Microencapsulation and characterization of polyamic acid microcapsules containing n-octadecane via electrospraying method

As an intermediate polymer of polyimide, polyamic acid (FAA) was synthesized by ternary copolymerization of pyromellitic dianhydride (PMDA), 4,4'-oxydianiline (4,4'-ODA) and 2,6-diaminoanthraquinone (2,6-DAAQ). Furthermore, spherical core/shell PAA/n-octadecane microcapsules were produced via coaxial electrospraying. The molecular structure of FAA was determined by fourier transformed infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (H-1 NMR). The morphology, microstructure and thermal properties of FAA microencapsulated n-octadecane were investigated by field-emission scanning electron microscopy (FE-SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. The diameters of microcapsules ranged from 0.5 to 3 mu m, and the surface morphology was influenced by the core (n-octadecane) solution feed rate. With core feed rate increasing, the surface of microcapsules turned from smooth and uniform to sags and crests, moreover, their mean diameter increased and then decreased. Meanwhile, the DSC results revealed that microcapsules had relatively high enthalpy (similar to 107 J/g) and associated by supercooling phenomenon. Due to the excellent thermal stability of shell materials, the 40% weight loss temperature of FAA microcapsules reached as high as 500 degrees C.