Dynamic mechanical investigations on drawn samples of isotactic polypropylene/EPM–Rubber blend

The physical behavior of isotropic and oriented samples of an isotactic polypropylene (iPP)/ethylene–propylene–copolymer (EPM) reactor blend was studied by performance of dynamic mechanical measurements over a wide temperature range (DMTA). The influence of thermal history and drawing procedure was examined. The results showed that with increasing draw ratio the uniaxial elastic modulus of the material was considerably enhanced, whereas the intensity and strength of the amorphous relaxations of both components were reduced. At a certain draw ratio, the glass transtions of iPP and EPM phenomenologically merged and appeared as a single relaxation. The crystalline relaxation of iPP emerged with increased draw ratio at higher temperatures and was better seperated and easier to detect. The effects observed were attributed to the orientation of the crystallites in a fibrillar structure and to the restricted molecular mobility in amorphous regions. Measurements by differential scanning calorimetry (DSC) and x-ray diffraction of several drawn samples were performed to determine the effects of drawing on the melting behavior and the crystal orientation in the semicrystalline polymer. For comparison, some results of analogous studies on neat isotactic PP are presented and discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1439–1448, 1997     

CO2 permeability and amorphous fractional free-volume in uniaxially drawn HDPE

The CO₂ permeability of uniaxially drawn high-density polyethylene (HDPE) was investigated and drastic changes were observed as a function of draw ratio. An estimation of variations in both density and fractional free volume of the amorphous component in the drawn polymers was made on the basis of experimental data. The dramatic variations in permeability resulting from drawing were shown to be the consequence of changes of fractional free volume in the amorphous phase of the drawn polymer. Good correlation exists between measured solubility and diffusion coefficients of drawn samples and the estimated fractional free volume; this correlation agrees with the existing free volume theory for diffusion in polymers.     

Orientation of the α- and γ-modification of elastic polypropylene at uniaxial stretching

New metallocene catalysts applied to propylene polymerization expand the range of properties of polypropylene (PP), resulting in semi-crystalline materials having crystallinities below 60% up to X-ray amorphous highly elastic ones. To date the origin of the unique elastic mechanical behavior of such low crystalline PP is not completely understood. Therefore, the microscopic orientation of those PPs due to uniaxial stretching was investigated using wide-(WAXS) and small-angle X-ray scattering (SAXS). The aim of this study was to correlate these orientations or changes in the developed fiber textures with the macroscopic stress-strain behavior. This includes efforts to come closer to the main question of the nature of the physical cross-links in these not chemical cross-linked homopolymers, which is the reason for the high elastic behavior. Therefore, high molecular weight metallocene PPs showing different crystallinities (0-36%) were stretched to several elongations and the structural changes during the deformation were recorded by X-ray scattering. Stress-strain measurements show the great potential of these PPs as a thermoplastic rubber material. For quantitative analysis and discussion of the polymer chain orientations, the orientation functions were calculated. Correlations between the orientation functions and the stress-strain curves allow an interpretation of the macroscopic behavior on a microscopic scale. A higher cross-linking density in elongated samples indicates that the network, which is responsible for the elasticity, mainly built up by strain-induced morphology changes and chain orientations.     

Microporous membranes obtained from polypropylene blends with superior permeability properties

A blend of two polypropylene resins, different in molecular structure, one with linear chains and the other with long chain branches, was investigated to develop microporous membranes through melt extrusion (cast film process) followed by film stretching. The branched component significantly affected the row‐nucleated lamellar crystalline structure in the precursor films. The arrangement and orientation of the crystalline and amorphous phases were examined by wide angle X‐ray diffraction and Fourier transform infrared spectroscopy methods. It was found that blending of a small amount of a long chain branched polypropylene improved the orientation of the both crystalline and amorphous phases in the precursor films. Annealing, followed by cold and hot stretching were consequently employed to generate and enlarge pores in the films as a result of lamellae separation. SEM micrographs of the surface of the membranes obtained from the blend revealed elongated thin fibrils and a large number of lamellae. The lamellae thickness for the blend was much shorter in comparison to that of the linear PP precursor film. The permeability of the samples to water vapor and N₂ was significantly enhanced (more than twice) for the blend system. The porosity of the blend membrane showed a significant improvement with a value of 53% compared to 41% for the linear PP membrane. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 148–157, 2008     

Partially miscible EVOH/copolyamide‐6/6.9 blends: The effect of stretching on the structure

