Effect Of Molecular Orientation Research Articles

The effect of molecular orientation and acetylene-enhanced crosslinking on the wear of UHMWPE in total artificial joints.

This paper investigates the benefits of combining roll-drawing and acetylene-enhanced crosslinking to alter the mechanical properties of the ultra high molecular weight polyethylene (UHMWPE) used in total hip and knee replacements, with the aim of improving its resistance to wear. UHMWPE was processed via crosslinking, roll-drawing and a combination of crosslinking and roll-drawing and subjected to gel content analysis, tensile tests, X-ray diffraction and wear tests using different types of motion and smooth and rough counterfaces. Purely roll-drawn materials with length and width draw ratios of lambda l x lambda w = 1.3 x 1.0 and lambda1 x lambdaw = 1.6 x 0.9 respectively, were found to have lower wear factors in a unidirectional motion test with a rough counterface when compared to the virgin material. The crosslinked roll-drawn material, with length and width draw ratios of lambda1 x lambdaw = 1.6 x 0.9, was seen to possess five crosslinks per initial number average molecule. This crosslinked and roll-drawn material showed 5.5 times less wear than the virgin material in a multidirectional motion test with a smooth counterface and 1.4 times more wear than the virgin material in a unidirectional motion test with a rough counterface. Hence this study supports previous work by the authors that acetylene-enhanced crosslinked materials may show benefits for a total hip replacement, but only where the femoral head remains smooth. The improvements in wear with the roll-drawn material in unidirectional tests were smaller, but may prove to have some benefits in the knee.

高分子液晶の二次元流れの数値シミュレーション

Finite difference solutions to the Doi equation with a quadratic closure approximation in two-dimensional channel flows, such as inlet developing flow between parallel plates and 2:1 contraction flow, were obtained. In the case of the inlet developing flow, a uniform velocity profile at the channel entrance was once overshot to become a profile similar to a Newtonian Poiseuille flow before reaching a fully-developed flat profile. The developing profile of velocity is in a wavy form because of the effect of molecular orientation. Because of the deceleration of the flow in the vicinity of channel wall near the entrance, the order parameter in the area became small. Afterwards, it recovered to a higher value owing to shear flow in the downstream, and a region of low order parameter was limited near the channel centerline, where the effect of the shear on the molecular orientation is small. In the developing region, the order parameter along the centerline was high because of extensional flow. It is quite interesting that the order parameter in an area between channel wall and the centerline near the entrance was lowered than the value of equilibrium state. For the contraction flow, the curvature of streamlines near the contraction was small compared to Newtonian flow, and resultantly larger vortices occured at channel corners. The order parameter in those regions was low and the effect of the secondary flow was reflected on the preferred angles.

Orientational averaging in the intense field tunnel ionization of molecules

The effect of molecular structure and orientation on the rate of tunnel ionization in intense laser fields is considered for linear molecules. The nonspherical nature of the molecular electrostatic potential near the classical electron turning point results in a nonisotropic distribution of tunneling rates. This must be considered when determining the overall ionization rate of a randomly oriented ensemble. Calculations are performed for two atom:molecule test pairs with similar ionization potential, Ar:N2 and Xe:O2, and the results are compared to experiments [Talebpour, Chien, and Chin, J. Phys. B 29, L677 (1996); Guo et al., Phys. Rev. A 58, R4271 (1998)].

Rotationally inelastic collisions of OH(X 2Π)+Ar. II. The effect of molecular orientation

Orientation effects in inelastic collisions of OH(X 2Π)+Ar at an energy of 746 cm−1 have been studied in a crossed molecular beam experiment. The OH(X 2Π) radicals were prepared in the v=0, Ω=32, J=32, f state by hexapole state selection and oriented with their O-end or H-end towards the Ar atom by a static electric field in the collision zone. The orientation-dependent probability density function has been determined by laser induced fluorescence spectroscopy using a narrow band dye laser and the Stark mixing induced P1′(1) transition. A degree of orientation of 〈cos θ〉=0.55 has been obtained. The steric asymmetry factor has been determined for rotational excitation up to the Ω=32, J=92 and Ω=12, J=52 states. Differences up to a factor of 8 in cross section were found between O-end and H-end impact. In general a slight preference for O-end impact was found for low rotational excitation, while a strong preference for H-end impact was found for excitation to high rotational states. The experimental results are compared to quantum scattering calculations on ab initio potential energy surfaces developed by Degli-Esposti and Werner [J. Chem. Phys. 93, 3351 (1990)] and Klos et al. [J. Chem. Phys. 112, 4952 (2000)]. The agreement between experiment and calculations based on the Klos and Chalasinski potential is excellent for transitions to states of A″ symmetry and good for transitions to states of A′ symmetry.

