Aliphatic polyimide

Under UV irradiation poly(dimethylacrylimide) (4.68) decomposes by elimination of isocyanic acid (HNCO), which results in main chain scission with the formation of olefin and ketene end-groups, as shown below [947]  

Microcapsules prepared by a coupling reaction of disuccinimido-4,4 -azobis(4-cyano-valerate) with L-lysine and/or polyallylamine show a light-responsive release of an inner content, sucdnylated bovine serum albumin, to the outer bulk phase [1207]. 

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However, the starting point of polyimide synthesis dates away back to the mid 1950s, when Edwards and Robinson had synthesized aliphatic-aromatic polyimides (hereafter referred to as aliphatic polyimides) by the melt polycondensation of nylon-salt-type monomers composed of aliphatic diamines and aromatic tet-racarboxylic acids or their diacid-diesters (Eq. 2) [6,7]. 

Nevertheless, further detailed information was unavailable on the polyimide synthesis from nylon-salt-type monomers that is referred to as salt monomer method , and this method was not really recognized as a simple synthetic method of both aromatic and aliphatic polyimides. In addition, many polyimide investigations have mainly been concentrated on aromatic polyimides, and little information is available about aliphatic polyimides [13-18] that are also potential candidates for engineering plastics. 

Thus, we have recovered the lost salt monomer method for a facile and versatile synthetic method for polyimides. This chapter reviews our recent findings on the rapid synthesis of a series of polyimides by the salt monomer method. This also includes the properties of newly synthesized aliphatic polyimides, and the application of the salt monomer method as well. 

Quite recently, we have investigated systematically the high-pressure polycondensation leading to the formation of a variety of polyimides, and polyben-zoxazoles as well [19,33]. Here we applied the salt monomer method to the high-pressure synthesis of aliphatic polyimides. 

Table 1. High-pressure synthesis and thermal behavior of aliphatic polyimides ...

The results of the high-pressure polycondensations of the aliphatic-aromatic salt monomers XPMA, XBPA, XTPE, XOPA, and XBTA under 200-600 MPa at 220-320°C for 3-30 h leading to the formation of a series of aliphatic polyimides (see Eq. 5) are summarized in Table 1 [22,24-26]. 

Contrary to the high-pressure polycondensation, when the polycondensation of the salt monomers was conducted in a molten state under atmospheric or reduced pressure for the preparation of the polyimides having Tm below 300°C, this often led to the formation of crosslinked aliphatic polyimides that were insoluble even in concentrated sulfuric acid. Therefore, the high-pressure polycondensation process provides a simple and effective method for the synthesis of the linear polyimides with well-defined structures that caused high crystallinity, compared with the other synthetic methods. 

The thermal behavior of a series of newly synthesized aliphatic polyimides having well-defined structures, obtained by the high-pressure polycondensation of... 

From these results, it may be concluded that the aliphatic polyimides, especially the pyromellitic acid-based polyimides P-XPM with Tm values around 300 °C, are promising candidates as moldable high-temperature polyimide resins useful for industrial applications. 

Here we have combined the salt monomer method with microwave-induced polycondensation for the synthesis of aliphatic polyimides P-XPM from salt monomers XPMA and XPME (see Eq. 5, X=6-12, Ar=PM, and R=H and ethyl) [28]. When DMI or CHP was used as the solvent, the polycondensation of both salt monomers proceeded quite rapidly, and only 2 min of microwave irradiation readily afforded the aliphatic polyimides with inherent viscosities around 0.7 dL/g or above. Under these microwave irradiation conditions, salt monomers XPME were found to be more reactive than salts XPMA, judging from the attained inherent viscosity values. 

