**3. Results and discussion**

#### **3.1. Monomer synthesis**

Scheme 1 illustrates the synthetic procedure for the substituted-tetralin alicyclic dianhy‐ drides. Four dianhydrides, including TDA (*R*=-H), MTDA (*R*=-CH3), TTDA (*R*=-C(CH3)3) and FTDA (*R*=-F) were synthesized by the Diels-Alder reactions of *R*-substituted-styrene com‐ pounds and maleic anhydride followed by the rearrangement reactions of the intermedi‐ ates. The yields are all higher than 70%. The styrene compounds were used as both of the reactant and the solvent. In the literature, TDA has been synthesized from styrene and male‐ ic anhydride with a molar ratio of 2.1:1 [13]. Toluene and a flowing air (15 liters per hour) were used as the solvent and catalyst, respectively. The obtained TDA dianhydride need‐ ed to be purified by recrystallization from a toluene/acetone mixture to remove the oxi‐ dized by-products. In our experiments, the reductive nitric oxide (NO) gas was used instead of the oxidative air. The obtained dianhydrides have good purity and can be used directly for polymerization. Also, they can be further purified by dissolution in a good solvent of ace‐ tonitrile and then precipitated slowly by adding a poor solvent of toluene although the yields might be slightly sacrificed. Due to the low cost of the starting materials and the high syn‐ thesizing yields for the new dianhydrides, this route is a promising procedure reducing the high cost of the present alicyclic PI alignment layers (usually hundreds of dollars per li‐ ter). In addition, this reaction is easily to expand to a large scale. A scale of kilograms per batch has been successfully achieved in our lab.

**Scheme 1** Synthesis of new alicyclic dianhydrides via a low-cost route

**2.4. Polyimide synthesis**

274 Optoelectronics - Advanced Materials and Devices

The resin was collected and dried at 80 o

C/1 h, and 220 o

**3. Results and discussion**

**3.1. Monomer synthesis**

lar procedure as mentioned above.

180 o

h, 200 o

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 *m*-cresol (100 g) to give a clear diamine solution. Then, FTDA (31.825 g, 0.1 mol) was added in one batch and an additional volume of *m*-cresol (112 g) was added to wash the residual dianhydride, and at the same time to adjust the solid content of the reaction system to be 20 wt%. After stirring in nitrogen for 1 h, a mixture of toluene (230 ml) and isoquinoline (catalytic amount) was then added. The reaction mixture was heated to

C and maintained for 6 h. During the reaction, the toluene-water azeotrope was distil‐ led out of the system and collected in the Dean-Stark trap. After cooling to room tempera‐ ture, the viscous solution was slowly poured into an excess of ethanol to yield a silky resin.

PI-8 resin (15 g) was dissolved in NMP (85 g) at room temperature to afford a 15 wt% solu‐ tion. The solution was filtered through a 0.45 μm Teflon syringe filter to remove any undis‐ solved 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 solu‐

Scheme 1 illustrates the synthetic procedure for the substituted-tetralin alicyclic dianhy‐ drides. Four dianhydrides, including TDA (*R*=-H), MTDA (*R*=-CH3), TTDA (*R*=-C(CH3)3) and FTDA (*R*=-F) were synthesized by the Diels-Alder reactions of *R*-substituted-styrene com‐ pounds and maleic anhydride followed by the rearrangement reactions of the intermedi‐ ates. The yields are all higher than 70%. The styrene compounds were used as both of the reactant and the solvent. In the literature, TDA has been synthesized from styrene and male‐ ic anhydride with a molar ratio of 2.1:1 [13]. Toluene and a flowing air (15 liters per hour) were used as the solvent and catalyst, respectively. The obtained TDA dianhydride need‐ ed to be purified by recrystallization from a toluene/acetone mixture to remove the oxi‐ dized by-products. In our experiments, the reductive nitric oxide (NO) gas was used instead of the oxidative air. The obtained dianhydrides have good purity and can be used directly for polymerization. Also, they can be further purified by dissolution in a good solvent of ace‐ tonitrile and then precipitated slowly by adding a poor solvent of toluene although the yields might be slightly sacrificed. Due to the low cost of the starting materials and the high syn‐ thesizing yields for the new dianhydrides, this route is a promising procedure reducing the high cost of the present alicyclic PI alignment layers (usually hundreds of dollars per li‐

tion in a flowing nitrogen according to the following heating procedure: 80 o

C in vacuo for 24 h. Yield: 47.54 g (96%).

