71 research outputs found
Non-diffusive dynamics in a colloidal glass: aging versus rejuvenation
The microscopic dynamics of spontaneously aged and rejuvenated glassy
Laponite is investigated through X-ray photon correlation spectroscopy. Two
different behaviours of the intensity autocorrelation functions are observed
depending on the history of the sample: stretched for spontaneously aged
samples and samples rejuvenated from a Wigner glass and compressed, typical of
anomalous dynamics, for samples rejuvenated from a DHOC glass. The relaxation
time behaviour in the three cases indicates a non-diffusive dynamics of the
particles. The present system offers therefore an overview of various dynamical
behaviours previously observed individually in several systems and the
possibility to pass from one to the other choosing ad hoc the time parameter.Comment: 5 pages, 4 figure
Study of network composition in interpenetrating polymer networks of poly(N isopropylacrylamide) microgels:the role of poly(acrylic acid)
Hypothesis: The peculiar swelling behaviour of poly(N-isopropylacrylamide)
(PNIPAM)-based responsive microgels provides the possibility to tune both
softness and volume fraction with temperature, making these systems of great
interest for technological applications and theoretical implications. Their
intriguing phase diagram can be even more complex if poly(acrylic acid) (PAAc)
is interpenetrated within PNIPAM network to form Interpenetrating Polymer
Network (IPN) microgels that exhibit an additional pH-sensitivity. The effect
of the PAAc/PNIPAM polymeric ratio on both swelling capability and dynamics is
still matter of investigation. Experiments: Here we investigate the role of
PAAc in the behaviour of IPN microgels across the volume phase transition
through dynamic light scattering (DLS), transmission electron microscopy (TEM)
and electrophoretic measurements as a function of microgel concentration and
pH. Findings: Our results highlight that aggregation is favored at increasing
weight concentration, PAAc content and pH and that a crossover PAAc content
C*_{PAAc} exists above which the ionic charges on the microgel become relevant.
Moreover we show that the softness of IPN microgels can be tuned ad hoc by
changing the PAAc/PNIPAM ratio. These findings provide new insights into the
possibility to control experimentally aggregation properties, charge and
softness of IPN microgels by varying PAAc content.Comment: preprint versio
A phase separation in diluted Laponite suspensions: evidence of empty liquid and equilibrium gel states
The relevance of anisotropic interactions in colloidal systems has recently
emerged in the context of rational design of novel soft materials. Theoretical
studies have predicted the possibility of a gas-liquid phase separation
confined at low densities and the formation of empty liquids and equilibrium
gels in low-valence systems. Here we provide experimental evidence of this
scenario in Laponite, a complex colloidal clay with discotic shape and
anisotropic interactions. We also report simulations of a patchy model for
Laponite platelets, able to reproduce the observed experimental phase diagram
and structural properties, confirming the crucial role of the reduced valence
Molecular mechanisms driving the microgels behaviour: a Raman spectroscopy and Dynamic Light Scattering study
Responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) exhibit
peculiar behaviours due to the competition between the hydrophilic and
hydrophobic interactions of the constituent networks. The interpenetration of
poly-acrilic acid (PAAc), a pH-sensitive polymer, within the PNIPAM network, to
form Interpenetrated Polymer Network (IPN) microgels, affects this delicate
balance and the typical Volume-Phase Transition (VPT) leading to complex
behaviours whose molecular nature is still completely unexplored. Here we
investigate the molecular mechanism driving the VPT and its influence on
particle aggregation for PNIPAM/PAAc IPN microgels by the joint use of Dynamic
Light Scattering and Raman Spectroscopy. Our results highlight that PNIPAM
hydrophobicity is enhanced by the interpenetration of PAAc promoting
interparticle interactions, a crossover concentration is found above which
aggregation phenomena become relevant. Moreover we find that, at variance with
PNIPAM, for IPN microgels a double-step molecular mechanisms occurs upon
crossing the VPT, the first involving the coil-to-globule transition typical of
PNIPAM and the latter associated to PAAc steric hindrance.Comment: preprint versio
Glass and Jamming Rheology in Soft Particles Made of PNIPAM and Polyacrylic Acid
The phase behaviour of soft colloids has attracted great attention due to the large variety of new phenomenologies emerging from their ability to pack at very high volume fractions. Here we report rheological measurements on interpenetrated polymer network microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc content as a function of weight concentration. We found three different rheological regimes characteristic of three different states: a Newtonian shear-thinning fluid, an attractive glass characterized by a yield stress, and a jamming state. We discuss the possible molecular mechanisms driving the formation of these states
Microglia-derived microvesicles affect microglia phenotype in glioma
Extracellular-released vesicles (EVs), such as microvesicles (MV) and exosomes (Exo)
provide a new type of inter-cellular communication, directly transferring a ready to use
box of information, consisting of proteins, lipids and nucleic acids. In the nervous
system, EVs participate to neuron-glial cross-talk, a bidirectional communication
important to preserve brain homeostasis and, when dysfunctional, involved in several
CNS diseases. We investigated whether microglia-derived EVs could be used to transfer
a protective phenotype to dysfunctional microglia in the context of a brain tumor.
When MV, isolated from microglia stimulated with LPS/IFNg were brain injected in
glioma-bearing mice, we observed a phenotype switch of tumor associated myeloid
cells (TAMs) and a reduction of tumor size. Our findings indicate that the MV cargo,
which contains upregulated transcripts for several inflammation-related genes, can
transfer information in the brain of glioma bearing mice modifying microglial gene
expression, reducing neuronal death and glioma invasion, thus promoting the recovery
of brain homeostasis
Local structure of temperature and pH-sensitive colloidal microgels
The temperature dependence of the local intra-particle structure of colloidal microgel particles, composed of interpenetrated polymer networks, has been investigated by small-angle neutron scattering at different pH and concentrations, in the range (299÷315) K, where a volume phase transition from a swollen to a shrunken state takes place. Data are well described by a theoretical model that takes into account the presence of both interpenetrated polymer networks and cross-linkers. Two different behaviors are found across the volume phase transition. At neutral pH and T 307 K, a sharp change of the local structure from a water rich open inhomogeneous interpenetrated polymer network to a homogeneous porous solid-like structure after expelling water is observed. Differently, at acidic pH, the local structure changes almost continuously. These findings demonstrate that a fine control of the pH of the system allows to tune the sharpness of the volume-phase transition
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