Dendronized Polyimides Bearing Long-Chain Alkyl Groups and Their Application for Vertically Aligned Nematic Liquid Crystal Displays

Polyimides having dendritic side chains were investigated. The terphenylene diamine monomer having a first-generation monodendron, 3,4,5-tris(n-dodecyloxy)-benzoate and the monomer having a second-generation monodendron, 3,4,5-tris[-3’,4’,5’-tri(n-dodecyloxy)benzyloxy]benzoate were successfully synthesized and the corresponding soluble dendritic polyimides were obtained by polycondensation with conventional tetracarboxylic dianhydride monomers such as benzophenone tertracarboxylic dianhydride (BTDA). The two-step polymerizations in NMP that is a general method for the synthesis of soluble polyimides is difficult; however, the expected dendritic polyimides can be obtained in aromatic polar solvents such as m-cresol and pyridine. The solubility of these dendoronized polyimides is characteristic; soluble in common organic solvents such as dichloromethane, chloroform, toluene and THF. These dendronized polyimides exhibited high glass transition temperatures and good thermal stability in both air and under nitrogen. Their application as alignment layers for LCDs was investigated, and it was found that these polyimides having dendritic side chains were applicable for the vertically aligned nematic liquid crystal displays (VAN-LCDs).

In this paper, the synthesis and characterization of a novel series of soluble polyimides having dendritic moieties (dendronized polyimides) based on the copolymerization of aromatic diamines having the first-or second-generation monodendrons bearing long-chain alkyl groups. The attachment of alkyl side chains to polyimides has recently been used to increase pretilt angles generated by the polyimides in alignment layer applications of liquid crystal displays (LCDs) and, thus, these studies are of great practical importance [29][30][31][32][33][34]. The application for the polyimide alignment layers based on dendronized polyimides for the vertically aligned nematic liquid crystal displays (VAN-LCDs) is finally described.

Measurements
The inherent viscosities of all polymers were measured using a Cannon Fenske viscometer at a concentration of 0.5 g/dL in NMP at 30 °C. Size exclusion chromatography (SEC) measurements were performed in chloroform at 40 °C with a TOSOH HLC-8020 equipped with a TSK-GEL ALPHA-M column. Number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity (Mw/Mn) were determined by TOSOH Multi Station GPC-8020 analysis system calibrated with a series of polystyrene standards having narrow polydispersities. The polyimide film samples for the measurement of ATR and differential scanning calorimeter (DSC) were prepared by the following casting method. About five wt % polyimide solution in appropriate solvents such as NMP, chloroform, m-cresol on aluminum cup or glass substrate and the solution were slowly evaporated by heating on a hotplate at appropriate temperature (ca. 50 °C for chloroform, ca. 150 °C for NMP and m-cresol) until the films were dried, then the films were dried in a vacuum oven at 100 °C for 12 h. In case the molecular weights of polyimides were lower, the polyimide films were tend to be brittle.
Differential scanning calorimetery (DSC) traces were obtained on a Shimadzu DSC-60 under nitrogen (flow rate 30 mL/min) at a heating rate of 20 °C/min using 30 mg of the film samples and the glass transition temperatures (Tg) were read at the midpoint of the heat capacity jump from the second heating scan after cooling from 250 °C. Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50 in air or under nitrogen (50 mL/min) at a heating rate of 10 °C/min using 5 mg of a dry powder sample, and 10% weight loss temperatures (Td 10 ) were calculated from the second heating scan after cooling from 250 °C. 1 H-and 13 C-NMR spectra were measured on a JEOL JNM-AL400 FT NMR instrument in CDCl 3 or dimethylsulfoxide-d 6 with tetramethylsilane (TMS) as an internal standard. IR spectra were recorded on a JASCO FT/IR-470 plus spectrophotometer. ATR Pro 450-S attaching Ge prism was used for the ATR measurements of polyimide films.
LCDs test cell properties were measured at Cheil Ind. Inc., Korea as follows: the polyimide solutions were spin-coated onto ITO glass substrates to a thickness of 0.1 μm, and cured at 210 °C for 10 minutes to produce liquid crystal alignment films. After the liquid crystal alignment films were subjected to a rubbing process, the alignment properties and the pretilt angles of the liquid crystal were measured. The surface of the alignment films were rubbed by means of a rubbing machine, two substrates were arranged anti-parallel to each other in such a manner that the rubbing direction of the each substrates were reverse, and the two substrates were sealed while maintaining cell gaps of 50 μm to fabricate liquid crystal cells. The liquid crystal cells were filled with the liquid crystalline compounds (Merk licristal). The alignment properties of the liquid crystal were observed under an orthogonally polarlized optical microscope. The pretilt angles of the liquid crystal were measured by a crystal rotation method.
