Elsevier

Polymer

Volume 44, Issue 8, April 2003, Pages 2509-2518
Polymer

Time-resolved X-ray scattering and calorimetric studies on the crystallization behaviors of poly(ethylene terephthalate) (PET) and its copolymers containing isophthalate units

https://doi.org/10.1016/S0032-3861(03)00130-7Get rights and content

Abstract

Time-resolved small-angle X-ray scattering (SAXS) measurements were carried out for PET and its copolymers undergoing isothermal crystallization. Wide-angle X-ray diffraction and differential scanning calorimetric measurements were also performed. Our data analysis of the SAXS results for PET and the copolymers clearly demonstrate that the one layer thickness l1 (derived directly from the correlation functions of the measured SAXS profiles) is the lamellar crystal thickness dc, not the amorphous layer thickness da. The observed dc values are found to be always smaller than da, regardless of polymer composition. dc is highly dependent on the crystallization temperature, showing that the degree of supercooling is the major factor determining the thickness of lamellar crystals. No thickening, however, occurs in isothermal crystallizations. The kinked isophthalate units in the copolymer are found to be mostly excluded from the lamellar crystals during the crystallization process, leading to an increase of the amorphous layer thickness. Moreover, the kinked, rigid nature of the isophthalate unit was found to restrict crystal growth along the chain axis of the copolymers and also to lower their crystallinity. Unlike dc, da decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in da is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the da distribution in the lamellar stacks and its variation with time, the number of lamellae in the lamellar stacks and their effect on the SAXS profile, and the relaxation of polymer chains in the amorphous layers.

Introduction

The physical properties of a polymer depend primarily on its morphological structure. However, the morphological structures of polymers are generally very complicated, because of their long chain lengths and many possible conformations. The morphological structure is thus sensitive to the conditions under which the polymer is processed, for example the quenching regime used in its crystallization. The ultimate properties of a polymeric system, therefore, correlate directly with the manner in which it is processed. The relationship between processing conditions, morphological structure and polymer properties has been extensively studied over the past few decades.

Research into semi-crystalline polymers has utilized their crystallization behavior as the key to understanding their morphological structures. A typical semi-crystalline polymer is poly(ethylene terephthalate) (PET), which is widely used as an engineering plastic material. The crystallization of PET has been extensively investigated by various analytical techniques [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. In particular, the small-angle X-ray scattering (SAXS) method has been widely employed to investigate the morphological structure of PET crystallized from melt [1], [8], [9], [10], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. It was concluded from these SAXS studies that the long period always decreases with increasing crystallization time [1], [8], [9], [10], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Similar SAXS results have been reported for poly(ether ether ketone)s [1], [19], [20], [21], [22], [23], [24], [25] and poly(aryl ether ether ketone)s [26]. These SAXS results are quite different from those observed for other common semi-crystalline polymers such as polyethylene, which undergo lamella thickening [32], [33]. Much research effort has been put into explaining these differences, resulting in the proposal of several models of morphological structure that have been widely debated [1], [8], [9], [10], [18], [19], [20], [21], [22], [23], [24], [25], [27], [28], [29], [30], [31]. The conflicts between these models have mainly arisen over different interpretations of the correlation functions calculated from the observed SAXS patterns. More research into the data analysis of SAXS studies of the crystallization of PET is needed in order to determine the morphological parameters correctly.

Unlike PET, poly(ethylene isophthalate-co-terephthalate) (PEIT), which is a copolymer based on PET, has been rarely studied [7], [15], [16], [17]. Hachiboshi et al. [7] investigated the crystallization of PEIT fiber specimens using SAXS and proposed that kinked isophthalate (IPT) units include into the lamellar crystals. However, they studied the crystallization of fiber-formed PEIT specimens rather than the crystallization of PEIT from melt. In fact, in the fiber formation process the polymer chains are preferentially aligned along the fiber drawing direction, moving favorably into lateral ordering, so kinked IPT units are more likely to be included into the resultant crystals. Such molecular crystallization in drawn fibers may be significantly different from crystallization from melt. The question remains whether kinked IPT units in PEIT include into or exclude from crystals formed by crystallization from melt.

To address the main two questions outlined above, in the present work we synthesized PET and PEIT copolymers containing 4.9–9.8 mol% IPT units and studied their crystallization. We conducted time-resolved SAXS measurements during the isothermal crystallization of PET samples and extended this procedure to the isothermal crystallization of PEIT copolymers. In addition, wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) measurements were performed on these samples.

Section snippets

Polymerization and sample preparation

PET was synthesized from ethylene glycol and dimethyl terephthalate by bulk polycondensation, as described elsewhere [15], [16], [17]. Using the same synthetic method, poly(ethylene isophthalate-co-terephthalate) copolymers with various compositions were prepared from ethylene glycol, dimethyl terephthalate, and dimethyl isophthalate (see Fig. 1) [7]. From the PET homopolymer and its copolymers, three samples rich in ethylene terephthalate units were chosen for study. These samples, referred to

SAXS analysis

PET, 5IPT and 10IPT have been previously determined to have equilibrium melting temperatures Tm0 of 275.4, 266.5 and 261.9 °C, respectively [15], [16], [17]. In addition, both copolymers have been determined to be random copolymers [15], [16], [17]. For PET, 5IPT and 10IPT, time-resolved SAXS measurements were conducted during their isothermal crystallizations from the melt over a temperature range of 170–240 °C. Fig. 2 shows typical SAXS patterns for PET and 5IPT polymers undergoing isothermal

Conclusion

The determination of morphological parameters, in particular the lamellar crystal and amorphous layer thicknesses (dc and da) in crystallized PET polymer and its copolymers, was successfully carried out by one-dimensional correlation function analysis of the SAXS profiles measured during crystallization. The comprehensive analysis in this study clearly demonstrated that the one layer thickness l1 derived from the correlation function analysis is the lamellar crystal thickness dc rather than the

Acknowledgements

This study was supported by the Korean Ministry of Science and Technology (MOST) (KISTEP—Basic Research Grant of Nuclear Energy). The synchrotron SAXS measurements were supported by MOST and POSCO.

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