Effect of crystallinity on enthalpy recovery peaks and cold‐crystallization peaks in PET via TMDSC and DMA studies

Poly(ethylene terephthalate) (PET) sheets of different crystallinity were obtained by annealing the amorphous PET (aPET) sheets at 110°C for various times. The peaks of enthalpy recovery and double cold‐crystallization in the annealed aPET samples with different crystallinity were investigated by a temperature‐modulated differential scanning calorimeter (TMDSC) and a dynamic mechanical analyzer (DMA). The enthalpy recovery peak around the glass transition temperature was pronounced in TMDSC nonreversing heat flow curves and was found to shift to higher temperatures with higher degrees of crystallinity. The magnitudes of the enthalpy recovery peaks were found to increase with annealing times for samples annealed ≤30 min but to decrease with annealing times for samples annealed ≥40 min. The nonreversing curves also found that the samples annealed short times (≤40 min) having low crystallinity exhibited double cold‐crystallization peaks (or a major peak with a shoulder) in the region of 108–130°C. For samples annealed long times (≥50 min), the cold‐crystallization peaks were reduced to one small peak or disappeared because of high crystallinity in these samples. The double cold‐crystallization exotherms in samples of low crystallinity could be attributed to the superposition of the melting of crystals, formed by the annealing pretreatments, and the cold‐crystallizations occurring during TMDSC heating. The ongoing crystallization after the cold crystallization was clearly seen in the TMDSC nonreversing heat flow curves. DMA data agreed with TMDSC data on the origin of the double cold‐crystallization peaks. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Variations in polymeric structure of ferroelectric poly(vinylidene fluoride) films during annealing at various temperatures

To clarify the thermal degradation mechanisms of uniaxially drawn poly(vinylidene fluoride) (PVDF) films, variations due to annealing in the polymeric structures of the films were investigated using the small‐angle X‐ray scattering (SAXS) and Fourier transform infrared (FTIR) spectroscopy. The films were composed of lamellar crystals that were stacked perpendicular to the stretch direction. Although the crystallinity of the films decreased during annealing in the temperature range above the preannealing temperature, the lamellar structure was maintained even after the annealing process. There are two kinds of irreversible relaxation mechanisms during the annealing process of the films, including both a decrease in crystallinity within the lamellae and also thickening of the lamellae. A significant lamella thickening effect was observed when the films were annealed above ～ 100°C. FTIR spectra suggested some disordered structures are developed during thickening of the lamellae. Furthermore, a long‐range periodic structure was formed in the films that were annealed above the melting temperature of PVDF. The polymeric structures formed during the fabrication process (including high‐order structures and disorders in molecular conformation) were clarified as having a significant influence on the annealing behavior of ferroelectric PVDF films. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Homogeneous nucleation and mesophase formation in glassy isotactic polypropylene

Differential fast scanning chip calorimetry has been employed to study nucleation/ordering during annealing the glass of quenched isotactic polypropylene. Initially non-ordered samples were annealed below the glass transition temperature for different periods of time, and the change of structure during isothermal annealing was then analyzed by monitoring the exchange of latent heat on heating. Primary result of this work is the proof of homogeneous nucleation of ordering and mesophase formation in the glassy state. It is suggested that only local non-cooperative mobility of molecular segments is required to form small, ordered domains, and that the classical nucleation theory, which restricts nucleation of the crystallization/ordering process of polymers to temperatures between the equilibrium melting temperature and the glass transition temperature, needs modification.     

Physical Aging of Arsenic Trisulfide Thick Films and Bulk Materials

Differential scanning calorimetry (DSC) measurements were performed at different heating rates under nonisothermal conditions to probe the glass transition kinetics of As₂S₃ bulk glass and thick films. We found that, for films, the glass transition temperature ( T _g) increases with increasing annealing temperature, and sharp crystalline-like melting endothermic peaks appear in samples annealed at 160°C. Such sharp endothermic peaks could also be found for samples annealed at 140°C for 300 h, indicating a very slow crystallization process. The activation energy for the molecular motions and rearrangements around T _g could be estimated as 2.12 eV for films and 2.3 eV for the bulk, respectively.     

