**3.3 Polymer syntheses**

Based on the results of the model reactions, several polyimides (PIs) were prepared from **3** and commercially available aromatic dianhydrides via a one-pot solution imidization method, as shown in **Figure 7**. The polymerizations of diamine

**Figure 4.** *Model reaction of 3 with phthalic anhydride.*

*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

**Figure 5.** *(a) FTIR, (b) <sup>1</sup> H, and (c) 13C NMR spectra of model compound (4) (NMR: DMSO-*d*6, 25°C) [48].*

monomer **3** with stoichiometric amounts of five different aromatic dianhydride monomers, BPDA (**PI-1**), BTDA (**PI-2**), ODPA (**PI-3**), 6-FDA (**PI-4**), and PMDA (**PI-5**), were carried out in *m*-cresol with catalytic amounts of isoquinoline at a solid content of about 16 wt%. The ring-opening polyaddition at room temperature for

### **Figure 6.**

*(a) Top view and (b) side view of displacement ellipsoid (50%) representations of 4. All hydrogen atoms are omitted for clarity.*

**Figure 7.** *Polymerization of 3 with aromatic dianhydrides.*

4 h yielded poly(amic acid) solutions. After dilution of the solution to 8 wt%, subsequent cyclodehydration by heating at 190°C with the azeotropic distillation of chlorobenzene for 12 h gave fully imidized and homogeneous PI solutions except for that of PMDA (**PI-5**). When PMDA was used as the dianhydride, the solution became turbid with phase separation as soon as the temperature reached 190°C. This was likely due to the most rigid chain characteristic of **PI-5**. At the end of the reaction, pure solid polymers were obtained by precipitation of the corresponding polymer solutions into ethanol.

**Table 1** shows the inherent viscosities and GPC data of the PIs. The inherent viscosities of the organosoluble PIs were in the range of 0.69–2.30 dL/g, as measured in DMAc at 30°C. Additionally, the PIs soluble in THF exhibited weightaverage molecular weights (*M*ws) in the range of 7.32–8.81 <sup>10</sup><sup>4</sup> relative to the polystyrene standard. The molecular weights of the PIs were high enough to obtain flexible and tough polymer films by casting from their DMAc solutions.

The formation and the structures of the polymers were verified by elemental analyses, FTIR, and <sup>1</sup> H NMR spectroscopy. The elemental analysis values of the PIs (listed in Experiments) were in good agreement with the calculated values of the proposed structures. The typical FTIR spectrum of **PI-1** is shown in **Figure 8**. All PIs exhibited characteristic imide group absorptions around 1780 and 1730 (typical of imide carbonyl asymmetrical and symmetrical stretching), 1370 (CdN stretching), and 730 cm<sup>1</sup> (imide ring deformation), together with a number of strong absorption bands in the regions of 1120–1200 cm<sup>1</sup> due to CdF stretching. The absence of amide and carboxyl bands indicates the virtually complete conversion of the poly (amic acid) precursors into PIs. The <sup>1</sup> H NMR spectra of the PIs are illustrated in **Figure 9**. All proton peaks were also assigned to the predicted structures without amide and acid protons, demonstrating the successful preparation of the PIs. Additionally, the proton peaks of the dianhydride units in the polymer chains were divided into different chemical shifts due to the unsymmetrical structure of the diamine unit in the PI main chains. The chemical shifts of protons closer to the CF3


*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

*a Measured in DMAc at a concentration of 0.5 g/dL at 30°C.*

*b Determined by GPC in THF at 35°C (relative to polystyrene standard).*

*c Insoluble.*

### **Table 1.**

*Inherent viscosities and elemental analyses results of the PIs.*

**Figure 8.** *FTIR spectrum of PI-1 (film).*

groups appeared further downfield in the NMR spectrum due to the deshielding effect of the electron-withdrawing CF3 groups. This result was consistent with the model reaction result.

