Page 481 - Handbook of Properties of Textile and Technical Fibres

454 Handbook of Properties of Textile and Technical Fibres

O O
C C

0.57 nm

Figure 13.17 Dimensions of terephthalate unit.
Slodovy J, Rutkowska A: Identifying the cause of destruction of textile linear structures,
AUTEX Res J 4:129e136, 2004.

coplanar arrangement of the benzene rings, carboxyl and aliphatic molecular groups in
the adjacent chains allow a side-by-side arrangement. The geometrical structure of the
terephthalate unit is shown in Fig. 13.17.
The cross-sectional area of a single PET chain is relatively small, equal to the
2
0.217 nm only (Kikutani, 2002). The cohesion of PET chains is a result of hydrogen
bonds and van der Waals interactions, caused by dipole interaction, induction, and
dispersion forces amongst the chains. The total magnitude of secondary forces in a
PET unit is 1.37 kJ/mol. Of this, 1.02 kJ/mol is due to the disperse forces induced
by the benzene rings. Thus the strength of PET fibers is determined in the first place
by the rigidity of the benzene ring (the secondary van der Waals forces decrease with
the sixth power of the distance), which forms an angle of only 12 degrees with the
plane of the ester bonds. The partial flexibility in the macromolecule of PET is mainly
due to the ethylene group. The tendency to crystallize depends on forces of attraction.
The interactive forces create inflexible tight packing among macromolecules resulting
in high moduli, strength, and resistance to moisture, dyestuffs, and solvents. The un-
usually high melting point of PET (compared to aliphatic polyesters) is attributed to
ester linkages. This means that PET is difficult to crystallize.
Rotation of chains around the C-O-C bonds in the ethylene glycol moiety of the
repeat unit results in the formation of two conformers, a planar trans-conformer and
a spatial gauche-conformer (see Fig. 13.18).
Trans-conformation corresponds to the arrangement with the longest elementary
unit. It is the state at which the so-called van der Waals distances between the chains
and the individual groups in the chains are maintained. It is known that the van der
Waals distances are distances at which the attractive and repulsive components of

(a) (b) 6
3
3 1 1
2 4 5 6 2 4 5

Figure 13.18 The glycol segment of polyethylene terephthalate in the (a) trans and (b) gauche
conformations (Saunders et al., 2007).