Structural studies of pressure-crystallized polymers. I. Heat treatment of oriented polymers under high pressure

A convenient pressure apparatus was designed for crystallization of high polymers under hydrostatic pressure up to 5000 atm. Melt crystallization as well as heat treatment under various temperatures and pressures was carried out on several polymers, and the effects of pressure on the molecular and crystal structures of the samples are discussed. Heat treatment of syndiotactic polypropylene under high pressure yields a new crystal modification rather than the previously known helix and planar zigzag modifications. Of the three modifications of poly(vinylidene fluoride), modification III was found as a high-pressure phase for specimens in the unoriented state, while modification I was obtained as the most stable one on heat treatment of oriented specimens under high pressure. Heat treatment under high pressure converts ordinary isotactic poly-4-methylpentene-1 with a lower density than the noncrystalline value, to a new crystal modification with higher density. As is reasonable, the dense modification is stable in a high-pressure range. For these three cases, the orientation of specimens was found to remain unchanged during the transitions, which must therefore occur in the solid state.     

Effects of pressure on the compressibility and crystallization of poly(ethylene terephthalate)

The effects of pressure on the compressibility and crystallization of poly(ethylene terephthalate) (PET) have been investigated. The Instron capillary rheometer was adapted as a high-pressure dilatometer to perform experiments up to 40,000 psi. Compressibilities of solid and molten PET were measured. The increases in compressibility with increase in temperature for the solid state are discussed in terms of free-volume theory. Results obtained for the melt are explained by invoking the second law of thermodynamics and the effect of pressure on the Gibbs free energy. The effects of temperature and compression rate on the pressure of crystallization (P_c) were also studied. As the crystallization temperature was increased from 240 to 286°C, P_c increased by about 16,000 psi. As the compression rate was raised from 1%/min to 8%/min, P_c increased 10,000 psi. At some undetermined compression rate above 8%/min it seemed impossible to induce crystallization in the melt, even with pressures up to 40,000 psi. Analysis of data on the kinetics of crystallization of PET melt under high pressures revealed low Avrami exponents, for which no unequivocal explanation is offered. It is possible, however, that crystallization at high pressure promotes the formation of a morphology made up of a certain percentage of “extended chains.” The alteration in the attendant spatial geometry involved in the crystallization might explain the lower Avrami exponents found. In another set of experiments, crystallization temperatures (T_c) were measured by slowly cooling PET melt under high pressures. As the pressure was raised from 3000 to 15,000 psi, T_c increased from about 246 to 282.5°C. These results are consistent with thermodynamic theory.     

Polymer crystallization in complex conditions. Towards more realistic modelling of industrial processes

Crystallization plays an important role in processing many industrial polymers. Crystallinity controls mechanical and sorptional properties of solid materials and strongly affects rheological properties of polymer melts and solutions. Therefore, realistic modeling of technological processes involving crystallizable polymers (melt spinning, film blowing, injection molding, etc.) requires that crystallization in the course of processing is taken into account. In contrast to idealized laboratory conditions usually applied in academic studies, crystallization in processing conditions covers wide range of rapidly changing temperatures and pressures. Simple models of non‐isothermal, and stress‐induced crystallization kinetics are discussed. Consideration of crystallization in the computer model of melt spinning reveals new effects, absent in earlier models, in which crystallization was ignored.     

Investigation on nonisothermal crystallization kinetics of the high‐flow nylon 6 by differential scanning calorimetry

The crystallization kinetics of the high‐flow nylon 6 containing polyamidoamine (PAMAM) dendrimers units in nylon 6 matrix was investigated by differential scanning calorimetry. The Ozawa and Mo equations were used to describe the crystallization kinetics under nonisothermal condition. The values of Avrami exponent m and the cooling crystallization function F(T) were determined from the Ozawa plots, which showed bad linearity, and were divided into three sections depending on different cooling rates. The plots of the m and log F(T) values versus crystallization temperatures were obtained, which indicated that the actual crystallization mechanisms might change with the crystallization temperatures. The high‐flow nylon 6 has higher values of m and log F(T) than those of pure nylon 6, which implied that the high‐flow nylon 6 had more complicated crystallization mechanisms and slower crystallization rate than those of pure nylon 6. The good linearity of the Mo plots verified the success of this combined approach. The activation energies of the high‐flow nylon 6 ranged from 157 to 174 kJ/mol, which were determined by the Kissinger method. The ΔE values were lower than those of pure nylon 6, and the ΔE values were affected by both the generation and the content of PAMAM units in the nylon 6 matrix. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2201–2211, 2008     

