Design and synthesis of hyperbranched polyimide containing multi-triphenylamine moieties for memory devices

Article abstract of DOI:10.1039/c6ra20353a

A novel triamine monomer, N,N′,N′′-tris(4-methoxyphenyl)-N,N′,N′′-tris(4-phenylamino)-1,3,5-benzenetriamine, was designed and synthesized. A hyperbranched polyimide (HBPI) was prepared by reacting the triamine monomer with 4,4-(hexafluoroisopropylidene)di

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Design and synthesis of hyperbranched polyimide containing multi-
triphenylamine moieties for memory device
Received 00th January 20xx,
Accepted 00th January 20xx
Ying Song,a Hongyan Yao,a Yunxia Lv,b Shiyang Zhu,a Shanyou Liu,a Shaowei Guan*a
A
novel triamine monomer, N,N',N''-tris(4-methoxyphenyl)-N,N',N''-tris(4-phenylamino)-1,3,5-benzenetriamine, was
DOI: 10.1039/x0xx00000x
designed and synthesized. A hyperbranched polyimide (HBPI) was prepared by reacting the triamine monomer with 4,4-
(hexafluoroisopropylidene)diphthalic anhydride for application in memory device. The resulting HBPI exhibited excellent
organo-solubility and high thermal stability. A memory device with a sandwich structure of indium tin oxide (ITO)/HBPI/Al
was fabricated by using HBPI as an active layer. The device exhibited a static random access memory (SRAM) behavior with
a relatively low switching voltage of -1.90 V. Moreover, the device showed good stability in both the OFF and ON states,
which could keep as long as 1 × 104 s under a constant voltage stress of -1.00 V with an ON/OFF current ratio reaching up
to 1 × 106. Molecular simulation results suggested that there existed efficient charge transfer between the triamine
moieties and hexafluoropropylidene phthalimides moieties in HBPI, which is responsible for the improved electrical
memory performance. Such a HBPI was expected to be potential useful in polymer memory application.
www.rsc.org/
Kang’s group designed and synthesized a linear PI with
triphenylamine (TPA) and phthalimide units as the electron
donor and acceptor moieties, respectively.11 The PI based
Introduction
Polymer materials have been considered as promising
candidates for next generation memory devices due to their
advantages of solution processability, outstanding mechanical
strength, rich structural flexibility, low-cost and feasible three-
dimensional multi-stack capability. As compared to the
traditional inorganic silicon-cell memory devices, in which
information is stored based on charging and discharging of the
silicon-cells, for polymeric memory devices information is
stored based on the transition of the polymer active layer from
low-conductivity state (OFF) to high-conductivity state (ON)
induced by an external electric field.1-5 Among the polymer
materials investigated, polyimides (PIs) received increasing
attentions due to their favorable electrical properties,
excellent thermal stability and chemical resistance.6-9
Generally, the phthalimide moiety in PI is effective to act as
electron acceptor (A) and a moiety which may act as an
electron donor (D) is necessary to be introduced to form a D-A
structure, which may promote the charge transfer (CT) and
thus the transition from OFF to ON state under the threshold
voltage.10-13 Triphenylamine (TPA) moieties as electron donors
were widely introduced into polyimides due to its charge
transport ability and electrochemical stability.14-20 In 2006,
device was Dynamic Random Access Memory (DRAM) in type,
with retention time only a couple of seconds, switching
threshold voltage of 3.2 V and ON/OFF current ratio of 1×105.
