Other Theories

A number of workers have suggested that emulsion polymerization may not occur homogeneously throughout a polymer particle but either at the particle surface (Sheinker and Medvedev, 1954) or within an outer monomer-rich shell surrounding an inner polymer-rich core (Grancio and Williams, 1970). The latter has been referred to as the shell or core-shell model and has been proposed to explain the apparent anomaly between the observed constant rate behavior up to about 60 percent conversion, which according to Eq. (6.202) requires [M] in the M/P particle to be constant, and the considerable experimental evidence indicating that emulsified monomer droplets (which serve as monomer reservoirs) disappear at 25 to 30 percent conversion and the monomer concentration drops thereafter. 

According to the core-shell model, the growing particle has a heterogeneous rather than a homogeneous composition, and it consists of an expanding polymer-rich (monomer-starved) core surrounded by a monomer-rich (polymer-starved) outer spherical shell (Grancio and Williams, 1970). It is the outer shell that serves as the major locus of polymerization by the Smith-Ewart (on-off) mechanism, while practically no polymerization occurs in the core as it is starved of monomer. Reaction within an outer shell or at the particle surface would be most likely to be operative for those polymerizations in which the polymer is insoluble in its own monomer or under conditions where the polymerization is diffusion-controlled such that a propagating radical cannot diffuse into the center of the particle. 

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The existence of the polyad number as a bottleneck to energy flow on short time scales is potentially important for efforts to control molecnlar reactivity rising advanced laser techniqnes, discussed below in section Al.2.20. Efforts at control seek to intervene in the molecnlar dynamics to prevent the effects of widespread vibrational energy flow, the presence of which is one of the key assumptions of Rice-Ramsperger-Kassel-Marcns (RRKM) and other theories of reaction dynamics [6]. 

Even better agreement between theory and experiment has been obtained in other theories by abandoning the notion of affine deformation and recognizing that shorter subchains experience a greater strain than do longer subchains for a given stress. We shall not pursue this development any further, however, and shall turn next to a consideration of other types of deformation. 

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

It should be pointed out that the view of the glass transition temperature described above is not universally accepted. In essence the concept that at the glass transition temperature the polymers have a certain molecular orientation time is an iso-elastic approach while other theories are based on iso-viscous. 

Other theories proposed dissipation of energy through crack interaction localised heating causing the material to be raised to above the glass transition temperature in the layers of resin between the rubber droplets and a proposal that extension causes dilation so that the free volume is increased and the glass transition temperature drops to below the temperature of the polyblend. 

Geometrically based theories, especially that of Labik et al., are remarkably accurate and are no more difficult to implement computationally than other theories that involve the correlations of order higher than h = 2. Their only disadvantage is that they are limited to the case of hard spheres. If a generalization to other systems could be found, this would be an important advance. 

Two other theories as to the mechanism of the benzidine rearrangement have been advocated at various times. The first is the rc-complex mechanism first put forward and subsequently argued by Dewar (see ref. 1 pp 333-343). The theory is based on the heterolysis of the mono-protonated hydrazo compound to form a n-complex, i.e. the formation of a delocalised covalent it bond between the two rings which are held parallel to each other. The rings are free to rotate and product formation is thought of as occurring by formation of a localised a-bond between appropriate centres. Originally the mechanism was proposed for the one-proton catalysis but was later modified as in (18) to include two-protons, viz. 

In addition to excess sodium intake, abnormal renal sodium retention may be the primary event in the development of hypertension, and it includes abnormalities in the pressure-natriuresis mechanism. In hypertensive individuals, this theory proposes a shift in the control mechanism preventing the normalization of blood pressure. The mechanisms behind the resetting of the pressure-natriuresis curve may include afferent arteriolar vasoconstriction, decreased glomerular ultrafiltration, or an increase in tubular sodium reabsorption.4 Other theories supporting abnormal renal sodium retention suggest a congenital reduction in the number of nephrons, enhanced renin secretion from nephrons that are ischemic, or an acquired compensatory mechanism for renal sodium retention.9... 

The cause of pruritus is unknown, although several mechanisms have been proposed. Vitamin A is known to accumulate in the skin and serum of patients with CKD, but a definite correlation with pruritus has not been established. Histamine may also play a role in the development of pruritus, which may be linked to mast cell proliferation in patients receiving hemodialysis. Hyperparathyroidism has also been suggested as a contributor to pruritus, despite the fact that serum PTH levels do not correlate with itching. Accumulation of divalent ions, specifically magnesium and aluminum, may also play a role in pruritus in patients with CKD. Other theories that have been proposed include inadequate dialysis, dry skin, peripheral neuropathy, and opiate accumulation.43... 

In one respect the valence shell electron-pair repulsion theory is no better (and no worse) than other theories of molecular structure. Predictions can only be made when the constitution is known, i.e. when it is already known which and how many atoms are joined... 

The Kuhn model is presented in detail (and in parts mathematically justified) in the articles referred to above. It is unclear why Kuhn s ideas have not always received due attention in the literature in comparison with other theories. Without going into detail, some features will be mentioned briefly. 

As regards the origin of the double layer the Helmholtz theory gives no information, but several other theories have been formulated. Modem electrical theories suppose every conducting substance to contain large numbers of negatively charged ions, called electrons, which are exactly alike no matter in what substance they are found. When two s.T. 5... 

The two-film theory considering molecular diffusion through stagnant liquid and gas films is the traditional way of understanding mass transfer across the air-water boundary. As briefly described, other theories exist. However, the two-film theory gives an understanding of fundamental phenomena that may lead to simple empirical expressions for use in practice. 

Please note that one theory-in-use might differ from other theories in use of the same transformation, because different individuals will hold different images of the prevailing theory-in-use, so they will judge specific situations in a distinct matter. Having set a definition of a deviation, the concept of a re-occurring deviation will be further explained in the following sub-Section. 

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