I. Chung, S. Kim
Apr 4, 2000
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Macromolecules
Abstract
Aromatic polyimides are well-known as highly heatresistant materials and have been widely used in many applications such as electronics, coatings, composite materials, and membranes.1 Polyimides are often insoluble and infusible in their fully imidized form due to their rigid chain characteristics, leading to processing difficulties. Thus, polyimide processing is generally carried out with poly(amic acid) intermediate and then converted to polyimide via rigorous thermal treatment.2 However, this process has several inherent problems such as emission of volatile byproducts (e.g., H2O) and storage instability of poly(amic acid) intermediate.3 To overcome these problems, much research effort has been focused on synthesis of soluble and processable polyimides in fully imidized form without deterioration of their own excellent properties.4 Several approaches to soluble polyimides including introduction of flexible linkage5 or bulky substituents6 and use of noncoplanar7 or alicyclic8 monomers have been developed in the past decade. For example, the polyimides containing unsymmetrically structured aromatic ring with perfluoroalkoxy6a or phenyl6b-d groups showed enhanced solubility behavior. Most of the above approaches for soluble polyimides are aimed at reduction of several types of chain-chain interaction, such as chain packing (e.g., crystallinity) and charge transfer and electronic polarization interactions.2,9 In this communication, we describe our approach for soluble polyimide by introducing a trifluoromethyl group unsymmetrically into the polymer chains that may reduce interactions between polyimide chains. 4,4′-Diaminodiphenyl ether (4,4′-DDE) is widely used in synthesis of polyimides, but the polyimides synthesized from this diamine are insoluble or have poor solubility depending on dianhydride used. To increase solubility, we designed a new diamine monomer having the same structure with 4,4′-DDE except one trifluoromethyl group at the 2-position of the benzene ring. The new diamine monomer, 2-trifluoromethyl-4,4′-diaminodiphenyl ether (1), was prepared from 2-chloro5-nitrobenzotrifluoride and 4-nitrophenol according to Scheme 1. 2-Chloro-5-nitrobenzotrifluoride was reacted with 4-nitrophenol in the presence of potassium carbonate to produce the dinitro compound, which was converted to the corresponding diamine monomer by reduction of the nitro groups. First, new polyimides were prepared from 1 and commercially available aromatic dianhydrides, such as PMDA, BPDA, BTDA, and ODPA, via a one-pot synthetic method as shown in Scheme 2. The polymerization was carried out by reacting stoichiometric amounts of diamine monomer 1 with aromatic dianhydrides at a concentration of 12% solids in N-methylpyrrolidone (NMP). The ring-opening polyaddition at room temperature for 8 h yielded poly(amic acid) solutions. After dilution of the solution to 8-10%, subsequent cyclodehydration by heating at 190 °C for 12 h gave the fully imidized polyimides except that of PMDA. While the polyimide prepared from 1 and PMDA (2b) was precipitated during the imidization process presumably due to the rigid chain characteristics, the rest remained in solution after imidization. The degree of * To whom correspondence should be addressed. Tel 82-42-8692834; Fax 82-42-869-2810; E-mail kimsy@cais.kaist.ac.kr. Scheme 1