Buu Dao

Water as Solvent in Polyimide Synthesis: Thermoset and Thermoplastic Examples

A combination of increasingly stringent environmental legislation and economic competition is driving industrial processes further and further towards the three “E”s of chemical manufacture: economy, efficiency and environmental impact. In this paper, we present a novel aqueous method for the synthesis of polyimides. The products resulting from this process perform similarly, through both qualitative and quantitative comparisons, to those conventionally produced using high boiling dipolar aprotic solvent. This highly efficient one-pot method potentially provides economic advantage through low solvent cost and environmental impact benefit from the manageable aqueous waste.

Preparation and Characterization of Some Novel Bismaleimide Monomers and Polymers Based on Diaminobisimides

New bismaleimide monomers, based on pure diaminobismides (DABIs) have been synthesized. In a number of cases the bismaleimide of the 2:1 (amine/anhydride) DABI adduct has been isolated as a pure compound, but where the starting DABI consisted of a mix of imide oligomers of the diamine and dianhydride (2:1, 3:2 and higher) the corresponding bismaleimide product was found also to have a similar composition ratio. This has been confirmed in one example by separation of the oligomer mix and characterization of the components. The utility of the various bismaleimides as monomers in composite matrices has been assessed by cocuring with the common coreactant, 3,3′-diallylbisphenol A. The physical properties and thermal stability of neat resin samples and laminates are reported as well as some mechanical properties.

Water as Solvent in Polyimide Synthesis II: Processable Aromatic Polyimides

Some representative thermal, chemical and mechanical characteristics of ‘commercial type’ polyimides are presented in this second paper in a series on using water as solvent in polyimide synthesis. The commercial types of polyimide that have been produced in this study are closely related to Kapton, Upilex R®, Upilex S®, Avimid N® and PMR-15. The chemical characterization of these materials using both nuclear magnetic resonance and Fourier Transform infrared indicate very pure and fully imidized products. The thermal and mechanical data from techniques such as thermogravimetric analysis, differential scanning calorimetry and dynamic mechanical thermal analysis reveal properties that are at least, equivalent to their commercial counterparts. Some improvement in processability is noted in pure materials, mixtures and chemically modified analogues of these well known polyimides. In addition this highly efficient one-pot method potentially provides economic advantages through low solvent cost and environmental impact benefits.

Important Factors Controlling Synthesis of Imides in Water

Four amine/anhydride models have been studied in order to understand the critical factors which control the synthesis of imides and polyimides in water. This synthesis method has been proposed recently as a convenient, environmentally friendly method of producing both thermosetting and thermoplastic polyimides. The key starting step in the synthesis is the formation of a salt/complex between the hydrolysed anhydride and the amine. The results from a combination of quantitative 1H NMR, HPLC and DSC analysis have shown that the reactions are reversible, and that the important factor in driving the reaction to completion in water is the insolubility of the imide product. Analysis of the precipitated imides shows them to be free of significant amounts of the considerably more water soluble amic acids (i.e. the uncyclized amide acids), which often contaminate imides made in conventional aprotic solvents. Of the models investigated, only the imidization of the second amino group in 2, 4, 6-trimethyl-1, 3-phenylene diamine (TMPDA) was subject to kinetic control. A general conclusion from these experiments is that where a different result is observed in water synthesis of imides compared with conventional aprotic solvent synthesis, it is likely to reflect the vastly different solvation powers of the two solvents rather than a difference in reaction mechanism.

Water as Solvent in Polyimide Synthesis III: Towards the Synthesis of Polyamideimides

Four different approaches to the synthesis of polyamideimides, which employ aqueous imidization, were studied in order to assess the viability of their preparation in water. Building on previous studies of synthesizing polyimides in water both one- and two-step methods were explored. The first direct method for polyamideimide synthesis forms both the amide and imide linkages simultaneously and is limited to appropriately activated diamine monomers. The second direct method uses commercially sourced diaminobenzanilide with standard tetracarboxylic acids in an imidization step to give the polyamideimide. This method, demonstrates the viability of using amide-containing diamines in a polyimidization step to synthesize the desired polymer. The third and fourth methods are both two-step processes and use aqueous imidization to produce imide-containing monomers with either amine or carboxylic acid terminal functionality. This is followed by standard amidation to produce high-quality polyamideimides.

