Melting irreversible

Figure 3 Tetramolecular quadruplex dissociation, (a) Example of an irreversible melting curve (TG4T) in 0.11 M Na recorded at 245 nm with a temperature gradient of 0.2°C min Directions of temperature changes are indicated by arrows. The small difference observed at high temperature between the heating and cooling profiles results from a partial evaporation of the sample, (b) Arrhenius representation of the dissociation rate (In (k ff) shown on the left Y-scale) and lifetime (right Y-scale) of the TG4T (DNA) and IIG4U (RNA) quadruplexes in 0.11 M Na" (ref 27)...

A simple analysis of an irreversible first-order transition is the cold crystallization, defined in Sect 3.5.5. For polymers, crystallization on heating from the glassy state may be so far from equilibrium that the temperature modulation will have little effect on its rate, as seen in Fig. 4.122. The modeling of the measurement of heat capacity in the presence of large, irreversible heat flows in Fig. 4.102, and irreversible melting in Figs. 3.89 and 4.123, document this capability of TMDSC to separate irreversible and reversible effects. Little needs to be added to this important application. 

Toda A, Tomita C, Hikosaka M (1998) Temperature Modulated DSC of Irreversible Melting of Nylon 6 Crystals. J Thermal Analysis 54 623-635. 

The discussion of metastable, semicrystalline phases of polymers and their irreversible melting is based on the two early papers Wunderlich B (1964) A Thermodynamic Description of the Defect Solid State of Linear High Polymers. Polymer 5 125-134 and The Melting of Defect Polymer Crystals. Polymer 5 611-624. A later review and expansion is given in Wunderlich B (1997) Metastable Mesophases. Macromol Symp 113 51-65. 

To estimate the ratio of the reversing to irreversible melting at a chosen temperature, the specific reversibility, TMDSC results were compared to standard... 

Poly(ethylene oxide) of high molar mass behaves similarly to the PET and PTT and other polymers analysed, although special effects are seen for many analysed polymers [78,82]. Figure 4.80 represents an example of PEO of a molar mass of 35,000 Da. As before, at low temperature, standard DSC and quasi-isothermal MTDSC give the same result. Most of the melting is irreversible and shows only in the total apparent heat capacity. A small amount, however, is reversing. The irreversible melting occurs at a temperature expected for 4 folds per molecule [52]. 

Although irreversible melting is commonly associated with oriented crystalline polymers, the possibility of conducting the transformation under reversible conditions that approach equilibrium cannot be disregarded. In fact, the treatment of this problem as one of phase equilibria lead to important relations between crystallization, deformation, and dimensional changes.(3,4)... 

Before further discussion, it is necessary to develop the equations for irreversible melting. An introduction to irreversible thermodynamics is given in Sect. 2.1.2. In the present discussion, equations are derived for the special case of fusion of small, lamellar crystals. This crystal morphology is frequently... 

On cooling a typical copolymer melt, one observes, after the customary supercooling, crystallization of pure A. The melt must thus increase to some degree in concentration B as predicted by the liquidus line of Fig. 4.23. But in copolymer systems, one neither reaches the liquidus concentration, nor observes the eutectic point. The system freezes to a metastable state before the eutectic temperature is reached. Usually only one component crystallizes in random copolymers. All of the component B and a large fraction of A remain in the amorphous portion of the semicrystalline sample. For a more extensive discussion of the irreversible melting of homopolymers and copolymers see Ref. 57, Chapters IX and X. 

The inevitable drawback of this implementation of DNP is the irreversible melting and dissolution of the sample, which limits the subsequent acquisition of NMR spectra to the life-time of the nuclear spins, i.e., the individual longitudinal relaxation times. While this seems to represent a major drawback, the huge enhancements observed for D-DNP may well compensate the loss of signal averaging, especially considering the availability of fast multidimensional acquisition schemes. 

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