Glass transition and structural relaxation in semi-crystalline poly(ethylene terephthalate): a DSC study

The aim of this work is to determine the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semi-crystalline poly(ethylene terephthalate) (PET) and compare them with those calculated in amorphous PET. Samples of nearly amorphous polymer were prepared by quenching and samples with different crystallinity fractions were prepared from the amorphous one using cold crystallisation to different temperatures. The differential scanning calorimetry (DSC) thermograms measured on samples rapidly cooled from temperatures immediately above the glass transition show a single glass transition which is much broader in the case of high-crystallinity samples than in the amorphous or low-crystallinity PET. To clarify this behaviour, the samples were subjected to annealing at different temperatures and for different periods prior to the DSC measuring heating scan. The thermograms measured in samples with low crystallinity clearly show the existence of two amorphous phases with different conformational mobility, these are called Phases I and II. Phase I contains polymer chains with a mobility similar to that in the purely amorphous polymer, while Phase II shows a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allows to determine the temperature dependence of Phase II relaxation times, which are independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature.     

Glass transition and cooperative rearranging regions in amorphous thermoplastic nanocomposites

The aim of this work was to study the effect of nanofillers on the structural relaxation phenomena occurring in amorphous poly(ethylene‐terephthalate)/poly(cyclohexane‐dimethanol terephthalate) copolymer (PET/PCHDMT) nanocomposites in correspondence with the glass transition temperature. PET/PCHDMT nanocomposites were prepared by melt mixing with an organic modified montmorillonite at different processing temperatures. Differential scanning calorimetry analysis revealed that addition of the organic modifier alone causes a decrease of the glass transition temperature and an increase of the specific heat discontinuity. Nanocomposites showed a higher glass transition temperature and a lower specific heat discontinuity compared with samples obtained by adding organic modifier to PET/PCHDMT. Both effects were more relevant for samples processed at lower temperatures. Therefore, the glass transition temperature was studied by introducing the concept of fictive temperature and relaxation time. It was found that nanocomposites have a higher apparent activation energy and an increased size of cooperatively rearranging regions compared with neat PET/PCHDMT. Both effects are more relevant for nanocomposites processed at lower temperatures. All the discussed effects are explained by considering the enhanced confinement of PET/PCHDMT macromolecules, due to the presence of intercalated lamellae of organofiller. The efficiency of intercalation is increased by decreased processing temperature, which involves an increase of the nano‐confinement area of the polymer. Copyright © 2012 Society of Chemical Industry     

Sorption kinetics and equilibria in annealed glassy polyblends

The sorption kinetics and equilibria of n-hexane in glassy polyblends of polystyrene and poly(2,6 dimethyl-1,4 phenylene oxide) were studied as a function of annealing conditions. Cast film samples were annealed 20°C above their respective glass transition temperatures for two hours and twenty-four hours. The rate of relaxation-controlled (Case II) sorption of n-hexane in these films was reduced markedly consequent to annealing. The effect of annealing on the sorption kinetics and the independently determined film densities was more pronounced for the poly(phenylene oxide)-rich samples. Although sorption rates were reduced by as much as a factor of 100, the sorption equilibrium was insignificantly affected by annealing. Super Case II transport was observed for the slow absorbing annealed samples whereas the more rapid sorption in the unannealed samples followed ideal Case II kinetics. The more pronounced effects of annealing for the poly(phenylene oxide)-rich samples on sorption rates and film densities were explained by considering the increasing difference between the film T_g and the drying temperature used in the original film preparation for the poly(phenylene oxide)-rich samples. These results suggest that glassy polymers, cast and dried well below their glass transition temperatures, will be subject to large long-term reductions in absorption rates and specific volume. Moreover, residual, excess free volume significantly affects relaxation-controlled absorption of vapors in partially annealed glassy polymers.     

