RAMAN APPLICATION IN EMULSION POLYMERIZATION SYSTEMS - Raman Spectroscopy for Soft Matter Applications - page 101

[NIR] and mid-range [MIR] spectroscopies) are sensors in the second
category that have been used to monitor polymerization reactors. The
advantage of these spectroscopic techniques relies on the noninvasive
nature of the measurement (the reactor content is not handled outside
of the reaction vessel). All these spectroscopic techniques have been
used to monitor emulsion polymerization and other related systems (see
earlier discussion) [19-30] and each presents advantages with respect to
other invasive (e.g. gas chromatographs and densitometers) or noninva-
sive (e.g. reaction calorimetry and ultrasounds) techniques that have
been used so far [31]. The multiple advantages that the Fourier transform
(FT) spectrometers offer, combined with the excitation in the NIR
region, have rediscovered Raman spectroscopy for polymer analysis. In
recent years, its use in the area of polymeric materials has grown con-
siderably, in part because all the problems regarding the existence of
fl uorescence (the main limitation of the conventional Raman for polymer
analysis) were solved. Furthermore, Raman spectroscopy has shown to
be better suited than absorption techniques (mainly MIR and NIR) to
monitor emulsion polymerization systems [20,32]. Some of these advan-
tages are related to the spectroscopic information while others to the
instrumentation itself.

5.2.2.1 Spectroscopic Advantages Water is an ideal medium
because its Raman spectrum is very weak. It does not absorb the laser
radiation, it does not heat up due to the radiation, and it does not
attenuate the emitted Raman radiation (which would cause a distortion
of the relative intensities of the spectrum, affecting the quantifi cation).
Figure 5.2 shows the Raman spectra of water and poly(methyl meth-

0.25

0.2

0.15

0.1

0.05

0

0 500 1000 1500 2000 2500 3000 3500

cm -1

Figure 5.2 Fourier transform (FT)-Raman spectra of water (solid line) and a PMMA/
PBA latex (dashed line) taken at room T, using a near-infrared (NIR) laser, with a
power of 500 mW, 8 cm − 1 resolution and accumulating 50 scans.