C/1 h. The other PI resin and films were prepared according to a simi‐

C/2 h, 150 o

C/1

**Figure 2.** FT IR spectra of tetralin dianhydrides

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 ap‐ peared at 2941 cm-1 for MTDA and TTDA. The 13C NMR and the two-dimensional 1 H-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 methyl‐ ene groups (H3,3' and H6,6') for both of the dianhydrides exhibited individual absorptions in their 1 H 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 prep‐ aration of the target dianhydrides.

**3.2. Polyimide synthesis**

the solvents.

tion at a temperature of 180 o

**+**

**H2N H2**

**O O**

**O**

**O**

**O**

**O O**

**O O O**

**O**

**CH3**

**H2**

**R=-H, TDA R=-CH3, MTDA R=-F, FTDA**

**N N**

**R**

**R**

**O**

**O O**

**O**

**O**

**O O**

**O**

**Scheme 2.** Synthesis of semi-alicyclic PIs

from 80-220o

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 *m*-cresol solu‐

mainly to induce the alignment of LC molecules [14-16]. The reaction proceeded smoothly during the polymerization, indicating good solubility of the reactants and the yielded PIs in

> **180 <sup>o</sup> C, 6h**

**C N N O**

**O**

*m***-cresol isoquinoline**

**nitrogen**

**C NH2 + H2N NH2**

*x*

**x (1-x)**

**MDA**

**O O**

**PI**

**O O O**

**O**

**F**

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 substitutedtetralin dianhydrides might exhibit different reactivities due to the asymmetrical molecular structures of the monomers. High polymerization temperatures might eliminate the reactivi‐ ty 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 in‐ herent 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 indi‐ cate 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 (*M*n) and weight (*M*w) molecular weights of the PI resins were higher than 17762 g/mol and 35356 g/mol, respectively. In addition, the PI resins exhibited a polydispersity index (*M*w/*M*n) low‐ er than 2.43. This indicates that the substituted-tetralin alicyclic dianhydrides exhibited good reactivity in polymerization with aromatic diamines. Flexible and tough PI films were obtained by casting their solutions in NMP followed by baking at elevated temperatures

**R**

**O O**

**O**

**O**

**O**

C. All the films exhibited creasable nature and good tensile properties. For in‐

stance, PI-8 showed a tensile strength of 76 MPa, an elongation at break of 6.2%, and a ten‐

C. Introduction of C16 long alkyl chain in the PI systems is

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 277

**O**

**16PDA**

**O** *1-x n*

*x***=1, 0.9, 0.8**

**Figure 3.** 1H-13C HSQC spectra of dianhydrides (a) TTDA; (b) FTDA.

#### **3.2. Polyimide synthesis**

their 1

aration of the target dianhydrides.

276 Optoelectronics - Advanced Materials and Devices

**Figure 3.** 1H-13C HSQC spectra of dianhydrides (a) TTDA; (b) FTDA.

H 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 prep‐

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 *m*-cresol solu‐ tion at a temperature of 180 o C. Introduction of C16 long alkyl chain in the PI systems is mainly to induce the alignment of LC molecules [14-16]. The reaction proceeded smoothly during the polymerization, indicating good solubility of the reactants and the yielded PIs in the solvents.

