Inherent viscosities and molecular weights of the PIs
In advanced thin film transistor liquid crystal display devices (TFT-LCDs), alignment layers (ALs) are playing an ever-increasing important role for achieving a high-quality optoelectronic display . The main function of AL materials is to align the rod-like liquid crystal (LC) molecules at a constant angle (so-called pretilt angle) to the local surface. Thus, when the electrical field is applied, the LC molecules can respond rapidly so as to result in a display effect. The characteristics of AL materials, including their abilities to achieve a proper pretilt angle for LC molecules, to achieve a high voltage holding ratio (VHR) and a low residual direct circuit voltage (RDC) value for the LCD devices, high thermal stability, high mechanical strength to resist rubbing process and their planarization ability are often taken into deliberate consideration in developing new generations of TFT-LCDs .
Currently, polyimides (PIs) are one of the most important AL materials for TFT-LCDs due to their intrinsic high thermal resistance, good mechanical properties and unique LC alignment ability . However, conventional wholly aromatic PIs suffer from their poor solubility in common solvents. Thus, in practice, they can only be used in the form of soluble precursors, poly(amic acid)s (PAAs). For example, in conventional twisted nematic LCD (TN-LCD) and super-twisted nematic LCD (STN-LCD) fabrications, wholly aromatic PAA solution is first spin-coated onto an indium tin oxide (ITO) substrate, and then the solution is imidized at elevated temperatures up to 300 oC to form the imidized PI alignment layer. However, the high curing temperature of PAAs often causes serious damage for the temperature-sensitive components in TFT-LCDs, such as the color filters which would be destroyed when the temperatures are higher than 230 oC . Hence, PI alignment layers with low curing temperatures (<220 oC) have been developed in the past decades [5-7].
Besides the curing temperature consideration, the high VHR and low RDC values of the devices are also highly desired for advanced TFT-LCD fabrication in order to achieve a high-resolution display (high contrast, low image sticking, etc) . VHR and RDC values of the TFT-LCD devices are influenced by many factors, including the characteristics of LC materials, the features of the PI alignment layers and the display modes of the devices. Among the factors, the effects of the chemical structures of the PI alignment layers are often critical. For instance, it has been well established that the highly conjugated molecular skeletons in wholly-aromatic PIs often lead to low VHR and high RDC values for the devices . Thus, PI alignment layers with low conjugated structures have been paid increasing attentions.
Considering the above-mentioned structure-property relationship for PI alignment layers used for advanced TFT-LCDs, alicyclic or semi-alicyclic PIs have been confirmed to be the best candidates as AL materials up to now. Especially, semi-alicyclic PIs derived from alicyclic dianhydrides and aromatic diamines possess the best combined properties, including good thermal stability, good solubility in organic solvents, acceptable mechanical properties, good optical transparency, high VHR and low RDC values. Thus, semi-alicyclic PIs have been widely investigated as AL materials for advanced TFT-LCDs [10-12]. Figure 1 illustrates the developing trajectory of PI alignment layers with different kinds of display modes.
In this chapter, a series of novel semi-alicyclic PI alignment layers were designed and synthesized. The designed novel PI materials are expected to show good solubility in organic solvents (thus low curing temperatures) and exhibit high VHR and low RDC values for TFT-LCDs. For this purpose, a class of substituted-tetralin alicyclic dianhydrides was synthesized first via a low-cost route using maleic anhydride and substituted styrene compounds as the starting materials (Scheme 1). Then, a series of semi-alicyclic PIs were synthesized from the newly-developed dianhydrides and commercially available aromatic diamines. The effects of the structures of the semi-alicyclic PIs on their thermal stability and optical properties were investigated. At last, a series of test LCD cells with fringe field switching (FFS) mode were fabricated using the novel PIs as the alignment layers. The electrical characteristics (pretilt angle, VHR and RDC) of the cells were preliminarily studied.
