Polymer solidification

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

The physical and mathematical description of the ribbon extrusion process was first given by Pearson [24], who simplified the conservation equations by using a onedimensional, isothermal, Newtonian fluid approach, and neglected the effects of polymer solidification. As in the case of blown film processes, several modifications and models have been proposed for the ribbon extrusion process (Table 24.2). 

The stretching force is a direct consequence of the distance of polymer solidification (DPS) in both processes (i.e., FLH in film blowing processes or cooling length in ribbon extrusion). In this sense, controlling DPS offers the possibility to control the final properties of films and ribbons. Table 24.3 lists typical values of stretching force as function of FLH and DR for blown films of LDPE. 

Hardening of cement mortars modified with water-soluble polymers comprises both cement hardening by hydration as well as polymer solidification by coagulation and film formation. Whereas the effect of polymer dispersions on the microstructure formation is frequently studied [1], only little information is available about the effect of polymer solutions. In contrast to polymer dispersions, water-soluble polymers are dissolved in the mixing water on a molecular scale and no surfactants are needed. However, the addition of small amounts of water-soluble polymers (usually below 4%) also influences the properties of the hardened material [2], This paper deals with the effect of the presence of water-soluble polymers on the microstructure. The study is made by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. This study was part of the doctoral research of E. Knapen [3],... 

In dry spinning, the dope is extruded through spinnerets located at the top of a tower. As the uncoagulated filaments flow down the tower, they are brought into contact with an inert gas heated above the boiling point of the dope solvent. The solvent evaporates from the filaments as they pass down the column and solidify. Conceptually, dry spinning can be considered to be a special case of melt spinning, in which the polymer solidification or crystallization temperature has been depressed by the solvent. The solidification temperature of the filaments will continuously increase as the solvent evaporates until solid filaments are formed. The filaments are continuously removed from the bottom of the tower, washed free of solvent, and then for the most part processed like the wet-spun fibers. 

For the sake of completeness, it should be conceded that the complexity of the investigation concerning polymer solidification imder processing conditions is even greater if the wide latitude of morphologies achievable is considered, esp>ecially when dealing with semicrystalline polymers. This would have to take into account also the complexity introduced by the presence of the crystallization process (Eder Janeschitz-Kriegl, 1997). 

The cooling rate was varied by changing the cooling fluid, its flow rate and temperature, or by changing the thickness of the sample assembly. However, the coolant temperature may not be crucial if it is sufficiently lower than the polymer solidification temperature. 

La Carrubba, V. (2001). Polymer Solidification under pressure and high cooling rate, Ph.D. Thesis, CUES, Salerno, ISBN ISBN 88-87030-27-8. 

Thus, also in this case an increase in the quantity and quaUty of the available experimental data can help to remove ambiguity and aid to understand the polymer solidification in more detail. Interesting examples of the complex morphology that can be achieved in a transformation process come from the structural analysis of injection molded samples in Syndiotactic Polystyrene [22-25]. 

Transition from liquid to glass and crystallization are both phenomena responsible for polymer solidification at the end of a processing operation. Since they are kinetic phenomena, they lead to an out-of-equilibrium thermodynamic state glassy polymers present an excess of unstable conformations and free volume semi-crystalline polymers are not totally crystallized, their melting point being largely lower (usually some dozens of degrees) than the equilibrium value. 

In discussing the cooling process, mention should also be made of another study by Kenig and Kamal [20], which combined a theoretical analysis and experimental data. The theoretical analysis was essentially an energy balance between the mold coolant, the mold, and the polymer. This approach included a term for polymer solidification (crystallization). The equations used (8-7-8-10), which follow, propose that there are four series resistances to heat transfer. First, the recirculating fiuid (at constant temperature cools the molten polymer cylinder. Thus, there is heat transfer at the coolant/steel interface ... 

There are, however, some drawbacks to this kind of structure. As a poly Schiff s base, one would expect hydrolytic instability and that is the case polyazomethine fiber, for the most part, under hot-wet conditions, loses physical properties quite rapidly. Secondly, most of these structures have a glass transition temperature (Tg) at about 100°C at or near this temperature, those fibers show a dramatic loss of tensile modulus. A third concern involves the difficulty in controlling polyazomethine molecular weight during polymerization and melt processing. If melt residence time is not carefully monitored, polymer solidification in the apparatus can become a problem. 

Dry spun fibers have a characteristic dumbbell or dog bone shape compared to wet spun fibers, which are mainly round or kidney bean shaped. This is the result of the solvent diffusion and evaporation stages that lead to polymer solidification. As the solvent evaporates from the polymer solution, the outer part of the forming fiber solidifies before the inner part. This causes radial inhomogeneity within each filament. The outer part collapses inwards to produce the characteristic shape (see Fig. 10.2). Dry spinning also typically produces fibers that have rough rather than smooth surfaces compared to fibers produced by melt extrusion. 

The hedgehogs in the centre of splay structures move to the edges (poles) with growing polymer network. With further polymerization, the polymer solidification quenches the growth and mobility of LC domains. The morphology remains more or less the same and at around 600 s, exhibits random optical axis orientations. It can be seen that the LC domains do not exhibit preferential alignment or orientation... 

In addition to the use of monomers, it is possible to form nanofibers directly from a polymer solutioa In this case, the polymer solution is filled into the hollow chaimels and solidified into nanofibers by removing the solvent. Nanofibers produced from the polymer solution typically have laiger diameters than those synthesized from monomers. This is because of the high viscosity of polymer solutions, which does not allow the use of hollow channels with very small diameters. Figure 13.14 shows a SEM image of PAN nanofibers prepared by filling the hollow channels of a porous aluminum oxide template with a PAN solution, followed by polymer solidification and template removal. 

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