Polyimides experimental materials

These plastics, also known as pyrrones, are experimental materials prepared from aromatic dianhydrides and aromatic tetraamines. The polymer syntheses provide soluble prepolymers that are converted to the pyrrone structures by thermal or chemical dehydration. The precursors can be used to cast films or coatings, or can be molded under very high pressures into filled or unfilled forms. The pyrrones combine some of the best properties of the polybenzimidazoles and polyimides. The pyrrone films are exceptionally radiation resistant and retain their strength properties after 10,000 megarads of 1-MeV electrons. 

Polyimide film Hitachi Corp. reports matched CTE, thin high temperature film, good dielectric (Note other polyimides have a CTE of 30 to 50 ppm/°C) Experimental material, moisture absorption, hygroscopic expansion, bonding problems... 

The first experimental material based on this type of model comprised a blend of two polymers—a low-MW polyimide (7) containing multiple 7r-electron-poor diimide units and a low-MW polysiloxane (8) containing tv-electron-rich pyrenyl end-groups (Figure 14). ... 

The sizes and concentration of the free-volume cells in a polyimide film can be measured by PALS. The positrons injected into polymeric material combine with electrons to form positroniums. The lifetime (nanoseconds) of the trapped positronium in the film is related to the free-volume radius (few angstroms) and the free-volume fraction in the polyimide can be calculated.136 This technique allows a calculation of the dielectric constant in good agreement with the experimental value.137 An interesting correlation was found between the lifetime of the positronium and the diffusion coefficient of gas in polyimide.138,139 High permeabilities are associated with high intensities and long lifetime for positron annihilation. 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

Fluorinated polyimides have achieved great importance as barrier materials during the last few years. Many experimental polyimides prepared from fluorine-containing monomers, mainly novel diamines, show an advantageous balance of permeability and selectivity for technical gases and vapours, which makes them very attractive for the fabrication of permselective membranes [119]. This is an application field showing very rapid expansion, where there exists a strong demand for new polymeric materials, and where soluble aromatic polyimides are considered as a real alternative [136-146]. 

The fracture behavior of an acetylene-terminated polyimide has been experimentally investigated. Engineering fracture methods were adapted and applied to small quantities of this material. Miniature compact-tension (CT) specimens were fabricated and subjected to a programmed series of post-cure cycles. Fracture tests were then conducted at room temperature and at several elevated temperatures in a nitrogen purged environment. The fracture toughness was found to vary with temperature and to depend upon post-cure. These Fracture tests clearly discriminate between the specimens resulting from the different cure cycles with only a small investment of material. 

The different models include many material parameters and several of these parameters are obtained from fitting of experimental data and have to be adjusted to fit each polymer [9,63]. It is worth mentioning in this context that polyimide is probably the most studied polymer in laser ablation and is also the material for which most ablation models are applied, but great care has to be taken for which type of polyimide the data have been obtained. Polyimide describes a whole group of polymer that can range from soluble polymers to insoluble films and even photosensitive polymers with very different properties [10]. Even products with the same name, such as Kapton , are not one polymer, but there are also many different types of Kapton that are defined with additional letters, for example, HN. 

White [25] investigated the transport properties of a series of asymmetric poly-imide OSN membranes with normal and branched alkanes, and aromatic compounds. His experimental results were consistent with the solution-diffusion model presented in [35]. Since polyimides are reported to swell by less than 15%, and usually considerably less, in common solvents this simple solution-diffusion model is appropriate. However, the solution-diffusion model assumes a discontinuity in pressure profile at the downstream side of the separating layer. When the separating layer is not a rubbery polymer coated onto a support material, but is a dense top layer formed by phase inversion, as in the polyi-mide membranes reported by White, it is not clear where this discontinuity is located, or whether it wiU actually exist The fact that the model is based on an abstract representation of the membrane that may not correspond well to the physical reality should be borne in mind when using either modelling approach. 

The experimental investigations of the mechanical properties of various organic materials after low temperature irradiation are very few, although many studies have been made after room temperature irradiation. Van de Voorde studied the influence of 77 K and 20 K irradiations on mechanical properties and reported that polyimides and aromatic-based epoxies have a good radiation resistance. The present studies show that polypropylene, polycarbonate, and Mylar have low radiation resistance due to chain fracture, and these materials are too brittle to permit mechanical testing. 

---