Orientation and crystallisation mechanisms during fast drawing of poly(ethylene terephthalate) (original) (raw)
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Orientation prior to crystallisation during drawing of poly(ethylene terephthalate
Polymer, 2000
Wide angle X-ray scattering data have been recorded during the drawing of poly(ethylene terephthalate) (PET) using a wide range of draw rates (0.05-12 s Ϫ1 ), temperatures (90-120ЊC) and draw ratios. The data were analysed to follow the development of molecular orientation and the onset of crystallisation. The molecular orientation prior to crystallisation has been characterised in terms of the orientation order parameter ͗P 2 cos u͘: The rate of increase of ͗P 2 cos u͘ with draw ratio decreases with both increasing temperature and decreasing draw rate. A superposition of all the data to a common reference temperature of 90ЊC was obtained using a WLF shift factor to provide a master curve showing the dependence of the development of ͗P 2 cos u͘ on draw rate. A comparison of the known chain relaxation motions of PET with the observed relation between draw rate and the onset of crystallisation provides an explanation of a previous discrepancy in the literature concerning the point of onset of crystallisation. For draw rates faster than the rate of the chain retraction motion, the onset of crystallisation is delayed until the end of the deformation process. For draw rates slower than the chain retraction motion, there is evidence of the onset of crystallisation occurring before the end of the deformation process. ᭧
Polymer, 1999
Wide angle X-ray scattering (WAXS) data recorded during the drawing of poly(ethylene terephthalate) (PET) under industrial processing conditions was analysed in terms of changes in the degree of polymer orientation and crystallinity over a wide range of draw temperatures and draw ratios. The actual draw rate and draw ratio at the region in the specimen from which X-ray data was recorded was determined independently with high consistency by direct video observation of strain and total X-ray scattering. In contrast to previous claims, the start of strain-induced crystallization coincided with the end of draw within the 40 ms time-resolution of the investigation. Crystallization has been shown to follow first order kinetics and the rate constant was determined over a range of temperatures from 85ЊC-125ЊC. Above 125ЊC little orientation is observed with no evidence of strain-induced crystallization. A detailed determination was made of the critical value of draw ratio below which strain-induced crystallization does not occur and where a relaxation in molecular orientation is observed after the extension process. The implications of these experimental observations for existing theory of strain-induced polymer crystallization are discussed. ᭧
Polymer, 2000
Using time-resolved X-ray diffraction at the European Synchrotron Radiation Facility we have observed a highly oriented weak transient diffraction peak which persists for about 0.2 s prior to strain-induced crystallization during the uniaxial drawing of poly(ethylene terephthalate) (PET) under industrial processing conditions. This structure may be identified with the mesophase structure proposed by a number of workers to occur during drawing of PET, poly(ethylene naphthalate) (PEN) and random copolymers of PET and PEN. In our studies, the transient structure was not observed at draw temperatures greater than 90ЊC nor when the draw conditions resulted in a degree of polymer orientation below a critical level. The possibility that this transient structure is a precursor of strain-induced crystallization is suggested by our observation of a correlation between the decay of the diffraction associated with it and an increase in the intensity of diffraction peaks associated with the development of crystallistion. ᭧
Polymer, 2003
Two types of SAXS and WAXS experiments have been made using synchrotron radiation to observe the transformation from smectic to crystalline phases in oriented poly(ethylene terephthalate) (PET). In step-anneal experiments, PET was drawn slowly at 30 8C and then observed after annealing at 5 8C steps up to 100 8C. In the other experiments, time-resolved observations were made while drawing at 90 8C at rates up to 10 s 21 . Up to 70 8C the WAXS data in the step-anneal experiments showed the smectic meridional reflection reducing in lateral width, indicating an increase in lateral long range order with annealing. Between 70 and 100 8C, there was a reduction in the intensity of the smectic reflection which correlated with an increase in the intensity of crystalline reflections. The SAXS from the step-anneal experiments showed an intense equatorial streak which has a correlation peak around 20 nm and which diminishes with annealing above 70 8C. It is concluded that this feature is a characteristic of the presence of the mesophase in oriented PET and is due to elongated domains of smectic mesophase with a length . 75 nm and with an interdomain spacing of around 20 nm. Between 70 and 100 8C the SAXS data showed additional diffuse diffraction which correlated quantitatively with the crystalline phase and evolved from a cross-like appearance to a well resolved four-point pattern. The time-resolved drawing experiments were limited by the time resolution of the SAXS detector. They showed the same development of four-point diffuse SAXS patterns as was observed in the step-anneal experiments and a very weak equatorial streak. Differences in phase transformation kinetics between the two types of experiment are attributed to the different chain relaxation processes available under different conditions. q