**Scheme 2.** Synthesis of semi-alicyclic PIs

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 substitutedtetralin dianhydrides might exhibit different reactivities due to the asymmetrical molecular structures of the monomers. High polymerization temperatures might eliminate the reactivi‐ ty 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 in‐ herent 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 indi‐ cate 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 (*M*n) and weight (*M*w) molecular weights of the PI resins were higher than 17762 g/mol and 35356 g/mol, respectively. In addition, the PI resins exhibited a polydispersity index (*M*w/*M*n) low‐ er than 2.43. This indicates that the substituted-tetralin alicyclic dianhydrides exhibited good reactivity in polymerization with aromatic diamines. Flexible and tough PI films were obtained by casting their solutions in NMP followed by baking at elevated temperatures from 80-220o C. All the films exhibited creasable nature and good tensile properties. For in‐ stance, PI-8 showed a tensile strength of 76 MPa, an elongation at break of 6.2%, and a ten‐


sile modulus of 2.2 GPa. Figure 4 presents the free-standing (left) and creased appearance of PI-8 film at a thickness of 25 μm.

the absorptions of C16 long alkyl chain protons are obviously observed at 0.83 ppm, 1.21 ppm, 1.89 ppm and 3.42 ppm, respectively, although some of the absorptions are overlap‐ ped by the absorptions of tetralin protons. In addition, the resonances observed at around 6.80 ppm (H15, H16 and H17) assigned to the absorptions of aromatic ring protons in 16PDA

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 279

moiety proved the successful preparation of the target polymer.

**Figure 5.** FT IR spectra of MTDA-PIs (PI-4~PI-6)

**Figure 6.** NMR spectra of PI-7 and PI-8

\* [η]inh: inherent viscosity measured with a PI resin at a concentration of 0.5 g/dL in NMP at 25 oC.

**Table 1.** Inherent viscosities and molecular weights of the PIs

**Figure 4.** Appearance of PI-8 film (left) and resin (right)

Figure 5 illustrates the typical FT IR spectra of the PIs. It can be obviously observed that the characteristic absorptions of imide moiety at 1778 and 1711 cm-1, due to the asymmetric and symmetric carbonyl stretching vibrations of the imide groups, and the absorptions at 1383 cm-1 assigned to the C-N stretching vibration of the imide structure are observed in all of the PIs. Typical 1 H NMR spectra of PI-7 and PI-8 are shown in Figure 6. For both of the PIs, the spectra are clearly divided into two parts. One part is the aliphatic, alicyclic and methylene protons in the upfield region; the other part is the aromatic protons in dianhydride moiety (H9, H10 and H11) and diamine moiety (H12 and H13) in the lowfield region. Similarly, the two pairs of protons in methylene groups (H2,3 and H5,6) exhibited individual absorptions in the spectra due to the different chemical environments of the protons in the polymers. For PI-8, the absorptions of C16 long alkyl chain protons are obviously observed at 0.83 ppm, 1.21 ppm, 1.89 ppm and 3.42 ppm, respectively, although some of the absorptions are overlap‐ ped by the absorptions of tetralin protons. In addition, the resonances observed at around 6.80 ppm (H15, H16 and H17) assigned to the absorptions of aromatic ring protons in 16PDA moiety proved the successful preparation of the target polymer.

**Figure 5.** FT IR spectra of MTDA-PIs (PI-4~PI-6)

sile modulus of 2.2 GPa. Figure 4 presents the free-standing (left) and creased appearance of

PI-2 90 10 0.90 22,319 53,387 2.39 PI-3 80 20 0.81 17,762 36,715 2.07

PI-5 90 10 0.94 25,319 61,528 2.43 PI-6 80 20 0.76 18,463 35,729 1.94

PI-8 90 10 0.88 27,834 57,339 2.06 PI-9 80 20 0.69 20,121 35,356 1.76

Figure 5 illustrates the typical FT IR spectra of the PIs. It can be obviously observed that the characteristic absorptions of imide moiety at 1778 and 1711 cm-1, due to the asymmetric and symmetric carbonyl stretching vibrations of the imide groups, and the absorptions at 1383 cm-1 assigned to the C-N stretching vibration of the imide structure are observed in all of the

spectra are clearly divided into two parts. One part is the aliphatic, alicyclic and methylene protons in the upfield region; the other part is the aromatic protons in dianhydride moiety (H9, H10 and H11) and diamine moiety (H12 and H13) in the lowfield region. Similarly, the two pairs of protons in methylene groups (H2,3 and H5,6) exhibited individual absorptions in the spectra due to the different chemical environments of the protons in the polymers. For PI-8,

