Preparation of various morphological films at nanoscale by phase separation method

Article abstract of DOI:10.1080/15421406.2020.1743429

A film consisting of nanopore and nanopillar structures was produced from a binary immiscible polymer solution of poly(amic acid) (PAA) and polyimide (PI), which can be used as a potential light extraction layer or flexible substrate in organic light-emit

Full text of DOI:10.1080/15421406.2020.1743429

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Preparation of various morphological films at
nanoscale by phase separation method
Thi Kieu Trang Tu, Quoc Thang Phan, Yong Hyun Kim, Yeong-Soon Gal &
Kwon Taek Lim
To cite this article: Thi Kieu Trang Tu, Quoc Thang Phan, Yong Hyun Kim, Yeong-Soon
Gal & Kwon Taek Lim (2020) Preparation of various morphological films at nanoscale by
phase separation method, Molecular Crystals and Liquid Crystals, 705:1, 127-134, DOI:
10.1080/15421406.2020.1743429
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
020, VOL. 705, NO. 1, 127–134
2
https://doi.org/10.1080/15421406.2020.1743429
Preparation of various morphological films at nanoscale
by phase separation method
a
a
a
b
Thi Kieu Trang Tu , Quoc Thang Phan , Yong Hyun Kim , Yeong-Soon Gal , and
Kwon Taek Lim
a
a
b
Department of Display Engineering, Pukyong National University, Busan, South Korea; Department of
Fire Safety, Kyungil University, Gyeongsan, South Korea
ABSTRACT
KEYWORDS
Carbazole-based dianiline;
nanopillars; nanopores;
phase separation; poly(amic
acid); polyimide
A film consisting of nanopore and nanopillar structures was pro-
duced from a binary immiscible polymer solution of poly(amic acid)
(
PAA) and polyimide (PI), which can be used as a potential light
extraction layer or flexible substrate in organic light-emitting diode
OLED) devices. A phase separation method was applied to create
(
uniform and random pattern structures in nanoscale via a simple
spin-coating technique. Firstly, the binary immiscible polymer solu-
tions of PAA and PI whose precursor is carbazole-based dianiline
were prepared with various volume ratios, and then the polymer
mixtures were spin-coated onto substrates to form transparent films
with various morphologies and dimensions, as observed by a field
emission scanning electron microscope technique. In addition, after
ꢀ
annealing PAA/PI films at 300 C, the homogenous and flexible char-
acteristic of nanopore and nanopillar structures could be achieved.
Introduction
Recently, flexible organic optoelectronic devices are a promising field with the ever-
growing rate of display and lighting application. Novel flexible materials must satisfy
several crucial requirements. Among various organic polymer materials, a polyimide
(PI) is one of the optimum candidates for flexible materials, especially an aromatic
PI [1]. PI has extraordinary properties which attract the interest of researchers, such as
excellent thermal, mechanical, electrical and chemical performance [2] although it con-
sists of rigid planar imide rings [3]. In addition, PI with heterocycles such as pyridine,
benzimidazole, pyrimidine, furan etc. in the backbone can increase their glass transition
temperature (T ) [4]. In optoelectronic parts, the flexible transparent PI is used largely
g
as a substrate instead of a glass in all flexible organic light-emitting diode (OLED) devi-
ces [5]. Meanwhile, carbazole and carbazole derivatives also gained much attention
from the researchers. They have been applied to OLED devices as an emitting layer or
hole transport layer with inherently incredible properties which are high thermal stabil-
ity [6], easy chemical functionalization, film-forming capability [7], excellent transpar-
ency [8], good refractive index [8,9] and high fluorescent quantum yield [10]. The
CONTACT Kwon Taek Lim
ktlim@pknu.ac.kr
Department of Display Engineering, Pukyong National University,
5
99-1 Daeyeon 3-Dong, Nam-Gu, Busan 48513, South Korea.
ß 2020 Taylor & Francis Group, LLC

