Orientation-induced crystallization of poly(ethylene terephthalate) fibers with controlled microstructure

Orientation-induced crystallization of PET fibers was studied by the in-situ wide-angle X-ray diffraction (WAXD) utilizing synchrotron radiation source combined with thermomechanical analysis. The noncrystalline as-spun fiber spun was heat-treated at 150, 165, 180 and 195 °C for 0.1 s under constrained condition. The heat-treatment allowed the fibers to have various amount of isotropic amorphous (IA), oriented noncrystalline (ON), and crystalline (Cr) phase. The structure evolution accompanying the crystallization of the fibers was then examined upon elevating temperature while the fiber length was held constant. The X-ray results clearly showed that the crystallization takes place first by ON phase (extended-chain crystallization) and then followed by the crystallization of IA phase (folded-chain crystallization). The on-set of extended-chain crystallization was dependent on the amount and degree of orientation of ON phase in the fiber that was derived from the various heat-treatment temperatures. It is also noted that the IA phase transforms into not only the CR phase but also the ON phase. The crystallization on the surface of preformed extended-chain crystals appeared to induce the spontaneous orientation of the chains. The thermomechanical data indicated that a stress emerges rapidly on fiber above glass transition temperature (T_g), which is associated with the entropic relaxation of the ON phase. The stress emerged on fiber then dropped drastically as the temperatures of fibers reached the temperatures of extended-chain crystallization, indicating that the stress drop is closely related with the extended-chain crystallization. The fibers heat-treated at the highest temperature showed the highest initial crystallinity but showed the slowest crystallization rate, resulting in the lowest final crystallinity among the fibers.     

The complicated influence of branching on crystallization behavior of poly(ethylene terephthalate)

The randomly branched poly(ethylene terephthalate) (BPET) was prepared by bulk polycondensation from dimethyl terephthalate (DMT) and ethylene glycol (EG), with 0.4–5.0 mol % (with respect to DMT) of glycerol (GL) as a branching agent. The glass transition and crystallization behavior was studied by differential scanning calorimetry (DSC). It was found that the glass transition temperature of BPET reduced with the increasing content of GL until 1.2 mol %, and then increases a little at high degrees of branching. When compared with a linear PET, the crystallization temperature of BPET from the melt shifted to higher temperature as GL content was smaller than 1.2 mol %, and then became lower while GL load was added. Nonisothermal crystallization kinetics was studied through the modified Avrami analysis. It was revealed that the overall crystallization rate parameter of BPET became larger when the GL content was less than 1.2 mol %, then turned to lower at higher branching degree. This indicated that low degree of branching could enhance the overall crystallization of poly(ethylene terephthalate) (PET), whereas high degree of branching in the range of 3.5–5.0 mol % would block the development of crystallization. On the basis of Hoffman's secondary crystallization theory, the product σσ_e of the free energy of formation per unit area of the lateral and folding surface was calculated. According to the change of the product σσ_e with the degree of branching, a possible explanation was presented to illuminate this diverse effect of different degrees of branching on crystallization. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

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.     

Liquid-induced crystallization of poly(ethylene terephthalate)

Amorphous unoriented poly(ethylene terephthalate) was crystallized at 25°C by various organic liquids. The crystalliznity induced in the amorphous polymer was measured by differential scanning calorimetry and infrared spectroscopy. The ability of liquids to interact with and induced crystallinity in the amorphous polymer was classified on the basis of their solubility parameters. Measurements of the density of liquid-crystallized 0.8-mil films of poly(ethylene terephthalate) indicated the presence of extensive internal voids in the semicrystalline polymer matrix. Comparison of differential scanning calorimetric thermograms and infared spectra of heat-crystalized and liquid-crystallized polymer indicated significant differences in the polymer morphologies induced by the two crystallization processes.     

Strain-induced crystallization of poly(ethylene terephthalate)

The fabrication of poly(ethylene terephthalate), PET, into fibers, films, and containers usually involves molecular orientation caused by molecular strain, which may lead to stress- or strain-induced crystallization (SIC). The SIC of PET was studied by the methods of birefringence, density, thermal analysis, light scattering, and wide-angle X-ray. The development of crystallinity is discussed in relation to the rate of crystallization, the residual degree of orientation, and stress relaxation. The experimental procedure involves stretching samples at temperatures above the glass transition temperature, Tg, to a given extension ratio and at a specific strain rate of an Instron machine. At the end of stretching, the sample is annealed in the stretched state and at the stretching temperature for various periods of time, after which the sample is quickly quenched to room temperature for subsequent measurements. During stretching, the stress strain and the stress relaxation curves are recorded. The results indicate that the SIC of annealed, stretched PET can proceed in three different paths depending on the residual degree of orientation. At a low degree of residual orientation, as indicated by the birefringence value, annealing of stretched PET leads only to molecular relaxation, resulting in a decrease of birefringence. At intermediate orientation levels, annealing causes an initial decrease in birefringence followed by a gradual increase and finally a leveling off of birefringence after a fairly long period of time. At higher orientation levels, annealing causes a rapid increase in birefringence before leveling off. The interpretation of the above results is made using the measurements of light scattering, differential scanning calorimetry, and wide-angle X-ray. The rate of the SIC of PET is also discussed in terms of specific data analysis.     