Binary blends of random copolymers, ethylene‐vinyl‐alcohol (EVOH) consisting of 38 mol % ethylene and Copolyamide‐6/6.9 with an approximate 1 : 1 comonomer ratio, were prepared via blown‐film extrusion and uniaxial stretching. The anisotropy induced by the uniaxial deformation of the polymer blends was characterized by X‐ray diffraction and birefringence measurements. The stretched films also were investigated via oxygen permeability. The results showed a sharp decrease in the apparent crystallite size throughout the entire composition range in comparison to the blown films. However, the order perceived within the amorphous phase in the EVOH‐rich blends increased (decrease in oxygen permeability), whereas in the copolyamide‐rich blends, orientation resulted in a decrease in the amorphous phase order (increase in permeability). Apparently, orientation destroyed the amorphous interpolymer complex. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 813–822, 2000     

Plastic deformation of poly(tetramethylene oxide). II. Molecular orientation as measured by x-ray diffraction and NMR measurements

The orientation of the crystalline and amorphous phases in uniaxially drawn samples of polytetramethylene oxide has been studied by x-ray and NMR methods. Pole figure measurements give the orientation of the crystalline phase. Its dependence on the draw ratio does not obey the pseudoaffine model. The crystalline fraction is derived from NMR measurements at temperatures between T_g and T_m, where the free induction decay (FID) of the unoriented sample can be analyzed in terms of a rigid (crystalline), a constrained, and an amorphous phase. Low temperature (T < T_g) measurements of the NMR second-moment anisotropy in protonated and deuterated samples give a mean orientation of the chains higher than that corresponding to the crystalline phase. The lack of anisotropy in the tail of the FID at temperatures between T_g and T_m indicates no appreciable anisotropy in the truly amorphous interlamellar phase. From this and from the ratio of the P₄/P₂ orientations, factors which obey the “most probable distribution,” it is concluded that the amorphous orientation is due to the layer of constrained chains at the surface of the lamellae.     

The transport of oxygen through oriented poly(ethylene terephthalate)

The transport of oxygen through oriented PET at 25 and 60°C has been studied using the dynamic diffusion method. It did not prove possible to determine the diffusion and solubility coefficients with reasonable accuracy, and only the permeability coefficients are discussed in terms of the structure of the samples. The results suggest that the lowering of the oxygen permeability on drawing is related to the production of additional material with the trans conformation of the glycol residue, either in crystallites or in amorphous regions, rather than solely to the overall development of orientation or crystallinity.     

An x-ray pole figure analysis on biaxially deformed polyethylene film

The orientational states induced upon two-step biaxially stretching low-density polyethylene at 25°C have been investigated. A pole figure analysis of the (200), (020), and (002) crystalline planes has been employed to elucidate the evolution of the molecular crystalline orientation as a function of biaxial stretching. The initial uniaxial-like orientation induced along the extrusion direction of the films was gradually lost upon transverse stretching and, consequently, replaced by a biaxial orientation as suggested by the orientation functions. In these cases, the a crystallographic axis was observed to be strongly oriented along the film normal, thus confining the c and b axes to the film plane. The pole figures clearly indicate that the c and b axes are preferentially aligned 45° with respect to the stretching directions. This unique orientational state of the orthorhombic unit cell of polyethylene has been termed a biaxial-double orientation. Birefringence measurements on the biaxial samples indicated that the amorphous and crystalline regions are simultaneously biaxially oriented. The evolution of the crystalline orientation as a function of stretching was conveniently followed on a White/Spruiell orientation triangle. Quantification was hindered, however, by the presence of different crystal populations in the biaxially stretched samples. © 1994 John Wiley & Sons, Inc.     

X-ray diffraction relaxation of polyethylene

The change in x-ray diffraction intensity with time following rapid stretching by various amounts is measured for a low-density polyethylene sample at several temperatures. An appreciable decrease in intensity with time at the meridian of the diffraction from the 110 and 200 planes is observed to occur within the first 5 sec following stretching. The change takes considerably longer times at lower temperatures. By use of a calibration curve, the change in crystal orientation functions for the a, b, and c crystal axes were calculated, and an increase in c-axis orientation was shown. From the results of birerringence measurements on the same sample, the crystalline and amorphous contributions to birefringence were calculated. It was shown that stretching is initially accompanied by a greater amount of amorphous than crystalline orientation, and that relaxation involves an increase in crystalline orientation accompanied by a decrease in amorphous orientation.     

