Melting of Crystals

When the temperature of a polymeric crystal is raised above the glass transition temperature, it can begin to melt. This process is the reverse of crystallization. 

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Tg can be determined by studying the temperature dependence of a number of physical properties such as specific volume, refractive index, specific heat, etc. First-order transitions, such as the melting of crystals, give rise to an abrupt change or discontinuity in these properties. However, when a polymeric material undergoes a second-order transition, it is not the primary property (the volume), but its first derivative with respect to temperature, (the coefficient of expansion), which becomes discontinuous. This difference between a first and second-order transition is illustrated in Figure 10. 

The melting of a crystalline-amorphous block copolymer of poly(tetrahydro-furan)-poly(isoprene) (PTHF-PI) was investigated using DSC by Ishikawa et al. (1991). They found a double melting peak, which was proposed to result from the semicrystalline structure of the crystalline PTHF layer, with less-ordered crystallites melting before those with well-ordered domains of chain-folded PTHF. Alternative explanations include fractionation of the polydisperse block copolymer or melting of crystals with different fold lengths. 

The peak temperature of the major endotherm (Tm) and the total enthalpy for the melting of crystals of artemisinin 1 were reported <1997P1209>. 

Entropy Compensation as a Competition Between Dynamics and Bonding The Relevance to Melting of Crystals and Biological Aggregates. 

Then as the temperature increases, pure A melts between and T. is the temperature at the end of the melting. For the corresponding DSC curve, an endotherm at the eutectic temperature is observed, then the melting of crystals A occurs. The effect of impurity on the DSC curve is a melting depression and a broadening of the melting curve (Fig. 13). 

Coupland and McClements further elaborate food emulsions in Chapter 10, reviewing the basic theory behind the propagation of ultrasound in emulsified systems and the mechanisms behind the thermal and viseo-inertial losses. The pros and cons of different experimental techniques are also reviewed. Crystallization (formation and melting of crystals) and influence of droplet eoneentration (individual droplets and floes), as well as droplet size and droplet eharge, are all parameters diseussed by the authors. 

The heat capacities of liquids are much more difficult to understand. The motion involves now also large-amplitude rotations and translations. Since, however, in the liquid state polymers are usually in equilibrium, measurements are more reproducible, as is shown in Fig. 2.58 using the example of poly(oxymethylene), POM. The graph is a direct copy of 36 runs of differently treated poly(oxymethylenes). The almost vertical approach of some curves to the liquid Cp is caused by the end of melting of crystallized samples. All curves superimpose when the samples are liquid. 

In this Chap. 5 of the book on Thermal Analysis of Materials, the link between microscopic and macroscopic descriptions of crystals is given in Sects. 5.1-3. This is followed by a thermodynamic analysis of melting of crystals and isotropization of mesophases in terms of entropy and enthalpy in Sects. 5.4 and 5.5. The final section deals with the properties of liquids and glasses (Sect. 5.6). 

Melting Temperatures and Entropies of Melting of Crystals with Spherical Motifs... 

The crystallization of liquids or melting of crystals is an important phase transition. Melting temperatures are defined as shown in Figure 4.78 (.see Sections 4.2.1 and 4.2.3). 

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