Intermolecular interactions, miscibility, blended

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]. 

In a fundamental sense, the miscibility, adhesion, interfacial energies, and morphology developed are all thermodynamically interrelated in a complex way to the interaction forces between the polymers. Miscibility of a polymer blend containing two polymers depends on the mutual solubility of the polymeric components. The blend is termed compatible when the solubility parameter of the two components are close to each other and show a single-phase transition temperature. However, most polymer pairs tend to be immiscible due to differences in their viscoelastic properties, surface-tensions, and intermolecular interactions. According to the terminology, the polymer pairs are incompatible and show separate glass transitions. For many purposes, miscibility in polymer blends is neither required nor de-... 

It appears that the main determinant in polyolefin miscibility is the way the chains pack together, a feature that controls the intermolecular interaction between molecules. The solubihty parameter approach is predictive in most cases and can be useful in designing polyolefin blends. 

We introduce the use of solid-state 2D FSLG 13C-3H HETCOR experiments for correlating intermolecular interactions in miscible, hydrogen bonded polypeptide blends in this section. The pulse sequence that is used... 

Static H 2D NOE spectroscopy was applied in a first experiment showing that the technique can be used to measure inter-chain interactions [44], This work was then continued by applying the technique under MAS to investigate the inter-molecular interactions responsible for the miscibility in polybutadiene/polyisoprene blends above the Tg [45]. It was shown that intermolecular association can be probed by this technique and the results reveal the existence of weak intermolecular interactions between the polyisoprene methyl group and the vinyl side chain of the polybutadiene. 

If there are more than two components in a mixture (as in a blend of a homopolymer with a copolymer), binary interaction parameters can be combined into a composite % parameter to describe the overall behavior of the system. For example, Choi and Jo [11] showed how the effects of copolymer sequence distribution in blends of polyethylene oxide) with poly(styrene-co-acrylic acid) can be described by an atomistic simulation approach to estimate the binary intermolecular interaction energies which are combined into a total interaction parameter for the blend. Their paper [11] also provides a list of the many preceding publications attempting to address the effects of copolymer composition, tacticity, and copolymer sequence distribution on polymer blend miscibility. In addition to the advances in computational hardware and software which have made atomistic simulations much faster and hence more accessible, work in recent years has significantly improved the accuracy of the force fields [12] used in such simulations. 

The crystallization of miscible and immiscible polymer blends can differ remarkably from that of the neat crystallizable component(s). In the case of crystallizable miscible blends (discussed in this section), important polymer characteristics with respect to crystallization are the chemical nature and molecular mass of the components, their concentration in the blend, and the intermolecular interactions between the components. 

Sphemlite growth of the crystallizable component in miscible blend (3.3.3) will be influenced by the type and molecular weight of the amorphous component (the former affecting the intermolecular interactions between both... 

Miscibility plays an important role in influencing chemical and solvent effects on a polymer blend. In a blend, where strong intermolecular interactions exist, the free energy of mixing, AGjjj, is given by the relation [Painter et al., 1988, 1989] ... 

Intermolecular interactions governing the miscibility in these systems were also considered from FT-IR difference spectra analysis which has been used to study the interaction of the constituents of polymer blends(7-9). 

Thermally processed WP and polyvinylalcohol (PVOH) blends have been studied by solid-state high resolution NMR spectroscopy. The intermolecular hydrogen bonding interactions between WP and PVOH induced some miscibility in the system at the nanometre scale, especially when the PVOH content was low. The TS and modulus of the blends were improved when compared to those of WP. However, the intermolecular interactions were relatively weak and could not be further enhanced by increasing the PVOH content, because such an increase enhanced the immiscible character of the blend composites [84]. 

One criterion to distinguish the miscibility of blends is the glass transition temperature (Tg) that can be measured with different calorimetric methods [95]. Tg is the characteristic transition of the amorphous phase in polymers. Below Tg, polymer chains are fixed by intermolecular interactions, no diffusion is possible, and the polymer is rigid. At temperatures higher than Tg, kinetic forces are stronger than molecular interactions and polymer chain diffusion is likely. In binary or multi-component miscible one-phase systems, macromolecules are statistically distributed on a molecular level. Therefore, only one glass transition occurs, which normally lies between the glass transition temperatures of the pure components. 

For nearly athermal systems, the proportionality factors, S. and Sy, are taken as equal to 1. Thus, for the systems without strong interactions, the binary parameters are weU approximated by the geometric and algebraic averages. For example, for PS/PVME blends, the assumption 5 = 5v = 1 resulted in 0.1 % deviation for the experimental values of the cross-parameters (Xie et al. 1992 Xie and Simha, 1997, private communication ). In contrast, it is to be expected that for systems with strong intermolecular interactions such mixture rules may fail and experimental values for the cross-factors may have to be found. However, least squares lit of Eqs. 2.42 and 2.43 to experimental values of CO2 miscibilities in PS (in a wide range of P and T) yielded values for and Sy close to 1 (Xie et al. 1997). 

Miscible polymer blends behave similar to what is expected of a single-phase system. Their properties are a combination of the properties of the pure components, and in many cases, they are intermediate between those of the components. The characteristics of the components affecting the properties of miscible blends are their chemical structure and molecular weight, their concentration, and their intermolecular interactions, including crystalfizabUity. 

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