Macromolecules 1997, 30, 1758-1764
Reversible Melting in Polymer Crystals Detected by Temperature-Modulated Differential Scanning Calorimetry
Iwao Okazaki† and Bernhard Wunderlich*
Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1600, and Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6197 ReceivedOctober 17, 1996X 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 560K. 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 thesurface of higher-melting crystals to permit crystallization during the cooling cycle without supercooling.
Introduction Melting and crystallization have been a subject of continued interest in the field of macromolecules. A first extensive discussion can be found already in the treatise of Stuart.1 A summary of the available information up to 1980 can be found in ref 2. Furthermore, unsolvedproblems of melting and crystallization were addressed in two discussion meetings that set milestones of progress. One was sponsored by the Faraday Society3 (Cambridge, 1979) and the other by NATO4 (Mons, 1992). Still, a number of questions concerning the melting and crystallization equilibrium of polymers remain open. Using the nomenclature derived by Ubbelohde,5 the structure of the melt of flexiblemolecules, such as macromolecules, is anticrystalline because of conformational isomerization6 introduced on melting. Crystals of rigid and spherical motifs, in contrast, are often quasi-crystalline in their liquid state, i.e., their structures can be related to the crystal structure and its lattice.5 The anticrystalline structure of the liquid state is at the root of the generally observedirreversibility of polymer melting.2 The research of this paper offers support to the suggestion that there exists reversibility of polymer melting, but only on a molecular or submolecular scale. As soon as a molecule is completely melted, reversibility is lost due to the need for molecular nucleation, a rate-determining step with a free-enthalpy barrier for the beginning of crystallization of amolecule or, at higher supercooling, part of a molecule on the surface of a crystal.2,7,8 Before melting of a molecule is complete, i.e., a part larger or equal to a molecular nucleus remains crystallized, melting and crystallization are shown in this paper to be reversible within a fraction of a kelvin. For the understanding of crystallization of macromolecules, molecular nucleation must be consideredin addition to crystal nucleation. Crystal nucleation is always the first step of any crystallization, but it can be easily avoided experimentally by the addition of nuclei.2 Molecular nucleation, in contrast, can only be affected by changing the molecular conformation in the melt or solution. The studies of this paper are based on temperaturemodulated differential scanning calorimetry (TMDSC),
a new thermal analysis technique.9-11 Specifically, a calorimeter of the heat-flux type, the Modulated DSC of TA Instruments, MDSC, was used to study the melting region of a series of differently crystallized poly(ethylene terephthalate)s (PET). The modulation of the temperature in MDSC consists of a sinusoidal oscillation, superimposed on a constant, underlying heating rate dTb/dt ) 〈q〉...