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Handbook of Size Exclusion Chromatography and Related ...

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(Sec. 7) <strong>and</strong> therefore they can be applied also to molecular characterization <strong>of</strong><br />

minor ( 1% or less) macromolecular admixtures that were added to major<br />

component(s) or which were created during processing (93).<br />

The highest separation selectivity <strong>of</strong> multicomponent polymer mixtures<br />

generally exhibits eluent gradient HPLC (Sec. 5.2).<br />

12.3 Statistical Copolymers<br />

Statistical copolymers were among the first <strong>and</strong> most popular complex polymer<br />

targets for chromatographic characterization (2,3). Initially,statistical copolymers<br />

were subject to tedious solubility-based cross-fractionations <strong>and</strong> only relatively<br />

recently have HPLC procedures have taken over thefield. Skvortsov<strong>and</strong> Gorbunov<br />

(103) stated that critical conditions apply only to sequenced complex polymers <strong>and</strong><br />

that the critical behavior is limited to the macromolecular chains that possess free<br />

ends.Onthecontrary,Brun(75,76)showedthatentropy–enthalpycompensationcan<br />

appearalsowithchains<strong>of</strong>statisticalcopolymers.However,LCCCiss<strong>of</strong>aronlyrarely<br />

used as a first dimension separation system for the latter species. Very good<br />

selectivities <strong>of</strong> statistical copolymer separations were obtained with eluent gradient<br />

HPLC procedures (3–5,81,82) where molar mass independent retention was<br />

frequentlyobserved,especiallywhenanadsorption<strong>and</strong>partitionretentionmechanism<br />

wasapplied(Secs3.2.1,3.2.2,<strong>and</strong>5.2).Seconddimensionseparationsystemcanbe<br />

SEC.<br />

12.4 Segmented Copolymers<br />

Di-, tri-, <strong>and</strong> multiblock copolymers, graft copolymers <strong>and</strong> star (miktoarm)<br />

copolymers are the most typical representatives <strong>of</strong> this group <strong>of</strong> complex<br />

polymers. Most common are diblock copolymers <strong>and</strong> so are the attempts for their<br />

characterization by means <strong>of</strong> HPLC. Skvortsov <strong>and</strong> Gorbunov (104) proposed<br />

application <strong>of</strong> LC CC for diblock <strong>of</strong> copolymers <strong>and</strong> first experimental<br />

measurements were published by Zimina et al. (105,106). Interestingly,Zimina<br />

et al. (105) stated in their paper from 1991 that “...we did not evaluate<br />

experimental data quantitatively because <strong>of</strong> large b<strong>and</strong> broadening ...” Later,<br />

Paschpublishedaseries<strong>of</strong>papersdescribingsuccessfulLCCC<strong>of</strong>di-<strong>and</strong>triblock<br />

copolymers without mentioning the b<strong>and</strong> broadening problems (4,5). Several<br />

further experimentally observed <strong>and</strong> anticipated problems connected with LC CC<br />

ingeneral<strong>and</strong>withitsapplicationtoblockcopolymersinparticularwerereviewed<br />

in Refs. (56,68,69 <strong>and</strong> 78) (see also Sec. 5.1). Mutual influence <strong>of</strong> chemically<br />

different blocks on their retention at critical conditions was recently confirmed by<br />

Lee et al. (107). Retention volumes <strong>of</strong> “critically retained” or “chromatographically<br />

invisible” homopolymers differ from VR <strong>of</strong> identical chains in the block<br />

copolymers.<br />

© 2004 by Marcel Dekker, Inc.

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