폴리벤즈옥사졸 공중합체의 합성 및 나노복합재료에 관한 연구

Abstract

ABSTRACT

Study on the synthesis of polybenzoxazole copolymers and their nanocomposites

by Lee, Eung-Jae Advisor: Professor Choi, Jae-Kon, PhD Department of Polymer Science & Engineering Graduate School, Chosun University

Aromatic polybenzoxazoles (PBOs) are aromatic heterocyclic polymers that show outstanding thermo-oxidative stability, high mechanical properties, good environmental resistance, and superior hydrolytic stability. Thus, the rigid-rod type of PBOs has found promising applications in the fields of high strength and high modulus fibers, aerospace materials, and photosensitive materials. Especially, PBO fibers are super fibers with superior thermal stability, flame resistance, ultra high strength, and high modulus, compared to other super fibers such as Kevelar, Vectran, Dyneema, and steel fiber. Unfortunately, however, it is not easy to produce the PBO fibers using a solution processing method because of the poor solubility of the PBO. Similar to polyimides, most of the PBOs have high melting temperature (Tm) and glass transition temperatures (Tg) due to the presence of rigid rod polymeric nature, and they show poor solubility in common solvents; they are soluble only in strong acids. Therefore, they cannot be fabricated into flexible, tough films or fibers with ease. This limits their applications of PBOs in a wide range of industrial fields. Thus, scientists keep trying to find methods to increase the solubility of the PBOs in common solvents.

One of the successful approaches to increase solubility and processability of PBOs without sacrificing their properties is the incorporation of flexible groups such as the bulky fluorinated alkyl chain (6F), aryl ether groups, or aryl sulfide groups into PBO precursors. Then, the PBO precursors are converted into PBO simply by heating via a cyclization reaction. It should be mentioned that released water molecules during the cyclization reaction as by-products act as a flame retardant agent.

In this work, a number of new PBO precursors were synthesized by using low temperature solution polymerization and direct polymerization methods, and the relationship between the chemical structure, processing, and thermal and physical properties of the PBOs were investigated. Firstly, we carried out the synthesis of aromatic PBO precursors, i.e. aromatic poly(o-hydroxyamide)s (PHAs) by the low temperature solution polycondensation reaction using two types of bis(o-aminophenol)s with various aromatic dicarboxylic acid chlorides and isophthaloyl chloride (IPC). The PHAs exhibited inherent viscosities in the range of 0.32－0.65 dL/g at 35 °C in DMAc solution. The hexafluoropropane (6F)-containing-PHAs derived from the 6F-containing bis(o-aminophenol)s showed relatively lower inherent viscosities, which might be attributable to low nucleophilicity of the fluorine-containing monomer caused by the presence of electron-withdrawing 6F groups. All PHAs, except for PHA 4, were readily soluble in aprotic solvents such as NMP, DMAc, and DMF. Only PHA 1 and PHA 2 could afford the flexible and tough film by solution casting. However, the other cast films of PHAs were cracked upon solution casting probably because of low molecular weights. The PBOs were quite insoluble in other solvents, but only partially soluble in sulfuric acid. The thermally converted PBOs showed relatively high Tg in the range of ca. 265－325 °C by the DSC thermograms. The maximum weight loss temperature and char yields of PHA 3 and 6F-PHA 3 showed the highest values of ca. 670 °C and 58.1% and ca. 575 °C and 55.1 %, respectively. The PBOs and 6F-PBOs did not show significant weight loss below 500 °C under nitrogen or air. On the other hand, PBO 4 and 6F-PBO 4 having less stable aliphatic groups by heating showed dramatic weight loss above 400 °C. The activation energy for the decomposition reaction of the PBOs were in the range of ca. 240－805 kJ/mol, which increased with a conversion rate.

Secondly, a series of aromatic PHAs were synthesized by direct polycondensaton of diacides containing di-imide ring with two types of bis(o-aminophenol)s including 3,3‘-dihydroxybenzidine and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. The PHAs exhibited inherent viscosities in the range of 0.34－0.65 dL/g at 35 °C in DMAc solution. The DPHA 1 and 6F-DPHA 1, introducing o-phenylene unit in the main chain, showed excellent solubilities in aprotic solvents such as NMP. However, the DPHA 3, having p-phenylene unit, was not even dissolved completely in aprotic solvents containing LiCl salt. The 6F-DPHAs were readily soluble at room temperature in that solvent. In addition, the 6F-DPHAs, except for the 6F-DPHA 3 were readily soluble at room temperature in aprotic solvents. However, they showed better solubility than DPHAs. The PBOs exhibited relatively high Tg in the range of ca. 305－310 °C. The maximum weight loss temperature and char yields of DPHA 3 and 6F-DPHA 3 showed the highest values of ca. 658 °C and 62.6%, and ca. 653 °C,and 62.1 %, respectively.

Thirdly, the PHAs having terephthaloyl chloride and/or 2,5-bis[ω-methoxy-poly(ethylene glycol)]terephthaloyl chloride (M-TPC) groups were synthesized by a solution polycondensation reaction at low temperature. The inherent viscosities of the PHAs measured at 35 °C in DMAc or DMAc/LiCl solution were in the range of 0.74－1.42 dL/g. The solubility of the precursors with higher M-TPC unit increased, but the PBOs were nearly insoluble in various solvents. The degradation temperature of the copolymer precursors were recorded in the ranges of ca. 410－665 °C under nitrogen, and char yields showed 13－59% values at 900 °C. The mechanical properties and flame retardancy of copolymer precursors decreased with higher M-TPC unit. The PHAs nanocomposites having various compositions of two different clays, organically modified montmorillonite (OMMT) and Cloisite 20A, were prepared by solution blending. From the TEM images of the materials, we confirmed that the clays were dispersed homogeneously in the PHAs matrix and showed partially exfoliated and intercalated. The thermal stability and char residues of the nanocomposites were increased with increasing the amount of two different organic clays. The tensile strength and initial modulus of the nanocomposite films increased with increasing each clay contents, and were significantly higher, compared to those of CP-5. The tensile strength and initial modulus of the nanocomposite containing 4 wt% OMMT, however, decreased to 3.9 MPa and 0.2 GPa, respectively, compared to 3wt% OMMT because of the poor dispersion of clays in the PHA matrix. The oxygen transmission rate of the PHA/OMMT decreased with increasing clay content from 1 to 3 wt%, but increased again for the nanocomposite containing 4 wt% clay, which might be due to the clay aggregation. The above results suggested that the nanocomposites prepared by using two different organic clays showed similar flame retardancy. Whereas, it was found that the Cloisite 20A was more effective than the OMMT for the enhancement of the thermal and the mechanical properties of the nanocomposites and the reduction of the oxygen transmission rate.

Key words: poly(hydroxyamide)s, poly(benzoxazole)s, thermal cyclization reaction, activation energy, LOI, nanocomposite