Emulsions dynamic moduli

Physical test properties on some cured rubber stocks prepared from lithium-catalyzed butadiene polymers are listed in Tables V and VI with appropriate controls. The results are only roughly indicative of the potential properties of rubbers made from lithium-catalyzed butadiene polymers because of the limited quantity of polymer available. The tensile data in Table VI indicate that compounded stocks from the lithium polymers are about equal or slightly inferior to the emulsion and sodium polymer controls in regard to these properties however, a hot tensile (lOO C.) on a cured compound from lithium polybutadiene was 325 pounds per square inch compared to 200 to 250 for an emulsion polybutadiene control. The internal friction of cured stocks from the lithium-catalyzed butadiene polymers is similar in magnitude to the emulsion or sodium polymer controls at 50 C. but higher at 100 °C. All lithium polymers, even those with low Mooney viscosities, gave cured compounds with high values of dynamic modulus. 

Paheme emulsion model failed to describe the dynamic modulus of the PP/EPDM blends after radiation, because the viscosity ratio increased significantly and the rubber phase changed from deformed droplets to hard domains after radiation (Cao et al. 2007). Intercoimections among inclusions of the dispersed phase (Shi et al. 2006) and the existence of multiple emulsion (emulsion-in-emulsion) structure exhibiting different relaxation domains in compatibilized systems are other factors contributing to the failure of Palieme s model (Friedrich and Antonov 2007 Pal 2007). 

For effective demulsification of a water-in-oil emulsion, both shear viscosity as well as dynamic tension gradient of the water-oil interface have to be lowered. The interfacial dilational modulus data indicate that the interfacial relaxation process occurs faster with an effective demulsifier. The electron spin resonance with labeled demulsifiers suggests that demulsifiers form clusters in the bulk oil. The unclustering and rearrangement of the demulsifier at the interface may affect the interfacial relaxation process. 

Figure 7.5 Dynamic mechanical spectra showing the variation of storage modulus (.S, 9) and loss modulus (E",0) with temperature for emulsion polymers prepared from styrene (50 wt%) and ethyl acrylate (50 wt%) using different processes (adapted from ref. 65).

It was discussed quite extensively, that interfacial dynamics and rheology are key properties of liquid disperse systems, such as foams and emulsions. The stability of such systems depends for example on the dilational elasticity and viscosity, however, surely not on the elasticity modulus (Borwankar et al. 1992). Here, the interfacial rheology with its frequency dependence comes into play, and data at respective frequencies will possibly correlate with the stability behaviour. 

Figure 35 4. Loss modulus, C, as a function of temperature for a poly(buta-diene) elastomer produced by emulsion polymerization, E>BR, an E>BR grafted with styrene E-Br-g-S, a poly(styrene), PS, and a high-impact poly(styrene) produced by in situ polymerization of styrene in a solution of E-BR in styrene, HIPS. The peaks give the dynamic glass transition temperatures. (After data by H. Willersinn.)...

Pasquini et al. reported that cellulose whiskers with high aspect ratio extracted directly from cassava bagasse were used to prepare NR nanocomposite films by mixing with the NR latex emulsion. The mixtures were cast on Teflon plates and dried overnight to obtain composite films. These NR nanocomposite films were not vulcanized by standard process. They found that a significant increase of the storage tensile modulus was observed upon filler addition by dynamic mechanical analysis. 

During a temperature cycle, emulsion stability can be evaluated using dynamic mechanical analysis (DMA) by means of viscosity measurements. Here, the sample is exposed to a small shear deformation with fixed amplitude and frequency and the complex dynamic shear modulus G = G - - G" is determined. G characterizes the elastic properties and G" the viscous properties of the material. A change in the modulus from cycle to cycle indicates structural changes induced by the applied thermal stresses [12,13]. 

The viscoelastic properties of partially miscible mixtures can be analyzed by applying incompressible emulsion models (Palierne 1990). Various studies (Graebling et al. 1993 Lacroix et al. 1998 Vinckier and Launn 1999) have confirmed the success of this model, providing information on the complex, typically bimodal, terminal relaxation of two immiscible or partially miscible polymers subjected to phase separation and capable of attaining dynamic equilibrium. Assuming that the droplet size is uniform, the complex modulus G (co) can be written as... 

These linear viscoelastic dynamic moduli are functions of frequency. For a suspension or an emulsion material at low frequency, elastic stresses relax and viscous stresses dominate with the result that the loss modulus, G", is higher than the storage modulus, G. For a dilute solution, G is larger than G over the entire frequency range but they approach each other at higher frequencies as shown in Fig. 3. 

Figure 8. Dynamic mechanical spectra (loss modulus) of vinyl acetate (A)-butyl acrylate (B) co polymers prepared by batch ) and semicontinuous (o) emulsion polymerization copolymers with [A]=89%(I), 71%(II), and 49%(III). (Pichot, Llamo, Pham, Ref. 25)...

Rheological studies can provide useful information on the stability and internal microstructure of the multiple emulsions. Some attention was given to this subject in recent years, and the results help clarify certain aspects of stability and release properties of the multiple emulsions (Benichou et al., 2002b). Oscillatory dynamic tests and steady-state analyses, and parameters such as shear or complex modulus (G ), the lag phase between stress and strain (5), the storage modulus (G ), and the loss modulus (G"), provide quantitative characterization of the balance between the viscous and elastic properties of the multiple emulsions. Oscillatory measurements generally indicate that multiple emulsions are predominantly viscous in that the loss modulus... 

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