Structural relaxation in poly(ethylene terephthalate) studied by positron annihilation lifetime spectroscopy

Positron lifetime technique was used to study the physical ageing process in poly(ethylene terephthalate) (PET). Positron lifetime results show that the structural relaxation processes in PET encompass two different time regimes, one short and the other long. The relaxation function constructed from the measured o‐Ps intensity I₃ exhibits non‐exponential character, which can be best fitted with two additive exponentials. The Narayanaswamy model (Kohlraush‐William Watt (KWW) function) is invoked to extract the stretching parameter β indicating the extent of deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seem to label the different kinetic units of PET structure participating in the relaxation process. © 2002 Society of Chemical Industry     

Dielectric relaxation spectroscopy of poly(ethylene terephthalate)

Contour maps of dielectric loss tangent within the ranges 0.1 Hz to 3 MHz and ?175 °C to +190 °C are presented for a commercial poly(ethylene terephthalate) (PET) in two initial states of crystallinity. Individual absorption regions resemble those for poly(butylene terephthalate) and are attributed to carbonyl‐driven α‐ and β‐relaxation processes and to Maxwell–Wagner–Sillars polarizations. Possible causes are considered for the asymmetry and structure apparent in the α‐peak of partially crystalline PET. © 2001 Society of Chemical Industry     

Enthalpy relaxation in poly(ethylene terephthalate) and related polyesters

Enthalpic relaxation data are presented on poly(ethylene terephthalate), poly(ethylene naphthalate) and their copolymers. Analysis of these data allows the determination of the amount of energy absorbed at the glass transition, Q_t, and the location of the enthalpic recovery peak, T_max, as a function of the time of ageing of the samples. Ageing measurements were carried out for periods of up to 2016 h and at temperatures between 40 °C and 110 °C, depending upon the chemical composition of the system being investigated. The enthalpic relaxation rates are influenced by the chemical structure and reflect the effects of local order pinning the chains and influencing the rate of enthalpic recovery. © 2000 Society of Chemical Industry     

Effects of microcrystallinity and morphology on physical aging of poly(ethylene terephthalate)

Physical aging characteristics of poly(ethylene terephthalate) have been evaluated in relationship to volume fraction levels of crystallinity up to 25%. Changes in the enthalpies of relaxation, monitored at aging temperatures from 55 to 65°C, are found to give good fits with the Cowie‐Ferguson model. Overall equilibrium enthalpy of relaxation values decrease linearly with increased crystallinity. They increase with decreased aging temperature, providing extrapolated lower temperature results that are validated in terms of specific heat relationships. Activation energies for enthalpic relaxations are found to increase from 337 to 361 kJ/mole as crystallinity increases up to 25%. Overall relaxation endotherms are further resolved into contributions from interspherulitic and intraspherulitic amorphous regions. Interspherulitic, equilibrium enthalpies of relaxation decrease with increased levels of crystallinity, while intraspherulic values show corresponding increases. Characteristic relaxation times of the intraspherulic regions increase greatly, as levels of crystallinity increase; however, interspherulitic relaxation times decrease very slightly. Dynamic differential scanning calorimetry results show two glass transitions in the case of a 25% crystalline sample and a single transition for noncrystallized material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Blends of poly(ethylene terephthalate) and poly(butylene terephthalate)

Commercial grade poly(ethylene terephthalate), (PET, intrinsic viscosity = 0.80 dL/g) and poly(butylene terephthalate), (PBT, intrinsic viscosity = 1.00 dL/g) were melt blended over the entire composition range using a counterrotating twin‐screw extruder. The mechanical, thermal, electrical, and rheological properties of the blends were studied. All of the blends showed higher impact properties than that of PET or PBT. The 50:50 blend composition exhibited the highest impact value. Other mechanical properties also showed similar trends for blends of this composition. The addition of PBT increased the processability of PET. Differential scanning calorimetry data showed the presence of both phases. For all blends, only a single glass‐transition temperature was observed. The melting characteristics of one phase were influenced by the presence of the other. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 75–82, 2005     

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.     

