Threading enthalpy

Threading driven by enthalpy is different from the statistical method in that an in-termolecular attractive force exists between the linear species and the cyclic to... 

The relevant thermodynamic and kinetic parameters are displayed diagrammatically in Figure 4. From the point of view of kinetics, AC is significant only in the case of slippage. From the point of view of thermodynamics, if AH° 0, the threading is controlled by entropy this is the case of the so-called statistical threading. If AH° < 0, the threading is controlled by enthalpy the threaded species is thermodynamically much more stable than the separated components. In this latter case, the equilibrium of scheme 1 can be rewritten as shown in scheme 2 ... 

As it runs like a thread through the present treatment, various cancellations and competitions (enthalpy/entropy, repulsion/attraction, etc.) appear to be conspiring to make molecular behavior in complex fluids seemingly simple. 

A main chain polyrotaxane, a mechanically interlocked structure that can be considered as a string of pearls, in which the strand is the polymer backbone and the pearls are the cychc species threaded onto the strand. The backbone polymer can be a polyester, polyamide, poly(ethylene oxide), or virtually any linear polymer. The cyclic species are typically crown ethers, cyclodextrins (CDs), cucurbimrils, and calix-arenes. Polyrotaxanes can be made by two different methods statistical threading or via the template approach (enthalpi-cally driven). The statistical approach utilizes le Chatelier s principle with an excess macrocycle during the polymerization. The template approach is driven by attractive interactions of the macrocyclic species with either the monomer... 

These results are in agreement with the above described inclusion mechanism if one assumes that the main binding force resides in the strain energy of the a-cyclo-dextrin molecule itself. In this case, the equilibrium constants and reaction enthalpies should be independent of the substrate molecule. The kinetic data reveal that, microscopically, there are indeed differences between the substrate molecules, the ones with bulkier substituents being included and released more slowly than the ones with no or small substituents - the substrate has to thread itself into the ring shaped a-cyclodextrin molecule [21]. 

In real systems, nonrandom mixing effects, potentially caused by local polymer architecture and interchain forces, can have profound consequences on how intermolecular attractive potentials influence miscibility. Such nonideal effects can lead to large corrections, of both excess entropic and enthalpic origin, to the mean-field Flory-Huggins theory. As discussed in Section IV, for flexible chain blends of prime experimental interest the excess entropic contribution seems very small. Thus, attractive interactions, or enthalpy of mixing effects, are expected to often play a dominant role in determining blend miscibility. In this section we examine these enthalpic effects within the context of thermodynamic pertubation theory for atomistic, semiflexible, and Gaussian thread models. In addition, the validity of a Hildebrand-like molecular solubility parameter approach based on pure component properties is examined. 

A small quantity of water is placed in a glass tube that has a narrow conical bore. The angle of contact between glass and water is zero. When the tube is held vertically it is found that the water is retained in the mid-position of the tube. Which end of the tube is uppermost when the water is retained in this way Calculate the minimum radius a drop of water can have if it is free to evaporate, but is thermally isolated. The specific enthalpy of vaporisation of water is 2270 X 10 J kg", its density is 1.0 X 10 kg m" and its specific surface free energy is 72 mJ m". A loop of fine thread is placed upon a plane liquid film and the... 

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