Chemistry Reference
In-Depth Information
relevant here have to do with the structure of the linear polymers or
cross-linked networks.
33-35
In this regard, NMR has been much used
to study the characteristics of the polysiloxanes, particularly with
regard to orientation,
36, 37
molecular motion,
38- 46
and the effect on the
diffusion and other properties of small molecules.
47-50
Small-molecule
diffusion is particularly relevant to the properties of silicone breast
implants, and NMR has been extensively used to image such im-
plants.
51-55
Aspects related to the structure of the networks include
the degree of cross linking,
56-58
the distribution of cross links,
59, 60
con-
centration of pendant chains,
61
elastic stress,
62
and topology.
63- 65
An-
other example is the use of NMR to clarify aging and phase separation.
66
Deuterium NMR has also been used to determine segment orientation
distributions in polymer networks, including those of PDMS.
67,
68
Dipolar coupling constant distribution analyses have been carried out
using proton multiple-quantum NMR on elastomers, including unimodal
and bimodal PDMS materials.
69
Also, a universal polymer analysis tech-
nique has been developed using
1
H NMR on complementary trimethylsilyl
end groups placed on the polymer.
70
Most elastomers require reinforcing fillers to function effectively, and
NMR has been used to characterize the structure of such composites as
well. One examples is the adsorption of chains onto filler surfaces,
71,
72
and the strong absorption of these chains into “bound rubber”—for ex-
ample, PDMS immobilized onto high surface area silica.
41, 46
Another ex-
ample is the use of NMR to image the filler or polymer itself.
73-80
NMR
has also been used to study the phase separation and order of water mol-
ecules and silanol groups in polysiloxane networks
81
and the activation of
transport and local dynamics in polysiloxane-based salt-in-polymer
electrolytes.
82
4.5 THERMOPOROMETRY
Thermooporometry involves measuring the crystallization temperatures
T
c
of small molecules constrained within a porous medium. The pores
keep the crystallites from growing beyond the size of the pore leading to
a large surface area to volume ratio. The interfacial free energy is positive,
which destabilizes the crystallites and decreases T
c
. The average values
can be used to estimate average pore sizes, and the distribution of T
c
s can
be used to determine the pore size distribution. The technique was origi-
nally applied to inorganic materials such as glasses,
83
mesoporous silica,
84
