Microgels are soft colloidal particles composed of networks of cross-linked
polymers used in a variety of industrial products such as pharmaceuticals,
cosmetics and personal care items. The attraction of using microgels is their
ability to tune the rheology of suspensions by reversibly swelling, often triggered
in response to stimuli such as pH or temperature. However, recently the research
is focussed on their emulsifying capabilities which is a result of their polymer
building blocks. Much of the literature is focussed on ‘model’ microgel systems
synthesised for research but there are no reports on the commercial microgel
considered here used in many personal care products. In this thesis, a commercial
microgel system composed of a block co-polymer called Sepimax Zen (SZ) was
analysed to understand its rheology modifying and emulsifying capabilities and
the results were discussed in the context of existing literature on model microgel
systems. In addition to bridging the gap between model and commercial systems,
we aim to understand how the polymer-colloid duality of microgels determines
the intrinsic functions of microgel particles.
The rheological profile of the SZ microgels was tested using a combination
of steady shear and oscillatory rheology. Steady state measurements revealed
that at sufficiently dilute concentrations the suspensions behave as Newtonian
liquids, however, once a critical concentration (0.03 wt%) has been reached they
reveal shear thinning behaviour. At higher concentrations (0.08 wt%) a yield
stress develops indicating the formation of a network structure. Oscillatory
measurements revealed the nature of these networks with strain measurements
indicating that microgels form networks by interpenetrating their polymeric
chains, similar to polymers. The resultant networks were found to be ‘soft’ set
gels when compared to other microgel systems with a cross link density of 1500
monomers per crosslink.
In order to understand the dynamics and to determine the size of the SZ microgels
Differential Dynamic Microscopy (DDM) was used. DDM measures the dynamics
of a population of colloids via image analysis, allowing the mobility and the size
of individual colloids to be characterised. From these measurements SZ microgels
were found to be 2.7 µm in radius at low concentrations < 0.03 wt%, above which
their mobility is greatly reduced, coinciding with the onset of shear thinning. The
reduced mobility is likely due to the attractive interactions between SZ microgels
where surface chains entangle, since similar results are predicted for adhesive
spheres. The response to salt was also investigated by exposing the microgel to a
range in concentrations of NaCl from 0.001 mM to 2000 mM. Unexpectedly, the
microgel is considerably tolerant of salt, only reducing in size at a high critical
concentration of 200 mM NaCl at which it reduces in size by a factor of 2; an
important observation when considering them in commercial formulations. In
order to simulate the effect crowding has on this microgel a non-excluding polymer
called Ficoll-400 was used to tune the osmotic pressure of suspensions. At the
highest Ficoll-400 concentration SZ microgel particles were found to deswell from
2.7 µm down to 0.52 µm, a much larger decrease than observed with other microgel
systems. As one of the first studies to analyse microgel particles using DDM these
results demonstrate how valuable DDM is to provide insight into the rheology of
these soft particle systems.
Using a combination of pendant drop tensiometry and cryo-SEM measurements
the stabilisation of a n-dodecane-water interface by SZ microgels was determined.
SZ microgels were found to significantly reduce the interfacial tension by building
up layers of microgels on the surface of the interface. These SZ-stabilised
interfaces are highly elastic, E
0 > 20 mN/m, E
00 < 1 mN/m, similar to colloid-laden interfaces. The interfacial elasticity was dependent on size of the microgel,
as deswollen microgels resulted in more effectively packed interfaces resulting
in more elastic interfaces, E
0
increases to almost 30 mN/m. A surprising
observation was that SZ-stabilised interfaces appeared to be immune to buckling,
a phenomenon observed when particle-laden interfaces are destabilised. We
conclude this effect is due to SZ microgel particles associative behaviour and
solubility. When significantly disturbed by an interface SZ microgel particles
detach in order to associate with other microgel particles in the bulk. This
phenomenon has been seen in a few microgel systems, however, this work is
the first to investigate it in detail. These findings could have a profound effect
on how microgels are made given their multifunctional properties
In the final chapter the effect SZ has on emulsion stability and rheology is
analysed. SZ-stabilised oil-in-water emulsions were found to make emulsion-filled gels. The oil drops initially behave as passive fillers and then appear to
behave as active fillers at φ ≈ 20 %, increasing the elasticity of the emulsion gels.
Furthermore, SZ-stabilised emulsions are stable only up to 30 wt%, considerably
lower than for other emulsion systems. We conclude that this is a result of the
large microgel particles associating only lightly at the interface and therefore the
emulsion becomes active when microgel-laden emulsion droplets are in contact
(at φ ≈ 20 %) and is unstable when emulsion droplets are closer than one
microgel particle apart. The stability of emulsions were tested by combining the
results of centrifugation and drying experiments. The centrifugation experiments
determined the critical disjoining pressure, the pressure at which the oil is
dispelled from the matrix, which was found to be similar to traditional surfactant
systems. The drying experiments revealed that emulsions dried down to a
partially coalesced emulsion film, separated by a thin polymeric film which is
similar to studies on surfactant-stabilised systems.
This thesis explores the properties of a commercial microgel system and contrasts
the results to model microgel systems. The SZ microgels were found to be
highly unusual giant microgels, much larger than previously studied microgel
systems, exhibiting both colloidal and polymeric behaviour. Through combining
the rheological and DDM measurements, this thesis provides a standard for
studying microgels and other soft matter systems. The results of this thesis
form a strong basis for developing a framework for designing microgels with
dual functionality, preventing emulsion coalescence and creaming, as required for
industrial applications. Future work could focus on understanding the adsorption
of microgels at the interfaces of different oils i.e. more polar oils
