Preparation And Thermal Property Of Polyimide Microspheres Synthesized From Various Monomers

Aromatic polyimides are considered to be a class of high-performance polymeric materials for their excellent thermal, mechanical, chemical stability and electrical properties. It has been generally recognized that polyimides can be prepared from thousands of dianhydrides and diamines with various chemical structures, synthesis technology of polyimides has attracted widespread attention according to application purpose. For instance, the monomers containing stiff structural elements and flexible linkages have been discussed for the high thermal stability and better solubility of polyimides, respectively. In order to meet the requirement of specific performance, choosing and/or designing monomer to synthesize different molecular structural polyimides are practical. Based on the previous research work, the non-aqueous emulsion, in which polymer concentration can be increased and microspheres agglomeration avoided, and different monomers are selected to prepare polyimide microspheres.In this thesis, non-aqueous emulsion is consist of N, N-dimethyl formamide/Pluronic-F127-Span85/Liquid paraffin. Poly(amic acid) precursors were fabricated by reacting with dianhydride (pyromellitic dianhydride; 3,3',4,4'-benzophenonetetracarboxylic dianhydride) and diamines (diaminonaphthalene; 1,4-phenylenediamine; 4,4'-oxydianiline) in non-aqueous emulsion, and corresponding polyimide microspheres were prepared by chemical and thermal imidization. The results of research were as follows:(1) In order to determine polymerization time, the viscosity during the polymerization process was observed. The result shows that polymerization time of PAA (BTDA/ODA), PAA (BTDA/PPD), PAA (BTDA/NDA) and PAA (PMDA/ODA) are 320min,280min,220min and 180min respectively.(2) Polyimide microspheres had prepared using monomers BTDA, NDA, PPD and ODA. It was found that the chemical structure of polyimides influenced the average size and morphology of particles formed. For the monomer BTDA-ODA, the better morphology of microsphere was got. The sequence of microspheres'average size is:PI (BTDA/ODA)< PI (BTDA/PPD)< PI (BTDA/NDA). In addition, TG was used to determine the thermal stability of polyimide microspheres as indicated by 5% weight losses at a high temperature above 495℃in both nitrogen and air atmospheres. And the glass-transition temperature (by DSC) of PI (BTDA/NDA), PI (BTDA/PPD) and PI (BTDA/ODA) are 223℃,245℃and 261℃respectively.(3) Polyimide microspheres had prepared using monomers BTDA, PMDA and ODA. It was found that the chemical structure of polyimides influenced the average size and morphology of particles formed. For the monomer BTDA-ODA, the better morphology of microsphere was got. The sequence of microspheres'average size is:PI (BTDA/ODA)< PI (PMDA/ODA). In addition, TG was used to determine the thermal stability of polyimide microspheres as indicated by 5% weight losses at a high temperature above 495℃in both nitrogen and air atmospheres. And the glass-transition temperature (by DSC) of PI (PMDA/ODA) and PI (BTDA/ODA) are 239℃and 223℃respectively. Meanwhile, the higher temperature of imidization, the thermal stability is better.(4) Polyimides were characterized by FTIR, which showed complete imidization can obtain by "two-step imidization" i.e., chemical followed by thermal imidization. Both thermal imidization time and temperature influenced the imidization degree.