Gravitational effects on the phase behaviour of dispersions

Gravity can influence the formation of highly ordered structures from monodisperse polymer colloid systems, i.e. crystallization. These effects are particularly significant in the formation of alloy type crystals from bimodal dispersions. In this situation the use of “time-average” gravity is beneficial for the formation of mixed crystals.     

The influence of temperature on the structural behaviour of sodium tri- and hexa-titanates and their protonated forms

The thermal stability of Na₂Ti₃O₇ and Na₂Ti₆O₁₃, together with that of their protonated forms was investigated. The influence of temperature on the structural behaviour of tri- and hexa-titanates was studied using DTG, XRD, FTIR and FT-Raman. These results were correlated with the nature of the exchangeable cations using elemental analysis and N₂-sorption. The tunnel structure of Na₂Ti₆O₁₃ revealed a higher thermal stability compared to the layered structure of Na₂Ti₃O₇ that shows traces of dimerisation above 800 °C. The sodium forms were observed to be more thermally stable compared to their protonated forms. However, analysis show that ‘H₂Ti₆O₁₃’ is much more thermally stable than the layered H₂Ti₃O₇ which go through structural changes above 250 °C and suffer a complete structural collapse into anatase/rutile above 800 °C. It was obvious that thermal stability was significantly influenced by the nature of the exchangeable cations and the degree of ion-exchange.     

The influence of non-ionic surface active agents on the stability of polystyrene latex dispersions

Summary The influence ofn-dodecyl hexaoxyethylene glycol monoether on the stability of dispersions of polystyrene latices has been investigated by measuring the rates of flocculation, by lanthanum nitrate and hydrochloric acid, in the presence of various concentrations of the surface active agent. The adsorption isotherm for the non-ionic surface active agent on the latex surface showed that saturation adsorption was reached at about the critical micelle concentration to give a monolayer of vertically orientated molecules. A determination of the thickness of the adsorbed layer by an ultracentrifuge method confirmed the vertical orientation. In order to explain the results a theory has been developed based on the interaction between the adsorbed layers which includes a term to account for the interaction between the surface active molecule and the solvent.

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Zusammenfassung Der Einfluß vonn-Dodecyl-hexaoxyäthylen-glycolmonoäther auf die Stabilität von Dispersionen von Polystyrol-Latices wird durch Messung der Flockungsgeschwindigkeit durch Lanthannitrat und hydrochlorische Säure in Gegenwart von verschiedenen Konzentrationen überflächenaktiver Agentien untersucht. Die Adsorptionsisothermen von nichtionischen oberflächenaktiven Agentien an die Latexoberfläche zeigt, daß etwa bei der kritischen Micellkonzentration Sättigungsadsorption erreicht wird, die eine monomolekulare Schicht senkrecht orientierter Moleküle gibt.Die Bestimmung der Dicke der adsorbierten Schicht durch eine Ultrazentrifugenmethode stellt die senkrechte Orientierung sicher. Um die Ergebnisse zu erklären, wird eine Theorie entwickelt, die auf der Wechselwirkung zwischen den adsorbierten Schichten beruht, einschließend einen Term, um der Wechselwirkung zwischen den oberflächenaktiven Molekülen und dem Lösungsmittel Rechnung zu tragen.

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With 7 figures and 3 tables     

The influence of the dry grinding on the thermal behaviour of pyrophyllite

The differences of the thermal curves (DTA-TG) of pyrophyllite measured before and after grinding have been studied to obtain quantitative information about the alteration of the structure of this layer silicate.The mechanical treatment by dry grinding of pyrophyllite produces a decrease in the particle size of the original material. This strongly influences the thermal behaviour of weight losses on the TG curves decrease, and the endothermic DTA effects shift to lower temperatures, until the mechanochemical limit in particle size reduction is reached.Further grinding produces a disruption of the 21 aluminosilicate structure giving an exothermic DTA effect, and also reagglomeration processes of the amorphous and activated material accompanied by a decrease in specific surface area.

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Zusammenfassung Es wurden vergleichbare Änderungen an thermischen Kurven (DTA, TG) von Pyrophyllit vor und nach Zerkleinerung untersucht, um quantitative Angaben über die Änderung dieses Schichtensilikates zu erhalten.Durch die mechanische, trockene Zerkleinerung von Pyrophyllit kam es gegenüber dem Ausgangsmaterial zu einer Abnahme der Partikelgröße. Durch diesen Umstand wird das thermische Verhalten des erhaltenen Produktes stark beeinflußt. Bis zum Erreichen der mechanochemischen Grenze der Minderung der Partikelgröße nehmen in den TG-Kurven die Temperaturen für die Massenverluste ab, und die endothermen DTA-Effekte werden zu niedrigeren Temperaturen verschoben.Weiteres Zerkleinern führt zu einer Störung der 21-Aluminosilikatstruktur, was einen exothermen DTA-Effekt verursacht, sowie auch zu Reagglomeration des sehr amorphen und aktivierten Materiales, begleitet von einer Minderung des spezifischen Oberfläche.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday

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The authors thank Professors J. Sanz and J. M. Trillo for their valuable discussions of some aspects of this paper, and Mr. E. Gomez-Asensio for technical assistance in the SEM work.

