Crystallisation and melting

The previous sections of this chapter have eoneentrated on deseribing the morphological features to be found in solid polymers and, in partieular, the predominance in crystalline polymers of the lamellar erystallite they have concentrated on what is observed. The present seetion deals with why some of the observed features are as they are and at what rate they appear. This involves a discussion of the melting temperature, the factors that determine the overall crystallinity achievable and the rate of 

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Di-m-tolyl methyl tellurium iodide,3 (C7H7)2(CH3)TeI, is obtained in quantitative yield from its components. It crystallises in four-sided columns, soluble in chloroform, insoluble in ether. It melts at 121° to 122° C.s splitting up again into its components. When boiled with water and silver oxide and the solution treated with picric acid, a picrate is formed this separates from alcohol with five molecules of solvent of crystallisation, and melts at 114° to 115° C. The platinichloride melts with decomposition at 154° to 155° C. 

Chromium di - p - bromotetraphenylene hydroxide, [Br(C6H4)4]2CrOH, appears to be present in the residues insoluble in alcohol and chloroform from the two preceding preparations. It is isolated with one molecule of carbon disulphide of crystallisation, and melts between 220° and 260° C., dissolving in carbon disulphide or benzene, insoluble in ether or alcohol. 

Wide-line and 2H NMR spectra and T2 relaxation experiments have been used to determine the composition of the phases in semi-crystalline polymers [133, 136, 138-144]. The experiments were also used to obtain real-time information on the kinetics of crystallisation and melting [143-148]. The use of high-resolution NMR methods to characterise semi-crystalline polymers is reviewed elsewhere [17, 18, 30, 34, 149]. 

A common mistake about butter is the assumption that because it appears to be a solid then the fat must be crystalline. In fact, to crystallise completely all of the fat in butter it must be stored at — 40 °C. [This should be compared with the normal temperature of a deep freeze at — 20 to — 18 °C (ca. 0 °F).] As butter does not become completely liquid until 38-40 °C this is an extremely wide crystallisation and melting range. 

The crystallisation and melting behaviour of palm stearin depends on the composition. Figures 3.6 and 3.7 show the behaviour of different palm stearins. PMF shows crystallisation exotherms which overlap into several peaks, while its melting thermogram shows one main endotherm with a shoulder, finally melting... 

Methylphenylaminophenyldichlorophosphine, (CH3)(CgH5)N. CgH4.PCl2, separates as a viscous, transparent oil, which cannot be distilled without decomposition even in vacuo. Sodium hydroxide converts it into the sodium salt of the phosphinous acid. This salt crystalhses in plates containing two molecules of water of crystallisation and melting at 265° C. it readily dissolves in water, is less soluble in alcohol and insoluble in chloroform or benzene. Hydrochloric acid transforms the sodium salt to the free phosphinous acid, which separates from water in small needles and from alcohol in pearly plates, M.pt. 150-5° C., easily soluble in benzene or chloroform. ... 

Figure 4.3. A schematic of the free enthalpy in the vicinity of the equilibrium melting temperature (left) and a plot of the linear crystallisation and melting rates of gaseous selenium (Sc2) (right). Selenium crystallizes or sublimes to and from selenium crystals made up of flexible, linear macromolecules. The process can be expressed as xSc2 (gaseous) Se2x...

Figure 4.5. Plot of the linear crystallisation and melting rates of polymers, oligomers and monomers. The shaded line indicates the shift that is possible for the melting of defect crystals.

The kinetics of processes with a latent heat, such as crystallisation and melting, can be measured either directly by isothermal or non-isothermal measurement of the latent heat or by observation of the change of the heat... 

In the framework of the thermal effects enumerated in Section 2.5, the increased reversible heat flow rate is due to the latent heat effect (3), which maybe caused by either isolated crystals, which crystallise and melt in a local equilibrium set-up within the network of primary and secondary crystals (5,6) after their rearrangement has ceased (4), or by reversible crystallisation and melting on the lateral surface areas of the crystals (5,6). 

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