Fractionation and gel permeation chromatography

In this section the theory of phase equilibria of polymer solutions is discussed as it is a simple practical illustration of the Flory-Huggins theory and can be extended to explain the principles behind the fractionation techniques which are used with polymer solutions. Finally the technique of gel-permeation chromatography, which is now widely used in polymer laboratories, is described in detail. 

A general principle of free energy-composition curves is that phase separation will occur if a tangent can be drawn which touches the curve at two points. This enables Tc to be calculated since it is the point at which the inflexion points merge. This can be expressed mathematically as the point 

For the polymer-solvent system i + = 1 and so d i = -d fc. Also the chemical potential of the solvent in the solution pti can be related to AG through Equation (3.39) and so the critical conditions for miscibility can be written in terms of chemical potentials as 

There are some aspects of the predictions of Equations (3.109) and (3.110) that are worth considering. Firstly, they can be used in conjunction with the Flory-Huggins theory to measure the Theta temperature of a polymer solution from studies of phase separation. It is possible to relate Tc and Xn to O by combining Equations (3.47) and (3.110) which give 

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Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

Polybutadiene samples were fractioned to obtain the stereospecificity distribution of AC. All the extracted fractions are sufficiently narrowly dispersed (M,yM < 2) and their yield was at least 95-96% (Table 3.9). There is satisfactory correlation between the average MW data of initial samples obtained from the MW of fractions and gel-permeating chromatography data, confirming successful fractionation [102]. 

A much better agreement between theory and experiment is found in the closely-related field of macrocyclisation equilibria. Investigations of the cyclic populations in ring-chain equilibrates set up in typical polymeric systems such as polyesters, polyethers, polysiloxanes, and polyamides take a major advantage from the relative ease with which the cyclic fraction can be separated from the linear fraction and analysed for the relative abundance of the individual oligomeric rings. This is conveniently done by means of modern analytical techniques such as gas-liquid and gel-permeation chromatography (Semiyen, 1976). 

Forss K, Schott O, Stenlund B (1967) Light absorption and fluorescence of lignosulfonates dissolved in water and dimethylsulfoxide Pap Puu 49 525-530 Forss K, Stenlund B (1969) Molecular weights of lignosulfonates fractionated by gel permeation chromatography Pap Puu 51 93-105... 

Non-volatile organic compounds can be characterized by c.g.c. if pyrolyzed directly into the injector chamber of a g.c. [56, 57 ]. It has been shown that size exlusion chromatography and gel permeation chromatography are adequate techniques for fractionation of non-volatile components from water samples. The fractions are then to pyrolysis-g.c.— mass spectrometry for the characterization of humic acid and fulvic acids, sugars, and proteins [57 ]. ... 

Ruggiero et al. (1978, 1981) have also examined the H NMR spectra of different molecular weight fractions of humic and fulvic acids isolated by adsorption and gel permeation chromatography. They found that there are differences in the aliphatic and aromatic regions of the spectrum between the different fractions. Their data are fragmentary and substantially more work... 

Examination of Polysulfide Fractions. Polysulfide fractions from the alloocimene, myrcene, limonene, styrene, and Thiokol LP-33 materials prepared at 140 °C have been fractionated by gel permeation chromatography. Other Thiokol products could not be examined because they were insoluble in all organic solvents used. In Tables VIII-XII the molecular weight and molecular formula of each fraction are given. Table... 

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