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Handbook of Size Exclusion Chromatography and Related ...

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As expected, pS is closer to the nominal value <strong>of</strong> 20%. The deviation in pc S is a<br />

consequence <strong>of</strong> the propagation <strong>of</strong> errors during the inversion operations.<br />

4 EXAMPLE 3: BRANCHING DISTRIBUTION<br />

Consider the analysis <strong>of</strong> a PB-graft-PS copolymer contained in high-impact<br />

polystyrene (i.e., a mixture <strong>of</strong> free PS, graft copolymer, <strong>and</strong> unreacted PB)<br />

(11,12,54). The high-impact polystyrene was produced in a solution<br />

polymerization <strong>of</strong> styrene in the presence <strong>of</strong> PB <strong>and</strong> a chemical initiator, <strong>and</strong><br />

the sample corresponds to a monomer conversion <strong>of</strong> 18%. Prior to the SEC<br />

analysis, the graft copolymer was isolated from the homopolymers through a<br />

solvent extraction technique (54). The copolymer branching points are mainly<br />

trifunctional, <strong>and</strong> are produced by a free radical attack to the double bonds <strong>of</strong> the B<br />

repeating units. Tetrafunctional branching points (or crosslinks) are neglected in<br />

the present analysis. The instantaneous CCD is broad, but the average composition<br />

does not change with the molar mass. For this reason, SEC alone is incapable <strong>of</strong><br />

determining the CCD. On the positive side, if one also assumes that in dilute<br />

solution the PS branches are noninteracting with the PB backbones, then the graft<br />

copolymer can be considered as a branched homopolymer from the<br />

chromatographic point <strong>of</strong> view. The rate <strong>of</strong> branching is proportional to the<br />

molar mass <strong>of</strong> the reacted PB chain. For this reason, the larger copolymer<br />

molecules are also the more highly branched, <strong>and</strong> a SEC fractionation by<br />

hydrodynamic volume also implies a fractionation by the number <strong>of</strong> branches.<br />

The instantaneous molar mass M(V ) <strong>and</strong> the instantaneous number-average<br />

number <strong>of</strong> grafted branches per molecule bn(V) were obtained from intrinsic<br />

viscosity measurements [h](V) combined with the universal calibration. The<br />

universal calibration was determined from narrow PS st<strong>and</strong>ards <strong>of</strong> known molar<br />

masses <strong>and</strong> intrinsic viscosities. At any given elution volume, fM [h]g is a<br />

constant. For a branched polymer, [h](V) is lower than its linear homolog, while<br />

the opposite is verified for M(V). To obtain bn(V), the following procedure<br />

(originally developed for branched homopolymers) was employed:<br />

1. Calculate the instantaneous intrinsic viscosity from:<br />

sIV(V)<br />

[h](V) ¼ kIV<br />

sDR(V)<br />

(10)<br />

where sIV(V) is the IV chromatogram, sDR(V) is the mass<br />

chromatogram, <strong>and</strong> kIV is a calibration constant.<br />

2. Calculate [h](M) from [h](V) <strong>and</strong> the universal calibration<br />

log M(V) [h](V) ¼ A BV.<br />

© 2004 by Marcel Dekker, Inc.

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