Lamella, melting

If thin hexagonal lamellae are to be stable within the orthorhombic-stable region of the phase diagram, it is necessary, for the same considerations advanced above, that an orthorhombic lamella melts at a lower temperature than a hexagonal one of the same thickness, i.e. from Eq. 2... 

Re crystallization is a process in which the initial, rather imperfect, lamellae melt and recrystallize to produce thicker and more perfect lamellae that as a consequence melt at a higher temperature. As a result of this process a double melting behavior may be observed. Recrystallization has been observed for neat polymers and blends. 

The DSC trace also gives information on the range of lamellar crystal perfection, since the thinnest, lowest molecular weight, lamellae melt some 30 °C below the final melting point. If a rapidly cooled polyethylene is subsequently annealed in this temperature range, the lamellae will thicken by a process of partial melting and recrystallisation, and the shape of the DSC trace will change. 

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. 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

Supramolecular structures formed during the crystallization of the melt under a tensile stress have already been described by Keller and Machin25. These authors have proposed a model for the formation of structures of the shish-kebab type according to which crystallization occurs in two stages in the first stage, the application of tensile stress leads to the extension of the molecules and the formation of a nucleus from ECC and the second stage involves epitaxial growth of folded-chain lamellae. 

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

The lower limit of this inequality is the minimum possible thickness lmin of a lamella for a given AF. Equivalently, a crystal of thickness l will be at its melting point when Eq. (2.3) is satisfied as an equality. It is common to express the average thickness as ... 

If 31/1 is small and AF is a known function of temperature, Eq. (2.3) can be used to determine the melting point and surface free energy of a lamella from experimental data. For long chain molecules there are several difficulties in choosing the relevant form for AF, Therefore we first consider the limiting case... 

The upper limit of this inequality gives the melting point of the lamella, Tm(l). Equations (2.11) and (2.12) are displayed graphically in Fig. 2.3. 

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