2.6 Mechanical Properties
71
(a)
(b)
(c)
n
n
n
Fig. 2.62 Polymers considered: a polyethylene, b polythiophene, and c polyselenophene
Table 2.19 Change in structural parameters and bond orders of polyethylene upon the 1%
elongation
Bond lengths and angles (in Å and
degrees, respectively)
Bond orders (Mulliken’s bond
population)
Before elongation After elongation Before elongation After elongation
C 1 C 2
1.5350
1.5443
0.738
0.736
∠C 1 C 2 C 3
113.62
114.31
–
–
Unit-cell length
2.5692
2.5949
–
–
together with change in the bond orders. It is seen that bond lengths are lengthened and
the bond angles made opened in all the polymers due to the elongation of the polymer
chains but that the change in the bond orders reveals some characteristic behaviors.
That is, although the bond orders are duly weakened in polyethylene, some of those
in polythiophene and polyselenophene become a bit larger at pentagonal thiophene
and selenophene rings. These peculiar behaviors of bond orders can only be found
by theoretical analyses including the electronic structures, which might be helpful
to understand mechanical properties of molecules in deeper sense.