Effect of stretching temperature on proton spin-lattice and spin-spin relaxation times of isotactic polypropylene film

Structural properties of isotactic polypropylene film during stretching were investigated mainly by the measurements of proton spin-lattice, T₁ and spin-spin, T₂, relaxation times. Both T₁ and T_2a, T₂ of the most mobile amorphous regions, of the sample stretched at 150°C are longer than those of the sample stretched at 130°C. These results indicate that in the sample stretched at 150°C the proportion of mobile amorphous region decreases, while the amorphous region achieves enhanced molecular mobility.     

Synthesis and characterisation of copolyesters from poly(ethylene terephthalate) and poly(hexamethylene terephthalate)

Copolyesters containing poly(ethylene terephthalate) and poly(hexamethylene terephthalate) (PHT) were prepared by a melt condensation reaction. The copolymers were characterised by infrared spectroscopy and intrinsic viscosity measurements. The density of the copolyesters decreased with increasing percentage of PHT segments in the backbone. Glass transition temperatures (T_g). melting points (T_m) and crystallisation temperatures (T_c) were determined by differential scanning calorimetry. An increase in the percentage of PHT resulted in decrease in T_g, T_m and T_c. The as-prepared copolyesters were crystalline in nature and no exotherm indicative of cold crystallisation was observed. The relative thermal stability of the polymers was evaluated by dynamic thermogravimetry in a nitrogen atmosphere. An increase in percentage of PHT resulted in a decrease in initial decomposition temperature. The rate of crystallisation of the copolymers was studied by small angle light scattering. An increase in percentage of PHT resulted in an increase in the rate of crystallisation.     

Morphology and crystallization of poly(ethylene terephthalate)

The melting behaviour and the morphology of poly(ethylene terephthalate) crystallized from the melt are reported. In general, dual or triple melting endotherms are seen, and single endotherms are seen when the samples are crystallized above 215°C for long times. The location of the uppermost endotherm was found to be constant below T_c = 230°C, and above that temperature the location depends on T_c. Therefore, we have shown that samples of PET which are crystallized above T_c = 230°C contain perfect crystals only; below T_c = 230°C, they contain perfect and imperfect crystals. Scanning electron microscopy showed that the perfect crystals are the dominant lamellae in the spherulitic structure, while the imperfect crystals are the subsidiary lamellae in the spherulitic structure, The amorphous regions are located between individual lamellae.     

PEN/PET共混物结晶行为研究

用差示扫描量热法(DSC)研究了不同共混比例PEN/PET共混物的熔体结晶行为,并进行了等温结晶动力学测定。结果表明:随着两种组分向中间比例(50/50)靠近,共混物的熔融温度越低,结晶速率也越慢。     

Extrusion drawn amorphous and semicrystalline poly(ethylene terephthalate): 3. Linear thermal expansion analysis

Thermal expansion analysis of uniaxially-oriented poly(ethylene terephthalate) (PET) films has been carried out from 123 K up to the PET glass transition temperature, T_g. The films are prepared by solidstate co-extrusion, from different premorphologies (amorphous, 33% and 50% crystalline), to draw ratios (EDR) up to 4.4, over a wide temperature range (T_ext). The coefficients of linear thermal expansion exhibit anisotropy: normal to the draw direction (α_⊥) it increases, whereas along the draw direction (α_∥) it always decreases with draw, independent of the initial morphology. Results are interpreted by treating PET as a simple two-phase composite structure. Tie-molecules occurring in the amorphous domains and bridging adjacent crystallites have a major influence. At EDR=4.4, a significant number of taut tiemolecules are developed, resulting in α_∥ becoming negative (α^max_∥=?1.0×10^?5°C^?1) at temperatures below ambient. This appears to be the first report of a negative α for a polymer of relatively low crystallinity. Temperature for extrusion draw significantly affects α. Normal to the draw direction, α_⊥ decreases with T_ext whereas α_∥ increases. The results show the thermal expansion of PET depends primarily on orientational effects. Only in the absence of anisotropy does per cent crystallinity have a dominant influence. In addition, the TMA thermograms display sharp transitions which are attributed to irreversible shrinkage of the oriented films. The shrinkage temperature (T_s) shows a strong dependence on both orientation and crystallinity, and it is discussed in association with T_g.     

