Repulsive interactions, miscibility

Intramolecular Repulsive Interactions. Miscible blends can also be achieved in absence of specific interactions, by exploiting the so-called intramolecular repulsive effect. This is observed in mixtures where at least one of the components is a statistical copolymer miscibility is restricted to a miscibility window, that is, it takes place within a well-defined range of copolymer composition. For example, poly(styrene-co-acrylonitrile) (SAN) and poly(methyl methacrylate) form miscible blends for copolymer compositions in the range 9-39% acrylonitrile (26,27). Miscibility in these systems is not a result of specific interactions but it is due to the intramolecular repulsive effect (28) between the two monomer units in the copol5uner such that, by mixing with a third component, these imfavorable contacts are minimized. The same situation is encoimtered in binary mixtures of two copol5uners (29). 

For example, taking the case illustrated in Figure 2.7, one may consider that PVDC is an alternate copolymer of units -CH - and -CCl -, whereas the aliphatic polyesters are composed of units -CH - and -COO-. Eqs 2.51-2.52, predict that even systems with all positive values of the binary interaction parameter, B > 0, (repulsive interactions) may have a window of miscibility, where the overall parameter, B < 0. The magnitude of this effect depends primarily on the value of the repulsive interactions within the copolymer molecule, B > 0. Schematic representation of Eq 2.52 is shown in Figure 2.11. 

If two polymers are mixed, the most frequent result is a system that exhibits complete phase separation due to the repulsive interaction between the components (i.e., the chemical incompatibility between the polymers). A necessary condition for miscibility to occur is that AG must be negative (AG < 0). This is a necessary requirement, but not a sufficient one, as the following expression must also be satisfied in order to obtain a stable one-phase system. The expression that describes the criteria for phase stability of binary mixtures of composition tp at fixed temperature T and pressure P is [2] ... 

An interesting layered structure, the so-called knitting pattern, was found in BCP SA [8]. The structure has been observed in frustrated ABC triblock terpolymers both experimentally and theoretically, where repulsive interactions between the two terminal blocks are weaker than those between middle block and terminal blocks. The morphology is regarded as a metastable state because the structure formation is driven by the relatively poor solvent miscibility of the middle block [8]. The middle block constitutes cylinders bridging wavy lamellae generated via agglomeration in a poor solvent. 

The decrease of Todt or Ts for MA-g-SEBS means that the repulsive interaction between styrene and ethylenebutylene is decreased by the MA modification. Since the molecular weight is small enough for the tacticity effect to be neglected in the miscibility problem, it is supposed that also the interaction between SPS and modified EB chains will be reduced by MA modification. 

At a given pressure, acetonitrile solubility in a copolymer is much higher than that in the corresponding homopolymers. This non-intuitive behavior is attributed to intramolecular repulsion between unlike segments of the copolymer. This repulsive interaction is weakend when acetonitrile molecules are in the vicinity of unlike copolymer segments, favoring copolymer+solvent miscibility. 

A commercially important example of the special case where one monomer is the same in both copolymers is blends of styrene—acrylonitrile, 1 + 2, or SAN copolymers with styrene—maleic anhydride, 1 + 3, or SMA copolymers. The SAN and SMA copolymers are miscible (128,133,144) so long as the fractions of AN and MA are neatly matched, as shown in Figure 4. This suggests that miscibility is caused by a weak exothermic interaction between AN and MA units (128,133) since miscibility by intramolecular repulsion occurs in regions where 02 7 can be shown (143) by equation 11. 

Another important feature of some random copolymers is the abihty to achieve miscibility in either a homopolymer or a second random copolymer. This "copolymer effect" has been shown empirically for quite some time, eg, PVC is miscible with random copolymers of ethylene and vinyl acetate (52). Such systems are effective because repulsions between the dissimilar segments in the copolymer are enough to overcome the repulsions between these segments and those of the second component in the mixture. In other words, in the above example, the ethylene units "hate" vinyl acetate units more than either of them "hate" PVC. Thus there is a net negative interaction energy and the two materials are miscible (53). 

Methanol, which has the shortest carbon chain, is more polar and soluble than other alcohols. SDS monomers are more easily solvated in an aqueous-methanol medium. This inhibits them from interacting and forming micelles. A similar behavior is expected for acetonitrile. Ethanol and propanol, which are also miscible with water, remain outside the micelles, dissolved in the bulk liquid, but interact with the micelle surface. Repulsion among the... 

Since this critical interaction parameter is very small for blends of long chains, most polymer blends have x > Xc and thus are phase separated over some composition range (within the miscibility gap). Only blends with either very weak repulsion (0 < x < Xc), or a net attraction between components of the mixture (x < 0) form homogeneous (single-phase) blends over the whole composition range. 

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