Rheology Rubbery

Crosslinking the PSA will increase the solvent resistance of the material and it will also have a significant effect on the rubbery plateau modulus of the polymer. Fig. 8 shows the effect of increasing amounts of a multifunctional az.iridine crosslinker, such as CX-100 (available from Avecia, Blackley, Manchester, UK) on the rheology of an acrylic polymer containing 10% acrylic acid. The amounts of crosslinker are based by weight on the dry weight of the PSA polymer. 

The Mittag-Leffler function, or combinations thereof, has been obtained from fractional rheological models, and it convincingly describes the behavior of a number of rubbery and nonrubbery polymeric substances [79, 85]. The numerical behavior of the Mittag-Leffler function is equivalent to asymptotic power-law patterns that are often used to fit experimental data, see the comparative discussion of data from early events in peptide folding in Ref. 86, where the asymptotic power-law was confronted with the stretched exponential fit function. 

Morales-Diaz and Kokini (1998) have used rheological models to describe the flow characteristics in glassy and rubbery state of 7S and 1 IS soy globulins. Glass transition temperatures of 7S fraction have shown values between 114°C and -67°C for moisture contents of 0-35%, and those of IIS fraction is determined between 160 C and -17°C at a moisture range of 0 0%, determined by differential scanning calorimetry (DSC) and mechanical spectroscopy (RMS) (Figure 7.11). 

Levine, H. and Slade, L. (1990). Influences of the glassy and rubbery states on the thermal, mechanical, and structural properties of doughs and baked produets. In H. Faridi and J.M. Faubion (Eds.), Dough Rheology and Baked Product Texture. Van Nostrand Reinhold, New York, pp. 157-330. 

The effects of molecular entanglements on the rheology of polymers above the glass transition temperature are large and have been recognized for a long One of the most prominent effects is the appearance of a rubbery... 

Concentrated aqueous solutions are used to prepare pastilles since on drying they form solid rubbery or glasslike masses depending upon the concentration used. Foreign policy changes and politically unstable conditions in Sudan, which is the principal supplier of acacia, has created a need to find a suitable replacement. Poloxamer 188 (12-15% w/w) can be used to make an oil/water emulsion with similar rheological characteristics to acacia. Other natural by-products of foods can also be used. Acacia is also used in the food industry as an emulsifier, stabilizer, and thickener. A specification for acacia is contained in the Food Chemicals Codex (FCC). 

In the glassy state, the polymer backbone is considered to be almost completely immobile, causing the material to be brittle. Between Tg and Tm the system is not to be considered as a highly viscous liquid but as an elastic material the transition near Tg is called a glass-rubber transition. The crystallites act as cross-links between flexible stretches of the polymer chains, causing the material to have a rubbery consistency, with an appreciable elastic modulus. Above Tm a viscous liquid is formed. Figure 16.3b illustrates the rheological relations. 

At service temperature and frequency, the solid-state rheological properties of a TPE and a conventional rubber are similar, although at large strains the former will generally have some permanent set, while the latter will not. Only by changing the temperature and/or the frequency do substantial differences between the two emerge. If the temperature is lowered or the frequency raised, the qualitative response of the two materials is still very similar. Both materials become more resistant to stress until a temperature is reached that corresponds to the Eg of the soft segment in a TPE or the rubbery matrix in a conventional... 

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