142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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192 J. Roovers, B. Comanita Scheme 7 drimers [13, 46]. Hopefully, a comparison of the analytical capabilities of mass spectrometric methods and SEC will be attempted. The least such a comparison will accomplish is to provide for an absolute method to evaluate the effect of column spreading in the determination of MW distributions (MWD) by SEC.
Dendrimers and Dendrimer-Polymer Hybrids 193 2.3 Size Exclusion Chromatography SEC by itself is not an absolute MW determination method but the analysis of the elution peak has been used extensively for estimating the molecular purity of dendrimers. If the shape of the elution peak of a size exclusion chromatography experiment is Gaussian, the total dispersion, s T of the curve is given by the sum of squares [47]: (s T ) 2 =(s c ) 2 +a(s d ) 2 (1) where (s c ) 2 is the sum of the squares of dispersions due to the injector, column spreading and detector, s d is the dispersion due to the polydispersity of the sample, and a is the experimental slope of the calibration of the elution volume, V e, against MW V e =B–a ln MW (2) (s d) 2 =ln(Mw/Mn). Mw/Mn is a measure of polydispersity of the sample. Since experimentally s T has almost always been substituted for s d, SEC has rarely given the true MW distribution, the deviation being larger the narrower the MWD of the sample. This is especially the case for near-monomolecular dendrimers. Methods have been described for obtaining the true MW distribution of narrow MWD polymers by SEC [47]. Furthermore, when the calibration is performed with a set of linear polymers, it can only be used directly with the same type of linear polymers. When other polymers are analyzed, the principle of universal calibration is invoked. V e =B'–a' ln [h]M (3) Analysis of polydispersity can still be made but only when the exponent in the Mark-Houwink relation [h]=KM a is identical for the calibrating polymer and the new polymer. As will be shown in the next section this is clearly not the case for highly branched dendrimers. The validity of the universal calibration principle for dendrimers has been questioned [48]. SEC analysis with multiple detectors, especially with a low angle laser light scattering detector, obviously removes many of these objections. The results of such experiments [48] will be described in the next section. SEC does not have the capability for analyzing small defects in a dendrimer sample. However, it is a quick method for quantifying the monomer-dimer and higher multiple content.
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142 Advances in Polymer Science Edi
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Branched Polymers I Volume Editor:
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Volume Editor Dr. Jacques Roovers I
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Preface While books have been writt
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Contents Synthesis of Branched Poly
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Synthesis of Branched Polymers by C
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72 N. Hadjichristidis, S. Pispas, M
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Poly(macromonomers): Homo- and Copo
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Poly(macromonomers), Homo- and Copo
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