H NMR spectra of PI-7 and PI-8 are shown in Figure 6. For both of the PIs, the

\* [η]inh: inherent viscosity measured with a PI resin at a concentration of 0.5 g/dL in NMP at 25 oC.

(dL/g)

MDA 16PDA *M*<sup>n</sup> *M*<sup>w</sup> *M*w/*M*<sup>n</sup>

100 0 1.03 31,326 73,798 2.36

100 0 1.04 30,321 68,823 2.27

100 0 0.97 33,192 80,281 2.42

Molecular weight (g/mol)

Diamine (mol %) [η]inh\*

PI-8 film at a thickness of 25 μm.

278 Optoelectronics - Advanced Materials and Devices

TDA

MTDA

FTDA

**Table 1.** Inherent viscosities and molecular weights of the PIs

**Figure 4.** Appearance of PI-8 film (left) and resin (right)

PI Dianhydride

PI-1

PI-4

PI-7

PIs. Typical 1

**Figure 6.** NMR spectra of PI-7 and PI-8

#### **3.3. Solubility**

The solubility of PIs is summarized in Table 2. All the PIs were easily soluble in polar aprot‐ ic solvents (NMP and DMAc), *m*-cresol, and γ-butyrolactone (GBL) at a concentration of 15 wt%. Among the PIs, those derived from MTDA (PI-4~PI-6) showed the best solubility due to the synergic effects of bulky tetralin moiety and pendant methyl substituent in the dia‐ nhydride unit, and flexible alkyl side chains in the diamine unit. PI-5 and PI-6 were even wholly soluble in dichloromethane at room temperature. In the same condition, the PIs de‐ rived from TDA or FTDA were only partially or not soluble in dichloromethane. The en‐ hancement of the solubility of MTDA-PIs can be attributed to the more loose packing of the molecular chains induced by the substituents mentioned above.

this work might be suitable because they all exhibit a relatively low absolute viscosities at a

The effects of asymmetrical *R*-substituted tetralin structure in the dianhydride units and the C16 long alkyl side chains in the diamine moieties on the thermal stabilities of the PIs were investigated by TGA and DSC measurements. Table 3 summarizes the thermal characteriza‐ tion of the polymers. Figure 8 depicts the thermogravimetry plots of PI films over a temper‐

Figure 8a that the current PIs possess good thermal stability with no significant weight loss

aliphatic or alicyclic moiety in the PIs did not apparently sacrifice their thermal stability. Among the series, FTDA-PIs (PI-7~PI-9) exhibited the best thermal stability, which might be due to the higher bond energy of C-F compared to those of C-H (TDA) and C-C (MTDA). As can be seen form Figure 8a that the PIs exhibited single-stage thermal decomposition behav‐ iors in nitrogen. However, they showed a two-stage thermal decomposition in air, as can be depicted in Figure 8b. For example, PI-2 showed a first thermal decomposition at about 400

C in air. When the temperature increased, a second thermal decomposition occurred at 580

C. The first thermal decomposition might be due to the oxidative cleavage of the C16 long alkyl side chains at elevated temperature and the second one was attributed to the decom‐ position of the residual molecular skeleton of PI-2. Although the thermal stability of the cur‐

C in nitrogen and PI-2 in nitrogen and in air. It can be seen from

C, the PIs lose their original weight rapidly, leaving a

C in nitrogen. This implies that the incorporation of

C. The 10% weight loss temperatures

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 281

solid content of 15 wt% (PI-2: 150 mPa.s; PI-5: 200 mPa.s; PI-8: 160 mPa.s).

