Viscosity polyimides

Epoxies are the most commonly used adhesives (qv). Silver and gold are sometimes added to an epoxy to improve its thermal conductivity. Polyimide, also used as an adhesive, has low shrinkage as well as low viscosity and can be cured at 180°C its primary drawback is a tendency to absorb water, as much as 6% by weight. 

Since successful commercialization of Kapton by Du Pont Company in the 1960s (10), numerous compositions of polyimide and various new methods of syntheses have been described in the Hterature (1—5). A successful result for each method depends on the nature of the chemical components involved in the system, including monomers, intermediates, solvents, and the polyimide products, as well as on physical conditions during the synthesis. Properties such as monomer reactivity and solubiHty, and the glass-transition temperature,T, crystallinity, T, and melt viscosity of the polyimide products ultimately determine the effectiveness of each process. Accordingly, proper selection of synthetic method is often critical for preparation of polyimides of a given chemical composition. 

The melt viscosity of a polymer at a given temperature is a measure of the rate at which chains can move relative to each other. This will be controlled by the ease of rotation about the backbone bonds, i.e. the chain flexibility, and on the degree of entanglement. Because of their low chain flexibility, polymers such as polytetrafluoroethylene, the aromatic polyimides, the aromatic polycarbonates and to a less extent poly(vinyl chloride) and poly(methyl methacrylate) are highly viscous in their melting range as compared with polyethylene and polystyrene. 

Linear phosphonitrilic chlorides (LPNCs), silicone fluids and, 22 573 Linear photodiode arrays, 19 153 Linear polyesters, 14 116 Linear polyethylene fibers, 20 398 Linear polyimides, synthesis of, 20 273 Linear polymers, 20 391 25 455 high molecular weight, 23 733 zero-shear viscosity of, 19 839 Linear poly(thioarylene)s, 23 705 Linear PPS, 23 704. See also... 

NASA LaRC-TOR series polyimides, 20 282 Nasal drug deUvery, 9 48-49 Nasal impact frequency (NIF), 11 520-521 Nasal sprays, 7 273t Nasicon compounds, 18 841 NaSPS ionomers, 14 469, 471, 480 melt viscosity of, 14 473 Nata di Coco, 5 364 Natamycin, 12 59 National Aeronautics and Space... 

In general polyimides derived from BTDA + p,p -DABP yielded rather poor quality, very brittle films which were due in part to the low viscosity of the resulting polyamic acid solution. 

Table 1 lists some of the metal compounds employed and the results obtained when attempts were made to cast films of the resulting metal ion filled polyimide derived from BTDA + m,m -DABP. Brittle films were produced in most cases regardless of whether the added metal ion was hydrated or anhydrous. The relatively low viscosities of the resulting polyamic acid-metal ion solutions no doubt accounted for this. Addition of AlCl3 6H20 or any simple aluminium salt to the polyamic acid produced immediately a rubbery material that could not be cast into a film. 

Further work in this area is underway employing polyamic acid systems which are known to produce higher viscosity solutions (e.g. polyimides derived from 4,4 -oxydianiline and either BTDA or pyromellitic dianhydride). This is being carried out in the belief that higher viscosity solutions will give rise to higher quality, less brittle films and will, thereby, enable a broader spectrum of metal systems to be studied regarding the adhesive and electrical conductance properties of metal ion filled polyimides. 

Influence of Water on Heat Resistance. It is known that the viscosity of polyimide prepolymer (polyamic acid solution) is reduced by the absorption of a-small amount of water(2). This is because the absorbed water reduces the molecular weight by hydrolysis. Therefore, it seems probable that absorbed water would reduce the heat resistance of cured PIQ film. For that reason, the influence of water contained in the starting materials on the heat resistance of cured PIQ film was investigated. 