Stress optical studies of oriented poly(methyl methacrylate)

A series of samples of oriented poly(methyl methacrylate) was produced by hydrostatic extrusion at temperatures below the glass transition temperature. The development of orientation in the process was monitored by the measurement of birefringence which was shown to depend on the extrusion temperature as well as on the applied deformation. Additional information to characterize the oriented state was obtained by measuring the shrinkage force which developed when the oriented sample, constrained to constant length, was heated to a temperature just above the glass transition temperature; specimens free to contract all recovered to the isotropic state and original dimensions on annealing at this temperature. Most measurements were made on specimens ‘as extruded’ with additional studies made on specimens annealed at temperatures above the extrusion temperature but below the glass transition. The data, which have strong implications with regard to deformation mechanisms, are interpreted both at a molecular level in terms of deviations from an ideal rubber network, and at a more phenomenological level in terms of the Mooney-Rivlin equation.     

The link between rigid amorphous fraction and crystal perfection in cold-crystallized poly(ethylene terephthalate)

The rigid-amorphous fraction (RAF) in cold-crystallized and subsequently annealed poly(ethylene terephthalate) (PET) was investigated as a function of crystallinity and crystal perfection. During cold-crystallization, the amount of RAF increases non-linearly with crystallinity to a maximum of 44% at a crystallinity of 24%. Vitrification of the RAF is almost complete at the cold-crystallization temperature. Increasing the crystallinity from 24% after cold-crystallization to 44% by subsequent annealing at higher temperatures decreases the RAF. The specific RAF, i.e. the ratio of RAF to crystallinity at the glass transition temperature, T_g, of the mobile-amorphous fraction decreases from almost 2.0 after cold-crystallization to 0.75 after the subsequent annealing. The decrease in the specific RAF is attributed to crystal perfection which decreases the strain transmitted to the amorphous phase. Analysis of the reversing specific heat capacity of the annealed samples on cooling leads to the conclusion that the remaining RAF at the temperature of annealing is still glassy up to at least 490 K. An observed excess heat capacity above 490 K is due to reversible melting and is discussed within the frame of the concept of the specific reversibility of melting.     

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.     

Temperature-resolved SAXS studies of morphological changes in melt-crystallized poly(hexamethylene terephthalate) and its melting upon heating

Temperature-resolved small-angle X-ray scattering (SAXS) on poly(hexamethylene terephthalate) (PHT) samples crystallized from the melt yields direct information about the morphological changes in lamellar crystals and interlamellar amorphous layers upon melt-crystallization and subsequent heating to melting. Absolute intensities of these SAXS patterns were further analyzed via one-dimensional correlation and interface distribution functions. These analyses indicate that melt-crystallization at low temperature produces lamellar crystals having diverse thicknesses whereas crystallization at high temperature tends to favor growth of thick lamellar crystals with a nearly uniform distribution of thickness. When heating the PHT samples in the melting temperature region, the melting of the lamellar crystals was found to correlate well with the sequential-melting features. When these crystals are heated to higher temperatures, structural alterations from stacked lamellae to isolated lamellar crystals evolve with increasing extent of sequential melting, but, upon re-crystallization during extended annealing, the isolated lamellar crystals can pass through a reversible transition back to stacked lamellae.     

Influence of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) on miscibility and properties of atactic poly(methyl methacrylate)

Miscibility and properties of two atactic poly(methyl methacrylate)‐based blends [containing 10 and 20% of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)] have been investigated as a function of thermal treatments. Differential scanning calorimetry and dynamic mechanical thermal analysis of blends quenched in liquid nitrogen or ice/water, after annealing at T > 190 °C, showed a single glass transition temperature, indicating miscibility of the components for the time‐temperature history. Two glass transition temperatures, equal to those of the pure components, are instead found for blends after annealing at T < 190 °C. Scanning electron microscopy confirmed the homogeneity for the former quenched blends and phase separation for the latter. These results indicate the presence of an upper critical solution temperature (UCST). Tensile experiments, performed on two series of samples annealed at temperatures above and below the UCST, showed that the copolyester induces a decrease of Young's modulus and stresses at yielding and break points, and a marked increase of elongation at break. Differences in tensile properties between the two series of annealed blends are accounted for by the physical state of the components at room temperature after annealing above or below the UCST. Copyright © 2004 Society of Chemical Industry     