In 2009, the same group reported an oxadiazole-based linear
PI. The device was static random access memory (SRAM) in
type, with retention time prolonged to a few minutes,
threshold voltage of -2.3 V and ON/OFF current ratio of
1×104.21 In 2013, Xu and coworkers reported a linear PI
containing rigid nonplanar conjugated tetraphenylethylene
moiety.22 The memory device with a sandwiched ITO/PI/Al
structure behaved as SRAM memory with a threshold voltage
of 2.7 V and ON/OFF current ratio of 1×104. Recent researches
of polyimide memory materials were almost focused on linear
polyimides. However, introduction of the conjugated electron
donor moieties into linear PI usually resulted in intensified
chain rigidity and promoted interchain packing due to the
formation of intermolecular CT complexes, which lowered the
fusibility and solubility of the PIs and made the processing of
the memory devices become tough. 23-28
Compared to linear PIs, hyperbranched polyimides (HBPIs)
usually exhibit better solubility and lower specific melt
viscosity due to their three-dimensional dendritic
architecture.29-33 In addition, hyperbranched polyimides can be
modified and functionalized from their core to periphery by
end capping, terminal grafting and hybrid blending, allowing
the design of PIs with complex structures and tailor-made
properties.34-37 However, the hyperbranched polyimide
memory devices are rarely studied. Qi and coworkers have
developed a hyperbranched polyimide. The memory device
a.National & Local Joint Engineering Laboratory for Synthesis Technology of High
Performance Polymer, College of Chemistry, College of Chemistry, Jilin University,
2699 Qianjin street, Changchun 130012, People’s Republic of China
b.Changchun Department for Product Supervision and Testing, 6888 Nanhu Road,
Changchun 130012, People’s Republic of China
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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with an Au/HBPI/ITO sandwich structure behaved as a disappeared, while the characteristic peaks of amino group at
nonvolatile write once read-many-times (WORM) memory 3458 and 3364 cm-1 appeared in the spectrum of MPPAB. It
with a witch voltage of 2.0 V, but the ON/OFF current ratio can be an evidence for the successful reduction of MPNPB.
(300) need to be further improved to meet the application 1HNMR spectra of MPNPB and MPPAB were diagrammatized in
requirements.38 In this study, we designed and synthesized a Fig. S4 in the supplementary information. The resonance signal
novel triamine monomer containing three TPA moieties, around 4.89 ppm was assigned to amino protons of the
N,N',N''-tris(4-methoxyphenyl)-N,N',N''-tris(4-phenylamino)-
triamine monomer. Differential scanning calorimetry (DSC)
1,3,5-benzenetriamine (MPPAB), and a hyperbranched trace of MPPAB is shown in Fig. S5 in the supplementary
polyimide (MPPAB-6FHBPI) was prepared by reacting the information. The melting point of MPPAB was about 231°C
triamine monomer with 4,4-(hexafluoroisopropylidene) with a melting range of 6 °C. All the data were well consisted
diphthalic anhydride (6FDA). The three TPA moieties were with the preconceived molecular structures, demonstrating
expected to promote the CT ability and improve stability of the the successful synthesis of MPPAB.
CT complexes. A sandwich device (ITO/MPPAB-6FHBPI/Al) in
Synthesis and characterization of MPPAB-6FHBPI
which MPPAB-6FHBPI was used as the active layer was
fabricated to evaluate its memory performance.
The synthetic pathway of the hyperbranched polyimide
MPPAB-6FHBPI is shown in Scheme 2. The FT-IR spectrum of
MPPAB-6FHBPI is shown in Fig. S5 in the supplementary
information. The characteristic bands at 1728 cm-1, 1783 cm-1
(C=O symmetrical stretching) and 1373 cm-1 (C–N stretching)
are assigned to the imide group of polyimide. No characteristic
band of polyamic acid around 1680 cm-1 was identified. These
results indicated that the polyimides were completed imidized.
The chemical structure of MPPAB-6FHBPI was confirmed by
1HNMR and the spectrum was illustrated in Fig. S6 in the
supplementary information. The bands in the range of 6.00–
7.50 ppm were attributed to the resonance absorption of the
phenyl hydrogen of MPPAB moieties, the bands around 7.50–
8.50 ppm were attributed to the phenyl hydrogen of 6FDA
moieties and those around 3.67 ppm were assigned to the
hydrogen of methoxy groups. All the data was in well
agreement with the expected structure. The inherent viscosity
of MPPAB-HBPI was 0.35 g dL-1 which was measured at a
polymer concentration of 0.5 g dL-1 in DMAc at 25 °C. The
molecular weights of MPPAB-6FHBPI were determined by gel
permeation chromatographic (GPC), the number-average
molecular weight (Mn) was 25078 and the weight-average
molecular weight (Mw) was 59291, corresponding to a
polydispersity index (PDI) of 2.36.
