Cross-aggregation

IV) The final stage involves aggregation, cross-linking, and possibly phase separation, caused by the high concentration of protein at the surface. These processes strongly influence the developing mechanical properties of the adsorbed layer. 

A diagonal stay or two will then be necessary and the heating surface will be reduced. The aggregate cross-sectional tube area should be one-ninth to one-tenth the grate area for hard coal and one ixth to one-seventh for soft coal. With horizontal and vertical pitches equal to p in., the number of tubes per square foot of tubed head area is 144 -i- p. The distance from the center line of the boiler to the center of the top row of tubes may vary from to lOK in. for shell diameters ranging from 48 to 72 in. The following pitches are typical ... 

Within the cell wall, fibrils may aggregate mtoflbril aggregates. The fibrils typically aggregate with main axes parallel or and-parallel. The shape of fibril aggregate cross-sections may be of my form with widths in the range of several tenths of nanometers (18-21). 

Polymers in Solution. Polyacrylamide is soluble in water at all concentrations, temperatures, and pH values. An extrapolated theta temperature in water is approximately —40° C (17). Insoluble gel fractions are sometimes obtained owing to cross-link formation between chains or to the formation of imide groups along the polymer chains (18). In very dilute solution, polyacrylamide exists as unassociated coils which can have an eUipsoidal or beanlike stmcture (19). Large aggregates of polymer chains have been observed in hydrolyzed polyacrylamides (20) and in copolymers containing a small amount of hydrophobic groups (21). 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

The mode of action of plasticizers can be explained using the Gel theory [35 ]. According to this theory, the deformation resistance of amorphous polymers can be ascribed to the cross-links between active centres which are continuously formed and destroyed. The cross-links are constituted by micro-aggregates or crystallites of small size. When a plasticizer is added, its molecules also participate in the breaking down and re-forming of these cross-links. As a consequence, a proportion of the active centres of the polymer are solvated and do not become available for polymer-to-polymer links, the polymer structure being correspondingly loosened. 

FIGURE 15.5 The cascade of activation steps leading to blood clotting. The intrinsic and extrinsic pathways converge at Factor X, and the final common pathway involves the activation of thrombin and its conversion of fibrinogen into fibrin, which aggregates into ordered filamentous arrays that become cross-linked to form the clot. 

Aggregation of floe for formation of cross-linking Covalent binding... 

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