Polymères Thermostables: Thermoplastiques Et Thermodurcissables, Nouvelles Tendances

Abstract

This paper deals with the recent developments and the trends in the field of the heat resistant linear and thermosetting polymers.Two driving forces can explain the recent important efforts of research and development. The first tar et is the improvement of the processabilityfor these materials and the second one is the strong demand foraeat resistant macromolecules with specific properties such as low thermal expansion coefficient, low loss factor, low permittivity, improved toughness ... Three types of material will be discussed in this paper : the linear polymers, the thermosetting resins and the semi-interpenetrated networks (semi IPN's).The heat resistant linear polymers are generally amorphous, their high viscosities above the glass transition Fg) limits the processability and the rather low Tg does not allow the high temperature application. In order to solve these problems, three main approaches have been studied. First the crystalline polyimides were prepared by introducing ether ketone sequences in the main chain so the high temperature properties are governed by the crystalline transitions. The second approach is the development of amorphous fully cyclized pol mers exhibiting high Tg. These polymers were obtained, with bulky or fluorinated group in the backbone. Finally a new concept is now developed, it is based on an intramolecular thermal rearrangement occuring in a flexible main chain and leading to a rigid one.Concerning the heat resistant thermosettin resins, the following roblems have to be solved : the low solubility of this type of telechelic oligomers, theirfilh melting point a n l as a consequence, the very narrow processability window and the brittle character oft e final network. For these purposes, the bis-maleimide were crosslinked with allylic resins, a new soluble precyclized nadimide resin was developed and a new concept of redox reaction for the formation of phthalocyanine network was discovered.Very promisin seems the semi IPN's networks in the field of the heat resistant pol mers. Starting from the same diamine k r the preparation of linear and thermosetting resins (bis-maleimi&s and end-capped acetylenic resins) al!ows a good miscibility of the mixture of linear and thermoset before curing.After crosslinking a phase segregation occurs. Good mechanical properties are observed above the Tg of the linear polyimide. The thermal stability of the semi-IPN is strongly dependent on the nature of the thermosetting resins, so the semi-IPN's with acetylenic are more thermostable than the semi IPN's with bis-maleimide.