The crystallization kinetics of filled poly(ethylene terephthalate)

The crystallization kinetics of poly(ethylene terephthalate) was measured under isothermal conditions by DSC in the presence of various fillers and with varying filler concentrations. The fillers used were carbon, titanium dioxide, glass fiber, and calcium carbonate. The kinetics was calculated using the slope of the crystallization vs. time plot, the times for 10% and 50% reduced crystallization, and the Avrami equation. Activation energies were determined from measurements under different isothermal temperatures. The fillers caused athermal nucleation to be inhibited as shown by the increased values of the Avrami exponent, n. Interactions between the polyester and filler were interpreted to reduce the mobility of the polymer in the melt. This decreased the rate of crystallization and increased its activation energy. The order of the filler effect in reducing crystallization was the following: no filler < carbon < titanium dioxide < glass fiber < calcium carbonate. The concentrations of filler were above those typically used for nucleation and more in the range expected for reinforcement or dilution of the polymer. © 1993 John Wiley & Sons, Inc.     

Non-isothermal crystallization kinetics of poly(ethylene terephthalate)/mica composites

The non-isothermal crystallization kinetics of pure poly(ethylene terephthalate) (PET), PET/mica and PET/TiO₂-coated mica composites were investigated by differential scanning calorimetry with different theoretical models, including the modified Avrami method, Ozawa method and Mo method. The activation energies of non-isothermal crystallization were calculated by Kissinger method and Flynn–Wall–Ozawa method. The results show that the modified Avrami equation and Ozawa theory fail to describe the non-isothermal crystallization behavior of all composites, while the Mo model fits the experiment data fair well. It is also found that the mica and TiO₂-coated mica could act as heterogeneous nucleating agent and accelerate the crystallization rates of PET, and the effect of TiO₂-coated mica is stronger than that of mica. The result is further reinforced by calculating the effective activation energy of the non-isothermal crystallization process for all composites using the Kissinger method and the Flynn–Wall–Ozawa method.     

Nonisothermal crystallization kinetics of poly(butylene terephthalate)/poly(ethylene terephthalate)/glass fiber composites

The influences of the glass fiber (GF) content and the cooling rate for nonisothermal crystallization process of poly(butylene terephthalate)/poly(ethylene terephthalate) (PBT/PET) blends were investigated. The nonisothermal crystallization kinetics of samples were detected by differential scanning calorimetry (DSC) at cooling rates of 5°C/min, 10°C/min, 15°C/min, 20°C/min, 25°C/min, respectively. The Jeziony and Mozhishen methods were used to analyze the DSC data. The crystalline morphology of samples was observed with polarized light microscope. Results showed that the Jeziony and Mozhishen methods were available for the analysis of the nonisothermal crystallization process. The peaks of crystallization temperature (T_p) move to low temperature with the cooling rate increasing, crystallization half‐time (t_1/2) decrease accordingly. The crystallization rate of PBT/PET blends increase with the lower GF contents while it is baffled by higher GF contents. POLYM. COMPOS. 36:510–516, 2015. © 2014 Society of Plastics Engineers     

High temperature crystallization of poly(ethylene terephthalate)

Annealing poly(ethylene terephthalate) (PET) at high temperature results in a crystalline phase stable to 10°C higher than the temperature previously regarded as the equilibrium melting point. Melting temperatures as high as 289°C can be attained, which is equivalent to the equilibrium melting point determined herein for PET. The high melting point and tendency to superheat suggest that the crystals possess a substantial extended chain structure, notwithstanding the magnitude of the infrared fold band.     

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.     

Nonisothermal crystallization of reprocessed poly(ethylene terephthalate)

Poly(ethylene terephthalate) was submitted to five reprocessing cycles by extrusion. The materials were analyzed with oligomer and after oligomer extraction. The nonisothermal crystallization of the five samples was investigated by differential scanning calorimetry. Samples with oligomer content and carboxylic end group concentrations between 44 and 98 eqw × 10⁶ g presented a nonlinear correlation with the crystallization temperature. After the oligomer extraction of the polymer, this correlation is linear. The nonisothermal crystallization results were analyzed using the Ozawa model. The polymers containing oligomers obey the Ozawa model for the first reprocessing cycle. After oligomer extraction, the polymers obey the Ozawa model from the first to the third reprocessing cycle. In both cases, the exponential n values are close to 2.0. For the other cycles, deviations from this model occur. The activation energy was calculated using the Kissinger and Varma models. The values obtained for the five reprocessed samples were inversely proportional to the molar mass when analyzed by both models. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 525–531, 2004     

Effects of a small amount of poly(ethylene glycol) on the non-isothermal cold crystallization of uniaxially oriented poly(ethylene terephthalate) fibers

The kinetics of non-isothermal crystallization of uniaxially oriented poly(ethylene terephthalate) fibers modified by poly(ethylene glycol)(PET-co-PEG) was investigated by using a DSC heating scanning method and analyzed by using a new non-isothermal equation. Two crystallization peaks appeared for PET and PET-co-PEG fibers. The kinetics parameters, such as the Avrami exponent, the activation energies of diffusion, and the weight fractions per sub-process, were obtained. Based on the Avrami exponent, peak position, and crystallization rate, the crystallization mechanism was proposed.     