Results and discussion
Synthesis and characterization of monomers
4-Methoxy-4′-nitrodiphenylamine was prepared according to a
previously reported procedure.39 Fourier transform infrared
(FT-IR) spectrum of 4-methoxy-4′-nitrodiphenylamine is
illustrated in Fig. S1 in the supplementary information. The
characteristic bands at 1598 cm-1 and 1323 cm-1 are attributed
to the asymmetric and symmetric stretching of nitro group.
1
The HNMR spectrum of 4-methoxy-4′-nitrodiphenylamine is
shown in Fig. S2 in the supplementary information. The
resonance signal around 6.21 ppm was assigned to amino
protons of 4-methoxy-4′-nitrodiphenylamine. All the data was
in well agreement with the expected structure. The synthetic
pathway of triamine monomer MPPAB is shown in Scheme 1.
1
The FT-IR and HNMR spectroscopy was used to identify the
structure of the targeted triamine monomer MPPAB. The FT-IR
spectra of the intermediate trinitro compound N,N',N''-tris(4-
methoxyphenyl)-N,N',N''-tris(4-nitrophenyl)-1,3,5-
benzenetriamine (MPNPB) and MPPAB are illustrated in Fig. S3
in the supplementary information. After the reduction of
MPNPB, the peaks attributed to the asymmetric and
symmetric stretching of nitro group at 1575 and 1310 cm-1
Scheme 1. Synthesis of MPPAB triamine monomer.
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Scheme 2. Synthesis of hyperbranched polyimide MPPAB-6FHBPI.
Soluble behaviors and thermal properties
Optical and electrochemical properties
MPPAB-6FHBPI exhibited outstanding solubility in several
common organic solvents at room temperature. MPPAB-
6FHBPI could be completely dissolved not only in polar aprotic
organic solvents such as N-methyl-2-pyrrolidone (NMP),
dimethylacetamide (DMAc) and N,N-dimethylformamide
(DMF), but also in low boiling-point solvent such as
trichloromethane (CHCl3), tetrahydrofuran (THF) and acetone.
Such excellent solubility should be attributed to the three-
dimensional dendritic architecture which limits the chain
packing and intermolecular interactions. The thermal
properties of MPPAB-6FHBPI were investigated by differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) as shown in Fig. 1. The glass transition temperature (Tg)
of MPPAB-6FHBPI was 316 °C and the 5% weight-loss
The UV-vis absorption spectrum of MPPAB-6FHBPI is shown in
Fig. 2a, the absorption maxima (λmax) of MPPAB-6FHBPI in
5
temperature (Td ) was 488 °C. The polymer afforded high char
yield of around 58% at 800 °C in a N2 atmosphere, indicating
the outstanding thermal properties of the MPPAB-6FHBPI. The
excellent solubility and thermal properties of MPPAB-6FHBPI
meet the criteria of easy-processing, large-scale fabrication
and operational stability for fabrication of memory devices.
Fig. 2 (a) UV-vis absorption spectrum of MPPAB-6FHBPI, (b) CV diagram of MPPAB-
6FHBPI film on ITO-coated glass substrate. The CV measurement was carried out in a
0.10 M acetonitrile (CH3CN) solution of tetrabutylammonium perchlorate (TBAP) with a
three-electrode cell which ITO (polymer films area about 0.5 cm × 1.0 cm) was used as
a working electrode, a platinum wire as the auxiliary electrode and an Ag/Ag+ as the
reference electrode. A scan rate of 50 mV s-1 was used.
Fig. 1 TGA thermogram of MPPAB-6FHBPI. Insert gives DSC trace of the polymer. The
measurements were carried out at a rate of 10 °C min-1 under nitrogen atmosphere.