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Thanks are extended to the Ministerio of Educacion y Ciencia for a grant to one of the authors (Pedro José Sánchez Soto) which enabled him to make this research.

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This paper was made possible through the financial support of the CICYT, research number GEO90-0060.     

The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

Thermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from T_g + 25°C to T_g + 80°C. Between T_g and T_g + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time-scale shift factors a_T different from those of the terminal (rubberlike to steady-state) dispersion. The a_T values calculated from zero-shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (T_g ? 35°C). The a_T shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The a_T obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some a_Ts which cannot be represented by the usual Doolittle free-volume expression.     

The influence of low-temperature crystallization on the tensile elastic modulus of natural rubber

When natural rubber crystallizes at low temperatures, there is an increase in elastic modulus of up to two orders of magnitude. This phenomenon has been studied at various temperatures in the range 0 to ?55°C for samples held at tensile strains of up to 500%. There is an induction period associated with the nucleation of crystallites, before any increase in modulus is observed. The induction period increases with decreasing strain and passes through a minimum with increasing temperature at ?25°C. The growth rate subsequent to nucleation is successfully described in terms of Avrami-type rate relationships. The Avrami rate coefficient is independent of temperature and follows a simple exponential function of strain. The equilibrium extent of the modulus increase has also been studied by means of experiments of up to three months' duration. The equilibrium modulus increases with decreasing temperature—as predicted by Flory's thermodynamic theory.     

The influence of core-shell structured modifiers on the toughness of poly (vinyl chloride)

The core-shell structured grafted copolymer particles of polybutadiene grafted polymethyl methacrylate (PB-g-PMMA, MB) were prepared by emulsion polymerization. The MB particles were used to modify poly (vinyl chloride) (PVC) by melt blending. The mechanical properties of the PVC blends were investigated. The micro-morphology of the PVC blends was observed by scanning electron microscopy (SEM). The results indicated that the samples with the best impact strength could be obtained when the core-shell weight ratio of PB to PMMA is lower than 93:7, the mechanical properties correlated well with SEM morphologies, the addition of modifier with the ratio core to shell of 93:7 could reduce the domain size of the dispersed phase. Furthermore, the compatibility and properties of the blends were greatly enhanced and improved. The modifier particles could be well dispersed in the PVC matrix.     

The influence of variations in composition on the creep behaviour of doloma

A study has been made of the high temperature creep behaviour of natural doloma and a wide range of synthetic CaOMgO samples. Over the stress and temperature ranges examined, the rate-determining process during creep of these two-phase ceramics is diffusion-controlled generation and movement of dislocations, independent of variations in the magnesia content (from 25 to 75% MgO), crystal sizes (from 1.5 to 11μm) and impurity levels (i.e. SiO₂, Al₂O₃ and Fe₂O₃ contents up to a total o f5.6 wt%). Under these conditions, whilst variations in crystal size have comparatively little effect, the creep strength can be significantly improved by decreasing the CaO/MgO ratio and by minimizing the impurity levels (particularly Fe₂O₃).     

The influence of the temperature on the formation of samarium nitrato complexes

The stability constants of the Sm(NO₃)²⁺ complex were determined at three temperatures, using the solvent extraction method. It was found that:K ₁ ⁰ =63.6 at 17°C, 30.3 at 35°C, 20.1 at 50°C. This corresponds with the formation of a Sm(H₂O)(NO₃)²⁺ complex at 17°C and a Sm(H₂O)₂(NO₃)²⁺ complex at 50°C.

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Der Einfluß der Temperatur auf die Bildung von Samarium Nitrato Komplexen

Zusammenfassung Die Stabilitätskonstanten von Sm(NO₃)²⁺ Komplexen wurden mittels der Lösungsmittelextraktionsmethode bei drei Temperaturen bestimmt. Dabei ergab sichK ₁ ⁰ =63.6 bei 17°C, 30.3 bei 35°C und 20.1 bei 50°C. Das entspricht der Bildung eines Sm(H₂O)(NO₃)²⁺ Komplexes bei 17°C und eines Sm(H₂O)₂(NO₃)²⁺ Komplexes bei 50°C.

