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     

Kinetics of structural relaxation of poly(ethylene terephthalate)

The results of an experimental study of the kinetics of structural relaxation of amorphous poly(ethylene terephthalate) are reported. Samples were prepared by ultraquenching the melt on rotating stainless-steel discs. Two types of measurements by differential scanning calorimetry were made: (1) the dependence of the “fictive” (or “structural”) temperature T_f(q^?) introduced by Tool, on the cooling rate q^? and (2) the dependence of the glass transition temperature T_g on the heating rate q⁺. In this way the value x = 0.47 was obtained for the dimensionless parameter proposed by Narayanaswamy.     

Infrared spectroscopic study of solvent-induced structural changes in poly(ethylene terephthalate) film

It is found from polarized infrared spectroscopy that treatment of oriented poly(ethylene terephthalate) (PET) film with strongly interacting solvents such as s-tetrachloroethane, nitrobenzene, dimethylformamide, and benzyl alcohol results in disorientation of polymer chains. Partial transformation from trans to gauche conformation takes place on thermal treatment and, to a larger extent, on solvent treatment. Increasing sharpness of the 988 cm^?1 band on solvent treatment indicates the formation of regular chain folds. Some new spectral features are discussed.     

X-ray determination of crystallinity and orientation in poly(ethylene terephthalate)

The measurement of crystallinity and orientation in poly(ethylene terephthalate) is discussed. A simple procedure is given for the estimation of orientation from the (1 05) plane, and it is shown that methods which use the equatorial planes are subject to certain disadvantages. In addition a method is given for the measurement of x-ray crystallinity. The technique is applied to fibers and films of various treatments and a linear relation is found between density and x-ray crystallinity.     

Visualization of strain-induced structural rearrangements in amorphous poly(ethylene terephthalate)

A direct microscopic observation procedure is applied to study the deformation of amorphous PET decorated with a thin metal layer when stretching is performed at different draw rates and at temperatures below and above the glass transition temperature T _g. Analysis of the formed microrelief allows stress fields responsible for the deformation of the polymer to be visualized and characterized. When tensile drawing is performed at temperatures above T _g, inhomogeneity of stress fields increases with the increasing draw rate; at high draw rates, the stress-induced crystallization of PET takes place. In the case of drawing the polymer at temperatures below T _g, direct microscopic observations make it possible to visualize the development of shear bands that appear in the unoriented part of the polymer specimen adjacent to the neck. The shear bands are oriented at an angle of about 45° with respect to the draw direction. When necking involves the unoriented part of the polymer, shear bands abruptly change their orientation and become aligned practically parallel to the draw axis.     

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.     

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.     

Fourier-transform infrared study of coextrusion-drawn poly(ethylene terephthalate)

The effects of initial morphology and extrusion temperature on the orientational anisotropy and conformational changes on coextrusion drawing of poly(ethylene terephthalate) (PET) have been determined by Fourier-transform polarized infrared spectroscopy. The samples were drawn from both amorphous and semicrystalline (50%) PET at 50 and 90°C. A strong influence of coextrusion drawing temperature was observed for overall chain orientation evaluated from the dichroic ratio of the 795-cm^?1 band for the samples prepared from the amorphous state: this dependence was less prominent in samples drawn from the semicrystalline state. Under the same drawing conditions, the dichroic ratio for the 973-cm^?1 trans band for samples prepared from the amorphous state was higher than from the semicrystalline state. Furthermore, in all samples, the relative intensity of this band was almost proportional to the degree of crystallinity. In all samples, the gauche content, evaluated from the 896-cm^?1 band, decreased with increasing draw ratio. However, the dichroic ratio of this band was near unity regardless of draw ratio, initial morphology, or extrusion temperature. From these results it is considered that all gauche units in the amorphous phase are almost isotropic in the extrusion-drawn samples with overall orientation arising largely from the crystalline chains possessing totally the trans conformation (973 cm^?1) in its content. In order to evaluate the deformation mechanism of the coextrusion drawing method, the relationship between the bulk and film surface orientation is also reported.     

Thermally induced structural changes in low-shrinkage poly(ethylene terephthalate) fibers

This work studies the behavior of low shrinkage PET fibers during free-ends thermal annealing. The interest in this type of sample stems from the fact that it possesses an interesting structure characterized by the presence of crystalline and amorphous domains both in a highly extended and oriented state. Furthermore, thermal annealing is not able to produce a significant increase in the crystalline content. Thus, the lack of crystallization allows to isolate the effect of chain recoiling on the observed phenomena. To follow changes at molecular and microstructural levels, Fourier transform infrared spectroscopy with photoacoustic detection, differential scanning calorimetry, wide-angle x-ray diffraction and scanning electron microscopy techniques were employed. By their use, substantial structural changes in the amorphous and crystalline domains were found which, finally, were related to the macroscopical behavior of the material, mainly the observed shrinkage and the mechanical properties. © 1996 John Wiley & Sons, Inc.     

Visualization of structural rearrangements during annealing of solvent-crazed poly(ethylene terephthalate)

A direct microscopic procedure is used for studying structural rearrangements during the annealing of PET samples after solvent crazing. Even at room temperature, solvent-crazed PET samples experience shrinkage which is provided by processes taking place in crazes. This shrinkage is observed at temperatures up to the glass transition temperature of PET and proceeds via drawing together of crack walls. Once the glass transition temperature is attained during annealing, the spontaneous self-elongation of the polymer sample occurs. The mechanism of this phenomenon is proposed. The low-temperature shrinkage of the polymer sample is related to the entropy contraction of highly dispersed material in crazes that has a lower glass transition temperature than that of the bulk polymer. This shrinkage cannot be complete, owing to crystallization of the oriented polymer in the volume of the crazes. As a result of crystallization, the oriented and crystallized polymer in the crazes coexists with the regions of the unoriented initial PET. As the annealing temperature approaches the glass transition temperature of the bulk PET, its strain-induced crystallization takes place. As a result, the regions of the unoriented polymer between crazes are elongated along the direction of tensile drawing and the sample experiences contraction in the normal direction.     

Quantitative structural characterization of the mechanical properties of poly(ethylene terephthalate)

The mechanical properties of poly(ethylene terephthalate) are examined to see if the quantitative morphological criteria previously used to define the structural state of isotactic polypropylene will lead to similar quantitative structure-property correlations when applied to PET. Tenacity, thermal shrinkage in oil and air, tensile modulus, dynamic loss modulus, long spacings, and small-angle x-ray intensities are included in the study. All of the data were obtained from the open literature. In all cases the physical properties correlate simply and quantitatively with the internal structure of the PET, and the character of the correlations leads to new insights into the nature of the internal mechanisms controlling the observed properties. The study shows that only by considering the structural state of a polymer, and not its fabrication parameters, can general, simplifying, quantitative, structure-property correlations be achieved.     

Laser-raman study of crystallinity changes in poly(ethylene terephthalate)

The bandwidth at one-half maximum intensity of the Raman-active C?O stretching vibration was found to correlate with density in poly(ethylene terephthalate). This correlation is valid for unoriented as well as highly oriented materials. It is suggested that a number of rotational states of the terephthalate segment occur in the amorphous polymer. Crystallization causes a resonance-stabilized planar conformation.     

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     

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.     

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.