Page 491 - Carrahers_Polymer_Chemistry,_Eighth_Edition

454 Carraher’s Polymer Chemistry

13.1.5 ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY

Electron paramagnetic resonance (EPR) spectroscopy, or electron spin resonance (ESR) spectros-
copy is a valuable tool for measuring the relative abundance of unpaired electrons present in mac-
romolecules. For example, macroradicals are formed by the homogeneous cleavage of nylon-66
chains when these fi laments are broken, with the concentration of macroradicals increasing as the
stress is increased.

13.1.6 X-RAY SPECTROSCOPY

X-ray diffraction is a widely used tool for structural identification for almost all solids under the

right conditions. X-ray diffractometers are generally either single-crystal or powder.
Single-crystal studies allow the absolute configurational determination of polymeric materials

that have high degrees of crystallinity. Such determinations are costly with respect to time because
of the complexity of polymeric materials.
Powder X-ray spectroscopy can employ smaller crystalline samples from 1 to several hundred

nanometers. These crystallites have broadened peak profiles as a result of incomplete destructive
interference at angles near the Bragg angle defi ned as
nλ = 2d sinθ (13.1)

where n is the order of a refl ection, λ is the wavelength, d is the distance between parallel lattice
planes, and θ is the angle between the incident beam and a lattice plane known as the Bragg angle.
This broadening allows determination of crystallite size and size distribution. (Note that this is not
particle size.)
X-ray analysis of proteins and nucleic acids is especially important as the absolute structure is
needed for many advances in the field of medicine and biochemistry.

13.2 SURFACE CHARACTERIZATION

Everything has a surface or an interface. These surfaces have their own kinetic and thermodynamic
features that affect their formation and behavior. Sperling notes that for most polymers, the end groups
reside perpendicular to the bulk of the polymers probably because the end is less hydrophobic to the
bulk and the polymer surfaces generally are “faced” with an air atmosphere that is more hydrophilic.
When a polymer solution is deposited onto a surface to “dry,” the concentration has an influence on the

orientation of the polymer chains at the surface in the dried solid. Thus, when the amount of polymer
is small, the polymer chain lays parallel to the surface in a so-called “pancake” form (Figure 13.1). As
the concentration increases, the surface is not able to accommodate the entire polymer chain and it
begins to form an inner tangled chain with only the end and some of the chain segments facing the sur-
face forming a “mushroom” shape. Finally, as the concentration of polymer increases, only the ends of
the polymer chains occupy the surface with the polymer ends forming “brushes.”
There is no exact, universally accepted structure of a surface. Here, the surface will be defi ned as
the outermost atomic layers, including absorbed foreign atoms. The chemical and physical composi-
tion, orientation, and properties of surfaces typically differ from those of the bulk material.
Current surface characterization techniques fall into two broad categories—those that focus
on the outermost few layers (to within the 10–20 atom layer boundary) and those whose focus
includes components present to several thousand angstroms into the solid (hundred to several hun-
dred layers).

Attenuated total refl ectance typically employs special cells fitted onto traditional IR, FTIR, or
ultraviolet (UV) instruments. While some outer surface aspects are gleaned from such techniques,
information to several thousand angstroms is also present in the spectra from ATF.

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