Inherent viscosity was measured using an Ubbelohde viscometer with a 0.5 g/dL NMP solution at 25 oC. Absolute viscosity was measured using a Brookfield DV-II+ Pro viscometer at 25 oC. Fourier transform infrared (FT IR) spectra were obtained with a Tensor 27 Fourier transform spectrometer. Ultraviolet-visible (UV-vis) spectra were recorded on a Hitachi U-3210 spectrophotometer at room temperature. The cutoff wavelength was defined as the point where the transmittance drops below 1% in the spectrum. Prior to test, PI samples were dried at 100 oC for 1 h to remove the absorbed moisture. Yellow index (YI) and haze values of the PI films were measured using an X-rite color i7 spectrophotometer with PI film samples at a thickness of 30-40 μm in accordance with the procedure described in ASTM D1925 “Test method for yellowness index of plastics” and in ASTM D1003 “Standard test method for haze and luminous transmittance of transparent plastics”, respectively. The color parameters were calculated according to a CIE Lab equation.
Solubility was determined as follows: 1.5 g of the PI resin was mixed with 8.5 g of the tested solvent at room temperature (15 wt% solid content), which was then mechanically stirred in nitrogen for 24 h. The solubility was determined visually as three grades: completely soluble (++), partially soluble (+), and insoluble (-). The complete solubility is defined as a homogenous and clean solution is obtained, in which no phase separation, precipitation or gel formation is detected.
The electrical characteristics, including VHR, RDC and pretilt angle values of the LC test cells were measured on a Toyo Model 6254 measurement system. VHR measurements were performed at LC test cells with a gap of 5-6 μm. The peak value of the square wave voltage and pulse duration was +5V and 60 μs, respectively. RDC measurements were performed using the “flick free” method. The test cells were first addressed with +5V direct circuit (DC) offset voltage for 3600 s. After the time, the +5V DC offset was switched off. The resulting flicker was monitored and the DC offset voltage was increased until the flickering was no longer visible. The compensating voltage was the residual DC voltage (RDC). Pretilt angles measurements were performed using the crystal rotation method with LC cells with a gap of 50 μm fabricated by anti-parallel rubbing process. The values of all measurements of VHR, RDC and pretilt angles are averages of at least 10 independent LC cells.
2.3. Monomer synthesis
Melting point: 229 oC (DSC peak temperature). FT IR (KBr, cm-1): 2941, 1855, 1778, 1506, 1412, 1223, 1076, and 916. 1H NMR (DMSO-
Melting point: 199 oC (DSC peak temperature). FT IR (KBr, cm-1): 2966, 1861, 1780, 1493, 1405, 1229, 1058, and 928. 1H NMR (DMSO-
Melting point: 218 oC (DSC peak temperature). FT IR (KBr, cm-1): 2962, 1859, 1789, 1504, 1410, 1222, 1055, and 925. 1H NMR (DMSO-
Melting point: 201 oC (DSC peak temperature). FT IR (KBr, cm-1): 2912, 1864, 1782, 1664, 1441, 1376, 1304, 1262, 1211, 1151, 1080, 1027, 967, 914, 870, 819, 754, 633, 594, 558 and 498. 1H NMR (DMSO-
2.4. Polyimide synthesis
The general procedure for the synthesis of PIs can be illustrated by the preparation of PI-8 (Table 1). Into a 250 mL three-necked, round-bottomed flask equipped with a mechanical stirrer, a Dean-Stark trap and a nitrogen inlet, MDA (17.8434 g, 0.09 mol) and 16PDA (3.457 g, 0.01 mol) was dissolved in
PI-8 resin (15 g) was dissolved in NMP (85 g) at room temperature to afford a 15 wt% solution. The solution was filtered through a 0.45 μm Teflon syringe filter to remove any undissolved impurities. Then, the solution was spin-coated on a clean silicon wafer or quartz substrate. The thickness of the PI film was controlled by regulating the spinning rate. PI-8 films with thicknesses ranged from 10~100 μm were obtained by thermally baking the solution in a flowing nitrogen according to the following heating procedure: 80 oC/2 h, 150 oC/1 h, 200 oC/1 h, and 220 oC/1 h. The other PI resin and films were prepared according to a similar procedure as mentioned above.
