Structural features of crystallized poly(ethylene terephthalate) polymers

Polyethylene terephthalate (PET) is a widely used polymeric material. In this work, the microstructural features before and after the solid‐state polymerization (SSP) of several DuPont PET products were investigated by low‐voltage scanning electron microscopy (LV‐SEM) and atomic force microscopy (AFM). The microstructural features on the cross section of various PET samples included crystallites, voids, boundaries, defects, and amorphous phases. The SEM images revealed layered and stepped structural features at the micron and 10‐micron scales that are highly crystallized at the near‐edge region of the cross section for both linear and branched PET samples after the SSP process. The AFM images demonstrate that the degree of crystallization for the linear and branched PET samples increases gradually from the central area to the edge on the cross section. The linear crystallized PET has a higher degree of orientation than the branched crystallized PET in the 10‐micron to micron scales, but their crystalline structures have no significant differences in the submicron to nanometer scales. The PET crystallization process occurs when the molecular chains in the amorphous phase are aligned and folded to form straight molecular chains at the nanometer scale, and small crystallites are formed. The crystallites aggregate and align together into a polygon rod‐like‐shaped crystallites at the submicron scale. Finally, large crystallites at the micron size are formed that appear on the edge area of the cross section. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 245–254, 2002     

DSC investigation of an isothermal crystallized poly(ethylene terephthalate)

Two samples of poly(ethylene terephthalate), oriented and unoriented (annealed for several times and temperatures), were scanned by DSC. The density of the samples was also determined. A thermal process was observed in the region between the glass transition and the melting temperature. The position of this thermal process is related to the annealing temperature. The model of the process was proposed as a simultaneous partial melting and recrystallization. The shape of the heating curve depends on the rate of both processes.     

Electron microscopy of high‐pressure crystallized poly(ethylene terephthalate)

The high‐pressure crystallized poly(ethylene terephthalate) samples were investigated with scanning electron microscopy. The striation appearance, which is the most common feature of polymer extended‐chain crystals, was clearly observed. Poly(ethylene terephthalate) extended‐chain crystals with thickness up to 17 m were obtained at high pressure. Fibrous crystals were also formed at high pressure. The fracture behaviors, which affected the exposure of the striations, were also discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1612–1616, 2000     

Diffusion studies of poly(ethylene terephthalate) crystallized by nonreactive liquids and vapors

The diffusion of vapors and liquids which induce crystallization in initially amorphous, unoriented poly(ethylene terephthalate) (PET) films was studied. It was determined that these vapors and liquids penetrate the polymer as distinct fronts, and the kinetics of this penetration and the weight uptake kinetics both follow apparent Fickian behavior. Distinct cavitation advanced into the polymer at the penetrant front–dry polymer interface in certain PET–liquid systems, and phenomenological explanations of its existence and of the general diffusion process observed are offered. Finally, the diffusion of the highly interactive liquids dioxane and methylene chloride into cold-drawn PET was studied and shown to occur considerably more slowly than does the diffusion of these liquids into unoriented films.     

On the location of chain-ends in rapidly crystallized poly(ethylene terephthalate)

Phenylmercurated poly(ethylene terephthalate) (PET) chain-ends are obtainable through the transesterification of PET by phenylmercury hydroxide or acetate in solution in nitrobenzene at 165 ± 10°C. The reaction results in an average of one mercury atom per chain. The phenylmercuration may be followed by infrared or x-ray fluorescence. Reaction with concentrated HCI Affords the elucidation of some structural parameters of the resultant partially crystalline tagged PET, through analyses of changes in viscosity and molecular weight, in percent crystallinity, and in the amount of mercury in the system. The chain-ends are almost completely excluded from the stem region of the PET crystal with no more than 2% remaining. The chain-ends are distributed unevenly throughout the amorphous phase. This is corroborated by sharp decreases in scattered intensity of small-angle x-ray measurements. Two models for the distribution of chain-ends in the amorphous phase are considered. The one in which the ends are pushed farthest from the crystal surface and concentrate halfway between crystallites is tentatively adopted. Analysis of the HCI hydrolysis kinetics and products leads to the following picture concerning the fold tightness and position. About one out of five folds extends significantly into the amorphous matter where it is mingled with cilia, tie molecules, and unattached molecules. Random scission throughout such an amorphous mass should lead to a preponderance of cleaved molecules whose length should be four to five times the lenght of the crystalline stem region, as is indeed observed.     

