Melting, intracrystalline

PVF displays several transitions below the melting temperature. The measured transition temperatures vary with the technique used for measurement. T (L) (lower) occurs at —15 to —20 " C and is ascribed to relaxation free from restraint by crystallites. T (U) (upper) is in the 40 to 50°C range and is associated with amorphous regions under restraint by crystallites (63). Another transition at —80° C has been ascribed to short-chain amorphous relaxation and one at 150°C associated with premelting intracrystalline relaxation. 

The higher melting point and lower solubility of a para isomer is only a special example of the general effect of molecular symmetry on intracrystalline forces. The more symmetrical a compound, the better it fits into a crystal lattice and hence the higher the melting point and the lower the solubility. Para isomers are simply the most symmetrical of disubstitut d benzenes. We can see (Table 12.1) that... 

The differences in structural order and in mobility of the constituents explain the general observation that, during thermal decompositions which are accompanied by melting, the reaction usually proceeds more rapidly in the liquid than in the crystalline state. Fusion diminishes the intracrystalline attractive forces which may confer stability on each lattice constituent. The greater disorder in the liquid also enables a reactant to adopt the reaction configuration more easily. 

The final stages of the above reaction overlapped with the onset of the nucleation and growth process that continued to complete the dehydration. Growth of three dimensional nuclei was confirmed microscopically. This second rate process was well described by the Avrami-Erofeev equation with = 2 and E, for crystals was 175 30 kJ mol (with a considerable scatter of data) below 460 K and a more reproducible reaction rate, with E, = 153 10 kJ mol, for powder. Above about 450 K there were some indications of intracrystalline melting of single crystals and the value of , increased markedly to 350 50 kJ mol (again with significant scatter of data). 

Reaction rates may be determined by the ease of intracrystalline transport to the surface, or by the chemical change on the surface. These surface reactions often resemble behaviour described in modelling heterogeneous catalytic processes and are usually reversible so that decomposition rates are sensitive to any gases present. Behaviour of reactants of the present group is similar to that of the oxides (Chapter 9), which are in the same class. Little information is available for other binary compounds, fluorides, chlorides, etc., which usually melt rather than decompose. 

The contrasting reaction mechanisms identified in these comparative studies [123,126] of two chemically similar compounds, decomposing in comparable temperature intervals, is surprising and exemplifies the difficulties inherent in the formulation of reaction mechanisms for these rate processes. Silver metal evidently promotes anion breakdown and this is the more electropositive element. This reaction proceeds in a marginally lower temperature interval than the decomposition of the copper salt where there is no metal formed in the first step and reaction may be promoted by relaxation of the intracrystalline forces of the solid on melting. 

Keywords melt reaction, intracrystalline reaction, cyanamide, cyanoguanidine... 

The attractive ion-conducting behavior of PEO-clay nanocomposites, which could be of potential interest for applications as solid electrolytes (122), led Vaia and coworkers (133) to utilize a new method of synthesis profiting from the fact that certain polymers can melt without chemical degradation (134). Insertion of PEO into alkaline metal homoionic montmorillonites is reached after 6 h of treatment at 80°C of a PEO-clay mixture. The chosen temperature is 10°C above the melting point of PEO, ensuring sufficient mobility of polymer chains to migrate toward the intracrystalline region of the clay. 

Polyvinyl fluoride has a number of transitions below the melting temperature, the values of which depend on the measurement techniques. The lower glass transition occurs at -15 to -20°C and is believed to relate to relaxation free from restraint by crystallites. The upper glass transition ranges from 40 to 50°C, apparently due to amorphous regions under restraint by crystalhtes.1 1 Yet another transition occurs at -80°C because of short-chain amorphous relaxation and another at 150°C associated with premelting intracrystalline relaxation. 

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