3. Results and discussion
3.1. Monomer synthesis
Scheme 1 illustrates the synthetic procedure for the substituted-tetralin alicyclic dianhydrides. Four dianhydrides, including TDA (
Figure 2 shows the FT IR spectra of the dianhydrides, in which the characteristic bands of carbonyl groups in the anhydride moiety were clearly observed at around 1863 and 1782 cm-1 for all of the compounds. In addition, the characteristic absorption of methyl group appeared at 2941 cm-1 for MTDA and TTDA. The 13C NMR and the two-dimensional 1H-13C heteronuclear single-quantum coherence (HSQC) spectra of TTDA and FTDA are illustrated in Figure 3, together with the assignments of the observed resonances. As depicted in Figure 3, 18 carbon signals are clearly revealed for TTDA and 15 signals for FTDA. The absorptions of the protons cohered well with those of the corresponding carbon signals. This result is consistent with their proposed structures. Interestingly, the two pairs of protons in methylene groups (H3,3' and H6,6') for both of the dianhydrides exhibited individual absorptions in their 1H NMR spectra due to the slightly different chemical environments of the protons in the dianhydrides. The protons in the aromatic ring (H13, H14, and H16) appeared at the lowest field in the spectra. In addition, elemental analysis results also revealed the successful preparation of the target dianhydrides.
3.2. Polyimide synthesis
As shown in Scheme 2, three series, totally 9 species of PIs were designed and synthesized from TDA (PI-1~PI-3), MTDA (PI-4~PI-6), FTDA (PI-7~PI-9) and aromatic diamines (MDA and 16PDA) by a one-step, high temperature polycondensation procedure in
Table 1 presents the chemical formulations, inherent viscosities and molecular weights of the obtained PIs. The utilization of high temperature polycondensation procedure in the present work is mainly based on the fact that the two anhydride moiety in the substituted-tetralin dianhydrides might exhibit different reactivities due to the asymmetrical molecular structures of the monomers. High polymerization temperatures might eliminate the reactivity differentia of the anhydride units so as to obtain the PI resins with higher molecular weights. White or pale-yellow fibrous PI resins were obtained quantitatively, which had inherent viscosities of 0.81~1.03 dL/g for TDA-PIs (PI-1~3), 0.76~1.04 dL/g for MTDA-PIs (PI-4~6) and 0.69~0.97 dL/g for FTDA-PIs (PI-7~9), respectively (Table 1). These values indicate that the current PIs possess moderate to high molecular weights, which can be further confirmed by the GPC measurements. As tabulated in Table 1, the average numerical (
|PI||Dianhydride||Diamine (mol %)||[|
|Molecular weight (g/mol)|
The solubility of PIs is summarized in Table 2. All the PIs were easily soluble in polar aprotic solvents (NMP and DMAc),
The effects of solid contents (
3.4. Thermal properties
The effects of asymmetrical
Glass transition temperatures (
3.5. Optical properties
It has been well established in the literature that the optical transparency of aromatic PI films can be efficiently improved by decreasing the formation of intra- and intermolecular charge transfer complexes (CTC) in the PI chains [17, 18]. When elaborately designed, colorless PI films can even be obtained [19-21]. Among various methodologies decreasing the CTC formations, introduction of alicyclic moiety either in the dianhydride or in the diamine moiety has been proven to be one of the most effective procedures.
In the present work, the tetralin-containing PI films were obtained as pale-yellow free-standing films. The optical properties of the films are summarized in Table 4 and the UV-Vis spectra of the MTDA-PI films are illustrated in Figure 10. The PI films showed good optical transparency in the ultraviolet-visible light region with cutoff wavelengths lower than 300 nm. Some of the PI films, such as those derived from FTDA showed transmittances around 80% at 450 nm wavelength at a thickness of 10 μm. The good optical transparency of the PI films, on one hand, is attributed to the asymmetrical and bulky alicyclic tetralin structure in the dianhydride unit through steric hindrance. On the other hand, the high electronegativity of fluoro substituents in the dianhydride moiety is also beneficial for improving the optical transparency of the PI films by reducing the CTC formation.
The yellow index (YI) measurement results are shown in Table 4 and Figure 11. YI is usually adopted as a criterion evaluating the color of a polymer film. This value describes the color change of a film sample form clear or white toward yellow. Lower YI value usually indicates a weak coloration for a polymer film . As shown in Table 5, PI-8 exhibited the lowest YI and Haze values compared with its analogs. This result correlated well with the results measured by UV-Vis measurements.
In summary, this low-color feature for the current PI films is desirable for their application as alignment layers for TFT-LCDs.
3.6. Application in TFT-LCD fabrication
In order to investigate the practical application of the newly-developed alicyclic PIs as alignment layers for TFT-LCDs, a series of fringe field switching (FFS) mode [23, 24] liquid crystal cells (FFS-LCDs) were fabricated. PI-8 was chosen as the alignment layer due to its good combined properties. Figure 12 illustrates the LC cells fabrication procedure form monomer synthesis to devices.
