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     

Equilibrium melting parameters of poly(ethylene terephthalate)

The equilibrium melting temperature, volume, and enthalpy and entropy changes on melting of poly(ethylene terephthalate) have been analyzed and heats of fusion have been newly measured with an automated scanning calorimeter to yield the following data: 553°K, 16.9 cm³/mole, 2.69 kJ/mole, and 48.6 J/deg/mole, respectively. A more detailed discussion of annealed samples obtained from etched starting materials shows that the density of the noncrystalline regions may be variable.     

Thermal history and melting behavior of poly(ethylene terephthalate)

Low molecular weight poly(ethylene terephthalate) samples were crystallized isothermally at 120–245°C from both the amorphous state and the melt. Isothermal annealing of these polymers at 215°C provided polymers which exhibited multiple melting peaks in thermal analysis, referred to as form I and form II, as assigned by Bell and Dumbleton. In these samples the peak temperature of the form II melting endotherm and the average crystallite size are dependent on the temperature of initial crystallization. This result requires a mechanism for retaining some structural feature during the conversion from morphological form I to form II. DSC thermograms obtained at varying heating rates on samples showing only form II endotherms support the assignment of superheating as the cause of the shift to higher peak temperatures with increasing heating rate.     

Brillouin scattering studies of the effect of orientation on mechanically deformed poly(ethylene terephthalate)

Brillouin scattering is used to study the internal structure of oriented films of poly(ethylene terephthalate). Splitting in the longitudinal spectrum is observed as the film is stretched, indicating that the crystalline region is developed gradually from the amorphous region. The hypersonic velocity data obtained from these two regions are used to draw directional maps of sound velocity propagation in different directions of the film. The results are discussed and correlated with a recently proposed model. The orientational parameter in the amorphous phase is calculated from the hypersonic velocity data as a function of stretch ratio. The results are found to be in good agreement with published values obtained by a different technique.     

Rigid amorphous fraction and melting behavior of poly(ethylene terephthalate)

The multiple melting behavior of poly(ethylene terephthalate) (PET) is generally attributed to the fusion of original crystals recrystallized during the heating at conventional scanning rate. In the present study, the triple and double melting behavior that is observed after isothermal crystallization at T _c lower and higher than 215 °C, respectively, is put in relation with the presence and absence of rigid amorphous fraction around the original primary crystal lamellae. The complex melting behavior is explained by assuming that two different morphologies of primary crystals develop during crystallization at temperatures lower than 215 °C, in a proportion that is a function of the crystallization temperature: chain cluster aggregations with a high percentage of rigid amorphous fraction on the boundaries and small crystals with a high percentage of adjacent reentry folding and reduced constraints at the amorphous/crystal interphase. These distinct morphologies differently transform upon heating at low scanning rate, originating two endotherms. On the contrary, after crystallization at T _c ?>?215 °C, all the primary crystalline structure, which probably are characterized by the same morphology made of tightly chain folded lamellae and absence of rigid amorphous fraction, undergo the same reorganization route, originating a single endotherm.     

Effect of iodine on small-angle X-ray diffraction from drawn poly(ethylene terephthalate) film

The effect of iodine sorption on the small-angle x-ray diffraction intensity is discussed in terms of the distribution of iodine sorbed in the amorphous regions of polymers. The change in the intensity of the peak corresponding to long periods is measured as a function of the iodine uptake in drawn, and annealed poly(ethylene terephthalate) films. A linear relation with slope K is observed between the square root of relative intensity and iodine uptake. The slope K varies with annealing conditions. Theoretical values calculated for models of the distribution of iodine in amorphous regions, are compared with the observed K. Results can be interpreted on the basis of a “concave” distribution of iodine, i.e., with sorption sites more numerous near a crystal surface than in the interior of an amorphous region.     

Surface studies of acid treated poly (ethylene terephthalate) filaments

Surface features of the sulphuric acid treated and untreated PET filaments, drawn at different draw ratios, have been studied through XPS(ESCA) and SEM. It is shown that the top surface of the polyethylene terephthalate fibres degrades, but the increase in draw ratio decreases the degradation.     

Infrared studies of cryogenic transitions in poly(ethylene terephthalate)

Measurements of infrared absorbance as a function of temperature down to 77°K have been made on poly(ethylene terephthalate). Transitions observed by other techniques such as torsion pendulum measurements are reflected in the infrared measurements by an abrupt change in the slope of the absorbance versus temperature. The infrared measurements are made on bands of known structural origin so the structures contributing to each transition can be determined. The β transition is shown by infrared to be a composite of two peaks with the lower-temperature portion arising from the amorphous regions and the higher-temperature portion from the crystalline regions.     