### **3.4 Polymer properties**

The solubility of the synthesized PIs is summarized in **Table 2**. The synthesized polymers retained good solubility in organic solvents, although their rigidity increased compared to those synthesized from 4-(4<sup>0</sup> -aminophenoxy)-3,5-bis (trifluoromethyl)aniline [40]. All PIs except for **PI-5** exhibited good solubility in *N*-methyl-2-pyrrolidone (NMP), *N*,*N*-dimethylacetamide (DMAc), *m*-cresol, and anisole at room temperature. In addition, **PI-2**, **PI-3**, and **PI-4** showed good solubility in *N*,*N*-dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, and ethyl acetate at room temperature. The good solubility of the PIs is attributed not only to the bulkiness of the two CF3 groups on the polymer chains but also to the unsymmetrical structure resulting from the diamine monomer. Given the increased chain flexibility, **PI-3** and **PI-4** were also soluble in chloroform and **PI-4** was soluble even

### **Figure 9.**

*1 H NMR spectrum of PI-1 (DMSO-*d*6, 100°C); <sup>1</sup> H NMR spectra of PI-2, PI-3, and PI-4 (DMSO-*d*6, 25°C).*


*Solubility: ++, soluble at room temperature; +, soluble on heating; +*�*, partially soluble;* �*, insoluble. Abbreviations: NMP,* N*-methyl-2-pyrrolidone; DMAc,* N*,*N*-dimethylacetamide; DMF,* N*,*N*-dimethylformamide; DMSO, dimethyl sulfoxide; THF, tetrahydrofuran; ODCB, 1,2-dichlorobenzene.*

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

in acetone at room temperature. Upon a comparison of the solubility behavior with PIs prepared from symmetrical benzidine, 2,2<sup>0</sup> -bis(trifluoromethyl)benzidine [17–21], the PIs described here showed enhanced solubility. The improved solubility can be attributed to the unsymmetrical and more twisted chain structure, which inhibits close packing and reduces intermolecular interactions. Meanwhile, **PI-5** showed poor solubility in the organic solvents tested, although it was partially soluble in NMP and DMAc.

*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

To clarify the cause of the poor solubility of **PI-5**, wide-angle X-ray diffraction (WAXD) studies were performed because a crystalline domain can influence the solubility of the polymer [31]. The X-ray diffractograms of **PI-1** and **PI-3** showed broad diffraction curves without obvious peak features, indicating that the PIs have an amorphous morphology in principle (**Figure 10**). On the other hand, the X-ray diffraction curve of **PI-5** exhibited a relatively unambiguous peak around 17°, which indicated that **PI-5** has a more ordered phase compared to the other soluble PIs. This is likely related to the highly linear and rigid chain structure of **PI-5**, which induced high intermolecular interactions, resulting in a decrease of the solubility.

The soluble PIs of **PI-1**, **PI-2**, **PI-3**, and **PI-4** could be processed into flexible and tough films conveniently by casting from the DMAc polymer solutions. While **PI-2**, **PI-3**, and **PI-4** produced transparent films, **PI-1** generated a turbid film which appeared as the solvent was evaporated. **Table 3** shows the mechanical properties of the PI films. The PI films had tensile strength levels, elongation at break values, and a Young's modulus in the ranges of 92–145 MPa, 26–55%, and 2.1–3.2 GPa, respectively. These values were comparable to the mechanical strength of the PIs prepared from 2,2<sup>0</sup> -bis(trifluoromethyl)benzidine [22] and also indicated that the PI films are strong enough for use.

The thermal properties of the PIs were evaluated by TGA, DSC, and TMA, and these results are summarized in **Table 4**. The dynamic TGA result showed high thermal stability in which 5% weight loss occurred for the PIs in the range of

**Figure 10.** *Wide-angle X-ray diffractograms of PI-1 (film), PI-3 (film), and PI-5 (powder).*


**Table 3.** *Mechanical properties of the PI films.*


*a 5% weight loss temperature measured by TGA at a heating rate of 10°C/min.*

*b Measured by DSC (the second scan) in N2 at a heating rate 10°C/min. <sup>c</sup>*

*A TMA analysis was conducted three times for each sample at a heating rate of 5°C/min in a heating range up to 300° C. Each CTE value was calculated from the mean coefficient of the linear thermal expansion over a specific*

