Polyimide systems

Relatively few processible polyimides, particularly at a reasonable cost and iu rehable supply, are available commercially. Users of polyimides may have to produce iutractable polyimides by themselves in situ according to methods discussed earlier, or synthesize polyimides of unique compositions iu order to meet property requirements such as thermal and thermoxidative stabilities, mechanical and electrical properties, physical properties such as glass-transition temperature, crystalline melting temperature, density, solubility, optical properties, etc. It is, therefore, essential to thoroughly understand the stmcture—property relationships of polyimide systems, and excellent review articles are available (1—5,92). 

Currendy, epoxy resins (qv) constitute over 90% of the matrix resin material used in advanced composites. The total usage of advanced composites is expected to grow to around 45,500 t by the year 2000, with the total resin usage around 18,000 t in 2000. Epoxy resins are expected to stiH constitute about 80% of the total matrix-resin-systems market in 2000. The largest share of the remaining market will be divided between bismaleimides and polyimide systems (12 to 15%) and what are classified as other polymers, including thermoplastics and thermoset resins other than epoxies, bismaleimides, cyanate esters, and polyimide systems (see Composites,polymer-matrix-thermoplastics). 

The lack of easy processability initially limited the utilization of this high-performance material. Several different modifications of the polyimide system have successfully produced... 

The concentration of the siloxane introduced into the copolymer is most easily determined by proton NMR for soluble systems, as briefly referred to in Sect. 3.1. This was demonstrated by Summers et al. [45] and Arnold and coworkers [46-50], as well as Rogers et al. [52]. Others have routinely conducted these experiments to establish the copolymer composition. Infrared spectroscopy has been particularly useful for demonstrating the transformation of the amic acid, which is often an intermediate for the final imide form. The assignments have been noted in many of the polyimide reviews referred to earlier [1-8]. In addition, it is useful to conduct an elemental analysis for silicon as complementary proof of copolymer composition. Solid-state NMR can be used even for intractable polyimide systems to provide a good estimate of the copolymer composition. 

Although some miscible systems exhibit Tg-composition dependencies as defined by these simple equations, many blends cannot be correlated by them or any of the other well known expressions such as the Kelly-Bueche, Gordon-Taly-lor or the Gibbs-Dimarzio relationships [126-128]. However, the existence of thermodynamic miscibility has not been proven for epoxy-polyimide systems. 

Polvimide-Metal Interfaces. Several technological applications including semiconductor packaging and metallization demand a reliable and durable adhesion properties of the metal films. In the development of multilayer devices consist of alternating layers of metal and polyimides several reliable techniques are needed to study both thin films and their interfaces. The usefulness of the nuclear scattering techniques to study the metallization and the associated interfacial elemental diffusion processes under the effects of various temperature and humidity treatments on the metal-polyimide systems, such as Al, Cu, N, and Au on Du Pont Kapton type H have already been reported (21., 22.). Only a couple of examples are presented here to illustrate the ERD application. 

GOLD. The deposition of gold onto polyimide (Fig. 4b and Table IV) is also observed to reduce the carbonyl Cls peak intensity (peak 2 of Cls in Table III) as well as that of the aromatic it - ir Cls (peak 1 of Cls), an effect analogous to that observed for the aluminum-polyimide system. New components also appear at lower energies in the 01s spectrum and there is an isoimide-like component in the Nls spectrum (peak 3). Gold is not generally known to exist in a stable complex form (marginal chemical reactivity) and is... 

The surface chemical structure of several thin polyimide films formed by curing of polyamic acid resins was studied using X-ray photoelectron spectroscopy (ESCA or XPS). The surface modifications of one of the polymer systems after exposure to KOH, after exposure to temperature and humidity, after exposure to boiling water, and after exposure to O2 and 02/CF plasmas were also evaluated. The results showed imide bond formation for all cured polyimide systems. It was found that (a) K on the surface of the polyamic acid alters the "normal" imidization process, (b) cured polyimide surfaces are not invarient after T H and boiling water exposures, and (c) extensive modifications of cured polyimide surfaces occur after exposures to plasma environments. Very complex surfaces for these polymer films were illustrated by the C Is, 0 Is, N Is and F Is line characteristics. 

