Chemistry Reference
In-Depth Information
3.4.5
Aqueous SEC
Most synthetic polymers are analyzed in organic solvents, using appropriate SEC
column packings in which the only interaction between the macromolecular solute
and the packing is steric. Separation of the polymeric species is inversely related to
their hydrodynamic volumes because the flow paths of the larger species are short-
ened by their inability to sample all the pores as they move with the flowing sol-
vent. The same basic SEC technique is used to characterize polymers that are
soluble primarily in water. Here, however, the procedure is more likely to be com-
plicated by polymer-packing interactions. The packings consist of derivatized sil-
ica or cross-linked hydrophilic gels, in contrast to the cross-linked polystyrene or
similar substrates used in organic phase SEC. Both the packing and solute contain
polar groups, and interactions may prevent purely steric separation. Efficient anal-
yses of different water-soluble polymers are often quite specific to the particular
material and more specialized references should be consulted for information.
3.4.6
Inhomogeneous Polymers
A polymer sample may consist of a mixture of species whose compositions differ
enough to affect the responses of both the concentration-dependent detector and
the molecular-weight-sensitive detector in a multidetector system. Examples are
mixtures of different polymers or copolymers (Chapter 9) whose composition is
not independent of molecular size. Conventional GPC cannot be used reliably to
characterize such mixtures, but an on-line viscometer can be employed to mea-
sure molecular weight averages independent of any compositional variations
[25]
.
Remember, of course, that such data characterize the mixture as a whole and not
just the major component.
Some polymers are homogeneous with respect to overall chemical composi-
tion but vary enough in branch frequency or comonomer spacing that important
physical properties may be affected. A prime example is copolymers of ethylene
and alpha-olefins (so-called linear low-density polyethylene, LLDPE). Here, the
relative frequency of comonomer placements is reflected in changes in branch fre-
quency, which influence the processing behavior of the polymer. In such cases,
even apparent identity of overall chemical composition and molecular weight dis-
tributions does not guarantee the same physical properties. A more complete anal-
ysis of the polymer structure then requires characterization of branch frequency
as well as SEC molecular weight data. A useful technique to assess branching of
such polymers is temperature rising elution fractionation
[26]
.
3.4.7
MALDI-MS
[27]
MALDI-MS refers to matrix-assisted laser desorption
ionization mass spectros-
copy. It is also called MALFI-TOF, because the mass spectrometer is a time-
of-flight version. In this technique, the polymer is mixed with a molar excess of a