In order to examine the electrical properties, the test cells were prepared by the same manner as above except the cell gap, 5 μm. The voltage holding ratios were measured with VHRM 105 (Autronic Melchers). To evaluate the VHR, the applied frequency and voltage was 60 Hz, 1V with pulse of 64 μsec. The voltage versus transmittance and optical response properties, such like contrast ratio, response time, image sticking, etc., were measured using computer-controlled system in conjunction with an tungsten-halogen lamp, a function/arbitrary waveform generator, photomultiplier. The residual DCs were measured by C-V method using impedance analyzer.

Preparation of poly(amic acid)s and polyimides
As a typical example of two-step polymerization in NMP, to a 30-100 mL flask was added the mixture of an equimolar amount of tetracarboxylic dianhydride and aromatic diamine. The system was purged with argon and NMP was added. Monomer concentration was kept at 20 wt%. The mixture was stirred at room temperature under argon for 12 h to allow viscosity to increase. The obtained poly(amic acid) solution was diluted with NMP, and polymer concentration was kept at 10 wt%. About one-third portion of the mixture was poured into a large amount of water or methanol. Precipitated poly(amic acid) was filtered, washed with excess methanol and dried at room temperature for 2 days. The residual two-third portion of the poly(amic acid) solution was used for the following polyimide preparation. To this poly(amic acid) solution were added five molar ratio of pyridine and four molar ratio of acetic anhydride to one molar ratio of tetracarboxylic dianhydride. The system was purged with argon and was stirred at 120 °C under argon for 4 h. Powdered polyimide was obtained by precipitation from a large amount of methanol, filtered, and washed with a large amount of methanol, and dried in a vacuum oven at 100 °C for 1 day. Polymerization conditions using solvents such as m-cresol and pyridine are mentioned in the next section. The structures of polyimides soluble in chloroform were confirmed by 1 H-NMR using CDCl 3 as a solvent. IR spectra were obtained by ATR measurement using the polyimide films samples. The detailed discussions of these characterizations are described in the next section. The obtained 1 H NMR and IR spectra data are as follows.
The structure and the synthetic methods of these diamine monomers having dendritic moiety have several characteristic futures as follows. As dendritic moieties have three long-chain alkyl groups, the enhancement of solubility of polyimides can be expected by the entropy effects of long-chain alkyl groups, and the generation of pretilt angles of liquid crystalline molecules can be expected in case the polyimides using these dendritic diamine monomers are applied for the alignment layers for LCDs. As dendritic moieties are bulky, aromatic spacers are connected for defending steric hindrance. Generally, aliphatic spacers are used for this purpose, however, aliphatic segments tend to lower the thermal stability that is the main feature of polyimides. Therefore, aromatic spacers are selected. A general method for the synthesis of diamine monomers is the reduction of corresponding dinitro precursor using a catalyst such as Palladium on Carbon (Pd/C). However, the second generation of Percec-type monodendron (12G2-AG) contains benzyl ether linkage that is easily cleaved by Pd/C. Consequently, the direct connection of aminophenyl groups using Suzuki coupling reaction are applied, and the terphenyl diamine monomers having the aromatic spacers and Percec-type dendritic moieties are successfully synthesized.

Polymer synthesis
The synthesis route for the polyimides and copolyimides based on BTDA, 6FDA, DDE and dendritic diamines bearing long-chain alkyl groups (12G1-AG-Terphenyldiamin, 12G2-AG-Terphenyldiamin) is illustrated in Figure 4, and the results of polymerization are summarized in Table 1.
