Flexible Display Substrates Research Articles

Polyimides with ultra-low coefficient of thermal expansion derived from diamine containing bisbenzimidazole and bisamide

To provide polyimide (PI) with high heat resistance and ultra-low coefficient of thermal expansion (CTE) for use in flexible display substrates, a novel diamine with bisbenzimidazole and bisamide groups, namely N,N'-(1H,1'H-[5,5'-bibenzo[d]imidazole]-2,2'-diyl)bis(4-aminobenzamide) (BZBA), was designed and successfully synthesized. Several poly(benzimidazole-amide-imide) (PBIAI) films were prepared by thermal imidization with 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-oxydiphthalic anhydride (ODPA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), and 4,4'-(hexafluoroisopropylindene)-diphthalic anhydride (6FDA), respectively. Among them, BZBA-BPDA has a high glass transition temperatures ( Tg = 345 °C) while achieving an extremely low coefficients of thermal expansion (CTE = 1.9 ppm K−1), meeting the processing requirements of polymer flexible substrates in OLED devices. The effects of different dianhydrides on the performance of PBIAIs were compared, providing a meaningful reference for further adjusting the PI molecular structure to meet specific requirements for industrial polymer films.

Enhancing the mechanical and dielectric properties of high thermally stable polyimides via a crosslinkable bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride monomer

Polyimides (PIs) have attracted great attention because of their excellent properties and applications in areas such as flexible display substrates, microelectronics, and integrated circuits. The effects of crosslinking on the thermal, mechanical, and dielectric properties of PIs, however, still require further investigation. In this work, we introduce a crosslinkable monomer, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid dianhydride (BTA), which contains double bonds, into the system consisting of cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA) and 4,4′-oxydianiline (ODA) for the synthesis of PI films. The obtained PIs are further crosslinked by free radical reactions using a crosslinking agent, 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TAIC) to form three crosslinked PI materials, PI-TAIC-5%, PI-TAIC-10%, and PI-TAIC-15%. The resulting all three crosslinked PI films exhibit excellent thermal properties with their glass transition temperatures (Tg) higher than 400 °C. For the mechanical properties, the formation of the rigid network structure leads to an improvement in their tensile strengths and Young's moduli. The introduction of 10 mol% TAIC in the PI (PI-TAIC-10%) shows the highest tensile strength (114.77 MPa), representing an 18.6% enhancement in comparison with that of the original PI material. In terms of dielectric properties, the crosslinking reactions effectively increase the free volumes between the polymer chains and reduce the molecular dipole moments. As a result, PI-TAIC-15% significantly reduces the dielectric properties, resulting in a decrease in the dielectric constant (Dk, from 3.42 to 3.15) and the dielectric loss (Df, from 3.67 × 10−2 to 2.58 × 10−2) under 10 GHz frequency. As far as the coefficient of thermal expansion (CTE) results, the formation of the network structure in PI-TAIC-15% restricts the movement of the polymer chains, leading to a 15% reduction in the CTE value than that of the non-crosslinked PI film. Overall, this work provides a promising strategy to optimize the mechanical and dielectric properties of high thermally stable PIs via crosslinking reactions.

Preparation and Characterization of Low CTE Poly(ethersulfone) Using Lignin Nano Composites as Flexible Substrates.

Polyethersulfone (PES) has outstanding thermal and dimensional stability. It is considered an engineering thermoplastic. However, its high coefficient of thermal expansion (CTE) hinders its use in automobiles, microelectronics, and flexible display areas. To overcome its high coefficient of thermal expansion (CTE), recent studies have focused on reducing its high CTE and improving its mechanical properties by adding nano-sized fillers or materials. The addition of nanofiller or nanofibrils to the PES matrix often has a positive effect on its mechanical and thermal properties, making it a flexible display substrate. To obtain ideal flexible substrates, we prepared polyethersulfone with lignin nanocomposite films to reduce CTE and improve the mechanical and thermal properties of PES by varying the relative ratio of PES in the lignin nanocomposite. In this study, lignin as a biodegradable nanofiller was found to show high thermal, oxidative, and hydrolytic stability with favorable mechanical properties. PES/lignin nanocomposite films were prepared by solution casting according to the content of lignin (0 to 5 wt.%). PES/lignin composite films were subjected to mechanical, thermo-mechanical, optical, and surface analyses. The results showed enhanced thermomechanical and optical properties of PES, with the potential benefits of lignin filler materials realized for the development of thermoplastic polymer blends.

