Chemistry Reference
In-Depth Information
M
Fi g u re 7. 27:
The network chain-length distributions shown in figure 7.15, with the addition of the
extremely broad “pseudo-unimodal” distribution obtainable by combining a number of
samples of the same polymer made in different polymerizations.
7.4 TRAPPING OF CYCLIC OLIGOMERS WITHIN NETWORK
STRUCTURES
7.4.1 Experimental Results
If cyclic molecules are present during the end linking of chains, some will be
trapped because of threading by the linear chains prior to the latter being
chemically bonded into the network structure (figure 7.28).
96,
288,
371,
373
he
fraction trapped is estimated from solvent extraction studies. Figure 7.29
shows schematically the fraction trapped as a function of degree of polym-
erization of the cyclic.
374
The results were independent of intermingling
time,
288
thus demonstrating the high mobility of the PDMS chains. As ex-
pected, very small cyclics don't get trapped at all, but the largest cyclics do.
The following section describes the interpretation of these data in terms of
the configurational characteristics of PDMS chains.
Cyclics can change the properties of the network. Since cyclics restrict
motion of the network chains, they should increase the modulus of an elasto-
mer. Small but significant increases in low-deformation modulus have in fact
been observed.
372
Also, when PDMS cyclics are trapped in a thermoplastic,
they can act as a plasticizer that is in a sense intermediate to the usual exter-
nal (dissolved) and internal (copolymerized) varieties. Interesting changes in
mechanical properties have been observed in materials of this type.
375,
376
7.4.2 Theoretical Interpretation
The trapping process has been simulated using Monte Carlo methods
based on a rotational isomeric state model
377-379
for the cyclic chains.
374
