Hierarchical Structures in Liquid Crystalline Polymers
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HIERARCHICAL STRUCTURES IN LIQUID CRYSTALLINE POLYMERS
Linda C. Sawyer and Michael Jaffe Hoechst Celanese Research Division, R. L. Mitchell Technical Center, 86 Morris Ave., Summit, New Jersey 07901 ABSTRACT
It is well known that the structure of highly oriented liquid crystalline polymers (LCPs) can be characterized by a hierarchical fibrillar structural model. Structure models were developed for the lyotropic aramid fibers and the thermotropic aromatic copolyester fibers during the last two decades showing the existence of fibrillar hierarchies. Hierarchies of structure have also been commonly observed for the biological materials. Concepts learned from the latter are useful in materials science studies today. The nature of the smallest nanostructure that aggregates, the combination of these small structures, typically microfibrils, into larger structures and the interaction of these hierarchical entities are important to understanding their behavior. The architecture of the whole of the polymer or the biological material is a further important variable as is the relation of the process with that architecture. This paper discusses details of the structure of LCPs and draws an analogy between the materials science and biological hierarchies. INTRODUCTION The morphology of highly oriented liquid crystalline polymers (LCPs) is well characterized as a hierarchy comprised of fibrillar elements [1-14]. Independently, models containing a hierarchy of fibrillar structures were developed for lyotropic aramid fibers in the late 1970's [1, 2, 5-7] and for thermotropic copolyesters in the mid-1980's [9-10, 12]. The scale, interaction and architecture of LCP microstructural elements has been the focus of continuing study over the past several decades. Of major interest is the development of structure-property relationships, especially the identification of those structural elements controlling mechanical performance of LCPs and, more generally, for all highly oriented polymeric fibers. Attention has also focused on biological hierarchies [i.e., 16-17] as a means to further the understanding and ultimately, as models for the design and control of synthetic hierarchies and their properties. Recently, imaging techniques with higher potential resolution have been applied to address the size, shape and organization of microfibrillar structures
Mat. Res. Soc. Symp. Proc. Vol. 255. ©1992 Materials Research Society
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found in LCPs [13-14]. A complete description of the data collected for the development of a refined structural model has recently been prepared [15]. In that work, field emission scanning electron microscopy (FESEM) and scanning tunneling microscopy (STM) were utilized, permitting imaging from the nanometer through the micrometer size scale. Complementary polarized light microscopy (PLM) and transmission electron microscopy (TEM) were also used to validate and aid interpretation of images from these new instruments. As a result, the general nature of the LCP fibrillar hierarchy has been elucidated in detail and a
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