Chemistry Reference
In-Depth Information
Si
O
Si
O
Si
O
O
O
O
Si
O
Si
O
Si
O
Rigid Polymers
Figure 5.3:
Some ways for making a polymer more rigid.
29
Reproduced by permission of John Wiley and Sons.
may provide considerable reinforcement for a network. Most polysiloxane
elastomers, however, have melting points that are too low to benefit from
this effect.
Figure 5.3 shows examples of ways in which a polymer can be made
more rigid.
88
Possibilities include combining two chains in a ladder struc-
ture (figure 5.4), insertion of rigid units such as
p
-phenylene groups into
the chain backbone and the addition of bulky side groups. Attempts have
been made to prepare the silsesquioxane ladder polymer shown in the
upper portion of figure 5.3 using trifunctional silanes.
89,
90
The basic strat-
egy is to decrease the entropy of fusion and thus increase the melting
point
T
m
(which is inversely proportional to the entropy of fusion). If the
chains are combined into a ladder structure they cannot disorder as much
as when they are separate, as shown in figure 5.5. The same argument
holds for the other two methods for increasing
T
m
. This decreased equilib-
rium flexibility is generally paralleled by decreased dynamic flexibility,
and thus by an increased glass transition temperature
T
g
. An advantage of
the ladder structure is its resistance to degradative chain scission. The
Ph
Ph
Ph
Ph
Ph
Ph
O
O
O
O
O
Si
Si
Si
Si
Si
Si
H
2
O
PhSiCl
3
O
O
O
O
O
O
Si
Si
Si
Si
Si
Si
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Figure 5.4:
A possible reaction for preparing a sesquisiloxane ladder polymer.
302
Reproduced by permission of Prentice Hall, Inc.
