Handbook of Size Exclusion Chromatography and Related ...

composition of the different deposited dried fractions. However, the poor film
morphology produced by the evaporative interface can seriously affect the FTIR
spectral accuracy, and a film posttreatment may be required (17–19). On-line
densimeters are, in general, less sensitive than DRs (20).
The instantaneous molar mass is also difficult to estimate. In the more
normal situation, in-line molar mass sensors are not available, and a molecular
weight calibration is employed. Since copolymer standards are, in general,
unavailable, the universal calibration is generally employed (21,22). The universal
calibration assumes that at any elution volume V, the hydrodynamic volume is
proportional to fM(V) [h](V )g, where M is the molar mass and [h] isthe
intrinsic viscosity. Unfortunately, this concept yields only approximate molar
masses. This is because fM(V) [h](V)g represents the hydrodynamic volume of
flexible molecules under Q-conditions, while good solvents are used in SEC.
Furthermore, to transform [h] into molar mass, the Mark–Houwink parameters of
the analyzed copolymer are required. Unfortunately, these parameters are
generally unknown because they depend on many variables (not only on the
solvent and the temperature, but also on the chemical composition, the molar
mass, the polymer microstructure, and the level of branching) (23,24). For block
copolymers, it has been suggested to estimate the Mark–Houwink parameters by
interpolation (with the chemical composition) between the Mark–Houwink
parameters of the corresponding homopolymers. This procedure includes a
correction term for statistical copolymers with many sequence alternations (25).
Consider the direct molar mass measurement through an intrinsic
viscometer (IV) or a light-scattering (LS) detector. Their signals are proportional
to the instantaneous molar mass (15,16,26–28), and for this reason the
measurements are insensitive to low molar masses (e.g., lower than 30,000g/mol).
LS sensors have the advantage of not requiring any molecular weight calibration.
However, the specific refractive index increment of the instantaneously analyzed
fraction must be a priori known, and this information is in general unavailable.
However, even if it were, only an apparent (rather than a true) molar mass would be
determined by LS (15,28). For the IV signal, either the universal calibration or the
Mark–Houwink parameters of the analyzed copolymer are required. Both
approaches only produce approximate molar masses, however. In spite of all their
limitations, IVs are generally preferred to LS sensors for analysing copolymers,
except for the rather special case where the specific refractive index increments of
both repeating unit types are identical (14). Through triple detection SEC
(i.e., DR þ IV þ LS sensor) it is in principle possible to characterize a
chromatographically complex polymer without resorting to any molecular weight
calibration (28,29). However, its applicability to copolymers with a varying
composition along the elution volume has not yet been fully demonstrated. Also,
M n may be directly obtained from the IV signal and the universal calibration,
without requiring an instantaneous mass measurement (30).
© 2004 by Marcel Dekker, Inc.