Microstructural Change

Stress-strain curves for PVC at 23 °C, for samples annealed at 60 °C and quenched from 90 °C. 

It is sometimes difficult to isolate the effect of cooling rate on mechanical properties from effects of residual stresses (Section 6.3.3) or molecular orientation (next section). 

If there is steady shear flow at a strain rate S until time t thereafter, Eq. (6.1) can be solved to give 

The quantity rj/G represents the melt relaxation time to, the time in which shear stress decays to He (34%) of its original value. The relaxation time depends on the molecular mass and the melt temperature, factors which 

In the blow moulding and thermoforming processes, solidification commences only when the melt contacts the cold mould wall, and flow stops. The time available for melt stress relaxation depend on the thickness of the product, and how close the melt is to the cold mould. There is likely to be high orientation in the thin side walls of a thermoformed disposable cup, made from a highly viscous glassy polymer that has solidified rapidly (hence the heat reversion shown in Fig. 1.16). Hence, if an empty cup is squeezed flat, vertical cracks tend to occur in the side walls, in the direction of orientation. 

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Grain growth and other microstructural changes. When a deformed metal is heated, it will recrysiallise, that is to say, a new population of crystal grains will... 

Passivation at the metal/active mass interface, or of the active mass itself can also lead to failure. Detrimental changes in the morphology of the active mass and microstructural changes in the grid material can also occur. 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

Combined analyses by XRD and TEM showed that the aurichalcite mineral was sufficiently similar to the synthetic aurichalcite to be used as a model compound, to study the microstructural changes occurring during the catalyst preparation procedures. Calcination of the mineral and synthetic samples led to highly preferred orientations of ZnO. ZnO electron diffraction patterns with [lOlO] and [3031] zone... 

Xie J, Wood DL, More KL, Atanassov P, Borup RL. 2005a. Microstructural changes of membrane electrode assemblies during PEFC durability testing at high humidity conditions. J Electrochem Soc 152 A1011-A1020. 

Denaturation and aggregation of whey proteins are affected by the pH of extrusion. When extruding WPI, alkaline conditions increase denaturation and solubility, decrease pasting properties, and produce more pronounced microstructural changes (Onwulata et ah, 2006). Denaturation in the extruder causes whey proteins to form small primary aggregates that combine to form large clusters. The clusters are then aligned by shear to form fibrous structures. 

Percolation in microemulsions and concomitant microstructural changes are the focal points of this review. A complete understanding of percolation phenomena in reverse microemulsions will require an understanding of droplet interactions and the associated thermodynamics of droplet fusion, fission, aggregation to form clusters of varying fractal... 

In addition to the chemical interactions described above, microstructural changes in the electrode can lead to performance degradation after long operation time. For example sintering of the porous structure can degrade electrode performance. In the case of LSM cathodes, the sintering ability is found to be related to the strontium dopant level and stoichiometric composition of (La, Sr)xMn03. In addition, LSM with A-site deficient compositions (x < 1) sinters more readily than their B-site deficient counterparts (x > 1) [198, 199],... 

Barat, J.M., Albors, A., Chiralt, A., and Fito, P. 1999. Equilibration of apple tissue in osmotic dehydration. Microstructural changes. Drying Technol. 17, 1375-1386. 

It is well known that the transfer of nonpolar molecules from nonpolar to polar surroundings results in a decrease in the partial molar volume of the solute. The dimerization studies also show that there is a similar volume decrease when two monomers form a dimer. This volume decrease is of the order of 20 cm3 mol-1. It is difficult to understand how there can be first a volume decrease when the nonpolar molecules are transferred from the nonpolar to the polar environment and then a further volume decrease when two molecules come together and partly reverse the first transfer. It is a little dangerous to speak of the partial reversal of a process we know so little about. It is believed that the hydrophobic hydration is a cooperative phenomenon, in which the exact microstructure of water is very important for the occupied volume. How this microstructure changes when two molecules associate in a hydrophobic interaction is not par-... 

Thermal aetivation by desorption/absorption cycling at elevated temperatures of premilled MgH reverts some of the microstructural changes induced by milling. ABCR powders initially milled, as shown in Fig. 2.13, were subjeeted to the following eyeling heating to 325°C for 15 min under 3.4 MPa to prevent desorp-... 

Desired organoleptic properties, particularly textural ones, are a direct consequence of these microstructural changes. A chip must be firm and snap easily when deformed, emitting a crunchy sound (Krokida et al., 2001a), whereas in thick products the better the contrast between a rich and soft inner structure and a crispy outside, the better the product (Moreira, 2006). Firmness is often related to starch swelling and gelatinization, as well as to the stability of pectic substances of the cell wall and middle lamellae. 

This complex unit operation involves significant microstructural changes in fact, most of the desirable characteristics of fried foods are derived from the formation of a composite structure a dry, porous, crispy and oily outer layer or crust, and a moist cooked interior or core, whose microstructures form during the process (Bouchon et al., 2001). The high temperatures (around 160 and 180°C) cause water evaporation, which is transferred from the food towards the surrounding oil, whereas oil is absorbed by the food replacing part of the released water. This process results in products with a unique flavor-texture combination (Mellema, 2003). 

Walker, C. T., Kameyama, T., Kitajima, S. Kinoshita, M. 1992. Concerning the microstructure changes that occur at the surface of U02 pellets on irradiation to high bumup. Journal of Nuclear Materials, 188, 73-79. 

Microstructure of Polybutadienes. Microstructure strongly influences the viscosity of the CTPB prepolymer. The viscosity of CTPB increases with increased vinyl content, but for CTPB prepolymers of the required molecular weight, an upper limit of 35% vinyl groups is satisfactory from the standpoint of propellant processing characteristics. It has also been found that the microstructure changes markedly with the synthesis process. Lithium-initiated polymerization yields prepolymers with slightly higher vinyl content than those produced by free radical initiation. 

The unique feature about anionic polymerization of diene to produce homopolymer was that the microstructure of the homopolymer could be altered and changed at will to produce unique physical and chemical properties. These microstructural changes can be introduced before, after or during the polymerization. For example, chelating diamines, such as tetramethyl ethylene and diamine (TMEDA) (18), with the alkyl-lithium catalyst have been used to produce polymer with 80 1,2 addition products, while the use of dipiperidine ethane (DPE),with same catalyst has produced polybutadiene with 100 1,2 addition product. 

Shen, D. Y., Hsu, S. L. Vibrational Spectroscopic Characterization of Rigid Rod Polymers. III. Microstructural Changes in Stressed Polymers, Polymer... 

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