Mechanical properties of oriented low‐density polyethylene with an oriented lamellar‐stack morphology

A quantitative study was undertaken of the anisotropy of low‐strain mechanical behavior for specially oriented polyethylene with controlled crystalline and lamellar orientation. The samples were prepared by the die drawing of injection‐molded rods of polyethylene and annealing. This produced a parallel lamellar structure for which a simple, three‐dimensional composite laminate model could be used to calculate the expected anisotropy. Experimental data, including X‐ray strain measurements of the lateral crystalline elastic constants, showed good quantitative agreement with the model prediction. The X‐ray strain measurements confirmed that the amorphous regions exert large constraints on the crystalline phase in the lateral directions, where an order of magnitude difference was found between the measured apparent lateral crystalline compliances in the lamellar‐stack sample and the expected values for a perfect crystal. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 755–764, 2000     

Molecular orientation and microstructure of roller-drawn sheets of poly(ether ether ketone)

The rolling and roller-drawing of poly(ether ether ketone) (PEEK) sheets were carried out in the roller temperature range of 165-262°C. The crystal orientation functions of the PEEK sheets were determined from the azimuthal intensity distribution of wide-angle x-ray diffraction, and the orientation behavior in the amorphous region was characterized by the measurements of sonic modulus and polarized fluorescence. The orientation functions increase monotonically with increasing draw ratio. The orientation function in the amorphous region is close to that of crystal orientation function of the same sample. The long period evaluated by small-angle x-ray scattering is almost constant over the draw ratio range studied, whereas the crystallite size along the 001 plane, D₀₀₁ tends to increase with increasing draw ratio. The value of the crystallite size exceeds the product of the crystallinity and the long period. The result suggests the formation of the crystalline linkages that penetrate the periodic layers. © 1994 John Wiley & Sons, Inc.     

Polarized reflection spectrum and molecular orientation in uniaxially drawn poly(ethylene terephthalate)

The polarized electronic spectrum of oriented poly(ethylene terephthalate) (PET) sheets was obtained from the specular reflection spectrum using the Kramers-Kronig relationship. The surface orientation function of drawn and drawn/annealed PET sheets was determined from the dichroic ratio of the second π* ← π transition observed at 41,000 cm^-1. The bulk orientation functions in the crystalline and amorphous regions were evaluated from wide-angle X-ray diffraction and sonic modulus measurements. On annealing of drawn PET sheets, the crystalline orientation and crystallinity were much improved, but the amorphous orientation function showed a minor decrease. The overall molecular orientation in the surface of the drawn PET sheet was shown to be approximately equivalent to the molecular orientation in the bulk.     

Molecular deformation mechanisms of isotactic polypropylene in α‐ and β‐crystal forms by FTIR spectroscopy

The orientation behavior of isotactic polypropylene (iPP) in α‐ and β‐crystal form was investigated by rheo‐optical Fourier transformed infrared (FTIR) spectroscopy. This method enabled quantification of the degree of orientation as a feature of structural changes during uniaxial elongation in not only the crystalline phase but also the amorphous one. Molecular orientation mechanisms can be successfully derived from experimental results. Generally, three mechanisms were detected for iPP: (1) interlamellar separation in the amorphous phase, (2) interlamellar slip and lamellar twisting at small elongations, and (3) intralamellar slip at high elongations. The third mechanism was favored by α‐PP, whereas β‐PP favored the second mechanism, which, in fact, was responsible for the different mechanical properties of both materials at the macroscopic level. On the other hand, crystallization conditions may have significantly affected the amorphous orientation. Nevertheless, for both iPP types the chains in the amorphous phase always oriented less than did those in the crystalline phase. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4478–4488, 2004     

Molecular dynamics simulation of dense carbon dioxide fluid on amorphous silica surfaces

Molecular dynamics (MD) simulations of dense carbon dioxide on the amorphous dehydroxylated silica surfaces have been carried out. The adsorption potential surfaces of the silica solids have been obtained in order to evaluate the characteristics of the amorphous surfaces. The atom density profiles, adsorption free energy profiles, surface orientation order parameters, and radial distribution functions for the CO2 molecules have been presented in order to study the effect of the amorphous surfaces on the microscopic interfacial structure properties of the CO2 molecules. The translational diffusion and orientation rotation at silica surfaces have also been investigated. It was observed that there is marked hindrance of the translational diffusion and orientation rotation of CO2 molecules near amorphous silica surfaces.     

