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

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 d_c, not the amorphous layer thickness d_a. The observed d_c values are found to be always smaller than d_a, regardless of polymer composition. d_c 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 d_c, d_a decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in d_a is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the d_a 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.     

Observations of structure development during crystallisation of oriented poly(ethylene terephthalate)

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 °C and then observed after annealing at 5 °C steps up to 100 °C. In the other experiments, time-resolved observations were made while drawing at 90 °C at rates up to 10 s⁻¹. Up to 70 °C 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 °C, 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 °C. 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 °C 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.     

Model of cold crystallization of uniaxially oriented poly(ethylene terephthalate) fibers

The differential scanning calorimeter heating curves of uniaxially oriented poly(ethylene terephthalate) (PET) fibers with three peaks were analyzed by using a newly proposed equation. The diffusion-controlled crystallization theory is suitable for describing cold crystallization of uniaxially oriented PET fibers. A crystallization model was proposed based on the kinetic parameters obtained. The model embraces the three sub-processes of crystallization corresponding to different growth geometries. The first sub-process corresponds to the nucleation of ordered molecular segments or the radial growth of preformed nucleus, resulting in the shorter bundle-like entities. The second sub-process corresponds to further growth of the bundle-like crystallites along chain direction, resulting in the longer bundle-like entities. The third sub-process corresponds to the three-dimensional growth of crystallites relating to the random segments, resulting in the spherical entities.     

Crystallization-induced order in polystyrene-poly(ethylene oxide) metallo-supramolecular diblock copolymer

The solid-state morphology of polystyrene-poly(ethylene oxide) metallo-supramolecular diblock copolymer PS₂₀-[Ru]-PEO₇₀, has been investigated by small-and wide-angle X-ray scattering and atomic force microscopy. Above the melting point of PEO the metal-ligand complexes and their associated counter ions are known to form aggregates within the still disordered polymer matrix of PS and PEO. Crystallization of PEO induces microphase separation between the PS and the PEO blocks. In addition, the metal-ligand aggregates are forced out of the crystalline PEO part and subsequently order at the interface in the amorphous PS block into a (short-range) square lattice.     

Crystallization of poly(ethylene terephthalate) under tensile strain: crystalline development versus mechanical behaviour

The crystallization of poly(ethylene terephthalate) (PET) under tensile strain was investigated using wide angle X-ray diffraction. Real-time investigation of the crystallization state, including the crystalline ratio and the crystallite orientation, of the material could be undertaken due to the high brilliance of the synchrotron X-ray source used in our study. Initially amorphous PET specimens were stretched at different strain rates and draw ratios at the same temperature (above and close to T_g). Our experimental set-up was designed to undertake simultaneous recording of the X-ray diffraction patterns and the mechanical parameters. Up to the draw ratio of 500% and draw rate of 0.75 s⁻¹, the crystalline development dynamics corresponded to three different regimes. (i) For small enough extension rates, there was no measurable crystallinity during the drawing process. The crystallization developed after cessation of deformation. (ii) For intermediate extension rates, the whole crystallization process took place during the deformation. (iii) For the highest extension rate involved in our experiments, the crystallization started during the drawing process and continued after cessation of deformation. The mechanical behaviour of the polymer was simultaneously recorded and correlated with the induced crystalline microstructure. In particular, we were able to discriminate the influence of crystallite orientation and crystallization growth on the mechanical behaviour of the material.     

Mesomorphic phase in oriented poly(pentamethylene 2,6-naphthalate)

The mesomorphic structure of poly(pentamethylene 2,6-naphthalate) (PPN) was investigated using a synchrotron X-ray scattering. The PPN fibers cold-drawn from the super-cooled amorphous state showed a smectic mesomorphic structure and further a crystalline phase at high strain. Based on the experimental evidence showing the split of amorphous halo up and down the equator and the conformational constraint suggested by the crystal structure refinement and computation, we suggested the smectic phase as S_CA where the mesogens are tilted against the layer surface normal and the tile direction is opposite between the neighboring layers.     

