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Handbook of Size Exclusion Chromatography and Related ...

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elations between molar masses <strong>and</strong> sizes <strong>of</strong> macromolecules for complex<br />

polymers are calculated from corresponding relations for homopolymers. The<br />

success <strong>of</strong> these approaches may be rather selective; in many cases they fail<br />

completely (1).<br />

Similarly,the “absolute” detectors, such as viscometers or light-scattering<br />

detectors, respond not only to molar mass but also to chemical structure <strong>and</strong> <strong>of</strong>ten<br />

alsotoarchitecture<strong>of</strong>macromolecules(Sec. 10).As aresult, it israrely possibleto<br />

determineexactlytwoindependentdistributionsfromonesingleSECmeasurement,<br />

even if using hyphenated detection (multidetector systems), except for mass<br />

spectrometry.Also,forthesamereasons,dataonmolarmassdistribution<strong>of</strong>complex<br />

polymers determined by simple SEC measurement will <strong>of</strong>ten be disturbed by the<br />

presence<strong>of</strong>furtherdistribution(s).Thesituationseemstobeeasierinpolymerblends<br />

in comparison with many other complex polymers. If two or several different<br />

detectors enable us to independently monitor concentrations <strong>of</strong> each blend<br />

constituent, calculation <strong>of</strong> molar mass/distribution data appears rather simple.<br />

However, the chemically different macromolecules can mutually affect their<br />

retentionvolumes(“concentrationeffects”)sothatthecalculatedMMDvaluesmay<br />

be inaccurate. Consequently,full separation polymer blend components is advised<br />

for exact determination <strong>of</strong> molar mass/distributionvalues (Secs 7<strong>and</strong> 12.2).<br />

SECis<strong>of</strong>tenappliedtodirectdetermination<strong>of</strong>meanmolar massvalues<strong>and</strong><br />

molar mass distribution <strong>of</strong> copolymers. For the above reasons, the data obtained<br />

canberegardedformostcasesasonlysemiquantitative.Theresultinginformation<br />

can be utilized in the investigation <strong>of</strong> various important tendencies in<br />

copolymerizationprocessesbuthardlyforanexactevaluation<strong>of</strong>copolymerization<br />

kinetics. For binary statistical copolymers, the situation is schematically<br />

representedinFig.2a,whichoriginatesfromconsiderations<strong>of</strong>Balke<strong>and</strong>Patel(2).<br />

Statistical copolymers, composed from two different monomer units, for<br />

example, A <strong>and</strong> B, usually exhibit distribution in their molar mass, chemical<br />

structure [various compositions between homopolymers poly(A) <strong>and</strong> poly(B)]<br />

<strong>and</strong> architecture (from an alternating copolymer ABABAB to a blockcopolymer<br />

AAAA–BBBB). Each molecular characteristic affects the size <strong>of</strong><br />

polymer species in solution in a different way. The resulting dependences <strong>of</strong><br />

polymer size on molecular characteristics can be represented by the contour plot<br />

shown in the center <strong>of</strong> the triangle in Fig. 2a. Evidently, the shape <strong>of</strong> the<br />

contour plot can be rather nonsymmetrical. If the sequence length distribution in<br />

a binary statistical copolymer is neglected, we arrive at a simplified scheme,<br />

which is shown in Fig. 2b. MMD <strong>and</strong> CCD are bimodal, though still continuous<br />

in this case. A scheme <strong>of</strong> a contour plot for a copolymer with discontinuous<br />

multimodal molar mass <strong>and</strong> chemical composition distributions is shown in Fig.<br />

2c. Although simplified, the schemes in Figs 2a–c objectively illustrate the<br />

necessity <strong>of</strong> determination <strong>of</strong> all molecular characteristics for complex polymer<br />

systems by two or several independent but complementary procedures. In this<br />

© 2004 by Marcel Dekker, Inc.

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