Photochromic and mechanochromic cotton fabric for flexible rewritable media based on acrylate latex with spiropyran cross-linker

Rewritable materials play a significant role in reducing paper production and consumption, information storage and anticounterfeiting, etc. Rewritable textiles have attained much attention due to the flexibility, easy availability and functionality. How to develop a photochromic and mechanochromic dual-stimuli responsive fabric as a flexible rewritable media or display substrate is still rarely reported. In the present work, we present the rational design and synthesis of stimuli-responsive acrylate latex via emulsion polymerization with a spiropyran cross-linker. Spiropyran was chemically bonded into the polymer network of latex particles. Then cotton fabrics were spraying coated by the latex with different composition and add-on percentage. Writing without ink was carried out to investigate the photochromic and mechanochromic properties of the functionalized cotton fabrics. The functionalized cotton fabric still could change color under writing without ink after soaping for 50 times and light irradiation for 20 cycles. Our developed flexible cotton fabric can be used as a long-term, efficient, and stable rewritable media or flexible display substrate, even in the harsh environment such as washing.

Disentangled ultrahigh molecular weight polyethylene thin film as a transparent substrate for flexible flat panel display

AbstractThe use of polymeric thin film as a substrate material in fabrication of flexible organic light‐emitting diodes (OLEDs) devices is well known. Polyethylene terephthalate (PET) and polyethylene naphtalate (PEN) thin films are used for such purpose commercially. In the present work, 10g of disentangled ultrahigh molecular weight polyethylene (D‐UHMWPE) was processed into 35 ± 5μ thin film by continuous solid state compaction process between two heated rollers of roll mill. The temperature of both rollers was kept at 125°C as well as 128°C and the thin‐film production rate was 1.5 m/min. Subsequently film was studied for substrate application in fabrication of flexible OLEDs. The primary requirement of OLED substrate material namely chemical stability, transparency, and dimensional stability of D‐UHMWPE thin film were studied. D‐UHMWPE film showed excellent chemical stability under various solvents (chloroform, chlorobenzene, dichlorobenzene, acetone, and isopropanol) treatments along with >80% transparency and dimensional stability up to 100 ± 10°C (machine and transverse directions 0.001% and 0% at 90°C and 0.003% and 0% at 110°C). Finally, OLED device was fabricated on D‐UHMWPE surface which showed excellent performance in terms of (light emitting) even when the devices were bended.

Preparation and properties of polyimide films derived from hexaazatriphenylenehexacarboxylic acid trianhydride

In this study, a new polyimide film were prepared using monomers with different proportions of hexaazatriphenylene-ternary-anhydride (HAT-T). This material had higher molecular weights (Mn =1.81–11.4 × 106; Mw=2.32–15.5 × 106) and narrow molecular weight distribution (PDI=1.10–1.36). With the introduction of HAT-T, the glass transition temperature (Tg) of the polyimide film exceeded 300 °C, and the temperature for 5% decomposition in N2 exceeded 520 °C. The polyimide exhibited good mechanical properties (TS = 63.39–77.81 MPa; EB = 4.50~7.72%). The polyimide film with HAT-T had a micro-crosslinked structure, and its coefficient of thermal expansion decreased by 97 ppm/K. This work has potential contributions to the study of flexible display substrate materials.

N-Phenyl-substituted poly(benzimidazole imide)s with high glass transition temperature and low coefficient of thermal expansion.

To obtain high thermostable materials for flexible display substrates, a series of copoly(benzimidazole imide)s was prepared using 5-amine-2-(4-aminobenzene)-1-phenyl-benzimidazole (N-PhPABZ) and 6(5)-amino-2-(4-aminobenzene)-benzimidazole (PABZ). Incorporating N-phenyl groups effectively healed the brittleness of the poly(benzimidazole imide)s (PBIIs) derived from pyromellitic dianhydride (PMDA), and the resultant homo- and copoly(benzimidazole imide)s displayed an outstandingly high glass transition temperature (Tg > 450 °C) and a low coefficient of thermal expansion (CTE < 10 ppm K−1). Furthermore, the influence of removing intermolecular hydrogen bonds on the properties of these poly(benzimidazole imide)s was systematically analyzed. These data provide a feasible method to prepare superheat-resistant poly(benzimidazole imide)s without H-bonding.