Phase behaviour in poly(ethylene oxide-b-t-butyl methacrylate) block copolymers

The lamellar thickness of the poly(ethylene oxide)-poly(t-butyl methacrylate) (PEO-PTBMA) diblock copolymer system, obtained by differential scanning calorimetry and small angle X-ray scattering investigations, is correlated with the degree of polymerization of the amorphous (PTBMA) and crystallizable (PEO) sequences. The non-equilibrium exponents obtained immediately after bulk crystallization are different to those from extrapolated equilibrium results. Within the experimental standard deviations, the theoretical predictions of DiMarzio et al. and of Whitmore and Noolandi could be confirmed. The molecular weights of PEO and PTBMA ranged from 250 to 21000 g mol⁻¹ and from 1500 to 17000 g mol⁻¹, respectively. Both the equilibrium lamellar thickness l and the PEO domain size d_PEO increase with increasing PEO and decreasing PTBMA degrees of polymerization Z according to d_PEO l Z^0.97±0.08_EOZ^{−(0.53±0.19)}_TBMA.     

Light scattering studies on the crystalline morphology of stretched poly(ethylene 2,6-naphthalate) film

The crystalline morphology of poly(ethylene 2,6-naphthalate) (PEN) film obtained by uniaxial stretching at 145 °C (T_g + 25 °C) was investigated by use of a light scattering photometer equipped with a CCD camera system. The Hv scattering showed a symmetric, circular pattern at a low stretch ratio of λ < 3. The intensity profile became sharper with an increase in λ, suggesting that anisotropic crystal rods are randomly assembled and that the length of the rods increases with λ. At a high stretch ratio of λ ≥ 3, a double-cross-type pattern consisting of a broad rod-like pattern and sharp cross streaks was observed. The rod-like pattern became smaller and the streaks became sharper with an increase in λ. By the model calculation of the scattering pattern, the double-cross-type pattern is explained by the stacking of anisotropic crystal rods oriented in the stretch direction. As λ increases, the thickness of the rods and the number per stack increase, and the stacks and rods are slightly oriented in the stretch direction. The change in the wide angle X-ray diffraction pattern suggested that the ordering of the molecular chain in the crystal rods increases with increasing λ.     

Ultra small-angle X-ray scattering studies on structural changes in micrometers upon uniaxial stretching of segmented polyurethaneureas

Ultra small-angle X-ray scattering (USAXS) experiments were conducted in order to examine structural changes in micrometers upon uniaxial stretching of elastomeric segmented polyurethaneureas at room temperature. It was possible to stretch the sample film up to α = 7.5 without break, where α designates the stretching ratio. Around α = 6.5 the sample became turbid, while it recovered transparency when the stress was removed. To understand this curious phenomenon, we conducted the USAXS experiments for the structural analyses in micrometers. Although a set of streaks appeared in the direction parallel to the stretching direction (SD) when the sample became turbid (around α = 7.2), more interesting result is that a set of streaks appeared in the direction perpendicular to SD much earlier around α = 5.0. Close examination of the scattering intensity profile revealed evolution of multiple interference peaks, which could be ascribed to the form factor of a lamellar particle. Since the streaks disappeared upon the removal of the stress, “lamellar particles” are considered to be crazes, which develop further into cracks in a subsequent stage. From the results of USAXS measurements, it is suggested that the lamellar-shaped crazes appeared around α = 5.0 being oriented parallel to SD, and further stretching created other lamellar-shaped crazes being oriented perpendicular to SD, which are co-existing with the preceding parallel crazes.     

