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
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.