In our laboratory, two-step polymerization systems consisting of poly(amic acid)s synthesis and chemical imidization have been generally performed. The poly(amic acid)s are obtained by reacting the mixture of an equimolar amount of tetracarboxylic dianhydrides and diamines at room temperature for 12 h under an argon atmosphere. Polyimides were obtained by chemical imidization at 120 °C in the presence of pyridine as base catalyst and acetic anhydride as dehydrating agent. These are the optimized synthesis conditions previously developed for the synthesis of soluble polyimides in our laboratory [35][36][37][38][39][40][41][42]. BTDA that is highly reactive and a common aromatic tetracarboxylic dianhydride is generally used as a dianhydride monomer, and DDE that is highly reactive and a common aromatic diamine is generally used as a diamine co-monomer in case the special functional diamine monomers such as the diamine monomers bearing long-chain alkyl groups are used. In the case of soluble polyimides, clear polyimide solutions are eventually obtained. In other cases, clear poly(amic acid) solutions are obtained, however, gelation or precipitation occur in the course of imidization process.
Thus, the polymerizations of 12G1-AG-terphenyldiamine in NMP were firstly investigated using BTDA, and co-diamine monomer, DDE under above conditions (Table 1). Although viscous poly(amic acid)s solution from the above monomers were obtained, precipitation occurred during the imidization process. It was speculated that the hydrocarbon and phenyl moiety of dendritic diamine monomers reduces the solubility of polyimides in NMP; therefore, a polar aromatic solvent, m-cresol, was used to improve the solubility of dendron moieties using a high temperature one-step polymerization system [43,[46][47][48]. The reaction conditions of one-step polymerization system are described in Table 1 and Figure 4. Thus, soluble copolyimides based on BTDA/12G1terphenyldiamine/DDE (100/50/50) and BTDA/12G1-terphenyldiamine/DDE (100/25/75) were obtained. These copolyimide have high molecular weight (η inh = 0.56-0.78 dL/g) and good film forming ability. On the other hand, homopolyimide based on BTDA/12G1-AG-Terpnenyldiamine was insoluble in m-cresol. Solubility of copolyimides, BTDA/12G1-terphenyldiamine/DDE (100/50/50) and BTDA/12G1-terphenyldiamine/DDE (100/25/75), may be improved by the randomizing effect based on copolymerization as well as the entropy effect of long-chain linear alkyl groups. This randomizing effect by copolymerization has been frequently observed in our research [38][39][40][41].   According to the previous paper [49] which described the synthesis and properties of polyimides containing multiple alkyl side chains using the diamine monomers such as 2,2-bis{4-[3,4,5-tris(ndodecane-1-yloxy)benzoate]}-4,4'-biphenyldiamine, m-cresol cleaved the ester linkages of the alkyl side chains from backbones and o-dichlorobenzene or 1-chloronaphthalene was used as one-step polymerization solvents. However, the authors confirmed the existence of long-chain alkyl ether groups in the polyimide based on 12G1-AG-Terphenyldiamine prepared in m-cresol by FT-IR (ATR) and TGA measurements described in the next section in detail. In 12G1-AG-Terphenyldiamine monomer, 3,4,5-tris(n-dodecyloxy)benzoate group is connected to the polymer backbone through the aromatic spacer (Figure 3), and the ester linkage is separated from the polymer backbone and probably stabilized by the aromatic spacer by the electronic effect. On the other hand, 3,4,5-tris(nalkloxy)benzoate group in aromatic diamine monomers used in ref. 49 are directly connected to the polymer backbone and the connected positions seem to be sterically hindered. Thus, the eater linkages in the side-chains of dendronized polyimides based on BTDA/12G1-AG-Terphenyldiamine are probably more stable against nucleophilic attack by m-cresol both sterically and electronically.
The dendronized copolyimides based on the second-generation dendritic monomer (BTDA/12G2-AG-Terphenyldiamine/DDE ) were not soluble in even m-cresol, and pyridine, an aromatic and more polar solvent, was necessary to dissolve these, probably due to the large aromatic moieties of secondgeneration dendrons. Thus the soluble hopopolyimide and the copolyimides based on BTDA/12G2-AG-Terphenyldiamine/DDE (100/100/0, 100/75/25, 100/50/50) were obtained in pyridine as a polymerization solvent. Molecular weights of these soluble polyimides were low judged form the value of inherent viscosity (η inh = 0.06-0.22 dL/g). It is spreculated that the huge second-generation dendritic moieties of 12G2-AG-Terphenyldiamine reduce the reactivity of the diamine monomer due to steric hindrance, thus lower the molecular weights of polyimides, even though the aromatic spacer existed.
