index and Excellent Thermal Dimensional Stability
GUEY-SHENG LIOU,1 PO-HAN LIN,1 HUNG-JU YEN,1 YANG-YEN YU,2 WEN-CHANG CHEN1,3
1Institute of Polymer Science and Engineering, National Taiwan University, 1 Roosevelt Road, 4th Section, Taipei 10617, Taiwan
2Department of Materials Engineering, Mingchi University of Technology, 84 Gungjuan Road, Taishan, Taipei 243, Taiwan
3Department of Chemical Engineering, National Taiwan University, 1 Roosevelt Road, 4th Section, Taipei 10617, Taiwan
Received 20 November 2009; accepted 27 December 2009
DOI: 10.1002/pola.23914
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: In this study, a novel synthetic route was developed
to prepare polyimide–nanocrystalline–titania hybrid optical films
with a relatively high titania content (up to 50 wt %) and thick-
ness (20–30 lm) from soluble polyimides containing hydroxyl
groups. Two series of newly soluble polyimides were synthe-
sized from the hydroxy-substituted diamines with various com-
mercial tetracarboxylic dianhydrides. The hydroxyl groups on
the backbone of the polyimides could provide the organic–inor-
ganic bonding and resulted in homogeneous hybrid solutions
by controlling the mole ratio of titanium butoxide/hydroxyl
group. AFM, SEM, TEM, and XRD results indicated the formation
of well-dispersed nanocrystalline-titania. The flexible hybrid
films could be successfully obtained and revealed relatively
good surface planarity, thermal dimensional stability, tunable re-
fractive index, and high optical transparency. A three-layer anti-
reflection coating based on the hybrid films was prepared and
showed a reflectance of less than 0.5% in the visible range indi-
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cated its potential optical applications. 2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 1433–1440, 2010
KEYWORDS: functionalization of polymers; nanocomposites;
polyimides; refractive index
INTRODUCTION Recently, with the rapid development of
nanotechnology and nanoscience, research on polymer–inor-
ganic hybrid materials have attracted considerable interest
owing to the enhanced mechanical, thermal, magnetic, opti-
cal, electronic, and optoelectronic properties when compared
to the corresponding individual polymer matrix or inorganic
component.1–3 In situ sol–gel hybridization approach made it
possible to manipulate the organic/inorganic interfacial
interactions at various molecular and nanometer length
scales, resulting in homogeneous structures and thus over-
coming the problem of nanoparticle agglomeration. For
optical applications of these hybrid materials, such as high
refractive index materials, optical waveguides, and antireflec-
tive films, the inorganic domains must be less than 40 nm to
avoid scattering loss and retain the optical transparency.4
Well-controlled morphology and phase separation are critical
issues in the preparation of transparent hybrid films; thus
sol–gel reaction was widely used for making transition metal
oxide solids with fine-scaled microstructures. Therefore, par-
ticle sizes less than a couple of nanometers could easily be
achieved in the derived gels, and the dimensions also could
be maintained when subsequently crystallized at elevated
temperatures.
(TiO2) hybrid materials as high refractive index materials.
However, the relatively poor thermal stability of PMMA often
limited their device applications, which might be overcome
by using thermally stable PI as the organic matrix. Recently,
Ueda laboratory have successfully prepared the PI–TiO2
hybrid film containing 45 wt % silica-modified anatase-type
TiO2 nanoparticles with a refractive index of 1.81 at 632.8
nm.9 Moreover, to control the titania domain size, the combi-
nation of in situ sol–gel processing with lower controlled
molecular-weight polymer, coupling agent (such as 3-amino-
propyl trimethoxysilane) or chelating agent (such as acetyl-
acetone) was commonly used to prepare the hybrid materi-
als.6 However, using lower molecular weight polymers and
additional coupling or chelating agents might suffer from
poor storage stability, processability, and higher optical loss
of the resulting hybrid materials. Furthermore, to obtain
materials with high refractive index,
a relatively large
amount of TiO2 nanocrystalline must be added. Thus, the
flexible, transparent, and homogenous PI–TiO2 hybrid thick
film films with high titania content could not be achieved so
far as we know.
In this contribution, a novel and simple synthetic route was
developed to fabricate aromatic PI–nanocrystalline–titania
hybrid thick films (up to 30 lm) with tunable titania con-
tents, refractive index, and good optical transparency. The
Poly(methyl methacrylate) (PMMA),5 polyimide (PI),6,7 and
others8 have been extensively used for polymer–titania
Additional Supporting Information may be found in the online version of this article. Correspondence to: G.-S. Liou (E-mail: gsliou@ntu.edu.tw)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1433–1440 (2010) 2010 Wiley Periodicals, Inc.
