Relaxation dynamics of entangled polymer liquids near solid substrates

We investigate relaxation dynamics of entangled 1,4-polybutadiene (PBD) mel ts and concentrated polystyrene/diethyl phthalate (PS/DEP) solutions near a n attractive glass substrate. Narrow molecular weight distribution PBD melt s and PS/DEP solutions covering a broad range of molecular weights (6.7 x 1 0(3) less than or equal to (M) over bar (w,PBD) less than or equal to 5.15 x 10(5); 1.3 x 10(5) < <phi>(M) over bar (w,PD) < 1-1 x 10(6) g/(mol)) were used in the study. A new experimental method, evanescent wave laser polari metry (EWLP), was developed to investigate nearsurface 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 tran sparent hemisphere and the polymer liquid to probe shear-induced changes in molecular orientation in a fluid layer within approximately 80 nm of the p olymer/substrate interface. A characteristic near-surface relaxation lime t au (surface) determined from step shear EWLP relaxation experiments using P BD melts was found to be a much stronger function of bulk polymer molecular weight, tau (surface) similar to M-w(4.2 +/-0.03), than the corresponding terminal properties in bulk, eta (0) similar to (M) over bar (3.4 +/-0.04)( w). In the case of PS/DEP solutions, tau (surface) was consistently larger than bulk terminal relaxation times but displayed the same dependence on po lystyrene molecular weight as tau (d0). A new gel-like surface relaxation p rocess was also identified in the PS/DEP solutions that is not present in b ulk. These experimental results are discussed in terms of the equilibrium s tructure of physically adsorbed polymer chains and classical relaxation mec hanisms available to entangled polymer liquids near rigid substrates.