Miscible blends, intermolecular

Now, let s look at a polymeric system. To begin with, the motion in polymer chains is hindered. The massive size of the polymer itself and the intermolecular forces within the chains create an inflexible system, especially when compared to the aqueous systems with which we are most familiar. Secondly, the entropy of mixing is not actually as great as that seen in typical solution formation. Polymers are inherently highly entropic, so the benefit of mixing them together is modest. Therefore, any two polymers that form a miscible blend depend primarily... 

In order to determine whether energy migration makes a significant contribution to the photophysical behavior of P2VN and PS in dilute miscible blends, it is instructive to calculate the expected exdmer-to-monomer fluorescence quantum yield ratio in the absence of energy migration. To do so, it is first necessary to assume that intermolecular and non-adjacent intramolecular EFS are absent. In addition, the adjacent intramolecular EFS are assumed to be frozen into the aryl vinyl polymer and must be excited by direct absorption of a photon. Since the absorption spectrum of an EFS is no different from that of non-EFS chromophores, then the calculated fraction of rings within EFS is sufficient to determine the fluorescence ratio. 

Intermolecular EFS and intramolecular EFS formed by rings separated by more than three backbone atoms can be neglected in dilute miscible blends. EFS are primarily located at meso dyads in the tt conformation. 

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... 

Nonuniform vulcanization networks in miscible blends of elastomers have a strong effect on tensile strength and elongation. These networks have an intermolecular distribution in crosslink density and are composed of different concentrations of crosslinkable sites in the components of the blend. Differences in the level of the enchained diene (5-ethylidene-2-norbomene) for EPDM... 

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. 

It can be noted that the observed miscible polymer systems may be due to intramolecular repulsion rather than intermolecular attraction as originally hypothesized. The miscible blends of random copolymer (AB) and a homopolymer (C) in AB/C type blends were found miscible over a range of compositions of comonomer 1 in copolymer AB. This indicates a possible effect of the chain sequence distribution on the miscibility. Even in cases where the homopolymers were not miscible, the copolymer and homopolymer were found to be miscible. The mean-field theory of random copolymer blends is an important development in the thermodynamics of polymer miscibility. The compositional window of miscibility has been found for systems where A, B, and C were found to be immiscible as homopolymers, but were found to be miscible as A-B copolymer and C homopolymer. 

Figure 3.19 Intermolecular cross-polarisation in polymer blends. The cross-polarisation intensity as a function of contact time is shown for (a-d) a mechanical mixture and (e-h) a miscible blend prepared by annealing. Reprinted with permission from ref. 21.

Chen C, Dong L, Yu PH (2006) Characterization and properties of biodegradable poly(hydroxyalkanoates) and 4,4-dihydroxydiphenylpropane blends intermolecular hydrogen bonds, miscibility and crystallization. Eur Polym J 42 2838-2848... 

Aqueous salt solutions such as saturated zinc chloride or calcium thiocyanate can dissolve limited amounts of cellulose. Two non- aqueous solvents are ammonium thiocyanate in ammonia and lithium chloride in N,N-dimethylacetamide. Cellulose solutions up to about 15% can be made with these solvents. Blends of cellulose and poly(vinyl alcohol) have been prepared in N,N-dimethylacetamide-lithium chloride, and exhibited good miscibility due to their mutual ability to form intra-intermolecular hydrogen bonds between hydroxyl groups [35]. Miscible blends of cellulose and poly(vinylpyrrolidone) have been prepared by dissolution in dimethyl sulfoxide-paraformaldehyde and blending with poly(vinylpyrrolidone) dissolved in dimethyl sulfoxide [36,37]. Cellulose has also been blended with poly(ethylene glycol) in dimethylsulfoxide and paraformaldehyde [38]. 

---