NANOCRYSTALLINE-TITANIA/POLYIMIDE HYBRIDS, LIOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
high molecular weight and organosoluble PIs with hydroxyl
groups (2,3-PHIc and 2,7-PHIc) were used to prepare their
titania hybrid materials. These new naphthalene-based PIs
revealed high refractive index values,10 which were compara-
ble with other heteroatom-containing polymers.11 In addi-
tion, the lateral hydroxyl groups also provided the organic–
inorganic bonding with titanium butoxide (Ti(OBu)4). There-
fore, no additional coupling agents or chelating ligands were
used and highly homogeneous hybrid films with different
titania contents could be successfully obtained. The morphol-
ogies of the prepared hybrid materials were characterized by
AFM, SEM, TEM, and XRD. The thermal properties, optical
transmittance, and refractive index dispersion of the pre-
pared hybrid films were also investigated and described
herein.
nol, and collected by filtration. The precipitate was dissolved
in 8 mL of DMAc, and the homogeneous solution was poured
into a 9-cm glass culture dish, which was placed in a 90 ꢀC
oven for 12 h to remove the solvent. Then, the obtained film
ꢀ
was further dried in vacuo at 180 C for 6 h. The FTIR spec-
trum of 2,3-PI film exhibited characteristic imide absorption
bands at 1771 (asymetrical C¼¼O), 1718 (symmetrical C¼¼O),
1396 (CAN), and 747 cmꢁ1 (imide ring deformation) with-
out the absorption bands in the region of 2500 to 3500
cmꢁ1 (OAH stretch).
Preparation of PI Films
A polymer solution was made by dissolving the PI sample in
DMAc. The homogeneous solution was poured into glass Pe-
ꢀ
tri dish, then placed in an 80 C oven overnight for the slow
release of the solvent, and then the semidried film was fur-
ther dried in vacuo at 180 ꢀC for 8 h. The obtained films
were 20–30-lm thick used for X-ray diffraction measure-
ments, solubility tests, and thermal analyses.
EXPERIMENTAL
Polymer Synthesis
Poly(o-hydroxy-imide)s PHIa-PHIe by One-Step Method
The synthesis of PI 2,3-PHIc was used as an example to
illustrate the general synthetic procedure. The stoichiometric
mixture of the hydroxyl diamine 2,3-3 (0.51 g, 1.36 mmol),
the dianhydride 4c (0.42 g, 1.36 mmol), and a few drops of
isoquinoline in m-cresol (10 mL) were stirred at ambient
temperature under nitrogen. After the solution was stirred
for 5 h, it was heated to reflux at 200 ꢀC for 15 h. During
this time, the water of imidization was allowed to distill
from the reaction mixture along with m-cresol. The m-cresol
was continually replaced so as to keep the total volume of
the solution constant. After the solution was allowed to cool
to ambient temperature, the viscous solution then was
poured slowly into 300 mL of stirred methanol. The polymer
that precipitated was collected by filtration, washed thor-
oughly with hot methanol, and dried under reduced pressure
Preparation of PI–Titania Hybrid Films
The synthesis of PI–titania hybrid 23TP50 was used as an
example to illustrate the general synthetic route procedure.
First, 0.12 g (0.18 mmole) of 2,3-PHIc was dissolved in 6.0
mL of DMAc, and then 0.20 mL of HCl (37 wt %) was added
very slowly into the PI solution and further stirred at room
temperature for 30 min. Then, 0.50 mL (1.46 mmole) of
Ti(OBu)4 dissolved in 0.50 mL of butanol was added drop-
wise into the above solution by a syringe and then stirred at
room temperature for 30 min. Finally, the resulting precur-
sor solution of 2,3TP50 was filtered through a 0.45-lm
PTFE filter and poured into a 6-cm glass Petri dish. The
hybrid optical film could be obtained by subsequent heating
program at 60 ꢀC for 4 h, 150 ꢀC for 3 h, and then 350 ꢀC
for 2 h under vacuum condition. After the curing process,
hybrid films were immersed into water to peel off from the
glass substrates and dried in vacuum. The obtained hybrid
films were about 20–25 lm in thickness. For thin film prepa-
ration, the above precursor solution was diluted by DMAc
and cast onto glass plate.