Influence of pressure and temperature on the physico-chemical properties of mobile phase mixtures commonly used in high-performance liquid chromatography

To fulfil the increasing demand for faster and more complex separations, modern HPLC separations are performed at ever higher pressures and temperatures. Under these operating conditions, it is no longer possible to safely assume the mobile phase fluid properties to be invariable of the governing pressures and temperatures, without this resulting in significantly deficient results. A detailed insight in the influence of pressure and temperature on the physico-chemical properties of the most commonly used liquid mobile phases: water-methanol and water-acetonitrile mixtures, therefore becomes very timely. Viscosity, isothermal compressibility and density were measured for pressures up to 1000 bar and temperatures up to 100 degrees C for the entire range of water-methanol and water-acetonitrile mixtures. The paper reports on two different viscosity values: apparent and real viscosities. The apparent viscosities represent the apparent flow resistance under high pressure referred to by the flow rates measured at atmospheric pressure. They are of great practical use, because the flow rates at atmospheric pressure are commonly stable and more easily measurable in a chromatographic setup. The real viscosities are those complying with the physical definition of viscosity and they are important from a fundamental point of view. By measuring the isothermal compressibility, the actual volumetric flow rates at elevated pressures and temperatures can be calculated. The viscosities corresponding to these flow rates are the real viscosities of the solvent under the given elevated pressure and temperature. The measurements agree very well with existing literature data, which mainly focus on pure water, methanol and acetonitrile and are only available for a limited range of temperatures and pressures. As a consequence, the physico-chemical properties reported on in this paper provide a significant extension to the range of data available, hereby providing useful data to practical as well as theoretical chromatographers investigating the limits of modern day HPLC.     

Polymer crystalline structure and morphology changes in nylon‐6/ZnO nanocomposites

The influence of ZnO nanoparticles on the crystalline structures of nylon‐6 under different crystallization conditions (annealing at different temperatures from the amorphous solid, isothermal crystallization from the melt at different temperatures, and crystallization from the solution) has been examined with differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, field emission scanning electron microscopy, and Fourier transform infrared. ZnO nanoparticles can induce the γ‐crystalline form in nylon‐6 when it is cooled from the melted state and annealed from the amorphous solid. This effect of ZnO nanoparticles increases with decreasing particle size and changes under different crystallization conditions. The effects of ZnO nanoparticles on the crystallization kinetics of nylon‐6 have also been studied with DSC. The results show that ZnO nanoparticles have two competing effects on the crystallization of nylon‐6: inducing the nucleation but retarding the mobility of polymer chains. Finally, the melting behavior of the composites has been investigated with DSC, and the multiple melting peaks of composites containing ZnO nanoparticles and pure nylon‐6 are ascribed to the reorganization of imperfect crystals. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1033–1050, 2003     

Effect of pressure on the crystallization of poly(ethylene terephthalate)

The effect of pressure on the crystallization of poly(ethylene terephthalate) (PET) was studied. The Instron capillary rheometer was adapted as a high-pressure/high-temperature dilatometer to carry on experiments up to 40,000 psi. Isothermal measurements of PET melt density were made with a precision of ±0.5%. Analysis of the kinetics of crystallization of PET melt at high pressures reaffirmed the existence of low Avrami exponents and their noninteger values. To rationalize the crystallization mechanism with the observed low exponents it is proposed that an appropriate increase in pressure would effectively reduce the free volume of a crystallizable substance to a point at which an alteration of the crystallization mechanism or nucleation mode could occur. It is further shown that PET clearly exhibits two different and sharply defined stages of crystallization behavior at pressures above 10,000 psi. Based on the Avrami equation, the fraction of uncrystallized polymer for the initial stage is defined as an empirically determined function of time and pressure. There is good agreement between the predicted and experimental values over the pressure range investigated.     

