Poly reversible melting

These data suggest that poly A, poly C, and poly U can exhibit some kind of an ordered configuration in neutral solutions. This structure can be reversibly melted by heating however, the transition is not sharp like that of DNA but occurs over a wide temperature range. To account for... 

A similar comparison of DSC and quasi-isothermal TMDSC for poly(ethylene terephthalate) is given in Fig. 3.92. The reversible melting is substantial and seems to contradict the schematic of melting developed with Fig. 3.76. More will be said about this topic in Chaps. 4—7 in connection with the description of temperature modulation in the melting range. The picture that has developed from this thermal analysis is that the globally metastable, semicrystalline, and flexible polymers in their... 

A similar analysis of a melt-crystallized poly(ethylene terephthalate), PET, of the typical molecular mass of a polyester showed a surprising reversing melting peak, as seen in Fig. 3.92. On comparison with an amorphous PET, one finds that the reversing peak depends on crystallization history, as is shown in Fig. 4.136. The change of the glass transition with crystallization is typical for polymers. It shows a... 

In this section the broad spectrum of melting of one-component macromolecular systems is described by means of several specific polymers. The description starts with polyethylene, the most analyzed polymer. It continues with two sections that present several special effects seen in the thermal analysis of polymers including some examples of detailed analyses by TMDSC, documenting the locally reversible melting and crystallization equilibrium within a globally metastable structure. Then illustrations of poly(oxymethylene) and PEEK are given as typical common polymeric materials. This is followed in the last section with the discussion of special effects seen in drawn polymers, as are commonly found in fibers and films. 

Poly(oxyethylene). The reversing melting of a poorly crystallized oUgomer of poly(oxyethylene) with mass 1500 Da is analyzed in Fig. 6.35 with quasi-isothermal TMDSC. A comparison of this sample with a standard DSC trace is shown in... 

Note that this decrease in reversing melting is similar to the behavior of poly(e-capro-lactone) in Fig. 6.58, and contrary to the behavior of PEN seen in Fig. 6.51. 

Di Lorenzo ML, Pyda M, Wunderlich B (2001) Reversible Melting in Nanophase-separated Poly(oligoamide-afe-oligoether)s and its Dependence on Sequence Length, Crystal Perfection, and Molecular Mobility. J Polymer Sci, Part B Polymer Phys 39 2969-2981. 

Figure 2.106. Reversible melting of poly(ethylene terephthalate) as recorded in a quasiisothermal step heating experiment. Comparisons can be made to a conventional DSC run performed at 10 °C/min. For the quasi-isothermal run, the conditions used were a period of 60s, an ampUtude of 1°C, and a sample mass of 5mg. [From Wunderlich et al. (1998) reprinted with permission from Elsevier Ltd.]...

Di Lorenzo M L, Pyda M and Wunderlich B (2001) Reversible melting in nanophase-separated poly(oligoamide-alt-oligother)s and its dependence on sequence length, crystal perfection and molecular mobility, J Polym Sci Polym Phys Ed 39 2969-2981. 

Certain organic compounds form reversible gels with poly(vinyl alcohol). Congo red, for example, yields a red gel that melts sharply at about 40°C. Other organic compounds that form temperature-reversible complexes with PVA include a2o dyes, resorcinol, catechol, and gaUic acid (168—170). 

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