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Handbook of Size Exclusion Chromatography and Related ...

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tetrafunctional) <strong>of</strong> branch points. This is defined in terms <strong>of</strong> another branching<br />

factor,<br />

g¼ Rg2 b<br />

Rg 2 l M<br />

(28)<br />

where Rg 2 is the mean-square radius <strong>of</strong> gyration. For different branching<br />

architectures, gcan be related to the number <strong>of</strong> branches per molecule (4).<br />

This branching factor gis related to the intrinsic viscosity branching factor<br />

g 0 by<br />

g 0 ¼g e<br />

(29)<br />

whereeisastructure factor notspecified by thetheory.Typical values for erange<br />

from 0.5 to 1.5. Experimentally determined values for avariety <strong>of</strong> polymer–<br />

solvent systems have been tabulated (68). Because <strong>of</strong> the uncertainty in e<strong>and</strong><br />

because SEC measurements are always made in good solvents, whereas gis<br />

defined for uconditions, there is too much uncertainty to use g 0 toobtain the<br />

number <strong>of</strong> branch points per molecule. In many cases, only the branching ratio g 0<br />

isreported,whereit servesasausefulmeasure<strong>of</strong>therelativedegree <strong>of</strong>branching<br />

<strong>and</strong> is auseful parameter for comparing variations among polymer samples.<br />

Kuo et al. (34) used this method to study r<strong>and</strong>omly branched <strong>and</strong> star<br />

polystyrene, as well as branched polyvinyl acetate. Figure 11 shows the Mark–<br />

Houwinkplotsforthelinear<strong>and</strong>branchedpolystyrenes<strong>and</strong>aplot<strong>of</strong>thebranching<br />

index g 0 for the branched polystyrene as afunction <strong>of</strong> molecular weight. As<br />

expected, g 0 for r<strong>and</strong>omly branched polystyrene decreases with increasing<br />

molecular weight. Siochi et al. (69,70) used this method to study model graft<br />

polymethylmethacrylates<strong>and</strong>foundthatinthiscase,g 0 increasedwithincreasing<br />

molecular weight. They speculated that this was possibly caused by adifference<br />

between macromer <strong>and</strong> the backbone monomer polymerization kinetics.<br />

Notethattheintrinsicviscosity-molecularweightdataforthecorresponding<br />

linear polymerarerequired tocalculateg 0 .Ideallythisshouldbedeterminedfrom<br />

alinear sample analyzed by SEC-viscometry.Alternatively,literature values for<br />

the Mark–Houwink parameters for the linear polymer may be used. If neither <strong>of</strong><br />

these data are available, the least branched sample or asecondary linear st<strong>and</strong>ard<br />

can be used as the control. Table 2lists selected references on the use <strong>of</strong> SECviscometry<br />

for branching studies.<br />

4.1.4 Mark–Houwink Coefficients<br />

An important application <strong>of</strong> SEC-viscometry in conjunction with universal<br />

calibration is to determine the Mark–Houwink coefficients for a given polymer<br />

© 2004 by Marcel Dekker, Inc.

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