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Temperature-rising elution fractionation (TREF) is a type of column fractionation technique and separates polymers according to the solubility difference by increasing column temperature. Inert materials, such as glass beads or diatomaceous earth (Chromosorb-P) are packed in a fractionation column for which temperature is controlled in a column oven or in a circulating oil bath. The polymer to be investigated is dissolved in a suitable solvent, such as xylene or 1,2,4-trichlorobenzene, at elevated temperature and the hot sample solution is in-
Table 3 Separation System for Normal-Phase and Reversed-Phase Liquid Chromatography
Type Column Gradient condition3 Copolymer3 Ref.
Normal Silica-NH2 28% THF in CHX to S-MMA 73
Normal Silica-CN 10% THF in CHX to S-MMA 73
Normal 5÷²1²Ãß CTP.1 60% THF S-MMA 74
3W„ THF in HX to
Normal An gel 35% chloroform in HX S-MMA 74
Normal AN gel to 85% chloroform SBR 75
Reversed Silica-phenyl 10% chloroform in HX S-MMA 73
Reversed Silica-ODS to 45% chloroform S-MMA 73
15% THF in ACN to
5% THF in ACN to
Reversed PS gel 20% DCM in ACN to S-MMA 76
aTHF, tetrahydrofuran; CHX, cyclohexane; HX, «-hexane; ACN, acetonitrile; DCM, dichloromethane; AN, polyacrylonitrile; PS, polystyrene; SBR, styrene-butadiene rubber.
troduced into a heated column. The temperature of the column is cooled slowly and the polymer sample is deposited onto the surface of the column packing. Elution is started with the same solvent by raising the column temperature gradually. The amorphous component that is more soluble at lower column temperature elutes first, followed by more crystalline components.
TREF was first performed by Wild and his co-workers . It is useful to characterize the degree of short-chain branching, which is related to the degree of crystallinity of polyeth-ylene. Nakano and Goto have developed an automatic cross-fractionation system (analytical TREF-SEC) which had a crystallizability fractionation device before a commercially available SEC apparatus [82J. The fractionation technique involves loading a sample onto a column packed with glass beads and slow cooling the sample solution in o-dichlorobenzene . Linear low-density polyethylene (LLDPE) has been examined by eluting stepwise from the column at 40-140°C. Each eluted polymer solution was automatically introduced into the SEC part of the system, which had an infrared detector. Chromatograms of elution temperature versus concentration were recorded and the biomodal distribution of LLDPE was observed.
A similar apparatus has also been developed by Hazlitt and applied for the measurement of the short-chain-branching distribution in ethylene-olefin copolymers . Mirabella prepared a TREF apparatus and characterized a commercial impact-grade polypropylene with a melt flow rate of 6 . This impact copolymer was composed of about 75% isotactic polypropylene. about 17% of a highly noncrvstalline ethylene-propvlene rubber, and about 8% of semicrystalline ethylene-propylene copolymers.
Field-flow fractionation (FFF) was invented by Giddings and has been developed by him and his group. It can be applied for the separation of macromolecules, colloids, and particles of diverse types extending over a mass range from a few hundred to 1018 Da . Separation in FFF takes place in a narrow tube or channel through which a solvent or a carrier liquid is
flowing. The separated components are subsequently eluted from the flow system and subjected to a detector. Because of the close relation to LC, FFF has instrumentation similar to that of LC. The only difference between FFF and HPLC (and SEC) is that FFF is carried out in an open tube or channel, and there are no support particles in the channel.
The FFF process is carried out in a thin open channel with externally generated fields applied across the thin dimension of the channel, in a direction perpendicular to flow. The applied field interacts with components of interest, driving them toward one of the walls. The larger particles are primarily driven toward the wall, resulting in a later elution than the smaller ones. There are several applied fields, and the FFF techniques of each different applied field has a different name: thermal FFF, sedimentation FFF, flow FFF, electrical FFF, and magnetic FFF. The particular field chosen depends on the material one wishes to separate.