111 research outputs found
Swelling of responsive-microgels: experiments versus models
No full textInterpenetrated Polymer Network (IPN) microgels of PNIPAM and PAAc have been investigated and the experimental data have been compared with theoretical models from the Flory–Rehner theory. We confirm that the swelling behavior of PNIPAM microgels is well described by this theory by considering the second order approximation for the volume fraction φ dependence of the Flory parameter χ(φ). Indeed the Volume-Phase Transition (VPT) of the PNIPAM-PAAc IPN microgel at neutral conditions and in D2O solvents can be well described only considering a third-order approximation. Interestingly we empirically find that sharper is the transition higher is the order of the χ(φ) relation which has to be considered. Moreover the VPT can be experimentally controlled by tuning the polymer/solvent interactions through pH and solvent allowing to directly modify the delicate balance between energetic and entropic contributions and to explore the swelling behavior in a wide range of environmental conditions. In particular we find that the most advantageous condition for swelling is in water at acidic pH
Rheological signatures of a glass-glass transition in an aging colloidal clay
No full textThe occurrence of non-equilibrium transitions between arrested states has recently emerged as an intriguing issue in the field of soft glassy materials. The existence of one such transition has been suggested for aging colloidal clays (Laponite® suspensions) at a weight concentration of 3.0%, although further experimental evidences are necessary to validate this scenario. Here, we test the occurrence of this transition for spontaneously aged (non-rejuvenated) samples by exploiting the rheological tools of dynamical mechanical analysis. On imposing consecutive compression cycles to differently aged clay suspensions, we find that quite an abrupt change of rheological parameters occurs for ages around three days. For Young’s and elastic moduli, the change with the waiting time is essentially independent from the deformation rate, whereas other “fluid-like” properties, such as the loss modulus, do clearly display some rate dependence. We also show that the crossover identified by rheology coincides with deviations of the relaxation time (obtained through x-ray photon correlation spectroscopy) from its expected monotonic increase with aging. Thus, our results robustly support the existence of a glass-glass transition in aging colloidal clays, highlighting characteristic features of their viscoelastic behavior
Aging behavior of the localization length in a colloidal glass
No full textThe localization length rloc associated with a fast secondary relaxation in glassy Laponite is determined by X-ray photon correlation spectroscopy (XPCS) through a Debye-Waller fit of the non-ergodicity parameter. Quantitative differences are observed between the time dependence (aging) of rloc in spontaneously aged and rejuvenated samples. This behavior is also reflected in the calculated shear modulus which matches well with data obtained by rheological measurements. © 2014 Elsevier B.V
Dynamic light scattering study of temperature and pH sensitive colloidal microgels
No full textMicrogel particles composed of Interpenetrated Polymer Network (IPN) of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc) dispersed in water have been investigated through dynamic light scattering. The study of the temperature, concentration and pH dependence of the relaxation time has highlighted the existence of a thermoreversible transition corresponding to the swollen-shrunken volume phase transition. The presence of PAAc introduces an additional pH-sensitivity with respect to the temperature-sensitivity due to PNIPAM and leads to interesting differences in the transition process at acid and neutral pH
Chemical-Physical Behaviour of Microgels Made of Interpenetrating Polymer Networks of PNIPAM and Poly (acrylic Acid)
No full textMicrogels composed of stimuli responsive polymers have attracted worthwhile interest as model colloids for theorethical and experimental studies and for nanotechnological applications. A deep knowledge of their behaviour is fundamental for the design of new materials. Here we report the current understanding of a dual responsive microgel composed of poly(N-isopropylacrylamide) (PNIPAM), a temperature sensitive polymer, and poly(acrylic acid) (PAAc), a pH sensitive polymer, at different temperatures, PAAc contents, concentrations, solvents and pH. The combination of multiple techniques as Dynamic Light Scattering (DLS), Raman spectroscopy, Small Angle Neutron Scattering (SANS), rheology and electrophoretic measurements allow to investigate the hydrodynamic radius behaviour across the typical Volume Phase Transition (VPT), the involved molecular mechanism and the internal particle structure together with the viscoelastic properties and the role of ionic charge in the aggregation phenomena
Isotopic effect on the gel and glass formation of a charged colloidal clay: Laponite
No full textThe time evolution of both dynamic and static structure factors of a charged colloidal clay, Laponite, dispersed in both H2O and D2O solvents has been investigated through multiangle dynamic light scattering (DLS) and smallangle X-ray scattering (SAXS) as a function of weight concentration. The aging phenomenology and the formation of arrested states, both gel and glass, are preserved in D2O, while the dynamics is slowed down with respect to water.
These findings are important to understand the role played by the solvent in the interparticle interactions and for techniques such as neutron scattering and nuclear magnetic resonance that allow for the extension of the accessible scattering vectors and time scales
Thermal Behaviour of Microgels Composed of Interpenetrating Polymer Networks of Poly(N-isopropylacrylamide) and Poly(acrylic acid): A Calorimetric Study
No full textStimuli-responsive microgels have recently attracted great attention in fundamental research as their soft particles can be deformed and compressed at high packing fractions resulting in singular phase behaviours. Moreover, they are also well suited for a wide variety of applications such as drug delivery, tissue engineering, organ-on-chip devices, microlenses fabrication and cultural heritage. Here, thermoresponsive and pH-sensitive cross-linked microgels, composed of interpenetrating polymer networks of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc), are synthesized by a precipitation polymerization method in water and investigated through differential scanning calorimetry in a temperature range across the volume phase transition temperature of PNIPAM microgels. The phase behaviour is studied as a function of heating/cooling rate, concentration, pH and PAAc content. At low concentrations and PAAc contents, the network interpenetration does not affect the transition temperature typical of PNIPAM microgel in agreement with previous studies; on the contrary, we show that it induces a marked decrease at higher concentrations. DSC analysis also reveals an increase of the overall calorimetric enthalpy with increasing concentration and a decrease with increasing PAAc content. These findings are discussed and explained as related to emerging aggregation processes that can be finely controlled by properly changing concentration, PAAc content an pH. A deep analysis of the thermodynamic parameters allows to draw a temperature–concentration state diagram in the investigated concentration range
Glass and Jamming Rheology in Soft Particles made of PNIPAM and polyacrylic acid
No full textThe 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
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