     

The influence of lateral substituents on the phase behaviour of nematogenic liquid crystals

The influence of lateral substituents or side chains attached to thermotropic nematogens is considered on the basis of the Flory lattice model. The rigid core of the nematogen is characterized by its axial ratio x and the characteristic temperature T* which measures the strength of the anisotropic dispersion forces operative between the cores. For the side chain an additional parameter zm which is proportional to its hard core volume enters the partition function. Given these quantities and the free volume as obtained from the thermal expansion coefficient, the nematic-isotropic transition temperature T_NI and the temperature dependence of the order parameter may be calculated. The results show that the lowering of T_NI by lateral substituents is caused by (1) their interference with molecules packing and (2) the dilution of the anisotropic dispersion forces between the central nematogenic cores. As an additional consequence of this the order parameter is lowered considerably. A survey of recent experimental data shows that the decrease of T_NI is correlated solely with the hard core volume of the substituent regardless of its chemical structure or polarity, thus corroborating the theoretical predictions.     

The influence of substituents on the orientational behaviour of novel azobenzene side-chain polyesters

A series of new mesogenic azobenzene propanediols with nitro, methoxy, cyano and no substituent have been synthesized and their mesomorphic behaviour mapped. The diols are employed in a melt transesterification with diphenyl tetradecanedioate to produce side-chain polyesters with high molar masses. Diols and polyesters have similar UV-visible spectra reflecting the acceptor character of the substituent. Polarization FTIR spectroscopy has been used to determine the mesogenic order parameter for the polyesters with different substituents at four laser irradiation wavelengths (413, 458, 488 and 515 nm). Diffraction gratings in thin polyester films have been recorded through polarization holography. Cyano and unsubstituted azobenzene polyesters are shown to exhibit the highest diffraction efficiencies on the order of 13%.     

The influence of surfactants on the behaviour of heavy metals in the system n-octanol-water

The partition of heavy metals between octanol and water in the presence of surfactants has been investigated. It is shown that metal ions can be extracted from water into the organic phase, n-octanol, as a model for biological materials in the presence of various anionic surfactants. Anions such as chromate can be extracted with the help of certain cationic surfactants. It was found that the extent of extraction depends on various parameters, such as the concentration of the surfactant, ionic strength and pH. With some anionic surfactants, the K_(ow) of lead or cadmium can reach values of 200 from waters of low ionic strength, decreasing as the ionic strength increases.     

Flow behaviour of titanium dioxide dispersions in the presence of 2-hydroxyethyl cellulose

 The effects of 2-hydroxyethyl cellulose (HEC) of weight-average molecular weight 15,000, 90,000 and 7,20,000 on the rheological properties of TiO₂ dispersions were evaluated. For all three HECs, the maximum yield stress, τ_ymax, (occurred at zero zeta potential), decreased with increasing HEC concentration. Interestingly, the largest reduction in τ_ymax was observed with the HEC with the highest molecular weight. This reduction was attributed to steric interaction arising from adsorbed HEC. Adsorbed high-molecular-weight HEC formed an effectively thicker steric barrier because of its larger size and higher adsorption capacity. Bridging interactions that were expected to be important for the high-molecular-weight HEC were found to be unimportant here. In the flocculated regime, HEC enhanced the shear-thinning characteristics of the TiO₂ dispersions. Received: 8 November 1999 Accepted: 20 December 1999     

The influence of exchangeable cation on thermal behaviour of ground vermiculite

Grinding and contact with water or salt solution increased the specific surface (ssa) but lowered the first dehydration effect (escaping up to 150°C) and increased the second dehydration effect (150 to 500°C). The dehydroxylation was moved to lower temperatures and was only ΔM(500-1100°C)=3.7±0.3 % as compared to 5.5% in the parent vermiculite (V). Except ΔM(20-150°C), the mass losses measured at the remaining T ranges, were consistent in the ground samples, thus the grinding for 2 min caused the homogenization of the crystal structure of vermiculite [ΔM(150-500°C)=7.6±0.7%]. DTA curves after grinding and cation exchange indicate an important exothermal peak at 795-870°C, its temperature depending on exchangeable cation. It indicates the formation of high temperature phases (enstatite, forsterite, spinel). The lowest temperature of the peak (795°C) was observed in V-gr-Li, here lithium silicate was formed. The highest peak temperature (870°C) was found in V-gr-K, where almost only forsterite developed. These exothermal peaks were very weak in unground V with various exchangeable cations. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of temperature on the ion exchange properties of phenolformaldehyde sorbents

The influence of temperature on cation exchange on phenol groups of macroporous materials based on phenolformaldehyde resins (PFRs) was studied for Amberlit XAD 761 with irregularly shaped grains obtained by breaking up polycondensation resin blocks and FFS-1.4/0.7 with spherical granules. An increase in temperature caused some decrease in the sorption capacity of PFRs in the alkaline region, but almost did not influence the selectivity of alkali metal ion exchange. The sorption capacity and selectivity of PFRs did not change after multiple (dozens of cycles) transfer of phenol groups from the hydrogen to salt form and back. Ion exchangers of this type can effectively be used for separating mixtures of alkali metal ions containing cesium and rubidium.