펜단트 그룹을 갖는 폴리 벤즈옥사졸 전구체의 합성 및 응용

Abstract

Aromatic polybenzoxazoles(PBOs) have been known since 1960's as high-performance polymers with excellent thermal stability, mechanical properties, and good chemical resistance. The excellent thermal stability and high flame resistance of PBO make it the material of choice for high-temperature application, for example, high modulus and high strength fibers, nonlinear optical materials, aerospace, photosensitive materials, and other industries. However, as with aromatic polymers they are generally difficult to process because of their high melting and glass-transition temperatures(Tg) and poor solubility in conventional organic solvents. As a consequence, potential applications have been limited. Therefore, attempts have been made to modify the backbone structure and improve their processibility. One successful approach is to introduce flexible linkages or bulky side groups into the polybenzoxazole backbone in order to increase processibility. These modifications lower the melting temperature and lead to soluble and amorphous polymers. In general, amorphous polymers have lower softening temperature and improved solubility compared to crystalline analogues, thus they may open applications in the area of films, coatings, engineering plastics, polymer blends, and composites. Aromatic PBOs were commonly prepared by either of two methods. The first is a two step method that involves the poycondensation of bis(o-aminophenol)s or their trimethylsilylated derivatives with aromatic dicarboxylic acid chlorides giving precursor poly(o-hydroxyamide)s, followed by thermal solid-state or solution cyclodehydration yielding PBOs. The second is a one-step process directly producing PBOs by the melt polycondensation of bis(o-aminophenol)s with aromatic dicarboxylic acid diphenyl esters and by the solution polycondensation with aromatic dicarboxylic acids in acidic media like polyphosphoric aicd. Precursors polymers have the advantage that they are easier to process, do not require strong solvents and can absorb large amounts of heat energy during cyclization process. When cyclized they liberate water or a flame retardant during the cyclization. Hence, the precursor will be converted to a high temperature heterocyclic polymer possibly containing a flame retardant. Polyhydroxyamides(PHAs) having poly(ethylene glycol)methyl ether(MPEG) and/or dimethyl- phenoxy pendant groups were synthesized by solution polycondensation at low temperature. This precursor polymers were studied by FT-IR, 1H-NMR, DSC, and TGA, Pyrolysis Combustion Flow Calorimeter(PCFC) and X-ray diffractometer. The inherent viscosities of the PHAs measured at 35 ℃ in DMAC or DMAc/LiCl solution were in the range of 0.51～2.31 dL/g. Solubility of the precursors with higher MPEG unit was increased, especially the polymer having MPEG(Mn=1100) was soluble or partially soluble in ethanol, methanol, and water as well as aprotic solvents, but the PBOs were nearly insoluble in a variety of solvents. PHAs were converted to polybenzoxazoles(PBOs) by thermal cyclization reaction with heat of endotherm. In case of the precursors having MPEG unit, the precursor polymers with a higher Mn were fully cyclized at a lower temperature than one with a lower Mn. The degradation temperatures of the polymers were recorded in the ranges of 396~482 ℃ in N2, and 276~397 ℃ in air. PCFC results showed that the heat release(HR) capacity and total heat release(total HR) values of the PHAs were increased with increasing molecular weight of MPEG. In case of PHA 4 annealed at 290 ℃, the values of HR capacity were siginificantly decreased from 253 J/gK to 42 J/gK, and 60% weight loss temperatures increased from 408 ℃ to 856 ℃ with an annealing temperature. The activation energy for the decomposition reaction of the PHAs showed in the range of 129.3～235.1 kJ/mol, which increased with increasing conversion. Tensile modulus of PHAs were decreased as increasing chain of MPEG, and showed an increase more than initial modulus after converted to PBOs. The blend of Poly(amic acid) and polyhydroxyamide having pendent was studied by thermal properties, mechanical properties, and morphology. Solubility of the blend was soluble in polar aprotic solvents such as DMAc, DMF, and DMSO. The 5% and max degradation temperatures of the blends were recorded in the ranges of 348～407 ℃ and 589～615 ℃in N2, as they were increased with increasing PAA content. After annealing, tensile strength and initial modulus of blend were showed in the ranges of 64.63～97.50 MPa, and 2.11～2.67 GPa, as they were increased with increasing PAA content. As morphology of blends, the domain size of PHA was 0.025～0.05 ㎛, degree of dispersion showed uniformly well, interfacial compatibility of two phase confirmed good. Nanocomposite was prepared from a PI and PBO precursor, PAA, PHA, and an organoclay. The nanocomposite were characterized by FT-IR, TGA, X-ray diffraction, sacnning electron microscopy(SEM), transmission electron microscopy(TEM), limitting oxygen index(LOI). The organoclay was formed by a cation exchange reaction between a Na+-montmorillonite clay and an ammonium salt of dodecylamine. The thermal decomposition temperature of nanocomposite was improved after adding an organoclay. X-ray diffraction, SEM, and TEM analyzes showed that a formation of nanocomposite as the organoclay was dispersed in the PAA matrix as a nanometer scale. The LOI values of nanocomposite showed in the range of 40～41.3, as they were increased with increasing an organoclay content.