Polyimide with improved processability

This review article deals with aromatic polyimides that are processable from the melt or soluble in organic solvents. Conventional aromatic polyimides represent the most important family of heat resistant polymers, but they cannot be processed in the melt, and their application in the state of soluble intermediates always involves a hazardous step of cyclodehydration and elimination of a non-volatile polar solvent. A major effort has therefore been devoted to the development of novel soluble and/or melt-processable aromatic polyimides that can be applied in the state of full imidation. The structural factors conducive to better solubility and tractability are discussed, and representative examples of monomers showing favourable structural elements have been gathered and listed with the chemical criteria. Experimental and commercial aromatic polyimides are studied and evaluated by their solubility, transition temperatures and thermal resistance. An example is also given of the methods of computational chemistry applied to the study and design of polyimides with improved processability. 

This book reviews some of the results of investigations of chemical conversions of chloral and TNT to new aromatic di(poly)amines and aromatic tetracarboxylic acid dianhydrides useful for the preparation of new polyimides combining good thermal, mechanical and electrical properties with improved processability. 

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. 

The variety of ways in which chloral and trinitrotoluene (TNT) derivatives can be used to prepare novel polyimides and polymeric materials is very promising. The use of chloral and TNT derivatives allows for the synthesis of a large number of monomers which, in turn, can impart a variety of useful properties to their respective polymers. The possibility of preparing, from available raw materials, high-molecular-weight compounds with increased heat and thermal resistance in combination with improved solubility and, consequently, easiness of processing is especially attractive and may provide impetus for further work in this field. 

One of the most important developments in the last decade was the increasing need for polyimides with various specific properties such as improved processability. New monomers were developed to meet such needs. New monomers as well as monomers which have become available recently because of new improved processes, are discussed below. 

Polyimides (PI) were introduced in 1962 as thermally non-processable Kapton . To improve processability, the main-chain flexibility was enhanced by incorporating segments with higher mobility, viz. polyamide-imide (PAl), polyether-imide (PEI), polyimide-sulfone (PISO), etc. These polymers are characterized by high T = 150-420°C and thermal resistance. They are blended with PPS to enhance its moldability, thermal stability and mechanical performance. 

The physical mixing of two or more polymers to crate a material with properties different from each of the components has become an increasingly popular route to new materials development. The resulting blend or alloy greatly reduces the associated time and costs while permitting improved processibility and enhanced properties tailored to specific application areas. Many commercial examples of two-phase polyblends consist of a matrix polymer impact modified by the addition of rubber particles. Recently, however, TLCPs have received increasing attention in the scientific and technical literature as in situ reinforcements in polymer blends and microcomposites. The matrices examined in the literature include polyimides, PES, PEI, PEEK, polycarbonate, PET, PPS, and polyarylate. 

There are several other related structures, some of which are thermosets. The significance of these polymers is their physical stability at peak temperatures of 300 C and above, together with high chemical stability. There are combinations of polyimides with polyamides and other modifications that aim at improved processability. The original polyimide is very difficult to process, being shaped only via compression or film casting, or by fiber spinning from solution. 

We report on the positive alkali-developable photosensitive polyimides based on an alkali-soluble polyimide precursor as a base polymer and diazonaphthoquinone (DNQ) sensitizer to improve process stability and sensitivity. Polyamic acid ester with pendant carboxylic acid (PAE-COOH) showed good dissolution behavior in aqueous alkali developer. The dissolution rate of PAE-COOH was controlled by the content of pendant carboxylic acid. It was found that a photosensitive system composed of butyl ester of PAE-COOH and a DNQ compound can avoid the residue at the edge of hole patterns (footing) after development, while that of methyl ester of PAE-COOH showed the residue. A DNQ compound containing sulfonamide derived from diaminodiphenylether renders improved sensitivity compared with DNQ compounds derived from phenol derivatives. 

To improve the processability of PI and to alleviate or eliminate volatiles, work began in the late 1960 s under NASA Lewis sponsorship (23), to end-cap oligomers with reactive groups. The first report involved the use of 3,6-endomethylene-l,2,3,6-tetrahy-drophthalic anhydride (5-norbornene-2,3-dicarbo ylic anhydride, nadic anhydride) and alkyl derivatives thereof (e.g. citraconic anhydride) and 1,2,3,4-tetrahydrophthalic anhydride (4-cyclohexene-1, 2-dicarboxylic anhydride) as the reactive end group on imide oligomers (23-25). This initial effort led to the development of P13N [with T for polyimide, 13 for Mn of 1300g/mole and N for nadic end cap] whose structure is shown below. 

The thermal polymerization of reactive polyimide oligomers is a critical part of a number of currently important polymers. Both the system in which we are interested, PMR-15, and others like it (LARC-13, HR-600), are useful high temperature resins. They also share the feature that, while the basic structure and chemistry of their imide portions is well defined, the mode of reaction and ultimately the structures that result from their thermally activated end-groups is not clear. Since an understanding of this thermal cure would be an important step towards the improvement of both the cure process and the properties of such systems, we have approached our study of PMR-15 with a focus only on this higher temperature thermal curing process. To this end, we have used small molecule model compounds with pre-formed imide moieties and have concentrated on the chemistry of the norbornenyl end-cap (1). 

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