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DMAc was 300 nm with the absorption edge extended to 387 The HOMO and LUMO energy levels are calculated to be -5.17
nm, from which the band gap (Eg) of the material was derived and -1.97 eV, respectively. The elevated HOMO energy level
to be 3.15 eV (Eg = hc/
MPPAB-6FHBPI was investigated by cyclic voltammetry (CV) MPPAB unit which contains three TPA moieties. The elevated
conducted by film cast on an ITO glass substrate as the HOMO energy level effectively descents the energy barrier
working electrode using 0.10 solution of between the ITO electrode and the HOMO, thus promoting the
λledge). The electrochemical behavior of was attributes to the strong electron-donating ability of
a
M
tetrabutylammonium perchlorate (TBAP) in anhydrous hole injection from ITO electrode to HOMO.27
acetonitrile electrolyte. As shown in Fig. 2b, the CV diagram
AFM morphology of MPPAB-6FHBPI thin film
showed two oxidation peaks and the onset oxidation located
at about 0.39 V. The highest occupied molecular orbital The morphology of the active polymer layer in the device is
(HOMO) and lowest unoccupied molecular orbital (LUMO) crucial for the device performance.40, 41 Fig. 3a and 3b reveals
energy levels can be estimated from the onset oxidation the surface structure of MPPAB-6FHBPI characterized by atom
potential [Eox(onset)] and the band gap of MPPAB-6FHBPI force microscopy (AFM). MPPAB-6FHBPI exhibits excellent
according to equation (1) and (2) based on the reference film-formation ability with
a
small root-mean-square
energy level of ferrocene (4.80 eV below the vacuum level, roughness about 0.50 nm. The smooth surface of the active
which is defined as zero)
HOMO = - [(Eox (onset) - Eferrocene) + 4.80)] (eV) (1)
LUMO = Eg + HOMO (2)
polymer film is profitable for injection of the holes and thus
the charge-transporting process. In addition, the faultless
morphology may suppress the permeation of Al into the
Fig. 3 (a) Tapping-mode AFM topography of MPPAB-6FHBPI thin film; (b) A typical cross-section profile of AFM topographic image of the MPPAB-6FHBPI thin film; (c) Configuration
of ITO/MPPAB-6FHBPI/Al sandwich memory device and the insert photograph of the ITO/MPPAB-6FHBPI/Al sandwich memory device. (d) Current–voltage (I–V) characteristic of
the ITO/MPPAB-6FHBPI/Al memory device; the first sweep was performed from 0 to -4.00 V to switch the device on, the fourth sweep was conducted about 23 min after turning
off the power to switch the device again to on, the others sweeps were performed to test the ON state of the device. The electrode contact area was 0.4 × 0.4 mm2. (e) The
retention time and stress tests of both the OFF and ON states of the ITO/MPPAB-6FHBPI/Al memory device, probed under a constant bias of -1.00 V for 1 × 104 s.
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the device. A high ON/OFF current ratio is profitable to
decrease the misreading rate during the information reading
process. MPPAB-6FHBPI based memory device in this work
owns a switch voltage of -1.90 V, which is much lower than
some polyimides memory devices in recent literatures as
shown in Table 1. The switch voltages of most polyimides are
higher than 2.50 V in Table 1. The memory devices using Au as
the electrode have switch voltages lower than 2.50 V but Au is
too expensive which will increase the cost of memory device
polymer layer during the vacuum deposition of the Al
electrode.
Characteristics of memory device
A
schematic sandwich structure comprising many top
electrodes (Al), a layer of MPPAB-6FHBPI thin film and the
bottom electrode (ITO) are shown in Fig. 3c. ITO was used as
common electrode and Al was employed as the electrode for
applying voltage during the sweep. To identify the electrical
performance, the electrical switching effect of the synthesized
MPPAB-6FHBPI was demonstrated in the current–voltage (I–V)
characteristics of the ITO/MPPAB-6FHBPI/Al sandwich device,
as shown in Fig. 3d. During the first sweep from 0 to -4.00 V, a
sharp current increase occurred at a threshold voltage of -1.90
V, indicating the transition from a low-conductivity state (OFF-
state) to a high-conductivity state (ON-state). This transition
from OFF state to ON state can serve as the “writing” process,
fabrication. The memory device with
a
sandwiched
ITO/MPPAB-6FHBPI/Al structure in this work exhibited not
only a low switch voltage but also a high ON/OFF ratio. In
addition, MPPAB-6FHBPI exhibits outstanding solubility and
thermal stability, which are beneficial to the widespread
application of memory devices. The excellent comprehensive
performance makes MPPAB-6FHBPI to be a potential polymer
material for memory applications.