ꢀ
at 150 C for 15 h. The inherent viscosity of the obtained PI
2,3-PHIc was 0.79 dL/g (measured at a concentration of 0.5
g/dL in DMAc at 30 ꢀC) The FTIR spectrum of 2,3-PHIc
(film) exhibited broad absorption bands in the region of
2500 to 3500 cmꢁ1 (OAH stretch) and characteristic imide
absorption bands at 1775 (asymetrical C¼¼O), 1719 (sym-
metrical C¼¼O), 1395 (CAN), and 743 cmꢁ1 (imide ring de-
formation). Anal. Calcd for (C38H20N2O9)n (648.57)n: C,
70.37%; H, 3.11%; N, 4.32%. Found: C, 68.54%; H, 3.59%;
N, 4.74%
To study the effect of the hydroxyl groups on the morphol-
ogy of the hybrid materials, a comparison film 2,3-PI30
without the hydroxyl group was also prepared using a simi-
lar procedure to that for 2,3TP30 except that 2,3-PI was
used to replace 2,3-PHIc. In addition, the 100 wt % of tita-
nia film (TP100) was prepared according to the previously
reported method.7
2,3-PI (without Hydroxyl Groups) by Two-Step Method via
Chemical Imidization
The synthesis of PI 2,3-PI was illustrated as follows: To a so-
lution of 0.51 g (1.36 mmol) of diamine 2,3-3 in 9.5 mL of
DMAc, 0.42 g (1.36 mmol) of 4c was added in one portion.
The mixture was stirred at room temperature overnight (ca.
12 h) to afford a viscous poly(amic acid) solution. The poly-
(amic acid) was subsequently converted to PI via a chemical
imidization process by addition of pyridine 2 mL and acetic
anhydride 5 mL, and the mixture was heated at 100 ꢀC for
2 h to effect complete imidization. The resulting polymer
solution was poured into 300 mL of methanol giving a fi-
brous precipitate, which was washed thoroughly with metha-
RESULTS AND DISCUSSION
Monomer Synthesis
The synthetic routes of bis(o-aminophenol) monomers are
outlined in Supporting Information Scheme S1. The bis(o-
aminophenol)s, 2,3-bis(4-amino-3-hydroxyphenoxy)naphtha-
lene (2,3-3) and 2,7-bis(4-amino-3-hydroxyphenoxy)naphtha-
lene (2,7-3), were synthesized by the potassium carbonate-
mediated nucleophilic substitution reaction of 2-benzyloxy-4-
fluoronitrobenzene12 (1) with 2,3-dihydroxynaphthalene
and 2,7-dihydroxynaphthalene, followed by simultaneous
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ARTICLE
deprotection and reduction, respectively. Elemental analysis
and FTIR spectroscopic techniques were used to identify the
structures of the intermediate dinitro compounds and the
hydroxyl diamine monomers, which were in good agreement
with the previous literature.13
Polymer Synthesis
Two novel series of 2,3-PHIs and 2,7-PHIs were synthesized
by the one-step method starting from hydroxyl-substituted
diamine monomers, 2,3-3 and 2,7-3, with aromatic tetracar-
boxylic dianhydrides in the presence of a catalytic amount of
ꢀ
isoquinoline at 200 C, respectively, (as shown in Scheme 1).
On the other hand, the synthesis of PHIs by two-step
method via chemical imidization was not suitable. Upon the
chemical imidization process, the hydroxyl groups on the
polymer backbone would react with acetic anhydride, thus
2,3-PI was also prepared for comparison investigation. The
formations of poly(o-hydroxy-imide)s were confirmed by ele-
mental analysis, FTIR, and NMR spectroscopic techniques.
The elemental analytic results of 2,3-PHIs and 2,7-PHIs are
summarized in Supporting Information Table S1. FTIR spec-
tra of 2,3-PHIc, 2,7-PHIc, and 2,3-PI are shown in Support-
ing Information Figure S1. The PHIs exhibited broad absorp-
tion bands in the region of 2500 to 3500 cmꢁ1 (OAH
stretch) and characteristic imide absorption bands at 1775
(asymetrical C¼¼O), 1719 (symmetrical C¼¼O), 1396 (CAN),
and 743 cmꢁ1 (imide ring deformation). With the reaction of
acetic anhydride and hydroxyl groups, 2,3-PI also showed
the characteristic imide bands but the absorption of hydroxyl
groups disappeared. NMR spectra of 2,3-PHIc are illustrated
SCHEME 1 Polymer Synthesis. [Color figure can be viewed in
the online issue, which is available at www.interscience.
wiley.com.]
SCHEME 2 Hybrid Synthesis. [Color figure
can be viewed in the online issue, which
is available at www.interscience.wiley.com.]