**Figure 7.** Viscosities as a function of solid contents for PI-2, PI-5 and PI-8 in NMP.

C. After 400 o

residual weight ratio in the range 1.8-32.7% at 600 o

(*T*10%) of the PIs are all higher than 410 o

**3.4. Thermal properties**

ature range of 50 to 750 o

up to approximately 400 o

o

o


\* ++: Wholly soluble at room temperature; +: Partially soluble; -: Insoluble; NMP: *N*-methyl-2-pyrrolidinone; DMAc: *N,N*-dimethylacetamide; GBL: γ-butyrolactone; BC: ethylene glycol monobutyl ether.

**Table 2.** Solubility of the PIs\*

The effects of solid contents (*S*c) on the viscosities (*η*) of the PIs were further investigated. This investigation is very important for applications of the PIs as alignment layers for TFT-LCDs because a proper *S*c-*η* relationship for PI would be beneficial for its orientation to LC molecules. Figure 7 illustrates the correlations between the absolute viscosities and solid contents of PI-2, PI-5 and PI-8 solution in NMP. Although these three PIs were all soluble in NMP, the *η* values of the solutions were quite different. For instance, at the same solid con‐ tent of 35 wt%, PI-2 had a *η* value of 26750 mPa.s, which was much lower than those of PI-5 (64440 mPa.s) and PI-8 (116000 mPa.s). Thus, in practical application, PI-2 is more suitable to be utilized to develop a soluble PI solution with a high solid content and at the same time a relatively low viscosity. However, for the applications as alignment layers for TFT-LCDs, in which the solid content of the PIs are usually lower than 15 wt%, all of the PIs developed in this work might be suitable because they all exhibit a relatively low absolute viscosities at a solid content of 15 wt% (PI-2: 150 mPa.s; PI-5: 200 mPa.s; PI-8: 160 mPa.s).

**Figure 7.** Viscosities as a function of solid contents for PI-2, PI-5 and PI-8 in NMP.

#### **3.4. Thermal properties**

**3.3. Solubility**

280 Optoelectronics - Advanced Materials and Devices

PI

\*

**Table 2.** Solubility of the PIs\*

The solubility of PIs is summarized in Table 2. All the PIs were easily soluble in polar aprot‐ ic solvents (NMP and DMAc), *m*-cresol, and γ-butyrolactone (GBL) at a concentration of 15 wt%. Among the PIs, those derived from MTDA (PI-4~PI-6) showed the best solubility due to the synergic effects of bulky tetralin moiety and pendant methyl substituent in the dia‐ nhydride unit, and flexible alkyl side chains in the diamine unit. PI-5 and PI-6 were even wholly soluble in dichloromethane at room temperature. In the same condition, the PIs de‐ rived from TDA or FTDA were only partially or not soluble in dichloromethane. The en‐ hancement of the solubility of MTDA-PIs can be attributed to the more loose packing of the

> Solvent NMP DMAc γ-BL BC *m*-cresol THF CH2Cl2

PI-1 ++ ++ ++ — ++ — — PI-2 ++ ++ ++ — ++ — + PI-3 ++ ++ ++ — ++ — + PI-4 ++ ++ ++ — ++ — + PI-5 ++ ++ ++ — ++ — ++ PI-6 ++ ++ ++ — ++ — ++ PI-7 ++ ++ ++ — ++ — — PI-8 ++ ++ ++ — ++ — + PI-9 ++ ++ ++ — ++ + +

++: Wholly soluble at room temperature; +: Partially soluble; -: Insoluble; NMP: *N*-methyl-2-pyrrolidinone; DMAc:

The effects of solid contents (*S*c) on the viscosities (*η*) of the PIs were further investigated. This investigation is very important for applications of the PIs as alignment layers for TFT-LCDs because a proper *S*c-*η* relationship for PI would be beneficial for its orientation to LC molecules. Figure 7 illustrates the correlations between the absolute viscosities and solid contents of PI-2, PI-5 and PI-8 solution in NMP. Although these three PIs were all soluble in NMP, the *η* values of the solutions were quite different. For instance, at the same solid con‐ tent of 35 wt%, PI-2 had a *η* value of 26750 mPa.s, which was much lower than those of PI-5 (64440 mPa.s) and PI-8 (116000 mPa.s). Thus, in practical application, PI-2 is more suitable to be utilized to develop a soluble PI solution with a high solid content and at the same time a relatively low viscosity. However, for the applications as alignment layers for TFT-LCDs, in which the solid content of the PIs are usually lower than 15 wt%, all of the PIs developed in

molecular chains induced by the substituents mentioned above.