Effect of poly(ethylene glycol) on the solid‐state polymerization of poly(ethylene terephthalate)

Poly(ethylene glycol) (PEG) and end‐capped poly(ethylene glycol) (poly(ethylene glycol) dimethyl ether (PEGDME)) of number average molecular weight 1000 g mol^?1 was melt blended with poly(ethylene terephthalate) (PET) oligomer. NMR, DSC and WAXS techniques characterized the structure and morphology of the blends. Both these samples show reduction in T_g and similar crystallization behavior. Solid‐state polymerization (SSP) was performed on these blend samples using Sb₂O₃ as catalyst under reduced pressure at temperatures below the melting point of the samples. Inherent viscosity data indicate that for the blend sample with PEG there is enhancement of SSP rate, while for the sample with PEGDME the SSP rate is suppressed. NMR data showed that PEG is incorporated into the PET chain, while PEGDME does not react with PET. Copyright © 2005 Society of Chemical Industry     

Morphology and melting behaviour of poly(ethylene terephthalate) crystallized from the glassy state

The influence of annealing conditions on the morphology and melting behaviour of poly(ethylene terephthalate) (PET) was studied. PET annealed under isothermal conditions often shows double melting endotherms depending on the annealing temperature (T_a) and the heating rate of the calorimeter. It was found that the morphological structure and the lower melting peak depend strongly on the annealing temperature, T_a. The increase of the lower melting peak temperature with T_a is due to an increase of the lamellar thickness within the spherulitic structure and to a higher crystallite perfection.     

Conditions of homogeneous deformation of poly(ethylene terephthalate) films during uniaxially drawing

Amorphous poly(ethylene terephthalate) film was uniaxially drawn over a wide range of temperatures from below to above the T_g at a constant strain rate. The geometry of the deformation in macroscopic dimensions of the sample demonstrates that homogeneous deformation can be obtained when the drawing temperature (T_def) is not lower than 69°C. The change of the cold crystallization peak temperature (T_cc) and crystallinity determined by differential scanning calorimetry and density measurement, respectively, were studied in terms of the T_def and the draw ratio (λ). The orientation, relaxation, and crystallization during drawing were investigated as a function of T_def as well as of λ. The results suggest that 69°C is the critical temperature at which the sample with the highest orientation and the least slippage of the molecular chain and without obvious crystallization can be obtained. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2044–2048, 2000     

Transesterification in poly(ethylene terephthalate) and poly(ethylene naphthalate 2,6-dicarboxylate) blends: model compounds study

Kinetics of transesterification reaction in poly(ethylene terephthalate)-poly(ethylene naphthalate 2,6-dicarboxylate), PET-PEN, blends resulting from melt processing was simulated using model compounds of ethylene dibenzoate (BEB) and ethylene dinaphthoate (NEN). The exchange reaction between BEB and NEN was followed by ¹H NMR spectroscopy using signals from the aliphatic protons of ethylene glycol moieties at 4.66 and 4.78 ppm, respectively. The first-order kinetics was established under pseudo-first-order conditions for both reactants. Thus, the overall transesterification reaction was second order reversible. The reversibility was confirmed experimentally by heating a mixed sequence of 1-benzoate 2-naphthoate ethylene (BEN) under similar conditions. Both forward reaction of the equimolar amounts of the reagents and reverse reaction came to equilibrium at the same molar ratio of the reactants and reaction products of roughly 0.25:0.50:0.25 for BEB, BEN, and NEN, respectively. The rate equation for the transesterification reaction in the model system was modified using half-concentration of BEN, which is the only effective in the intermolecular exchange. Direct ester-ester exchange was deduced as a prevailing mechanism for the transesterification reaction under the conditions studied, and the values of equilibrium and rate constants, as well as other basic thermodynamic and kinetic parameters were determined. The use of Zn(OAc)₂ as a catalyst resulted in a significant decrease in the activation enthalpy of transesterification, which might be due to the partial switch of the reaction mechanism from primarily pseudo-homolytic to more heterolytic where Zn^II acts as a Lewis base which binds to the ester carbonyl oxygen.     