Influence of temperature and fiber structure on the dielectric properties of polypropylene fibrous structures

In this work, dielectric spectroscopy was conducted on five commercial woven polypropylene‐based fabrics. Measurements of dielectric loss tangent, the effective relative dielectric permeability and ac electrical conductivity were performed over a wide range of temperature and frequency. The results in temperature range from 250 K to 355 K showed that the samples with lower value of volume fraction whose yarn is made from a short fiber have a lower value of the above mentioned dielectric parameters than the samples with bigger value of volume fraction and filament yarn along the weft and the warp lines. Based on the results gained from the measurements in the vacuum and ambient conditions, it can be concluded that the samples with a lower value of volume fraction, whose yarn is made from a short fiber, showed stability of dielectric properties in the measurement interval. Copyright © 2014 John Wiley & Sons, Ltd.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

Oriented noncrystalline structure in PET fibers prepared with threadline modification process

The development of an oriented noncrystalline phase in a semicrystalline polymer filament has been studied via X-ray scattering. These unique PET fibers contain a relatively high noncrystalline content and also have high tenacity, high modulus, and low breaking elongation. Fiber properties were found to be very responsive to the oriented amorphous phase content. This phase was utilized for interpreting noncrystalline orientation in PET fibers produced by a new extrusion technique. Here, the oriented noncrystalline regions in a series of PET fibers varies from 6% to 63%, depending strongly on the production conditions. In particular, samples produced with a newly developed threadline modification process possess a high content of oriented noncrystalline polymer. Measurements such as dynamic and static mechanical properties have been performed on various samples, and these properties are related to the oriented noncrystalline phase. The results provide direct evidence for the existence of highly oriented noncrystalline material in these unique PET fibers spun with a threadline modification process. © 1996 John Wiley & Sons, Inc.     

Amorphous Li2O2: Chemical Synthesis and Electrochemical Properties

When aprotic Li–O₂ batteries discharge, the product phase formed in the cathode often contains two different morphologies, that is, crystalline and amorphous Li₂O₂. The morphology of Li₂O₂ impacts strongly on the electrochemical performance of Li–O₂ cells in terms of energy efficiency and rate capability. Crystalline Li₂O₂ is readily available and its properties have been studied in depth for Li–O₂ batteries. However, little is known about the amorphous Li₂O₂ because of its rarity in high purity. Herein, amorphous Li₂O₂ has been synthesized by a rapid reaction of tetramethylammonium superoxide and LiClO₄ in solution, and its amorphous nature has been confirmed by a range of techniques. Compared with its crystalline siblings, amorphous Li₂O₂ demonstrates enhanced charge‐transport properties and increased electro‐oxidation kinetics, manifesting itself a desirable discharge phase for high‐performance Li–O₂ batteries.     

Carbon-13 NMR studies of local dynamics in polymer solutions and melts

Carbon-13 spin-lattice relaxation time measurements have been performed at two experimental frequencies on a series of amorphous polymers in the bulk state at temperatures well above the glass-transition temperatures and in solution. The high experimental values of the T₁ minimum as a function of temperature cannot be accounted for only by the specific orientation autocorrelation functions developed for polymers. They indicate the existence of an additional fast anisotropic motion, which we have assigned to librations of limited but significant extent of the internuclear CH vectors about their rest position. Moreover, in most cases, the temperature dependence of the segmental motions proves that they are involved in the glass-rubber transition phenomena.     

Oxygen and water-vapor diffusion through biaxially oriented poly(ethylene terephthalate)

The gas barrier properties of a wide range of biaxially oriented polyethylene terephthalate films have been investigated. The permeability and diffusion coefficients for oxygen and water vapor transmissions were determined for films prepared by both simultaneous and sequential biaxial stretching. The effect of annealing was also studied and a comparison made with previous results for uniaxially oriented films. It was found that the permeability correlated well with the density of the sample, but that the dependence on the gauche/trans conformer ratio shown for uniaxial material was not so clear. A good empirical correlation was also obtained between permeability and a proposed measure of molecular orientation obtained from refractive index measurements. A more detailed interpretation, based on infrared measurements of orientation, showed that there is a systematic reduction in permeability as the planes of the terephthalate residues become parallel to the film surface.