*N,N*-dimethylacetamide; GBL: γ-butyrolactone; BC: ethylene glycol monobutyl ether.

The effects of asymmetrical *R*-substituted tetralin structure in the dianhydride units and the C16 long alkyl side chains in the diamine moieties on the thermal stabilities of the PIs were investigated by TGA and DSC measurements. Table 3 summarizes the thermal characteriza‐ tion of the polymers. Figure 8 depicts the thermogravimetry plots of PI films over a temper‐ ature range of 50 to 750 o C in nitrogen and PI-2 in nitrogen and in air. It can be seen from Figure 8a that the current PIs possess good thermal stability with no significant weight loss up to approximately 400 o C. After 400 o C, the PIs lose their original weight rapidly, leaving a residual weight ratio in the range 1.8-32.7% at 600 o C. The 10% weight loss temperatures (*T*10%) of the PIs are all higher than 410 o C in nitrogen. This implies that the incorporation of aliphatic or alicyclic moiety in the PIs did not apparently sacrifice their thermal stability. Among the series, FTDA-PIs (PI-7~PI-9) exhibited the best thermal stability, which might be due to the higher bond energy of C-F compared to those of C-H (TDA) and C-C (MTDA). As can be seen form Figure 8a that the PIs exhibited single-stage thermal decomposition behav‐ iors in nitrogen. However, they showed a two-stage thermal decomposition in air, as can be depicted in Figure 8b. For example, PI-2 showed a first thermal decomposition at about 400 o C in air. When the temperature increased, a second thermal decomposition occurred at 580 o C. The first thermal decomposition might be due to the oxidative cleavage of the C16 long alkyl side chains at elevated temperature and the second one was attributed to the decom‐ position of the residual molecular skeleton of PI-2. Although the thermal stability of the cur‐ rent PIs is lower than that of common aromatic PIs (usually with a *T*10% values >500 o C), it is high enough for their applications in TFT-LCD fabrications.

*T*gs of the PIs due to the rigid nature of *meta*-phenylenediamine skeleton in the diamine; however, overloading of the diamine would conversely decrease the *T*gs of the PIs due to the

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 283

PI λ (nm) *T*450 (%) *a* \* *b* \* *L* \* Haze YI PI-1 319 66.7 -2.80 17.46 92.06 2.50 29.32 PI-2 302 82.7 -1.74 7.84 94.31 5.85 13.39 PI-3 331 52.3 —\*\* — — — — PI-4 304 79.4 -2.76 12.02 93.76 1.86 19.90 PI-5 298 82.0 -2.05 10.10 94.11 2.03 17.07 PI-6 306 78.5 -3.63 21.98 92.42 1.69 35.76 PI-7 299 82.2 — — — — — PI-8 297 84.3 -1.30 7.71 94.56 0.78 13.26 PI-9 303 81.4 — — — — —

> , *b*\* , *L*\*

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, color‐

: see 2.2 measurement; \*\* ND: not detected.

flexible long alkyl chains and ether linkage in the diamine.

**Figure 9.** DSC curves of the FTDA-PI films (10 oC/min, in nitrogen)

\* λ: cutoff wavelength; *T*450: transmittance at 450 nm; *a*\*

**3.5. Optical properties**

**Table 4.** Optical transparency and yellow indices of PI films\*


\* *Tg*: glass transition temperature; *T*5%, *T*10%: temperatures at 5% and 10% weight loss, respectively; *R*w600: residual weight ratio at 600 oC in nitrogen.