Viscoelastic Behavior of 1-Dodecanol Monolayers Undergoing a Liquid−Solid Phase Transition. A Surface Quasielastic Light Scattering Study

A surface quasielastic light scattering (SQELS) and electrocapillary wave (ECW) study of a monolayer of 1-dodecanol, obtained by placing a reservoir drop of the alcohol on top of a water surface, has been carried out as a function of temperature at different wavevectors. This monolayer goes through a liquid−solid phase transition at 312.1 K, more than 14 K above the freezing of bulk 1-dodecanol. The fit of the SQELS spectra gives the frequency, ωc, and the temporal damping, Δωc, of thermally induced capillary waves. In all the temperature range ωc follows Kelvin-like behavior (i.e., ωc ∼ q3/2) leading to values of the apparent dynamic surface tension lower than the static ones. In addition the damping follows, Δωc ∼ q1.2-1.4 very far from the expected ∼q2 dispersion behavior. Both wave parameters show temperature trends compatible with the expected liquid−solid 2D phase transition at around 312 K. The expansion of the frequency range by using the ECW technique, shows that our experimental results cover the regions q ≤ qR ≤ q, where qR is the wavevector at which capillary-dilational resonance takes place. Within the classical elasto-hydrodynamic theory, our results can be accounted for if the dilational viscosity, κ, takes a negative sign. In these conditions mode mixing takes place for q ≥ qR. Alternatively the experimental results can be quantitatively explained with κ = 0 if an extra hydrodynamic coupling is included in the dispersion equation. From a physicochemical point of view this coupling may arise from the vicinity of the phase transition and/or molecular orientational effects promoted by the dilational wave. The small values of the dynamic elasticity, ε(ω), found in this work can be rationalized in terms of a monolayer−subsurface material transport, within the Lucassen−Van de Tempel model.

Influence of temperature and chain orientation on the crystallization of poly(ethylene terephthalate) during fast drawing

Three regimes of poly(ethylene terephthalate) (PET) deformation are identified from a detailed analysis of time-resolved wide angle X-ray scattering (WAXS) data recorded during the mechanical deformation of PET at a wide range of draw rates (0.05–12s−1), temperatures (90–120°C) and draw ratios. These are:(i) where the onset of crystallization occurs after the end of deformation with a tendency for preferred alignment of the (100) plane in the plane of the sample;(ii) where the onset of crystallization occurs during deformation and with increasing tilt of the crystal chain axis away from the draw direction; and(iii) where there is no oriented crystallization.A comparison of the crystallization rate with the orientation parameter 〈P2(cosθ)〉 at the onset of crystallization shows a strong dependence of the crystallization rate on both temperature and molecular orientation. It is proposed that a previously observed insensitivity of crystallization rate to temperature was a consequence of fortuitous choice of drawing conditions in which the opposing effects of temperature and molecular orientation produced similar net crystallization rates. The effects of the temperature dependence have been factored out using a WLF shift factor to reveal an overall dependence of the crystallization rate on approximately the fourth power of 〈P2(cosθ)〉. At high draw temperatures where the rate of the chain retraction motion is faster than the draw rate, there is a significant tilt of the crystal chain axis away from the draw direction. During the crystallization process both the degree of crystal tilt and the half-width of the crystal reflections remain essentially unaltered over the whole crystallization process.

Homogeneous and Heterogeneous Electron Transfer Dynamics of Osmium-Containing Monolayers at the Air/Water Interface

from 3.0 to 7.0. Over the range 3.0 e pH e 7.0 a single pKa of 4.3 ( 0.3 is observed for the unbound Qbpy pyridine nitrogens when confined within a Langmuir monolayer. In solution, over the range 1.6 e pH e 7.0 two pKa so f 3.2( 0.2 and 4.3 ( 0.2 are observed. Horizontal touch voltammetry, performed using microelectrodes, has been used to determine the apparent charge transport diffusion coefficients, Dapp ,a s the mean molecular area is systematically varied. For surface concentrations, i, between approximately 1.0 10 -10 and 1.4 10 -10 mol cm -2 , the monolayer is in a liquid state and Dapp increases linearly with increasing surface concentration. Although surface concentration dependent physical diffusion of the complexes prevents an accurate determination of the self-exchange rate constant, kex, analyzing these data in accordance with a 2-D Dahms-Ruff approach provides a lower bound of 1.0 10 8 M -1 s -1 for kex. When the monolayers are further compressed so that the molecular area is less than approximately 100 A 2 , charge transport proceeds via a percolation mechanism and the maximum value of the apparent diffusion coefficient, Dapp,max, yields a rate constant for electron self-exchange between Os 2+ and Os 3+ of 1.6 10 9 M -1 s -1 . Dapp,max depends on the solution pH in a sigmoidal manner, increasing from approximately 6.6 ( 0.2 10 -6 to 8.9 ( 0.4 10 -6 cm 2 s -1 as the subphase pH is increased from 3.0 to 7.0. To probe the effect of molecular orientation on interfacial electron transfer rates, we have used chronoamperometry conducted on a microsecond time scale to measure the heterogeneous electron transfer rate, k, for both Langmuir and spontaneously adsorbed monolayers. In this way, the effect of making electrical contact through the Qbpy or dpp ligands on k can be probed. Although both rates are large, the rate measured at an overpotential of 50 mV for Langmuir monolayers (1.7 ( 0.2 10 5 s -1 ) is approximately an order of magnitude smaller than that found for spontaneously absorbed systems (1.2 ( 0.3 10 6 s -1 ), suggesting that the local microenvironment, e.g., packing density and molecular orientation, influence the rate of heterogeneous electron transfer in these systems.

