Crystal, defect, point melting,

Ionic solids, such as lithium fluoride and sodium chloride, form regularly shaped crystals with well defined crystal faces. Pure samples of these solids are usually transparent and colorless but color may be caused by quite small impurity contents or crystal defects. Most ionic crystals have high melting points. 

We believe that the different crystal habits were caused by the solvents used for crystallization. The dissimilar melting points are probably due to the poor transfer of heat caused by the larger crystal size or to possible crystal defects."... 

The usual and therefore most important situation where polymers crystallize is in melts eooled below the point of the fusion of a crystallite of infinite dimensions. Then, crystallization occurs by the nucleation and growth of spherulites. Another crystallization process is sometimes encountered in oriented melts and glasses. In such systems, the crystallization seems to occur at once in the whole sample and not at the interface between the growing crystallites and the amorphous matrix. Despite munerous studies, the crystallization process is not fully understood. Scattering measurements suggest a preliminary spinodal decomposition of the undercooled isotropic melt in phases with and without chain ends and chain defects before the formation of the crystallites [32]. 

Short-chain crystals have lower melting points, which can be regarded as a result of high content of chain-end defects in the large-size crystal. The defects bring an effective depression in the melting points. 

The lower limit of segregation is set by the hypothetical equilibrium of crystallization. It is assumed that dynamic equilibrium is achieved between fully extended-chain crystals and the surrounding melt. At equilibrium, the molecular length of the crystallizable species corresponds closely to the lamellar thickness, and molecules too short or too long introduce defects and increase the free energy and are thus rejected from the crystal. The equilibrium melting point of a given molecular species is dependent not only on its molar mass but also on the molar masses of the other difFerent species present in the blended melt ... 

The effect of different types of comonomers on varies. VDC—MA copolymers mote closely obey Flory s melting-point depression theory than do copolymers with VC or AN. Studies have shown that, for the copolymers of VDC with MA, Flory s theory needs modification to include both lamella thickness and surface free energy (69). The VDC—VC and VDC—AN copolymers typically have severe composition drift, therefore most of the comonomer units do not belong to crystallizing chains. Hence, they neither enter the crystal as defects nor cause lamellar thickness to decrease, so the depression of the melting temperature is less than expected. 

Crystals with Frenkel or Schottky defects are reasonably ion-conducting only at rather high temperatures. On the other hand, there exist several crystals (sometimes called soft framework crystals ), which show surprisingly high ionic conductivities even at the room or slightly elevated temperatures. This effect was revealed by G. Bruni in 1913 two well known examples are Agl and Cul. For instance, the ar-modification of Agl (stable above 146°C, sometimes denoted also as y-modification ) exhibits at this temperature an Ag+ conductivity (t+ = 1) comparable to that of a 0.1m aqueous solution. (The solid-state Ag+ conductivity of a-Agl at the melting point is actually higher than that of the melt.) This unusual behaviour can hardly be explained by the above-discussed defect mechanism. It has been anticipated that the conductivity of ar-Agl and similar crystals is described... 

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