Rheooptical investigation of the crystallization of poly(ethylene terephthalate) under tensile strain

The crystallization of poly(ethylene terephthalate) under uniaxial tensile strain at different extension rates was investigated with optical polarimetry in a temperature range between the glass-transition temperature and the quiescent crystallization temperature. The evolution of the optical properties of the polymer, including the turbidity, birefringence, and dichroism, were monitored simultaneously with the mechanical parameters. To complete the semicrystalline microstructure characterization of the polymer under strain, an online wide-angle X-ray diffraction (WAXD) technique was used in separate experiments, which were performed under the same thermomechanical conditions. For real-time measurements, a high-energy synchrotron radiation source was used. The optical properties provided information about both the crystalline and amorphous phases, whereas the WAXD patterns essentially gave information about the crystalline phase. The two experimental techniques were then used in a complementary way to characterize the semicrystalline microstructure. Significant deviations from the stress-optical rule were found. This was attributed to both transient effects and the appearance of crystallites, which consisted of highly oriented molecular segments that could contribute to the optical anisotropy but not necessarily to the stress. The behavior of the optical dichroism was found to be qualitatively different from that of the birefringence. The latter monotonically increased with the strain, whereas the former first increased with the strain, passed through a maximum, and then decreased to a steady-state value. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1915–1927, 2004     

Photolysis of poly(ethylene terephthalate)

The photolysis of poly(ethylene terephthalate) films was studied in vacuo with light of wavelengths 2537 and 3130 A. A very stable filter system which cuts out the 3025 A. line was developed to isolate 3130 A. from a mercury spectrum. Despite the fact that the penetration of 2537 A. light was limited to a depth of a ca. 10³ A. whereas 3130 A. light was more uniformly absorbed it was possible to demonstrate that the quantum yields for CO and CO₂ formation were in agreement for the two wavelengths. Quantum yields for fractures and crosslinks were estimated by sol-gel analysis. An absorption maximum which develops near 13 μ after exposure of poly(ethylene terephthalate) to light or γ-rays was attributed to the formation of groups formed by elimination of CO and CO₂. ESR spectra for trapped radicals were tentatively assigned to the components p-C₆H₃· and ·O? CH₂? CH₂? . It is suggested that the former radicals combine to form crosslinks. Quantum yields (× 10⁴) with 3130 A. light are: CO, 6; CO₂, 2; crosslinks, 5.5; trapped radicals, 1.5; With 2537 A. light, quantum yields are: CO, 6–9; CO₂, 2–3; the network formed was not characterized as to crosslinks and fractures; trapped radicals were observed to exist but not determined.     

Adsorption properties of cermet and track-etched poly(ethylene terephthalate) membranes

The adsorption properties of cermet and track-etched poly(ethylene terephthalate) (PET-TM) membranes are compared with respect to proteins and water-soluble dyes. It is shown that the cermet membranes have a noticeably higher adsorption capacity (calculated per unit surface area) than the PET-TM. In this case, the character of adsorption of these substances on both types of membranes is quite similar and determined by the combination of ionic and hydrophobic interactions. The adsorption values of basic dyes are considerably higher than acid one because of the negative charge at the membrane surfaces. The isotherm of adsorption of the basic dye rhodamine 6G on PET-TM from aqueous solution is characterized by the inflection in the concentration range of lower than 1 µmol/l due to the presence of highly active adsorption sites on the surface. The adsorption of dyes considerably lowers on adding isopropanol to the aqueous solution. Using basic protein cytochrome C as an example, it is established that its adsorption on cermet membranes can be prevented by increasing solution ionic strength.Translated from Kolloidnyi Zhurnal, Vol. 67, No. 1, 2005, pp. 124–127. Original Russian Text Copyright © 2005 by Khataibe, Khokhlova, Trusov, Mchedlishvili.     

Electrosurface and structural properties of poly(ethylene terephthalate) track membranes

The electrosurface characteristics are studied for poly(ethylene terephthalate) (PET) track membranes (TMs) with pore radii of 6.5?C60 nm, which are used for ultra- and microfiltration. The data obtained enable one to indirectly assess the structure of tracks and variations in the pore space structure of TMs with an increase in the pore radii. Higher porosity values obtained for TMs from the data on their electrical resistance in comparison with those derived from the filtration data lead one to state that the PET pore surface has a loose structure. The thermal treatment of TMs makes the porosity values determined by the methods of electrical resistance and filtration closer to one another. The regularities of variations in the isoelectric point, ?? potential, and surface charge suggest that the properties and structure of PET pore surface depend on the pore radius. The data obtained may be used to predict the separating power of TMs.     

