Spatial heterogeneity of solvent dynamics in multicomponent polymer solutions

Oscillatory electric birefringence has been used to examine the rotational dynamics of the solvent Aroclor 1248 (A1248), in dilute and semidilute polymer solutions containing both polystyrene (PS) and poly(1,4-butadiene) (PB) segments. Three systems were employed: an 8.0 × 10⁴-8.0 × 10⁴ PS-PB block copolymer, 0 ≤ c ≤ 0.162 g/mL; a 40-60 wt % PS-PB random copolymer, 0 ≤ c ≤ 0.126 g/mL; a 50-50 wt % homopolymer blend of 7.8 × 10³ PS and 5 × 10³ PB, 0 ≤ c ≤ 0.251 g/mL. Measurements were performed for six temperatures ranging from -17.00 to 2.81°C, and data were collected for frequencies from 1 Hz to 100 kHz. Previous extensive measurements of solvent rotation in homopolymer PS/A1248 and PB/A1248 solutions have established that PS acts to retard, and PB to accelerate, the solvent relaxation. Thus, in the multicomponent solutions examined there is competition between the effects of the different segments, and the results of this competition give insight into the length scale associated with the influence of the polymer on the solvent dynamics. In the block copolymer and blend solutions, the distribution of solvent relaxation times is very broad, demonstrating that the solvent dynamics are spatially inhomogeneous. The results are qualitatively consistent with a combination of the separate influences of PS and PB, but in general the PB component exerts a stronger influence on the solvent. Computer simulations were employed to extract an estimate of the correlation length associated with the influence that a given polymer segment has on the solvent dynamics; the results are consistent with a correlation length of approximately one solvent diameter.