Relaxation Dynamics of Entangled Polymer Liquids near Solid Substrates

We investigate relaxation dynamics of entangled 1,4-polybutadiene (PBD) melts and concentrated polystyrene/diethyl phthalate (PS/DEP) solutions near an attractive glass substrate. Narrow molecular weight distribution PBD melts and PS/DEP solutions covering a broad range of molecular weights (6.7 × 10 3 ≤ M̄ w,PBD ≤ 5.15 × 10 5; 1.3 × 10 5 < φ M̄ w,PD < 1.1 × 10 6 g/(mol)) were used in the study. A new experimental method, evanescent wave laser polarimetry (EWLP), was developed to investigate near-surface relaxation dynamics in these materials. The method relies on total internal reflection of phase-modulated laser light at an interface between a high refractive index transparent hemisphere and the polymer liquid to probe shear-induced changes in molecular orientation in a fluid layer within approximately 80 nm of the polymer/substrate interface. A characteristic near-surface relaxation time τ surface determined from step shear EWLP relaxation experiments using PBD melts was found to be a much stronger function of bulk polymer molecular weight, τ surface ∼ M w 4.2 ± 0.03, than the corresponding terminal properties in bulk, η 0 ∼ M̄ w 3.4 ± 0.02 and τ d 0 ∼ M̄ w 3.4 ± 0.04 . In the case of PS/DEP solutions, τ surface was consistently larger than bulk terminal relaxation times but displayed the same dependence on polystyrene molecular weight as τ d 0 . A new gel-like surface relaxation process was also identified in the PS/DEP solutions that is not present in bulk. These experimental results are discussed in terms of the equilibrium structure of physically adsorbed polymer chains and classical relaxation mechanisms available to entangled polymer liquids near rigid substrates.