Reversible Melting in Polymer Crystals Detected by Temperature-Modulated Differential Scanning Calorimetry

Abstract

A small amount of locally reversible melting and crystallization in poly(ethylene terephthalate) (PET) has been detected by temperature-modulated differential scanning calorimetry (TMDSC). Extended-time TMDSC was used in the quasi-isothermal mode. Studied were melt-crystallized, quenched, and a biaxially-oriented film of PET in temperature steps of 2 K from 320 to 560 K. The integral of the endothermic and exothermic latent heat contributions to the reversible melting and crystallization is less than 10% of the total heat of fusion and decreases further with time over many hours. The new observations support the concept that “molecular nucleation” is the reversible and rate-determining step in crystallization. On TMDSC, partially-melted chains remain on the surface of higher-melting crystals to permit crystallization during the cooling cycle without supercooling.

The influence of molecular weight on the properties of polyacetal/hydroxyapatite nanocomposites. Part 1. Microstructural analysis and phase transition studies

“…Poorer crystallized molecules have higher reversing melting contributions and perfect crystals show none. The origin of this reversing melting was linked to a small amount of melted polymer that could not be extracted with solvents [21].…”

Crystallization and melting of poly(oxyethylene) analyzed by temperature-modulated calorimetry

“…The result is shown in Figure 9. Its crystal- The earlier observed, also small, amount of rethe characteristics of the standard DSC of Figure 4 and the difference between Figures 6 and 8 from versing melting on higher molar mass PEO 18 and PET 5,6 mentioned in the Introduction is, however, Figure 9, and finally the disappearance of the effect on more perfect crystallization 18 make it quite largely different from the here-discussed case of low molar mass PEO. It is not a local equilibrium, certain that reversing crystallization and melting of poor crystals is at the root of the observed effect.…”

“…The first observations of temperature-modulated calorimetry (TMC) of poly(ethylene terephthalate) these initial studies of the melting and crystallization of polymers with TMDSC two special effects were observed. 5,6,18 One was a small amount of locally reversible melting of molecules that were left incompletely melted at the temperature of maximum oscillation 6,18 and decreased in amount over time periods of many hours. 5 The other occurred at the low-temperature end of the melting peak and was an increasing amount of reversing crystallization and melting that seemingly reached a steady state.…”

Reversible melting of polyethylene extended‐chain crystals detected by temperature‐modulated calorimetry

“…64,65 Assuming that reversible recrystallization can occur only for partially melted molecules, with the remaining crystalline portion able to perform the function of a molecular nucleus, a decrease in reversible melting is expected for PE-ECC melting at higher temperature. Once fully melted, a molecule can recrystallize only much below the modulation range of the temperature because a new molecular nucleus must be formed.…”