Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.     

The low-temperature endotherm in poly(ethylene terephthalate): partial melting and rigid amorphous fraction mobilization

A detailed investigation of the low-temperature endotherm of poly(ethylene terephthalate) (PET) performed by temperature-modulated differential scanning calorimetry is presented. The origin of the small endotherm, generally observed a few degrees above the crystallization temperature in PET and in many other polymers, is a widely discussed matter. The most frequent interpretation considers it the result of partial fusion with superposition of a recrystallization process even if it has also been proposed that it can originate from enthalpic recovery connected to mobilization of the rigid amorphous fraction. In an attempt to resolve the question, a new method for the interpretation of the modulated heat-flow-rate curve resulting from a temperature modulation program is proposed. The procedure consists of the analysis of the initial points of the steady-state heat-flow-rate signals in the heating and cooling semiperiods with the temperature modulation being performed with a sawtooth profile. The study conducted in parallel on the reversing specific heat capacity and the heat-flow-rate curves, observed on heating after isothermal crystallization at various temperatures, showed that multiple processes, involving both the crystalline and the rigid amorphous fraction, overlap in the temperature range in which the low-temperature endotherm is observed. The origin of the endotherm under investigation is therefore connected with both partial fusion of the crystalline portions and enthalpy recovery subsequent to structural relaxation of the rigid amorphous fraction. An estimation of the relative percentages of the two different processes is presented and discussed.     

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.     

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.     

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     

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     

Crystallization and melting behavior of liquid crystalline copolyesters based on poly(ethylene terephthalate)

A series of copolyesters were prepared by the incorporation of p‐hydroxybenzoic acid (HBA), hydroquinone (HQ), and terephthalic acid (TA) into poly(ethylene terephthalate) (PET). On the basis of viscosity measurements, high molar mass copolyesters were obtained in the syntheses, and ¹H‐NMR analyses indicated the total insertion of comonomers. They exhibit nematic phase above melting temperature, as observed by polarized light microscope (PLM). Their crystallization and melting behaviors were also studied by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). It was found that these copolyesters are more crystalline than copolyesters prepared from PET and HBA. Introduction of HQ/TA disrupts longer rigid‐rod sequences formed by HBA, and thus enhances molecular motion and increases crystallization rate and crystallinity. Isothermal crystallization at solid phase polymerization conditions (up to 24 h at 200°C) resulted in increased copolymer randomness (by NMR) and higher melting point, the latter attributed to structural annealing. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 369–377, 1999     

Elastic behavior of poly(ethylene terephthalate) chains

The Monte Carlo (MC) method based on the rotational-isomeric-state (RIS) model is adopted in studying the elastic behavior of poly(ethylene terephthalate) (PET) chains in this paper. The mean-square end-to-end distance 〈R²〉, the mean-square radius of gyration 〈S²〉, and the ratio of 〈R²〉/〈S²〉 all increase with elongation ratio λ. The interior conformations are also investigated through calculating the a priori probability of rotational state in the process of tensile elongation. The radius of gyration tensor S is introduced here in order to measure the shape of PET chains, and increases with elongation ratio λ, however, some different behaviors are obtained for . Here , and are the eigenvalues of the radius of gyration tensor . The average energy per repeat unit 〈U〉 and the average free energy per repeat unit 〈A〉 are also calculated, and we find that the average energy decreases with elongation ratio λ, however, the average free energy per repeat unit increases with elongation ratio λ. Elastic force f, energy contribution to force f_U, and entropy contribution to force f_S are also investigated. Both elastic force f and entropy contribution to force f_S increases with λ, however, energy contribution to force f_U and the ratio f_U/f decreases with λ. The ratio of f_U/f is less than zero and almost independent of chain length. The results of these microscopic calculations may explain some macroscopic phenomena of rubber elasticity.     

Non-isothermal crystallization and melting behavior of compatibilized polypropylene/recycled poly(ethylene terephthalate) blends

Binary blends of polypropylene (PP)/recycled poly(ethylene terephthalate) (r-PET), r-PET/maleic anhydride grafted PP (PP-g-MA), r-PET/glycidyl methacrylate grafted PP (PP-g-GMA), and ternary blends of PP/r-PET (80/20 w/w) compatibilized with various amounts (2-10 wt%) of PP-g-MA or PP-g-GMA were prepared on a twin-screw extruder. The non-isothermal crystallization and melting behavior, and the crystallization morphology were investigated by DSC and POM. The chemical reactions of r-PET with PP-g-MA and PP-g-GMA were characterized by FT-IR. DSC results show that the crystallization peak temperatures of r-PET and PP increased when blending them together, due to the heterogeneous nucleation effect on each other. The of r-PET increased with increasing the content of PP-g-MA while slightly influenced by the content of PP-g-GMA in the binary blends of r-PET with grafted PP, implying different reactivity of r-PET with PP-g-MA and PP-g-GMA. The of PP in the ternary blends retained or slightly decreased, dependent on the compatibilizers and their contents. The melting peak temperature of r-PET in PP/r-PET blends compatibilized by PP-g-MA was lower than that of compatibilized by PP-g-GMA, indicating that PP-g-MA had stronger reactivity towards r-PET compared to PP-g-GMA. The crystallization and melting behavior of blends was influenced by the pre-melting temperature, especially the melting behavior of r-PET in the blends. The crystallization behavior of PP in the blends was also evaluated by Mo’s method. POM confirmed the heterogeneous nucleation effect of r-PET on PP.     