The three-phase structure and mechanical properties of poly(ethylene terephthalate)

The relation between the mechanical properties and the microstructure of PET has been investigated, combining results from WAXS, SAXS, FTIR, DSC, and uniaxial compression tests. The rigid amorphous fraction in the PET was explicitly taken into consideration in interpreting structure–property relations. WAXS results prove that glass crystallized PET with a high volume fraction of rigid amorphous material and small crystal size, on uniaxial compression shows a considerable loss in crystalline fraction. FTIR results in combination with these WAXS results suggest that during this loss in crystallinity, short-range conformational order is retained, while long-range structural order is lost. At the same time, material with small crystals and a high amount of rigid amorphous material was found to show unexpectedly low yield stress. It is concluded that in the interpretation of these phenomena it is necessary to take the three-phase structure of PET, including the rigid amorphous fraction into account. This is expected to hold for other semicrystalline polymers, where a rigid amorphous fraction is prominent, such as PHB, PBT, PEN, PEEK, etc. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2092–2106, 2004     

Segmented block copolymers of poly(ethylene glycol) and poly(ethylene terephthalate)

A new series of segmented copolymers were synthesized from poly(ethylene terephthalate) (PET) oligomers and poly(ethylene glycol) (PEG) by a two‐step solution polymerization reaction. PET oligomers were obtained by glycolysis depolymerization. Structural features were defined by infrared and nuclear magnetic resonance (NMR) spectroscopy. The copolymer composition was calculated via ¹H NMR spectroscopy. The content of soft PEG segments was higher than that of hard PET segments. A single glass‐transition temperature was detected for all the synthesized segmented copolymers. This observation was found to be independent of the initial PET‐to‐PEG molar ratio. The molar masses of the copolymers were determined by gel permeation chromatography (GPC). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4448–4457, 2004     

Mesophase structures in poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(ethylene naphthalate bibenzoate)

Films of poly(ethylene naphthalate) (PEN) and poly(ethylene naphthalate bibenzoate) (PENBB) have been drawn under a variety of conditions of temperature and strain rate to determine the conditions under which a nematic-like mesophase structure can be produced. In PEN the combination of low temperature and high-strain rate encourages mesophase formation, while in PENBB the mesophase was formed under all conditions where it proved possible to draw the material at all. A molecular modelling study of the mesophase in PEN and in poly(ethylene terephthalate) (PET) offers possible structures for the mesophase and showed that the mesophase structure could be stable once formed © 1997 John Wiley & Sons, Ltd.     

Incorporation of an abnormally high amount of poly(ethylene glycol) into poly(ethylene terephthalate) during tensile drawing in liquid media

Tensile drawing of PET via the mechanism of solvent crazing in adsorption-active media containing poly(ethylene glycol) with M < 1 × 10⁶ is accompanied by their penetration into the porous structure of the matrix polymer. In this case, the amount of PEG in PET exceeds its concentration calculated on the assumption that the porous structure is filled with the polymer solution. This excess is evidently due to the adsorption of PEG on the highly developed surface of crazes.     

Branching and cross-linking of poly(ethylene terephthalate) and its foaming properties

The branching and cross-linking of poly(ethylene terephthalate) were investigated using two chain extenders: glycidyl methacrylate-styrene copolymer (GS) and poly(butylene terephthalate)-GS (PBT-GS) in order to improve the melt viscosity and melt strength of poly(ethylene terephthalate). An obvious increase in torque evolution associated with chain extending, branching and cross-linking was observed during the process. The properties of modified poly(ethylene terephthalate) were characterized by intrinsic viscosity and insoluble content measurements, rheological and thermal analysis. The intrinsic viscosity and rheological properties of modified PET were improved significantly when using PBT-GS, indicating that PBT-GS should be a better chain extender. Good foaming of poly(ethylene terephthalate) materials were obtained using supercritical CO₂ as blowing agent. The average cell diameter and cell density were 61 μm and 1.8 × 10⁸ cells/cm³, respectively.     