Frictional surface temperature determination of high-temperature-resistant semicrystalline polymers by using their double melting features

Most semicrystalline polymers exhibit multiple melting peaks in the course of normal differential scanning calorimetry (DSC) measurements. When their amorphous versions are annealed above the glass transition temperatures, the lower endothermic temperatures (T_m1) appearing on the subsequent DSC heating traces are highly dependent on the annealing temperature (T_a). In consideration of the fact that temperature is the critical environmental factor controlling polymer crystallization, thermal history experienced by the material during annealing in the DSC cell is basically equivalent to that under frictional heating, and the surface temperature prevailing under sliding wear can be estimated from DSC scans taken on the worn surface. In this case, the lower melting peak temperature observed (which can be correlated with the annealing temperature) serves as an indicator for the flash temperature. In addition, this thermoanalytical method can also provide information about microstructural changes due to wearing. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 589–593, 1997     

Processing–structure–property relationships in poly(ethylene terephthalate) blown film

An investigation was undertaken to establish processing–structure–property relationships in poly(ethylene terephthalate) (PET) blown film. For the study, a commercial grade of PET was used to fabricate the film specimens by means of a tubular film blowing process. In this process, the stretch temperature was accurately controlled by an oven. The annealing treatment of the oriented specimens involved clamping the sample in an aluminum frame and then putting the clamped sample in an oven, controlled at a temperature between the glass transition temperature (70°C) and the melting point (255°C) of PET, for a specified annealing period. The structure of the blown film samples was characterized by density, bulk birefringence, flat plate wide-angle X-ray scattering, and pole figure analysis. The processing variables, namely, takeup ratio, blowup ratio, and stretch temperature were found to significantly affect the bulk birefringence and density of the oriented PET blown film samples. It was found that both the bulk birefringence and density of the specimens increased upon annealing at an elevated temperature. Both the crystalline and amorphous orientation functions were calculated from the data of bulk birefringence, density, and the pole figure analysis. Compared to the amorphous orientation functions, the crystalline orientation functions were found to be relatively insensitive to the processing variables. It was concluded that equibiaxially oriented PET films can be produced via a tubular film blowing process by judiciously controlling the processing and annealing conditions. It has also been observed that the tensile stress-at-break of equibiaxially oriented PET film increases with decreasing stretch temperature and increasing annealing temperature.     

Fatigue-fracture mechanism of slowly notched poly(ethylene terephthalate) polymers

Summary Fatigue fracture behavior of slowly notched polyethylene terephathalate (PET) polymers were investigated at temperatures close to their transition temperatures up to well above their glass transition temperatures. Detailed characterization on the morphology of the notched roots showed that the crack tip during crack propagation became more dull with increasing testing temperature. The failure cycle (N_f) of these samples increased with increasing temperatures until it reached the transition temperatures of PET polymers, and most of the increase in N_f is due to the increased time consumed in the initiation period. On the other hand, the initial crack growth rate increased significantly and N_f of these samples decreased dramatically as the temperature increased well above the glass transition temperature. This interesting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature range.     

Strain‐induced crystallization in uniaxially drawn PETG plates

The copolyester poly(ethylene glycol‐co‐cyclohexane‐1,4‐dimethanol terephthalate) (PETG) is used industrially as an uncrystallizable polymer, whereas PET is an inherently crystallizable polymer. Nevertheless, a crystalline phase could appear in the material. To create a strain‐induced crystalline phase in an initially amorphous PETG material, plates were placed in the heating chamber of a tensile machine at 100°C and uniaxially drawn to obtain different samples with various draw ratios. During DSC analysis of highly drawn samples, perturbations of the baseline appear above the glass‐transition temperature, consisting of weak exothermic and endothermic phenomena. Comparison of DSC and X‐ray diffraction analysis of drawn PETG and PET shows that a strain‐induced crystalline phase appears in this copolyester. A spherulitic superstructure could also appear after lengthy annealing. Analysis of this semicrystalline material allowed estimation of the degree of crystallinity, about 3% after a drawing at high draw ratio and about 11% for undrawn annealed material. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3405–3412, 2001     

