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

Abstract

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)diphthalic anhydride for application in memory devices. 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 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 be retained 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 efficient charge transfer between the triamine moieties and hexafluoropropylidene phthalimides moieties in HBPI exist, which is responsible for the improved electrical memory performance. Such a HBPI was expected to be potentially useful in polymer memory applications.

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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 a 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, 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 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 with an Au/HBPI/ITO sandwich structure behaved as a nonvolatile write once read-many-times (WORM) memory with a switch voltage of 2.0 V, but the ON/OFF current ratio (300) need to be further improved to meet the application requirements.³⁸ In this study, we designed and synthesized a novel triamine monomer containing three TPA moieties, N,N′,N′′-tris(4-methoxyphenyl)-N,N′,N′′-tris(4-phenylamino)-1,3,5-benzenetriamine (MPPAB), and a hyperbranched polyimide (MPPAB-6FHBPI) was prepared by reacting the triamine monomer with 4,4-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). The three TPA moieties were expected to promote the CT ability and improve stability of the CT complexes. A sandwich device (ITO/MPPAB-6FHBPI/Al) in which MPPAB-6FHBPI was used as the active layer was fabricated to evaluate its memory performance.

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 ESI.† The characteristic bands at 1598 cm⁻¹ and 1323 cm⁻¹ are attributed to the asymmetric and symmetric stretching of nitro group. The ¹HNMR spectrum of 4-methoxy-4′-nitrodiphenylamine is shown in Fig. S2 in the ESI.† 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. The FT-IR and ¹HNMR 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 ESI.† After the reduction of MPNPB, the peaks attributed to the asymmetric and symmetric stretching of nitro group at 1575 and 1310 cm⁻¹ disappeared, while the characteristic peaks of amino group at 3458 and 3364 cm⁻¹ appeared in the spectrum of MPPAB. It can be an evidence for the successful reduction of MPNPB. ¹HNMR spectra of MPNPB and MPPAB were diagrammatized in Fig. S4 in the ESI.† The resonance signal around 4.89 ppm was assigned to amino protons of the triamine monomer. Differential scanning calorimetry (DSC) trace of MPPAB is shown in Fig. S5 in the ESI.† The melting point of MPPAB was about 231 °C with a melting range of 6 °C. All the data were well consisted with the preconceived molecular structures, demonstrating the successful synthesis of MPPAB.

Scheme 1 Synthesis of MPPAB triamine monomer.

Synthesis and characterization of MPPAB-6FHBPI

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 ESI.† 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 ESI.† 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 (M_n) was 25 078 and the weight-average molecular weight (M_w) was 59 291, corresponding to a polydispersity index (PDI) of 2.36.

Scheme 2 Synthesis of hyperbranched polyimide MPPAB-6FHBPI.

Soluble behaviors and thermal 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 (T_g) of MPPAB-6FHBPI was 316 °C and the 5% weight-loss temperature (T⁵_d) 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. 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.

Optical and electrochemical properties

The UV-vis absorption spectrum of MPPAB-6FHBPI is shown in Fig. 2a, the absorption maxima (λ_max) of MPPAB-6FHBPI in DMAc was 300 nm with the absorption edge extended to 387 nm, from which the band gap (E_g) of the material was derived to be 3.15 eV (E_g = hc/λ_ledge). The electrochemical behavior of MPPAB-6FHBPI was investigated by cyclic voltammetry (CV) conducted by film cast on an ITO glass substrate as the working electrode using a 0.10 M solution of tetrabutylammonium perchlorate (TBAP) in anhydrous acetonitrile electrolyte. As shown in Fig. 2b, the CV diagram showed two oxidation peaks and the onset oxidation located at about 0.39 V. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels can be estimated from the onset oxidation potential [E_ox(onset)] and the band gap of MPPAB-6FHBPI according to eqn (1) and (2) based on the reference energy level of ferrocene (4.80 eV below the vacuum level, which is defined as zero)

HOMO = −[(E_ox(onset) − E_ferrocene) + 4.80)] (eV) (1)

LUMO = E_g + HOMO (2)

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.

The HOMO and LUMO energy levels are calculated to be −5.17 and −1.97 eV, respectively. The elevated HOMO energy level was attributes to the strong electron-donating ability of MPPAB unit which contains three TPA moieties. The elevated HOMO energy level effectively descents the energy barrier between the ITO electrode and the HOMO, thus promoting the hole injection from ITO electrode to HOMO.²⁷

AFM morphology of MPPAB-6FHBPI thin film

The morphology of the active polymer layer in the device is crucial for the device performance.^40,41 Fig. 3a and b reveals the surface structure of MPPAB-6FHBPI characterized by atom force microscopy (AFM). MPPAB-6FHBPI exhibits excellent film-formation ability with a small root-mean-square roughness about 0.50 nm. The smooth surface of the active 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 polymer layer during the vacuum deposition of the Al electrode.

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.

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, with an ON/OFF current ratio of the studied memory device as high as 1 × 10⁶, meaning a low misreading rate in memory 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 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 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 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.

Table 1 The switch voltage and ON/OFF current ratio of some polyimides memory devices based on recent literatures

Memory device	Vonset^a (V)	ON/OFF	Type
a The switch voltage from the OFF state to the ON state.
ITO/3SOH-6FPI/Al^44	−4.90	1 × 10^7	DRAM
ITO/PI-b/Al^45	−4.70	1 × 10^4	FLASH
ITO/3STP-7/Al^44	−4.50	1 × 10^8	SRAM
ITO/PI-6FDA/Al^25	3.20	1 × 10^5	SRAM
Al/PI:PCBM/Al^46	3.00	1 × 10^4	WORM
ITO/PI-6FDA/Al^22	2.70	1 × 10^4	SRAM
Al/PI(DAT-6FDA)/Al^47	−2.50	1 × 10^4	SRAM
ITO/HBPI/Au^38	2.00	1 × 10^2	WORM
ITO/6F-CzTPA PI/Au^48	−1.80	1 × 10^5	SRAM
ITO/PI(BTFBPD-DPBPDA)/Al^42	−1.30	1 × 10^3	FLASH
ITO/MPPAB-6FHBPI/Al (this work)	−1.90	1 × 10^6	SRAM

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Theoretical analysis and switching mechanism

To better understand the switching behavior of MPPAB-6FHBPI 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 contributed by the three 6FDA phthalimide groups as well as the twisted steric structure between the MPPAB and the 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.

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.

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.

Conclusions

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.

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, ≥99.8%) were purchased from Aladdin Reagents 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.

Synthesis of MPPAB

The synthesis of MPPAB is illustrated in Scheme 1. 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, 10.98 g (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 (11 g, 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. Yield: 5.12 g (65.0%). ¹H 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 (0.36 g, 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) 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.

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 60 s, 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 the polymer surface at about 2 × 10⁻⁶ Torr with rate ranging 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 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 Nicolet Impact 410 FT-IR spectrometer. ¹HNMR 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 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 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).

Acknowledgements

The authors gratefully acknowledge the financial support of this research through the Chinese Natural Science Foundation (No. 51573067) for financial support of this work. The authors 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 Synthesis Technology of High Performance Polymer, Jilin University, China) and Shipan Wang (State Key Laboratory of Supramolecular Structure and Materials, Jilin University, China) in memory device fabrication.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20353a

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