with an ON/OFF current ratio of the studied memory device as Theoretical analysis and switching mechanism
high as 1 × 106, meaning a low misreading rate in memory
To better understand the switching behavior of MPPAB-6FHBPI
applications. The device kept in this ON-state during the
subsequent second sweep from 0 to -4.00 V and then the third
sweep from 0 to 4.00 V, which corresponded to the reading
process. After turning off the power for 23 min, the device
turned back to the OFF-state, and was reswitched to the ON-
state at the threshold voltage about -1.90 V (sweep 4) and
remained in the ON state during the following negative (sweep
5) and positive (sweep 6) scans. The device with a long
retention time yet volatile showed a SRAM characteristic,
which may contribute a low power consumption in practical
applications. To further evaluate the stability of the SRAM
memory behavior, the retention characteristics of both the ON
and OFF states under a constant stress (-1.00 V) were
measured as shown in Fig. 3e. Initially, the memory device was
set to a high (ON) or low (OFF) conductivity state. Under a
based memory device, molecular simulation on the basic unit
was explored through density functional theory (DFT). The
resulting HOMO, and LUMOs surfaces are shown in Fig. 4a. The
HOMO is primarily located on the MPPAB moiety (donor),
while the three LUMOs are mainly located on the three 6FDA
phthalimide moieties (acceptor). It is noted that the three
LUMOs in MPPAB-6FHBPI are in degeneracy, whereas they are
in splitting in the linear PI with phthalimide units as the
electron acceptor. Such a degeneracy of the LUMOs should be
profitable to improve the efficiency of CT due to the
suppressed indirect CT processes.23 Under the application of
an external electric field, electrons at HOMO could be excited
to the LUMOs effectively. As a result, the holes generated in
the HOMO and delocalized on the conjugated triamine
moieties which are effective to form an open channel for the
carriers to migrate through. When the applied electric field
reached the threshold voltage, current density of the MPPAB-
6FHBPI film increased sharply due to the effective transition of
the electrons and thus the transition from the OFF state to the
ON state occurred. Usually, the excited state is unstable and
the electrons tend to go back from LUMOs to HOMO when the
power was turned off. The strong electron-withdrawing ability
constant stress of -1.00 V, no obvious degradations of current
were observed in both ON and OFF states for at least 1×104 s
and the ON/OFF current ratio were kept around 1×106 during
the measurements.
As we know, the threshold voltage and ON/OFF current ratio
are key parameters of the polymeric memory device.42, 43
A
low switch voltage would not only reduce the power
consumption but also probably decrease the risk of puncturing
Table 1. The switch voltage and ON/OFF current ratio of some polyimides memory devices based on recent literatures.
a
Memory device
ITO/3SOH-6FPI/Al 44
ITO/PI-b/Al 45
Vonset (V)
-4.90
-4.70
-4.50
3.20
ON/OFF
1×107
1×104
1×108
1×105
1×104
1×104
1×104
1×102
1×105
1×103
1×106
Type
DRAM
FLASH
SRAM
SRAM
WORM
SRAM
SRAM
WORM
SRAM
FLASH
SRAM
ITO/3STP-7/Al 44
ITO/PI-6FDA/Al 25
Al/PI:PCBM/Al 46
3.00
ITO/PI-6FDA/Al 22
2.70
Al/PI(DAT-6FDA)/Al 47
ITO/HBPI/Au 38
-2.50
2.00
ITO/6F-CzTPA PI/Au 48
ITO/PI(BTFBPD-DPBPDA)/Al 42
ITO/MPPAB-6FHBPI/Al (this work)
-1.80
−1.30
−1.90
a
The switch voltage from the OFF state to the ON state.
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.
Fig. 4 (a) Molecular orbitals and energy levels of the basic unit of MPPAB-6FHBPI. (b) Optimized geometry of the basic unit of MPPAB-6FHBPI. (c) LUMO and HOMO energy levels of
MPPAB-6FHBPI along with the work function of the ITO and Al electrodes.
contributed by the three 6FDA phthalimide groups as well as
the twisted steric structure between the MPPAB and the
Conclusions
phthalimide moiety (Fig. 4b) is profitable to stabilize the CT
complexes. Therefore, the MPPAB-6FHBPI based memory
device exhibited a SRAM behavior with a pretty long retention
time.