NANOCRYSTALLINE-TITANIA/POLYIMIDE HYBRIDS, LIOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Reaction Composition and Properties of the 2,3-PHIc Hybrid Films
Reactant Compostition
(wt %)
Hybrid Film TiO2 Content
(wt %)
Experimentala
h (nm)b
Rq (nm)c
nd
vD
e
Sample
2,3-PHIc
Ti(OBu)4
Theoretical
2,3-PHIc
2,3TP10
2,3TP30
2,3TP50
2,3TP70
TP100
a
100
67.8
35.4
19.0
9.1
0
0
10
0
10.0
29.6
49.4
69.9
100.0
d
498
248
393
238
131
132
2.029
2.568
2.335
0.375
0.208
–
1.67
1.75
1.81
1.92
1.96
1.99
17.8
12.0
12.0
12.0
14.2
17.0
32.2
64.6
81.0
90.9
100
30
50
70
0
100
Experimental titania content estimated from TGA curves.
h, Film thickness.
n, Refractive index at 633 nm.
b
c
e
Abbe’s number is given by vD ¼ (n589 ꢁ 1)/(n486 ꢁ n656).
Rq, The root mean square roughness.
in Supporting Information Figure S2 and agree well with the
proposed molecular structures. These obtained PHIs had in-
herent viscosities in the range of 0.35–0.90 dL/g, and the
representative 2,3-PHIc, 2,7-PHIc had high weight-average
molecular weights (Mw) of 51,900 and 48,200 Da, respec-
tively, relative to polystyrene standards (Supporting Informa-
tion Tables S1 and S2).
Refractive indices of the PIs could be calculated using the
Lorentz–Lorenz equation:
n2 ꢁ 1 4p qNA
4p a
¼
a ¼
(1)
n2 þ 2
3 Mw
3 Vmol
where n is the refractive index; q, the density; NA, Avogadro’s
number; Mw, the molecular weight; a, the linear molecular
polarizability; and Vmol, the molecular volume. As shown in
eq 1, the molecular volume/density and polarizability were
main factors of the refractive index. As the result, the lowest
n values of 2,3-PHIe and 2,7-PHIe films (Supporting Infor-
mation Figure S4) in the analogous series could be attrib-
uted to the lower electronic polarizability and a larger
molecular volume of the hexafluoroisopropylidene groups.
Considering the transparency and refractive index, 2,3-PHIc
and 2,7-PHIc were used to prepare the titania optical hybrid
materials for further investigations.
Polymer Properties
The solubility behavior of PIs was tested qualitatively and
summarized in Supporting Information Table S3. All the PIs
revealed high solubility in polar solvents such as N-methyl-
2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), and
N,N-dimethylformamide (DMF), and the enhanced solubility
could be attributed to the introduction of flexible ether links,
kinked bulky naphthalene moieties, and hydroxyl groups into
the polymer main chain. Thus, the excellent solubility makes
these polymers potential candidates for practical applications
by spin- or dip-coating processes and further sol–gel
process.
Synthesis of PI–Titania Hybrid Materials
The reaction route of PI and titania precursors is shown in
Scheme 2, and the reaction compositions for 2,3TPX and
2,7TPX were summarized in Table 1 and Supporting Infor-
mation Table S5, respectively. The flexible, transparent, and
homogeneous PI–TiO2 hybrid optical films with different tita-
nia contents have been successful prepared and the appear-
ance is shown in Figure 1.
The thermal properties of the PIs were investigated by TGA
and summarized in Supporting Information Table S4. All the
aromatic PIs exhibited good thermal stability with insignifi-
cant weight loss up to 400 ꢀC in nitrogen. Their 10%
weight-loss temperatures in nitrogen and air were recorded
ꢀ
ꢀ
at 460–515 C and 465–515 C, respectively. The carbonized
residue (char yield) of these aromatic polymers was more
than 53% at 800 ꢀC in nitrogen atmosphere. The high char
yields of these polymers could be ascribed to their high aro-
matic content.
Properties of PI–Titania Hybrid Films
Structural Characterizations
The FTIR spectra of 2,3TP50 and 2,7TP50 films are shown
in Supporting Information Figure S5; the absorption peak at
3000–3500 cmꢁ1 of PI–TiO2 thin film was attributed to the
hydroxyl groups of the titania crystalline. In addition, the
inorganic TiAOATi band was also observed at 650–800
cmꢁ1 and similar to that in the previous report.7
The optical properties of the PIs were investigated by UV–vis
and ellipsometer spectroscopy. The cutoff wavelengths (ko)
from the UV–vis spectra are shown in Supporting Informa-
tion Figure S3. The PIs 2,3-PHIc and 2,3-PHIe obtained
from ether and hexafluoroisopropylidene linkages-containing
dianhydrides exhibited higher transparent when compared
with other ones. These results were attributed to the
reduced intermolecular charge-transfer complex between
alternating electron-donor (diamine) and electron-acceptor
(dianhydride) moieties.