Polymer International, 2006
Three types of poly(ethylene terephthalate) (PET) were investigated: linear (unprocessed) bottle-grade PET (intrinsic viscosity, IV ∼ 0.82 dL g −1 ); a branched PET produced from linear PET by reactive extrusion with 0.4% w/w pyromellitic dianhydride and pentaerythritol in 5:1 molar ratio (IV ∼ 0.97 dL g −1 ); and a control sample produced from the same linear PET by extrusion under the same conditions without the reactive agents (IV ∼ 0.71 dL g −1 ). A key finding is that the reactive extrusion process, presumably as a consequence of branching and branch distribution, significantly modifies the crystallisation kinetics and changes the final morphology. Using small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC), the crystallisation kinetics of PET was monitored from the melt (270 • C) to a crystallisation temperature of either 205 or 210 • C. The IV of the branched PET was ∼ 21% greater than that of the unprocessed PET, and the rate of melt crystallisation (from DSC measurements) was 510 s for the branched, 528 s for the control, and 640 s for the unprocessed PET. The lamellae spacings measured from the equilibrium SAXS patterns were ∼160 ± 10Å for the branched PET and ∼180 ± 10Å for the unprocessed PET. Such properties offer the potential for new applications requiring high-melt-strength PET.
Polymer, 2004
The isothermal cold crystallization of poly(ethylene terephthalate) was investigated by simultaneous small and wide angle X-ray scattering (SAXS and WAXS) and dielectric spectroscopy (DS). By this experimental approach (SWD), simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization. The main features which are directly derived from our experiments can be explained assuming the formation of a heterogeneous multiple lamellar population arrangement. The rigid amorphous phase can be associated with the intra-lamellar stack amorphous phase. The restriction of the amorphous phase mobility mainly occurs in the inter-lamellar stacks regions probably due to the formation of secondary lamellae. q
Journal of Polymer Science Part B: Polymer Physics, 2000
After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastable melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable melt can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The crystallization kinetics of this metastable melt can be determined by means of DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n ϭ 1.0 ϳ 1.2, probably reflecting one-dimensional or irregular line growth of the crystal occurring between the existing main lamellae with heterogeneous nucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thicknesses.
Polymer, 1999
The structural changes occurring when amorphous cold-drawn poly(ethylene terephthalate) films are annealed at different temperatures (50ЊC-240ЊC) for different annealing times (10-10 4 s) were investigated by means of X-ray diffraction and microhardness techniques. The X-ray results reveal the appearance of smectic order at 60ЊC with a period of 10.7 Å . At 70ЊC, a layer structure in the scale of 110 Å emerges. Finally, triclinic order is observed above 80ЊC. The appearance of a layer structure prior to the development of triclinic crystals is associated with a density difference along the molecular direction produced by a molecular tilting mechanism. The microhardness behaviour of annealed cold-drawn PET films is correlated to the developing morphologies. At high annealing temperatures (Ͼ100ЊC), the plastic component of hardness is shown to vary with the occurring microstructural changes. Results indicate that the hardness of the amorphous intrafibrilar regions is higher than that of a fully amorphous material. The indentation anisotropy, DH, which is related to the elastic recovery of the material shows a conspicuous decrease at T a t 70ЊC, which is explained in terms of a relaxation of the fibrils in the chain direction. ᭧ Polymer 40 (1999) 6475-6484 0032-3861/99/$ -see front matter ᭧
Synchrotron studies of polymers at high spatial and temporal resolution
Developments in X-ray synchrotron radiation sources and electronic area detectors have allowed the development of new X-ray diffraction techniques to investigate polymeric materials at high temporal and spatial resolution. These developments are illustrated by studies of poly(ethylene terephthalate) (PET). The fabrication of films and containers of PET typically involves mechanical deformation at elevated temperatures close to its T g . Such processing can have major effects on the degree of polymer orientation and crystallinity and hence on the physical properties of the material. This paper describes the time-resolved X-ray diffraction technique developed to investigate the strain-induced crystallisation in PET under industrial processing conditions with 40 milliseconds time resolution. The investigation of the structural variation in orientation and crystallinity across the wall of a container fabricated from PET using an X-ray beam as small as 2 µm diameter is also described. The potential for more general application of these techniques in the study of other polymer materials is indicated.