The Study Of Structure And Properties Of Polyimide Based On Benzidine

Abstract: Aromatic polyimides (PIs) have been extensitvely studied fortheir synthetic simplicity, and outstanding properties. Fully rigidpoly(4,4—biphenylene pyromellitimide) PMDA-BZD exhibits highthermal stability, low moisture absorption and coefficient of thermalexpansion, but it's very brittle. Homopolyamic and copolyamic acid wereprepared from: pyromellitic dianhydride, 3,3'4,4'-oxydiphthalicanhydride and benzidine(BZD) in N-methyl-2-pyrrolidone. Polyamicacids were imidazed by different technics to obtain in the correspondingpolyimides. The effects of macromolecular chain and imidization technicson the crystal morphology, crystallinity and the degree of order of PIpowders and films were studied using WAXD, POM, SEM, etc. Themechanical properties of different films were obtained through an InstronUniversal Tester, and the morphology of the break surface was observed by SEM. Further, the effects of PI aggregation structure on the moistureabsorption ability and the dimension stability of PI films wereinvestigated.Four imidazation technics were employed: imidazation in PAAsolution at 180℃(Technic 1); imidazation of PAA coating film at hightemperature(Technic 2); imization of PAA coating film by stagedtemperatures(Technic 3); mixed imidazation of PAA (Technic 4).Homo-PI and co-PI powders were prepared by Technic 1. SEM showedthat ODPA-BZD particles were strip-like, while PMDA-BZD particleswere elliptical balls. The morphology of co-PI powders gradually changeswith increase of the content of ODPA in molecular chains. Thecystallinity of different samples are as follow: PMDA-BZD PI, 75.84％PMDAODPA(8/2)-BZD PI, 65.68％; PMDA/ODPA(6/4) PI, 50.51％;PMDA/ODPA(5/5) PI, 52.06％; PMDA/ODPA(4/6) PI, 30.82％;PMDA/ODPA(2/8) PI, 40.19％; ODPA-BZD PI, 55.08％. It could be seenthat as the proportion of ODPA increase, the cystallinity first drops, andthen rises when PMDA/ODPA=2/8. The crystal ability of PMDA-BZD isbetter than ODPA for its fully rigid structure.Deformed films of PMDA-BZD PI and ODPA-BZD PI wereprepared by Technic 2. By controlling film preparing condition, both PIfilms formed spherulits.PI-1 series and PI-2 series films, whose heating procedure wasdifferent from each other, were prepared Technic 3. PMDA-BZD homo-PIfilm is very brittle; ODPA-BZD homo-PI film is flexible; the brittlenessof PMDA-BZD is successfully improved by introducing into ODPA.WAXD and Mechanical Test revealed that, with the increase of ODPA, forboth kinds of films, the degree of order first decreased, and then went up,respectively; tensile strength, initial modulus and break elongation allrose. PI-2 films'the degree of order, tensile strength and intial modulus,which was prepared by slower heating procedure were a little than that of corresponding PI-1 films which were prepared by faster heatingprocedure. That is to say, films with have different order can be preparedby controlling heating procedure; tensile strength and initial modulus ofthe PI-2 films are a little higher than corresponding PI-1 films, but theformer's break elongation is a little lower than the latter. ForPMDA/ODPA (5/5)-BZD alternating copolyimide and block copolyimide,the former has higher the degree of order and mechanical properties.Tensile strength, initial modulus and elongation were 196.97MPa,2.99GPa and 9.70％, respectively.The moisture absorption and CTE data of PI-2 series films areobtained. With the content of ODPA increasing, saturation moistureabsorption first increased and reached the maximume when the content ofODPA reached 60％, then dropped. The change is related to the degree oforder of films. CTE data became smaller and smaller with the content ofPMDA increasing. The trend is related to the compactness degree ofmacromolecular chains.Finally, Pc series films are studied, which are prepared by Technic 4.First, PMDA/ODPA (5/5)-BZD block copolyamic acid was prepared.Second, chemical imidazated of PAA was carried out in solution usingacetic anhydride/pyridine and PAA films were made. Third, three kinds ofPAA films whose acetic content were 40％, 50％and 60％, were imidizedat high temperature to produce Pc series films. Films' tensile strengthreached 200MPa, initial modulus reached 4.4GPa, and elongations werearound 9％, The relationship between mechanical properties and thecontent of acetic needed to be further studied.