Viscoelasticity of oriented poly(ethylene terephthalate)

Time–temperature superposition can be successfully applied to both the stress relaxation and dynamic mechanical properties of oriented PET fibers. Two curves result; one is the time dependence of the modulus at constant temperature, while the other is the shift, log aT, of this curve along the time scale as a function of temperature. This temperature dependence is less than that for both unoriented PET and typical amorphous polymers above T_g. It is about the same as that for oriented nylon 66 and unoriented glassy poly(methyl methacrylate). The isothermal modulus has the same time dependence as that of the unoriented PET; however, it is a factor of 3.3 larger. The modulus curve is almost identical in both shape and magnitude with that of oriented nylon 66. However, a temperature of 82°C. is required to place the viscoelastic dispersion region of PET at the same time scale as nylon 66 at 25°C. This temperature increase is the major difference in viscoelasticity between these two oriented polymers.     

Fast crystallization of poly(ethylene terephthalate)

The crystallization of poly(ethylene terephthalate) (PET) was studied in the presence of nucleating agents and promoters. The effect of both by themselves and in concert was investigated using differential scanning calorimetry. The aim of this work is to find conditions of fast crystallization of PET. Sodium benzoate(SB) and Surlyn® (S) substantially increase the crystallization rate of PET at higher temperature owing to a reduction in the energy barrier towards primary nucleation, but they accelerate crystallization even more at lower temperature with an additional improvement of the molecular mobility of PET chains. Chain scission of PET caused by the reaction with the nucleating agents was proven by determination of molecular weight. The addition of S alone led to a lower reduction in molecular weight. A series of N-alkyl-p-toluenesulfonamides (ATSAs) were shown to effectively promote molecular motion of the PET chains, leading to an increase in crsytallization rate at lower temperature. A remarkable acceleration of crystallization of PET was attained at lower temperature when S and ATSA were added together. When the content of ATSA is low, S has the dominant influence due to its dual effect of decreasing energy barrier towards nucleation and promoting molecular motion of PET chains. A further increase of crystallization rate of PET was found only after an addition of ATSA of above 5 wt.%.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayThis work was supported by State Science and Technology Commission, and partially by National Science Foundation.     

Small angle X-ray scattering studies of poly(ethylene terephthalate) and poly(butylene terephthalate)

Summary Small angle X-ray studies and density measurements were carried out on isotropic PET and PBT samples. PET samples were crystallized between 60 and 260 °C, and PBT between 60 and 225 °C. The aim of these studies was to investigate the dependence of the amorphous density, the degree of crystallinity and the average transmission path through the regions of the two-phase system on the crystallization temperature. It could be shown that PET and PBT crystallize with sharp phase boundaries.Since for the evaluation of the amorphous density the knowledge the exact crystal density is very important, additional measurements of the wide angle X-ray behaviour were made. Both the crystal and the amorphous densities of PET and PBT show specific differences dependent on the crystallization temperature, which can be explained by the higher mobility of the PBT chain.The degrees of crystallization, evaluated with the individual values of crystal density and amorphous density determined on each sample, are principally higher than those calculated with the usually used values of crystal and amorphous density. Investigations of the background scattering have shown that both the specific amorphous and specific crystalline scattering background are constant.For PET and PBT the average transmission path through the amporhous regions firstly decreases with increasing crystallization temperature. This can be explained by new formation of crystallites. At higher crystallization temperatures increases. The average transmission path through the crystalline regions increases over the whole range of crystallization temperature.

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Zusammenfassung An isotropen PET- und PBT-Proben, kristallisiert bei Temperaturen zwischen 60 und 260 °C bzw. 60 und 225 °C wurden Röntgenkleinwinkel- und Dichtemessungen durchgeführt, mit dem Ziel, die amorphe Dichte, die Volumenanteile und die mittleren Durchschußlängen durch die Phasen in Abhängigkeit von der Kristallisationstemperatur zu bestimmen.Da für die Bestimmung der amorphen Dichte die Kenntnis der genauen Kristalldichte sehr wichtig ist, wurden zusätzliche Röntgenweitwinkelmessungen durchgeführt.Es konnte gezeigt werden, daß sowohl PBT als auch PET mit scharfen Phasengrenzen kristallisiert.Die Kristalldichte und die amorphe Dichte von PET bzw. PBT zeigen in Abhängigkeit von der Kristallisations-temperatur spezifische Unterschiede, die durch die höhere Beweglichkeit der PBT-Kette erklärt werden können.Die Kristallisationsgrade, die mit den von uns bestimmten Kristalldichten und amorphen Dichten ermittelt wurden, liegen generell höher als die mit den bekannten Werten von _c und _a berechneten. Untersuchungen des Streuuntergrundes zeigten, daß sowohl der spezifische amorphe als auch der spezifische kristalline Streuuntergrund konstant ist.Bei PET und PBT nehmen die mittleren Durchschußlängen durch die amorphen Phasenanteile bei geringen Kristallisationstemperaturen ab, was durch die Neubildung von Kristalliten erklärt wird, und nehmen bei höheren Kristallisationstemperaturen wieder zu.Die mittleren Durchschußlängen durch die kristallinen Phasenanteile nehmen über den gesamten Temperaturbereich zu.

