Dilatational rheological studies on the surface micelles of Poly(styrene)-b-Poly(4-vinyl pyridine) block copolymer at the air-water interface

A thin film of an amphiphilic block copolymer, poly(styrene)-b-poly(4-vinyl pyridine), is investigated at the air-water interface. The surface pressure - area per molecule isotherm and the static compressional modulus provide evidence of two phases, assigned as L-1 and L-2, respectively. The surface topography and the phase images of these phases obtained using atomic force microscope reveal the presence of surface micelles of different sizes and densities. The dilatational rheology of these phases was investigated using the oscillatory barrier method for different strain amplitudes (1-5%) and qualitatively assessed using Lissajous-Bowditch curves. It points out the emergence of nonlinearity (asymmetric shape and distorted ellipse) with an increase in strain amplitude. Using a fast Fourier transform, the amplitudes and the phases of different harmonics of the stress-strain curves were extracted, and the total harmonic distortion was determined. In contrast to L-1 phase, the total harmonic distortion for the L-2 phase increases monotonically with strain amplitude. Fixing the strain amplitude at 1% (small amplitude regime), the temperature as well as frequency dependence studies of the dilatational moduli were carried out. The dynamic storage moduli of the L-1 phase is found to be insensitive to temperature while it decreases for the L-2 phase suggesting softening. Further, evidence for solid-like viscoelastic response emerges (explained using the Kelvin-Voigt model) for the L-1 phase, whereas the L-2 phase show a cross-over behaviour.