Page 289 - 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

798 Polymers, Thermally Stable

semiconductor devices. By “tailoring” properties in the on PI/silica ceramers (polymer–ceramic hybrids). These
ways already described, the major in-built advantages ceramers contain nanoscale silica domains trapped in the
of the PIs—high thermal stability, excellent mechanical PI matrix, the composites exhibiting lower thermal expan-
properties, and low dielectric constant—can be reinforced sion and higher thermal stability compared with carbon
by improved processibility (including highly planar coat- fiber/PI-only composites.
ings), increased adhesion, and low thermal expansion co- Maintaining the thermal/thermo-oxidative stability of
efficient. Recently reported are polypyrrole-PI compos- PIs, while simultaneously increasing their solubility and
ite films which combine high electrical conductivity with processibility, is an ongoing aspect of R&D. The in-
high thermal stability. Two types of film are available: troduction of large bulky units (e.g., the adamantane
Type 1, a polypyrrole-coated PI with maximum conduc- group) pendant to or in-chain continue to feature in
tivity of 10 Scm −1 and Type 2, a PI film loaded with finely breaking up the regular symmetric structure of aromatic
divided polypyrrole particles, producing a maximum con- PIs.
−1
ductivity of 5 × 10 −4 Scm . Both types having a ther- Significant advances in processibility have also been
◦
mal stability up to 350 C. Applications of the condensa- achieved in those thermoplastic PIs formed from the
tion PIs as matrix materials for structural composites are complex ether-linked dianhydride and diamine pre-
more limited than for additiontype PIs. A thermoplastic PI cursors examples of which are featured below. The
(LARC-TPI) has been developed as an adhesive or coating “cranked/twisted” nature of these precursors confers a
product from the (XXXIII) system above. It has also been noncoplanar structure, which again inhibits chain pack-
commercialized (Duramid) as a tough and strong (ten- ing in the polymer structure. The thermoplastic polyimide
sile strength 153 MPa, tensile modulus 4.3 GPa) molding New TPI (XLVI) exhibits crystallization and melting be-
resin. havior similar to high-performance poly(aryl ether ke-
Perhaps the most novel reported application is that of tones). However, both bulk crystallization and linear crys-
film produced from a blend of PBI and a poly(siloxane- tallization rates are significantly slower and the T g higher
imide) copolymer. Conventional Kapton (PI film), used in New TPI probably due to to a decreased chain mobility.
as a thermal blanket and protective coating on the space Blends of New TPI with other high-performance polymers
shuttle, is rapidly eroded by the action of atomic oxygen. have been reported.
In the blended film, however, the siloxane component con- Polyimides have featured prominantly in the devel-
tinuously migrates to the film surface, where reaction with opment of rigid-rod molecules and related molecu-
atomic oxygen converts it to a protective silica coating. lar composites. Typically, a molecular composite has
During the past decade continued efforts have been comprised a rigid-rod component (XXXIII), A =
made further to capitalize on the stability and applica- CO CH 2 and a flexible matrix (XLVII). A
tional development of condensation-type polyimides. high-modulus composite film has been obtained in which
The link between polyimides (PIs) and silica referred to cross-linking of ethynyl groups afforded enhanced perfor-
above has been extended to carbon fiber composites based mance at higher temperatures.

CO CO
O O
CO O CO O
O CO O CO
O O
CO CO
e.g.,
H 2 N H 2 N O

O . NH 2 O NH 2

CO CO
N N (XLVI)
CO CO
. O O .
n