Recent Advances in Morphology and Mechanical Properties of Rigid-Rod Molecular Composites
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RECENT ADVANCES IN MORPHOLOGY AND MECHANICAL PROPERTIES OF RIGID-ROD MOLECULAR COMPOSITES STEPHEN J. KRAUSE* AND WEN-FANG HWANG** *Dept. of Chemical, Bio, and Materials Engineering, Arizona State University, Tempe, AZ 85287 **Dow Chemical Co., Central Research Laboratories - Advanced Polymeric Systems, Midland MI 48674 ABSTRACT Rigid-rod molecular composites are a new class of high performance structural polymers which have high specific strength and modulus and also high thermal and environmental resistance. The concept of using a rigid-rod, extended chain polymer to reinforce a ductile polymer matrix at the molecular level has been demonstrated with morphological and mechanical property studies for aromatic heterocyclic systems, but new materials systems and processing techniques will be required to produce thermoplastic or thermoset molecular composites. Improved characterization and modeling will also be required. In this regard, new results on modeling of mechanical properties of molecular composites are presented and compared with experimental results. The Halpin-Tsai equations from "shear-lag" theory of short fiber composites predict properties reasonably well when using the theoretical modulus of rigid-rod molecules in aromatic heterocyclic systems, but newer matrix systems will require
consideration of matrix stiffness, desired rod aspect ratio, and rod orientation distribution. Application of traditional and newer morphological characterization techniques are discussed. The newer techniques include: Raman light scattering, high resolution and low voltage SEM, parallel EELS in TEM, synchrotron radiation in X-ray scattering, and ultrasound for integrity studies. The properties of molecular composites and macroscopic composites are compared and it is found that excellent potential exists for use of molecular composites in structural applications including engineering plastics, composite matrix resins, and as direct substitutes for fiber reinforced composites. INTRODUCTION Since the concept of a self-reinforcing molecular composite of rigid-rod and flexible coil polymer
components was first proposed by Helminiak et al. [1, 2], and first successfully applied by Hwang et al. [3], a variety of candidate systems have been studied. The advantages of rigid-rod molecular composites over macroscopic fiber reinforced composites are based upon the elimination of discrete fiber/matrix interfaces and upon the intrinsically superior properties of the aromatic heterocyclic chemistry of the rod molecules. These advantages include: higher specific mechanical properties; higher environmental and thermal resistance; and the potential for a
wider choice of processing options. Possible applications for molecular composites include: engineering plastics; high performance fibers; composite matrix resins; and direct substitutes for fiber reinforced composites.
The choice of the reinforcing molecule for a molecular composite is critical, in order to maintain both high aspect ratio (ratio of length to diameter) for efficient rein
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