Handbook of Size Exclusion Chromatography and Related ...

1,4-trans;and8%1,2-vynil.TheglobalCCDisnarrowbecause(a)bothreactivity
ratios are close to 1, and (b) the average chain length is much larger than the
averageSorBsequence.Twopotentialcomplicationsare(a)thepolymerexhibits
some degree of branching, and (b) the high molecular weight fraction may be
totally excluded from the column pores and/or subject to degradation. These
difficulties are expected to be unimportant, however, and will be neglected in
the present analysis. Consequently,both M wand the polydispersity M w=M nare
expected to give slightly underestimated results.
Figure 1a shows the baseline-corrected chromatograms sDR(V)and sUV(V).
The UV signal was shifted with respect to the DR signal, to account for the time
lag between detectors. The chromatograms are seen to be almost proportional to
eachother.Theinstantaneousmass w(V)andtheinstantaneousmassfractionofS
pS(V)wereobtainedthroughEqs(3)and(4)(Fig.1b).Thefollowingisobserved:
(a) w(V)is proportional to both chromatograms, and (b) pS(V)is essentially
constant in the midchromatogram region (and close to the global nominal
composition of 24.5%), while deviations are observed at the chromatogram tails.
The molecular weights were calculated from Eq. (5) with pS(V)ffi0:245,
resulting in logM¼ 0:1821 Vþ12:5941 (Fig. 1c). From this expression and
w(V), the MWD of Fig. 1d was obtained. This distribution is relatively broad and
unimodal, with M w=M n¼3:17. This indicates that acorrection for instrumental
broadening is not required. Also, since logM(V) is linear, the ordinates of
w(logM)are proportional to the ordinates of w(V).
3 EXAMPLE 2: CHEMICAL COMPOSITION DISTRIBUTION
Figure 2represents the SEC analysis of anarrow-distributed linear SB diblock
copolymer (45). The calibrations of Example 1 are here readopted. The sample
was produced in a sequential anionic polymerization. First, the butadiene solution
was slowly loaded into the initiator solution. Then, the styrene solution was slowly
added until almost complete conversion. The nominal weight fraction of S in the
copolymer was 20%. The impurities contained in the stock comonomer solutions
produced a continuous deactivation of living ends along the polymerization, and
for this reason the copolymer S content increases with the molar mass.
The sUV(V ) and sDR(V) chromatograms are represented in Fig. 2a. The
instantaneous mass and mass fraction of S [w(V) and pS(V), respectively]
were directly calculated from the chromatograms and Eqs (3) and (4) (Fig. 2b). In
the midchromatogram region, pS(V) increases monotonically with the molar mass,
while oscillations are observed at the chromatogram tails as a result of the
propagation of errors. In summary, the copolymer exhibits a broad CCD and a
continuous variation of the chemical composition with the molecular weight.
© 2004 by Marcel Dekker, Inc.