Page 157 - Carrahers_Polymer_Chemistry,_Eighth_Edition

120 Carraher’s Polymer Chemistry

In general, more crystalline nylons are used as fibers, while less crystalline nylon materials are

more used as plastics. The amount of crystallinity is controlled through a variety of means, includ-
ing introduction of bulky groups and asymmetric units, rapid cooling of nonaligned melts, and
introduction of plasticizing materials. The theme of using asymmetric units was used by Grace
and Company in developing Trogamid T, an amorphous transparent nylon, from the condensation
of terephthalic acid with a mixture of 2,2,4- and 2,4,4-trimethylhexamethylene diamines.

4.8 POLYIMIDES

The initial announcement for the commercial preparation of polyetherimides (PEIs) was made by

General Electric in 1982 under the trade name of Ultem. The final reaction involves the imidiza-
tion of a diacid anhydride through reaction with a diamine, here m-phenylenediamine (4.67). The
“ether” portion of the polymer backbone results from the presence of ether linkages within the
diacid anhydride.
Ultem is used in medical and chemical instrumentation. Products made from Ultem are high
melting, offer good stiffness, transparency, impact and mechanical strength, high fl ame resistance,
low smoke generation, and broad chemical resistance. Some of these properties are expected. The
high flame resistance is at least in part derived from the presence of already partially or largely

oxidized atoms in the product. The low smoke generation is partially derived from the largely cyclic
structure with other cyclic structures predictable from the product structure if exposed to suffi cient
heat. These cyclic structures often give products that are not evolved with good char formation when
the material is exposed to ordinary fl ame conditions.

O

O H C
3
O
R N CH 3 N
(4.67)
O
O
O

R
Polyetherimide

The general good mechanical properties are a result of the presence of strong double bonds pres-
ent within polycyclic structures composing the polymer backbone plus the presence of strongly polar
bonding units that allow the formation of good interactions between chains. Further, the structure is
largely rigid with good dimensional stability along the polymer backbone. Any flexibility is gained

largely from the presence of the ether linkages for the PEI and the presence of methylene units for the
polyimides. These products offer good stable melt viscosities even after recycling several times. They
can be processed using a variety of techniques including formation of sections as thin as 5 mil.
While Ultem is an example of thermoplastic polyimides, there are a number of thermoset poly-
imides. These are known for good chemical resistance and mechanical properties as well as good

thermal stability. They are employed in electronic applications as insulating film on magnetic wire,
for flexible cables, and for medical tubing. On a laptop computer, the cable connecting the main

logic board to the display is often a polyimide with copper conductors. Polyimides are also used as
high-temperature adhesives in the semiconductor sector.

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