1
28
T. T. K. TU ET AL.
strategy for introducing carbazole rings into PI backbones have been proposed in the
report of H. M. Wang et al. for the purpose of a hole-transporting and electrochromic
materials [11].
Here, we used poly(amic acid) (PAA) and PI made from carbazole based dianiline
(CDA) to prepare phase-separated films by a simple and low-cost fabrication process,
spin-coating technique. The phase separation phenomenon has been described earlier in
our previous reports [12, 13]. The binary polymer solutions were prepared with various
ratios to obtain nano-pattern films which were either nanopores or nanopillars. The
phase-separated PI films have thermostability, transmittance, high refractive index, flexi-
bility and good light scattering. With these properties, the phase-separated film may be
used as a flexible substrate or light out-coupling layer in optoelectronic devices.
Experiment details
Materials
Carbazole (>97.0%), 4-fluoronitrobenzene (>98%), hydrazine monohydrate (79%),
acetic anhydride (AA) (>98%), pyridine (>99%) and K CO (>99%) were purchased
2
3
0
from Tokyo Chemical Industry. 4,4 -(Hexafluoroisopropylidene)-diphthalic anhydride
6FDA) (99%) and palladium 10% on carbon (10% Pd/C) were obtained from Sigma
Aldrich. Dimethyl formamide (DMF) (>99%), ethanol (>99%), and dimethyl acetamide
DMAC) (>99%) were purchased from Duksan (South Korea) and used as received.
(
(
Preparation of monomers
Step 1: synthesis of 9-(4-nitrophenyl)-9H-carbazole
0.1 mol (16.72 g) of carbazole and 0.1 mol (14.11g) of 4-fluoronitrobenzene were dissolved
in 100 mL of DMF in a 250 mL two-neck round-bottom flask equipped with a stirring
hot plate, a condenser and an inlet of inert gas. 0.15mol (20.7 g) of K CO was gradually
2
3
ꢀ
added into the solution and the reaction was carried out at 100 C for 12h. After cooling
the reaction solution, crystals were filtered, washed with water and recrystallized by etha-
nol. The obtained product was light-yellow needle crystals (yield 95%) (Scheme 1).
Step 2: synthesis of 9-(4-aminophenyl)-9H-carbazole
0.08 mol (23.05 g) of 9-(4-nitrophenyl)-9H-carbazole and 0.016 mol (1.7 g) of 10% Pd/C
were dispersed in 100 mL of ethanol in a 250 mL two-neck round-bottom flask equipped
with a stirring hot plate, a condenser and an inlet of inert gas. The reaction mixture was
stirred under nitrogen for 1 h at room temperature before 0.8 mol (40g) of hydrazine
ꢀ
monohydrate was slowly added. The reaction mixture was refluxed at 90 C for 12h. The
reaction mixture was filtered to get rid of Pd/C, and then ethanol was removed by a vac-
uum evaporator to leave the white gel product (yield 90%) (Scheme 1).
Step 3: synthesis of N-4-(9H-carbazol-9-yl)-4-nitro-N-(4-nitrophenyl)aniline
The process is the same as that of step one but with the ratio of reactants; one equiv.,
two equiv., and 1.5 equiv. for 9-(4-aminophenyl)-9H-carbazole, 4-fluoronitrobenzene

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
129
Scheme 1. The structures of PAA and PI prepared from CDA.
and K CO , respectively. The obtained product was dark-yellow needle crystals (yield
2
3
9
1.4%) (Scheme 1).
1
1
Step 4: synthesis of N -(4-(9H-carbazol-9-yl)phenyl)-N -(4-aminophenyl)benzene-
,4-diamine
The process is the same as that of step two but with the ratio of reactants; 1 equiv.,
1
0
.Two equiv., and 20 equiv. for N-4-(9H-carbazol-9-yl)ꢁ4-nitro-N-(4-nitrophenyl)ani-
line, 10% Pd/C and hydrazine monohydrate, respectively. The obtained product was
light-grey needle crystals (yield 89.3%) (Scheme 1).
Preparation of polymers
Step 1: preparation of PAA
1
1
0
1
2
.05 mol (22.01 g) of N -(4-(9H-carbazol-9-yl)phenyl)-N -(4-aminophenyl)benzene-
,4-diamine and 0.05 mol (22.22 g) of 6FDA were dissolved in 80 mL of DMAC in a
50 mL two-neck round-bottom flask equipped with a stirring hot plate, a condenser
ꢀ
and an inlet of inert gas. The reaction mixture was stirred under nitrogen at 0 C for
1
h. Then, stirring was continued at room temperature for 24 h. Finally, PAA was col-
lected by precipitation of the one-half reaction solution into excess deionized water
Scheme 1).
(

