Page 487 - Carrahers_Polymer_Chemistry,_Eighth_Edition

450 Carraher’s Polymer Chemistry

13.1 SPECTRONIC CHARACTERIZATION OF POLYMERS

13.1.1 INFRARED SPECTROSCOPY
The infrared (IR) spectral range spans the region bound by the red end of the visible region to the
microwave region at the lower frequencies. Molecular interactions that involve vibrational modes
correspond to this energy region. IR is one of the most common spectronic techniques used today
to identify polymer structure. Briefly, when the frequency of incident radiation of a specifi c vibra-

tion is equal to the frequency of a specific molecular vibration, the molecule absorbs the radiation.

Today, most IR machines are rapid scan where the spectra are Fourier transformed. For the most
part, IR band assignments for polymers are analogous to those made for small molecules.
In Fourier transform infrared spectroscopy (FTIR), the light is guided through an interferome-
ter where the signal undergoes a mathematical Fourier transform giving a spectrum identical to the
conventional dispersive IR.
With the advent of femtosecond infrared laser pulses, two-dimensional infrared correlative spec-
troscopy has become a new tool. Here, pump pulses are applied to the sample. After some time,
that can be from zero to several picoseconds to allow the sample to relax, a second pulse is applied.
The result is a two-dimensional plot of the frequency that resulted from the initial pump pulse and
a second plot resulting from the relaxed state spectrum. This allows the coupling of various vibra-
tional modes. In some ways, this is similar to two-dimensional nuclear magnetic resonance (NMR)
spectroscopy in that the spectrum is spread out in two dimensions allowing certain “cross-peaks”
to be observed.
Following are brief discussions of some of the more important techniques used specifi cally for
polymer analysis.
Attenuated total refl ectance IR (ATR-IR) is used to study films, coatings, threads, powders,

interfaces, and solutions. (It also serves as the basis of much of the communications systems based
on fiber optics.) ATR occurs when radiation enters from a more-dense material (i.e., a material with

a higher refractive index) into a material that is less dense (i.e., with a lower refractive index). The

fraction of the incident radiation reflected increases when the angle of incidence increases. The

incident radiation is reflected at the interface when the angle of incidence is greater than the critical
angle. The radiation penetrates a short depth into the interface before complete refl ection occurs.
This penetration is called the evanescent wave. Its intensity is reduced by the sample where the
sample absorbs.

Specular refl ectance IR involves a mirror-like reflection, producing reflection measurements of

a reflective material or a reflection–absorption spectrum of a film on a reflective surface. This tech-

nique is used to look at thin (from nanometers to micrometers thick) fi lms.
Diffuse refl ectance infrared Fourier transform spectroscopy (DRIFTS) is used to obtain spectra
of powders and rough polymeric surfaces such as textiles and paper. IR radiation is focused onto the

surface of the sample in a cup resulting in both specular reflectance (which directly reflects off the

surface having equal angles of incidence and reflectance) and diffuse reflectance (which penetrates
into the sample subsequently scattering in all angles). Special mirrors allow the specular refl ectance
to be minimized.
Photoacoustic spectroscopy IR (PAS) is used for highly absorbing materials. In general, modu-
lated IR radiation is focused onto a sample in a cup inside a chamber containing an IR-transparent
gas such as nitrogen or helium. The IR radiation absorbed by the sample is converted into heat
inside the sample. The heat travels to the sample surface and then into the surrounding gas causing
expansion of the boundary layer of gas next to the sample surface. The modulated IR radiation thus
produces intermittent thermal expansion of the boundary layer creating pressure waves that are
detected as photoacoustic signals.
PAS spectra are similar to those obtained using ordinary FTIR except truncation of strong absorp-
tion bands occurs because photoacoustic signal saturation often occurs. PAS allows the structure to

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