Mechanical compatibility

The normal course of a kinetic investigation involves postulating likely mechanisms and comparing the observed rate law with those expected for the various mechanisms. Those mechanisms that are incompatible with the observed kinetics can be eliminated as possibilities. Let us consider aromatic nitration by nitric acid in an inert solvent as a typical example. We will restrict the mechanisms being considered to the three shown below. In an actual case, such arbitrary restriction would not be imposed, but instead all mechanisms compatible with existing information would be considered. 

As we have seen, rate expressions for reactions must be determined experimentally. Once this has been done, it is possible to derive a plausible mechanism compatible with the observed rate expression. This, however, is a rather complex process and we will not attempt it here. Instead, we will consider the reverse process, which is much more straightforward. Given a mechanism for a several-step reaction, how can you deduce the rate expression corresponding to that mechanism ... 

The thermal, mechanical, and morphological behaviors of two binary blends, HDPE-E-plastomer (Engage 8200) and iPP-E-plastomer (Engage 8200) have been investigated to compare the compatibility and molecular mechanistic properties of the blends. Both systems are thermodynamically immiscible but mechanically compatible. Thermal studies indicate that both blends exhibit two distinct melting peaks and there is depression of the HDPE melting peak in the blend with high... 

Polymer blends have been categorized as (1) compatible, exhibiting only a single Tg, (2) mechanically compatible, exhibiting the Tg values of each component but with superior mechanical properties, and (3) incompatible, exhibiting the unenhanced properties of phase-separated materials (8). Based on the mechanical properties, it has been suggested that PCL-cellulose acetate butyrate blends are compatible (8). Dynamic mechanical measurements of the Tg of PCL-polylactic acid blends indicate that the compatability may depend on the ratios employed (65). Both of these blends have been used to control the permeability of delivery systems (vide infra). 

It is apparent that the existence of a second-order term in the rate expression does not of itself offer any proof of associative or dissociative activation, for there are two possible alternative mechanisms compatible. These are ... 

Bonfield, W., Grynpas, M.D., Tully, A.E., Bowman, J. and Abram, J. (1981) Hydroxyapatite reinforced polyethylene - a mechanically compatible implant. Biomaterials, 2, 185-186. 

Blends of ethylene-vinyl acetate (EVA) copolymer with metallocene-catalysed elastomeric ethylene-alpha-olefin copolymer were investigated and were found to be immiscible in the melt and solid state but mechanically compatible. The morphology (SEM), thermal (DSC), rheological (viscosity), mechanical (including tensile, shear thinning and elastic behaviour) and optical properties of EVA-rich and ethylene-alpha-olefin copolymer-rich blends were studied and the results are discussed in terms of processibility in film applications. 24 refs. 

Composites provide an atPactive alternative to the various metal-, polymer- and ceramic-based biomaterials, which all have some mismatch with natural bone properties. A comparison of modulus and fracture toughness values for natural bone provide a basis for the approximate mechanical compatibility required for arUficial bone in an exact structural replacement, or to stabilize a bone-implant interface. A precise matching requires a comparison of all the elastic stiffness coefficients (see the generalized Hooke s Law in Section 5.4.3.1). From Table 5.15 it can be seen that a possible approach to the development of a mechanically compatible artificial bone material... 

Problem 6.32 (a) What does each of the following observations tell you about the mechanism of the addition of Brj to an alkene (i) In the presence of a Cl salt, in addition to the vic-dibromide, some vic-bromochloroalkane is isolated but no dichloride is obtained, (ii) With cis-2-butene only rac-2.3-dibromobutane is formed, (iii) With rra/u-2-butene only m o-2,3-dibromobutane is produced, (h) Give a mechanism compatible with these observations. ... 

For each kinetic scheme in Scheme 9.4, the rate law obtained by applying the Bodenstein approximation to the intermediate (I) is presented and, for this discussion, we consider that the reactant R is the component whose concentration can be easily monitored. The reactions are all expected to be first order in [R], but the first-order rate constants show complex dependences on [X] and, in two cases, also on [Y]. All the rate laws contain sums of terms in the denominator, and the compositions of the transition structures for formation and destruction of the intermediate are signalled by the form of the rate law when each term of the denominator is separately considered. This pattern is general and can be usefully applied in devising mechanisms compatible with experimentally determined rate laws even for much more complex situations. 

Either equation (143) or equations (149) to (150) (with (150) as the rate-limiting step) represent reaction mechanisms compatible with the observation of general acid catalysis (Bell, 1941b). 

On the other hand, some mechanically compatible blends as well as some dispersed two-phase systems have made respectable inroads into the commercial scene. Many of these are blends of low-impact resins with high-impact elastomeric polymers examples are polystyrene/rubber, poly (styrene-co-acrylonitrile) /rubber, poly (methyl methacrylate) /rubber, poly (ethylene propylene)/propylene rubber, and bis-A polycarbonate/ ABS as well as blends of polyvinyl chloride with ABS or PMMA or chlorinated polyethylene. 

The composition dependence of the flexural strength of the aMS/AN-copolymer blend with polyarylether K appears in Figure 13. As the composition of the copolymer increases, the strength first increases, reaches a maximum, and then decreases. It actually exhibits a minimum at about 80% < MS/AN. This behavior can only substantiate earlier suggestions regarding the possible immiscibility of these systems. All of the other mechanical properties indicate that mixtures with polyarylether K may not be miscible but are mechanically compatible. Finally, it is interesting to note that at least one of the pendant chemical groups present on K exists on either of the a-methyl styrene interpolymers. It... 

A type of mechanism compatible with the form of the hypothetical rate equation is the cycle... 

Evidence in favour of direct initiation involving the attack of an undissociated Lewis acid ito an olefin is essentially confined to systems containing titanium tetrachloride. This section is devoted to an analysis and discussion of that evidence and to the proposal of an overall mechanism compatible with it. 

Although it is impossible to demonstrate by kinetics alone, the only mechanism compatible with the experimental facts is one involving an oxythallation adduct which decomposes to give the observed products. 

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