1
30
T. T. K. TU ET AL.
Step 2: preparation of PI
0
.025 mol (2.55 g or 2.36 ml) of AA and 0.025 mol (1.98 g or 2.02 ml) of pyridine were
ꢀ
added into the rest of the PAA solution. The reaction was continually stirred at 100 C
for 24 h to produce PI. Finally, PI was collected by precipitation of the reaction solution
into methanol (Scheme 1).
Step 3: preparation of phase separated film. PAA and PI were separately dissolved into
DMAC, where the concentration of solutions was 5%. The PAA and PI solution were
mixed with the following ratios: 1:9; 3:7; 5:5; 7:3 and 9:1. The mixtures were spin-coated
on a glass substrate with a speed of 1200 rpm for 30 s in the first step and 2500 rpm for
ꢀ
3
s in the second step. The phase-separated films were annealed at 50 C for 10 min
ꢀ
under vacuum, and the temperature was gradually increased to 150 C at intervals of
1
0 min/time to form polymer films on the glass substrate. Afterwards, the temperature
ꢀ
ꢀ
was gradually increased at 50 C/time and kept for 60 min/time until up to 300 C to
gain phase-separated PI films.
Characterization
1
Proton nuclear magnetic resonance ( H-NMR) spectra were measured on a JEOL JNM
ECP-600 MHz. Fourier-transform infrared spectroscopy (FT-IR) spectra were recorded
on an Agilent Tech spectrometer. Thermogravimetric analysis (TGA) was conducted on
a Perkin - Elmer Pyris 1 analyzer (USA). Field emission scanning electron microscopes
(FE-SEM) images were performed on a JEM-2100F (JEOL). Ultraviolet-visible spectros-
copy (UV-Vis) spectra were carried out on an Optizen POP UV-Vis spectrophotometer.
Refractive index (RI) was characterized by a J. A. Woollam M-2000D Spectroscopic
Ellipsometer.
Results and discussion
To prepare PAA and PI from CDA, we have referred to the report of H.M. Wang
et al. [11] (Scheme 1). Carbazole was transformed repeatedly into the corresponding
nitro and amino compounds in sequence by using simple and mild processing condi-
tions to produce CDA. In the next step, the step-growth polymerization was conducted
by the reaction between CDA and 6FDA, resulting in PAA. The chemical imidization
reaction was used to produce PI powder for the preparation of phase separated films.
The thermal imidization was also used, however, in the step of completing the homoge-
neous PI film.
The FT-IR spectra were studied to examine the presence of the functional groups on
the backbone of polymers. The principal difference of structure features between PAA
and PI is the appearance and disappearance of O–H and N–H links, as shown in Fig. 1.
The spectrum of PAA shows the O–H stretch and N–H stretch (secondary amine) in
ꢁ
1
the range 3683–3166 cm , which almost overlap the C–H stretch at 3070 and
ꢁ
1
2
932 cm . The characteristic imide group absorptions are exhibited by the presences of
ꢁ
1
imide carbonyl symmetrical stretch (C ¼ O) around 1784 cm
and tertiary amine
ꢁ
1
stretch (C–N) around 1382 cm . It is also clearly observed that the intensity of imide

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
131
Figure 1. FTIR spectra of PAA and PI.
ꢁ
1
carbonyl asymmetrical stretch (C ¼ O) around 1728 cm increases strongly and the
ꢁ
1
N–H bend in secondary amine around 1512 cm disappears.
1
The H-NMR spectra of PAA and PI confirmed the polymer structures (Fig. 2). The
most contrast difference between the two polymers figured out in the spectra is the
presence of a proton peak at 4.32 ppm that characterizes the proton of amide groups on
the PAA backbone. The proton peak of the carboxyl group in PAA is hardly detected
because the strong hydrogen bonding between the carboxyl group and deuterium in
DMSOd After the chemical imidization, the protons in aromatic rings of 6FDA moi-
6
.
eties (position a, d, k) and aniline moieties (position j) are not much affected by ones
in the amide groups. Thus, all these protons are shielded, in other words, their chemical
shifts move to the lower field.
To investigate the optical characteristics of PAA and PI materials, we measured the
refractive index and transparency of their samples which were coated on a silicon wafer
or glass substrate with 5% concentration (Fig. 3). The refractive index was collected by
the spectroscopy ellipsometry using the Cauchy and Si_jaw layer in the range wave-
length from 400 to 900 nm. The index of PAA is measured to be like common polymer
materials (around 1.68), whereas the index of PI jumps to around 1.73 which is similar
to the value of typical organic layers in OLEDs. Besides, UV-vis spectra were recorded
in the wavelength range from 300 to 1000 nm to determine the percentage of transmit-
tance of the light ray through the PAA and PI films coated on the glass substrate. As a
result, the transmittances starting from 450 nm onwards are 80% and 85% for PAA and
PI, respectively. Although these results are not high as compared to other polymer

1
32
T. T. K. TU ET AL.
1
Figure 2. H-NMR spectra of PAA and PI.
Figure 3. (a) Refractive index of PAA and PI in the wavelength range from 400 nm to 900 nm,
(b) The percentage of transmittance of PAA and PI in the wavelength range from 300 nm to 1000 nm.
materials, the combination of high refractive index and transparency of the PI film is
suitable enough for optoelectronic devices.
The thermal stability was evaluated by TGA, as shown in Fig. 4. Both TGA curves of
PAA and PI exhibited similar shapes and good thermal stability. While the weight loss
ꢀ
curve of PI remained until 400 C under nitrogen, the curve of PAA diminished over
4
% by weight. The curves of both polymers continued to fall sharply at the same tem-
ꢀ
ꢀ
perature 417 C. It means that during the first 400 C the thermal imidization has
occurred to transform PAA to PI. The first weight loss of PI and the second weight loss
of PAA started to take place at 417 C. Approximately 16% by weight of PAA and over
ꢀ