Thermal Properties
The thermal properties of the PI–TiO2 hybrid materials were
evaluated by TGA, DSC, TMA, and DMA, and summarized in
Table 2. The TGA curves of the hybrid materials 2,3TPX and
2,7TPX are shown in Supporting Information Figures S6 and
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ARTICLE
FIGURE 1 The photograph shows appear-
ance of the high transparent, flexible PI–
nanocrystalline–titania hybrid optical films.
S7, respectively, revealing high thermal stability of all hybrid
materials. The titania contents in the hybrid materials could
be estimated based on the char yields under air flow, which
were in good agreement with the theoretical content and
ensured the successfully incorporation of the nanocrystal-
line-titania. In addition, an initial 13.5% weight loss of PI
2,3-PHIc observed between 380 and 490 ꢀC showed the
expulsion of two molecules of carbon dioxide per repeat unit.
The proposed sequence is shown in Supporting Information
Scheme S2, where the hydroxy-imide rearranged to a carboxy-
benzoxazole intermediate followed by decarboxylation above
350 ꢀC to give the fully aromatic benzoxazole product.14 On
the other hand, the initial weight loss could not be found in
the PI–titania hybrid materials, which provided another evi-
dence of completely organic–inorganic bonding.
these hybrid materials was because the nanocrystalline tita-
nia limited the mobility of the PI segment. Therefore, TMA of
these materials was measured and the softening temperature
increased from 273 to 386 ꢀC with the increasing titania
contents. The typical TMA thermogram of 2,3TP10 is illus-
trated in Supporting Information Figure S9. In addition, the
dynamic mechanical thermal properties of the PI and hybrid
thick films such as Tg, storage modulus E0, and tan d were
also measured and shown in Figure 2. The Tg and storage
modulus E0 increased gradually with increasing titania con-
tent because the segmental chain mobility was restricted by
crosslinking between PI chains and titania clusters. More-
over, 2,3TP50 still exhibited good damping behavior (tan d
> 0.3) even with a high titania content.
The coefficiency of thermal expansion (CTE) is also one of
important designing parameters for the application of poly-
mer films in microelectronic field; therefore, CTE of the pure
The DSC curves of the hybrid materials are shown in Sup-
porting Information Figure S8. However, the unobvious Tg of
TABLE 2 Thermal Properties of 2,3-PHIc Hybrid Materials
Tg(ꢀC)a
Td (ꢀC)d
5
Td (ꢀC)d
10
Polymer
DSC
DMA
Ts (ꢀC)b
CTE (ppm/K)c
N2
Air
N2
Air
Rw800 (%)e
2,3-PHIc
2,3TP10
2,3TP30
2,3TP50
a
271
296
340
391
273
321
333
373
273
301
313
386
81.3
72.1
59.1
45.5
435
505
515
580
430
430
430
430
460
570
590
680
465
465
465
465
53
73
78
87
Midpoint temperature of the baseline shift on the second DSC heating trace (rate: 20 ꢀC/min) of the sample after quenching from 400 ꢀC to 50 ꢀC
(rate: 200 ꢀC/min) in nitrogen. Dynamic mechanical thermal analysis (DMA) was performed on PI film specimens (15-mm long, 8-mm wide, and 40–
50-lm thick) at a heating rate of 3 ꢀC/min with a load frequency of 1 Hz in air.
b
Softening temperature measured by TMA with a constant applied load of 10 mN at a heating rate of 10 ꢀC/min by penetration mode.
c
The CTE data were determined over a 50–200 ꢀC range by expansion mode.
d
Temperature at which 5% and 10% weight loss occurred, respectively, recorded by TGA at a heating rate of 20 ꢀC/min and a gas flow rate of
30 cm3/min.
e
Residual weight percentage at 800 ꢀC in nitrogen.
NANOCRYSTALLINE-TITANIA/POLYIMIDE HYBRIDS, LIOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
was less than 0.15%, implying the excellent surface planarity of
the hybrid films could be obtained. Furthermore, the TEM
image of the 2,3TP50 film shown in Figure 3(a) exhibited the
titania nanocrystallites with the average size of 3–5 nm well
dispersed in the hybrid material. The XRD patterns of the
hybrid films shown in Figure 3(b) revealed that the matrix PI
was amorphous, and the intensity of a titanina crystalline peak
gradually increased in the range 2h ¼ 23ꢀ–27ꢀ with increasing
titania content suggesting that the titania clusters were well
dispersed in PIs because of hydrolysis-condensation reactions
occurred between Ti(OBu)4 and pendant hydroxyl groups of PI.
