Crystalline phase, melting temperature

Analysis of the thermodynamic properties of crosslinked polymer networks shows the crystalline phase melting temperature rising with growth in the tension extent [27,46, 50]. The results stated in the present chapter suppose that this effect can be... 

Eig. 15. Time—temperature transformation ia a thin-phase change layer during recording/reading/erasiug (3,105). C = Crystalline phase A = amorphous phase = melting temperature = glass-transition temperature RT = room temperature. 

Nakazawa et al. (1984) argued that when starch-water mixtures (30-50% starch) are held at a certain temperature (55-80 °C), for a certain period (0-45 h), and depending on the time-temperature combination, starch granules increase their amorphous portion and decrease their crystalline portion. These amorphous and crystalline phases melted sequentially during DSC phase transition experiments. Their experiments... 

Figure 12 shows the temperature variation of the in-phase Young s modulus E of an EVA foam (Verdejo, unpublished). E falls as the amorphous phase goes through a glass transition close to -10 °C, then again as the crystalline phase melts at 80 °C. The peaks in tan S at these temperatures are labelled a and p. 

A low melting point is preferable in order to avoid metastable, monotropic liquid crystalline phases. Low-temperature mesoporphic behaviour in general is technologically more useful, and alkyl terminal groups promote this. 

Figure 41.1 shows the gel-to-liquid crystalline phase transition temperatures (Tm) of DPPC-cholesterol mixtures as a function of the cholesterol-lipid molar ratio. The Tm of fully hydrated DPPC is 42°C (Crowe and Crowe, 1988 Vist and Davis, 1990 McMullen et al., 1993 Ohtake et al., 2004). Upon the addition of cholesterol, the transition enthalpy decreases continuously imtil it is no longer observable at 50 mol% cholesterol. The disappearance of the melting transition has been attributed to strong interactions between cholesterol and DPPC (McCoimell, 2003). Upon dehydration, the Tm for DPPC increases from 42 to 105°C (Crowe and Crowe, 1988 Ohtake et al., 2004). This Tm increase is caused by the reduction in the spacing between the phospholipids, which allows for increased van der Waals interactions between the lipid hydrocarbon chains (Koster et al., 1994). Between 10 and 70 mol% cholesterol, two endothermic transitions are observed, both lower than the Tm of the pure phospholipid (Figure 41.1). High-sensitivity DSC studies on fully hydrated DPPC-cholesterol systems reported endotherms consisting of two components, suggesting the existence of domains enriched/depleted in cholesterol (Vist and Davis, 1990 McMullen et al., 1993). The two peaks present in our freeze-dried systems also suggest the...

Table 1.2 shows the temperature T , at which the crystalline phase melts, or, for non-crystalline polymers, the glass transition temperature Tg at which the glass changes into a melt. Samples can be dragged across the surface of metal hotplates, set to a range of temperatures. However, when the polymer is just above Tm, some polymers leave a streak of melt, while others of higher viscosity just deform. Therefore, transition temperatures can be overestimated. 

PVC is not stable at temperatures of 220-230 °C at which the crystalline phase melts. As the crystallinity of PVC is only of the order of 10%, processing in the semisolid state is not an insuperable problem, but the apparent viscosity is much higher than for most other polymer melts. Tertiary chlorine atoms, which occur at long-chain branches in PVC, are weak points, where the elimination of a hydrogen chloride molecule can occur. 

For the crystalline phase, at temperatures below the melting point. 

In the case of melting of ChMAA-5,6,8,10 monomers, an isotropic liquid is formed at temperatures of 108, 98, 85 and 84 C, respectively. Rapid cooling of these melts results in the formation of liquid crystalline phase whose temperature existence interval depends on the rate of cobling. At slow cooling, either growth of solid crystals (ChMAA-10) or polymerization of monomers (ChMAA-5,6,8) is observed. 

While the sample PTMO-2000-MDI-31-CA displays the existence of both crystalline and melting temperature transitions, no such activity is observed in PTMO-2000-TDI-31-CA. The Tg of the soft segments in TDI-based materials is lower than that of MDI-based materials, suggesting better phase separation in the latter. It appears that the unsymmetric configurations of TDI units cause the the formation of random hard segment structures which inhibit, but not necessarily eliminate, the crystallization of soft segments under identical thermal histories. 

Copolymerization is one of the most efficient synthetic techniques to decrease the crystallinity and melting temperature of a given polymer. The melting-point depression occurring in copolyesters (Vectra) based on p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid (HNA) is in fact relatively moderate in comparison with that of other copolymers (Fig. 6.35). The minimum in melting point occurs at about 40 mol% HNA. The decrease in isotropization temperature in the copolyesters is moderate, leading to the desired expansion of the temperature region of a nematic phase. 

The transition from the solid to liquid crystalline phase (melting) corresponds to the melting of the flexible chains the aromatic cores retain positional and orientational order. The transition from the mesophase to the isotropic liquid (clearing) corresponds to the destruction of the columns. There are two possible means of influencing the transition temperatures extension or branching of the aliphatic chains depresses the transition tem-... 

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