Page 154 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 117

TABLE 4.6
General Physical Properties of a Typical Aramid
Heat-deflection temperature (1,820 kPa; C) 260

o
o
Maximum resistance to continuous heat ( C) 150
Crystalline melting point ( C) >370
o
o
–5

Coefficient of linear expansion (cm/cm- C, 10 ) 2.6
Compressive strength (kPa) 2 × 10 5
Flexural strength (kPa) 1.7 × 10 5
Impact strength (Izod: cm-N/cm of notch) 75
Tensile strength (kPa) 1.2 × 10 5
Ultimate elongation (%) 5
Density (g/mL) 1.2

and tougher than nylons but they are also more difficult to solubilize and fabricate. Because the
presence of the aromatic groups causes the aramids to be stiff, they often form liquid crystals (LC)
that are present in a nematic liquid crystal state in concentrated solution.
Aramids are generally prepared by the solution or interfacial polycondensation of meta- and
para-substituted diacid chlorides and/or diamines. In some systems, synthesis is achieved under
rapidly stirred conditions where the polymer is quasi-soluble in the system. The polymer mixture

is forced through a small opening into a nonsolvent forming a fiber without the need to dissolve
the polymer before fiber formation. General properties of an amide such as Nomex are given in

Table 4.6. Notice the generally higher “strength” values and the greater stiffness (less elongation)
in comparison to nylon-66.
Poly(m-phenylene isophthalamide) (Equation 4.64), sold under the trade name of Nomex, exhib-

o
its good thermal stability decomposing above 370 C. It is used in flame-resistant clothing. It is also
used in the form of thin pads to protect sintered silica-fi ber mats from stress and vibrations during
the flight of the space shuttle.

R
HN
Cl O
R
O O
H N NH
2 2
O Cl
+ O NH

Isophthaloyl chloride + m -Phenylenediamine → Poly(m -phenylene isophthalamide)
(4.64)

Nomex is sold as both a fiber and a sheet. The sheet material is often a laminate with paper and

used in electrical applications such as circuit boards and transformer cores. Firefi ghter gear gener-
ally includes a Nomex hood to protect the face not protected by the helmet. Race car drivers are

required to wear flame-resistant underwear, balaclava, socks, shoes, and gloves that are typically

made from Nomex fiber. Military pilots also wear flight suits that are often Nomex.

Because of the meta orientations of the reactants, Nomex chains do not realign on fi ber forma-
tion, resulting is a material that is weaker than the corresponding para-oriented polymer, Kevlar.
The corresponding aramid produced using the para reactant in place of the meta gives
poly(p-phenylene terephthalamide) (PPT) produced under the trademark of Kevlar and developed

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