Relaxation Behavior and Nonlinear Surface Rheology of PEO-PPO-PEO Triblock Copolymers at the Air-Water Interface

Surface dilatational viscoelasticity of adsorbed layers of pluronics triblock copolymers at the air-water interface was measured using the oscillating barrier technique. The effect of molecular architecture and concentration on surface viscoelasticity was explored for two different types of pluronics with different degrees of hydrophobicity, Pluronic F-108 (M_w ≈ 14 600 g/mol) and Pluronic P-123 (M_w ≈ 5800 g/mol), the former exhibiting a larger hydrophilic to hydrophobic block length ratio. Frequency sweeps in the linear regime suggested that interfacial films of F-108 have higher surface limiting elasticity and larger in-plane and out-of-plane relaxation times at the same bulk concentration (the former possibly related to in-plane microstructure rearrangements, the latter to surface/bulk diffusion). Increasing the bulk concentration of pluronics from 1 to 100 μM led to a decrease in both in- and out-of-plane relaxation times. Large amplitude oscillatory dilatation (LAOD) tests were performed to capture nonlinear behavior of these interfacial films by means of elastic and viscous Lissajous plots. Nonlinearities in elastic responses were quantified through calculation of the strain-stiffening indices in extension S_E and compression S_C. Both pluronics exhibited strain softening in extension. In compression, P-123 showed strain-hardening and F-108 displayed a relatively linear response. Apparent strain hardening in extension was observed for the P-123 adsorbed film, at high strain, at a bulk concentration of 100 μM. However, at these strains, the response was dominated by the viscous contribution and calculation of strain rate-thickening factors in extension and compression showed that the overall response was strain rate-thinning in extension and strain rate-thickening in compression.