Page 284 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology En012c-604 July 26, 2001 16:2

Polymers, Thermally Stable 793

difficulties associated with the processing and fabrication conventional organic solvents from which such materials
of conventional laminates together with those of fiber– could readily be processed into films, coatings, or fibers.
resin property mismatch (e.g., difference in expansion co- While soluble in strong acids, e.g., methanesulfonic acid
efficients) is of considerable importance. (MSA), such a corrosive medium presents extremely dif-
The heteroaromatic rigid-rod molecular composites ficult processing problems, and attempts to resolve the
continue to excite considerable research interest. Molec- dilemma have been made. PBT, is, for example, soluble
ular composites are composed of binary blends of re- (up to 10 wt%) in nitroalkanes or nitrobenzene which
inforcement having a high aspect, high strength, rigid contain Lewis acids (e.g., FeCl 3 , AlCl 3 ). Solvent cast-
structure dispersed in a flexible coil polymer matrix. Re- ing onto glass, sapphire, or silicon wafers has given films
search has continued into PBT, PBO, PBI, and BBL of PBT/MX n complexes which after immersion in non-
(see Section II.C.2) as the rigid-rod polymers. Ordered solvent produce clear coatings of pure PBT. An alterna-
polypyromellitimides produced from highly phenylated tive approach to the solubility dilemma has been to intro-
diamines (XXVII and XXVIII) or alkoxy-substituted di- duce hydroxy, alkoxy, alkyl, and sulfonic acid side groups
amines (XXIX) have been described. into the basic PBT system. Highly ordered rigid-rod poly-
imide structures have been introduced into flexible coil
matrices via a DMAC-soluble polyisoimide intermedi-
ate. The isoimide and flexible coil matrix (polysulfone or
acetylene-terminatedpolyimidethermosets)wereblended
in solution. Heat treatment after coagulation and film-
casting converts the isoimide to the rigid-rod polyimide
structure. Characterization of these systems is under way.
Despite the ability of rigid-rod PBT to be spun into
fibers having a nearly perfect uniaxial orientation with
state-of-the-art tensile strength and modulus, it suf-
fers from a relatively low axial compression strength
PBT, however, remains the most frequently investigated (∼450 MPa). The highly oriented polymer chain buck-
of the rigid-rod polymer systems—heat-treated samples les under compressive loading, and attempts have been
have demonstrated a tensile modulus of 330 GPa and a made to “tailor-in” resistance to this compressive weak-
tensile strength of 3 GPa—acting in combination with ness. Such techniques have included the introduction of
a variety of flexible coil polymers such as nylon-6, 6-, bulky main-chain pendant groups e.g., (XXX) in order to
poly-2,5(6)-benzimidazole, PEEK, and BBB (see Sec- disrupt the nematic packing order. In another approach,
tion II.C.2). One major barrier which has prevented the ex- cross-linking via reactive fluorene units in-chain (XXXI)
tensive characterization of structure and solid-state prop- has been investigated. In neither of these approaches have
erties, as well as application of the rigid-rod/flexible coil improvements yet been observed to the compressive be-
matrix system, has been the problem of insolubility in havior of the basic PBT system.