Melting and reorganization of poly(ethylene terephthalate) on fast heating (1000 K/s)

For poly(ethylene terephthalate) (PET) and other polymers the origin of the multiple melting peaks observed in differential scanning calorimetry (DSC) curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. Developments in thin-film (chip) calorimetry allow scanning rates as high as several thousand Kelvin per second. We utilized a chip calorimeter based on a commercially available vacuum gauge, which is operated under non-adiabatic conditions. The calorimeter was used to study the melting of isothermally crystallized PET. Our results on melting at rates as high as 2700 K/s give clear evidence for the validity of a melting-recrystallization-remelting process for PET at low scanning rates (DSC). At isothermal conditions PET forms crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of different populations of crystals with significantly different stability (melting temperatures) under isothermal conditions. Superheating of the crystals is of the order of 10 K at 2700 K/s.     

On the kinetics of isothermal crystallization of branched polyethylene

Isothermal crystallization of branched polyethylene was studied by time-dependent small-angle X-ray scattering experiments and dilatometry. A strict proportionality between scattering intensities and density changes was found for all times. This result indicates complete absence of crystal thickening and perfectioning processes. Isotherms directly reflect the increase in specific inner surface resulting from nucleation and lateral growth of lamellae. The shape of isotherms suggests a time-dependent non-uniform internal structure of spherulites with a decrease in specific inner surface with increasing distance from centre.     

Recrystallization processes in cold-crystallized poly(ethylene terephthalate): Interplay between structure evolution and conformational relaxation

The changes induced in the amorphous regions of poly(ethylene terephthalate) as a consequence of recrystallization processes, taking place after cold-crystallization at T_c = 100 °C, are analyzed by means of isothermal dynamical mechanical spectroscopy and microindentation hardness. Overall, a recrystallization process at either 115 or 125 °C causes an increase of the rigidity within the amorphous domains confined by the crystals. Microhardness measurements carried out at room temperature reveal that recrystallization leads to an enhanced mechanical performance of the amorphous regions. The analysis of isothermal segmental relaxation patterns recorded in a frequency interval of 10⁻³-60 Hz indicates the appearance of two distinct contributions, which find correspondence with observations by broad band dielectric spectroscopy on the same systems. The faster one is ascribed to segmental relaxation within the amorphous domains where the confinement by crystals is relatively weak. The slow relaxation mode is associated with regions where the conformational dynamics is strongly restricted by the crystals. A relative increase of the slow process is detected upon recrystallization. A recently developed relaxation function model is employed to estimate the size of the static cooperatively rearranging regions for both, the slow and the fast modes. It is found that this size increases either upon decreasing the temperature or as an effect of recrystallization. In addition, the number of monomers involved in a conformational rearrangement turns out to be significantly larger in the regions associated to the slow mode process.     

Effect of hydrolytic degradation on the microstructure of quenched,amorphous poly(glycolic acid): an X-ray scattering study of hydrated samples

The effect of hydrolytic degradation on the microstructure of unoriented, quenched poly(glycolic acid) (PGA) was investigated using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS). Samples were analysed immediately after removal from the degradation media in order to prevent dehydration. Analysis showed that the material initially contained a small degree of crystallinity. On degradation, the material rapidly crystallized, developing a broadly similar morphology to samples crystallized from the melt. The behaviour of these new structures on degradation was similar to that observed in the precrystallized samples previously reported. The crystal density remained constant and little change was seen in the lateral extent of the crystal lamellae. Both the crystallinity and SAXS scattering power (or invariant) increased during the first 30 days which may be due to the preferential removal of amorphous material and further crystallization of amorphous chains. The crystallization of amorphous material was facilitated by plasticization due to the ingress of water and the cleavage of amorphous chains. In both quenched and precrystallized material, the average lamellar spacing fell and then rose during degradation. It is not possible to interpret this unambiguously from the SAXS data alone. It may be partially the consequence of a two-stage removal of amorphous material. Alternatively, the behaviour may be explained by changes in the osmotic potential of the amorphous layer on degradation, together with insertion crystallization. © 1999 Society of Chemical Industry     

Ac conductivity and dielectric permittivity of poly(ethylene glycol) during crystallization: Percolation picture

Dielectric conductivity and permittivity of poly(ethylene glycol) were measured in the frequency range between 10¹ and 10⁶ Hz during non-isothermal crystallization and melting with different cooling/heating rates (5, 10 and 20 K/h). The time development of the conductivity and permittivity spectra during crystallization and melting is discussed in terms of charge carrier diffusion in a percolation network formed by the amorphous phase of the semi-crystalline polymer.     

