Page 497 - Carrahers_Polymer_Chemistry,_Eighth_Edition

460 Carraher’s Polymer Chemistry

TABLE 13.1
Techniques Employed to Study the Amorphous
Regions of Polymers
Short-range interactions Long-range interactions
Magnetic birefringence Electron diffraction
Raman scattering Small-angle X-ray scattering
Depolarized light scattering Electron microscopy
Rayleigh scattering Density
Bruillouin scattering Small-angle neutron scattering
NMR relaxation
Wide-angle X-ray scattering

Birefringence measures order in the axial, backbone direction. The birefringence of a sample

can be defined as the difference between the refractive indices for light polarized in two directions
o
90 apart. Thus, a polymer sample containing polymer chains oriented in a preferential direction
by stretching or some other method will exhibit a different refractive index along the direction of
preferred chain alignment compared to that obtained at right angles. This change in birefringence
gives information concerning the amount of order, thus, information about disorder.
Small-angle neutron scattering (SANS) results indicate that vinyl polymers exist in random coils
in the amorphous state. Results from electron and X-ray diffraction studies show diffuse halos con-
sistent with the nearest-neighbor spacings being somewhat irregular. It is possible that short-range
order and long-range disorder exists within these amorphous regions.

13.4 MASS SPECTROMETRY

There are a number of mass spectrometry (MS) techniques available and directly applicable to poly-
mers. Since high polymers have high molecular weights, determination of unbroken chains is not
usual. Even so, determination of structures of ion fragments of segments of the polymers is straight
forward and a valuable tool in determining the unit structure.
The exception to this is the application of matrix-assisted laser desorption/ionization mass

spectrometry (MALDI MS). In 1981, Barber and Liu and coworkers independently introduced the
concept of employing matrix-assisted desorption/ionization where the absorption of the matrix is
chosen to coincide with the wavelength of the employed laser to assist in the volatilization of mate-
rials. In 1988, Tanaka, Hillenkamp and coworkers employed the laser as the energy source giving
birth to MALDI MS.
Matrix-assisted laser desorption/ionization mass spectrometry was developed for the analysis
of nonvolatile samples and was heralded as an exciting new MS technique for the identifi cation
of materials with special use in the identification of polymers. It has fulfi lled this promise to only

a limited extent. While it has become a well used and essential tool for biochemists in exploring
mainly nucleic acids and proteins, it has been only sparsely employed by synthetic polymer chem-
ists. This is because of lack of congruency between the requirements of MALDI MS and most
synthetic polymers.
Classical MALDI MS requires that the material be soluble in a suitable solvent. A “suitable sol-
vent” means a solvent that is suffi ciently volatile to allow it to be evaporated before the procedure.
Further, such a solvent should dissolve both the polymer and the matrix material. Finally, an ideal
solvent will allow a decent level of polymer solubility, preferably a solubility of several percentage

and greater. For most synthetic polymers, these qualifications are only approximately attainted.
Thus, traditional MALDI MS has not achieved its possible position as a general use modern char-
acterization tool for synthetic polymers. By comparison, MALDI MS is extremely useful for many

9/14/2010 3:42:15 PM
K10478.indb 460 9/14/2010 3:42:15 PM
K10478.indb 460