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application notes & technical articles
Rheology as a Tool for Immiscible
Polymer Blends Characterisation:
Interfacial Tension and
Compatibilisation
Polymer blends have gained an increasing
popularity in the field of polymer science and
industry during the last thirty years. The growth in
the use of polymer blends is mainly due to their
ability to combine the properties of their phases
in a unique product, making polymer blends
broadly employed in automobile, aerospace,
packing, and general goods industries. Currently,
about 40% of the world polymer market is
involved with the production of polymer blends,
representing trades of billions of US$ a year. The
final properties of polymer blends are directly
related to the quality of their morphology, which in
turn depends on the rheological properties of the
phases of the blend, on the composition of the
blend, on the processing conditions of the blend,
and on the interfacial tension between the
polymers forming the blend. However, most
polymer blends are incompatible, resulting in
materials with coarse morphology, weak adhesion
among phases and poor mechanical properties. In
fact, the future of polymer blends is based on
biennial 07-08
polymer industries
methods to achieve stable morphologies, so that
the enhanced and optimized properties of
polymer blends do not change during the product
processing due to phenomena such as
coalescence and breakup. This matter still
remains an industrial challenge and is the object
of intense research.
Interfacial tension between molten polymers has
received considerable experimental and
theoretical attention due to its implication in
several industrial applications such as coating,
composites and especially in polymer blends.
Interfacial tension is the single most accessible
parameter that describes the thermodynamic
state and structure of an interface in polymer
blends, therefore directly related to compatibility
and hence morphology of polymer blends. Also,
interfacial tension can be considered as a key
parameter that measures the state of miscibility
between incompatible polymers. The compatibility
between the phases of a blend can be improved
by the addition of compatibilisers which result in
a finer and more stable morphology, better
adhesion between the phases of the blends and
consequently better properties of the final
product. The exact definition of the optimum
concentration of compatibiliser to be added is
associated with the costs minimisation, since
most compatibilisers are much more expensive
than ordinary polymers.
In this work, rheological measurements are
shown to be a useful technique to study polymer
blends. In particular, they can be used to
determine the interfacial tension between
immiscible polymers and define the optimum
concentration of compatibiliser to be added to the
blend. This study is based on the execution of
small amplitude oscillatory shear (SAOS) tests in
the linear viscoelasticity region of the materials.
The determination of interfacial tension is related
to the study of relaxation spectrum of a polymer
blend, which is a result of the contribution of the
phases and the interface. The definition of the
concentration of compatibiliser is determined
studying the applicability of time-temperature
superposition (TTS) for the blend as a function of
compatibiliser concentration.
A full copy of the paper is available by
quoting reference no. RF- 110902-br-01
application notes & technical articles
Rheological and Microstructural
Analysis of a Model Rennet Casein
Gel as Influenced by Cooling Rate
food industries
Cooling is a final process during process cheese
production and is therefore critical when
determining the texture and functional properties
of the product. It is well established that a slower
cooling yields a firmer cheese. However, there is a
lack of quantitative description of this operation,
and the mechanism behind the observation is
needed to better control the cooling process. The
rheological data for process cheese cooled at
different rates was quantified and was consistent
with the industrial observation. To illustrate
physical changes during cooling, rennet casein
gels were studied as our first step in
understanding the cooling effects on process
cheese texture and microstructure. A similar trend
in storage modulus to process cheese was
observed when the casein gels were cooled at
different rates. To explain this behavior, a
schematic illustration was used as the physical
bases for microstructure of colloidal gels that can
result in different rheological properties. Fractal
dimension, floc size, and floc order in the protein
network were treated as three possible variables.
The theory developed by Shih et al. (1990) was
used to determine the floc fractal dimension, and
a confocal laser scanning microscope observed
the floc size and order. The results showed that
the floc fractal dimension and size were not
significantly different when casein gels were
cooled at different rates. However, the cooling did
impact floc arrangement in the protein network. A
higher order at a slower cooling rate resulted in a
higher storage modulus, a smaller limit of linear
viscoelastic range, and an increase in gelation
temperature. The rheological data were consistent
with the microscopic images, and the
hypothesised variables sufficiently explained the
physical changes in casein gels during cooling at
different rates.
A full copy of the paper is available by
quoting reference no. RF- 200902-US-01
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