Chemistry Reference
In-Depth Information
Bimodal
Unimodal
Time
Fi g u re 7. 25:
Dependence of relative length of a sample on cyclic compressive stress as a function of
time for a unimodal and bimodal elastomer.
Nuclear magnetic resonance (NMR) has also been used to characterize
chain-segment orientation in PDMS bimodal materials.
279
Another method, “thermoporometry,” is based on the fact that solvent
molecules constrained to small volumes form small crystallites and there-
fore exhibit lower crystallization temperatures.
280-
286
Differential scanning
calorimetry on solvent molecules constrained in the pores of PDMS elasto-
mers shows several crystallization temperatures, which could be indicative
of an unusual distribution of pore sizes.
287
The effects are most pronounced
for trimodal networks, which are discussed later in this chapter.
Calorimetric measurements on bimodal poly(ethylene oxide) networks
indicate that the short chains decrease crystallinity in the unstretched
state.
257
This is an intriguing result since short chains
increase
the extent
of crystallization in the stretched state.
When cyclic molecules are present during the end-linking reaction, the
larger molecules tend to get trapped by being threaded with chains subse-
quently bonded into the network structure. Such experiments have also been
carried out using a bimodal distribution of end-linkable PDMS chains.
288
Additional insights into the dynamics and structure of bimodal elasto-
mers come from experiments using dynamic light scattering,
289
NMR
spectroscopy,
290,
292
neutron scattering,
213
and computer simulations on
chain orientations and network mechanical properties.
214
7.3.2.7 Inadvertent Bimodal Networks
Elastomers cured with sulfur frequently have improved mechanical prop-
erties when the curing conditions are chosen to give cross links that
