Study Of The Toughening Of Penylethynyl-terminated Polyimide And Its Composite Materials

Advanced polymer-based materials have been widely used because of its high strength, high modulus, corrosion-resistant, low density and good dimensional stability. These materials can be used as the structure materials in the aircraft industry due to the low weight reduction of 20-30% as compared to other materials. With the development of high speed aircrafts, harsh atmospheric conditions require high temperature performance-grade materials. Exploration of new type of high temperature performance-grade materials has become a very challenging task.Polyimide is a kind of aromatic heterocyclic polymers with the molecular structure containing imide bonds. It possesses excellent high temperature performance, excellent mechanical properties, thermal dimensional stability and dielectric properties. It is an integrated high performance specialty engineering plastics that can be used for a long time at 250oC. Thermosetting polyimide has attracted considerable attention because of its application at high temperatures. BMI Resins are a class of thermosetting polymers e.g. PMR-15 that are gaining acceptance by industry because they can be used at elevated temperatures 288-316 oC and in wet environments.Thermosetting cross-linked polyimide (PI) prepolymer was synthesized from 4, 4'-Bisphenol A dianhydride (BPADA) and 3-aminophenylacetylene(APA) through a two-step polycondensation. FT-IR spectrophotometer was used to monitor the hydrogen in acetylene for studying the degree of curing at different temperatures in the curing process. It exhibited low melting temperature before curing, thus a wide processing window and excellent processing performance were expected. After crosslinking it showed good thermal and dimensional stability so that its potential application as resin-based composite materials may be realized in the field of aerospace.However, thermosetting resins after curing became more brittle with lower impact resistance and stress cracking because of higher cross-linking density, which limited its application. Therefore, toughening of thermosetting resins became a subject of interest.There are many ways to toughen thermosetting resins in which the introduction of thermoplastic resin in the thermosetting resins provides the most outstanding comprehensive performance. By controlling the curing conditions, the system can acquire a suitable phase structure such as interpenetrating polymer network (IPN) or phase inversion. This is an effective way to improve the toughness of polymers, which not only increases the toughness of the resin, but also maintains good thermal,chemical and dimensional stability. There are several factors for phase structure: 1. structure and positions of functional groups of the polymers as well as molecular weight: the structure of thermoplastic polymers, the MW and existence of functional groups; 2. curing conditions including temperature, time and solvent selection etc. 3. The compatibility between two kinds of polymers, because compatibility is decisive factor in phase separation and the final phase structure.Therefore, we first selected polyether ether ketone with fluorinated flexible group as a toughening agent to blend with thermosetting resins. The reason for selecting this structure is the low molecular weight and ether moieties in the main chair and fluorinated groups which make the distribution of the molecular chain easy. By studying the processing conditions, we found a suitable solid content of the toughening agents; Blends formed the structure of phase separation or phase inversion by controlling curing conditions. The results of tests showed that the two kinds of polymers are incompatible or partially compatible. SEM studies showed further validates after dissolving toughening parts. Although the thermal stability at high temperatures were affected of some extent, but they still exhibited their high temperature nature. As the concentration of blend films was low, mechanical properties of thin films cannot be tested. We selected three different thermoplastic polyimides as toughening agents, using a diamine and three different anhydrides to study different molecular structures that affect toughening. It was found that the introduction of flexible groups could lead to better properties such as phase separation and phase inversion, thus better toughening effect could be obtained. Molecular symmetry of toughening agents also has effect on results. Research and testing showed that two kinds of polymers are incompatible or partially compatible; they maintained high-temperature thermal stability, the thermal weight loss temperatures of part of the system were increased; Phase separation or phase inversion occurred in blends; The transmittance of films has also been improved at the same time.Using thermoplastic polyimides with side groups of benzene act as a toughening agent to introduce thermosetting polyimide system, we studied the effect of toughening with different solutions. The results showed that solvent have little influence on toughening. The phase separation was easier because of the large space volume and size caused by side group of benzene; thermal stability was not affected and transmittance of films was improved.To further study the toughness effect of carbon fiber reinforced resin material with the thermoplastic toughening agent, we prepared three carbon-fiber reinforced composite materials with resin had been toughness. We had preliminary research about the processing technology and the toughening mechanism, and study the toughening effect and phase morphology. CAI value showed that the toughness effect was improved by 30%; tensile tests also proved that the mechanical properties of composite materials have been improved; in the morphological study it was found that the phase separation between layers of the resin composites; thermal stability retained the same standard. As a conclusion, it has potential research value and applications in the aerospace fields.