The impact of new crystalline lamellae formation during annealing on the properties of polypropylene based films and membranes

Cast films, based on polypropylene (PP), were prepared via melt extrusion and, then, annealed below the melting temperature. The effect of annealing conditions on the properties of the films and the microporous membranes formed by stretching was investigated. It is shown that annealing is an effective method to improve the physical properties of semi-crystalline polymers by promoting chain rearrangement and creating secondary lamellae in the amorphous region. DSC results for annealed samples revealed the appearance of a shoulder endothermic peak and a new peak in the correlation function from the SAXS patterns was observed. The annealed films exhibited double yield points in the tensile deformation curves. A direct linear relationship was found between the strength of the second yield point and the fraction of the lamellae. From mercury porosimetry and SEM images of the membranes larger pore sizes and porosity were observed as the annealing time and temperature increased.     

Annealing effect on the microstructure of poly(vinyl chloride)

The effect of annealing on the microstructure of commercial grade poly(vinyl chloride) was investigated by calorimetric, X-ray and viscoelastic measurements. The degree of crystallinity increases with increasing annealing temperature from above the glass transition temperature up to 130°C, at which point the degree of crystallinity takes on a maximum value. Also, the crystal melting temperature increases with increasing annealing temperature. Thermal analysis and X-ray study suggest that the crystallite of poly (vinyl chloride) decomposes by thermal degradation when annealed, above 170°C. The isothermal crystallization process is analyzed using Avrami's equation employing the degree of crystallinity as a function of annealing time at various annealing temperatures. The crystallization rate has a maximum value at around 140°C. It is expected that the crystalline texture grows in the shape of a lineal-like habit, judging from the magnitude of Avrami's constant and from a study of the X-ray intensity distribution. The α_f-transition was observed to occur at temperatures 5 to 10°C lower than the crystalline melting temperatures for annealed specimens of poly(vinyl chloride) using a dynamic spring analysis. The α_f-transition may be attributed to thermal molecular motions with a long time scale, resulting from the cross-link points introduced by the small crystallites.     

Studies on the surface of as-grown and annealed polyethylene single crystals by the spin-probe method

Studies on molecular motions at the surface of as-grown and annealed polyethylene single crystals in relation to the surface structure have been made by dispersing electron paramagnetic probes on the surface of the crystals and detecting them by the electron spin resonance method. A relatively large scale molecular motion supposedly associated with the glass transition was detected as a change in hyperfine patterns at around −50°C. Rotational correlation frequency of the paramagnetic probe was found to rapidly increase above the crystallization temperature suggesting that the new mode of molecular motion takes place by thickening of crystals. For the annealed crystals the frequency was much depressed compared with that of the as-grown crystals indicating the decrease in mobility of the surface by annealing. Extensive study revealed that the mobility of the surface of annealed crystals is uniquely determined by the increased portion of the lamellar thickness by annealing.     

Finite size effects in multilayered polymer systems: Development of PET lamellae under physical confinement

The glass transition temperature and the crystallization behaviour of poly(ethylene terephthalate) PET ultra-thin layers (a few tens of nm) within multilayered PET/polycarbonate (PC) coextruded films are investigated as a function of layer thickness by means of calorimetric measurements. Results are discussed in terms of reduced thickness and interface effects. The appearance and evolution of lamellar orientation upon isothermal crystallization of ultra-thin PET layers from the glassy state are explored based on real time small-angle X-ray scattering (SAXS) studies. Analysis of the SAXS measurements reveals that finite size effects hamper the crystallization process. However, the final lamellar structure is similar in both, the nanolayered PET and the bulk material. Results suggest that not only lamellar insertion but also some lamellar thickening contribute to the development of the final lamellar structure. Room temperature SAXS and wide-angle X-ray diffraction (WAXS) measurements indicate that two lamellar populations develop: edge-on lamellae are proposed to appear close to the interphases while flat-on lamellae, arising as a consequence of confinement, should be preferentially located in the layers core.