Aromatic polyimides preparation

Tan et al. investigated polymers made from bis-benzocyclobutenes [13-15]. As the benzocyclobutane is analogous to tbe dien, tbe Diels-Alder addition takes place. This reaction is applied to the preparation of polyimides. The advantage of this system is that the resultant polymer is oxidized to form thermally stable aromatic polyimides (Fig. 7). 

Makino, H., Y. Kusuki, H. Yoshida, and A. Nakamura, Process for Preparing Aromatic Polyimide Semipermeable Membranes, U.S. Patent No. 4,378,324, March 1983. 

The classical synthetic pathway to prepare polyimides consists of a two-step scheme in which the first step involves polymerization of a soluble and thus processable poly(amic acid) intermediate, followed by a second dehydration step of this prepolymer to yield the final polyimide. This preparative pathway is representative of most of the early aromatic polyimide work and remains the most practical and widely utilized method of polyimide preparation to date. As illustrated in Scheme 4, this approach is based on the reaction of a suitable diamine with a dianhydride in a polar, aprotic solvent such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), or AT-methylpyrrolidone (NMP), generally at ambient temperature, to yield a poly(amic acid). The poly(amic acid) is then cyclized either thermally or chemically in a subsequent step to produce the desired polyimide. This second step will be discussed in more detail in the imidization characteristics section. More specifically, step 1 in the classical two-step synthesis of polyimides... 

Aromatic polyimides have excellent thermal stability in addition to their good electrical properties, light weight, flexibility, and easy processability. The first aromatic polyimide film (Kapton, produced by DuPont) was commercialized in the 1960s and has been developed for various aerospace applications. The structure of a typical polyimide PMDA/ODA prepared from pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA), which has the same structure as Kapton, is shown in (1). Aromatic polyimides have excellent thermal stability because they consist of aromatic and imide rings. 

Aromatic polyimides have many anisotropic imide rings and benzene rings, and they are easy to orient by a film-forming process. Molecular orientation in polyimide films causes in-plane/out-of-plane birefringence (AnJ. Russell et al. have reported the A/ / of conventional PMDA/ODA. On the other hand, spin-coated polyimide films just after preparation do not cause Ann, as noted in the... 

PFMB can be used to prepare aromatic polyimides that display solubility in ketone, ether, and polar aprotic solvents. This unusual solubility can be utilized in die facile preparation of thin films that display anisotropy in their structures and properties. The anisotropy in the optical properties of the films makes them promising candidates for use as compensation layers in liquid-crystal displays. Their low dielectric constants and CTEs in combination with their outstanding thennal and thermooxidative stabilities make diem candidates for dielectric layers in microelectronics applications. 

Considerable research effort has been devoted in recent years to the use of chloral derivatives for the synthesis of linear heterocyclic polymers. Of these, the most common are aromatic polyimides [1-12], Many of these polymers have been synthesised from compounds like 4,4 -diaminobenzophenone, and other diamines, which, as demonstrated in the previous chapter, can be obtained from chloral. Polyimides prepared from these diamines were largely synthesised by the conventional two-step procedure [11, 12] involving mild reaction of the diamines with the bis(phthalic)anhydrides, isolation of poly(o-carboxy)amide (PCA) prepolymers, and then processing into products followed by thermal or chemical imidisation [13—16] (Scheme 3.1). Some properties of polyimides prepared from 4,4 -diaminobenzophenone are provided in Table 3.1. 

The effect of aromatic diamine residues on the heat resistance of polyimides is less clear cut. It was only by comparing thermomechanical curves of polypyromellitimides obtained from 3,3-diamino-4,4 -dichlorodiphenylmethane, and l,l-dichloro-2,2-di(3-amino-4-chlorophenyl)-ethylene that the system containing the 1,1-dichloroethylene group was found to be less heat resistant. With polyimides prepared from other dianhydrides this rule holds true as well but is less distinct. 

Trinitrotoluene (TNT)-based aromatic diamines are used for the preparation of substituted polyimides. Interest in substituted polyimides is very understandable due to the known poor tractability of unsubstimted aromatic polyimides [1—4], As is known, the introduction of methoxy substituents to macromolecules of polyimides enhances their solubility in organic solvents [5]. One of the simplest TNT derivatives, 3,5-diaminoanisole, is used for the preparation of methoxy-substituted polyimides [6]. 

Aromatic polyimides are most useful super engineering plastics which exhibit excellent thermal, electrical, and mechanical properties, and have been used widely in aerospace, electronics, and other industries over the past three decades [ 1 -4]. Aromatic polyimides are generally prepared through a two-step procedure by the ring-opening polyaddition of aromatic diamines to aromatic tet-racarboxylic dianhydrides in NMP (or DMAc) solution giving soluble polyamic acids, followed by thermal cyclodehydration (Eq. 1) [1-5]. 

Figure 5 represents the temperature dependence of inherent viscosity of the polyimide prepared by the polycondensation of salt monomer ODPMA for 1 h [27]. From the DSC and TG studies, the aromatic salt monomer was found to undergo polycondensation at the endothermic peak temperature of 220 °C. The polymerization at a temperature lower than 220 °C resulted in the polymer with low viscosity value. The inherent viscosity of the polymer increased with in-... 

Fortunately, aromatic polyimides could be used as materials because they can be prepared through a multistep process, being applicable in the state of soluble polymeric intermediate. Nevertheless, the transformation into polyimides at the moment of application is an approach far from being optimal in most cases, and it can be said that, for many years, aromatic homopolyimides could be successfully applied only in the form of films or coatings [2,3]. 

Table 3 shows the Tg values and solubility of some selected polyimides among those prepared from monomers of Tables 1 and 2. The combination of non-pla-nar dianhydrides and non-planar, raefa-oriented aromatic diamines containing flexible linkages provides the structural elements needed for solubility and melt processability. Some aromatic polyimides marketed as thermoplastic materials are based on these statements [9,57-60]. 

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]. 

Deuterated aromatic polyimides having excellent mechanical, thermal, and optical properties and exhibiting infrared absorption transparency between 2500 and 3500 cm have been prepared. These materials are useful in optical transmission and optical signal processing because of minimum optical losses associated with absorption. 

Aromatic polyimides possess outstanding thermal stability as well as being unusually high melting, intractable and insoluble (1). Polyimides are prepared either by polyamide salt techniques, by condensation of dianhydrides with diisocyanates (2) or by reaction of an aromatic diamine with a dianhydride to give a poly(amic acid) followed by dehydration to give the polyimide. The polyimides from a variety of diamines have been reported and the dianhydride unit has been varied widely (1). 

Kim TH, Ki CD, Cho H, Chang T, Chang JY (2005) Facile preparation of core-shell type molecularly imprinted particles molecular imprinting into aromatic polyimide coated on silica spheres. Macromolecules 38(15) 6423—6428... 

S. Katsuki and H. Mii. Process for preparing metal-coated aromatic polyimide film. US Patent 7232610, assigned to UBE Industries, Ltd. (Yam-aguchi, JP), June 19, 2007. 

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