Quasi-melting

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). 

The transition to the bulk structure appears for several metals at relatively large sizes, 17,000 atoms for Ni [39] and about 50,000 atoms for Cu [42]. It is important to notice that the energy difference between these structures is often very small. This explains the very rapid fluctuations between different structures observed by HRTEM [43] that we call quasi-melting [44]. 

NASA Lewis have developed LARC 160, a melt processable PMR version by replacing the MDA with Jeffamine AP22 (a eutectic blend of MDA type amines) to give a quasi melt resin [41,46]. 

According to the structure of this equation the quantity cp indicates the influence of the filler on yield stress, and t r on Newtonian (more exactly, quasi-Newtonian due to yield stress) viscosity. Both these dependences Y(cp) andr r(cp) were discussed above. Non-Newtonian behavior of the dispersion medium in (10) is reflected through characteristic time of relaxation X, i.e. in the absence of a filler the flow curve of a melt is described by the formula ... 

The theory of shape selection has been examined by many investigators concerned with solidification from the melt, and its status has recently been reviewed by Caroli and Muller-Krumbauer [63], The problem is to find stable, quasi-stationary solutions to the diffusion equation where a propagating branch maintains a constant shape and velocity. If the interface is assumed to have a uniform concentration, a family of such solutions exists, but there is no unique solution owing to the lack of a characteristic length. The solutions fix the peclet number. 

A quasi-racemate or pseudo-racemate is a true racemate like molecular compound formed between optical antipode of different (but related) compounds. The quasi-racemate also has a melting point ciin c resembling the curve of a true racemate but with quasi-racemic compounds the curves are unsymmetrical, because the melting points of the components are different as shown in Fig. (9.3). The curve A represents the melting point of a true-racemate formed by mixing (+) mandelic acid XXII and (-) hexahydromandelic acid XXIII while B represents that of a mixture of (+) XXII and (+) hexa hydro-mandelic acid XXIII. 

In an ionic melt, coulombic forces between charges of opposite sign lead to relative short-distance ordering of ions, with anions surrounded by cations and vice versa. The probability of finding a cation replacing an anion in such ordering is effectively zero and, from a statistical point of view, the melt may be considered as a quasi-lattice, with two distinct reticular sites that we will define as anion matrix and cation matrix. 

This quasi-lattice formulation of fused salts is known as Temkin s equation. Its application to silicate melts was provided by Richardson (1956), but it is inadequate for the compositional complexity of natural melts, mainly because, in a compositionally complex melt, the types of anions and consequently the entity of the anion matrix vary in a complicated way with composition. 

Figure 6.2 Gibbs free energy of mixing in the quasi-chemical model of Toop and Samis (1962a,b), compared with values experimentally observed in Pb0-Si02 and Ca0-Si02 melts. Reprinted from Toop and Samis (1962b), with kind permission of ASM International, Materials Park, Ohio.

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