Mechanical property studies of poly(ethylene terephthalate) crystallized by nonreactive liquids

The mechanical properties of initially amorphous, unoriented poly(ethylene terephthalate) (PET) films crystallized in acetone and dioxane at room temperature were studied under tensile testing conditions. It was found that the nature of the liquid, the morphology it induced, and its significant residual level trapped within the PET following this type of crystallization all influenced the observed mechanical behavior. Further, thermally treating PET at 70, 90, and 174°C prior to the liquid treatments appeared to inhibit the effects of the liquids on the resultant mechanical behavior. Finally, because these liquids penetrated the polymer as distinct fronts, laminate-type structures were generated in the films. It then was shown that an ideal parallel element laminate could adequately model the mechanical behavior of the films.     

Transport properties of poly(ethylene terephthalate) crystallized from the glassy state

Poly(ethylene terephthalate) was crystallized from the glassy state at 120 and 200°C. The structural organization of samples, after the primary and secondary crystallizations, was analyzed by density, X-rays, infrared and transport properties of dichloromethane vapor. The values of crystallinity derived with different methods do not agree, indicating that the crystallized samples cannot be considered simply biphasic. Since the fraction of the impermeable phase is much higher than the fraction of the crystalline phase, it suggested that the presence of mesomorphic form, impermeable to the vapors at low activity. With this hypothesis, the complete composition of the crystalline samples, in terms of fraction of crystalline, amorphous and mesomorphic form was derived. A value of density of the mesomorphic form of 1.39 g/cm³ was also derived.     

Molecular segregation and nucleation of poly(ethylene oxide) crystallized from the melt. III. Morphological study

Solvent extraction followed by DSC, optical microscopy, and transmission electron microscopy show morphological evidence of molecular segregation for binary mixtures of poly(ethylene oxide)s (PEO). Two segregation types, namely microscopic and macroscopic segregation, have been found. A special etching method for PEO has been developed, and the crystalline textures of bulk samples crystallized at different temperatures for different molecular-mass mixtures are reported. A linkage between optical and electron microscopy is suggested. The morphology of PEO binary mixtures is crystallization-temperature and molecular-mass dependent.     

Molecular segregation and nucleation of poly(ethylene oxide) crystallized from the melt. I. Calorimetric study

Thermal analysis is used to elucidate the fusion of fractions of poly(ethylene oxide) of from 3500 to 5000000 molecular weight. In addition, the equilibrium and nonequilibrium phase diagrams of binary mixtures of these fractions have been studied. Complete segregation from high-molecular-weight polymer is possible up to at leat 20000 molecular weight. This segregation is not governed by the melting/crystallization equilibrium, but rather by molecular nucleation. Over a broader crystallization temperature range three regions can be identified. At low crystallization temperature, solid solutions result, at higher temperature one finds an intermediate type of crystallization, and ultimately one reaches complete, eutectic segregation. This work will be followed by analysis of the kinetics of crystallization.     

Molecular segregation and nucleation of poly(ethylene oxide) crystallized from the melt. IV. Computer modeling

A crystal surface is modelled and Monte Carlo simulations are attempted to learn about conformations and motion of a macromolecule that could lead to crystallization. Chains with up to 200 mobile segments have been analyzed. The diffusion distance of a given chain segment increases with positioning closer to the chain end, and completion of crystallization accelerates with decreasing length of remaining amorphous chain ends and increasing temperature.     

Oxygen barrier properties of crystallized and talc‐filled poly(ethylene terephthalate)

The improvement in oxygen barrier properties of poly(ethylene terephthalate) (PET) by incorporation of an impermeable phase such as crystallinity or talc platelets was examined. Crystallinity was induced by crystallization from the glassy state (cold crystallization). Microlayering was used to create talc‐filled structures with controlled layer architecture. The reduction of permeability in crystallized and talc‐filled PET was well described by Nielsen's model. Changes in permeability of crystalline PET could not be ascribed to the filler effect of crystallites only. Our data on solubility, obtained on the basis of measurements of the oxygen transport coefficients, confirmed a previous finding that the amorphous phase density of PET decreases upon crystallization. The data were amenable to interpretation by free volume theory. Talc‐filled materials processed by different methods showed the same permeability; however, much better mechanical properties were achieved by microlayering. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 847–857, 1999     

Structure and properties of poly(ethylene terephthalate) crystallized by converging flow and high pressure

The structure and properties of a poly(ethylene terephthalate) extrudate, prepared by converging flow and high pressure, were examined. Relatively high levels of crystallinity and orientation were produced by the combination of an orienting flow and high-pressure quench. The fine structure was examined in detail by x-ray diffraction, negative staining–transmission electron microscopy, and amine etching–gel permeation chromatography. The morphology was directly relatable to this unique preparative technique. Modulus was determined by a compressive Rheovibron technique. Low shrinkage and the compressive modulus were related to the structure in the development of a structural model.     