It is well known that 6FDA produces a soluble polyimide in combination with most aromatic diamines because of the effect of two trifluoro methyl groups. The polyimide based on 6FDA/12G1-AG-Terphenyldiamine was readily soluble in NMP, however 12G2-AG-Terphenyldiamine monomer itself was insoluble in NMP. Therefore, THF that have high affinity for hydrocarbon moiety was added, and the polyimide based on 6FDA /12G2-terphenyldiamine was eventually obtained in NMP/THF (1/1, volume ratio), using the same conditions as two-step polymerization systems in NMP (Table 1).

Polymer characterization and properties
The solubility in various organic solvents of the obtained soluble polyimides are summarized in Table 2. It is interesting that the solubility of these dendronized polyimides were dramatically changed and these were insoluble or less soluble in polar aprotic solvents such as NMP, DMF and DMSO, but easily soluble in less polar solvents such as dichloromethane, chloroform, toluene and THF. This solubility change is probably affected by hydrocarbon moieties of dendritic side chains of polyimides. As four dendronized polyimides out of the seven soluble dendronized polyimides obtained from these experimental are soluble in chloroform (Table 2), 1 H-NMR measurements using CDCl 3 as a solvent and SEC measurements using chloroform as an eluent can be performed. For example, Figure 5 shows the 1 H-NMR spectra of the polyimides based on 6FDA/12G2-AG-Terphenyldiamine, 6FDA/12G1-AG-Terphenyldiamine, and the monomer, 12G2-AG-Terphenyldiamine. As 1 H-NMR spectra of polyimides shows broad peaks characteristic of a polymer NMR spectrum, the polymer formation is consequently confirmed. In the 1 H-NMR spectrum of the polyimide based on 6FDA/12G2-AG-Terphenyldiamine, broad methylene proton peak of Ar-O-CH 2 -Ar existing in the second generation of dendoron appears, and the intensity of aromatic protons becomes very low in comparison with the peaks based on nine long-chain alkyl chains. Thus, the structures of dendronized polyimides were confirmed by 1 H-NMR measurements. Representative ATR spectrum of dendronized polyimides based on 12G1-AG-Terphenyldiamine and 12G2-AG-Terphenyldiamine were shown in Figure 6 and these spectrum show the strong absorptions based on C-H bonds of long-chain alky groups and the strong absorptions of C-O bonds of alkyloxy groups, and these absorption intensities become stronger with the increase of long-chain alkyl ether segments in the polyimides.
The results of SEC measurements in chloroform as an eluent are summarized in Table 1 and SEC traces are shown in Figure 7. The inherent viscosities of four dendronized polyimides, soluble in chloroform, were not high, in the range of 0.06-0.28 dLg -1 . Therefore, the weight average molecular weights determined SEC measurements were also not so high, in the range of 12,800-43,600. Although the molecular weights were not so high, SEC traces indicates the formation of polymers, because these SEC traces display typical molecular weight distributions (Mw/Mn = 1.3-1.9) for polycondensation polymers. In addition, peaks around 27-28 min (elution time) considered as unreacted monomers are very small.
The glass transition temperatures (Tg) of these dendronized polyimides were determined by DSC measurements, and the thermal stabilities were evaluated in terms of 10% weight-loss temperatures (Td 10 ) in TGA measurements. The data are summarized in Table 3, and the representative TGA traces are shown in Figure 8 and Figure 9. It is sometimes difficult to recognize Tg of rigid polymers such as polyimides. Therefore, the DSC measurements were performed at the higher heating rate (20 °C/min) and using relatively much film samples (30 mg) to read heat capacity jump easier, referring the previous papers [41,50 ].   As can be seen from Table 3, the Tg values of these polyimides that were recognized above measurements were in the range from 249-311 °C, and are 100-150 °C lower than those of the conventional fully aromatic polyimides, however, are 100-150 °C higher than the commodity thermoplastics. Consequently, these soluble dendritic polyimides based on 12G1-AG-Terphenyldiamne and 12G2-AG-Terphenyldiamine can be ranked as heat resistant polymers. Table 3. Thermal properties of polyimides and copolyimides based on 12G1-AG-Terphenyldiamine, 12G2-AG-Terphenyldiamine.   The Td 10 values of these dendronized polyimides bearing long-chain alkyl groups in Table 3 were in the range 349~455 °C in air and 375~449 °C under nitrogen, showing similar values observed in soluble polyimides obtained from our laboratory (ca. 400~500 °C). Although these Td 10 values of the polyimides obtained in our laboratory are 100~200 °C lower than those of wholly aromatic polyimides; however, these soluble dendritic polyimides based on 12G1-AG-Terphenyldiamne and 12G2-AG-Terphenyldiamine still can be ranked as heat resistant polymers. Figures 8 and 9 show the TGA traces of dendronized polyimides based on BTDA/12G1-AG-Terphenyldiamine (100/50/50), BTDA/12G2-AG-Terphenyldiamine (100/50/50), respectively. These TGA traces showed steep weight loss at the intial stage of degradation, and these weight loss percent almost correspond the calculated value of the weight percent of alkyl groups in polymer segments. Therefore, it is considered that the degradation of long-chain alkyl groups occurred at the initial stage of thermal degradation. Furthermore, these TGA traces also show the evidence that the long-chain alkyl groups exist in the polyimides and the cleavage of alkyl groups did not occurred during the polymerization.

