Thermoplastic polyester elastomers (copolyesters,
COPEs) are multi-block copolymers, which can be
represented by a generalized formula (–A–B–)n [1].
They are essentially copolyether esters with alternating,
random-length sequences of either long-chain
or short-chain oxyalkalene glycols connected by
ester linkages [2]. Structurally, they are related to
polyurethanes (Chapter 9) and polyamide elastomers
(Chapter 10) in that they also contain repeating highmelting
blocks that are capable of crystallization
(hard segments) and amorphous blocks with a relatively
low glass transition temperature (soft segments).
Typically, the hard segments are composed
of multiple short chain ester units such as tetramethylene
terephtalate units and the soft segments are
derived from aliphatic polyether and polyester glycols
[2]. At useful service temperatures these materials
resist deformation because of the presence of a network
of microcrystallites formed by partial crystallization
of hard segments. These microcrystallites
function as physical cross-links. At processing temperatures
the crystallites melt and form a viscous
melt that can be shaped by usual melt-processing
methods. After cooling the hard segments recrystalize
and keep their shape. The ratio of hard to soft segments
determines the characteristics of the product.
Thus, the products range from soft elastomers to
hard elastoplastics [2].
An example of the composition of such copolymer
is [3]:
— [4GT]x [BT]y [4GBT]z —
where 4G is butylene glycol, B is poly(tetramethylene
ether glycol), and T is dimethyl terephtalate.
Copolyesters were commercialized in the
early 1970s by DuPont under the trade name
Hytrel® and by Toyobo under the trade name
Pelprene®. Within the following decade several
other U.S. manufacturers developed their own versions
of copolyester elastomers; GAF Corporation
(Gaflex®), Eastman Chemical Products (Ecdel®),
General Electric Company (Lomod®). They have
been often referred to as engineering thermoplastic
elastomers because of their high flexural modulus
and other characteristics (see below) [4].