Temperature-induced gelation of concentrated silicon carbide suspensions

Due to the steric barrier provided by the adsorption of the dispersant hypermer KD1 (a polyester/polyamine condensation polymer), stable and low-viscosity suspensions of SiC, Y(2)O(3), and Al(2)O(3) powder mixtures could be prepared in methyl ethyl ketone (MEK)/ethanol (E) solvent with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. Steady shear viscosity and oscillatory measurements were performed as a function of temperature for suspensions with different solids loading. The viscosity and elastic modulus of suspension increased with decreasing temperature and became more sensitive with the increase of solids loading. The suspensions with solids loading higher than 40 vol% could be solidified with decreasing temperature, but gelation temperature and gelation stiffness decreased with decreasing solids loading. The 60 vol% solid-loaded suspension was a stable and free-flowing fluid at 20 degrees C and gradually transformed to a very highly viscous and elastic system upon cooling to about 13 degrees C. Complete solidification occurred when the temperature was decreased to 5 degrees C. The gelation mechanism was mainly based on the collapse of the adsorbed layer as the temperature decreases, which induced incipient flocculation and formed a stiff network. The gelled body was further strengthened by separation of the dispersant from the suspension.     

Temperature-induced association and gelation of aqueous solutions of pectin. A dynamic light scattering study

Dynamic light scattering (DLS) measurements were carried out on aqueous solutions of low-methoxyl pectin at different temperatures and polymer concentration. Low temperature and increased polymer concentration promote the formation of multichain aggregates. The time correlation data obtained from the DLS experiments revealed, for all polymer solutions, the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. In the semidilute regime, a temperature reduction induced enhanced chain associations in the solutions with high values of the slow relaxation time and a strong wave vector dependence of the slow mode. These features could be rationalized in the framework of the coupling model of Ngai. At low temperatures (10 °C), gelation occurs in the semidilute regime and a transparent gel is formed. In this state, the profile of the correlation function changes and nonergodic signs are observed. The conjecture is that the association complexes and the gel network are stabilized through intermolecular hydrogen bonds, which are broken-up at higher temperatures. The hydrogen-bonded structures are formed in a process where the polymer chains have been “zipped” together in a cooperative manner.     

The gelation and rigidity modulus of dilute poly(vinyl chloride) gels in phthalate plasticizer solutions

Static elastic moduli of gels of PVC have been obtained in various dialkyl phthalate plasticizers at times between 0.5 and 200 hr after formation, at temperatures between 27 and 45° and for polymer concentrations between 4 and 10.55%. The moduli increase linearly with log (time) and they increase with decreasing temperature, with increasing polymer concentration and with increasing size of the alkyl group of the plasticizer. These changes follow the same pattern as those observed with plasticized PVC at concentrations normally used in commerce. The moduli obtained at the present concentration have been interpreted using rubber-like elasticity theory and a simplified theory of network equilibrium.     

Temperature-induced phase transitions in BaTbO3

The crystal structures of BaTbO₃ have been investigated over a wide temperature range between 40 and 773 K using high-resolution time-of-flight neutron powder diffraction. Two-phase transitions were observed. Below about 280 K, BaTbO₃ adopts an orthorhombic perovskite structure (space group Ibmm), which is characterized by rotation of TbO₆ octahedra about the pseudocubic two-fold axis. Above 280 K, BaTbO₃ undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm), in which the tilting of the octahedra is around the pseudocubic four-fold axis. As the temperature is further increased, BaTbO₃ adopts the primitive cubic aristotype at about 623 K. This later phase transformation is characterized by a gradual decrease of the rotation angle, indicating a continuous phase transition, which is described by a critical exponent β=0.35.     

Hybrid nanofluids

Journal of Thermal Analysis and Calorimetry -     

Temperature-induced reversible changes in long spacing in oriented polyethylene

The possible explanations for the temperature-induced reversible changes in long spacing in polymers are reviewed. The observation of particularly large changes in certain irradiated samples of oriented low-density polyethylene is reported. By combining these results with those obtained by DSC and other means it is concluded that the spacing changes are caused by partial melting of small lamellae within the lamellar stacks which alters the mean periodicity. The requirement of an irregular lattice explains why the effect is observed primarily in bulk samples and especially in materials which contain intrinsic irregularities.     

