Quasi-crystalline melts

Alkali metal halide melts are characterized by a quasi-crystalline structure originating by dilatation of the crystal structure and by the occurrence of different kinds of positional disordering. Cations and anions are preferably surrounded by ions of the opposite charge... 

Few studies have been made on transport processes involving concentrated solutions. In the concentrated solutions, in the range of dehydrated melt formation, incompletely hydrated melts and anhydrous salt melts, various structural models are described to define their properties, i.e. the free-volume model, the lattice-model and the quasi-crystalline model. Measured and calculated transport phenomena do not always represent simple ion migration of individual particles, but instead we sometimes find them to be complicated cooperative effects (27). 

VI. Melts for which Quasi-Crystalline Models are Inadequate. . . 467... 

Prefreezing phenomena are less easily summarized than premelting ones partly because they are more diversified and partly because they have been less well characterized up to now. In quasi-crystalline melts, minute concentrations of crystalline regions can be inferred on the basis of observations on spontaneous nucleation at the critical supercooling temperatures P, which for many quasi-crystalline melts show a constant ratio to the... 

VI. MELTS FOR WHICH QUASI-CRYSTALLINE MODELS ARE INADEQUATE... 

A second mechanism for increasing disorder on melting which cannot be conveniently represented by a quasi-crystalline model for the melt involves the formation of association complexes. Quite generally, these can be defined as clusters of the units of structure (e.g., molecules or ions) in the crystal which have approximately the same distance between nearest neighbours as in the crystal lattice, but which need not have the full regularity of crystal packing. As already stated, only one particular form of cluster, the crystal nucleus can normally be extended indefinitely... 

Attempts have also been undertaken to improve the processability of PMR imide resins through molecular weight adjustments and exchange of the monomers employed. LARC 160 as an example here Jeffamine AP22, a eutectic blend of MDA type amines, was used as a polyamine instead of the crystalline MDA. This modification provided a quasi melt processable PMR resin (15). Other modifications were studied with the aim of improving the thermal oxidative stability by using hexafluoroisopropylidene dipthalic anhydride as a monomer (16). 

---