Premelt crystallization

The melting temperature of monomer depends on relative molecular mass. A broad endotherm observed at around 25 °C is attributed to premelt crystallization. 

One potential, and perhaps unexpected, application of conductive polymers is for the definition of conductive tracks on printed circuit boards. Rubner et al. [98] exploited the thermal properties of PPS to enable the selective introduction of conductivity by doping. By using a heat gun or a CO2 laser beam and a suitable masking procedure, the PPS could be made to undergo a premelting crystallization at 150 C, such that there... 

These contact angles can be related to the physical state of the surface. The 100 facet is better wetted than the 111 one because the 100 surface is partly premelted. But, the liquid-like disordered monolayer is too thin to have the properties of the macroscopic liquid, and this "adsorbed liquid layer" coexists with a non-wetting macroscopic liquid. This so-called "incomplete surface melting" has also been observed on a pure single crystal of ice. ... 

Melting curves from binary phase studies ( 0, H., j ) have been interpreted in terms of a crystal transition near 800 C, although the most recent paper (13) Indicated no transition. Two high-pressure polymorphs have been observed (J ) but there is no evidence of their stability at atmospheric pressure. Enthalpy data (8) showed no obvious transition near 800 C points at 829 and 815 C showed reasonable premelting contributions of about 200 and 300 kcal mol", respectively. Although this evidence does not preclude the existence of a transition, definite evidence would be needed to establish such a transition. 

Clark et al. [30] considered premelting transitions in the context of a general theory of disorder in plastic crystals. Entropy changes were attributed to contributions from two sources increasing rotation of the molecular constituents of the crystal and the expansion of the lattice, against the van der Waals forces, necessary to accommodate this increased disorder. The observed changes were collated with molecular shapes. 

Examples of premelting have sometimes been suspect, because impurities soluble in the melt but insoluble in the crystals lead to the formation of small amounts of the actual liquid phase at... 

When any likely impurities in the system readily form solid solutions in the crystals, they may be very difficult to remove by conventional methods. Effects of such foreign molecules in the crystal lattice can profoundly modify barriers to molecular rotation, and can thereby enhance premelting. In favourable cases, this can be tested by the direct addition of the appropriate impurity, whose structure must be so close to that of the primary molecules that the foreign species can easily be accommodated in the crystals. Some experimental examples have been investigated by Ubbelohde, Oldham and Ubbelohde, and Thompson and Ubbelohde (cf. Reference 27). 

Another aspect of high chain symmetry is the possibility of molecular motion within the crystal lattice contributing to higher T. For example, polyethylene and polytetrafluoroethylene are both sufficiently symmetrical to be considered as smooth, stiff cylindrical rods. In the crystal, these rods tend to roll over each other and change position when thermally agitated. This motion within the crystal lattice, called premelting, effectively stabilizes the lattice. Consequently, more thermal energy is required to break down... 

The premelting temperature, T u, may have a profound influence on the crystallization temperature of the matrix, Tc, during cooling from the melt (Table 3.16). 

Several factors can influence the fractionated crystallization behavior. An important parameter that has already been discussed is the thermal history of the sample. CrystaUizable dispersed droplets that were submitted to premelting at higher temperatures or longer times generally display a shift in the heterogeneous... 

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