Temperature-induced changes in the composition of floatglass surfaces

Floatglass is an important kind of commercial glass, comprising the main body of modern flat glass used in buildings and vehicles. The stoichiometry of the surface layer differs from that of the bulk and will usually change due to subsequent high-temperature process steps or attack by water or humidity. Glass samples have been investigated by means of ion beam analysis. Using resonant nuclear reaction analysis (¹⁵N technique), hydrogen concentration profiles have been determined. Profiles of the heavier glass constituents, especially sodium and tin, have been obtained by RBS. Changes in the composition of the floatglass surfaces caused by subsequent temperature treatment (up to 700^°C) and by controlled hydration treatment are reported. Possible mechanisms of hydrogen uptake and release are discussed.     

Temperature-induced changes in the composition of floatglass surfaces

Floatglass is an important kind of commercial glass, comprising the main body of modern flat glass used in buildings and vehicles. The stoichiometry of the surface layer differs from that of the bulk and will usually change due to subsequent high-temperature process steps or attack by water or humidity. Glass samples have been investigated by means of ion beam analysis. Using resonant nuclear reaction analysis ((15)N technique), hydrogen concentration profiles have been determined. Profiles of the heavier glass constituents, especially sodium and tin, have been obtained by RBS. Changes in the composition of the floatglass surfaces caused by subsequent temperature treatment (up to 700 ( degrees )C) and by controlled hydration treatment are reported. Possible mechanisms of hydrogen uptake and release are discussed.     

Temperature-induced growth of wormlike copolymer micelles

We study the temperature-induced growth of polymer micelles based on Pluronic P84 in brine (2 M NaCl) using small-angle neutron scattering, static and dynamic light scattering, and viscometry as a function of temperature and polymer concentration. Spherical micelles below 30 degrees C are shown to grow between about 30 and 40 degrees C into wormlike micelles long enough to enter the semidilute regime for polymer volume fraction larger than 0.005. The entanglements in this regime are responsible for a huge increase in the viscosity. Above about 41 degrees C, the micellar aggregates become denser as the cloud point is approached and the viscosity drops.     

Amylose gelation in low-concentration conditions

Amylose gels of low concentration (1–2 wt.-%) were studied by means of a rheological method and turbidity measurements. The effects of temperature and concentration were investigated. The kinetics of turbidity evolution and of the storage modulus G' variations were strongly dependent of both parameters. The overall gelation process could be described in three steps: first, an induction period, followed by a sharp variation of G' and turbidity, and finally a slow variation of G'. These observations confirm that amylose gelation proceeds through a phase separation process.     

Globular protein gelation

In this paper, significant advances in the structure and rheological properties of globular protein gels have been described. The achievements of scattering methods, which have provided information over a range of distances from the monomer to the micron scale, are reviewed. The kinetics of gelation measured by rheological methods have also been discussed, including currently applied models for the gelation time, the gel elastic modulus and the critical gel concentration.     

Applications of nanofluids in porous medium

Journal of Thermal Analysis and Calorimetry - This investigation provides a review on the applications of nanofluids in porous media. The transport phenomena in porous media have been of continuing...     

Temperature-induced self-assembly of triple-responsive triblock copolymers in aqueous solutions