The salt monomer method was successfully applied to the preparation of the electrically-conducting polyimide-carbon black composites [62]. The composites are prepared as follows An aqueous solution of salt monomer 9PMA was mixed with carbon black, giving a suspension. This was evaporated to dryness under reduced pressure to afford a homogeneously-mixed powder composed of the salt monomer and carbon black. The powder was subjected to solid-state thermal polycondensation in the form of a pellet at 240 °C for 1 h under atmospheric pressure. The semiconducting aliphatic polyimides (P-9PM, Tm=315 °C) having electric conductivity of about 10"6 S/cm was readily obtained by mixing only 1 wt% of carbon black based on the polyimide. 

The high-pressure polyimide synthesis from salt monomers was extended to the development of a new approach for hybrid materials composed of aliphatic polyimides and silica, wherein this method was combined with the sol-gel process [63,64]. The preparation process is outlined in Scheme 2, where aliphatic polyimide P-9PM was used as the polyimide component. 

Aliphatic polyimides are in general fusible and soluble in many solvents, and they can often be made in one step. Aromatic polyimides, however, are usually made in two steps. 

Polyimides with aliphatic units in their main chain are generally obtained by thermal polycondensation by heating salts of aromatic tetracarboxylic acids and aliphatic diamines. Edwards and Robinson were the first to describe the preparation of an aliphatic polyimide by the fusion of a diamine with a diester of a tetracarboxylic acid. The reaction for the polyimide formation may be represented as follows [2] ... 

Aliphatic polyimides as alignment layers for surface stabilized ferroelectric smectic C liquid crystal cells, Figure 1, B.O. Myrvold, Liquid Crystals, 7 (2), p. 262 (1990). Reproduced by permission of Taylor Francis. 

As a result, two composite systems derived from partially aliphatic polyimides— 5-(2,5-dioxotetra-hydrofurfuryl)-3-methyl-3-cyclohexene-l,2-dicarboxylic acid anhydride reacting with 4,4 -oxydianiline (DOCDA-ODA) and with iron oxide as Fe304—were prepared at different temperatures, 250 and 300°C (Nica et al. 2015). Correlations among the structural, morphological, thermal, and magnetic properties were established for possible application as humidity sensor devices. In addition, the influence of humidity on the electrical properties of poly(DOCDA-ODA)/iron oxide composite were discussed, which led to an analysis of the humidity sensitivity of pure polyimide and of a poly(DOCDA-ODA)/Fe304 with different filler contents of iron oxides at various values of relative humidity (RH). 

Eichstadt A E, Ward T C, Bagwell M D, Farr I V, Dunson D L, McGrath J E (2002) S3tnthesis and Characterizahon of Amorphous Partially Aliphatic Polyimide Copolymers Based on Bisphenol-A Dianhydride. Macromolecules 35 7561-7568. 

Koning C, Delmotte A, Lamo P, Van Mele B. Influence of pol)tmerization conditions on melt crystallization of partially aliphatic polyimides. Polymer 1998 39 3697-3702. 

Eichstadt AE, Ward TC, Bagwell MD, Farr IV, Dunson McGrath JE. Structure-property relationships for a series of amorphous partially aliphatic polyimides. J. Polym. Sci. Polym. Phys. Ed. 2002 40 1503-1512. 

The comparison of the results obtained in this study for the LCP degradation imder processing temperatures with the peculiarities of some thermally stable polyheteroaiylenes degradation [14] brings to light some common features carbonization of the structure, H2 evolution, improvement in thermo-oxidative stability with transition metal compoimds. That is why we took into accoimt the stabilization of polysulfones, aiyl-aliphatic polyimides, polyamides etc. The approach to such stabilization is based on the following proposals on the mechanisms of the above said polymer degradation ... 

Delozier, D. M., K. A. Watson, J. G. Smith, T. C. Clancy, and J. W. Connell (2006). Investigation of aromatic/aliphatic polyimides as dispersants for single wall carbon nanotubes. Macromolecules 39(5) 1731-1739. 

Furthermore, aromatic polyimides can produce better alignment than aliphatic polyimides [6]. 

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