Thermal imidization

Tsai then applied thick films of the polyamic acid of PMDA and 4-BDAF to polished silver substrates and thermally imidized the films. The substrates were immersed into liquid nitrogen, causing the films to delaminate and XPS was used to examine the polyimide and silver fracture surfaces (see Fig. 33). The C(ls) spectra of the silver fracture surface were very similar to those of neat polyamic acid, indicating that imidization was inhibited by interaction of the polyamic acid with the silver substrate. This was evident from the observation of two peaks near... 

The thermal imidization of a polyamic acid film (PMDA-ODA or BPDA-ODA) obtained by casting an NMP solution leads to an amorphous polyimide. Two different teams have shown that a polyamic acid solutions in NMP heated at 200°C for a short time (20 min) gives polyimide particles fully cyclized and highly crystalline, as shown by X-ray diffraction and solid 13C NMR spectroscopy.151152 The chemical imidization of the same solution gives only amorphous particles. The difference between the cyclization of a solution and a casted film in the same solvent is intriguing. In the case of the solution, the temperature and the heating time are lower than in the case of the casted film as a consequence, a less organized structure would be expected for the particle. 

Thermal gravimetric analysis (tga), 18 851 Thermal imaging, of plastics, 19 589 Thermal imidization, 20 269-270, 282 Thermal indicators, virtual two-way SMA devices in, 22 348-349 Thermal iniferters, 14 297 Thermal insulation... 

Cobalt(II) chloride was dissolved in poly(amide acid)/ N,N-dimethylacetamide solutions. Solvent cast films were prepared and subsequently dried and cured in static air, forced air or inert gas ovens with controlled humidity. The resulting structures contain a near surface gradient of cobalt oxide and also residual cobalt(II) chloride dispersed throughout the bul)c of the film. Two properties of these films, surface resistivity and bullc thermal stability, are substantially reduced compared with the nonmodified condensation polyimide films. In an attempt to recover the high thermal stability characteristic of polyimide films but retain the decreased surface resistivity solvent extraction of the thermally imidized films has been pursued. 

Flexible copper-clad laminates with nonthermoplastic polyimides were prepared in a two-step process entailing isolating the polyamic acid and then thermally imidizing to the corresponding polyimide. Plasma or thermal treatments of selected polyimides generated materials with excellent bonding strength when evaluated on laminated copper foil. 

In most of the previous work with polyimide fibers, the fibers were spun from poly(amic acid) precursors, which were thermally imidized in the fiber form. However, high degrees of imidization were not achieved. Thus, tensile properties of these polymers were not as good as those of high-performance fibers. Work in our laboratories has shown that when the fibers are spun directly from preimidized polymers, it is possible to achieve tensile properties that are as good or even better than those of poly(p-phenyleneterephthalamide) (PPTA or Kevlar ) fibers. For example, fibers have been prepared from m-cresol solutions of BPDA-PFMB using a dry-jet wet-spinning method. The as-spun fibers were then extensively drawn and annealed above 400°C to achieve excellent mechanical properties. 

Thermal imidization presents an interactive array of desired and undesired reactions, which may lead to complete imidization under driving conditions. On the other hand, chemical imidization presents a different set of synthetic challenges on which we will soon publish. 

Shirai et al. [71] also synthesized a novel class of polyimides containing M(II) phthalocyanine rings by solution condensation in iV-Me-2-pyrrolidone of M(II) [2,9, or 10,2,16 or 17-bis(3,4-dicarboxy benzoyl)] phthalocyanine dianhydride with 2,6-diaminopyridine followed by thermal imidation [71] (Fig. 23). 

An all aromatic polyetherimide is made by Du Pont from reaction of pyromellitic dianhydride and 4,4,-oxydianiline and is sold as Kapton. It possesses excellent thermal stability, mechanical characteristics, and electrical properties, as indicated in Table 3. The high heat-deflection temperature of the resin limits its processibility. Kapton is available as general-purpose film and used in applications such as washers and gaskets. Often the resin is not used directly rather, the more tractable polyamide acid intermediate is applied in solution to a surface and then is thermally imidized as the solvent evaporates. 

The rate is slower in basic aprotic amide solvents, and faster in acidic solvents such as / -cresol. In general, the imidization reaction has been shown to be catalyzed by acid (14,32,33). Thermal imidization of poly(amic acid)s is catalyzed by tertiary amines (34). High temperature solution polymerization in -cresol is often performed in the presence of high boiling tertiary amines such as quinoline as catalyst. Dialkylaminopyridines and other tertiary amines are effective catalysts in neutral solvents such as dichlorobenzene (35). Alkali metal and zinc salts of carboxylic acids (36) and salts of certain organophosphorus compounds (37) are also very efficient catalysts in one-step polycondensation of polyimides. 

