Shear thinning and frequency dependent behaviour of adsorbed polymer layers

Abstract

Nanorheological measurements were carried out using an oscillatory AFM technique to investigate the viscoelastic properties of adsorbed hydroxypropyl guar (HPG) layers. The oscillations were performed at frequencies between 300 Hz and 1 kHz, with applied oscillation amplitudes of 2 nm. Qualitative data analysis was carried out using complex viscosity and complex modulus transfer functions based on a hydrodynamic lubrication model. The results indicated viscous behaviour at large surface separations and viscoelastic behaviour in the region of polymer layer overlap, as would be expected for adsorbed polymer layers. However, the adsorbed HPG layers also showed an indication of frequency dependent viscoelastic behaviour and shear thinning (reduction of viscosity with frequency). Furthermore, there appeared to be an unanticipated correlation between the apparent thickness of the adsorbed layers and the viscoelastic properties of the system, which may be attributed to the shear thinning behaviour of the layers.

The role of copolymer composition on the specific ion and thermo-response of ethylene glycol-based brushes

“…Suraya et al suggest that this may result from a small population of dilute tails which affect the flow of solvent without significantly contributing to the entropic penalty in the early stages of compression. 58 This explanation is consistent with the data in Figure 4b where the onset of repulsion upon approach is within the length of chains extended during retraction. At small separations the restricted conformation of the polymer chains is also expected to lead to a stiffer layer.…”

“…Measurements utilising dynamic or oscillating surface forces apparatus and AFM are able to quantify the mean viscoelastic properties of the brush and surrounding solvent by treating them as a homogeneous medium. 58 Experiments on adsorbed polymer show that an increase in the apparent viscosity of up to 2 orders of magnitude is possible as the separation is decreased. 56,58,59 The higher apparent viscosity was attributed to hindered drainage through the brush and increases at smaller separations due to the increasing local polymer concentration.…”

“…58 Experiments on adsorbed polymer show that an increase in the apparent viscosity of up to 2 orders of magnitude is possible as the separation is decreased. 56,58,59 The higher apparent viscosity was attributed to hindered drainage through the brush and increases at smaller separations due to the increasing local polymer concentration. Importantly, the onset of the increase in the viscosity occurs at larger separations than the onset of repulsion for the quasiequilibrium case.…”

Shear thinning and frequency-dependent behaviour of adsorbed polymer layers

“…One reason for the effectiveness of the model is the large factor R/h 0 that appears in the expression for the force (9). In the experimental work, this factor is of order 100 or more, justifying the assumption that the force is dominated by the lubrication flow in the narrow gap, so that other contributions, such as the drag on the cantilever itself, can be neglected.…”

“…Hence, to obtain meaningful results from the raw data, a mechanical model must be used to interpret the detected motions in terms of physical stress and strain. In a companion paper [9] hereafter referred to as Part I, the "hydrodynamic lubrication equation" was applied to provide qualitative interpretation of nanorheological data.…”