Apparent relaxation-time spectrum cutoff in dilute polymer solutions: An effect of solvent dynamics

Citation
Sc. Peterson et al., Apparent relaxation-time spectrum cutoff in dilute polymer solutions: An effect of solvent dynamics, J POL SC PP, 39(22), 2001, pp. 2860-2873
Citations number
53
Categorie Soggetti
Organic Chemistry/Polymer Science
Journal title
JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS
ISSN journal
08876266 → ACNP
Volume
39
Issue
22
Year of publication
2001
Pages
2860 - 2873
Database
ISI
SICI code
0887-6266(20011115)39:22<2860:ARSCID>2.0.ZU;2-4
Abstract
The apparent short time cutoff of the relaxation-time spectrum at surprisin gly long times for polymers in solution is a well known but not yet underst ood observation. To elucidate its origins we revisit viscoelastic and oscil latory flow birefringence data for solutions and melts of two linear polyme rs (polystyrene and polyisoprene) and present new measurements of oscillato ry flow birefringence of the latter. Previous measurements have suggested t hat the "flexibility" of both polymers in solution is smaller than in the m elt on the basis of the breadth of the relaxation-time spectrum of the solu tion as compared with that of the melt. Our new measurements have explored a higher effective frequency range than was previously possible. This has a llowed us to observe the effect of the rotational relaxation time of the so lvent on the dynamics of the solution at high frequencies. To obtain the po lymer global motion contribution, one now needs to subtract from the soluti on properties a frequency-dependent complex solvating environment contribut ion. We show that the decrease in apparent "flexibility" for solutions aris es from the presence of a solvent that exhibits a rotational relaxation tim e and thus simple viscoelastic behavior somewhat near the frequency window of the experiment. Although recent predictions of a model for a chain in a solvent with a single relaxation time are in qualitative agreement with our results, our data suggest that the solution results may reflect the influe nce of solvent on the development of the "entropic spring" forces at short times. (C) 2001 John Wiley & Sons, Inc.