Synthesis and characterization of new alkali-soluble polyimides and preparation of alternating multilayer nano-films therefrom

Abstract

New alkali-soluble aromatic polyimides were prepared by a direct one-step polycondensation of 3,6-di(4-carboxyphenyl)pyromellitic dianhydride with common aromatic diamines in the presence of isoquinoline at 170 °C. The obtained polymers with inherent viscosities of the 0.62–1.01 g/dl range were all amorphous and highly soluble in dilute aqueous NaOH and tetramethylammonium hydroxide. Upon heating in TGA, the carboxylic acid groups degraded away at lower temperatures and the chain backbone did at higher temperatures. In DSC, no glass transition could be detected before decomposition. The alternating multilayer nano-films from the polyimides and poly(ethyleneimine) were prepared by the molecular self-assembly method and characterized by ellipsometry and X-ray reflectivity.

Introduction

Aromatic polyimides have been steadily widening their application fields from the conventional heat-stable structural materials to functional polymers such as photo-sensitive resists [1], gas-permeation membranes [2], nano-porous materials with low dielectric constant [3] and liquid crystal-aligning films [4]. In most of the applications the polyimide products are processed through the poly(amic acid) precursors, because they have extremely low solubility in organic solvents. The precursor processing procedures are often tied up with formation of the undesirable structures and properties such as micro-void generation, shrinkage or deformation in the final products [5]. To obtain highly soluble polyimides, their chain structure should be appropriately designed and synthesized [6].

To our best knowledge, only a few aromatic polyimides soluble in aqueous solvents have been reported [7], [8], [9]. In the present article, two carboxy groups were incorporated onto the aromatic polyimide backbones and investigated their solubility in aqueous alkalis or tetrametylammonium hydroxide (TMAH). The polyimides' solubility in alkali is advantageous not only in direct processing to the final products in imide form but also in improving the environmental friendliness by replacing polluting organic solvent. The solubility in TMAH solution is of particular interests since it is the developing solvent in micro-lithography for integrated semiconductor circuits, and such polyimides could be used as photoresist material [7].

The carboxy functionality is desirable in developing new applications such as fabrication of nano-structures by molecular self-assembly [10], [11], ion exchange membranes, polymer electrolytes or ionomers [6]. The multilayer nano-films are recently drawing a lot of attention for constructing polymeric nano-structures simply by alternative dipping in two polymer solutions. In these structures, the carboxy group forms the basis of the multi-layer generation via its strong attraction with the polymers with basic functionality. Thus, ultrathin films by self-assembly have been extensively investigated for decades [12], [13], [14].

In the present work, new aromatic polyimides having carboxy functional groups in side branch were prepared and characterized and their multilayer films with commercial poly(ethyleneimine) (PEI) were made by sequential dipping in a polyimide solution in NMP and PEI in water.