The clear titania crystallization could be observed in 2,3TP70,
four peaks, 25.5ꢀ, 38.4ꢀ, 48.3ꢀ, and 54.8ꢀ, corresponding to the
(101), (112), (200), and (211) crystalline planes of the anatase
titania phase, respectively.15,16 The broad width of the peaks
was because the scattering of X-ray resulted from the small size
of the titania nanocrystalline grain, which was also well agreed
with the results from TEM images. The average crystallite sizes
could be estimated to be ꢂ5 nm for 2,3TP50 by using the
Debye-Scherrer equation:
FIGURE 2 (a) Storage modulus and (b) Tan delta curves of 2,3-
PHIc hybrid materials. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
PI film and PI–TiO2 hybrid films was measured and summar-
ized in Table 2. Inorganic reinforced components often
revealed much lower CTE values than that of organic matrix,
which suppressed CTE of the resulting hybrid materials.
Thus, CTE of the organic–inorganic hybrids decreased with
increasing of volume fractions of inorganic reinforcement.
0:94 ꢃ k
B ¼
(2)
d ꢃ cos h
Note that B is the peak’s full width at half maximum
(FWHM), h is the peak position, and d is the average diame-
ter of the crystallite in nanometer.
Morphology Analyses
The SEM images of 2,3-PHIc and 2,7-PHIc hybrid films are
shown in Supporting Information Figures S10 and S11, respec-
tively, which exhibited uniform surface without any apparent
microstructural separation or significant titania aggregates. In
addition, the SEM image of 2,3-PI30 in Supporting Information
Figure S10(d) shows severe phase separation with a large
aggregated domain. These results demonstrate that the
hydroxyl groups on PI played important roles for not only
enhancing the solubility and providing bonding sites with TiO2
but also effectively improving the resultant dispersity and mor-
phology stability of these hybrid materials. The AFM images of
the 2,3-PHIc and 2,7-PHIc hybrid thin films are shown in Sup-
porting Information Figure S12 and S13, and the analyzed root
mean square surface roughness (Rq) for the hybrid films is
listed in Table 1 and Supporting Information Table S5, respec-
tively. The ratio of surface roughness to film thickness (Rq/h)
Optical Properties
UV–vis spectra of the hybrid thin films and thick films are
shown in Figure 4. The cutoff wavelengths of the hybrid
films in the UV region were contributed from the chromo-
phores of PI, and all the thick hybrid films exhibited much
higher optical transparency than Kapton [Fig. 4(b)]. In addi-
tion, the absorption intensity of the cutoff wavelengths was
enhanced and the corresponding band edge was red-shifted
(Supporting Information Table S6) with increasing the titania
content. Such band shift could usually be observed for titania
sizes less than 10 nm,7 indicating that highly homogeneous
PI–nanocrystalline–titania hybrid materials were obtained.
The refractive index dispersion results are shown in Figure 5
and Supporting Information S14, and the variation in
FIGURE 3 (a) TEM images of the
2,3TP50 hybrid film and (b) XRD
patterns of the 2,3-PHIc hybrid
materials. [Color figure can be
viewed in the online issue, which
is available at www.interscience.
wiley.com.]
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ARTICLE
FIGURE 5 Variation of the refractive index of the 2,3-PHIc
hybrid materials with wavelength. The insert figure shows the
variation of refractive index at 633 nm with titania content.
[Color figure can be viewed in the online issue, which is avail-
able at www.interscience.wiley.com.]
ure 6. The glass substrate revealed a refractive index (n ¼
1.52) higher than air (n ¼ 1.0) and leaded an average reflec-
tance of 4.4% in the visible range. The reflectance could be
reduced significantly via the three-layer antireflection coating
consisted of Colloid SiO2, 2,3TP50, and 2,3TP10 for the
first, second, and third layer, respectively. To reduce reflec-
tion through adjusting phase of light, the optical thickness
(physical thickness ꢃ refractive index) was designed to be
0.25 ko, 0.5 ko, and 0.25 ko (ko ¼ 550 nm) for the three-layer
structure. Thus, the prepared film thickness and refractive
index of Colloid SiO2, 2,3TP50, and 2,3TP10 were 96 nm
and 1.29; 140 nm and 1.96; 80 nm and 1.77, respectively. As
shown in Figure 6, the reflectance of prepared antireflection
FIGURE 4 Transmittance UV–visible spectra of 2,3-PHIc hybrid
materials: (a) thin films (thickness: 100–500 nm) and (b) thick
films (thickness: 20–30 lm). [Color figure can be viewed in the
online issue, which is available at www.interscience.wiley.