Swollen-state polymerization of poly(ethylene terephthalate) in fibre form

The swollen-state polymerization of poly(ethylene terephthalate) in fibre form was performed in hydrogenated terphenyl as the swelling solvent. Ultra-high-molecular-weight poly(ethylene terephthalate) (CHMW-PET) fibre with an intrinsic viscosity of 3–4dl g⁻¹ (M_n = 2–3 × 10⁵) was obtained. The polymerization rate of as-spun PET fibres in the swollen state was greater than that of PET granules in the swollen state. It was clarified that the polymerization rate was related to the chain mobility of the starting materials. The chain mobility was influenced by various conditions, such as changing rigidity of the segments during copolymerization, the chain orientation of the starting fibre before swollen-state polymerization and the temperature of pretreatment with the solvent. Pretreatment with solvent before polymerization was effective in increasing the chain mobility. The relation between chain mobility and polymerization rate was examined by wide-angle X-ray diffraction, density, differential scanning calorimetry, solvent content and viscoelastic measurements. Undrawn UHMW-PET fibres could be drawn 10 times or more by the zone drawing technique in spite of their high crystallinity, and the drawn fibre showed high tensile strength (12 g d⁻¹) and high modulus (240 g d⁻¹).     

An infra-red study of the structure of oriented poly(ethylene terephthalate) fibres

Procedures have been developed for quantitative infra-red spectroscopic measurements on poly(ethylene terephthalate) fibres in conventional yarns. Following computer reconstruction methods already established for films, measurements of molecular orientation and trans/gauche conformer content have been carried out for a wide range of fibres produced by different processing routes. The trans bands can be separated into load-bearing and non-load-bearing conformations, where the former govern the modulus. It is also shown that a quantitative measure of amorphous orientation can be obtained from the infra-red measurements. While there are similarities between the development of overall molecular orientation and changes in molecular conformation for high wind-up speed yarns and drawn yarns produced from a low wind-up speed yarn, there are also major differences, which confirms previous work showing that these two classes of fibres are basically different in structure. These differences are shown by the relationships between the load-bearing trans conformations and the amorphous orientation with the overall orientation.     

Volatile products of poly(ethylene terephthalate) thermal degradation in nitrogen atmosphere

Thermal degradation of PET was studied in a nitrogen atmosphere at 200–700°C. The experiments were carried out in a tubular furnace under isothermal conditions. The volatile substances evolved from PET were identified and quantified. Weight losses of PET during the thermal degradation in different temperatures were determined. The results are presented on plots as a function of the degradation temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1894–1901, 2000     

Extrusion drawn amorphous poly(ethylene terephthalate): 4. Irreversible spontaneous elongation

Uniaxially-oriented poly(ethylene terephthalate) (PET) films prepared by solid-state co-extrusion exhibit irreversible spontaneous elongation (rather than shrinkage) under specific conditions. Results for these conditions show that marked elongation (up to 20%) can occur during annealing of unconstrained samples. This phenomenon, which is not commonly observed for polymers, depends strongly on the prior conditions of extrusion draw. There is significant increase in length for films prepared with extrusion draw ratio (EDR) at 2.0 in the extrusion draw temperature (T_ext) range 80–100°C. The extrusion rate is also significant. Lower extrusion rates favour spontaneous elongation on subsequent heating. In addition, the annealing temperature (T_a) also affects elongation. Samples extruded at T_ext=80°C to EDR of 2.0 show maximum elongation at T_a=180–190°C. However at higher temperatures, e.g. at 10°C below the melting temperature and higher, shrinkage occurs instead. Moreover, annealing at T_a=90°C for different periods of time (t_a) shows that prior to the elongation a moderate amount of shrinkage occurs (t_a ? 30 s). The results suggest a correlation between spontaneous elongation and crystallization during anealing.