Isothermal crystallization kinetics and melting behavior of poly(ethylene terephthalate)/barite nanocomposites

Poly(ethylene terephthalate) (PET)/Barite nanocomposites were prepared by direct melt compounding. The effects of PET‐Barite interfacial interaction on the dynamic mechanical properties and crystallization were investigated by DMA and DSC. The results showed that Barite can act as a nucleating agent and the nucleation activity can be increased when the Barite was surface‐modified (SABarite). SABarite nanoparticles induced preferential lamellae orientation because of the strong interfacial interaction between PET chains and SABarite nanoparticles, which was not the case in Barite filled PET as determined by WAXD. For PET/Barite nanocomposites, the Avrami exponent n increased with increasing crystallization temperature. Although at the same crystallization temperature, the n value will decrease with increasing SABarite content, indicating of the enhancement of the nucleation activity. Avrami analyses suggest that the nucleation mechanism is different. The activation energy determined from Arrhenius equation reduced dramatically for PET/SABarite nanocomposite, confirming the strong interfacial interaction between PET chains and SABarite nanoparticles can reduce the crystallization free energy barrier for nucleus formation. In the DSC scan after isothermal crystallization process, double melting behavior was found. And the double endotherms could be attributed to the melting of recrystallized less perfect crystallites or the secondary lamellae produced during different crystallization processes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 655–668, 2009     

Relations between dynamic mechanical properties and melting behavior of nylon 66 and poly(ethylene terephthalate)

Nylon 66 films exhibiting form I melting behavior show the γ mechanical relaxation at ?140°C. Samples which have form II melting behavior do not show this relaxation. The γ relaxation disappears when material having form I behavior is converted to material having form II behavior by annealing or by cold drawing. The form I and form II types of melting behavior are also found in poly(ethylene terephthalate); the interconversions and thermal behavior of the forms are analogous to the nylon 66 case. In poly(ethylene terephthalate), the β relaxation at ?40 to ?60°C is present only when form I melting behavior is found. Conversion to form II melting behavior by annealing or drawing (80°C) again causes the relaxation to disappear. No β relaxation was found in amorphous polymer. The γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) can therefore be associated with the crystalline structure responsible for form I melting behavior. Form I melting behavior has been associated with foldedchain crystals based on previous work. It is therefore postulated that the γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) are associated with motions in the chain folds. This assignment is not inconsistent with the change in the γ dispersion of nylon 66 with the number of backbone CH₂ units, since these will affect the fold structure.     

Morphology development and crystallization behavior of poly(ethylene terephthalate)/poly(trimethylene terephthalate) blends

The spherulite morphology and crystallization behavior of poly(ethylene terephthalate) (PET)/poly(trimethylene terephthalate) (PTT) blends were investigated with optical microscopy (OM), small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS). The thermal analysis showed that PET and PTT were miscible in the melt over the entire composition range. The rejected distance of non-crystallizable species, which was represented in terms of the parameter δ, played an important role in determining the morphological patterns of the blends at a specific crystallization temperature regime. The parameter δ could be controlled by variation of the composition, the crystallization temperature, and the level of transesterification. In the case of two-step crystallization, the crystallization of PTT commenced in the interspherulitic region between the grown PET crystals and proceeded until the interspherulitic space was filled with PTT crystals. The spherulitic surface of the PET crystals acted as nucleation sites where PTT preferentially crystallized, leading to the formation of non-spherulitic crystalline texture. The SALS results suggested that the growth pattern of the PET crystals was significantly changed by the presence of the PTT molecules. The lamellar morphology parameters were evaluated by a one-dimensional correlation function analysis. The blends that crystallized above the melting point of PTT showed a larger amorphous layer thickness than the pure PET, indicating that the non-crystallizable PTT component might be incorporated into the interlamellar region of the PET crystals. With an increased level of transesterification, the exclusion of non-crystallizable species from the lamellar stacks was favorable due to the lower crystal growth rates. As a result, the amorphous layer thickness of the PET crystals decreased as the annealing time in the melt state was increased.     

Morphology and melting behavior of semicrystalline poly(ethylene terephthalate). II. Annealed PET

Annealing of poly(ethylene terephthalate) samples crystallized under isothermal conditions above the crystallization temperature has a marked influence on their morphology and results in increased thermal stability of the crystalline structure as indicated by the melting point increase of the samples. The morphological transformation processes induced by annealing are very complex and depend on the thermal history of the samples, i.e., crystallization temperature and heating procedure. Depending on the nature of the processes occurring during annealing, various parameters characterizing the semicrystalline state of the samples can be affected such as the degree of crystallinity, the long spacing, the thickness of amorphous and crystalline layers, the crystal perfection, the fold-surface structure, and the mosaic structure of the crystalline lamellae. Annealing involves a solid-state transformation of the original crystalline structure including crystal perfection without thickening or a melting followed by recrystallization with crystal perfection and crystal thickening. The combination of differential scanning calorimetric (DSC) measurements and small-angle x-ray scattering is a powerful analytical tool to detect morphological changes and helps in deciding on the processes which are involved in the transformation of the microstructure upon annealing.     

Low-frequency glow discharge treatment of amorphous poly(ethylene terephthalate) films

The influence of treatment in a low-frequency glow discharge on the surface properties of an amorphous poly(ethylene terephthalate) (PET) film was studied. It was shown that, at identical external discharge parameters, changes in the wettability of plasma-treated PET films depended on its morphological structure—the amorphous film had higher values of the contact angle and lower values of the surface charge density than a biaxially oriented poly(ethylene terephthalate) of the PET-E brand.     

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.