Solid-State Replication of Relief Structures in Semicrystalline Polymers

The use of semicrystalline polymers for surface structuring allows multiple embossing and structuring of oriented polymer films, in contrast to amorphous polymers. The Figure shows an oriented perfluorinated polymer film (FEP), on which a lamellar grating has been superimposed by solid-state embossing. Polarized optical microscopy reveals the combined optical effects of surface structure and molecular orientation.

Solid-State Replication of Relief Structures in Semicrystalline Polymers

The use of semicrystalline polymers for surface structuring allows multiple embossing and structuring of oriented polymer films, in contrast to amorphous polymers. The Figure shows an oriented perfluorinated polymer film (FEP), on which a lamellar grating has been superimposed by solid-state embossing. Polarized optical microscopy reveals the combined optical effects of surface structure and molecular orientation.

Effect of Molecular Orientation on the Elastic Constants of Polypropylene

The Brillouin spectroscopic measurements of elastic properties of polypropylene films fabricated by different processing techniques are described. We find that the elastic symmetry and the associated elastic constants are dependent on the molecular orientation brought about by the processing conditions used to produce the films. We have shown that Brillouin scattering techniques can successfully be used to track the molecular orientation induced by uniaxial stretching. We find a direct correspondence between the Brillouin measurements and optical birefringence measurements, illustrating that molecular orientation plays a dominant role in determining the mechanical anisotropy in these materials.

Comparative assessment of slow strain rate, 4-pt bend and constant load test methods for measuring environment stress cracking of polymers

The environment stress cracking (ESC) susceptibility of polycarbonate and acrylonitrilebutadiene-styrene (ABS) in a number of environments has been evaluated using slow strain rate, four-point bend and constant load tests. Testing was conducted for the materials in the as-processed and annealed state. For the as-processed materials, the longitudinal direction of the specimen was generally perpendicular to the melt flow direction. Nevertheless, a few tests were carried out with the specimen orientation parallel to the flow direction. The ESC techniques gave consistent ranking of aggressivity of environments although the extent of relaxation of the ABS when exposed to particular environments limited the significance of results from the bend tests. Otherwise, the results validate further the use of the slow strain rate technique as an accelerated test method for assessing the ESC of plastics, albeit primarily as a ranking technique. The threshold stress for crazing was highest for the slow strain rate method which can be attributed partly to the particular strain rate adopted. There was no effect of molecular orientation on ESC for the limited range of test conditions evaluated. The difference in threshold stress measured for the annealed and as-processed specimens indicated the existence of a net tensile residual stress in the very near-surface region, not detected by the layer removal method of residual stress measurement.

The effects of molecular orientation on the physical aging and mobility of polycarbonate?solid state NMR and dynamic mechanical analysis

Solid-state NMR and dynamic mechanical (DMA) measurements were performed on a series of uniaxially hot-drawn bisphenol-A polycarbonate samples in order to determine the effects of stretching on the structure, mobility, and local orientation environment. Proton spin-lattice relaxation times, 1H T1ρ, for the phenylene carbon protons were fitted to a biexponential decay function, and both the long and short relaxation times initially increased with stretching. Intensity data indicated an increase in the number of short relaxation time protons and a decrease in the number of long relaxation protons with orientation. Similarly, DMA spectra showed that the β-relaxation strength also increased with drawing, which implied an increase in the number of localized segmental relaxations. It is theorized that the long and short 1H T1ρ relate to protons within tightly packed “cooperative domains,” and to those with greater localized free-volume, respectively. Stretching is known to distort the free-volume distribution, causing a decrease in the mean free-volume but an increase in the number of larger, more elliptical holes. This is expected to cause a decrease in the α-transition mobility (due to larger cooperative domains) and an increase in the β-mobility (due to the increase in the number of β-relaxing segments associated with the larger free-volume holes). These predictions are consistent with results recently reported by Shelby and Wilkes on the physical aging and creep behavior of these samples (M. D. Shelby & G. L. Wilkes, Polymer 1998, 39, 6767; M. D. Shelby & G. L. Wilkes, J Polym Sci Part B: Polym Phys 1998, 36, 2111). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 32–46, 2001