First, PI-8 resin was dissolved in a mixture solvent of NMP/GBL/BC (55/30/15, volume ratio) at a solid content of 6.0 wt%. BC (butyl cellosolve) was used in the mixed solvents as a planarization agent so as to obtain a uniform PI coating. The obtained PI-8 solution was purified by filtering through a 0.25 μm Teflon filter and had a viscosity of 55 mPa.s. The purified PI-8 solution was spin-coated on an ITO glass substrate (10 cm×10 cm) at a rotating speed of 3000 rpm. Then the ITO glasses with PI-8 coating were placed on a hot plate at 80 oC for 30 min, followed by curing in a nitrogen oven at 230 oC for 1h. The thicknesses of PI-8 film was measured with a profilometer (Dektak XT, Bruker) and found to be 553.7 Å, as shown in Figure 13.
The cured PI-8 film was then rubbed with a nylon cloth-packed rubbing roller using the following parameters: radius of rubbing roller: 50 mm; rotation speed: 1000 rpm; pile impression: 0.32 mm and stage speed: 30 mm/s. Then, the FFS-LCD cells were assembled by two individual rubbed ITO glasses and the rubbing direction of the two glasses was anti-parallel to each other. For the pretilt angle measurements, the cell thickness was controlled to be about 50 μm by spraying spacers or glass fibers (50 μm) on PI-8 film surface; whereas for VHR and RDC measurements, the thickness was about 6 μm. The fabricated LC cells were sealed with a UV-curable epoxy sealant with two small filling holes left. LC molecules were then filled into the filling holes between the ITO plates via capillary action. The rod like LC molecules are thought to be oriented along the long alkyl chains pre-aligned by the rubbing treatments , as illustrated in Figure 14.
Table 5 tabulates the electrical properties of the fabricated LC cells. The LC cells exhibited an average pretilt angle of 2.7°, which could meet the demands of the FFS-LCD applications. Meanwhile, a VHR of 98.2% and a RDC of 230 mV were also obtained. This result indicated that PI-8 alignment layer could endow the LC cells with a high VHR level; however, a few charge accumulations occurred in the LC cells. The RDC value of 230 mV is a bit higher than the desired value (<100 mV) for FFS-LCD applications. It has been well established that a high residual DC voltage value might result in an image sticking for the LCD devices . Thus, the formulation and structural characteristics of PI-8 should be modified to reduce the RDC values as low as possible. As we know, as compared to the pre-imidized PI alignment agents, their precursors, poly(amic acid)s usually exhibited much lower RDC levels . Thus, it might be a compromised procedure to combine the imidized PI-8 and its PAA solution so as to achieve a balance of high VHR and low RDC levels for the TFT-LCDs. The detailed investigations are being performed in our laboratory.
|LC cells mode||Cell gap (μm)||Pretilt angle (°)*||VHR (%, 30 oC)||RDC (mV, 30 oC)|
As one of our continuous endeavors developing low-cost and high performance PI alignment layers for TFT-LCDs, a systematic experiment was performed from monomer design and synthesis, PI preparation and characterization to final devices fabrication. First, several novel semi-alicyclic dianhydrides containing substituted-tetralin moiety were synthesized via a low-cost route with good yields. Then, a series of PIs were prepared from the dianhydrides and aromatic diamines. The asymmetrical alicyclic structures in the dianhydride units destroyed the regularity of the PI molecule chains; thus enhances their solubility in common solvents. Meanwhile, this irregular packing of the PI chains is beneficial for the penetration of UV and visible light. Thus, a good optical transparency is achieved in the present PIs. Incorporation of the bulky alicyclic tetralin moiety in the dianhydride units did not deteriorate their thermal stability. More importantly, the current PIs showed good orientating ability to LC molecules. The LC cells with PI-8 as the alignment layer exhibited good optoelectrical properties.
Thus, the present semi-alicyclic PIs are considered to be promising alignment layers for TFT-LCDs. Research on the applications of the PIs in large area TFT-LCD monitors are in progress now.
Financial support from the National Natural Science Foundation of China (51173188 and 50403025) is gratefully acknowledged.