Small-molecule probe diffusion in polymer solutions: Studies by Taylor dispersion and phosphorescence quenching

Marc B. Wisnudel, John M Torkelson*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Scopus citations

Abstract

The most comprehensive study to date of the effects of size, shape, and flexibility on the translational diffusion of small probe molecules in polymer solutions has been completed by Taylor dispersion, which directly yields Dprobe, and phosphorescence quenching, which yields kq, the concentration dependence of which is identical to that of Dprobe for appropriate conditions. Diffusion of 16 probes ranging by a factor of 6 in molar volume was investigated using both Taylor dispersion in solutions of up to 400 g/L polystyrene in tetrahydrofuran and phosphorescence quenching in solutions of up to 700 g/L polystyrene in tetrahydrofuran, cyclohexane, and carbon tetrachloride. Results were compared quantitatively to modified Vrentas-Duda free volume theory for ternary solutions to obtain probe jumping unit sizes relative to the solvent, ξprobe,s, which correlate with probe volume. With the exception of 3,4-hexanedione (a highly flexible and small probe), the PS concentration dependencies of Dprobe and kq were approximately equal to or greater than that of solvent (0.9 ≤ ξprobe,s ≤ 1.75). The data fell into two types of behavior: when ξprobe,s was plotted against the ratio of probe to solvent molar volume, Ṽ(0)probe/Ṽ(0)s, the vast majority of data fell around a line of slope 0.13, while for two of the probes ξprobe,s fell near a line of slope unity. Literature data for five probes in several polymer-solvent systems could also be described by these two types of behavior. The former behavior indicates that for most probes the concentration dependence can be described by modified free volume theory, with the understanding that the critical hole free volume for a jump unit for these probes is but a fraction of the probe molar volume. The apparent dichotomy in the probe volume dependence of ξprobe,s raises the question of whether only two dependencies are possible or whether, by virtue of the probes selected, only these two distinct behaviors are observable. Small effects of flexibility and shape on Dprobe for probes with large aspect ratios were also observed and discussed in terms of anisotropic diffusion. A comparison of concentration dependence data with limited temperature dependence data from the literature shows a consistency based on the modified free volume picture. This, along with an understanding of the "bimodal" ξprobe,s data, indicates that the modified free volume theory for ternary systems forms a reasonably robust picture by which to interpret probe diffusion in polymer solutions.

Original languageEnglish (US)
Pages (from-to)6193-6207
Number of pages15
JournalMacromolecules
Volume29
Issue number19
DOIs
StatePublished - Sep 9 1996

ASJC Scopus subject areas

  • Organic Chemistry
  • Polymers and Plastics
  • Inorganic Chemistry
  • Materials Chemistry

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