The effect of processing conditions on structure evolution in melt spun poly(ethylene terephthalate) fibers

An investigation has been made into the structural evolution in poly(ethylene terephthalate) fibers as a function of wind-up speed (WUS) and quench air temperature (QAT). Analysis of the mechanics and mechanisms of cold drawing reveals that the fiber structure is heterogeneous. Evidence from differential scanning calorimetry (DSC) and density measurements indicates the presence of crystallites which are imperfect according to wide-angle diffraction (WAXD). Increased thread-line stress (higher WUS and higher QAT) appears to induce larger and more oriented crystallites in a progressively oriented matrix. A radially layered structure is disclosed from high resolution scanning electron microscopy (SEM) micrographs.     

Nonisothermal crystallization kinetics of nucleated poly(ethylene terephthalate)

Studies of the nonisothermal crystallization kinetics of poly(ethylene terephthalate) nucleated with anhydrous sodium acetate were carried out. The chemical nucleating effect was investigated and confirmed with Fourier transform infrared and intrinsic viscosity measurements. The Avrami, Ozawa, and Liu models were used to describe the crystallization process. The rates of crystallization, which initially increased, decreased at higher loadings of the additive. The activation energy, calculated with Kissinger's method, was lower for nucleated samples. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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.     

DSC analysis of crystallization and melting behavior of polypropylene in model composites with glass and poly(ethylene terephthalate) fibers

Isothermal and nonisothermal crystallization of maleic anhydride grafted polypropylene (PP), which is used for the production of split warp knit composite preforms,¹ are analyzed in model composites to determine the influence of reinforcement glass fibers (GF) and poly(ethylene terephthalate) (PET) binding yarns on the crystallization kinetics. Basic energetic parameters of crystallization are determined, and the melting behavior of PP in model composites is analyzed. The crystallization of PP carried out in nonisothermal and isothermal regimes is facilitated in the presence of GF, and the additional effects of PET fibers are also shown. Better conditions for nucleation, resulting in lower energy for formation of a stable nucleus and lower critical dimensions, are proposed as a reason for this. The crystal structure of PP in model composites exhibits lower lamellae thickness and is less disposed to recrystallization. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 239–246, 1999     

Solidification of poly(ethylene terephthalate) with incomplete crystallization

Cooling is a critical step in any crystalline polymer molding or extrusion process. A simulation is proposed which will predict the transient temperature and crystallinity profiles developed when a finite polymeric slab comes in contact with a cooling fluid. A generalized, phenomenological model of the crystallization kinetics of polymers is incorporated to account for the effect of the latent heat of crystallization on the thermal history as well as on the crystalline structure at any point in the slab. The model assumes heterogeneous nucleation and temperature-dependent radial growth of spherulites. DSC cooling thermograms for the polymer are used to verify the kinetic model for comparing experimental measurements against simulated results. Observed spherulite sizes should also be matched by the simulation. Kinetic data have been obtained for two grades of poly(ethylene terephthalate) which have the same growth rate expression but different nucleation characteristics. Crystallinity of these two polymers decreases rapidly as either quench temperatures or nucleation densities are decreased independently. Calculations have been carried out for 1/16 in. thick sheets of polymer exposed to a cooling medium with a heat transfer coefficient of 100 Btu/hr/ft²/°F. Temperature gradients are also presented. The simulation can be used for optimizing quench conditions in polyester film extrusion.     

Effect of shearing on crystallization behavior of poly(ethylene terephthalate)

The shear‐induced crystallization behavior of PET was investigated by measuring the time‐dependent storage modulus (G′) and dynamic viscosity (η′) with a parallel‐plate rheometer at different temperatures and shear rate. The morphology of shear‐induced crystallized PET was measured by DSC, X‐ray, and polarizing optical microscopy. When a constant shear rate was added to the molten polymer, the shear stress increased with the time as a result of the orientation of molecular chains. The induction time of crystallization is decreased with frequency. Moreover, the rate of isothermal crystallization of PET was notably decreased with increasing temperature. The shape of spherulites is changed to ellipsoid in the direction of shear. In addition, aggregation of spherulites is increased with increasing frequency. Particularly, the row nucleation morphology could be observed under polarized light for ω = 1. From the results of DSC, the melting point and enthalpy have a tendency to decrease slightly with increasing frequency. The crystallite size and perfectness decreased with frequency, which was confirmed with X‐ray data. The unit length of the crystallographic unit cell of the PET increased and the (1 0 3) plane peak increased with increasing frequency. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2640–2646, 2001