Page 267 - 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

776 Polymers, Thermally Stable

weight, cross-linking, or cyclization. As a general crite- away from an emphasis on predominantly military ap-
rion, degradation adversely affects those polymer prop- plications into aspects of multidisciplinary studies. Thus,
erties critical for commercially viable plastics, fibers, while research into high-temperature polymers has contin-
or rubbers. ued to be directed toward aerospace requirements, there
Reinforcedplastics Compositenonmetallicmaterialsfor has also been a move toward environmental objectives,
which the basic resin system is combined in varying which has led, for example, to studies into dielectrics
proportion with a fibrous additive in order to improve for electronic applications, polymer sensors, resist materi-
the mechanical strength and modulus of the matrix. als, and polymeric membranes. Associated basic studies,
Rigid-rod polymers Formed from benzenoid and hetero- while continuing to examine the design and synthesis of
cyclic moieties, the molecules are colinearly arranged thermally stable systems, have concentrated efforts into
and have rotational flexibility within the polymer back- those areas of polymer chemistry linked to chain architec-
bone only at the junction of phenyl and heterocyclic ture and the control of end-group structure. The previous
rings. overriding emphasis on stability, both thermal and thermo-
Thermal analysis Measurement and evaluation of poly- oxidative, which frequently led to brittle “brick dusts,” has
mer thermal stability by monitoring a specific polymer shifted noticeably toward efforts to provide an optimum
property with respect to temperature. Analytic data in- thermal/thermo-oxidative stability allied with a capacity
cludes polymer weight loss measured by dynamic or to produce processible materials that can provide manu-
isothermal thermogravimetric analysis (TGA); glass factured products (see Section 1.C) by melt, solution, or
transition temperature (T g ) measured calorimetrically dispersion processing.
by differential scanning calorimetry (DSC)/differential
thermal analysis (DTA) or mechanically by tor-
sional braid analysis (TBA)/thermomechanical analy-
sis (TMA); monitoring volatile by-products by effluent I. THERMALLY STABLE POLYMERS
gas analysis (EGA).
Thermoplastic polymer Relates to a plastic material that A. Essentials of Polymer Stability
can be repeatedly softened when heated and hardened Two basic mechanisms control property deterioration of
when cooled. polymers at elevated temperature. One, linked predom-
Thermosetting resin Polymer capable of being modified inantly to thermoplastics, involves a reversible wholly
into a predominantly and permanently infusible and in- temperature-dependent softening phenomenon; the other
soluble material due to the effects of heat or chemical an irreversible degradative process time, temperature, and
processes. environment dependent relates to thermosets.
To increase the polymer softening point, or more criti-
cally the glass transition temperature (T g ), it is necessary
THE KEY Requirement for a successful “working” ther- to maximize contributions from both interchain attractive
mally stable (heat-resistant) polymer is that function- forces and chain regularity:
ing within clearly defined parameters of temperature,
time, and environment it should retain a high propor-
tion of those practically useful properties in material form
Interchain forces Chain regularity
(e.g., film, fiber, resin matrix, or metal-to-metal adhesive)
that were demonstrated under ambient conditions. The Polar side groups Crystallinity
temperature–time limitations most frequently imposed in- Hydrogen bonding para-Linked cyclics
◦
◦
clude 230–260 C for several thousand hours, 360–370 C Cross-linking Extensive orientation
for hundreds of hours, 550–560 C for 1 hr, and 750–
◦
800 C (ablative conditions) for minutes only. The envi-
◦
ronment within which the polymer must function will Observations from the purely thermal degradation of
include the prevailing atmosphere surrounding the poly- several polymers with closely related structures have em-
mer (inert gas, air/oxygen, or vacuum), possible exposure phasized the following features as important for high ther-
to chemical attack and/or radiation, and the individual mal stability:
or combined mechanical stress factors (tensile, compres-
sive, shear) encountered under static or dynamic loading 1. Maximum bond strength via resonance stabilization
conditions. 2. Minimum of low-energy paths allowing
The 1990s saw significant changes in the direction taken rearrangement processes
by developments in the field of thermally stable materials, 3. Maximum influence of the polybonding effect