Transparent poly(ether sulfone) nanocomposite film with low thermal expansion coefficient for flexible display substrates

Optically transparent poly(ether sulfone) (PES) nanocomposite films with low coefficients of thermal expansion (CTE) were synthesized for use as flexible display substrates. Boehmite (AlOOH) nanowires ~10 nm in diameter were synthesized and applied as fillers to reduce the CTE of the PES film. PES molecules were slightly sulfonated to enhance their compatibility with AlOOH nanowires via hydrogen bonding. The aspect ratio of the AlOOH nanowires had a significant effect on the CTE of nanocomposite films. At a fixed filler concentration of 15 wt%, the AlOOH nanowires with an aspect ratio of 100 reduced the CTE of the PES film by 73.6 % from 65.5 ppm/°C to 17.3 ppm/°C, while those with an aspect ratio of 20 reduced it only by 28 %. Although such a huge reduction of CTE was provided by AlOOH nanowires, the optical transmittance was still 96 % of the pristine PES film because the refractive indices of PES and AlOOH nanowires were the same. Introduction of AlOOH nanowires also enhanced the thermal and dimensional stabilities of the PES film.

Heat‐resistant polyimides with lowCTEand water absorption through hydrogen bonding interactions

AbstractA novel diamine, 1H,1′H‐(2,2′‐bibenzimidazole)‐5,5′‐diamine (DPABZ), containing bisbenzimidazole unit was successfully synthesized, and used to prepare a series of copolyimides BPDA:(ODAm/DPABZn) by polycondensation with 4,4‐diaminodiphenyl ether (ODA) and 4,4‐biphthalic anhydride (BPDA). For comparison, a series of copolyimides BPDA:(ODAm/PABZn) based on another benzimidazole diamine 5‐amino‐2‐(4‐aminobenzene)‐benzimidazole (PABZ) was also prepared. As a result, with the increase of PABZ or DPABZ content, the heat resistance (TgandTd) and mechanical properties (σandE) of the resulting polyimide (PI) films increased, while the coefficient of thermal expansion (CTE) decreased. Overall, the DPABZ‐based PIs showed higherTgvalues and much lower CTE values than PABZ. As the content of PABZ increased, the water absorption of PABZ‐based PIs increased obviously, but no significant change in DPABZ‐based PIs. The intramolecular hydrogen bonding in DPABZ‐based PIs caused by the diamine DPABZ was believed to be the reason for the aforementioned differences. The BPDA: DPABZ film with low‐water adsorption of 2.1%, high‐Tgvalue of 436°C and low‐CTE value of 5.4 ppm/°C could be a promising new generation of flexible display substrates.

Homopolyimides containing both benzimidazole and benzoxazole with highTgand low coefficient of thermal expansion

AbstractA pair of isomer diamines containing both benzimidazole and benzoxazole and derived homopolyimides were synthesized for the first time. Due to rich rigid and linear benzoazole units, as well as the strong intermolecular interactions from the hydrogen bonding and the charge transfer complexation (CTC), the obtained polyimides (PIs) exhibited outstanding thermal properties, including high thermal weight loss temperature (Td5% = 540–561°C), high glass transition temperature (Tg = 392–421°C), and low coefficient of thermal expansion (CTE = 1.3–20.9 ppm/°C). In addition, the obtained PIs also showed ideal mechanical properties (TS = 189–240 MPa,TM = 4.1–5.0 GPa andEB = 2.9%–4.7%). These potential novel PI films with highTgand low CTE were expected to be applied to next generation of flexible display substrate material.

Selection of Surfactants for Hydrogen peroxide-oxalic acid Polishing Slurry in Chemical Mechanical Polishing of 304 Stainless Steel

Stainless steel material will become one of the main materials for flexible large-size display substrates. Chemical mechanical polishing (CMP) will be one of the most practical processing technologies to achieve super smooth, non-damaged stainless steel surface. In this paper, the effects of different surfactants on material removal rate and surface roughness were studied. The results show that when adding the surfactant of the sodium hexadecyl sulfate in CMP slurry, the MRR was significantly higher than the other two kinds of surfactants. When the content of the sodium hexadecyl sulfate was 0.04wt%, the MRR reached the Maximum. When the content of the sodium hexadecyl sulfate was 0.2 wt%, the surface roughness reached the Minimum. So, the sodium hexadecyl sulfate is selected as the surfactant of the hydrogen peroxide-oxalic acid type CMP slurry.

Selection on Measuring Points of the Surface Roughness on Large Size Ultra-Thin Stainless Steel

Stainless steel will become one of the main substrate materials of flexible large-scale display. The biggest problem of stainless steel as flexible display substrate is that the surface roughness is too large, so it is necessary to be ultra-precision machining on its surface, such as the chemical mechanical polishing (CMP). How to select the most representative measurement points in measuring surface roughness after CMP is the first problem to be solved. Based on the idea of orthogonal design, taking a roll of ultra-thin flexible stainless steel substrate as an example, the method and algorithm of selecting 12 measuring points on a 600mm×100mm rectangular area are obtained. This method avoids the blindness in random selection of measurement points, and reduces the workload of selecting measurement points. It can provide a reference for the selection of measurement points on the large-scale surface after ultra-precision machining.

Study on Dispersant of Hydrogen Peroxide-Oxalic Acid Polishing Slurry in Chemical Mechanical Polishing of 304 Stainless Steel

Flexible display has become a research hotspot for next-generation display technology. Stainless steel materials will become one of the main materials for flexible large-size display substrates, and chemical mechanical polishing (CMP) technology will be one of the most practical processing technologies to achieve super-smooth, non-destructive surface of stainless steel material. In this paper, through a series of experiments, the material removal rate and surface roughness of hydrogen peroxide oxidizer under different dispersants and different dispersant contents were studied. The results shown that when the content of dispersant sodium hexametaphosphate is 1.2% wt, the material removal was the largest, which was 146nm / min. the surface roughness after CMP was relatively low, Ra = 10nm. Sodium hexametaphosphate was selected as the dispersant of the solution. The research results provided a reference for further study of 304 stainless steel chemical mechanical polishing fluid.

Superheat-resistant polyimides with ultra-low coefficients of thermal expansion

To achieve the superheat-resistant polymers for flexible display substrates, two new diamines, namely 6-amine-2-(4-aminobenzene)-1-methyl-benzimidazole (m-MePABZ), 5-amine-2-(4-amino-benzene)-1-methyl-benzimidazole (p-MePABZ) containing N-methyl benzimidazole (BI) were synthesized and a serials of poly(benzimidazole imide) (PBII) films were prepared through thermal imidization. Among them, the flexible polyimide (PI) film derived from m-MePABZ and pyromellitic dianhydride (PMDA) has an ultra-low coefficient of thermal expansion (CTE) down to 2.5 ppm K−1 and an outstanding high glass-transition temperature (Tg) up to 431 °C. Furthermore, the employment of N-methyl overcame an issue of the high H2O-absorption of traditional PBIIs, and the effect of “para-” and “meta-connecting” methyl on molecular packing and properties of these PIs was investigated. These data provided a useful guide to develop high performance PBIIs by incorporating N-methyl.

Optically Transparent and Thermal‐Stable Polyimide Films Derived from a Semi‐Aliphatic Diamine: Synthesis and Properties

AbstractA novel semi‐aliphatic diamine with bulky cyclohexyl and ortho‐substituted tertiary butyl groups (4,4'‐(cyclohexylmethylene)bis(2‐(tert‐butyl)aniline), CHMBTBA) is synthesized via Mannich and rearrangement reactions of cyclohexanecarboxaldehyde and 2‐tert‐butylaniline. Afterward, several semi‐aliphatic polyimides are prepared by the high‐temperature one‐pot polymerization of CHMBTBA with various aromatic dianhydrides respectively. The resulting PIs exhibit outstanding solubility in common organic solvents (i.e., trichloromethane, etc.) and are easily processed into light color or colorless transparent films (thickness: 25 ± 1 µm) by the blade coating of polymer solution. The transmittance of all resulting films is above 86% in the visible light region (400–760 nm). Their glass transition temperatures exceed 310 °C and reach even to 374 °C (by differential scanning calorimetry). They also display low water absorption (0.29–0.87%) and good mechanical properties (tensile strength: 50.9–87.7 MPa, Young's modulus: 2.0‐2.5 GPa, elongation at break: 3.8–7.6%). Thus, they are promising potential candidates for flexible display substrates.