com.]
refractive index at 633 nm with titania content was depicted
in insert figures. The refractive index increased linearly with
titania contents suggesting that the TiAOH groups of the
hydrolyzed precursors condensed progressively to form the
TiAOATi structures and resulted in an enhanced refractive
index. It also indicated that using a soluble PI with hydroxyl
groups on each repeating units is a successful approach for
preparing titania hybrid materials. Besides, the evaluated
color coordinates of the hybrid films summarized in Support-
ing Information Table S6 also suggested excellent optical
transparency in the visible region. Combining the issues of
thickness, flexibility, and optical transparency, the PI–titania
hybrid optical thick film 2,3TP50 (20–30 lm in thickness)
in this article demonstrated the highest refractive index
(1.92) to the best of our knowledge comparing with other
polymer–titania hybrid materials.5–8
FIGURE 6 Variation of the reflectance with wavelength: optical
glass and the three-layer antireflection coating. The insert
figure shows the structure of the three-layer antireflection
coating. [Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]
Multilayer Antireflection Coatings
The structure of three-layer antireflective coating on the
glass substrate and the reflectance spectra are shown in Fig-
NANOCRYSTALLINE-TITANIA/POLYIMIDE HYBRIDS, LIOU ET AL.
1439

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Chiang, C. L.; Teng, C. C.; Yu, Y. H. J Polym Sci Part A: Polym
Chem 2008, 46, 803–816.
coatings was less than 0.5% in the visible range (400 nm to
700 nm), which was significantly smaller than that of the
glass with 4.4%. It suggested the potential application of the
prepared PI–titania hybrid films in optical devices.
4 Althues, H.; Henle, J.; Kaskel, S. Chem Soc Rev 2007, 36,
1454–1465.
5 (a) Chen, W. C.; Lee, S. J.; Lee, L. H.; Lin, J. L. J Mater Chem
1999, 9, 2999–3003; (b) Lee, L. H.; Chen, W. C. Chem Mater
2001, 13, 1137–1142; (c) Yuwono, A. H.; Zhang, Y.; Wang, J.;
Zhang, X. H.; Fan, H.; Ji, W. Chem Mater 2006, 18, 5876–5889.
CONCLUSIONS
Two series of novel soluble PIs were synthesized from the
hydroxy-substituted diamines with various commercial tetra-
carboxylic dianhydrides, respectively. Furthermore, high
transparency and tunable refractive index PI–titania hybrid
optical films were successfully synthesized from soluble
hydroxyl-substituted PIs with titanium butoxide by control-
ling the organic/inorganic mole ratio. The refractive index of
hybrid thin films revealed good surface planarity, high ther-
mal stability, tunable refractive index, and high optical trans-
parency in the visible range. Three-layer antireflective coat-
ing based on the hybrid films possessed reflectance of less
than 0.5% in the visible range, suggesting potential optical
applications of the novel PI–titania hybrid optical films. Fur-
thermore, the thick titania hybrid films could also be
achieved even with the relatively high titania content (50
wt %) and refractive index (1.92). To the best of our
knowledge, the refractive index and titania content were the
highest among the polymer–titania hybrid optical thick films
(20–30 lm in thickness).
6 (a) Nandi, M.; Conklin, J. A.; Salvati, L.; Sen, A. Chem Mater
1991, 3, 201–206; (b) Chang, C. C.; Chen, W. C. J Polym Sci
Part A: Polym Chem 2001, 39, 3419–3427; (c) Guevel, X. L.;
Palazzesi, C.; Prosposito, P.; Giustina, G. D.; Brusatin, G.
J Mater Chem 2008, 18, 3556–3562.
7 Su, H. W.; Chen, W. C. J Mater Chem 2008, 18, 1139–1145.
8 (a) Wang, B.; Wilkes, G. L.; Hedrick, J. C.; Liptak, S. C.;
McGrath, J. E. Macromolecules 1991, 24, 3449–3450; (b) Chen,
W. C.; Lee, L. H.; Chen, B. F.; Yen, C. T. J Mater Chem 2002, 12,
3644–3648; (c) Lu, S. R.; Zhang, H. L.; Zhao, C. X.; Wang, X. Y.
Polymer 2005, 46, 10484–10492; (d) Liu, J. G.; Ueda, M. J Mater
Chem 2009, 19, 8907–8919; (e) Vestberg, R.; Piekarski, A. M.;
Pressly, E. D.; Berkel, K. Y. V.; Malkoch, M.; Gerbac, J.; Ueno,
N.; Hawker, C. J. J Polym Sci Part A: Polym Chem 2009, 47,
1237–1258.