The solution behavior of poly(amic acids) was until recently, probably the least understood aspect of the soluble polyimide precursor. However, the advent of sophisticated laser light scattering and size exclusion chromatography instrumentation has allowed elucidation of the solution behavior of poly(amic adds). In the early days of polyimide chemistry, when most molecular weight characterization was based on viscosity determinations, a decrease in viscosity was associated with molecular weight degradation [15, 28, 29]. Upon combination of the two monomers an increase in the viscosity to the stoichiometric equivalence point is observed, followed by a decrease in the solution viscosity as a... 

An approach for improved processing of PMR polyimide is the addition of jV-phenylnadimide to the precursor solution of the monomeric reactants. After the in-situ condensation, a PMR-15 resin is obtained which is diluted with JV-phenylnadimide in order to improve the rheological properties of the system (118). The amount of JV-phenylnadimide (PN) added was in the range of 4 to 20 mol %. Just 4 mol % caused a significant and disproportionate reduction of the minimum viscosity with no concomitant loss of thermal stability. 

Certain thermoplastic polyimides possess excellent resistances to high temperatures and chemicals, with Tgs ranging from 217 to 371 °C. Certain polyimides also exhibit excellent toughness and dielectric properties. The melt blending process of polyimides with other thermoplastic polymers is difficult due to polyimides high Tg, high melt viscosities, and incompatibility. A solution process is used, therefore, to achieve a semi-interpenetrating polyimide network... 

Polyimides with better viscosity characteristics have been obtained from 3,3 -diamino-4,4 -bis(dimethylamino)-benzophenone and different aromatic tetracarboxylic acid dianhydrides [21] (Scheme 3.4). 

Even higher viscosities are typical for polyimides based on tetranuclear diamines, e.g., 4,4 -di-(p-aminophenoxy)-benzophenone and l,l-dichloro-2,2 di(/ -aminophenoxy)-ethylene. Polyimides were prepared according to the method for the synthesis of PCA by low-temperature polycondensation in NMP. Polyimides were prepared by catalytic cyclodehydration of PCA directly in the reaction solutions using the catalytic system pyridine-acetic acid anhydride (1 1) [22] (Scheme 3.5). 

An alternative order of introducing acylating reagents onto the bis[3-amino-4-(p-aminophenoxy)]-benzophenone, that is, the addition of first bis(phthalic anhydride) and then phthalic anhydride to the tetramine, yields polyimides containing predominantly N-phthalimide o-substituents (Scheme 3.7). These polyimides are characterised by higher viscosities and softening temperatures (Table 3.8) but poorer solubilities as compared to those polymerised according to Scheme 3.6. 

Viscosity of polyimides prepared by catalytic imidisation of PCA using a pyridine-acetic anhydride complex viscosity measured in trichloroethane (TCE)/phenol. 

Viscosity of polyimides solution (in TCE/phenol) polyimides synthesised by high-temperature condensation in m-cresol. ins insoluble... 

The attempt to synthesise this polyimide in t77-cresol using high-temperature condensation was not very successful since, although the synthesis proceeded in a homogeneous system, the viscosity of the polymer solution in w-cresol was significantly lower than that of the corresponding polyimide prepared using the catalytic PCA-cyclisation process (Table 5.1). 

Comparison of the properties of the polyimides listed in Table 5.7 demonstrates that polyimides based on 3,5-diaminodiphenylsulfone have much lower reduced viscosity when compared with the related polyimide based on 3,5-diaminodiphenylsulfide. This difference is probably due to the negative influence of the electron-withdrawing phenylsnlfone group on the nucleophilic reactivity of the amino groups. 

High-temperature (160-180 °C) polycyclocondensation of the starting monomers in w-cresol, using quinoline as catalyst. In almost all cases polyimides obtained in w-cresol demonstrated higher solution viscosities than the same polymers synthesised in NMP. 

Synthesis of polyimides containing benzothiazole-2-sulfide groups was carried out using high-temperature polycyclocondensation in w-cresol in accordance with Scheme 5.7 [41, 44]. All the polyimide synthesis reactions were homogeneous and yielded polymers having reduced viscosities Pred = 0.29-0.42 dl/g, Tg = 215-265 °C and 10% weight loss temperature = 325-400 °C (Table 5.9). 

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