Study on polishing slurry of hydrogen peroxide-oxalic acid in CMP 304 stainless steel

Stainless steel will become the substrate material of the flexible display, requirements of the flexible substrate in the surface quality and performance are very strict. Chemical mechanical polishing (CMP) is one of the most appropriate technologies to achieve the surface processing of ultra-thin stainless-steel flexible display substrate with ultra-smooth and damage-free. In this paper, the design of CMP slurry of 304 stainless steel on the hydrogen peroxide-oxalic acid type was proposed. Through experiment and analysis, the basic ingredients of CMP slurry was obtained. The research results showed that the hydrogen peroxide can increase the hydrophilicity of the stainless steel surface, and the Fenton type Haber-Weiss reaction can occur on the surface of the fresh metal substrate of stainless steel. The trivalent iron ions generated by the Fenton type reaction combined with oxygen to generate iron oxide and promoted the removal of the surface material. Under acidic conditions, the oxalic acid can decrease the stability of the oxide film on the stainless steel surface, promote the diffusion of oxygen into the metal interface, improve the oxygen reduction reaction, and increase the material removal rate. The results will provide an important reference for the next researching the CMP slurry of 304 stainless steel.

Dramatically enhanced mechanical properties of diamond-like carbon films on polymer substrate for flexible display devices via argon plasma pretreatment

Hydrogen-free diamond-like carbon (DLC) films have been deposited on polyimide (PI) substrate at room temperature using the sputtering with a novel nanodiamond target. Also, argon plasma pretreatment procedure for PI substrate was optimized to improve the adhesion and characteristics of the as-prepared DLC film. The deposited DLC film on the PI substrate pretreated with −400 V of bias voltage was found to possess the highest sp3 bonding fraction of 53%, the pencil hardness of 3H, and the optical transmittance of 82%, respectively. The research work demonstrates prosperous applications of DLC films in the protective coating for commercialized optoelectronic devices.

Colorless PI structure design and evaluation for achieving low CTE target

To obtain the colorless polyimide (PI) film with low coefficient of thermal expansion (CTE) and high thermal stability, the rigid biphenyltetracarboxylic acid dianhydride (BPDA) and 4,4'-(Hexafluoroisopropylidene) diphthalic anhydride (FDA) were selected and copolymerized with 2,2'-bis (trifluoromethyl)-[1,1'-biphenyl]-4,4'-diamine (TFDB). The physical properties of PIs were effectively regulated and optimized by adjusting the ratio of BPDA to FDA. When the ratio of BPDA to FDA is 6:4, the thermal imidized film (PI-6) performance is the best. It has a glass transition temperature (Tg) of 363 ℃, a 5% weight loss temperature (Td5) of 533 ℃, a CTE of 18.8 ppm/K, and a tensile strength of 140 MPa. The transmittance of PI-6 at 760 nm and 430 nm are up to 89.6% and 83.2% respectively. Further, the PI-6's precursor, PAA-6 were treated by chemical imidization. The obtained film has a transmittance of 92.6% at 760 nm, a decreased CTE of 15.5 ppm/K, and a tensile strength of 227 MPa, which is expected to be used as flexible display substrates. Finally, the effects of molecular and aggregation structure on the properties of PIs were studied.

Synthesis of highly transparent and heat‐resistant polyimides containing bulky pendant moieties

AbstractA new diamine with bulky pendant biphenyl and ortho‐position dimethyl structures, 4,4′‐((1,1′‐biphenyl)‐4‐ylmethylene)bis(2,6‐dimethylaniline), was synthesized via a one‐pot reaction of 4‐biphenyl carboxaldehyde and 2,6‐dimethylaniline. The diamine was employed to polymerize with several dianhydrides via one‐step condensation under high‐temperature conditions. The light yellow or colorless polyimide (PI) films obtained were found to have cut‐off wavelengths in the range 286–358 nm and transmittance over 80% in the visible region (400–780 nm). Meanwhile, these PIs possessed excellent solubility in common organic solvents, even in low‐boiling‐point solvents such as chloroform (CHCl3), dichloromethane (CH2Cl2) and tetrahydrofuran. The glass transition temperatures (Tg) of the PIs were determined to exceed 343 °C, even to 456 °C. All PI films were flexible with a tensile strength of 78–119 MPa, a Young's modulus of 2.0–2.5 GPa and elongation at break of 4.0%–8.2%. Therefore, these colorless PIs can be used as candidate materials for flexible display substrates. © 2019 Society of Chemical Industry

60‐3: High‐Performance MoS2 Thin‐Film Transistors for Flexible OLED display

We demonstrated Molybdenum disulfide (MoS2) based thin‐film transistors on polyimide substrate for flexible OLED display. And we fabricated practical MoS2 OLED driving circuit using large area MoS2 thin film grown by chemical vapor deposition (CVD) method. Our device showed high‐performance for display driving circuit over 10 μA driving current, on‐off ratio of 109. And the 2T OLED driving operation test was also conducted. In this work, we will show results suitable for practical flexible OLED display.