Page 266 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer
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P1: GPQ Final Pages/GNB  P2: GTV  Qu: 00, 00, 00, 00
 Encyclopedia of Physical Science and Technology  En012c-604  July 26, 2001  16:2




















                                           Polymers, Thermally Stable






              J. P. Critchley
              Royal Aerospace Establishment



               I. Thermally Stable Polymers
              II. Carbocyclic Aromatic Systems
              III. Heterocyclic Aromatic (Heteroaromatic) Systems








              GLOSSARY                                            These entanglements restrict the segmental motion
                                                                  such that the creep and flow phenomena of the par-
              Ablative coating Polymeric material providing thermal  ticipating polymers can be radically reduced.
                protectionwherebyhighlevelsofheatfluxareabsorbed  Ladder (double-strand) systems Relates to linear as op-
                in the course of the pyrolytic thermal decomposition of  posed to network polymers in which two molecular
                the exposed surface layer.                        strands are linked often via complex ring systems. “Per-
              Creep (cold flow) Reversible change in shape sustained  fect” ladder polymers require the rupture of at least two
                by thermoplastic materials under sustained loading.  bonds within the same ring before major reduction in
              Cross-linking Formation of three-dimensional or net-  molecular weight can occur.
                work polymer systems by polymerization of monomers  Liquid crystallinity Is exhibited in melt (thermotropism)
                with functionality greater than two or by covalent bond-  or solution (lyotropism) by polymers having rigid rod-
                ing between preformed linear molecules.           like structures; such polymers showing spontaneous
              Cyclopolycondensation Process in which a stable hete-  anisotropy and readily induced orientation in the liquid
                rocyclic ring is formed as part of the polymer chain  crystalline state. Processing (i.e., fiber spinning) from
                backbone with elimination of condensation volatiles.  this state can lead to materials with great strength in
                The process can occur via chain extension or postpoly-  the direction of orientation.
                merization of a preformed precursor polymer.    Molecular composites Utilize the extended chain struc-
              Interlaminar shear strength (ILSS) Design critical fea-  ture of a rigid-rod segment as reinforcement, copoly-
                ture of fiber-reinforced composites relating to the ab-  merized or blended at molecular level with the flexible
                solute stress level required to separate individual layers  matrix component which may be, typically, a thermo-
                of reinforcement. For optimum levels of fiber strength  plastic polymer. To achieve optimum properties for the
                and loading a valuable analytic tool for predicting com-  composite, there should be little or no phase separation
                pressive and flexural strengths of individual composites  between rigid-rod and matrix components.
                formulated from different resin matrices.       Polymer degradation Involves the chemical modifica-
              Interpenetrating network system (IPN) An intimate   tion of main-chain backbone and/or side groups of the
                mixture of two or more polymeric networks held to-  macromolecule. The process requires rupture of pri-
                gether predominantly by permanent entanglements.  mary valence bonds and results in reduced molecular



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