*temperature range between 50 and 250°C in the second and third runs, respectively. d Measured by TMA at a heating rate of 5°C/min.*

*e Not detected.*

*f Not measured.*

### **Table 4.**

*Thermal and optical properties of the PIs.*

535–605°C in nitrogen and 523–594°C in air (**Figure 11a** and **b**, respectively). DSC experiments were conducted at a heating rate of 10°C/min in nitrogen (**Figure 11c**). A survey of all of the PIs by DSC revealed that no endothermic peaks associated with melting were observed up to the temperature region investigated here. Moreover, while the glass-transition temperatures (*T*g) of **PI-2**, **PI-3**, and **PI-4** were clearly detected, **PI-1** and **PI-5** did not show any discernible glass transition on the DSC thermograms. Therefore, the *T*<sup>g</sup> of **PI-1** was measured by the TMA method after preparation of the polymer film (**Figure 11d**). All PIs exhibited high *T*<sup>g</sup> values above 340°C which depended on the chemical structure of the aromatic dianhydride component. **PI-1** obtained from BPDA showed the highest *T*<sup>g</sup> value (366°C) among the soluble PIs owing to the absence of a flexible linkage between the phthalimide units. Compared with PIs derived from 4-(4<sup>0</sup> -aminophenoxy)-3,5 bis(trifluoromethyl)aniline [40], the PIs based on 2,6-bis(trifluoromethyl)benzidine possess higher *T*<sup>g</sup> values due to their greater chain rigidity. Even when compared to the PIs derived from 2,2<sup>0</sup> -bis(trifluoromethyl)benzidine [18–22, 25], the PIs synthesized here showed similar or higher *T*<sup>g</sup> values. For example, the PIs based on BPDA and 6-FDA with 2,2<sup>0</sup> -bis(trifluoromethyl)benzidine exhibited *T*<sup>g</sup> values of 287–373°C and 335°C, respectively. This result could be attributed to the unsymmetrical introduction of the two CF3 groups, which further increases the rotational barrier of the polymer chains compared to the symmetrically introduced case.

The coefficients of thermal expansion (CTEs) of the PI films were found in the range of 6.8–63.1 ppm/°C. In general, polymers consisting of rod-like backbone structures together with a high chain alignment toward the direction parallel to the film plane have shown relatively low CTE values [24–28]. The relationship between the chain rigidity/degree of in-plane orientation and the CTE value can be applied to this study. The CTE value of the PIs decreased from 63.1 to 6.8 ppm/°C with an increase in the chain rigidity and the degree of in-plane orientation, as identified through the birefringence value (**Table 5**). Although the birefringence value of **PI-1** could not be measured due to the low transparency in this case, it can be speculated that **PI-1** has the highest degree of in-plane orientation because **PI-1** possesses the most rigid chain structure among the soluble PIs [18, 19, 24, 25, 28–30]. Therefore, **PI-1** gave the lowest CTE value (6.8 ppm/°C), displaying good dimensional stability.

*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

### **Figure 11.**

*TGA curves of the PIs in (a) nitrogen and (b) air at a heating rate of 10°C/min, (c) DSC curves of the PIs (the second heating run ranging from 0 to 400°C at a heating rate of 10°C/min in N2) and (d) TMA CURVE Of PI-1 (measured at a heating rate of 5°C/min).*


*a Measured at a wavelength of 633 nm at room temperature.*

*b* <sup>n</sup>*TE: the in-plane refractive index. <sup>c</sup>*

<sup>n</sup>*TM: the out-of plane refractive index. <sup>d</sup>*

<sup>n</sup>*av: the average refractive index (*n*av = (2*n*TE <sup>+</sup>* <sup>n</sup>*TM)/3). <sup>e</sup>*

*Δ*n*: birefringence (*n*TE* n*TM).*

*f Dielectric constant estimated from the refractive index: ε 1.10*n*av<sup>2</sup> . <sup>g</sup>*

