Free volume increase mechanism

For diffusion of liquid through rubbery polymer composites, Fickian and non-Fickian diffusion theories are frequently used to describe the mechanism of transport, but for gas or vapour, other models have been developed to fit experimental data of diffusion profiles. The models of gas transport include Maxwell s model," free volume increase mechanism," solubility increase mechanism," nanogap hypothesis," Nielsen model, " " Bharadwaj model, ° Cussler model " " and Gusev and Lusti model, " etc. 

Free volume increase mechanism was proposed by Cohen and Turnbull. The concept of this model deals with the effect of polymer free volume on penetrant diffusion coefficient. The model equation express for penetrant diffusion coefficient, D as following given ... 

Fig. 2 The crankshaft mechanism is a simple way of considering the motions of a polymer chain permitted by increases in free volume. The molecule is visualized as a series of balls and rods, and these move as the free volume increases. (View this art in color at www.dekker.com.)...

Within the context of this model, c and f are adjustable constants that vary from polymer to polymer. The ratio f /c is a crude measure of the average interchain separation. As polymer chain stif ess increases and free volume decreases, c should increase (20), and as fractional free volume increases, /c should increase. This theory is based on four hypotheses (1) the solution-di sion mechanism (equation 2) governs gas transport (2) diffusion obeys an Arrhenius expression (equation 8) (3) the linear free energy relation (equation 12) is valid and (4) the effect of penetrant size on activation energy is given by equation 13. 

The Tg or glass transition temperature is an important and measurable parameter giving information about the retrogradation behavior. Besides that, the Tg is also of great importance for the mechanical properties of the material. Beneath the glass transition temperature the material exists in an amorphous, frozen liquid structure with stiff and brittle behavior. Below its Tg the intermolecular bonds are not broken, due to the small amounts of room left for Brownian movement, as is stated in Eyring s free volume theory [6]. The specific volume increases relatively slowly with increasing temperature. When the material is heated up, at temperatures close to Tg the cohesive forces decrease drastically, the polymer expands, and the free volume increases to such an extent that there is... 

Plasticization has been explained by a variety of theories in an attempt to explain how the plasticizer reduces the rigidity of the final part. All theories rely on the premise that the plasticizer reduces the strength of the intermolecular forces between the polymer chains. The theories fall into two broad categories interference mechanisms and expansion mechanisms. The interference mechanisms state that plasticizer molecules interact only weakly with the polymer chains after separating the chains from one another, thereby reducing the overall cohesion of the material. The expansion mechanisms state that the reduced rigidity arises from an increase in the free volume of the system as the system expands to incorporate bulky,... 

As an example of composite core/shell submicron particles, we made colloidal spheres with a polystyrene core and a silica shell. The polar vapors preferentially affect the silica shell of the composite nanospheres by sorbing into the mesoscale pores of the shell surface. This vapor sorption follows two mechanisms physical adsorption and capillary condensation of condensable vapors17. Similar vapor adsorption mechanisms have been observed in porous silicon20 and colloidal crystal films fabricated from silica submicron particles32, however, with lack of selectivity in vapor response. The nonpolar vapors preferentially affect the properties of the polystyrene core. Sorption of vapors of good solvents for a glassy polymer leads to the increase in polymer free volume and polymer plasticization32. 

Various photochemical reaction mechanisms lend themselves for variation of the above parameters, and the more important ones are summarized above. In general, one can expect for the volume necessary for the reaction to occur that it decreases in the sense Excimer > TICT > Butterfly > ESIPT mechanism. Therefore, for a given size distribution of microscopic free volume voids, the fraction of the total free volume usable for the reaction and thus amenable to probing increases in the same sense with the ESIPT mechanism being the outstanding extreme, because this reaction cannot even be stopped in a rigid matrix at very low temperature. 39 ... 

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