Oscillatory shear polymer networks

Jeyaseelan and Giacomin (1995) examine the use of large-amplitude oscillatory-shear (LAOS) rheology of filled polymer melts (HDPE filled with carbon black) and use transient-network theory (which separates filler and polymer entanglement effects) to describe the non-linear flow behaviour. 

These differences are shown in the following examples where measurements of the dynamic moduli, G and G" are used to monitor the structure of gel networks. Measurements are performed by imposing an oscillatory shear field on the material and measuring the oscillatory stress response. The stress is decomposed into a component in phase with the displacement (which defines the storage modulus G ) and a component 90 out of phase (which defines the loss modulus G"). The value of G indicates the elastic and network structure in the system (15, 17, 18) and can be interpreted by using polymer kinetic theories. 

The rheological behavior of these materials is still far from being fully understood but relationships between their rheology and the degree of exfoliation of the nanoparticles have been reported [73]. An increase in the steady shear flow viscosity with the clay content has been reported for most systems [62, 74], while in some cases, viscosity decreases with low clay loading [46, 75]. Another important characteristic of exfoliated nanocomposites is the loss of the complex viscosity Newtonian plateau in oscillatory shear flow [76-80]. Transient experiments have also been used to study the rheological response of polymer nanocomposites. The degree of exfoliation is associated with the amplitude of stress overshoots in start-up experiment [81]. Two main modes of relaxation have been observed in the stress relaxation (step shear) test, namely, a fast mode associated with the polymer matrix and a slow mode associated with the polymer-clay network [60]. The presence of a clay-polymer network has also been evidenced by Cole-Cole plots [82]. 

For example, oscillatory shear experiments can be performed on a rotational rheometer (Fig. 4.17), where the polymer network is placed between two parallel plates in order to obtain the complex shear modulus G ... 

With dynamic rheometry, the measurement of the dynamic moduli G and G" in small amplitude oscillatory shear is exploited. The gelation point is reported to be the intersection point of the curves of storage and loss moduli, i.e. the moment at which tan 8 equals one [49]. However, the crossover is observed to correspond to the gel point only for stoichiometrically balanced network polymers and networks with excess crosslinking agent at temperatures much above Tg [50]. 

Oscillatory shear rheology of EPDM plasticized with resol was used to determine an equilibrium shear modulus Ge), relaxation in compression and strain recovery. Ge was analysed with consideration of crosslink density and permanent entanglements, including evaluation of plasticizer and soluble polymer fraction. Relaxation data were modelled with the empirical Chasset-Thirion equation and it was proposed that longer relaxation times were associated with chains pendant from the network. Relaxation times increased with crosslink density. When the crosslink density was low and pendant chains were longer and more numerous, relaxation times were increased and elastic recovery diminished. ... 

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