A series of triple-thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (PNPAM, A), poly(methoxydiethylene glycol acrylate) (PMDEGA, B), and poly(N-ethylacrylamide) (PNEAM, C) was synthesized by sequential reversible addition-fragmentation chain transfer polymerizations. Polymers of differing block sequences, ABC, BAC, and ACB, with increasing phase transition temperatures in the order A < B < C were prepared. Their aggregation behavior in dilute aqueous solution was investigated using dynamic light scattering, turbidimetry, and NMR spectroscopy. The self-organization of such polymers was found to dependent strongly on the block sequence. While polymers with a terminal low-LCST (lower critical solution temperature) block undergo aggregation above the first phase transition temperature at 20-25 °C, triblock copolymers with the low-LCST block in the middle show aggregation only above the second phase transition. The collapse of the middle block is not sufficient to induce aggregation but produces instead stable, unimolecular micelles with a collapsed middle block, as supported by NMR and fluorescence probe data. Continued heating of all copolymers led to two additional thermal transitions at 40-55 and 70-80 °C, which could be correlated to the phase transitions of the B and C blocks, respectively. All polymers show a high tendency for cluster formation, once aggregation is induced. The carrier abilities of the triple responsive triblock copolymers for hydrophobic agents were probed with the solvatochromic fluorescence dye Nile Red. With passing through the first thermal transition, the block copolymers are capable of solubilizing Nile Red. In the case of block copolymers with sequences ABC or ACB, which bear the low-LCST block at one terminus, notable amounts of dye are solubilized already at this stage. In contrast, the hydrophobic probe is much less efficiently incorporated by the BAC triblock copolymer, which forms unimolecular micelles. Only after the collapse of the B block, when reaching the second phase transition at about 45 °C, does aggregation occur and solubilization becomes efficient. In the case of ABC and ACB polymers, the hydrophobic probe seems to partition between the originally collapsed A chains and the additional hydrophobic chains formed after the collapse of the less hydrophobic B block.     

Temperature-induced sedimentation to dispersion of ionic vesicles

We present a temperature-induced sedimentation/dispersion transition of ionic vesicles in the system of alkyldimethylamine oxide hemihydrochloride (CnDMAO.1/2HCl) with a hydrocarbon chain length of 12-16 (n = 12, 14, and 16) and sodium 2-naphthalenesulfonate (NaNphS). The temperature-sensitive sedimentation/dispersion of ionic vesicles took place around a temperature of 50 degrees C, which was weakly dependent on the alkyl chain length. The combined effect of the thermally induced dissociation of the counterions from the vesicle and a hydrogen bonding between the nonionic and the cationic head groups is likely to be responsible for this unique behavior.     

Temperature-induced protein release from water-in-oil-in-water double emulsions

A model water-in-oil-in-water (W1/O/W2) double emulsion was prepared by a two-step emulsification procedure and subsequently subjected to temperature changes that caused the oil phase to freeze and thaw while the two aqueous phases remained liquid. Our previous work on individual double-emulsion globules1 demonstrated that crystallizing the oil phase (O) preserves stability, while subsequent thawing triggers coalescence of the droplets of the internal aqueous phase (W1) with the external aqueous phase (W2), termed external coalescence. Activation of this instability mechanism led to instant release of fluorescently tagged bovine serum albumin (fluorescein isothiocyanate (FITC)-BSA) from the W 1 droplets and into W2. These results motivated us to apply the proposed temperature-induced globule-breakage mechanism to bulk double emulsions. As expected, no phase separation of the emulsion occurred if stored at temperatures below 18 degrees C (freezing point of the model oil n-hexadecane), whereas oil thawing readily caused instability. Crucial variables were identified during experimentation, and found to greatly influence the behavior of bulk double emulsions following freeze-thaw cycling. Adjustment of these variables accounted for a more efficient release of the encapsulated protein.     

Thermophysical properties of interfacial layer in nanofluids

Although recent experiments have revealed that nanofluids have superior thermal conductivities to base fluids, the inherent physics are not fully understood. In this study, an interfacial layer, competing with Brownian motion as a corresponding mechanism, is conceptually connected with the surface-charge-induced electrical double layer. By applying colloidal science, the first explicit equations for the thickness and thermal conductivity of the layer are obtained. A fractal model including the new concept of the layer is developed. The model predictions are compared with experimental data for effects of pH, temperature, volume fraction, and primary particle size of CuO-water nanofluids.     

Viscosity divergence and gelation

A new model of gelation is presented in which the viscosity is observed to diverge without the introduction of shear elasticity when a critical concentration of suspended particles is reached. This viscosity divergence results from the hydrodynamic interaction between the suspended particles and can be explained using the Random Resistance Network model. A comparison of the data and with other models is also presented.