I. Vinckier et Hm. Laun, Manifestation of phase separation processes in oscillatory shear: droplet-matrix systems versus co-continuous morphologies, RHEOL ACT, 38(4), 1999, pp. 274-286
Phase separation processes in mixtures of poly-alpha-methylstyrene-co-acryl
onitrile (P alpha MSAN) and poly-methylmethacrylate (PMMA) with lower criti
cal solution temperature (LCST) behavior have been studied, focusing on the
manifestation of the interface in oscillatory shear measurements. By using
blends of different composition, systems with a droplet-matrix morphology
or a co-continuous structure are generated during the phase separation proc
ess. The feasibility of probing this morphology development by rheological
measurements has been investigated. The development of a disperse droplet p
hase leads to an increase in the elasticity of the blend at low frequency,
showing up as a shoulder in the plot of storage modulus versus frequency. H
ere, the droplet growth is unaffected by the shear amplitude up to strains
of 0.2; therefore the resulting dynamic data are suitable for quantitative
analysis. In contrast, for blends in which phase separation leads to a co-c
ontinuous structure, the storage modulus shows a power law behavior at low
frequency and its value decreases as time proceeds. For the latter systems,
effects of the dynamic measurement on the morphology development have been
observed, even for strain amplitudes as low as 0.01. To probe the kinetics
of morphology evolution in droplet-matrix systems, measurements of the tim
e dependence of the dynamic moduli at fixed frequency should be performed (
for a whole series of frequencies). Only from such measurements, curves of
the frequency dependence of the moduli at a well defined residence time can
be constructed. From fitting these curves to the emulsion model of Paliern
e, the droplet diameter distribution at that particular stage in the phase
separation and growth process can be obtained. It is not appropriate to use
a simplified version of the Palierne model containing only the average dro
plet size, because a morphology with too broad a size distribution is gener
ated.