Showing papers in "Advanced Materials in 1992"

Synthesis, processing sand properties of thermotropic liquid-crystal polymers

Abstract: Thermotropic liquid-crystal polymers for high-performance applications are typically based on wholly aromatic polyester and polyamide architectures. The linear character of the aromtic monomers produces polymer chains with stiff, extended conformations. As a result, the chains organize themselves into a nematic melt and readily orient in response to processing flow fields. The close coupling between chain orientation and flow fields produces both a rich variety of materials and a high degree of controllability of their structure and properties (such as the tensile strength, elastic modulus and coefficient of thermal expansion). In this paper, we describe synthetic efforts to develop liquid-crystal polymer structures and the relationships between chain molecular structure, processing, and properties of this class of polymers in two distinct processing situations: Injection molding and fiber formation. The special importance of the orientational flow field in developing high orientation and excellent mechanical properties will be highlighted.

The quest for magnetic polymers—caveat emptor

Abstract: Numerous reports claiming ferromagnetic behavior in organic polymers exist; however, the reported materials have been plagued by ill-defined compositions, low yields on a magnetic basis, and poor reproducibility. Such low values of saturation magnetization (i.e., ≪ 5600 emuG/mol spin) cause serious concern as to whether the magnetic behavior arises from extrinsic sources and thus obligate the researcher to provide a persuasive case that when a material has a low saturation magnetization, suggesting thrtt the magnetic fraction is only a small portion of the bulk, that the magnetic behavior is in fact intrinsic and not of extrinsic origin. In several cases, when reinvestigated, the magnetic behavior of reported polymeric magnetic materials has been attributed to extrinsic sources. In the cases where extrinsic sources have not been identified, verification of the magnetic behavior has not been reported by independent laboratories. Thus, to date there have not been any reports of a bulk polymer that have stood the test of confirmation for ferromagnetic or even ferrimagnetic behaviour. Hence, caveat physics—scientist beware.