9 Liu, J. G.; Nakamura, Y.; Ogura, T.; Shibasaki, Y.; Ando, S.;
Ueda, M. Chem Mater 2008, 20, 273–281.
The authors are grateful to the National Science Council of the
Republic of China for financial support of this work.
10 Kawasaki, S.; Yamada, M.; Kobori, K.; Jin, F.; Kondo, Y.;
Hayashi, H.; Suzuki, Y.; Takata, T. Macromolecules 2007, 40,
5284–5289.
REFERENCES AND NOTES
11 (a) Liu, J. G.; Nakamura, Y.; Shibasaki, Y.; Ando, S.; Ueda,
M. Macromolecules 2007, 40, 4614–4620; (b) Liu, J. G.; Naka-
mura, Y.; Suzuki, Y.; Shibasaki, Y.; Ando, S.; Ueda, M. Macro-
molecules 2007, 40, 7902–7909; (c) You, N. H.; Suzuki, Y.;
Yorifuji, D.; Ando, S.; Ueda, M. Macromolecules 2008, 41,
6361–6366.
1 (a) Beecroft, L. L.; Ober, C. K. Chem Mater 1997, 9,
1302–1317; (b) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J. P.
Adv Mater 2003, 15, 1969–1994; (c) Laine, R. M. J Mater Chem
2005, 15, 3725–3744; (d) Zelcer, A.; Donnio, B.; Bourgogne, C.;
Cukiernik, F. D.; Guillon, D. Chem Mater 2007, 19, 1992–2006;
(e) Zhou, Z.; Franz, A. W.; Hartmann, M.; Seifert, A.; Muller, T.
J. J.; Thiel, W. R. Chem Mater 2008, 20, 4986–4991; (f) Pereira,
F.; Valle, K.; Belleville, P.; Morin, A.; Lambert, S.; Sanchez, C.
Chem Mater 2008, 20, 1710–1718; (g) Lin, Y. Y.; Chen, C. W.;
Chu, T. H.; Su, W. F.; Lin, C. C.; Ku, C. H.; Wu, J. J.; Chen, C. H.
J Mater Chem 2007, 17, 4571–4576.
12 Imai, Y.; Park, K. H.; Kakimoto, M. A. J Polym Sci Part A:
Polym Chem 1998, 36, 1987–1994.
13 Chern, Y. T. U.S. Patent 20,060,241,187, 2006.
14 Likhatchev, D.; Gutierrez-Wing, C.; Kardash, I.; Vera-Gra-
ziano, R. J Appl Polym Sci 1996, 59, 725–735.
2 (a) Functional Hybrid Materials; Gomez-Romero, P.; Sanchez,
C., Eds.; Wiley-VCH: Weinheim, 2004; (b) Kickelbick, G. Hybrid
Materials: Synthesis, Characterization, and Applications; Wiley-
VCH: Weinheim, 2007.
15 (a) Yuwono, A. H.; Xue, J.; Wang, J.; Elim, H. I.; Ji, W.; Li,
Y.; White, T. J. J Mater Chem 2003, 13, 1475–1479; (b) Yuwono,
A. H.; Liu, B.; Xue, J.; Wang, J.; Elim, H. I.; Ji, W.; Li, Y.; White,
T. J. J Mater Chem 2004, 14, 2978–2987.
3 (a) Li, T. L.; Hsu, S. L. C. J Polym Sci Part A: Polym Chem
2009, 47, 1575–1583; (b) Ye, Y. S.; Yen, Y. C.; Chen, W. Y.;
Cheng, C. C.; Chang, F. C. J Polym Sci Part A: Polym Chem
2008, 46, 6296–6304; (c) Wahab, M. A.; Mya, K. Y.; He, C. J
Polym Sci Part A: Polym Chem 2008, 46, 5887–5896; (d) Liu, Y.
L.; Tseng, M. C.; Fangchiang, M. H. J Polym Sci Part A: Polym
Chem 2008, 46, 5157–5166; (e) Yuen, S. M.; Ma, C. C. M.;
16 (a) Eder, D.; Windle, A. H.
J
Mater Chem 2008, 18,
Mater Chem 2008, 18,
2036–2043; (b) Miyauchi, M.
J
1858–1864; (c) Yu, A.; Lu, G. Q.; Drennan, J.; Gentle, I. R. Adv
Funct Mater 2007, 17, 2600–2605; (d) Fattakhova-Rohlfing, D.;
Wark, M.; Brezesinski, T.; Smarsly, B. M.; Rathousky, J. Adv
Funct Mater 2007, 17, 123–132.
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