The crystallization of oriented poly(ethylene terephthalate)

The rate of crystallization of oriented poly(ethylene terephthalate) has been measured at 100°, 120° and 150°C using carefully prepared amorphous fibre samples. The samples were held to length during crystallization so that shrinkage did not occur, and the course of crystallization was followed by measuring the changes in density and boiling water shrinkage of the samples. The results show that the rate of crystallization is strongly dependent on the degree of orientation. Nucleation and initial growth of crystallites occur in times of the order of milliseconds at 120°C in samples of birefringence 0.08 compared with times of several minutes in isotropic material. It was found that crystallization in oriented material cannot be described by the Avrami equation.     

Liquid induced crystallization of poly(ethylene naphthalate) oligomers

Amorphous poly(ethylene naphthalate) (PEN) oligomer has been crystallized at room temperature by treating in liquids having solubility parameter, δ, in the range of 15-29 (J cm⁻³)^1/2. It has been observed that the liquids having δ in the range of 18.5-25 are able to crystallize PEN oligomer efficiently. A direct correlation has been observed between δ and crystallite size calculated from WAXS technique.     

Carbon nanotubes induced crystallization of poly(ethylene terephthalate)

We have investigated the crystallization characteristics of melt compounded nanocomposites of poly(ethylene terephthalate) (PET) and single walled carbon nanotubes (SWNTs). Differential scanning calorimetry studies showed that SWNTs at weight fractions as low as 0.03 wt% enhance the rate of crystallization in PET, as the cooling nanocomposite melt crystallizes at a temperature 10 °C higher as compared to neat PET. Isothermal crystallization studies also revealed that SWNTs significantly accelerate the crystallization process. WAXD showed oriented crystallization of PET induced by oriented SWNTs in a randomized PET melt, indicating the role of SWNTs as nucleating sites.     

Morphology and crystallization of poly(ethylene terephthalate)

The melting behaviour and the morphology of poly(ethylene terephthalate) crystallized from the melt are reported. In general, dual or triple melting endotherms are seen, and single endotherms are seen when the samples are crystallized above 215°C for long times. The location of the uppermost endotherm was found to be constant below T_c = 230°C, and above that temperature the location depends on T_c. Therefore, we have shown that samples of PET which are crystallized above T_c = 230°C contain perfect crystals only; below T_c = 230°C, they contain perfect and imperfect crystals. Scanning electron microscopy showed that the perfect crystals are the dominant lamellae in the spherulitic structure, while the imperfect crystals are the subsidiary lamellae in the spherulitic structure, The amorphous regions are located between individual lamellae.     

Effects of curing temperature on poly(ethylene terephthalate)

The effect on poly(ethylene terephthalate) (PET) of thermal curing in a particular temperature range (T max =280-350C) in air have been studied. The changes in the structure were monitored using various characterization techniques such as differential scanning calorimetry, thermogravimetric analysis, optical microscopy equiped with hot-stage, and scanning electron microscopy. It was observed that when (PET) is cured at very high temperature above its original melting point, cross-linking of the (PET) samples occurs. The cross-linking takes place in the melt in this case. With increasing the curing temperature, the area of the higher melting peak temperature decreases due to the increase in cross-linking of (PET). In terms of spherulitic texture, it was found that with increasing the curing temperature more inter-lamellar intra-spherulitic inclusions are observed in the material.