Small amplitude oscillatory shear rheology is employed in order to
investigate the linear
viscoelastic behavior of the lower critical solution temperature blend
polystyrene/poly(vinyl methyl ether),
PS/PVME, as a function of temperature and composition. At low
temperatures, where the mixture is
homogeneous, the dependence of the zero shear viscosity
(η0) on concentration is measured and is
well-described by means of a new mixing rule, based on surface fractions
instead of volume fractions. Shift
factors from time-temperature superposition (TTS) exhibit a
Williams−Landel−Ferry (WLF) behavior.
As the macrophase separation temperature is approached (the phase
diagram being established by
turbidity measurements), the blend exhibits a thermorheologically
complex behavior. A failure of TTS
is observed at low frequencies, both in the homogeneous pretransitional
and in the two-phase regimes.
Its origin is attributed to the enhanced concentration
fluctuations, which exhibit a critical slowing down
near the phase boundary in the homogeneous regime, and in the two-phase
morphology inside the phase-separated regime. The anomalous pretransitional behavior can be
quantified using a recent mean field
theory, yielding the spinodal temperature. Furthermore, in the
two-phase region an intermediate region
of enhanced moduli at low frequencies is observed, followed by flow at
even lower frequencies, which is
attributed to the two-phase structure. The linear viscoelastic
properties of the phase-separated blends
are, to a first approximation, adequately described by a simple
incompressible emulsion model considering
a suspension of droplets of one coexisting phase in the matrix of the
other phase.
With an increasing consumption of lipids nowadays, decreasing the fat content in food products has become a trend. Chocolate is a fat-based suspension that contains about 30%wt fat. Reducing fat content causes an increase in the molten chocolate viscosity. This leads to 2 major issues: difficulties in the process and a loss of eating quality in the final product, reported to have poor in-mouth melting properties, remain hard, and difficult to swallow. Literature shows that optimizing the particle size distribution (PSD), that is, having one with an increased packing fraction, can decrease the viscosity of highly concentrated suspensions. This study focuses on the impact of the PSD and fat content on the rheological properties, melting behavior, and hardness of chocolate models (dispersions of sugar in fat). We show that optimizing the PSD while reducing the fat content to a critical amount (22%wt) can decrease the viscosity of the molten material and reduce the hardness of the crystallized chocolate models. Melting in the mouth, characterized by an in vitro collapse speed, is faster for the samples with an optimized PSD. The decrease in the viscosity by optimizing the PSD in systems with a constant fraction of medium phase is based on the decrease of interparticle contact, reducing the particle aggregates strength, and structure buildup during flow or meltdown. In its crystallized state, the particle network is less interconnected, providing less resistance to breakage and meltdown.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.