Chemistry Reference
In-Depth Information
alcogels,
85
or zeolites.
86
Similar decreases in T
c
are observed in nanocrys-
tals or metal clusters prepared for use as “quantum dots.”
87,
88
Swollen elastomers have domains of solvent between the network
chains that may also be considered “pores.” Thermoporometry is useful to
characterize the pore size and size distribution in such gels. Examples in-
clude natural rubber,
89,
90
styrene-divinyl benzene polymers,
91
poly(ethylene
oxide),
92
polyethylene and polypropylene,
93
and ethylene-propylene-diene
polymers and
cis
-polycyclo-octene.
94
Relevant here are the corresponding studies on polysiloxane gels such
as PDMS of various degrees of cross linking (giving various pore sizes).
The samples are either unfilled95-97
95-97
or filled with silica.
96
Aged PDMS has
also been studied.
98
In some cases, PDMS networks were prepared by end-
linking chains, to have known values of the molecular weight between
cross links. These model elastomers had chain-length distributions that
were unimodal,
99, 100
bimodal,
99
or trimodal.
99
The effects of constraints in general on the freezing temperature have
been studied theoretically, specifically in an entanglement/frozen tube
model,
101
and by molecular dynamics simulations.
102
Confinement of liq-
uids also affects the glass transition temperature, which typically de-
creases as the pore size decreases.
103-105
4.6 SCATTERING OF LIGHT, X-RAYS, AND NEUTRONS
Static and dynamic light scattering have been used to investigate PDMS in
both liquid and supercritical carbon dioxide.
106
The solvent quality of the
CO
2
was found to be adjustable by independently varying temperature or
density. The results give the theta temperature and strength of excluded
volume interactions.
107
Small-angle scattering techniques have been applied to polysiloxane
materials. One important example is the characterization of fillers intro-
duced into polysiloxane elastomers, or the reverse, the incorporation of
such elastomers into ceramic matrices (in both cases to improve mechan-
ical properties).
3, 108, 109
Another example is characterization of the ani-
sotropy induced by strain in silica-PDMS composites.
110
Chapter 9
describes some of this work. Elastic neutron scattering can be illustrated
by the characterization of polysiloxane blends,
111
and quasielastic neu-
tron scattering by studies of the dynamics of PDMS.
112
There have also
been Monte Carlo calculations of PDMS particle scattering functions,
including how they varied with chain length, chain structure, and
temperature.
113
