In this work, the linear viscoelastic properties of the cetyltrimethylammonium tosilate (CTAT)−water system are examined in detail. This system forms elongated micelles at low and intermediate concentrations, and it yields a hexagonal phase above 27 wt % CTAT at 25 °C. Rheological behavior at low frequencies in a small-amplitude oscillatory shear experiments or at long times in stress relaxation measurements is governed by a single dominant relaxation time, although deviations from the limiting slope of the elastic modulus in the terminal region are observed at high CTAT concentrations. For higher frequencies, however, there is an additional mechanism whose dependence on frequency is analyzed with several rheological models. Analysis of data in terms of the theory of Cates demonstrates that the system consists of flexible micelles in the slow-breaking limit and it exhibits a constant entanglement density along the whole micellar region, even though the average micellar length decreases monotonically with concentration. Under these conditions, reptation speed up by the kinetics process of breaking and re-forming is the controlling relaxation mechanism.

The partial phase behavior of CTAT/water is investigated here as a function of temperature by WAXS, DSC, polarizing microscopy, conductometry, 1H-NMR, and FTIR spectroscopy. Oscillatory strain and temperature sweeps are also reported. The Krafft temperature (7k) of CTAT/water is 23 °C. Below this value, triclinic crystals of CTAT coexist with an isotropic solution. Above 7k and at low concentrations, spherical micellar solutions are Newtonian and exhibit low viscosities. At higher concentrations (ct), cylindrical micelles form and viscosity increases dramatically with CTAT concentration, but no elastic effects are noticed. When micelles are long enough to entangle (0.9-27 wt % at 25 °C), clear viscoelastic solutions form. At higher concentrations and up to 47 wt %, an hexagonal phase appears. This phase exhibits yield stress and viscoelasticity. At higher concentrations, a nonelastic, viscous solid paste forms. Micellar solutions and hexagonal phase depicts three regimes of viscoelasticity with temperature. These regimes are bounded by T\ and by the temperature (TV) at which the system exhibits its main relaxation time. 77 moves to lower temperatures as CTAT concentration increases indicating that the main relaxation time decreases upon increasing concentration.

The nonlinear viscoelastic behavior of the cetyltrimethylammonium p-toluenesulfonate (CTAT)−water system is investigated in steady and unsteady shear flow as a function of surfactant concentration and temperature. A rheo-optical study which includes measurements of dichroism, birefringence, and turbidity under flow at various shear rates is also discussed. The shear viscosity data in steady shear agree with the complex viscosity in the limit of low deformation rates. For moderate deformation rates, in the shear thinning region, the Cox-Merz rule is not followed. In all cases, a limiting stress or plateau stress was observed at shear rates that exceed one-half of the reciprocal of the main relaxation time [(2τd)-1]. At the stress plateau, the micellar solution most likely undergoes an isotropic-to-nematic phase transition induced by shear. However, our results do not conclusively exclude the possibility of a constitutive instability with respect to shear banding, in which simultaneous shear rates coexist under controlled stress experiments. In unsteady shear flow, CTAT−water micellar solutions exhibit a slow transient behavior in which the system achieves steady state in starting up experiments after tens to hundreds of Maxwell relaxation times. This is consistent with the existence of shear banding. Metastable branches are also observed in thixotropic loops produced under exponential shear. The time scale of this branch coincides with that of the inception of shear flow just before the overshoot peak. Moreover, the system exhibits a quasilinear rheological behavior at long times characterized by an exponential relaxation with a single time constant. A simple model consisting of the co-deformational Maxwell constitutive equation and a kinetic equation for construction and destruction of structure is proposed to predict distinct features of the complex rheological behavior of the elongated micellar solutions.

No abstract

DNA dynamics and flow properties are of great importance for understanding its functions. DNA is a semiflexible polymer chain characterized by having a large persistence length of around 50 nm and high charge density; DNA chains are interacting efficiently at high concentrations, in dependence of the ionic concentration. In relation with DNA molecular characteristics, it is also known that DNA solutions are able to form liquid crystalline phases over a critical polymer concentration. In this work, the supramolecular organization in calf-thymus DNA solution, with low degree of entanglement, appearing under flow was studied in a wide DNA concentration range from 2 to 10 mg/mL, at a pH of 7.3 and 20 °C. The rheological behavior of the system was studied using steady state flow and oscillatory measurements. Transient regimes were also tested by imposing controlled shear rates on a short time up to steady state. Furthermore, a combination of visual observations and flow birefringence measurements was proposed to reach a better understanding of the obtained rheological behavior. The presence of a shear-induced texture is revealed under flow for the calf-thymus DNA solutions at C DNA> 5 mg/mL and attributed to organized domains of DNA molecules, named in the text as crystalline parts, which are progressively oriented under shear. Finally, at high shear rates (over 100 s–1), it is shown that for the DNA solutions the orientation of these organized DNA domains and connecting chains under flow goes to an anisotropic monodomain.

Aluminum alloys 6061, 2024, and 7075 were heat treated to various tempers and then subjected to a range of plastic strain (stretching) in order to determine their strain limits. Tensile properties, conductivity, hardness, and grain size measurements were evaluated. The effects of the plastic strain on these properties are discussed and strain limits are suggested.

Due to their slow kinetics, the dynamical pathways of block copolymer micelles are as important as the copolymer architecture, particularly when making self-assembled nanostructures.Two types of dynamical pathways could govern these dynamics: insertion/expulsion of copolymer chains and fusion/fission of micelles. However, quantifying and understanding the fusion and fission processes remains very limited in copolymer micelles, especially at equilibrium. In this article, it was demonstrated the use of a fluorescent technique, using randomization of a highly hydrophobic pyrene derivative between micelles, as a tool to quantify the fusion and fission at equilibrium in a series of triblock copolymers micelles Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) in aqueous solution. The temperature dependence of fusion and fission is investigated for copolymers with various core block lengths (NPPO) in the spherical regime. Fusion and fission rates were found to strongly decrease with increasing the core block length. The dependence of the fission rate on NPPO is analyzed in terms of thin corona and starlike micelle models which suggest that fission is mainly dominated by the core interfacial tension as predicted by Halperin et al. The comparison between fission and expulsion kinetics and their dependence on PPO block is also reported. Finally, fusion is found to follow the same temperature and core length dependencies as fission, which is an indication that the interfacial tension plays a relevant role in the fusion kinetic.

Tissue engineering provides solutions that require medicine to restore damaged tissues or even complete organs. This discipline combines biologically active scaffolds, cells and molecules; being the addition of nanoparticles into the scaffolds, one of the techniques that is attracting more interest these days. In this work, Hydroxyapatite Nanorods (HA) were added to the network of Gelatin hydrogel (GE), and the particular properties resulting from their interaction were studied. Specifically, viscoelastic properties were characterized as a function of gel and nanoparticle concentration, varying ratios and temperatures. Oscillatory Time Sweeps (OTS) provided the necessary information about how the timeresolved material property/structure alteration. A wide variety of Continuous Flow Tests and Frequency Sweeps were used to describe the mechanical properties of the material, proving that the presence of nanoparticles led to a reinforcement of the gel network, mechanical stiffness and strength. The thixotropic nature of the gels was also evaluated and the most common theoretical models were described and commented. The attributes inferred from the data, showed a material that can allow the natural growth of bone tissue whilst withstanding properly the mechanical efforts; resulting in a material with an outstanding suitability to be used in regenerative medicine.