In summary, we have successfully synthesized a novel triamine
monomer
N,N',N''-tris(4-methoxyphenyl)-N,N',N''-tris(4-
phenylamino)-1,3,5-benzenetriamine containing multi-TPA
moieties. A hyperbranched polyimide MPPAB-6FHBPI was
prepared by reacting the triamine monomer with 4,4-
(hexafluoroisopropylidene)diphthalic anhydride. The three-
dimensional architecture of MPPAB-6FHBPI greatly reduced
the chain packing thus enhanced the solubility in common
organic solvents. The memory device based on MPPAB-6FHBPI
exhibited a SRAM memory storage behavior with a long
retention time about 23 min. The device could be switched
from the initial low-conductivity (OFF) state to the high-
conductivity (ON) state at a relatively low switch voltage of -
1.90 V with an ON/OFF current ratio reaching up to 1 × 106.
Moreover, the device showed excellent stability with an
operation time of 1 × 104 s at continuous applied voltage of -
1.00 V. Such a hyperbranched polyimide is expected to be
potential candidate for fabrication of memory devices.
The work functions of the Al and ITO electrodes as well as the
HOMO and LUMO energy levels of MPPAB-6FHBPI which are
determined by UV–vis spectrum and CV measurement are
illustrated in Fig. 4c. The HOMO and LUMOs of MPPAB-6FHBPI
are -5.17 eV and -1.97 eV respectively, which were in well
consistent with the results of the molecular simulation. When
the positive sweep was conducted, the large energy gap (0.97
eV) between the top electrode Al (-4.20 eV) and the HOMO (-
5.17 eV) of MPPAB-6FHBPI makes it difficult for the holes to be
injected from the top Al electrode into the HOMO. On the
contrary, the holes can be readily injected from the bottom
ITO electrode into the HOMO due to the narrow energy barrier
(0.37 eV) between ITO (-4.80 eV) and the HOMO (-5.17 eV).
Thus, the memory device could be switched to the ON state
during the negative sweep. The elevated HOMO energy level
descends the energy barrier for injection of the holes, thus
giving rise to a lowered threshold voltage.
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solution. Subsequently, chemical imidization was processed by
the addition of triethylamine (1.00 g) and acetic anhydride
(3.00 g), and then, the reaction mixture was stirred at 80 °C for
another 8 h. After cooling to room temperature, the mixture
was precipitated from ethanol (200 mL) and the precipitate
was thoroughly washed with ethanol. Finally, the polymer was
collected by filtration and dried under vacuum at 80 °C for 24
h.
Experimental
Materials
All commercially available reagents or anhydrous solvents
obtained from suppliers were used without further purification
unless otherwise noted. 4-Fluoronitrobenzene, p-anisidine and
dimethylacetamide (DMAc) (GC,
from Aladdin Reagents
≥
99.8%) were purchased
Co., Ltd. 4,4-
(Hexafluoroisopropylidene) diphthalic anhydride (6FDA) and
1,3,5-tribromobenzene (GC, >98.0%) were purchased from
Tokyo Chemical Industry (TCI). Acetonitrile was purified by
distillation before used and kept in a vacuum drier.
Tetrabutylammonium perchlorate (TBAP; Energy Chemical)
was recrystallized twice by ethanol under nitrogen atmosphere
and then dried in vacuo before used.
Fabrication and measurement of MPPAB-6FHBPI memory
device
The memory device was fabricated with the configuration of
indium tin oxide (ITO)/MPPAB-6FHBPI/Al as shown in Fig. 3c.