Alignment layer application for VAN-LCDs
The alignment layer application for VAN-LCDs using polyimides having dendritic side chains was performed at Cheil Ind. Inc., Korea. The polyimide alignment layers containing 8 mol % of 12G1-AG-Terphenyldiamine were utilized for the vertical alignment mode (VA-mode). The synthesis of polyimide alignment layers containing 8 mol % of 12G1-AG-Terphenyldiamine was carried out in NMP as a solvent by conventional two step polymerization method regularly used for the synthesis of polyimide alignment layers for TN-LCDs , and 12G1-AG-Terphenyldiamine monomer was used as one of the diamine components. LCDs test cell properties are summarized in Table 4. PIA-DEN represents the test cell using the polyimide alighnment layers containing 8 mol % of 12G1-AG-Terphenyldiamine, and TN represents the test cell using the regular polyimide alignment layers. The pretilt angles of LC molecules were over 89° in PIA-DEN test cells, which are the suitable values for VAN-LCDs. It is speculated that an extremely bulky and hydrophobic dendritic moieties affects the generation of pretilt angles between the surface of polyimide and liquid crystalline molecules as illustrated in Figure 10. The considerably lower surface energy value of the PIA-DEN alignment film in comparison with the one of TN mode also indicate that the surface of polyimides containing dendritic moieties is more hydrophobic.  The various important properties of PIA-DEN test cells such as voltage holding ratio (VHR), response time, contrast ratio, residual DC, and image sticking are equivalent or advantageous in comparison with those of regular TN test cell. Figure 11 shows a V-T (voltage-transmittance) curve of these test cells, and shows a dramatic change of T. Consequently, it is convinced that the dendritic monomers, and dendritic polyimides developed by our research can be applied for the alignment films for VAN-LCDs. Figure 11. Voltage-transmittance curves of LCD test cells using dendronized and conventional polyimides.

Conclusions
The novel dendritic diamines monomers bearing long-chain alkyl groups (12G1-AG-Terphenyldiamine, 12G2-AG-Terphenyldiamine) were successfully synthesized. The characteristic future of synthetic methods is that Suzuki coupling reaction is applied for the connection of aminophenyl groups. This method is considered the versatile synthetic method for aromatic diamine monomer synthesis because the reduction of corresponding nitoro compound, sometime cause a sidereaction, is not nessesary. The corresponding soluble dendritic polyimides were obtained by polycondensation with tetracarboxylic dianhydride monomers such as benzophenone tertracarboxylic dianhydride (BTDA). The common two-step polymerizations in NMP are difficult; however, the expected dendritic polyimides can be obtained in aromatic polar solvents such as m-cresol and pyridine. It is speculated that the hydrocarbon and phenyl moiety of dendritic diamine monomers reduces the solubility of polyimides in NMP, but raise the solubility in aromatic polar solvents. The solubility of these dendoronized polyimides is characteristic; soluble in common organic solvents such as dichloromethane, chloroform, toluene and THF. The hydrocarbon moieties of dendritic segments may help the increment of solubility in these solvents. These dendronized polyimides exhibited high glass transition temperatures and good thermal stability in both air and under nitrogen. According to TGA measurements, it is considered that the thermal decompositions of long-chain alkyl groups firstly occur at the initial stage of decomposition. Their application as alignment layers for LCDs was investigated, and it was found that these polyimides having dendritic side chains were applicable for the vertically aligned nematic liquid crystal displays (VAN-LCDs). It is speculated that an extremely bulky and hydrophobic dendron moiety affects the generation of vertical alignment.