Adhesion of polyimides to inorganic substrates is of great importance to the microelectronics industry [1, 2]. The polyimide films are deposited most often by spin coating the polyamic acid (PAA) usually from a TV-methylpyrrolidone (NMP) solution onto the substrate surface followed by thermal imidization at temperatures up to 400<>C. The most studied polyimide is the pyromellitic dianhydride-oxydianiline (PMDA-ODA), which exhibits excellent mechanical and dielectric properties, but not so good adhesion characteristics. The latter has been generally overcome by application of an adhesion promoter, such as y-aminopropyltriethoxysilane [3-7]. The reactions of APS (coated from water solution) with the silicon dioxide surface as well as with polyamic acid have been well characterized by Linde and Gleason [4] however, we do not have such detailed information available on APS interaction with other ceramic surfaces. 

The substrates used in this study were (0001) sapphire (A1203), (001) magnesia (MgO), and amorphous fused silica (Si02). All substrates were obtained with surface finish to 0.025 / m, and were cleaned with isopropylalcohol (IPA) prior to PA A or APS application. The surfaces and interfaces after peel test were characterized using X-ray photoelectron spectroscopy (XPS). The PMDA-ODA PAA was cast from NMP solution. Figure l shows the structure of the PAA and the thermally imidized PMDA-ODA polyimide. 

Figure 1. Structure of PMDA-ODA polyamic acid and the corresponding imide after thermal imidization.

Not only do the chemical structure and the molecular weight affect the processability but also the method of synthesis, in particular the imidation step. Thermally imidized polyimides are always less tractable than solution imidized polyimides. That is because thermally imidized polyimides can undergo cross-linking, and because thermal treatment (about 300 °C) favour chains packing and provide higher molecular order than that achievable by solution imidation. Therefore, solution imidation is always preferable when thermoplastic polyimides are to be developed. 

Bott RH, Summers JD, Arnold CA, Blankenship CP Jr, Taylor L T, Ward TC, McGrath JE (1988) Poly(imide siloxane) segmented copolymer structural adhesives prepared by bulk and solution thermal imidization. SAMPE J 24(4) 7... 

Twelve samples of each type of polyimide were prepared by spinning the polyamic acid precursor on 3" quartz wafers followed immediately by thermal imidization of the films. Half of the samples of each type of polyimide were cured supported and the other half were cured unsupported. A 14% solution of the PI-2545 polyamic acid and a 19% solution of the PI-2555 polyamic acid in l-methyl-2-... 

The solubility of the polyimide dictates, to a large extent, the synthetic route employed for the copolymerization. The ODPA/FDA and 3FDA/PMDA polyimides are soluble in the fully imidized form and can be prepared via the poly(amic-ac-id) precursor and subsequently imidized either chemically or thermally. The PMDA/ODA and FDA/PMDA polyimides, on the other hand, are not soluble in the imidized form. Consequently, the poly(amic alkyl ester) precursors to these polymers were used followed by thermal imidization [44]. For comparison purposes, 3FDA/PMDA-based copolymers were prepared via both routes. The synthesis of the poly(amic acid) involved the addition of solid PMDA to a solution of the styrene oligomer and diamine to yield the corresponding poly(amic acids) (Scheme 8). The polymerizations were performed in NMP at room temperature for 24 h at a solids content of -10% (w/v). Chemical imidization of the po-ly(amic-acid) solutions was carried out in situ by reaction with excess acetic anhydride and pyridine at 100 °C for 6-8 h. The copolymers were subjected to repeated toluene rinses in order to remove any unreacted styrene homopolymer. 

Another group of thermally stable polymeric materials for electronic application includes poly(ether-imide-benzoxazolejs. Mercer and McKenzie [228] prepared the polymer by polycondensation of 2,2 -bis[2-(4-aminophenoxy)benzoxazole-6-yl] hexafluoropropane with pyromellitic, biphenyl, benzophenone etc., followed by thermal imidization of polyamic acid films. The polymers showed an onset temperature for polymer degradation in the range of 424°-456°C and glass transition temperatures in the range of 299 -337°C. 

M. Hasegawa, T. Matano, Y. Shindo, and T. Sugimura, Spontaneous molecular orientation of polyimides induced by thermal imidization. 2. In-plane orientation, Macrorrwlecules, 29, 7897-7909 (1996). 

Certain methods for making PI foams utilize solutions of diamines and dianhydrides or dianhydride derivatives in a low-molecular-weight alkyl alcohol solvent. The precursor solutions or powders are then processed into foams through the expulsion of water and alcohol during a thermal imidization process. Some processes require the application of microwave radiation to initiate the foaming process. 

---