The ITO glass used for the memory device was cleaned by
ultrasonication with water, acetone, and isopropanol for 15
min respectively. A DMAc solution of MPPAB-6FHBPI (20 mg
mL-1) was first filtered through a PTFE membrane syringe filter
of 0.22 μm pore size. Then 250 μL of the filtered solution was
spin-coated onto the ITO substrate at a spinning rate of 1000
rpm for 60s, followed by solvent removal in a vacuum oven at
10-5 Torr and 80 °C for 6 h. The thickness of the MPPAB-6FHBPI
film was about 50 nm, as determined by Vecco Dektak 150
surface profiler. The roughness of the film was investigated by
AFM. An aluminum (Al) top electrode layer with thickness of
about 100 nm was thermally evaporated and deposited onto
Synthesis of MPPAB
The synthesis of MPPAB is illustrated in Scheme1. 4-Methoxy-
4′-nitrodiphenylamine was prepared according to a previously
reported procedure.39 The trinitro compound MPNPB was
prepared by the Ullmann coupling reaction. In a 250-mL three-
neck round-bottom flask equipped with a stirring bar, 10.98g
(45.00 mmol) of 4-methoxy-4′-nitrodiphenylamine, 3.38 g
(11.00 mmol) of 1,3,5-tribromobenzene, 17.39 g (126.00
mmol) of powdered anhydrous potassium carbonate, 4.00 g
(63.00 mmol) of copper powder and 0.84 g (3.20 mmol) of 18-
crown-6-ether were stirred at 160 °C in o-dichlorobenzene
(150 mL) under nitrogen atmosphere for 24 h. The hot reaction
mixture was filtered to remove the copper and inorganic salts,
and then; the product was precipitated into methanol. The
product was purified by recrystallization from o-
dichlorobenzene and methanol to give a dark red powder. The
triamine monomer MPPAB was synthesized by hydrazine Pd/C-
catalyzed reduction of the trinitro compound MPNPB. MPNPB
(11g, 13.60 mmol) and 0.48 g of 10 % Pd/C were
dissolved/suspended in 180 mL of tetrahydrofuran
(THF)/ethanol mixed-solvent in a 250-mL three-neck flask
under nitrogen atmosphere. The suspension solution was
heated to reflux, and 8 mL of hydrazine monohydrate was
added slowly to the mixture, then the solution was stirred at
reflux temperature for 24 h, the solution was filtered to
remove Pd/C. After the removal of solvent, a white powder
was collected and purified by silica column chromatography.
the polymer surface at about 2
from 3~5 Å s-1 through a shadow mask. The current-voltage (I-
V) data reported was based on device units of 0.4
0.4 mm2 in
×
10-6 Torr with rate ranging
×
size, under ambient conditions, using a Keithley 2636B
semiconductor parameter analyzer. ITO was used as common
electrode and Al was the electrode for applying voltage during
the sweep.
Measurements
Fourier transform infrared (FT-IR) spectra were recorded on a
1
Nicolet Impact 410 FT-IR spectrometer. HNMR spectra were
measured on a Bruker AVANCE NMR (1H, 300 MHz)
spectrometer in DMSO-d6, using tetramethylsilane as an
internal reference. Gel permeation chromatographic (GPC)
analysis was carried out on
a
high temperature
chromatography PL-GPC 220 system (DMF as the eluent at a
flow rate of 1.00 mL min-1 and polystyrene as the standards).
Atom force microscopy (AFM) measurements were obtained
with a NanoScope IIIa AFM at room temperature. Commercial
silicon cantilevers (Nanosensors, Germany) with typical spring
constants of 21-78 N m-1 were used to operate the AFM in
tapping mode. Thermogravimetric analysis (TGA) was
conducted with a PerkinElmer TGA-7, heated in flowing
nitrogen (heating rate of 10 °C min-1 and flow rate of 20 mL
min-1). Differential scanning calorimetry (DSC) analysis was
performed on a Mettler Toledo DSC821e instrument at a scan
rate of 10 °C min-1 in flowing nitrogen (20 mL min-1). UV–vis
spectrum was acquired on a Shimadzu 3600 UV–vis–NIR
spectrophotometer. Electrochemistry was recorded on a
CHI660D Electrochemical Workstation using a three-electrode
1
Yield: 5.12 g (65.0%). H NMR (300 MHz, DMSO-d6, δ, ppm):
3.67 (s, 9H,-OCH3), 4.89 (s, 6H,-NH2), 5.78 (s, 3H), 6.43 (d, J=8.7
Hz, 6H), 6.66 (d, J=8.4 Hz, 6H ) 6.71 (d, J=9.0 Hz, 6H), 6.80 (d,
J=9.0 Hz, 6H).
