Handbook of Size Exclusion Chromatography and Related ...

of any other constituent. One might be tempted, therefore, to assume that a
copolymer composed of various combinations of two monomers could be treated
as if the different monomeric components scattered light in the absence of the
other monomeric constituents. Thus, were it possible to know the relative
composition of each homopolymer at each slice, then one might obtain a weightaverage
value of each constituent eluting in that slice and, therefrom, a weightaverage
of the copolymer in that slice. Unfortunately, the polarizability of a
copolymer molecule, and therefore its effective dn/dc value, depends critically
upon the composition of the molecule. The problem is far more complicated
here for an unfractionated sample than for the case discussed earlier of a
homogeneous copolymer whose composition is independent of molar mass. The
difficulties and complex means by which the weight-average molar mass of such
an unfractionated sample be determined was described by Benoit and Froelich
(20). It should be emphasized that even after separation by SEC, a particular
slice still contains a distribution of molar masses since the different molar
masses present can have a large variation despite the fact that they share an
equivalent hydrodynamic size.
Because MALS has proven to be so successful a means for the
determination of molar mass for homogenous copolymers, attempts always
persist to find a means by which molar mass may be deduced even for such
complex systems of heterogeneous copolymers. Since the thermal conductivity
of a copolymer molecule depends on its composition, the possibility of using
thermal FFF as a separation technique (16) remains a possibility to be explored.
Unfortunately, the actual result of such separation in terms of molar mass or size
remains unknown. Other attacks on the problem have been suggested in the past
including finding a solvent that is isorefractive for one component, thereby
permitting the measurement of the “visible” component. From the measured
concentration of this component, one could change solvents to obtain a measure
of the sum of the contributions and from those, attempt to determine the sample’s
weight-average molar mass. However, note all the additional complications such
an approach would entail. As each solvent is introduced, the dn/dc values for
each homopolymer constituent would change, as would the separation
mechanism itself. Alternatively, one might try to find a solvent whereby the
differential refractive index of each homopolymeric constituent would be of the
same magnitude but of different sign. Thus the (dn=dc) 2 factor of K in Eq. (1)
would be the same for each component. Yet, the concept of separating the
molecules by molar mass remains elusive and unpredictable for such
heterogeneous copolymers. The objective of determining the molar mass
distribution often remains elusive.
Generally, attempts to separate compositional distributions from molar mass
distributions for such heterogeneous copolymers have been all but abandoned by
the decision to treat all such copolymers as having the properties of a
© 2004 by Marcel Dekker, Inc.