Effect of polymer surface morphology on adhesion and adhesive joint strength. II. FEP Teflon and nylon 6

Heterogeneous nucleation and crystallization of FEP Teflon and nylon 6 melts against high energy surfaces (i.e., gold) produce an interfacial region, in these polymers, of high mechanical strength. Dissolution of the metal substrate rather than removal by mechanical means results in a polymer surface which is amenable to conventional structural adhesive bonding. Nucleation and crystallization of the polymer melts in contact with phases of low surface energy (e.g., vapor) result in the generation of weak boundary layers.     

Study of thermomechanical properties of hybrid tire cord

Thermomechanical properties of tire cords, which have a considerable influence on tire functions, were investigated in the current research. The nylon and polyester cords are the most commonly used polymers in the tire. Many efforts have been made to produce a new tire cord having the capability of integrating the desired properties of these two polymers into one cord. In this study, a new cord structure was developed using nylon 6,6 and polyester tire yarns. The effect of thermal treatment was studied at the temperatures of 180°C, 200°C, and 220°C under two different loads of 600 and 1800gf on thermomechanical properties of the nylon 6,6/polyester hybrid tire cord. Shrinkage and shrinkage force as thermal properties, as well as static and dynamic mechanical properties, were investigated. The relationship between the aforementioned properties and variation in the crystal structure and fiber orientation obtained from Differential scanning calorimetry (DSC) and Wide‐Angle X‐Ray Scattering (WAXS) analysis were examined too. The results showed that the higher tension leads to the higher initial modulus and storage modulus due to the crystallization of polymers during heat treatment. The shrinkage and shrinkage force also increased, as the tension increased. In addition, a decrease in residual shrinkage and shrinkage force were acquired because of an enhancement in the temperature.     

Bridging the pressure gap in model systems for heterogeneous catalysis with high-pressure scanning tunneling microscopy

A high-pressure scanning tunneling microscope (HP-STM) enabling imaging with atomic resolution over the entire pressure range from ultrahigh vacuum (UHV) to one bar has been developed. By means of this HP-STM we have studied the adsorption of hydrogen on Cu(110), CO on Pt(110) and Pt(111), and NO on Pd(111) at high pressures. For all of these adsorption systems we find that the adsorption structures formed at high pressures are identical to high-coverage structures formed at lower pressures and temperatures. We thus conclude that for these systems the so-called pressure gap can be bridged, i.e. the results obtained under conventional surface science conditions can be extrapolated to higher pressures. Finally, we use the HP-STM to image the CO-induced phase separation of a Au/Ni(111) surface alloy in real time, whereby demonstrating the importance of catalyst stability in the study of bimetallic systems.     

Formation of spherulites in polyamides. V. “odd-odd” polyamides

The crystallization of a series of “odd-odd” polyamides has been examined for a variety of fusion conditions and crystallization temperatures. Diverse kinds of spherulites and other crystalline structures have been formed in these nylon polymers by direct crystallization from the melt and by melt-seeding techniques. The structures formed in this way have been characterized primarily with the aid of optical microscopy. In this series of polymers, characteristic textural features yield a fairly unified pattern of crystallization behavior. Crystalline aggregates are formed near the respective melting points of these polymers. In very thin sections (ca. 0.1μ), plateletlike crystals of high crystallinity exhibit optical and diffraction behavior characteristic of single crystals.     

Thermodynamic characterization of the mixture (1-butanol + iso-octane): Densities,viscosities, and isobaric heat capacities at high pressures

The densities at high pressures of 1-butanol and iso-octane were measured in the range (0.1 to 140) MPa at seven different temperatures, from (273.15 to 333.15) K, and their mixtures were measured in the range (0.1 to 50) MPa at four different temperatures, from (273.15 to 333.15) K. The measurements were performed in a high-pressure vibrating tube densimeter. The pressure–volume–temperature behavior of these compounds and their mixtures was evaluated accurately over a wide range of temperatures and pressures. The data were successfully correlated with the empirical Tamman–Tait equation. The experimental data and the correlations were used to study the behavior and the influence of temperature and pressure on the isothermal compressibility and the isobaric thermal expansivity.Also, the isobaric heat capacities were measured over the range (0.1 to 25) MPa at two different temperatures (293.15 and 313.15) K for the pure compounds and their mixtures. The measurements were performed in a high-pressure automated flow calorimeter. The excess molar heat capacities were assessed for the mixture and a positive deviation from the ideality was obtained.     

