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Curing and viscoelasticity of vitrimers
No full textWe present an experimental investigation of the curing kinetics and viscoelasticity for a number of “vitrimers” recently developed by Leibler and coworkers.1-3 Vitrimers are covalently crosslinked networks that can relax stress at elevated temperatures due to thermoreversible bond-exchange reactions. The chosen formulations are composed of diglycidyl ether of bisphenol A, commercial fatty acid mixtures and an appropriate catalyst. The effects of the catalyst and functionality of the curing agents on the kinetics of the curing reactions were systematically investigated using rheometry. The curing kinetics followed the Arrhenius law and the catalyst drastically accelerated the reactions. Time-temperature superposition was used to construct master curves of the small-strain amplitude oscillatory shear moduli over wide ranges of frequencies for the cured networks. Terminal relaxation was not reached in oscillatory experiments for temperatures up to 130 °C and creep and stress relaxation experiments were used to probe the long-time relaxation. The shift factors displayed a Williams-Landel-Ferry dependence on temperature which could be divided into two regions, one above 70 °C, where the dynamics appeared to be controlled by the catalyst, and one below, controlled by the monomeric friction and the free volume of the network. The moduli of the vitrimers obeyed the classical rubber theory well, indicating that the curing reactions proceeded to completion. Furthermore, we systematically and reproducibly observed a double relaxation behavior for the vitrimers, i.e. next to the rubbery plateau at high frequencies, the storage modulus displayed a secondary plateau at lower frequencies before reaching terminal relaxation at even lower frequencies. Interestingly, 70 °C was found to be the transition point in agreement with the shift factors. To the best of our knowledge, the double relaxation behavior has not been previously reported in experimental works and recent theories do not incorporate an explanation for this behavior. Consequently, future investigations concerning the viscoelasticity of other “vitrimer-chemistries” are important to assess if the double relaxation is a universal fingerprint for vitrimers or if it is specific to the here-investigated formulations based on commercial fatty acid mixtures. © The Royal Society of Chemistry
Adding salt to a surfactant solution: Linear rheological response of the resulting morphologies
No full textThe micellar system composed of Cetylpyridinium Chloride-Sodium Salicylate (CPyCl-NaSal) in brine aqueous solutions has been studied by systematically changing the salt concentration, in order to investigate the rheology of the arising morphologies. In particular, the zero-shear viscosity and the linear viscoelastic response have been measured as a function of the NaSal concentration (with [CPyCl] = 100 mM). The Newtonian viscosity shows a nonmonotonic dependence upon concentration, passing through a maximum at NaSal/CPyCl approximate to 0.6, and eventually dropping at higher salt concentrations. The progressive addition of salt determines first a transition from a Newtonian to a purely Maxwell-like behavior as the length of the micelles significantly increases. Beyond the peak viscosity, the viscoelastic data show two distinct features. On the one hand, the main relaxation time of the system strongly decreases, while the plateau modulus remains essentially constant. Calculations based on the rheological data show that, as the binding salt concentration increases, there is a decrease in micelles breaking rate and a decrease in their average length. On the other hand, in the same concentration region, a low-frequency elastic plateau is measured. Such a plateau is considered as the signature of a tenuous, but persistent branched network, whose existence is confirmed by cryo-transmission electron microscopy images
Migration and alignment of spherical particles in sheared viscoelastic suspensions. A quantitative determination of the flow-induced self-assembly kinetics.
No full textFlow-Induced Self-Assembly (FISA) is the flow-driven formation of ordered structures in complex fluids. In this paper the effect of shear flow on the microstructure formation of dilute sphere suspensions in a viscoelastic fluid has been studied experimentally by optical microscopy techniques. The system is formed by Polymethylmethacrylate beads suspended in 20. wt.% aqueous solutions of Hydroxypropylcellulose at volume fractions ranging between 0.1% and 1.0%. Experiments show that, under the action of flow, beads migrate from the bulk to the shear walls, there forming strings aligned along the flow direction. Strings grow with time eventually reaching a steady-state final length. The alignment kinetics have been quantified by means of an alignment factor, which is a measure of the average length of the strings. The experimental results indicate that both shear rate and particle concentration are relevant factors in determining the alignment factor kinetics. In particular, it is shown that, upon increasing shear rate, strings grow both faster and longer. As a consequence, the characteristic time of the overall alignment process remains roughly constant. It is also shown that an increase in particle volume fraction determines effects similar to an increase of shear rat
Effects of matrix viscoelasticity on the rheology of dilute and semi dilute suspensions of non Brownian rigid spheres
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Effect of polymer concentration and thermal history on the inverse thermogelation of hydroxypropylcellulose aqueous solutions
No full textWe report on the inverse thermogelation of aqueous solutions of high molecular weight HydroxyPropylCellulose (HPC). The gelation process is investigated at different polymer concentrations, in the range 4÷16% (w/w). For each concentration, different heating rates are considered, in order to explore the effects on the sol-gel transition. Our findings corroborate the scenario proposed in literature for the thermoreversible gelation of cellulose derivatives, according to which the sol-gel transition is governed by an interplay of liquid-liquid phase separation and gelation. Gelation occurs in the polymer-rich phase via enhanced hydrophobic interactions between polymer chains at high temperature. At low heating rates, the phase separation proceeds significantly before the gelation occurs, creating more coarsened phases. As a consequence, the polymer network is less percolated and results in a weaker gel. Such an effect is more evident in less concentrated solutions characterized by a shorter terminal relaxation time, that is, faster polymer diffusion. As HPC concentration and heating rates are increased, the dissipative capacity of the final gel, expressed by the value of the phase angle at high temperatures, becomes nearly independent on polymer concentration and heating rate. However, the absolute values of the viscoelastic moduli of the formed gel increase with both HPC concentration and heating rate. The temperature window where transition occurs slightly narrows down with increasing concentration
Rheology of dilute and semidilute noncolloidal hard sphere suspensions
No full textThe dissipative behavior of model suspensions composed of non Brownian, inertialess, rigid spheres immersed in Newtonian and viscoelastic matrices is investigated in the range of volumetric concentrations up to 10%, thus encompassing both the dilute and semidilute regimes. Polymethylmethacrylate (PMMA) beads are dispersed into Polyisobutylene (PIB), characterized by a Newtonian rheology, and into two viscoelastic Polydimethylsiloxanes (PDMS). Both steady state viscosity and oscillatory shear loss modulus measurements are performed. As expected, the presence of the filler increases both the viscosity and the loss modulus of all suspensions. Following the hydrodynamic calculations of Batchelor, the concentration dependence is described by a second order polynomial expansion in the volume fraction. For low concentrations, the linear Einstein and Palierne predictions for Newtonian and viscoelastic fluids are found to be well obeyed by both the Newtonian and the viscoelastic suspensions. In the semi-dilute regime, the experimental data for the Newtonian suspension show an excellent quantitative agreement with Batchelor’s calculations. Conversely we find that, out of possible experimental errors, the viscoelastic suspensions show more pronounced deviations from the linear dilute behavior, resulting in a second order polynomial coefficient substantially larger than that predicted by Batchelor for Newtonian systems
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