Synthesis and characterization of high-temperature polymers containing two trifluoromethyl groups and mono-functionalized poly(3-hexylthiophene)s

Abstract

This thesis deals with the synthesis of high-temperature polymers containing two trifluoromethyl groups and mono-functionalized poly(3-hexylthiophene)s (P3HT). Chapter 1 introduces general aromatic polymers, and Chapter 2-3, Chapter 4-6, and Chapter 7-8 describe poly(biphenylene oxide)s (PBPO), polyimides (PI), and poly(amide imide)s (PAI), respectively.

In Chapter 2 and 3, high-molecular-weight PBPOs containing trifluoroemthyl ($CF_3$) groups were pre-pared through a meta-activated nitro displacement reaction. Although several literatues reported the preparation of polymers via meta-activated nucleophilic aromatic substitution ($S_NAr$) reaction, the in-depth study about the reaction is deficient. Therefore, the reactivity of meta-activated $S_NAr$ reaction was investigated in Chapter 2. The increase in reaction temperature, nucleophilicity of nuclophiles, and the number of activating groups in-creased the reaction rate. Solvents also affected the reaction rate in the following order of DMPU $\gtr$ NMP $\thickapprox$ DMSO $\geq$ DMF. The trend that affects the reaction was similar to ortho- or para-activated $S_NAr$ ractions. Based on these results, new PBPOs were prepared from AB-type monomers, 4′(3′)-hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl in Chapter 3. The obtained polymers have weight-average molecular weight of 20,800−143,300 g/mol and molecular weight distribution of 1.68−2.85. While two homopolymers showed a semicrystalline morphology, copolymers were amorphous and dissolved in a wide range of organic solvents. The polymers possessed a glass transition temperature (Tg) in the range of 169 to 208 $^\circ$C depending on their structure and good thermal stability with 10% weight loss temperatures from 486 to 542$^\circ$C in nitrogen and from 465 to 516$^\circ$C in air. Moreover, PBPOs showed low refractive indices in the range of 1.4979−1.5052 as well as low bire-fringence values of 0.0095−0.0148 due to the $CF_3$ groups.

In Chpater 4, new unsymmetrical diamine monomer, 4-(4’-aminophenoxy)-3,5-bis(trifluoromethyl)aniline, which contains one ether linkage and two CF3 substituents was prepared for the preparation of a series of new PIs. The monomer was polymerized with typical dianhydrides using a one-pot synthetic method to obtain corresponding PIs. Interestingly, PI containing PMDA moiety as a dianhydride unit also showed good solubility despite its highly rigid nature. All of the PIs were readily solution-cast into transpar-ent, flexible, and tough films. The films had a UV–vis absorption cut-off wavelength at 340–375 nm and light transparencies of 87–91% at a wavelength of 550 nm. Unsymmetrical incorporation of two CF3 groups into rigid PIs not only improved their solubility and transparencies but also increased their $T_gs$ above 300 $^\circ$C due to the high barrier against free chain-rotation. The polymers also exhibited high thermal stability with 5% weight loss at temperatures ($T_{d5}$) ranging from 534 to 593 $^\circ$C in nitrogen and from 519 to 568 $^\circ$C in air.

In Chapter 5, another diamine monomer, 2,6-bis(trifluoroemthyl)benzidine, was designed for the preparation of a series of new PIs. While the PI synthesized with PMDA as a dianhydride unit was insoluble, other PIs were soluble in various organic solvents and could be solution-cast into flexible and tough films. All PIs exhibited high thermal stability with Td5 values ranging from 535 to 605 $^\circ$C in nitrogen and from 523 to 594 $^\circ$C in air, and high Tg values in the range of 345 – 366 $^\circ$C due to rigid chain structure as well as high rotational barrier of polymer chains. The PI prepared from BPDA as a dianhydride unit showed low CTE value of 6.8 ppm/$^\circ$C due to rigid chain structure though it had poor optical property with a light transparency of 34 % at a wavelength of 550 nm.

During the study of the PIs, it was observed that the PIs containing BTDA and ODPA as dianhydride units showed thermo-responsive behavior in organic solvents (Chapter 6). With increasing temperature, the intra-chain interactions were weakened and inter-chain interactions were enhanced, leading to the occurrence of the phase separation in solution state. The temperature at which the phase transition occurs depended on the chain flexibility and the type of solvents.

In Chapter 7, a new unsymmetrical diacid monomer was synthesized from 2,6-bis(trifluoroemthyl)benzidine with trimellitic anhydride and then, a series of soluble PAIs were prepared from the polymerization of the diacid monomer with commercially available diamine monomers. All the polymers were readily soluble in many organic solvents and could be solution-cast into transparent, flexible, and tough films. The films had a UV-vis absorption cut-off wavelength at 371 – 424 nm and light transparencies of 74 – 88% at a wavelength of 550 nm depending on their structure. Also, the PAIs exhibited high softening temperatures above 330 $^\circ$C and low CTE values in the range of 4.2 – 27.3 ppm/oC. Importantly, the PAI synthesized from 2,2’-bis(trifluoroemthyl)benzidine as a diamine monomer showed a nice balance of properties. This polymer showed the best solubility and transparency (88%), no appearance of Tg, and the lowest CTE value of ~4 ppm/$^\circ$C at the same time.

Further experiments about the PAI possessing low CTE value together with high transparency were treat-ed in Chpater 8. DFT calculation suggested that the smallest inter-molecular distance change between symmet-rical and unsymmetrical units upon heating contributed the low thermal expansion of the PAI. Based on the result, the CTE value of the PAI was controlled by the regioisomerization of the polymer chains, in which the position of $CF_3$ groups was regulated. Therefore, the CTE value of the PAIs was controlled under the range of 4.2 – 17.5 ppm/$^\circ$C. They also showed no distinct $T_gs$ and good optical properties including a UV-vis absorption cut-off wavelength at 364 – 371 nm and high light transparencies of 88 – 89% at a wavelength of 550 nm.

In the final chapter, we have demonstrated a highly efficient and simple approach for the modification of P3HT end group. Mono-functionalized P3HTs were synthesized via thiol-ene click reation, in which allyl functional group positioned at the end of the polymer chain was well converted to thiol-, hydroxyl-, carboxylic acid-, and methyl ester groups. P3HTs functionalized with thiol- and carboxylic acid groups showed good binding property on gold and $TiO_2$ surfaces, respectively.