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With 22 figures and 3 tables     

Reorganization of poly(ethylene terephthalate) structures and conformations to alter properties

The solid‐state morphologies, structures, and chain conformations of poly (ethylene terephthalate) (PET) have been reorganized/altered from those normally produced by solution and melt processing. This has been achieved by two distinct methods: (1) formation of a crystalline inclusion compound (IC) between guest PET and host γ‐cylodextrin (γ‐CD), followed by removal of the host γ‐CD and coalescence of the guest PET (c‐PET) and (2) rapid precipitation of PET from a warm trifluoracetic acid solution into a large excess of rapidly stirred acetone (p‐PET). Our prior observations (FTIR, NMR, DSC, X‐ray) demonstrated that c‐PET processed in this manner has a morphology, structure, and non‐crystalline chain conformations that are quite distinct from those of as‐received PET (asr‐PET). Where possible to compare, here we find that c‐ and p‐PETs behave very similarly, but very distinctly from asr‐PET. The reorganized c‐ and p‐PETs were found to be repeatedly rapidly crystallizable from the melt with a high level of crystallinity, and in their non‐crystalline regions to have tightly packed chains predominantly adopting highly extended kink conformations, which evidence no glass‐transition behavior. What is most unusual and somewhat puzzling is that their contrasting structures, morphologies, conformations, and thermal responses were observed to be independent of melt annealing, and persisted even after holding both samples above T_m for extended periods (hours). p‐PET, which can be produced in larger quantities than c‐PET, was utilized to measure additional macroscopic properties, such as melt viscosities, densities, and the stress‐strain and thermal shrinkage of melt‐pressed films, for comparison to those of asr‐PET. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 735–746, 2007     

Organocatalytic depolymerization of poly(ethylene terephthalate)

We describe the organocatalytic depolymerization of poly(ethylene terephthalate) (PET), using a commercially available guanidine catalyst, 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD). Postconsumer PET beverage bottles were used and processed with 1.0 mol % (0.7 wt %) of TBD and excess amount of ethylene glycol (EG) at 190 °C for 3.5 hours under atmospheric pressure to give bis(2‐hydroxyethyl) terephthalate (BHET) in 78% isolated yield. The catalyst efficiency was comparable to other metal acetate/alkoxide catalysts that are commonly used for depolymerization of PET. The BHET content in the glycolysis product was subject to the reagent loading. This catalyst influenced the rate of the depolymerization as well as the effective process temperature. We also demonstrated the recycling of the catalyst and the excess EG for more than 5 cycles. Computational and experimental studies showed that both TBD and EG activate PET through hydrogen bond formation/activation to facilitate this reaction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Heterogeneous nucleation of poly(ethylene terephthalate)

The effect of various metal salts as nucleating additives for poly(ethylene terephthalate) (PET) has been investigated. In the case of sodium benzoate and probably for all other effective nucleating additives, the nucleation process can be divided into a “heterogeneous particle nucleation” performed by the unreacted salt and a “homogeneous nucleation” due to the polymer–sodium (metal) salt formed during the extrusion. This polymer–sodium (metal) salt is the major nucleating agent in these systems. We have also shown the fundamental difference between the concept of a nucleating additive and that of a nucleating agent.     

