Chemistry Reference
In-Depth Information
characterized by fluorescence polarization, deuterium nuclear magnetic
resonance (NMR), and polarized infrared spectroscopy.
1, 3, 12
Infrared spectroscopy has been used to characterize the structures of
silica-filled polydimethylsiloxane (PDMS).
9,
13-15
Other optical and spectroscopic techniques are also important, includ-
ing positron annihilation lifetime spectroscopy,
16,
17
spectroscopic ellip-
sometr y,
18
confocal Raman spectroscopy,
19
and photoluminescence
spectroscopy.
20
Surface-enhanced Raman spectroscopy has been made
tunable using gold nanorods and strain control on elastomeric PDMS
substrates.
21
4.3 MICROSCOPIES
A great deal of information is now being obtained on filler dispersion and
other aspects of elastomer structure and morphology through the use of
scanning probe microscopy, which consists of several approaches.
22-27
One
approach is that of scanning tunneling microscopy (STM), in which an ex-
tremely sharp metal tip on a cantilever is passed along the surface while
measuring the electric current flowing through quantum mechanical tun-
neling. Monitoring the current then permits maintaining the probe at a
fixed height above the surface. Display of probe height as a function of sur-
face coordinates then gives the desired topographic map. One limitation of
this approach is the requirement that the sample be electrically conductive.
Atomic force microscopy (AFM), on the other hand, does not require a con-
ducting surface. The probe simply responds to attractions and repulsions
from the surface, and its corresponding downward and upward motions are
directly recorded to give the relief map of the surface structure. The probe
can be either in contact with the surface or adjacent to it, sensing only Cou-
lombic or van der Waals forces.
Both transmission electron microscopy and AFM have been used to
characterize the structures of silica-filled PDMS.
7,
28
Another example of
an application to polysiloxane elastomers is the characterization of
binodal and spinodal phase-separated structures occurring in model
PDMS networks.
29-31
4.4 NUCLEAR MAGNETIC RESONANCE
Although NMR has been used to characterize some aspects of polym-
erization (e.g., copolymerization kinetics),
32
the applications most
