Miscibility polymer-solvent pairs

Table 4. Miscibility of polymer-solvent pairs as a function of the difference in solubility parameters A(5...

Mumal solubilities of liquids vary greatly at ambient temparature. water and ethyl alcohol are miscible in all proportions, water and benzene are only very slightly soluble in one another, while benzene and mercury show essentially no mumal solubility. For most liquid pairs, mutual solubility increases with rising temperature, but meny exceptions are kaown a few pairs are completely soluble in one another at low temperatures and at high tempeimares with limited miscibility in between, while others (notably polymer-solvent systems) show complete miscibility only between lower and upper temparature limits. 

Up to now we have only discussed the structure of interfaces that have reached equilibrium. However, in many processes only a finite amount of time is available for an interface to be formed. For example, when a pair of polymers is coextruded, the time available for the interface to develop is limited to the time at the process temperature. Alternatively, if one polymer is applied as a solution to coat a second pol)mier, then molecular motion of the polymers near the interface will often only be possible before all the solvent has evaporated. This question of kinetics becomes particularly important when we come to consider interfaces between miscible polymers here there is no equilibrium interface width at aU and the width of the interface that is achieved in practice... 

Polymer miscibility—with solvents, other polymers, or in block copolymers— can be treated with integral equation methods such as PRISM (255-257) or density functional theory (252,258). Theories of this kind inevitably required approximations that are difficult to assess by independent methods, with the variety of closure relations having been developed in integral equation theories (259) illustrating the case in point. Molecular modeling can provide the detailed molecular-level information, such as pair correlation fimctions, that is needed to assess the validity of these t5q>es of theories (260,261). 

Solvent effect Phase separation can be seen when a miscible pair is cast from a common solvent. Show that this effect is caused by an asymmetry in the polymer-solvent interactions leading to a closed region of immiscibility in the ternary solvent-polymer-polymer phase diagram [20]. 

BHA Bhattacharyya, C., Maiti, N., Mandal, B.M., and Bhattachaiyya, S.N., Sensitivity of ternary phase diagrams of miscible polymer pairs in common solvents to traces of water Solvent-poly(ethyl aciylate)-poly(vinyl propionate) and solvent-poly(methyl aciylate)-poly(vinyl acetate) systems. Macromolecules, 22,487, 1989. 

Whereas examples in Fig. 19.6A,B pertain to immiscible polymer melts. Fig. 19.6C pertains to a miscible polymer pair [31]. Submicron layers were formed in a blend of 30% by volume EVOH and polyamide (PA). To improve phase contrast in the scanning electron image, samples were etched in a solvent so that some or portions of EVOH layers were removed. Thicknesses of layers were determined by separate transmission electron examinations to be less than 0.5 pm. The potential amount of layer refinement obtained with a smart blender is demonstrated for a blend consisting of 15% by volume LDPE and high density polyethylene (HDPE) [5]. Because interfacial tension was very low due to... 

Homopolymers are sometimes modified by a mechanical admixture of another homopolymer. As only about 5 % of pairs of all known polymers are mutually miscible, compatibility may be a problem in mixtures (blends). Copolymerization is technically applied to overcome, for example, the brittleness of polystyrene, polypropylene and PVC. It is also applied for improving the curing properties and modifying the viscoelastic properties of rubbers. By copolymerization, the relation between the hydrophobic and hydrophilic properties of macromolecules can also be modified. Their resistance to solvents may be enhanced. 

Biopolymer incompatibility is a general phenomenon typical of aU polymers. Biopolymer incompatibility occurs even when their monomers would be miscible in all proportions. For instance, sucrose, glucose and other sugars are normally cosoluble in the common solvent, water, while different polysaccharides usually are not miscible. The transition from a mixed solution of monomers to polymers corresponds to the transition from good to limited miscibility. Normally, a slight difference in composition and/or structure is sufficient for incompatibility of macromolecules in common solvent (Tolstoguzov 1991, 2002). Compatibility or miscibility of unlike biopolymers in aqueous solutions has only been exhibited by a few biopolymer pairs (Tolstoguzov 1991). 

Other examples of solvent effects in casting blends include epoxy resin/copoly-ester/tetrachloroethane polyethersulphone/poly(ethylene oxide)/cyclohexanone and mixtures of PVC with various polyacrylates in solvents such as THF One particular pair of polymers PVC/poly(ethyl acrylate) appear to be miscible but no suitable solvent has been found as yet. Homogeneous blends can only be prepared by in situ polymerisation though it is possible that miscibility is enhanced by small amounts of graft copolymer which is inevitably formed by this technique. 

The parameter ji defined in Eq 2.81 is a measure of the polymer-polymer miscibility — negative values indicate immiscibility, positive the miscibility. Three series of blends were examined (1) PVC/PMMA, (2) PiBMA/PMMA, and (3) PiBMA/PVC. In agreement with the calculated values of the parameter the first of these three blends was found miscible, whereas the two other immiscible in the full range of composition. However, the method is, at best, qualitative. For example, the effect of solvent on the parameter was not investigated, but fundamentals of intermolecular interactions make it dubious that non-polar and strongly polar solvents will lead to the same value of the parameter The author observed that the method breaks down for polymer pairs that form molecular associations. Intrinsic viscosity measurements were also used to evaluate intermolecular interactions in blends of cellulose diacetate with polyvinylpyrrolidone [Jinghua et al, 1997]. 

Some polymers have been found miscible with many other resins, or in other words there are many immiscible blends whose components are miscible with the same polymer. Addition of this polymer can be used to partially homogenize the system, i.e., to compatibilize the blend. The added polymer is a co-solvent. Of particular interest are systems in which presence of a co-solvent makes it possible for the two immiscible components to form three-body interactions. In this case, the blend is indeed compatibilized, with the co-solvent being located in the interphase. For the thermodynamic reasons, mostiy copolymers belong to this type of co-solvents. In the left hand side column of Table 4.1 there are polymers that may be used as co-solvents for pairs of resins listed in the other column. Some of the latter resins may show local miscibihty (e.g., PS with styrenic copolymers), but the vast majority is immiscible. 

Just as it is useful to know how miscible any pair of solvents are when they are in a liquid state it is also often necessary to screen a list of solvents to know which are likely to be effective in dissolving resins and polymers. 

For most polymer pairs to be miscible, an exothermic interaction is required. Nandi et al. [40] studied the miscibility of poly(methyl acrylate) (PMA) and poly(vinyl acetate) (PVAc) in several solvents by the inverse... 

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