Intrinsic birefringence of poly(ethylene terephthalate)

In the existing literature various values are given for the intrinsic birefringence of the crystalline and the amorphous phases in poly(ethylene terephthalate) (PET). These values have either been calculated theoretically or obtained from experimental data on the basis of certain models. In this investigation, using the Samuels two-phase model which correlates sonic modulus with structural parameters, intrinsic birefringence values for the crystalline (Δn_c) and amorphous (Δn_a) phases have been determined by studying 30 PET samples prepared by heat setting to have a wide range of structures; the results are Δn_c = 0.29 and Δn_a = 0.20. These values are discussed along with others in the literature and it is concluded that in the light of the present work, the values used by many authors need reexamination.     

Stress-induced crystallization of poly(ethylene terephthalate)

Quenched amorphous films of poly(ethylene terephthalate) (PET) are stretched at temperatures less than T_g; changes in density, wide-angle x-ray diffraction, and small-angle light scattering are observed. The density increase upon stretching is attributed to an increase in crystallinity accompanied by an increase in the intensity of somewhat diffuse wide-angle x-ray diffraction and of both V_V and H_V small-angle light scattering patterns. The formation of oriented rodlike superstructure may be discerned from small-angle light scattering. Annealing of these samples increases the crystallinity as measured from density and leads to an increase in the perfection of crystalline and supercrystalline structure as measured by wide-angle x-ray diffraction and small-angle light scattering. The rodlike morphology changes to form spherulitelike aggregates as observed by small-angle light scattering and light micrographs. A model is proposed to explain the observations. Studies are extended to stretching films of PET above their T_g and observing changes in birefringence, density, wide-angle x-ray diffraction and small-angle light scattering as a function of elongation and stretching temperature. The formation of defomed spherulitelike superstructure may be discèrned from light micrographs. Results are compared with those obtained upon stretching films below T_g.     

Light-scattering characterization of poly(ethylene terephthalate)

By using a closed-circuit filtration system, we have succeeded in clarifying poly(ethylene terephthalate) (PET) dissolved in hexafluoroisopropanol (HFIP). Such static properties as the radius of gyration R_g, the weight-average molecular weight M_w, and the second virial coefficient A₂ and such dynamic properties as the translational diffusion coefficient D, or its equivalent hydrodynamic radius R_h, and the second (diffusion) virial coefficient k_d were determined for several PET samples of different molecular weights by using light-scattering intensity and linewidth measurements. An empirical relation between D_o (or R_h) and M_w was established: R_h = (1.77±0.15)X10^?2 M with R_h and M_w expressed in units of nanometers and grams per mole, respectively. The empirical exponent α_D(ca. 0.58±0.01) is in good agreement with the less precisely determined intrinsic viscosity/molecular weight exponent α_η (ca. 0.71±0.02). Several intensity correlation functions were measured very precisely using long accumulation times. A Laplace inversion was performed using the singular-value decomposition technique. The approximate molecular weight distribution (MWD) determined by light-scattering spectroscopy was in reasonable agreement with a completely independent determination of MWD using gel permeation chromatography (GPC). It was interesting to note, though not surprising, that GPC showed emphasis on lower-molecular-weight fractions, while light-scattering emphasized higher-molecular-weight fractions. The agreement further strengthens some complementary aspects of the two techniques.     

Synthesis and hydrolytic degradation of poly(ethylene succinate) and poly(ethylene terephthalate) copolymers

The conditions of synthesis of statistical poly(ethylene succinate-co-terephthalate) copolymers (2GTS) and high molecular weight poly(ethylene succinate) (PES) with good hydrolytic and optical parameters, designed for the production of biodegradable products and resins, are presented in this article. Copolymers were prepared by melt polycondensation of bis-(β-hydroxyethylene terephthalate) (BHET) and succinic acid (SA) with excess of ethylene glycol (2G) in the presence of a novel titanium/silicate catalyst (C-94) and catalytic grade of germanium dioxide (GeO₂) as cocatalyst. The chemical structure and physical properties of those materials were characterized by ¹H NMR, FT-IR, dynamical-mechanical thermal analyses (DMTA), differential scanning calorimetry (DSC), solution viscosity and spectroscopic methods. The hydrolytic degradation was performed in a water solution with variable pH, also in garden soil and in compost. The highest hydrolytic degradation rate was observed for pH 4 and for compost. Better hydrolytic degradation values in compost medium were observed for copolyester prepared in the presence of GeO₂ as polycondensation cocatalyst. The copolyester with 40 mol% of aliphatic units was chosen for industrial syntheses which were performed in ELANA and subsequently the processing parameters and compatibility with potato starch of this polyester were checked by BIOP Biopolymer Technologies AG.