Scaling relations, polymer blend phase separation

Nakano et al) reported a C CP/MAS NMR study on poly(L-Alanine) (PLA)/Polyglycine (PC) blends. The comparison between the C CP/MAS spectra of the blends and those of the parent polymers discloses that upon blending, new conformations of PLA and PC are fonned, which is closely related to the presence of intermolecular hydrogen-bonding interactions. Tip measurements demonstrate that the major parts of PC and PLA in the blends, which are in 3i-helix and a-helix conformation forms respectively, are phase separated. On the other hand, the p-sheet forms of PC and PLA, which are newly formed upon blending, exhibit similar values, demonstrating that these two parts are miscible at the scale of 3-4 nm. 

We now present briefly more explicit calculations of the mutual virial coefficients obtained with the use of des Cloizeaux direct renormalization method for blends of linear flexible polymers in a common good solvent, a common 0-solvent and a selective solvent and for blends of rodlike polymers and flexible polymers in a 0-solvent (marginal behavior). These calculations enable one to find (universal) prefactors relating the mutual virial coefficient to the chain volume (in Eq. 7) in the asymptotic limit. Moreover they give the corrections to the scaling behavior which explicitly depend on the interactions between unlike monomers and are actually responsible for the phase separation of flexible polymer blends in a good solvent. 

The types of polymer blends are quite varied and comprise many diverse combinations of polymeric materials of both academic and industrial interest. The primary differentiation of polymer blends involves their phase behavior specifically, miscibility versus phase separation. Miscibility is related to mixing approaching the molecular dimension scale such that the properties observed are that expected of single phase materials. Miscibility does not imply ideal mixing at the molecular scale. Miscibility was initially believed to be an extremely rare observation and, in fact, most random combinations of binary blends are indeed phase separated. However, many miscible combinations have been noted and the rationale for miscibility is well-established. The primary advantage of miscible versus phase separated polymer blends is the blend property profile, which is generally intermediate between that of the... 

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