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
133
ꢀ
ꢀ
Figure 4. TGA diagram of PAA and PI in temperature range from 50 C to 1200 C.
Figure 5. SEM images of phase-separated PI films from various PAA/PI binary solutions with volume
ratios (a) 1/9; (b) 3/7; (c) 5/5; (d) 7/3; (e) 9/1.
ꢀ
2
0% by weight of PI were lost steadily over a quite long temperature range from 417 C
ꢀ
to 776 C, which was attributed to the contribution of the high aromatic ring content in
the backbone of PAA and PI. Meanwhile, the remaining 80% weight loss of PAA and
PI took place at the next stage from ca. 800 C. As a result, the PAA and PI possess
ꢀ
inherently excellent thermal stability.
The topography of surficial phase-separated PI films was surveyed by using the FE-
SEM technique after transforming the PAA:PI blend films to PI films by thermal imid-
ization. The formation of nanopores and nanopillars at various ratios of binary polymer
mixtures were observed on SEM images as shown in Fig. 5. The surficial morphologies

1
34
T. T. K. TU ET AL.
that were formed from the PAA: PI solution with a volume ratio of 1:9 exhibited nano-
pore patterns whose size and shape are not uniform and well distributed (Fig. 5a). At
the volume ratio of 3:7, sharper and larger nanopores were formed on the surface of
polymer films as shown in Fig. 5b. However, inside and outside the big pores, there
were countless nanopillars which were not the same size. Interestingly, the morpholo-
gies of the film surface became opposite in comparison to ones with the lower PAA
contents when the PAA content increased until equal to or greater than the PI content
in the binary polymer solution. That is, the shapes of patterns were nanopillars from
the PAA: PI solutions with volume ratios of 5:5 and 7:3. Although the surfaces were
nanopillars, their distributions, tightness and sizes of nanopillars were different from
each other. For the volume ratio of 5:5, the nanopillars were ununiform and had vari-
ous sizes in the range from 50 to 200 nm. Whereas, nanopillars of well-distribution and
smaller size were formed for 7:3. Finally, a nearly flat surface was observed in the case
of the PAA:PI mixture of 9:1.
Conclusion
In conclusion, the nanopore and nanopillar patterned surfaces of PI films were prepared
successfully by the simple and low-cost spin-coating technique. The PI film showed out-
standing properties with the high refractive index of 1.73, good transparency of 85%,
excellent thermal stability and flexibility. It is supposed that the PI can be one of prom-
ising materials for the optoelectronic or photovoltaic fields in general and OLED devices
in particular.
Funding
This work was supported by a Research Grant of Pukyong National University (2019).
References
[1] M. Lian, X. Lu, and Q. Lu, Macromolecules 51, 10127 (2018). doi:10.1021/acs.macromol.
8b02282
[
[
[
[
[
[
[
[
2] J. Tan et al., J. Macromol. Sci. A 55, 75 (2018). doi:10.1080/10601325.2017.1387492
3] Z. Wu et al., Ind. Eng. Chem. Res. 58, 6992 (2019). doi:10.1021/acs.iecr.9b00674
4] Z. Xu et al., Org. Electron. 44, 225 (2017). doi:10.1016/j.orgel.2017.02.013
5] R. Mertens, The OLED Handbook, 2019 ed. (Lulu.com, 2019).
6] P. Zassowski et al., Dyes Pigm. 149, 804 (2018). doi:10.1016/j.dyepig.2017.11.040
7] M. Reig et al., Dyes Pigm. 137, 24 (2017). doi:10.1016/j.dyepig.2016.09.062
8] X. Liang et al., Dyes Pigm. 139, 764 (2017). doi:10.1016/j.dyepig.2017.01.012
9] M. Rajeshirke et al., J. Phys. Chem. C 122, 14313 (2018). doi:10.1021/acs.jpcc.8b02937
[
10] Q. Dong et al., Dyes Pigm. 137, 84 (2017). doi:10.1016/j.dyepig.2016.10.003
[
11] H. M. Wang and S. H. Hsiao, J. Polym. Sci. Part A: Polym. Chem. 52, 1172 (2014). doi:10.
1002/pola.27103
[
12] T. T. Tu et al., Optik, 192, 162944 (2019). doi:10.1016/j.ijleo.2019.162944
[
13] T. T. Tu et al., Mol. Cryst. Liq. Cryst. 679 (1), 80 (2019). doi:10.1080/15421406.2019.
1597549