**Table 3.** Thermal properties of PI films

**Figure 8.** TGA curves of PI films. (a) PI films in nitrogen; (b) PI-2 in nitrogen and in air

Glass transition temperatures (*T*g) values were obtained from the second heating scans of PI samples at a heating rate of 10 o C /min in nitrogen. The data were summarized in Table 3 and the typical DSC curves of FTDA-PIs are shown in Figure 9. All the PIs exhibit good ther‐ mal stabilities with the *T*g values in the range of 233~270℃, depending on the rigidity of the polymers. It is observed that for the same dianhydride, PI containing 10% molar ratio of 16PDA exhibited the highest *T*<sup>g</sup> values. For example, PI-2, PI-5 and PI-8 showed *T*g values of 257 o C, 261 o C and 270 o C, respectively, which were all the highest one among their series. This indicates that introduction of 16PDA at a low proportion, such as 10%, can increase the *T*gs of the PIs due to the rigid nature of *meta*-phenylenediamine skeleton in the diamine; however, overloading of the diamine would conversely decrease the *T*gs of the PIs due to the flexible long alkyl chains and ether linkage in the diamine.

**Figure 9.** DSC curves of the FTDA-PI films (10 oC/min, in nitrogen)


\* λ: cutoff wavelength; *T*450: transmittance at 450 nm; *a*\* , *b*\* , *L*\* : see 2.2 measurement; \*\* ND: not detected.

**Table 4.** Optical transparency and yellow indices of PI films\*

#### **3.5. Optical properties**

rent PIs is lower than that of common aromatic PIs (usually with a *T*10% values >500 o

PI-1 235 399 422 5.3 PI-2 257 420 436 1.8 PI-3 233 407 425 6.7 PI-4 244 412 434 13.3 PI-5 261 428 447 8.5 PI-6 252 396 414 16.0 PI-7 248 408 427 22.6 PI-8 270 428 441 24.2 PI-9 225 390 411 32.7 \* *Tg*: glass transition temperature; *T*5%, *T*10%: temperatures at 5% and 10% weight loss, respectively; *R*w600: residual

oC) *T*10% (

high enough for their applications in TFT-LCD fabrications.

282 Optoelectronics - Advanced Materials and Devices

PI *T*g (oC) *T*5%(

**Figure 8.** TGA curves of PI films. (a) PI films in nitrogen; (b) PI-2 in nitrogen and in air

Glass transition temperatures (*T*g) values were obtained from the second heating scans of PI

and the typical DSC curves of FTDA-PIs are shown in Figure 9. All the PIs exhibit good ther‐ mal stabilities with the *T*g values in the range of 233~270℃, depending on the rigidity of the polymers. It is observed that for the same dianhydride, PI containing 10% molar ratio of 16PDA exhibited the highest *T*<sup>g</sup> values. For example, PI-2, PI-5 and PI-8 showed *T*g values of

This indicates that introduction of 16PDA at a low proportion, such as 10%, can increase the

C /min in nitrogen. The data were summarized in Table 3

C, respectively, which were all the highest one among their series.

weight ratio at 600 oC in nitrogen.

**Table 3.** Thermal properties of PI films

samples at a heating rate of 10 o

C and 270 o

257 o

C, 261 o

C), it is

oC) *R*w600 (%)

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, color‐ less 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-stand‐ ing 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.

**Figure 11.** Yellow indices of PI films

synthesis to devices.

at 80 o

553.7 Å, as shown in Figure 13.

