Kovacs theory

Theories assuming on the formation of a free volume and mobility of the molecules within the holes (Williams, Landel, Ferry, Tobolsky, Kovacs and others)2. ... 

Important theoretical and experimental considerations of the use of macromolecular theories for the description of coal network structures have been recently analyzed (1). Relevant equations describing the equilibrium swelling behavior of networks using theories of modified Gaussian distribution of macromolecular chains have been developed by Kovac (2 ) and by Peppas and Lucht (3) and applied to various coal systems in an effort to model the relatively compact coal network structures (1 4). As reported before (1), Gaussian-chain macromolecular models usually employed in the description of polymer networks (such as the Flory... 

Ptitsyn n indique aucune valeur numerique pour ces differents para-metres, aussi l identification ne peut etre poussee plus loin. Notons que ces expressions contiennent six paramktres, lies entre eux par trois relations que nous n avons pas explicitees ici, de sorte que le comporte-ment de ce systkme ne depend, en demtere analyse, que de trois para-metres ind pendants, comme c est le cas des theories precedentes. Toute-fois, ces parantetres ont des significations moins directes, que ceux qui entrent dans les theories de Tool ou de Kovacs. 

Kovacs AJ, Aklonis JJ, Hutchinson JM, Ramos AA (1979) Isobaric volume and enthalpy recovery of glasses II. A transparent multiparameter theory. J Polym Sci Polym Phys Ed 17 1097-1162... 

Several researchers tried to replace the single-shear plane model by a shear zone model. Lee and Shaffer (1951) provided a slip-line solution by applying the theory of plasticity. In the slip-line model, the metal is assumed to flow along the line of maximum shear lines. The slip-line field solution cannot be applied easily to three-dimensional as well as strain-hardening cases. Sidjanin and Kovac (1997) applied the concept of fracture mechanics in chip formation process. Atkins (2003) demonstrated that the work for creation of new surfaces in metal cutting is significant. He also points out that Shaw (1954) has shown this work to be insignificant. However, when this work is included based on the modem ductile fracture mechanics, even the Merchant analysis provides reasonable results. 

We want to pursue the subject by starting with a brief review for the present purposes of the essentials of lattice-hole theory, then follow with a consideration of free-volume mobility connections, and continue with some comparisons of experiment versus theory. Finally, we propose and sketch modifications of the theory. These may open the way to generalizations and more insightful relations to empirical formulations, such as the KAHR model [Kovacs et al., 1977, 1979] for volume relaxation. 

FIGURE 4.6 Same experiment as in Figure 4.5 with the theory adjusted for retarded motion in the glass at low temperatures. (Adapted from Robertson et al. 1988 data from Kovacs [1963].)... 

Kovacs, A. J., Hutchinson, J. M., Aklonis, J. J., and Ramos, A. R., Isobaric volume and enthalpy recovery of glasses 2. Transparent multi-parameter theory, J. Polym. Sci. Polym. Phys. Ed., 17,1097-1162(1979). 

Robertson et al. [1984] developed a stochastic model for predicting the kinetics of physical aging of polymer glasses. The equilibrium volume at a given temperature, the hole fraction, and the fluctuations in free volume were derived from the S-S cell-hole theory. The rate of volume changes was assumed to be related to the local free volume content thus, it varied from one region to the next according to a probability function. The model predictions compared favorably with the results from Kovacs laboratory. Its evolution and recent advances are discussed by Simha and Robertson in Chapter 4. 

Hermann, T., Kovacs, Z., and Varady, T. Special applications in Surface Fitting, Geometric Modeling Theory and Practice, W. Strasser, R. Klein, and R. Rau (eds). Springer, Berlin, 1997. 

Also, in further developments of this kind of treatment, specific structureforming and structure-breaking effects of the organic adsorbate must be considered in relation to the orientation of solvent molecules, e.g., as indi-cated by entropies and heats of adsorption of pyridine at Hg electrodes. Analysis of various types of isotherms which represent organic molecule adsorption has been made by Kovac and Bockris and by Wroblowa and Mueller in relation to the theory presented above. 

The kinetics of this behaviour have been described by various theories. In particular, Kovacs et al. (2) have proposed a multiparameter model which describes most of the experimentally observed effects. It is based on the free volume concept and assumes that the overall free volume influences the contribution of the structural parameters on the recovery behaviour. 

The hole theory of liquids — based on the conception that within a liquid there are statistically distributed vacancies or holes, whose number and volumia decrease with decreasing temperature has been applied by Fox and Flory (127), Hirai and Eyring 128), Kanig (121), Kovacs (129), Someynsky and Simha (130), and others, to explain various aspects of the glass transition in polymers. In its simplest form the theory ddines the molar V and occupied volume F ( = N y ) as they are related to the number of holes (N) in the following way ... 

In real systems the flexibility of protein structures adds another dimension to the problem of surface recognition. Inst as with the interactions between proteins and small molecnles discnssed above, the induced-fit hypothesis is an invalnable concept for understanding interactions between protein snrfaces. The balance between induced fit and conformational selection remains a topic of considerable interest in biology and is potentially of considerable importance in the development of the next generation of pharmaceutical agents. Kovacs et al. have pointed out that game theory... 

Kovacs et al. concluded that the basic kinetic theories of extended chain crystal growth require some modifications, although they appear to be satisfactory for fast growth at high supercooling. 

FIG. 10-20. Logarithmic plot of against 7T /2 for narrow-distribution polystyrene with = 180,000. Different symbols refer to five different temperatures from 142° to 174.5°C. Curve drawn from Graessley theory. (Penwell, Graessley, and Kovacs. ) Reproduced, by permission, from the Journal of Polymer Science. 

For much higher pressure changes, a marked departure from linearity may be expected because of the diminution of free volume under pressure. The effects of such volume reduction on shear relaxation times (and dielectric and nuclear magnetic relaxation times) have been described in Sections D and E of Chapter 11 the bulk relaxation and retardation times are presumably similarly affected. The origin of this kind of nonlinearity can be understood qualitatively on the basis of a theory developed by Kovacs for a single retardation time. 

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