Fluid Transition

The rubber-fluid transition is associated with the change in the global mobility of molecules. This transition is practically important to the molding of polymer materials, for it is a prerequisite condition of permanent deformation. In 1979-1981, Boyer suggested that corresponds to a dynamic transition called as Til (a liquid-liquid transition above Tg), reflecting the frozen and de-frozen of the whole polymer chain (Boyer 1979, 1980, 1981). From the molecular weight dependence curve illustrated in Fig. 6.3, one may see that Tf appears as an extrapolation from glass transition temperatures of small molecules. The properties of Til ate quite similar to Tg. 

Til appears as general to non-crystalline polymers and their copolymers  

The chains are more rigid, Tn becomes higher the interchain interactions are stronger, Tu becomes higher as well  

When the molecular weight is large, the molecular weight dependence of Tu becomes saturated, as 

Tu has the equivalent-free-volume phenomenon, as well as of viscosity  

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Whereas microscopic models for bulk systems incorporate the amphiphihc character and often the orientational properties of the surfactants as basic ingredients, models for bilayers and monolayers are constructed to reproduce internal transitions, such as the gel-fluid transition, and therefore concentrate on rather different aspects of the surfactant structure. 

The Pink model is found to exhibit a gel-fluid transition for lipids with sufficiently long chains, which is weakly first order. The transition disappears in bilayers of shorter lipids, but it leaves a signature in that one observes strong lateral density fluctuations in a narrow temperature region [200,201]. In later studies, the model has been extended in many ways in order to explore various aspects of gel-fluid transitions [202]. For example, Mouritsen et al. [203] have investigated the interplay between chain melting and chain crystallization by coupling a two-state Doniach model or a ten-state Pink model to a Potts model. (The use of Potts models as models for... 

For many valve constructions, it is reasonable to approximate the fluid transition from the valve inlet to the minimum cross section of the flow stream as an isentropic or lossless process. Using this approximation, the minimum pressure pvc can be estimated from the... 

Fig. 3.8 Comparison of the effects of pentachlorophenol (PCP) on the gel-to-fluid transition of DPPC (open symbols) and DMPC (closed symbols) membranes. The circles represent the action of ionized PCP obtained at pH 10 and the triangles that of the unionized...

Furthermore, DSC can be useful for modeling the toxic effects of xenobiotics such as phenols and especially chlorinated phenols. A detailed study was performed by Smejtek et al. [44]. Among other observations, it was found that pentachlorophenol induces changes in the -potential and the gel to fluid transition temperature in model lecithin membranes. The ionized form was more potent in abolishing pretransition. The unionized species induced an increase in melting transition width (Figure 3.8). 

Seddon, J.M., Cevc, G., and Marsh, D. (1983) Calorimetric studies of the gel-fluid transition (Lj) —> La) and lamellar-inverted hexagonal (La — H ) phase transition in dialkyl- and diacyl-phosphatidylethanolamines. Biochemistry 22 1280-1289. 

Shin, S. Wang, Z.G. Rice, S.A. An interpretation of the multiple fluid-fluid transitions in liquid supported amphiphile monolayers. J. Chem. Phys. 1990, 92 (2), 1427. 

Polyethylene is a man-made homopolymer. Its chemical synthesis is well understood. It is a random walk polymer with little secondary or tertiary structure. A batch can largely be characterised by its molecular weight distribution, and its rheology can be related to these parameters by developed rules of polymer behaviour. The action of specific chemicals as plasticisers can be used to modulate these bulk properties in a predictable way, allowing the nature and characterisation of its glass to fluid transition to be predicted. 

Figure 10 The average width of the NMR spectrum (Mi) as a function of temperature and pH. Closed circles pH 5.2 open circles pH 6.2 closed triangles pH 7.4. Repetition time 50 sec at all temperatures where the sample contained some solid component, then 1 sec at higher temperatures. (In practice, the quadrupolar echo intensity was checked every few degrees at the 1-sec repetition time toward the completion of the solid-to-fluid transition, and when the echo intensity differed by < 1% between the short and long repetition times, a l-sec repetition time was used for all higher temperatures.)...

Figure 3. (a) Two-dimensional, bond orientational order parameter average values in the molecular fluid layers of LI ecu confined in a multi-walled carbon nanotube of diameter D=9norder parameter values for the contact, second, third and fourth layers, respectively. The dotted line represents the bulk solid-fluid transition temperature, (b) Positional and orientational pair correlation functions in the unwraiqred contact layer of U CCU confined in a multi-walled carbon nanotube of diameter D=9.1< (5 nm) showing liquid phase at 7=262 K and crystal phase at 7=252 K. 

There is a fluid transition between adaptation and alteration, with complete reversibility being possible if the damage does not exceed a certain limit. 

Two situations must be distinguished, (i) assays of pure enzyme and (ii) assays of cell extracts. When purified enzyme preparations are available, no labeled substrate is required. Natural or synthetic sphingomyelin is prepared, pure or mixed with other lipids, in the form of extruded large unilamellar vesicles (LUV) ca. 100 nm in diameter. When pure sphingomyelin vesicles are used extrusion must take place at a temperature close to or above the gel-fluid transition temperature of the lipid, i.e. often 45-50 °C. LUV and enzyme are mixed in the appropriate assay buffer and aliquots are removed at fixed time intervals. The aliquots are mixed with chloroform-methanol and, after phase separation, phosphorous (from phosphorylcholine) is assayed in the aqueous phase. The procedure has been described in detail by Ruiz-Arguello et al. [91]. 

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