**3.6. Application in TFT-LCD fabrication**

**Figure 12.** LC cells fabrication procedure from monomer synthesis to devices

In order to investigate the practical application of the newly-developed alicyclic PIs as align‐ ment layers for TFT-LCDs, a series of fringe field switching (FFS) mode [23, 24] liquid crys‐ tal 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

First, PI-8 resin was dissolved in a mixture solvent of NMP/GBL/BC (55/30/15, volume ra‐ tio) 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.

purified PI-8 solution was spin-coated on an ITO glass substrate (10 cm×10 cm) at a rotat‐ ing speed of 3000 rpm. Then the ITO glasses with PI-8 coating were placed on a hot plate

es of PI-8 film was measured with a profilometer (Dektak XT, Bruker) and found to be

C for 30 min, followed by curing in a nitrogen oven at 230 o

s. The

C for 1h. The thickness‐

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 285

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 indi‐ cates a weak coloration for a polymer film [22]. As shown in Table 5, PI-8 exhibited the low‐ est 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.

**Figure 10.** UV-Vis spectra of PI films derived from MTDA (PI-4~PI-6)

**Figure 11.** Yellow indices of PI films

less 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

In the present work, the tetralin-containing PI films were obtained as pale-yellow free-stand‐ ing 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

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 indi‐ cates a weak coloration for a polymer film [22]. As shown in Table 5, PI-8 exhibited the low‐ est YI and Haze values compared with its analogs. This result correlated well with the

In summary, this low-color feature for the current PI films is desirable for their application

moiety has been proven to be one of the most effective procedures.

transparency of the PI films by reducing the CTC formation.

results measured by UV-Vis measurements.

**Figure 10.** UV-Vis spectra of PI films derived from MTDA (PI-4~PI-6)

as alignment layers for TFT-LCDs.

284 Optoelectronics - Advanced Materials and Devices

#### **3.6. Application in TFT-LCD fabrication**

In order to investigate the practical application of the newly-developed alicyclic PIs as align‐ ment layers for TFT-LCDs, a series of fringe field switching (FFS) mode [23, 24] liquid crys‐ tal 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.

**Figure 12.** LC cells fabrication procedure from monomer synthesis to devices

First, PI-8 resin was dissolved in a mixture solvent of NMP/GBL/BC (55/30/15, volume ra‐ tio) 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 rotat‐ ing speed of 3000 rpm. Then the ITO glasses with PI-8 coating were placed on a hot plate at 80 o C for 30 min, followed by curing in a nitrogen oven at 230 o C for 1h. The thickness‐ es of PI-8 film was measured with a profilometer (Dektak XT, Bruker) and found to be 553.7 Å, as shown in Figure 13.

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 [26]. 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 [27]. Thus, it might be a compromised procedure to combine the imidized PI-8 and its PAA solu‐ tion so as to achieve a balance of high VHR and low RDC levels for the TFT-LCDs. The de‐

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 287

LC cells mode Cell gap (μm) Pretilt angle (°)\* VHR (%, 30 oC) RDC (mV, 30 oC) FFS 5.6 2.7 98.2 230

As one of our continuous endeavors developing low-cost and high performance PI align‐ ment 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 dianhy‐ drides 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 orien‐ tating ability to LC molecules. The LC cells with PI-8 as the alignment layer exhibited good

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

Financial support from the National Natural Science Foundation of China (51173188 and

tailed investigations are being performed in our laboratory.

\* Measured with LC cells with 50 μm spacers.

**4. Conclusions**

optoelectrical properties.

**Acknowledgements**

50403025) is gratefully acknowledged.

progress now.

**Table 5.** Electrical characterization of FFS-LCD cells

**Figure 13.** Thickness of PI-8 alignment layer

The cured PI-8 film was then rubbed with a nylon cloth-packed rubbing roller using the fol‐ lowing parameters: radius of rubbing roller: 50 mm; rotation speed: 1000 rpm; pile impres‐ sion: 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 [25], as illustrated in Figure 14.

**Figure 14.** Schematic diagram of LC alignment on PI surfaces

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 [26]. 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 [27]. Thus, it might be a compromised procedure to combine the imidized PI-8 and its PAA solu‐ tion so as to achieve a balance of high VHR and low RDC levels for the TFT-LCDs. The de‐ tailed investigations are being performed in our laboratory.


**Table 5.** Electrical characterization of FFS-LCD cells