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.     

The triple melting behavior of poly(ethylene terephthalate): Molecular weight effects

The melting behavior of isothermally crystallized PET has been studied using linear heating in a differential scanning calorimeter (DSC). Variables such as crystallization temperature, crystallization time, heating rate, and average molecular weight are the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET is a function of programmable DSC variables such as crystallization temperature, crystallization time, and heating rate. However, in testing the hypothesis that there was a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC-independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involves secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1757–1774, 1997     

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.     

A small-angle neutron scattering study of adsorbed poly(ethylene oxide) on Laponite

The adsorption of poly(ethylene oxide) (PEO) on synthetic anisotropic clay particles (Laponite) has been investigated as a function of the molecular weight. Contrast variation small-angle neutron scattering (SANS) measurements were used to characterize the distribution and adsorbed amount of polymer on the particles. These experiments show not only that polymer is present on the face of the clay particle but that it also extends or "wraps" over the edges. The edge layer was thicker than the face layer for all the molecular weights studied. The polymer layers are unusually thin, with a thickness and adsorbed amount that show little variation with molecular weight.     

New insight into melting and crystallization behavior in semicrystalline poly(ethylene terephthalate)

After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastable melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable melt can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The crystallization kinetics of this metastable melt can be determined by means of DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n = 1.0 ∼ 1.2, probably reflecting one‐dimensional or irregular line growth of the crystal occurring between the existing main lamellae with heterogeneous nucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thicknesses. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 53–60, 2000     

Compatibility studies of blends of polycarbonate and poly(ethylene terephthalate)

Blends of bisphenol-A polyarbonate (PC) and poly(ethylene terephthalate) (PET) has been investigated by differential scanning calorimetry and scanning electron microscopy. Blends were prepared by screw extrusion and solution casting with weight fractions of PC in the blends varying from 0.90 to 0.10. From the measured glass transition temperature (T_g) and apparent weight fractions of PC and PET dissolved in each phase, it appears that PET dissolves more in the PC-rich phase than does the PC in the PET-rich phase. The composition-dependent values of the Flory–Huggins polymer–polymer–interaction parameter were determined and found to be from 0.054 to 0.037 for extruded blends at 275°C and from 0.058 to 0.040 for solution casting at 25°C. The interaction parameter decreases with increasing PET concentration. This result is consistent with the values of the T_gs, the microscopy study, and the measured extrudate swell ratios which show that compatibility increases more in the PET-rich compositions than in the PC-rich compositions. The PC–PET blends are not microscopically miscible for all the blend compositions.     

Static light scattering from poly(ethylene oxide) in methanol

Static light scattering measurements were performed on dilute solutions of monodisperse poly(ethylene oxide) (PEO) in methanol at 25°C. PEOs of five different molecular weights ranging from nominal M_w = 8.6 × 10⁴ to 9.13 × 10⁵ were used. Linear Zimm plots were obtained for all the PEO samples: no downturn was observed at small angles, indicating that no large aggregates of PEO molecules exist in the solution. From the plots, values of the weight-average molecular weight, M_w, the radius gyration, R_G, and the second virial coefficient, A₂, were successfully determined for respective PEOs. Observed relationship between R_G and M_w indicates that methanol is certainly a good solvent for the polymer. © 1996 John Wiley & Sons, Inc.     

A small-angle neutron scattering study of poly(ethylene oxide) microstructure in aqueous poly(styrenesulfonate sodium) solutions

Dilute aqueous solutions of d-PEO and PSSNa mixtures were studied by (2)H NMR spectroscopy and small-angle neutron scattering (SANS). The interactions between d-PEO and PSSNa were found to be negligible both in the presence and absence of NaCl. At very dilute concentration (0.7 mg mL(-1)), d-PEO chains were still found to be slightly collapsed at ambient temperature in water. Upon the addition of PSSNa, aggregates of d-PEO were observed with d-PEO coils loosely associated with each other. The average centre to centre distance between d-PEO coils, which was calculated from the maxima in SANS spectra, was similar to the size of the individual coils. The effect of a simple salt, NaCl, on d-PEO-PSSNa interaction was investigated. Salt addition induced a breakdown of the dilute d-PEO aggregates.     

Reversible melting in poly(butylene terephthalate)

Crystallized samples of poly(butylene terephthalate) (PBT), examined in the melting region by means of temperature modulated differential scanning calorimetry (TMDSC), show reversible fusion. The analysis of the complex heat capacity reveals that the fusion of poor crystallites can follow temperature modulation more easily than perfect crystals, in agreement with the findings recently reported in the literature, and that the amount of reversible melting decreases with increasing the modulation frequency.