The Utility of 2-Naphthol Derivatives As Diels–Alder Based Co-Reactants for Bismaleimides

A number of 2-naphthols were found to undergo a Diels–Alder addition reaction with maleimides. This reaction can be used to bring about the cure of bismaleimides. The simplest co-reactant prepared in this work was 7-allyloxy-2-naphthol and satisfactory cures were obtained with appropriate bismaleimides. However, the laminate coupons made using this system had lower thermal stability than those of a comparable system using the commercial co-reactant Matrimid 5292B. The performance of this new chemistry was further tested by incorporating the naphthol/maleimide Diels–Alder addition structure into two other co-reactants. The most successful of these compounds (the diallyl ether of the adduct made from Diels–Alder addition of 2, 7-dihydroxynaphthalene and 4-hydroxyphenylmaleimide) produced cured neat resin samples having T gs 30 °C higher than those of a comparable system cured with the standard bismaleimide co-reactant (Matrimid B) and had slightly higher tensile strength and modulus than this system. The fact that these laminate coupons had better thermal performance than the system using commercial co-reactant showed that the presence of the Diels–Alder adduct structure in the resin backbone was not detrimental to the normal performance of ene cured bismaleimides. On the other hand the same structure incorporated into phenylethynyl end-capped resins was found to compromise its thermal stability.

PMR Type Polyimides by Aqueous Cyclization Methods Chemistry

PMR-15 has long been the leading resin system for high performance composite applications requiring long-term use temperatures up to 300°C. A mixture of carboxylic esters and acids, aromatic diamine and norbornyl end caps capable of crosslinking by a reverse Diels—Alder mechanism in methanol is pre-pregged into cloth and under careful processing conditions produces high quality composite parts. Some problems with handling and processing remain in this commercial system and in an attempt to overcome these problems we have synthesized a series of fully cyclized PMR type prepolymers under aqueous imidization conditions. Their solubility and cure behavior have been determined with a view to producing resins that will form good quality, high performance composites. While resins with these characteristics have been obtained, the best of them utilize significant proportions of relatively expensive co-monomers. Almost all of the materials presented in this study have very high temperature stability and high glass transition temperatures, although no one system out performs PMR-15 on a monomer cost basis, the advantages of water as the synthesis solvent includes both its low cost and low toxicity and its ability to fully react the monomers to cyclic imide structures.

Methylnadimide End-Capped Polyimides for Use in High Temperature Composites—A New Approach to Their Synthesis

Two methylnadimide end-capped polyimides have been prepared by a new water synthesis procedure and their properties compared to similar polyimides synthesized in the conventional solvents, cresol and NMP. The water method has been shown to be a practical alternative for the synthesis of a wide range of polyimides with no evidence of contaminating polyamic acids. The present results showed that with appropriate adjustment of the synthesis conditions very useful methylnadimide end-capped polyimides could be obtained. It was found that the amount of methylnadimide end-cap was reduced under 180°C synthesis conditions, and even at lower temperatures with prolonged heating, and that the water reaction is best carried out at 165°C for these polyimides. One of the uncured resins, Polyimide 1 was quite soluble in common solvents allowing complete characterization. It was found that the amount of endcap introduced by either conventional or water synthesis procedures was less than the theoretical maximum. Examination of the cured neat resin from polyimide 1 showed it to consist of a portion of insoluble, possibly cross-linked, material and a considerable amount of soluble polymer of wide molecular weight distribution. The potential usefulness of these polymers as high-temperature composite matrices was indicated by the excellent tensile properties of polyimide 2B, which on curing produced a tough voidless polymer with a tensile strength of 120 MPa, a modulus of 3.9 GPa and a strain at break of 9.7%.