Page 483 - Handbook of Properties of Textile and Technical Fibres

456 Handbook of Properties of Textile and Technical Fibres

(180e190 C), and melting point (256 C). When amorphous PET is saturated by water
a reduction in T g of 15 C occurs (Illers and Breuer, 1963).

The cohesion energy DE k of amorphous PET and copolymers can be calculated
from the molar constant of attraction by the molar contribution method (Van Krevelen,
1990). For PET, a value DE k ¼ 74.29 kJ /mol was computed (Militký et al., 1991).
Typical PET is 40% crystalline. Another factor for crystallization is the position of
the benzene rings. If the benzene rings are placed on the chain axis (c) then close
packing of the molecular chains helps polymer crystallization. The elastic modulus
of crystalline regions of PET in the direction parallel to the chain axis is 108 GPa.
The calculated Poisson’s ratio is around 0.34.

13.3.4.2 Supramolecular structure
Polyester fibers may be considered to be composed of crystalline, oriented noncrystal-
line (mesophase, tie molecules), and noncrystalline (amorphous) regions. There is a
maximum crystallization rate at 180 C for crystallization from the quiescent melt,

which is independent of PET characteristics and measurement technique. The shortest
half-times for crystallization are in the range of 16e50 s (Estes and Schweizer, 1987).
The quenched fiber does not show any development of crystallinity. As the crystal-

lization of PET does not start before a temperature of 85 C is attained, the undrawn
fibers (spun below 4000 m/min), which are relatively rapidly cooled below this
temperature after extrusion from the spinning nozzle, are nearly amorphous. Drawing
conducted at lower temperatures can give a highly oriented structure with a low level
of crystallinity. However, as drawing is generally conducted at higher temperatures,
these fibers exhibit a relatively high crystallinity (about 15% of crystalline phase).
During subsequent tempering processes additional crystallization takes place so that,
eventually, commercial fibers are about 40% crystalline.
The triclinic unit cell of crystalline PET is equal to more than 98% of the theoretical
extended length of the monomer repeat unit. There is a little molecular extensibility
remaining in a PET crystal resulting not only in a high modulus but also in a relatively
short extension range over which the crystal can be recovered elastically. The length of
the polymer chain within a crystalline region is typically around 20 repeat units. The
crystalline regions of PET are composed primarily of folded chain segments, so that
the length of any given crystalline region is fairly small before being interrupted by
an amorphous region. Crystalline regions are of different sizes and the size and distri-
bution of these crystallites contribute to some fiber properties.
A number of basic structural models are required to represent the different states of
the fiber: amorphous (no orientation) after extrusion, amorphous (oriented) after cold
drawing, and crystalline oriented after thermal treatment and after hot drawing, stretch-
ing, and annealing. The crystalline oriented form can also be obtained by high-speed
spinning. It is important that the rate of crystallization for oriented fibers under tension
is thousands of times faster than for unoriented melts.
The basic structural element of all semicrystalline fibers is the microfibril. In PET
fibers the microfibril thickness is around 10 nm and the length is comparable to that of
macromolecular chains: around 1 mm. Microfibrils are thin, long elements of elliptical