Exclusion effect of carbon dioxide on the crystallization of polypropylene

We investigated the crystallization growth of isotactic polypropylene under carbon dioxide (CO₂) at various CO₂ pressures and temperatures by in situ observation with a digital high‐fidelity microscope and a specially designed high‐pressure visualized cell. The fibrils within the spherulite were distorted and branched by crystallization under CO₂ at pressures higher than 2 MPa, and this suggested the exclusion of CO₂ from the growth front of the fibrils. The spherulite growth rate (G) at 140 °C increased with the CO₂ pressure, attained a maximum value around 0.3 MPa, and then decreased. Above 6 MPa, it became slower than that under air at the ambient pressure. An analysis of the crystallization kinetics by the Hoffman–Lauritzen theory revealed that the pressure dependence of G could be ascribed to the change in the transportation rate of crystallizable molecules (β_g) with pressure; that is, β_g increased and then decreased with pressure. The increase in β_g at a low pressure was caused by the plasticizing effect of CO₂, whereas the decrease in β_g at a high pressure was due to the exclusion of CO₂ from the crystal growth front. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1565–1572, 2004     

Crystallization of polyethylene at high pressure: A kinetic view

A model for the crystallization kinetics of polymers is outlined and is used to interpret observations of the crystallization of polyethylene at high pressures. This model introduces a distinction between σ_e the lamellar surface energy which controls the lamellar thickness, and σ_e′, the surface nucleus surface energy which controls the growth rate. Differential scanning calorimetry and electron microscopy data for several polyethylenes crystallized at pressures of up to 8 kb are presented. From the dependence of lamellar thickness on the crystallization undercooling at 5 kb, it is found that σ_e increases markedly with pressure leading to the formation of very thick crystals at high pressures. The magnitude of the increase in σ_e is in agreement with σ_e values calculated from the dependence of melting temperatures on pressure. The nucleus surface energy σ_e′ is not expected to vary significantly with pressure, and estimates of growth rates of 5 kb which indicate that the growth rate does not vary significantly with pressure at constant under-cooling confirm this. Fractionation effects and the differences in behavior between different polyethylenes are also discussed.     

Polymorphism of dense, hot oxygen

The phase diagram and polymorphism of oxygen at high pressures and temperatures are of great interest to condensed matter and earth science. X-ray diffraction and Raman spectroscopy of oxygen using laser and resistively heated diamond anvil cells reveal that the molecular high-pressure phase ε-O(2), which consists of (O(2))(4) clusters, reversibly transforms in the pressure range of 44 to 90 GPa and temperatures near 1000 K to a new phase with higher symmetry. The data suggest that this new phase (η') is isostructural to a phase η reported previously at lower pressures and temperatures, but differs from it in the P-T range of stability and type of intermolecular association. The melting curve increases monotonically up to the maximum pressures studied (～60 GPa). The structure factor of the fluid measured as a function of pressure to 58 GPa shows continuous changes toward molecular dissociation.     

High-pressure phases in polymers. I. Spherulitic growth kinetics in cis-polyisoprene

This is the first paper in a series dealing with the crystallization of high polymers at elevated pressures. Through an extension of the thin-film technique using osmium tetroxide staining, the measurement of lamellar growth kinetics at high pressures as well as the observation of micromorphological changes has proved possible. Three different morphological species of cis-polyisoprene have been detected in the pressure–temperature plane. A review is given of the conditions governing their formation and the growth kinetics of lamellar crystals are discussed in detail. Secondary nucleation theory is obeyed, the major effect of pressure being to alter the kinetics through the thermodynamic transitions. Variations in lamellar thickness with pressure may be explained simply in terms of the elevation by pressure of the equilibrium melting point.     

The effect of pressure on the mechanical properties of polymers. IV. Measurements in torsion

Stress relaxation was studied in torsion under superposed hydrostatic pressure using a newly constructed device. Two materials, a styrene–butadiene rubber and a butadiene rubber, were studied in the range from ?60 to 30°C at pressures up to 500 MPa. The time–temperature–pressure superposition of the data can be described by the FMT form of the free-volume theory which uses input from relaxation experiments at high pressure. The applicability of the Havlí?ek–Ilavský–Hrouz form of the Adam–Gibbs theory was also examined. This theory, which does not require information from high-pressure relaxation experiments, offers less flexibility than the FMT theory and is less successful in predicting superposition. The data are also examined in light of an adaptation of the Simha–Somcynsky equation of state to the free-volume theory. Prediction of the effect of pressure by the theory requires a single adjustable parameter. It was possible to calculate the free-volume parameters of several polymers without use of information from high-pressure experiments.     

Effect of Electron Beam Irradiation on the Kinetics of Non-isothermal Crystallization of Nylon 66

The non-isothermal crystallization kinetics was studied by differential scanning calorimetric analysis on nylon 66 and e-beam irradiated nylon 66 at different cooling rates. The Modified Avrami equation, the Ozawa equation and the Combined Avrami-Ozawa equation were applied to study the kinetics of non-isothermal crystallization of nylon 66. The crystallization behavior of pristine nylon 66 polymer was compared with that of e-beam irradiated nylon 66 and observed that the kinetics of non-isothermal crystallization of nylon66 was affected largely upon e-beam irradiation. E-beam irradiation not only decreased the crystallization temperature of nylon 66, but influenced the mechanism of nucleation and crystal growth and reduced the overall crystallization rate of nylon 66 also. The crystallization activation energy calculated by the Kissinger method for irradiated nylon 66 was lower than that of pristine nylon 66.     

Cold crystallization and postmelting crystallization of PLA plasticized by compressed carbon dioxide

The cold crystallization at temperature T_cc (melting > T_cc > glass transition) and the postmelting crystallization of polylactic acid plasticized by compressed carbon dioxide (CO₂) were studied using a high-pressure differential scanning calorimeter. The kinetics of the two kinds of crystallization were evaluated by the Avrami equation as a function of pressure at certain temperatures. The effects of using talc as a nucleation agent on the two types of crystallization under pressure were also investigated. The results show that compressed CO₂ increased the mobility of the polymer chains in solid state, resulting in an increased rate of cold crystallization. The decreased rate of postmelting crystallization was mainly in the nucleation-controlled region, which indicates that the number of nuclei was decreased by the compressed CO₂. The growth rate of the two crystallization types followed the Avrami equation, but the kinetics of each depended upon temperature and pressure. The inclusion of talc accelerated postmelting crystallization but had little effect on cold crystallization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2630–2636, 2008     

Phenomenological approach to compare the crystallization kinetics of isotactic polypropylene and polyamide‐6 under pressure

Reliable experimental data for semicrystalline polymers crystallized under pressure are supplied on the basis of a model experiment in which drastic solidification conditions are applied. The influence of the pressure and cooling rate on some properties, such as the density and microhardness, and on the product morphology, as investigated with wide‐angle X‐ray scattering (WAXS), is stressed. Results for isotactic polypropylene (iPP) samples display a lower density and a lower microhardness with increasing pressure over a wide range of cooling rates (from 0.01 to 20 °C/s). Polyamide‐6 (PA6) samples exhibit the opposite behavior, with the density and microhardness increasing at higher pressures over the entire range of cooling rates investigated (from 1 to 200 °C/s). A deconvolution technique applied to iPP and PA6 WAXS patterns has allowed us to evaluate the final phase content and to assess the crystallization kinetics. A negative influence of pressure on the α‐crystalline phase crystallization kinetics can be observed for iPP, whereas a slightly positive influence of pressure on the crystallization kinetics of PA6 can be noted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 153–175, 2002