Synthesis of the hyperbranched polyimide MPPAB-6FHBPI
MPPAB-6FHBPI was synthesized from 6FDA and MPPAB, the
synthetic pathway is shown in Scheme 2. Firstly, a dianhydride
(6FDA) (0.44 g, 1.00 mmol) was dissolved in DMAc (8 mL) in a
thoroughly dried 100 mL three-necked flask under a nitrogen
flow. To the mixture, MPPAB (0.36g, 0.50 mmol) in DMAc (9
mL) was added dropwise through a dropping funnel over 1 h
under magnetic stirring at room temperature. The reaction
was further conducted for 20 h to afford a poly(amic acid)
cell in which ITO (polymer films area about 0.5 cm × 1.0 cm)
was used as a working electrode and a platinum wire as the
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auxiliary electrode at a scan rate of 50 mV s-1 against a Ag/Ag+
reference electrode in a 0.10 M acetonitrile (CH3CN) solution
of tetrabutylammonium perchlorate (TBAP).
21 Y. L. Liu, K. L. Wang, G. S. Huang, C. X. Zhu, E. S. Tok, K. G.
Neoh and E. T. Kang, Chem. Mater., 2009, 21, 3391-3399.
22 Y. Liu, Y. Zhang, Q. Lan, Z. Qin, S. Liu, C. Zhao, Z. Chi and J. Xu,
J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 1302-1314.
23 H. J. Y. C. J. Chen, W. C. Chen and G. S. Liou, J. Polym. Sci.,
Part A: Polym. Chem, 2011, 49, 3709-3718.
Acknowledgments
24 C. J. Chen, H. J. Yen, W. C. Chen and G. S. Liou, J. Mater.
Chem., 2012, 22, 14085-14093.
The authors gratefully acknowledge the financial support of
this research through the Chinese Natural Science Foundation
25 Y. Liu, Y. Zhang, Q. Lan, S. Liu, Z. Qin, L. Chen, C. Zhao, Z. Chi,
J. Xu and J. Economy, Chem. Mater., 2012, 24, 1212-1222.
(No.51573067) for financial support of this work. The authors 26 H. J. Yen, C. J. Chen and G. S. Liou, Adv. Funct. Mater., 2013,
23, 5307-5316.
also express their deep gratitude for assistances of Prof.
Huiling Liu (Laboratory of Theoretical and Computational
Chemistry, Jilin University, China) in molecular simulation, Dr
Jiyong Fang (National & Local Joint Engineering Laboratory for
27 D. M. Kim, S. Park, T. J. Lee, S. G. Hahm, K. Kim, J. C. Kim, W.
Kwon and M. Ree, Langmuir, 2009, 25, 11713-11719.
28 K. L. Wang, Y. L. Liu, I. H. Shih, K. G. Neoh and E. T. Kang, J.
Polym. Sci., Part A: Polym. Chem., 2010, 48, 5790-5800.
Synthesis Technology of High Performance Polymer, Jilin 29 X. Lei, M. Qiao, L. Tian, Y. Chen and Q. Zhang, The Journal of
Physical Chemistry C, 2016, 120, 2548-2561.
30 X. Lei, Y. Chen, M. Qiao, L. Tian and Q. Zhang, J. Mater.
Chem. C, 2016, 4, 2134-2146.
31 X. F. Lei, M. T. Qiao, L. D. Tian, P. Yao, Y. Ma, H. P. Zhang and
Q. Y. Zhang, Corros. Sci., 2015, 90, 223-238.
University, China) and Shipan Wang (State Key Laboratory of
Supramolecular Structure and Materials, Jilin University, China)
in memory device fabrication.
32 X. Lei, M. Qiao, L. Tian, Y. Chen and Q. Zhang, Corros. Sci.,
2015, 98, 560-572.
33 X. F. Lei, Y. Chen, H. P. Zhang, X. J. Li, P. Yao and Q. Y. Zhang,
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Graphical Abstract
A hyperbranched polyimide (HBPI) was synthesized from a designed triamine monomer and the
HBPI based memory device presented a SRAM (static random access memory) behavior with a low
switch voltage and high ON/OFF current ratio.