SUPERPOSITION OF OSCILLATIONS ON STEADY SHEAR-FLOW AS A TECHNIQUE FORINVESTIGATING THE STRUCTURE OF ASSOCIATIVE POLYMERS

The viscoelastic properties over a range of steady shear conditions of an alkali-swellable associative polymer have been determined using th e technique of superposition of oscillations upon steady shear now, th us enabling the structure of the polymer to be investigated. The assoc iative polymer studied consists of a backbone of metharcylic acid and ethyl acrylate to which is attached macromolecules containing C-20 hyd rophobes via an ethylene oxide-isocyanate linkage. A 1 wt % solution w ith its pH adjusted to 9.5 was used. At high pHs, the polymer solubili zes to form a network of both intra- and intermolecular associating hy drophobic junctions. The solution shows a non-power-law sheer-thinning behavior: the viscosity now curve, when plotted against sheer stress, shows two distinct regions where network rupture is prominent, at str ess of 2 Pa and between 40 and 50 Pa By superimposing small amplitude oscillations on to shear now at constant stresses, the network structu re of the polymer is unperturbed and linear viscoelastic properties of the polymer under the applied stress conditions can be obtained. At a n applied stress of 2 Pa and above, bath the storage and loss moduli o f the polymer are greatly reduced at low frequencies, with the G' appr oaching second-order behavior and eta' tending toward constant values. By analogy to Maxwell relaxation time, an estimate of the relaxation time of the associative polymer at different stress conditions can be made. The results show that the relaxation time is reduced by up to 4 orders of magnitude as the stress is increased from 1 to 60 Pa, while a much smaller decrease in viscosity is observed. At sufficiently high frequencies, both the storage and loss moduli show an increase above their linear viscoelastic values as the strain amplitude is increased. This behavior is believed to be dependent on the relaxation time of t he polymer which is a function of the state of network disruption. Thu s the technique may prove to be a powerful tool for probing the struct ure of network polymer in solution.