ISOTHERMAL IMIDIZATION OF AN AROMATIC POLYIMIDE PRECURSOR STUDIED BY FLUORESCENCE SPECTROSCOPY

1, 2-Bis(2, 4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) or 1-palmitoyl-2-(2, 4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (POPC) was dispersed in water by sonication to form liposomes. DODPC liposomes were polymerized by UV-irradiation or with radical initiator such as azobis(2-amidinopropane)dihydrochloride (AAPD) at 60°C and methanolyzed to obtain THF-soluble polymers. The molecular weight distribution obtained from a gel permeation chromatography reveals that the UV-irradiation provides low-molecular-weight products and a small amount of polymers, while free radical polymerization gives polymers having relatively high-molecular-weight. The molecular weight of the phospholipid polymers is also influenced by the assembling state. The constrained lipid packing of small unilamellar POPC liposomes provides relatively low-molecular-weight polymers in comparison with those of larger liposomes. In addition, POPC liposomes polymerized at the temperature above the gel-to-liquid crystalline phase transition temperature (Tc) show the higher molecular weight of polymers than those polymerized below the TcIsothermal imidization of an aromatic polyimide (PI) precursor, poly(amic acid) (PAA) derived from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylene diamine (PDA) was studied in solution and the solid state using intermolecular charge-transfer (CT) fluorescence reflecting sensitively the degree of molecular aggregation of polyimides. Isothermal imidization was carried out in a hand-made furnace which makes possible to jump the temperatures of samples to the established reaction temperatures by inserting the samples into a slit in the temperature-controlled heating block. The degree of imidization (i) was determined by IR spectroscopy. When imidized at 110°C in solution (10 wt%), the fluorescence of the as-cast films reduced rapidly and became very weak at about 30% in conversion. In the solid state, as imidization proceeded, the fluorescence of the PAA reduced rapidly as well as in solution, and then the CT fluorescence of the PI increased. The relationship between the intensity and the conversion clearly shows that the fluorescence intensity at a same conversion depends strongly on the imidization temperature (Ti). For example, when imidized at 150°C the intensity increases only slightly as the reaction proceeds, while at higher Ti such as 230 or 270°C the intensity increases rapidly. This indicates that the degree of the molecular order in the PI formed on isothermal imidization depends strongly on T. © 1990 The Society of Polymer Science, Japan.