The use of torsional braid analysis to predict rheological properties of polyimides
In recent years a number of new heterocyclic type polymers have been synthesized in an attempt to obtain polymers with improved thermal stability at high temperatures. The environmental factors that must be met by any material at high temperatures demand improved processing and characterization to insure that the material will meet these conditions. As in many polymer systems, the way the polymer is synthesized determines much of the approach to its characterization. For the most part the polyimide is prepared as a prepolymer that is first imidized and then cures to an intractable polymer at some high temperature. While one can relate these general concepts of processing in rather simple terms, there are numerous complexities in arriving at a polyimide that will perform to the requirements needed for high temperature application. A basis of these complexities is in the imidization step. While soluble as a prepolymer in a variety of solvents, in most cases it becomes insoluble after imidization. Such a situation precludes many traditional ways of characterization of the polymer, particularly in terms of molecular weights. One technique that has been used to characterize these polymers is the Torsional Braid Analysis (TBA). This approach has the advantage following the polymer from its prepolymer form through the imidization step to the curing and in most cases a post curing step. Once this has been done the same sample can be exposed to a number of additional thermal histories and these effects measured.
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