*Film thickness for the refractive index measured.*

### **Table 5.**

*Refractive indices of the PIs.*

The corresponding UV-vis spectra of the PI films with a thickness of about 60–80 μm are shown in **Figure 12a**. While the light transparencies of **PI-2**, **PI-3**, and **PI-4** were as high as 87–90% at a wavelength of 550 nm, **PI-1** film exhibited extremely low transparency of 34%. This outcome is attributable to the high degree of in-plane orientation caused by the most rigid chain structure compared to other PIs. The cutoff wavelengths (λo) of the PI films listed in **Table 4** range from 354 to 398 nm, indicating that they are light-colored films. The color of the PI films decreased in the following order: **PI-1** > **PI-2** > **PI-3** > **PI-4**. As shown in **Figure 12b**, **PI-3** and **PI-4** produced fairly transparent and almost colorless PI films compared to the other PIs. The good optical properties of **PI-3** and **PI-4** were attributed to the poor CTC formation ability caused by the ether chain of ODPA and the bulky hexafluoroisopropylidene group of

**Figure 12.** *(a) Transmittance UV-vis spectra and (b) photographs of the PI films.*

6-FDA, respectively. The increased yellowness of **PI-2** resulted from the presence of the carbonyl group, attracting electrons in the dianhydride units.

The refractive indices and birefringence values of the **PI-2**, **PI-3**, and **PI-4** were measured using a prism-coupling method with a laser beam having a wavelength of 633 nm. The measurement of the **PI-1** film could not be performed due to the low transparency of the film. As shown in **Table 5**, the PIs showed low refractive indices (*n*av) in the range of 1.534–1.614 due to the incorporation effect of the two CF3 groups, which led to low molecular polarizability and density levels. The birefringence (Δ*n*) values of the PIs were determined as the difference between *n*TE and *n*TM and were in the range of 0.042–0.072. The birefringence values among the PIs tested increased with an increase of the chain rigidity because the high chain rigidity led to high chain alignment in the direction parallel to the film plane [28–30]. **PI-4** showed the lowest birefringence value, indicating that the linear polarizability and segmental orientation of **PI-4** are the most isotropic among the PIs. The dielectric constant (ε) can be estimated from the refractive index *n* according to Maxwell's equation, ε ≈ *n*<sup>2</sup> . The ε value at 1 MHz was determined to be ε ≈ 1.10 *n*av2 , including an additional contribution of approximately 10% due to infrared absorption [49]. The ε values of the PI films estimated from the average refractive indices ranged from 2.59 to 2.87. The low dielectric constants are also attributed to the existence of two CF3 groups in the main chain.

In our previous work, the thermo-responsive behavior of PIs containing CF3 groups was observed in acetyl-containing solvents [41]. Likewise, **PI-2** and **PI-3** also showed LCST behavior in the solvents used here. The transmittance of the ethyl acetate solutions of **PI-2** and **PI-3** was followed as a function of the temperature at a heating/cooling rate of 0.5°C/min. With an increase of the solution temperature, the solutions clearly turned turbid and opaque, indicating a phase transition. The clouding point temperatures (*T*cp) of the **PI-2** and **PI-3** solutions were 44 and 48°C, respectively, at a concentration of 0.1 wt%, and the *T*cp values decreased with an increase of the polymer concentrations. Compared to the polymers synthesized from 4-(4<sup>0</sup> -aminophenoxy)-3,5-bis(trifluoromethyl) aniline, **PI-2** and **PI-3** have lower *T*cp values, presumably due to lower solubility originating from the increased chain rigidity. The solutions became clear and transparent again when they were cooled.

## **4. Conclusion**

New fluorinated PIs were prepared from the benzidine monomer containing two trifluoromethyl groups on one aromatic ring, 2,6-bis(trifluoromethyl) benzidine.

*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

Due to the rigid and twisted structure of the diamine monomer, the resulting PIs showed good solubility together with high thermal stability and excellent mechanical properties. The PIs also possessed low refractive indices and low dielectric constants due to the high fluorine contents. These PIs can be considered as promising processable high-temperature materials that can find applications in flexible electronics including substrates of flexible and rollable AMOLED displays and lowk dielectrics for microelectronics.
