Variations in strength

The polysulfide impression materials can be formulated to have a wide range of physical and chemical characteristics by modifying the base (polysulfide portion), and/or the initiator system. Further changes may be obtained by varying the proportion of the base to the catalyst in the final mix. Characteristics varied by these mechanisms include viscosity control from thin fluid mixes to heavy thixotropic mixes, setting-time control, and control of the set-mbber hardness from a Shore A Durometer scale of 20 to 60. Variations in strength, toughness, and elasticity can also be achieved. 

Manual sampling techniques can introduce error by virtue of variations in strength and size of the human hand, from analyst to analyst. As a result, the pulling velocity through the filter may vary considerably. Too rapid a movement of liquid through the filter can compromise the filtration process itself. 

The first clear definition of acidity can be attributed to Arrhenius, who between 1880 and 1890 elaborated the theory of ionic dissociation in water to explain the variation in strength of different acids.3 Based on electrolytic experiments such as conductance measurements, he defined acids as substances that dissociate in water and yield the hydrogen ion whereas bases dissociate to yield hydroxide ions. In 1923, J. N. Brpnsted generalized this concept to other solvents.4 He defined an acid as a species that can donate a proton and defined a base as a species that can accept it. This... 

Tissues are composites of macromolecules, water, ions, and minerals, and therefore their mechanical properties fall somewhere between those of random coil polymers and those of ceramics. Table 6.1 lists the static physical properties of cells, soft and hard tissues, metals, polymers, ceramics, and composite materials. The properties listed in Table 6.1 for biological materials are wide ranging and suggest that differences in the structure of the constituent macromolecules, which are primarily proteins, found in tissues give rise to the large variations in strength (how much stress is required to break a tissue) and modulus (how much stress is required to stretch a tissue). Because most proteins are composed of random chain structures, a... 

Figure 7.5. Variation in strength of CNT-PMMA composites with different CNT loadings. Two results from same author represents composites prepared by improved techniques. (Horizontal lines shown in the legend represent secondary Y-axis).

Summary Introduction of a chiral ligand to a series of pentacoordinate silicon complexes led to the assignment of two intramolecular rate processes Si-N cleavage and pseudorotation. Linear correlations between and Si chemical shifts of the complexes, as well as between the latter and the N-methyl exchange barriers were attributed to variation in strength of N—>Si coordination. 

It is now well recognized that the term hydrogen bonding covers a wide range of interactions with a corresponding variation in strengths of interaction. Table 9.4 lists representative examples. 

In addition to change of concentration by mixing processes, the relative concentrations of lanthanides may be changed by oxidation-reduction reactions followed by diflFerential mineral uptake (as for manganese nodules (4, JO), or phosphorites (4)), or by diflFerential solubility or ion-exchange processes aflFected by variations in ionic radius. The lanthanides are the group of elements for which such processes are known to be least effective only two, cerium and europium, are expected to exhibit stable ions other than trivalent, and as summarized by Moeller et aL (13), lanthanide complexes are typically weak, and show within the series, only modest variations in strength. 

Biesiadecki, B.J., Brand, P.H., Metting, P.J., Koch, L.G., and Britton, S.L. Phenotypic variation in strength among eleven inbred strains of rats. Proc. Soc. Exp. Biol. Med. 219 126-131,1998. 

Moreover, in the vicinity of the LCST (or UCST) the insoluble core is soft enough to undergo structural transformations in response to variations in strength of ionic interactions in the corona. In [135], micellization of poly(iV-isopropylacrylamide)- /ocfc-poly(DMAEMA) in aqueous solution was studied as a function of pH and temperature. Predicted by the theory [19], a pH-induced, jump-wise transition between spherical micelles with distinctively different aggregation numbers was observed by combination of DLS and AFM at temperatures T > LCST for the core-forming iV-isopropylacrylamide block. To the best of our knowledge, to date this study is the only experimental evidence of abrupt structural transformations in spherical micelles with annealing PE corona. 

This has been demonstrated for a commercial glass ceramic [38], and for alumina [39]. It has been suggested that the microstructure and the defect populations themselves may vary throughout the volume of a component, causing variations in strength unpredictable by Weibull analysis [40,41]. An attempt to verify the minimal specimen size required for strength determination was carried out lately [42]. 

We now turn our attention from considerations of stiffness to stress-strain and tensile behavior. Variations in strength and modulus as a function of direction (Broutman and Krock, 1967, Chapter 12) have been treated by several investigators for example, Tsai (1965) and Brody and Ward (1971). Even though the polymer matrix typically has such a low modulus that it does not contribute much overall to the composite modulus, the matrix can by no means be neglected, because failure often involves catastrophic crack growth in the matrix (see below). Stress-strain curves for unidirectional composites are typically fairly linear up to failure for loading in the direction of the fibers (Broutman and Krock, 1967, p. 370), but quite nonlinear transverse to the fiber direction. The stress to rupture is also very low in the latter case, presumably due to a high concentration of stress in the matrix. 

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