Hoffman-Lauritzen parameters for non-isothermal crystallization of poly(ethylene terephthalate) and poly(ethylene oxide) melts

Summary By applying an advanced isoconversional method to DSC data one can evaluate a dependence of the effective activation energy (the temperature coefficient of the growth rate) on the relative extent of melt crystallization. The conversion dependence can further be converted into a temperature dependence and parameterized in terms of the Hoffman-Lauritzen equation. For poly(ethylene terephthalate) (PET) we observe a transition from regime I to II. Poly(ethylene oxide) (PEO) crystallization appears to begin in regime II and then undergoes 2 consecutive changes that however cannot be clearly interpreted as regime III. The K_g and _e parameters obtained for regime I and II (PET) and regime II (PEO) are consistent with the respective parameters reported for isothermal crystallization.     

X-ray structural study of poly(ethylene terephthalate) after solid-state postpolycondensation

The structure of PET samples exposed to multistep annealing below the melting temperature T _m in high vacuum (the so-called solid-state postpolycondensation) has been studied by wide-angle X-ray diffraction. The sizes of crystallites have been calculated through the analysis of half-widths of corresponding reflections via the Rietveld, Scherrer, and Hosemann methods. As the molecular mass (M _η) of PET is increased from 4.5 × 10⁴ to 3 × 10⁵, the sizes of crystallites increase in three crystallographic directions (100, 010, and 001). An increase in the thickness of the crystal core of a lamella (the fold length in direction $\bar 1$ 05) $D_{\bar 105} $ from 40 to 58 Å (the Rietveld method) in the molecular mass range under study is accompanied by a rise in T _m by 19°C. The role of the $D_{\bar 105} $ size and intercrystallite tie links formed in the course of postpolycondensation in the rise in T _m with molecular mass is discussed. The free surface energy σ _e of the crystal end face (the surface of folds) has been calculated through the Tomson-Gibbs equation. The values of σ _e for PET samples with M × 10^?3 = 45, 100, and 300 have been estimated as 24.3, 23.5, and 15 mJ/m², respectively. These values turn out to be comparable with the lateral surface energy of crystallites available from the literature (13–19 mJ/m²). It has been inferred that the proportion of tie bridges in the intercrystallite space is appreciably higher than the proportion of folds on the face end surface of PET with not only M = 300 × 10³, but also with M = 45 × 10³ and 100 × 10³.     

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.     

Studies on thermo-acoustic parameters of poly(ethylene glycol)- 400 at different temperatures

The ultrasonic velocity and density have been measured at different temperatures between 299 and 363 K for the pure liquid sample, poly(ethylene glycol) with average molecular mass 400 g mol^?1 (PEG 400). From these, isentropic compressibility (β), intermolecular free length (L _f), acoustic impedance (Z), molar volume (V _m), Schaff’s available volume V _a(s), molar sound velocity (R _a), and molar compressibility (W) have been evaluated. The variations of these parameters with the temperature of the sample have been studied. Data so obtained are employed to compute other thermodynamic parameters. Variations in various parameters with respect to temperature are discussed in the light of the results obtained.     

Small-angle x-ray scattering and crystallization kinetics of poly(ethylene terephthalate) crystallized in a liquid environment

By small-angle x-ray scattering, a systematic investigation was performed of the long spacing of poly(ethylene terephthalate) (PET) crystallized in a liquid environment. The results indicated that the measured long spacings were temperature dependent and apparently relatively insensitive to liquid type under the conditions studied. The kinetic nucleation model of polymer crystallization was found to adequately explain this dependence. The differences in the long spacings between thermal and liquid-induced crystallization were in part rationalized in terms of the suspected supercoolings involved in the respective processes. Calculation of the spherulite growth rates for liquid-induced crystallization was made on the basis of the kinetic nucleation model and the classic theory of polymer–diluent crystallization. The results were shown to agree with inferential experimental observations of these growth rates and to elucidate the physics underlying liquid–induced crystallization. Finally, use of this growth rate theory in conjunction with a previous model for overall crystallization kinetics was shown to adequately describe and predict the diffusion-limited kinetics observed experimentally for most liquid-induced crystallization situations.