Page 282 - 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 791

blends, significant improvements have been observed for A staged heating (220–280 C) under nitrogen produces
◦
thermal/thermo-oxidative stability and hot-wet mechani- an oligomeric prepolymer with evolution of phenol and
cal strength. water; above 350 C(in vacuo) complete conversion to
◦
Epoxy resins are miscible with, and coreact with, CE the high-molecular-weight, intractable PBI occurs via a
resins resulting in hybrid products with improved prop- solid-state process. Most PBI variants have been produced
erties, e.g., increased T g and better electrical resistance by this melt–polycondensation route; however, a limited
compared with the parent epoxy. number are prepared in solution using high boiling polar
A number of epoxy/CE systems have been used in ap- solvents. Examples are the formation of the substituted
plications where the relative cheapness of the epoxy resin PBIs (XXIV) via the “open-chain” poly(aminoamide) in-
makes a significant difference to the commercial viability termediates:
of the product. Blending CEs with BMIs has enhanced
the toughness characteristics of the latter resin systems
R
and a range of blended materials (Skyflex BT resins) has
been produced by Mitsubishi Gas Chemical Corporation.
Originally it was believed that the CE/BMI resin systems
coreacted on blending. However, it is now considered that
an interpenetrating network (IPN) is established, indicated and interaction of aromatic tetraamines with bisor-
by the fact that two disinct T g values are observed origi- thoesters to produce high-molecular-weight film- and
nating from the independent CE and BMI networks. The fiber-forming PBIs (XXV).
lower valued of these T g values governs the “use” temper- Structural modifications to the wholly aromatic PBI
ature of the blend. system have involved incorporation of both relatively sim-
The introduction of reactive allyl groups into the CE ple midchain flexible groups [( (CH 2 ) n ), SO 2 , O ]
monomer has been reported to facilitate copolymerization and more complex units such as siloxanes
with the BMI component, producing a linked interpene-
trating network (LPN) with a single high T g and a G IC
higher than for either homopolymer.

B. Bi- and Tricyclic Polymers
1. Polybenzazoles—Polybenzimidazoles
The extension of conventional polycondensation pro-
cesses from simple aliphatic systems to high-temperature
heteroaromatic polymers was first observed for the
polybenzimidazoles (PBIs). Practical, semicommercial
applications—adhesives, fibers, composites—were soon
evaluated, but this initial progress was not sustained and
the aromatic polyimides soon took, and retained, “center-
stage” as the most important applicational heteroaromatic
polymer system. However, techniques of synthesis, and
structure versus property data evaluated for PBIs have
been of considerable value in the development of other
polyheteroaromatics.
The main synthetic route to PBIs involves the reaction
of aromatic tetraamine with dicarboxylic acid esters un- In general, improvements that these groups make to the
der melt conditions, typified below for the formation of solubility/processibility of the polymers are linked with

poly(2,2 -m-phenylene-5,5 -bibenzimidazole) (XXIII) reductions in thermal/thermooxidative stability. However,

the substitution of the imidazole N—Hby N-phenyl
(XXIV) provides an improvement to both processibility
and long-term thermooxidative stability (Fig. 9). A com-
parison of weight loss data for typical wholly aromatic
heterocyclic polymers, first under inert conditions (TGA)
then in air (ITGA), highlights the basic oxidative instabil-
ity of the PBI system: