Page 459 - Handbook of Properties of Textile and Technical Fibres

432 Handbook of Properties of Textile and Technical Fibres

are the same. These very simplified calculations are based on the assumption of an
ideal arrangement of copolymer chains in an unlimited length. Real fibers contain,
of course, segments of higher or lower local comonomer concentrations. Moreover,
the polymer chains have a finite length. Corresponding to a polymerizing degree of
80 is an average PET chain length of 86 nm. It is evident that some chains will not
contain any modification component at all.

13.2.2.2 Effect of modification on the state of crystalline phase

The presence of a comonomer has an influence on the rate of crystallization and on the
whole crystalline structure. The following possibilities exist:
• mixed crystals are formed (the comonomer is built into the crystallites);
• crystallites are formed by the monomer units of the basic homopolymer only (comonomer
units remain in the amorphous phase only);
• crystallization is prevented (in this instance, however, the copolyester is unsuitable for prep-
aration of textile fibers);
• two independent crystalline structures are formed (this comes into consideration with block
copolymers only).
It is evident that increasing the relative molecular mass of the polymer leads to a
retardation of crystallization. But the temperature corresponding to the maximum crys-
tallization rate, T cm , does not change in practice (Jackson and Longmann, 1969). The
addition of a comonomer will shift T cm towards lower values. Likewise the crystalli-
zation rate will be generally markedly decreased. Owing to the effect of a reduced glass
transition temperature T g and shifted T cm , the crystallization rate of copolymers can be
even higher. Privalko (1978) has found that T cm (K) is related to the glass transition
temperature of polymers T g (K) by the empirical relation T cm ¼ 1.26 T g .
Isomorphous modification components increase the rate of crystallization when
used at lower concentrations. But at higher concentrations above 5 M% with adipic
acid, even such modification components will decrease the crystallization rate. These
conclusions are supported by the results of measurements of the dependence of the
cold crystallization temperature T c on the content of different comonomers (Militký
et al., 1991). The higher the value of T c the more difficult will be the crystallization
of a given system. An increase in isophthalic acid content leads to a marked increase
of T c . In copolyesters containing a sodium salt of 5-sulfoisophthalic acid, T c increases
proportionally to the modifying component content. On the other hand, increasing the
content of adipic acid, DEG, and PEG (Veena et al., 1979) decreases the T c .
The equilibrium degree of crystallinity at a given temperature is affected by the
presence of comonomers to a much lesser extent. Thus, for example, it needs 10%
content of isophthalic acid to cause a slight decrease in equilibrium crystallinity
(Militký et al., 1991). A more distinct decrease in the equilibrium degree of crystal-
linity is caused by comonomers that increase the rigidity of chains (e.g., sodium salts
of 5-sulfoisophthalic acid).
The different crystallization rates of copolymer and homopolymer can play an
important role in the course of fiber production, mainly during drawing and setting