The triple melting behavior of poly(ethylene terephthalate): Molecular weight effects

Abstract

The melting behavior of isothermally crystallized PET has been studied using linear heating in a differential scanning calorimeter (DSC). Variables such as crystallization temperature, crystallization time, heating rate, and average molecular weight are the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET is a function of programmable DSC variables such as crystallization temperature, crystallization time, and heating rate. However, in testing the hypothesis that there was a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC‐independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involves secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1757–1774, 1997

Crystallization and melting behavior in syndiotactic polypropylene: Origin of multiple melting phenomenon

“…In fact, various investigators reported similar observations on a number of semicrystalline polymers, including some flexible polymers: polyethylene, 11,12 isotactic polypropylene (i-PP), 13,14 trans-1,4-polyisoprene, 15 and poly(butylene succinate) 16 ; and some semistiff polymers: aliphatic polyamides, [17][18][19] isotactic polystyrene (i-PS), 20 syndiotactic polystyrene and its blends, 21 poly-(ethylene terephthalate), [22][23][24][25][26][27] poly(butylene terephthalate), 28 -31 poly(phenylene sulfide), 32,33 and poly(aryl ether ether ketones). 34 -50 A number of hypotheses were proposed to explain the occurrence of multiple melting endotherms.…”

“…Figure 5 illustrates some representative DSC melting thermograms of s-PP (recorded at 20°C min Ϫ1 ) after isothermal crystallization at a T c of 75°C for 1.0, 1.5, 1.7, 2.0, 2.5, and 8.0 min. Figure 6 shows DSC melting thermograms (recorded at 20°C min Ϫ1 ) after isothermal crystallization at a T c of 95°C for 15,20,25,30,40, and 50 min. At a T c of 75°C, a time interval of at least 0.5 min was required for a melting peak to be observed in the subsequent melting endotherm (not shown) whereas it was at least 5 min in the case of crystallization at a T c of 95°C.…”

Recrystallization studies on isotropic cold-crystallized PET: Influence of heating rate

“…The intensity distribution scans as a function of s were obtained from the azimuthal integration of the two-dimensional SAXS patterns over the angular range 89.5e90. 5 . Calibration against a rat tendon tail standard was accomplished.…”

Influence of the lignin on thermal degradation and melting behaviour of poly(ethylene terephthalate) based composites

“…PET exhibits multiple melting endotherms depending on its thermal history, as revealed by DSC investigation [16,17,19,21]. It can be observed that after the first annealing step both samples exhibit a single endothermic event.…”

“…Opposite interpretations have been reported about PET structural organization and thermal behaviour [12][13][14][15][16][17][18][19][20][21][22].…”