Multiple melting behaviour of poly(ethylene terephthalate)
Introduction
The melting of polymers is different from that of low molar mass materials in that melting generally occurs over a wider temperature range and is greatly dependent on sample thermal history. The presence of multiple melting endotherms as observed by differential scanning calorimetry (DSC) is very common and has been observed with many semi-crystalline polymers, copolymers and blends [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The phenomenon has been extensively studied but conflicting interpretations have been made. A variety of effects have been invoked to explain the phenomenon, i.e. to the presence of more than one crystallographic forms (polymorphism); to the presence of melting/re-crystallisation and re-melting; to changes in morphology, such as lamella thickening and crystal perfecting; to changes in orientation; and to the effect of molecular weight distribution [1], [24], [25]. It is more likely that one of these mechanisms alone cannot explain the observation of multiple endotherms.
PET is an important engineering polymer, whose properties are markedly dependent on the degree and quality of crystallinity. PET crystallises over a wide temperature range, and samples crystallised to the same degree of crystallinity at different temperatures have different melting characteristics. These samples frequently exhibit multiple melting endotherms, which depend on the thermal history. Bell et al. [12], [26] using differential thermal analysis (DTA), birefringence and dynamic mechanical measurements observed two endotherms and proposed that the lower one represented the melting of imperfect or smaller crystals with partially extended chains, while the higher endotherm was associated with the melting of folded chain crystals. They considered that the folded chain crystals were kinetically preferred and the more extended crystals were thermodynamically preferred [12]. On annealing during heating the folded chain crystals converted to extended chain crystals. On the other hand, Robert [27] attributed the lower melting endotherm to the melting of folded chain crystals and the higher one to bundle-like crystals. Both conclusions were based on the assumption that melting endotherms were directly related to the structures that developed on crystallisation, and the effect of heating scan was not considered. Holdsworth and Turner-Jones later [28] suggested that partial melting and recrystallisation took place during heating to the melt, and observed two endotherms. The lower temperature one was due to the melting of crystals formed at the crystallisation temperature Tc and the other at higher temperature to the melting of crystals produced by annealing on heating. The DSC measured endotherms did not directly reflect the structures produced during isothermal crystallisation. Groenickx et al. [29] came to similar conclusions but they attributed annealing to crystal perfection or lamella thickening on heating.
Zhou and Clough [30] were the first to report three melting endotherms in the melting of PET although others have observed it earlier in other materials [31], [32]. They labelled the endotherms I–III in order of increasing melting point. Endotherm I, which normally appeared about 10 K above the crystallisation temperature, was attributed to the melting of the crystals formed during secondary crystallisation, endotherm II to the melting of the crystals formed during primary crystallisation stage and endotherm III to those formed as a result of recrystallisation on heating. This view has also been accepted by others [23], [33], [34].
More recently, Medellin-Rodriguez et al. [25], [35] have studied the melting behaviour of PET by using DSC, polarised light microscopy and SAXS. They found that melting was the morphological reverse of crystallisation with respect to the primary and secondary structures produced. On the basis of the branching lamella model for spherulites, they suggested that spherulites consisted of dominant and subsidiary branches. The subsidiary ones were formed later in the crystallisation and between the dominant branches. Endotherm I was due to the melting of small metastable branches, endotherm II to the melting of main metastable branches, and endotherm III was associated with dominant branches, which underwent some recrystallisation on heating. This is obviously different from the view of Zhou and Clough [30].
MTDSC, in which a small sinusoidal perturbation is superimposed on the conventional linear heating programme, is a useful thermal analysis technique for separating reversing and non-reversing thermal events [36], [37], [38]. The temperature and heat flow profiles can be deconvoluted by performing a Fourier transform analysis [39], [40], [41] from the rate of change of temperature and heat flow data. This separated the total heat flow, which is equivalent to that obtained by conventional DSC, into reversing and non-reversing components. The non-reversing component is kinetic in nature and can be attributed to non-reversing melting and crystallisation on heating. Using a quasi-isothermal model, Okazaki and Wunderlich [42] found that locally reversible melting and non-reversing crystallisation existed on heating PET to the melting point largely due to molecular nucleation.
Conventional DSC and MTDSC have been used to study the melting behaviour of PET in this paper in an attempt to resolve some of these apparent differences in the assignment of the multiple endotherms.
Section snippets
Experimental
Poly(ethylene terephthalate) (PET) was obtained from DuPont, USA, as moulding pellets. It has a number average molecular weight of 19.6 kg mol−1 and weight average molecular weight of 36.4 kg mol−1. The PET was dried in vacuum at 373 K for 12 h. prior to moulding into 100×100×1.0 mm3 plaques at 553 K (2 min at a pressure of 7.5 MN m−2). The plaques were quenched into ice/water to obtain amorphous samples, as determined by DSC and wide angle X-ray diffraction (WAXD). The same procedure was used to prepare
The melting behaviour of bulk PET samples
The DSC response of PET samples crystallised isothermally at different temperatures is shown in Fig. 1. To prevent further crystallisation occurring on cooling, the samples were heated from the crystallisation temperature to the temperature of the last trace of crystallinity. Three melting endotherms were observed, namely I–III with increasing temperature, respectively, in samples crystallised at temperatures from 448 to 473 K. In samples crystallised below 448 K two endotherms were present, and
Conclusions
The melting characteristics of PET samples are very complex depending on the experimental conditions chosen for the measurements, isothermal temperature or non-isothermal crystallisation conditions, thermal history and heating rate. It is confirmed that when two endotherms, I and II, are present that these are due to the presence of a dual lamella thickness distribution produced during crystallisation. Three endotherms have also been observed in the samples crystallised at intermediate
Acknowledgements
One of the authors, Y. Kong, acknowledges the award of a research scholarship from ORS Committee and the Department of Metallurgy and Materials of the University of Birmingham during the tenure of this work. The authors would like to express their appreciation to Mr F. Biddlestone for his technical support and assistance.
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