FRACTIONATION OF POLYMERS BY
Vol. 5 , No. 2, March-April 1972
TLC
227
Fractionation of Polymers by Thin-Layer Chromatography. 111. Phase Separation and Thin-Layer Gel Permeation Chromatography E. P. Otocka,* M. Y. Hellman, and P. M. Muglia Bell Laboratories, Murray Hill, New Jersey
07974. Received February IO, I972
ABSTRACT: Further studies of the polymer fractionation mechanism by thin layer chromatography indicate that precipitation is the prime fractionation mechanism. Inadequacies of the present theories are discussed. Where precipitation is circumvented, fractionation by molecular size (GPC) can be carried out on thin layers of macroporous adsorbents.
A
number of investigations have recently been carried out which demonstrate that thin-layer chromatography (tlc) is capable of rapid accurate determination of molecular weight distributions for polystyrene. Several hypotheses have been put forward for the mechanism by which the fractionation occurs during tlc.2-6 The two basic approaches may be called the precipitation mechanism 3 s 7 , s and the adsorption mechanism.1s2'5 In the former, it is pointed out that the phase ratio, Le., the weight of solvent per unit weight of adsorbent, is a decreasing function of distance from the dip line.8s9 This results i n a continuous increase in the concentration of polymer in the mobile phase as a band travels away from its starting point. At some point, depending on the molecular weight of the polymer, the solubility limit is exceeded and the polymer precipitates. Another type of deviation from ideal behavior occurs when mixed solvents are employed as eluents. The more polar component of the eluent is preferentially adsorbed and the composition of the mobile phase as well as the phase ratio becomes a function of distance from the dip line. Changes in mobile phase composition become significant only when the polar component is originally present in minor amounts. In adsorption mechanisms, the main considerations are the well-known dependencies of adsorption and desorption rates and equilibrium coverage on polymer molecular weight. 10-12 While the energy per attachment between the polymer molecule and the substrate is invariant for homopolymers, the number of attachments per molecule increases with molecular weight, thereby providing the separation mechanism. In addition to the postulated adsorptive action, several investigators employing porous adsorbents have claimed to observe gel permeation effects modified by adsorptive interaction.5~1~ In this report we wish t o describe several experiments which indicate that Inagaki's precipitation fractionation mechanism is largely correct. Several additional experiments show that (1) E. P. Otocka and M. Y . Hellman, Macromolecules, 3,392 (1970). (2) E. P. Otocka, ibid., 3, 691 (1970). (3) F. Kamiyama, H . Matsuda, and H. Inagaki, Poljm. J., 1 , 518 (1970). -, \ - -
(4) V. G . Belenkii and E. S . Gankina, Dokl. Akad. Nauk SSSR, 186, 857 (1969). (5) V. G . Belenkii and E. S.Gankina, J . Chromatogr., 53,3 (1970). (6) E. P . Otocka, P. M. Muglia, and H . L. Frisch, Macromolecules, 4 , 512 (1971). (7) H . Inagaki, F . Kamiyama and T. Yagi, ibid., 4, 133 (1971). (8) F. Kamiyarna and H. Inagaki, Poljm.J., in press. (9) J . C. Giddings, G . H . Stewart, and A. L. Ruof, J . Chromatogr., 3, 239 (1960). (10) J. Koral, R. Ullman, and F. R. Eirich, J . P h j s . Chem., 62, 541 (1958). (11) R . R. Stromberg, D. J. Tutas, and E. Passaglia, ibid., 69, 3955 (1965). (12) M. J. Shick and E. N. Harvey, Poljrn. Lett., 7,495 (1969). (13) H . Halpaap and I
