Handbook of Size Exclusion Chromatography and Related ...

y SEC alone (11,12,46). In this case, the following is required: (1) the
instantaneous distributions of the molecular weights and of the number of
branches per molecule are both narrow, (2) the average number of branches per
molecule increases monotonically with the molar mass, and (3) the CCD
is narrow, or (at least) the average composition does not change with the molar
mass (11,12). The first condition is again the most important. All three conditions
are approximately verified in a copolymerization where both reactivity ratios are
close to 1, and where long branches are produced by reaction with the accumulated
polymer.
In the remaining sections, three styrene–butadiene (SB) copolymers are
analyzed by SEC alone. In Example 1, the aim is to determine the MWD of a
statistical SBR obtained in an emulsion process. In Example 2, the aim is to
determine the MWD and CCD of a linear diblock SB rubber obtained in a
sequential anionic polymerization. In Example 3, the aim is to determine the
MWD and BD of a graft SB copolymer contained in high-impact polystyrene.
Examples 2 and 3 have been previously presented with greater detail (11,12,45),
but in this work they will be reconsidered in a more general fashion. In all three
examples, the measurements were carried out with a Waters ALC244 size
exclusion chromatograph fitted with a DR, a UV sensor at 256nm, and a full set of
6m-Styragel w columns. In all three cases, the carrier solvent was tetrahydrofurane
(THF) at 1mL/min and 258C. In example 3, an in-line IV (Viscotek Corp.,
Houston, Texas) was added to the dual-detection system. The detector signals were
sampled as follows: every 0.118mL in Example 1, every 0.150mL in Example 2,
and every 0.027mL in Example 3.
2 EXAMPLE 1: MOLECULAR WEIGHT DISTRIBUTION
Let us first discuss the more general problem of analysing an SB copolymer by
SEC with standard dual-detection and a set of narrow PS and PB standards of
known molecular weights. A UV sensor at 256nm was used. This detector “sees”
only the phenyl groups of the S repeating units, but not the B repeating units.
The following equations can be written for the baseline-corrected UV and DR
chromatograms [sUV(V) and sDR(V), respectively] (1,6,43–45):
sUV(V) ¼ kUVpS(V)w(V ) (1)
sDR(V) ¼ kDR nPSpS(V) þ nPB[1 pS(V )] w(V) (2)
where w(V) is the instantaneous mass, pS(V) is the instantaneous mass fraction of
S; kUV, kDR are the UVand DR sensor gains; and nPS, nPB are the specific refractive
index increments of PS and PB, respectively. From Eq. (2), w(V) is proportional
© 2004 by Marcel Dekker, Inc.