Melting region

Irreversible transformations, from more disordered toward more ordered modifications of a given form have been, for instance, observed for the a form of i-PP [39,40,43,44] (see Sect. 2.4), as well as for the most common form of i-PS [74], In these cases the transitions occur by recrystallization processes in the respective melting regions. 

Toomey DR, Wileoek WSD, Conder JA, Forsyth DW, Blundy JD, Parmentier EM, Hanunond WC () Asynunetrie mantle dynamics in the MELT region of the East Pacific Rise. Earth Planet Sci Lett 200 287-295... 

Toomey DR, Wilcock WSD, Solomon SC, Hammond WC, Orcntt JA (1998) Mantle seismic structure beneath the MELT region of the East Pacific Rise from P and S wave tomography. Science 280 1224-1227... 

With decreasing temperature, the density oscillation becomes very pronounced and grows into a deeper melt region. At 300 K, for example, we can see at least 5 layers after 1.28 ns. Within the layers, as will be shown later, definite order in chain orientation and chain packing is observed suggesting the growth of polymer crystals. 

Proton NMR relaxation parameters have also been determined for polyethylene ( ) and polyethylene oxide (39) in the melting region. The apparent contradiction between the proton spin-lattice relaxation parameter for a high molecular weight linear polyethylene sample at its melting point, with the relaxation measurements, has previously been pointed out. (17) This discrepancy is still maintained with the more detailed results reported here for both types of polyethylene. For the proton relaxation a small, but distinct, discontinuity is reported at the melting teirperature. (38)... 

The diagram shows the homogeneous liquid (melt) region (with/= 2) as well as the three distinct two-phase regions (each with/= 1). The junction of these regions is the eutectic point (triangle), a triple point (with/= 0). 

One possible reason suggested by Flory and Mandelkern (1956) for or contribution to the discrepancies indicated in Table 2 is the inability of the penetrometer method to detect the maximum temperature of melting because of its relative insensitivity and because of the upper "tail to the crystallinity-temperature curve as deduced from the copolymer crystallinity theory of Flory (1955). Dole and Wunderlich (1957, 1959) found, however, that two melting regions each of the quenched 80/20 and 60/40 copolymers could be observed in the calorimetric experiments. The specific heat curves of these two copolyesters are illustrated in... 

Figure 8 Effect of warming rate on the DSC thermograms of glycine/water in the melting region of ice.

Indeed, over the last decade, the area of melting of active compacted particulate assemblies in twin-rotor equipment has received a good deal of experimental attention. This body of experimental work utilizes both glass windows on sections of the barrel for on-line observations f39—43) and, more often, extracted solidified carcasses of the processed stream, which are sectioned along the downstream direction in the melting region (3,44-50). This body of work has confirmed the existence, and elucidated the natures of PED, FED, and DMM and, most importantly, has confirmed the evolution of melting in twin-rotor devices mentioned earlier. Such evolution, based on extensive carcass analyses for both polypropylene (PP) pellets and powder feeds in Co-TSEs, is shown in Fig. 5.14 (3,51). 

The screw is placed within a barrel of diameter Df, Ds I 25f, where 6y is the radial flight clearance. This is shown schematically in Fig. 9.2. The figure shows a pelletizing extruder, but the discussion that follows is valid for any melt extruder equipped with any kind of die, and for the melt region in a plasticating extruder as well. 

Disturbances in a crystal can be brought about by the above mentioned or by other causes. These disturbances are arbitrarily distributed over the crystallites, so that a spread in AH occurs, resulting in a spread in Tm, a melting region, which has, as an upper limit, the melting point of the best crystal. 

The melting point is also somewhat dependent on the crystal size, in particular, when the crystals are very small, Tm is lower. This is, i.a., caused by the restrictions in chain conformations near the crystal surface. A spread in crystal size, therefore, also contributes to the presence of a melting region. This is clearly demonstrated by the (new) PE grades with very low density (copolymers of ethylene with e.g. 1-octene) the crystallites are so small that for a grade with d =.86 the melting point is only 30 °C ... 

A partially crystalline polymer could follow curve c at Tm the amorphous part passes into a rubber, the crystalline part is unaffected. The actual curve resembles more curve d, i.a. as a result of a broad melting region. 

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