Polyimides with Bulky Groups: Synthesis, Characterization, and Physical Properties

ABSTRACT

Aromatic polyimides are an important class of polymers used in modem technology, including membrane gas separation. Synthesis and study of polyimides with bulky groups in the elementary unit, high free volume, and attractive transport characteristics are modem trends in membrane materials science. The present study describes the methods of synthesis of polyimides used to obtain new materials with bulky or pendant, noncoplanar, kink, spiro, cardo, branched substituents, flexible, or unsymmetrical linkages in the backbone. Particular attention is paid to the gas separation characteristics of polyimides of this class. In permeability-selectivity diagrams, polyimides with bulky groups are located above the upper bound of the distribution. The reason for this behavior is an increase in the free volume and the growth of chain conformational rigidity due to hindering rotation of the fragments of the main chain, which is proved by numerical methods. Conformational factors lead to an increase in the permeability and overall selectivity of poly- imides of this class and to a decrease in then permittivity. Thus, polyimides with bulky groups are promising materials for various applications.

INTRODUCTION

Aromatic polyimides (Pis) are known as high-performance polymers. This class of polymers attracted great attention because of their excellent set of physical and chemical properties. Though synthesis of aromatic PI was first reported by M. T. Bogert and R. R. Renshawas early as in 19081; real interest to them appeared only in 1950 after the development of the two- step PI synthesis in DuPont Co.2 Prepared polymers displayed outstanding thermal, mechanical, and electrical properties and found numerous applications in aerospace and electronic industries as adhesives and matrixes for composites.3-6 Also, these polymers were gifted with high chemical and solvent resistant and excellent film forming ability that allowed their uses in different membrane-based applications.7'10 Nonporous PI membranes characterized by high selectivity of gas separation.7-8 It is for these reasons that more than 40% of the presented polymers (about 400) belong to this class11 in the TIPS RAS Database on the transport properties of glassy polymers.12 The relationship between chemical structure and membrane properties of Pis is discussed in a number of review's and books.7813'15 On the basis of methods of computer modeling16-17 and additive schemes of prediction of transport characteristics18'21, the basic elements of the chemical structure of the elementary link7-8-14-21 and the structure of the chain817 responsible for the gas separation characteristics of Pis are revealed. Thus, it is known that the features of PI packaging, providing high selectivity of gas separation and, at the same time, good mechanical properties and high thermal stability, are determined by the increased cohesion energy due to the interaction of flat phenylimide cycles (so-called stacking). However, the same features of the packaging lead to a decrease in gas permeability. A well-known design element for obtaining highly permeable Pis is the use of structural elements with voluminous groups, or rigid kinks, which prevent the chain rotation and lead to the destruction of the stacking interaction. Thus, the most permeable in this gr oup of polymers are Pis with structural elements characteristic of the so-called polymers “with intrinsic microporosity”.22 Less pronounced effects, but similar in conformational features of the chain, are observed for

Pis with rigid diamines with voluminous substituents.23 On the other hand, poor solubility of Pis in organic solvents and exceptionally high glass transition or melting temperatures restrict their processing in melt and solution routes. Numerous approaches have been adopted to improve the processing characteristics of intractable Pis. One of the successful approaches to ease the processing characteristics of Pis is the incorporation of bulky side groups or bulky units in the polymer backbone.9-2324

SYNTHESIS OF THE POLYIMIDES

Pis are prepared by condensation polymerization reaction of organic diamines with organic dianhydrides by one-step and two-step polymerization process.3"5-9-25-26 A comprehensive route of synthesis of Pis from an aromatic dianhydride and an aromatic diamine is shown in Figure 2.1.

General reaction scheme of synthesis of polyimides

FIGURE 2.1 General reaction scheme of synthesis of polyimides.

 
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