Crystal, Crystallization melting temperature

The second consideration, anticipated in Sect. 3.1, is that whereas Sm and Sc increase little with temperature (see Fig. 8) so that in the literature the temperature dependence of Gc and Gm is often approximated by straight lines, Sl varies moderately up to the glass transition temperature Tg but will increase substantially between Tg and the isotropization temperature. The behavior of Sl is due to the fact that, as the temperature increases, polymer chains are progressively more likely to bend sharply in the melt, whereas they are forced to remain straight in the mesophase and in crystals. The downward curvature of Gl in Fig. 8 shifts to lower temperatures with inherently more flexible chains. With very flexible polymers Tml becomes therefore smaller than the crystal-melting temperature Tcl and a stable mesophase cannot form. 

Thermoplastic elastomers (TPE), 9 565-566, 24 695-720 applications for, 24 709-717 based on block copolymers, 24 697t based on graft copolymers, ionomers, and structures with core-shell morphologies, 24 699 based on hard polymer/elastomer combinations, 24 699t based on silicone rubber blends, 24 700 commercial production of, 24 705-708 economic aspects of, 24 708-709 elastomer phase in, 24 703 glass-transition and crystal melting temperatures of, 24 702t hard phase in, 24 703-704 health and safety factors related to, 24 717-718... 

Fig. 16a and b. Phase diagrams of poly(ethylene terephthalate-co-oxybenzoate). a. Crystal melting temperatures b. Glass transition temperatures. All samples with two glass transitions are most likely two-phase samples. The black circles up to 63 mol- % represent one-phase samples. Filled triangles W. J. Jackson, Jr. and H. F. Kuhfuss, J. Polymer Sci., Polymer Chem. Ed. 14, 2043 (1976). Open triangles R. W. Lenz and K. A. Feichtinger, Polymer Preprints, Am. Chem. Soc. Div. Polymer Chem. 20, 114 (1979). Filled circles Ref. 20). Open circle J. Menczel and B. Wunderlich, J. Polymer Sci, Polymer Phys. Ed., 18, 1433 (1980)... 

Fig. 20. Phase diagram of polyethylene. The difference between folded and extended chain crystal melting temperatures is largely a size effect (1 kbar = 100 MPa). Curve based on data of Ref. I19>...

From a knowledge of the crystal-melting temperature, I m, the glass temperature can be estimated with some degree of conhdence, at least for many of the excipients commonly used for freeze-dried pharmaceutical preparations, where it has been found empirically that Fg/rm 0.73. Furthermore, for the same preparations, the temperature of zero mobility can be approximated by Fm/2. Also, for many materials, ACgTg x 100 kJ g At T , the viscosity of most glasses is assumed to lie between... 

Tm, crystal melting temperature Tg, glass transition temperature. 

Powdered raw material is condensed in molds at a high temperature (190-350°C) and pressure of 10-80 N/mm and sintered, sometimes for hours. Sintering occurs above the crystal melt temperature. The cooling time determines the degree of crystallinity and thus the material properties... 

It was later confirmed by Ide and Chung [119] that these observations were also valid for thermotropics. Small-diameter rods of a longitudinal PLC (a copolyester based on 2,6-hydroxynaphthoic acid, terephthalic acid and p-aminophenol) were annealed above its crystal melting temperature in the nematic state for different periods of time. The dependence of the Young modulus at room temperature on the... 

Differential thermal analysis (DTA) has also been exploited, mainly to determine polymer crystal melting temperatures but also (less frequently) to determine crystallization kinetics. Crystallite formation also changes the optical... 

For thermotropic LCs, the transition to the liquid crystal state is induced by a purely thermal process. Thermotropic LCs form thermally activated mesogenic phases that extend from the crystal melting temperature, Tm, up to the clearing or isotropic temperature, 7]. Thermotropic LCs have found wide application in the electro-optical displays [1,2]. For lyotropic LCs, the transition is induced by the influence of solvents. Lyotropic LCs exhibit their phase transitions through the addition or removal of solvent [1,2]. 

Crystallisable polymers will crystallise at temperatures (crystallisation temperatures T(-) between the crystal melting temperature (T ) and the glass transition temperature (Tg), i.e. 

For polymer blends in which one component is crystalline the melting behaviour depends on circumstances. For immiscible blends, where the components are phase separated (prior to crystallisation) and act independently, the crystal melting temperature will be that of the homopolymer. In miscible blends, where the amorphous phase contains both components, the melting temperature will be lower than the equilibrium melting temperature for the crystallisable homopolymer, i.e. the crystalline polymer exhibits a melting point depression as discussed above. The Nishi and Wang approach (Sect 3.2) has been used to estimate the magnitude of the interaction parameters in a niunber of blends (Sect. 7). Poly(e-caprolactone) blends are often semi-crystalline and the above considerations, therefore, apply to many PCL blends. 

Crystal melting temperatures were also observed to increase with increasing T. for a given blend composition. Hoffman-Weeks type extrapolations gave values of equilibrium melting points (T ) for each composition which decreased with increasing SAN content (Fig. 32). 

For the 50 50 and 25 75 PCL/CAB blends, the latter group found double crystal-melting temperatures which were attributed to two populations of crystals arising from a morphological effect. It was also found that at low CAB (PCL>50 wt %) in as cast samples the Tj and AH for PCL were raised relative to those for pure PCL. This effect (which has been found in two other systems) was attributed to an improvement of crystal quality and increased lamellar thickness relative to those found for pure PCL. (This effect is inconsistent with melting point depression but the components are not miscible in the melt.) The effect, which was not explained, persisted after thermal cycling in blends with low CAB contents (20-30 wt %). 

Blends with copolymers having lower PCL contents were less miscible with PCL. Glass-transition temperatures of quenched blends were almost independent of composition and close to those of the copolymers themselves blend TgS changed by about 10 °C over the composition range 0-90 wt % PCL. Thus, it is seen that PCL was phase-separated from the copolymers at high temperatures. In such blends the crystal-melting temperatures of samples crystallised as above were very close to those of pure PCL and heats of fusion per gram of PCL... 

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