Melting point composition curve

Fig. 17. Melting point—composition curves for random copolyamides of nylon-6,6.

For a number of the systems, comparisons were made between the effects of enantiomeric composition in the monolayer and corresponding melting-point-composition curves for the crystals. All of the latter gave clear evidence of racemic compound formation in the crystals, and this type of pattern was repeated in the monolayer properties. 

Similarly, by melting together polyvinyl and polyvinylidene fluoride at all relative compositions a unique crystalline phase is observed, which is identical with the structure of crystalline polyvinylfluoride (49) and also with the structure of one of the crystalline forms of polyvinylidene fluoride (21). Since the lattice constants of these two forms are quite close, no variation is observed in the X-ray spacings of the solid mixtures throughout the whole range of compositions. The existence of a true co-crystallization is shown by the melting point/composition curve, which shows no minimum. 

Many binary mixtures (two components) will form a compound which will be in equilibrium with liquid of the same composition. In many cases, such as with benzophenone and diphenylamine, compound formation may be detected by the appearance of a species with a new melting point in the melting point-composition curve (Fig. 2-15), and... 

In certain cases, the melting point of the compound is not observed, because the mixture decomposes completely at a temperature below its melting point. Such a compound is said to have an incongruent melting point. A typical melting point-composition curve for this type... 

Composition Fig. 2-18. Melting point—composition curve tor a mixture giving a minimum melting point. 

Fig. 2-19. ( ) Melting point-composition curve tor a portioliy miscibie mixture giving a eutectic system, [b] Melting point—composition curve for a partially miscible mixture giving a perieutectic system.

Points Tfus, A and 7fus, b represent the melting points of components A and B. The lines starting from the individual melting points represent curves of primary crystallization, i.e. their liquidus curves. Both the liquidus curves meet at the eutectic temperature Te in the eutectic point with the composition Xg. The eutectic temperature is the lowest temperature at which the liquid phase is present in the system. 

A somewhat different method of plotting the results will help the reader to appreciate the significance of the eutectic temperature. In Fig. 1,11, 2 melting points are plotted against composition. The curve AC portrays the decreasing melting point of a-naphthol as naphthalene is added up to a mol fraction of 0 605. The curve BG represents the... 

System in which the solid phases consist of the pure components and the components are completely miscible in the liquid phase. We may now conveniently consider the general case of a system in which the two components A and B are completely miscible in the liquid state and the solid phases consist of the pure components. The equilibrium diagram is shown in Fig. 1,12, 1. Here the points A and B are the melting points of the pure components A and B respectively. If the freezing points of a series of liquid mixtures, varying in composition from pure A to pure B, are determined, the two curves represented by AC and BC will be obtained. The curve AC expresses the compositions of solutions which are in equilibrium, at different temperatures, with the solid component A, and, likewise, the curve BC denotes the compositions... 

The general case of two compounds forming a continuous series of solid solutions may now be considered. The components are completely miscible in the sohd state and also in the hquid state. Three different types of curves are known. The most important is that in which the freezing points (or melting points) of all mixtures lie between the freezing points (or melting points) of the pure components. The equilibrium diagram is shown in Fig. 7, 76, 1. The hquidus curve portrays the composition of the hquid phase in equihbrium with sohd, the composition of... 

H approaches the curve, and at a certain temperature meets it. When this occurs, the hydrate is in equilibrium with one solution of its own composition, and the temperature is the melting-point of the hydrate. 

From these considerations, the general form of the concentration-temperature curve is easily deduced. It is cut in two points by a perpendicular to the T-axis and is bounded on the side of higher temperature by a tangent line also perpendicular to the temperature axis. The abscissa of this point of contact denotes a maximum temperature which is the melting-point of the hydrate, and its ordinate represents the composition. 

We have found it possible to formulate a simple treatment of the lead-thallium alloys that accounts satisfactorily for the existence of a maximum in melting-point displaced from the composition PbTls of the ordered structure, and that also accounts in a reasonably satisfactory way for the shapes of the liquidus and solidus curves throughout the range 0—75 atomic percent thallium (Fig. 1). The maximum in these curves occurs at a composition near that for a compound Pb2Tl3 or a compound PbTl2. If either of these compounds existed, it would have to be considered as forming solid solutions with lead and with thallium. The data, however, give no evidence for the existence of such compounds. 

Copolyesters of poly(3HB-co-3HV) have approximately the same degree of crystallinity as the homopolymer PHB and all copolymers show similar conformation characteristics as those observed for PHB [24,26,65]. They show a minimum in their melting point versus composition curve at a 3HV content of approximately 40 mol%. The apparent ability of the two different monomeric units to cocrystallize might result from the fact that the copolymers are prone to show isodimorphic behavior [21, 26, 66-70]. However, the considerable reduction of the heat of fusion upon 3HV inclusion, as reported by Bluhm et al. 

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