The purpose of this investigation was to observe the effects on solubility of changing isomeric points of attachment of phenoxy units in the diamine portion of several all-aromatic polyimide systems. Hexafluoropropane- and oxygen-containing dianhydrides were used in this study because of their known value at contributing to polyimide solubility.(6-7)... 

Chemical vapor deposition of polynapthalene differs from the parylene and polyimide systems in terms of the deposition process. As mentioned in earlier sections deposition of parylenes and polyimides occurs on cold surfaces, and the deposition rate decreases with increasing substrate temperature. In other words, deposition is done in a "hot wall" reactor. In contrast, CVD of polynapthalenes is performed in a "cold wall" reactor, meaning that the substrate is maintained at a high temperature ( 350°C) while the surrounding wall temperature is kept at near room temperature. A schematic of the CVD reactor setup employed by Lang et al. can be found in Ref. 28. 

To study the polymer structural effects on PAP, including the size of the chromophore and the mode of connection of the chromophore to the main chain, SHG-PAP studies have been performed both on a true side hain polyimide system in which the chromophore is attached to the polymer backbone via flexible tedier (PI-3a in Figure 8.4 Tg = 228 C), and on donor-embedded polyimide syst s (PI-1 and PI-2 in Figure 8.4 Tg = 350 and 252°C, r pectively), in which the azo chromophore is incorporated into the polymer backbone through the donor substituent without any flexible connector or tether. Details of SHG-PAP can be found in reference 14. In both the side-chain and the donor-embedded polyimides, polar order can be generated by thermal poling... 

Aromatic imines are usually intensely yellow. Other possible byproducts which may be formed in the polyimide system are the previously discussed isoimide and imide-imine formed by the following reaction. 

Bis(maleimides) have been nsefnl in many applications, including the electronic indns-tries, as materials for printed circnit boards and insulators, and in the aerospace industries in matrix resins for strnctnral composites. Alternative polyimide systems have been investigated which are formed via the free-radical polymerization of the maleimide ring. 

Such TTT diagrams have also been useful in describing the cure of polyimide systems, as shown in Palmese (1987), which shows the TTT diagram of a polyamicacid/polyimide system. The TTT diagram of a polycyanurate system is developed by Simon (1993) on the basis of FT-IR, DSC and torsional braid measurements. Kim et al (1993) developed a TTT diagram for a thermoset-thermoplastic blend, specifically a tetrafunctional epoxy-resin/poly(ether sulfone)/dicyandiamide thermoset-thermoplastic blend (Figure 2.9). 

A dynamic method for acquiring and treating infrared spectroscopic data from the imidization of a number of polyimide systems is presented. In situ FT-IR analysis of polymer reactions is preferred when doing comparitive studies on a number of polymer systems. For systems where these reactions occur at relatively high temperatures, it is often difficult to obtain good isothermal data for determining kinetic parameters. Kinetic data for several polyimide systems are shown and compared. 

Activation energies for the polyimide systems tested are shown in Table I, and the calculated reaction rates for imidization are included in Table II. There are some differences in the kinetic... 

That work has been extended to provide even further reductions in the dielectric constants of aromatic condensation polyimides by physically incorporating selected diamic acid additives into low dielectric constant polyimide systems. The monomer selection for the synthesis of these additives was based on the results of the Langley structure-property studies on lowering the dielectric constant of polyimides.CL)... 

Two of the additives were also incorporated into a more conventional polyimide system, pyromellitic dianhydride (PMDA)/4,4 -ODA, to see if the diamic acid additives were effective in lowering the dielectric constant of a higher dielectric constant polyimide also. The evaluation was preliminary in that only two additives were screened at one concentration. As is shown in Table IV the dielectric constant was lowered, though not as significantly as in the BDSDA/4-BDAF system. 

HexceL [Hercules] Themioset phenolic, polyester, bismaleimide, or polyimide systems. 

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