Molecular orientation-photoconductivity relationship study of phthalocyanine polymer-oriented thin films

The molecular orientation–photoconductivity relationships of several kinds of phthalocyanine polymer (PPc)-oriented thin films have been studied in double-layered photoreceptor devices, where the charge-generation layers (CGLs) are phthalocyanine polymer-oriented thin films and the charge-transportation layers (CTLs) are composed of hole transporting materials of tetraphenyl benzidine or hydrazone. The oriented thin films containing PPc dispersed in polyvinyl difluoride (PVDF) were prepared by the electric field orientation. The results showed that the photosensitivities of the phthalocyanine polymer (PPcs)-oriented thin films were higher than those of the unoriented PPcs thin films, and varied with their molecular structures and the molecular stacking in the films. This was thought to be due to the molecular orientation effect, which was demonstrated by the analyses of the polarized fluorescence, DSC, FTIR reflection absorption spectroscopy (FTIR-RAS), and angle-dependent XPS. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2331–2339, 2000

TRADE-OFFS IN BLOWN FILM LLDPE TYPE RESINS FROM CHROMIUM, METALLOCENE AND ZIEGLER-NATTA CATALYSTS

In this paper, the processing-structure-property (P-S-P) behavior and trade-offs observed with three LLDPE resins made from chromium, metallocene and Ziegler-Natta catalysts were examined and critically compared. The basic molecular and rheological properties were examined. The blown film performance of these resins was examined with respect to the effects of film thickness and molecular orientation on film properties. Also, the processability of these resins was compared through an examination of the typical extrusion and film blowing processing parameters. Finally, an attempt was made to highlight the advantages and disadvantages of each resin type as they apply to blown film applications. The results demonstrate that LLDPE resins from these various catalysts are dramatically different in nature, thereby resulting in different trade-offs with respect to their P-S-P behavior in film blowing.

Applications of Liquid Crystals to Variable-Focusing Lenses

Liquid Crystal (LC) lens cells with variable focusing properties are fabricated using nematic LC materials and applicable to optical devices, and our recent work on these cells is described. First, the LC lens cells are prepared using lens-shaped substrates coated with transparent electrodes. Their focal length can be continuously varied between the values for an ordinary ray and an extraordinary ray by changing the voltage passing across them. Methods of improving properties and some applications of the lens-shaped LC lens are briefly described. The lens properties of these cells with plane-paralleled structure are then demonstrated, where the refractive index is graded to a quadratic distribution resulting from an axially symmetric non-uniform electric field. LC cells with axially distributed tilt angles are constructed using a pair of circular hole-patterned electrode substrates and very small LC lens (LC microlens) with variable focusing can be fabricated. Optimizing the electrode structure, device parameters, and material parameters of the LCs, excellent focusing properties can be obtained. The properties of the LC microlens are improved by using the polymer stabilization technique. The LC microlens with a divided electrode structure shows three-dimensional beam steering and focusing properties, and the astigmatic aberration caused by the molecular orientation effect can be compensated. Applications of the LC microlens to optical devices and systems are introduced.

Ferroelectricity and piezoelectricity of nylon 11 films with different draw ratios

The first studies by Shuford et al. (Polym Eng Sci 1976, 16, 25) of the effect of draw ratio on the piezoelectric properties of poled poly(vinylidene fluoride) films concluded that the effect of molecular orientation due to draw prior to film polarization was to increase greatly the piezoelectric response. In fact, unoriented films (being crystallized in the nonpolar crystal form, phase II) showed negligible piezoelectricity. It was clear that the effect of increased draw ratio prior to film polarization was to increase the measured piezoelectric coefficients, because draw gave rise to a phase transition to a polar crystal phase, phase I, and molecular chain orientation in the plane of the film, subject to high electric poling fields perpendicular to the plane of the film, resulted in a preferred dipole orientation perpendicular to the plane of the film by a rotation about molecular stems. A detailed study of nylon 11 films and other odd nylons has shown that these polymers also can exhibit ferroelectricity, piezoelectricity, and pyroelectricity of comparable or greater magnitude than that exhibited by poly(vinylidene fluoride) with a greater thermal stability. For these polymers, the effect of draw on piezoelectricity and ferroelectricity is not complicated by the effect of crystal phase transitions. For this reason, a study of the effect of draw ratio on the ferroelectric and piezoelectric properties of nylon 11 was initiated in order to provide some insight into the direct effect of molecular orientation (without accompanying phase changes) on piezoelectric and ferroelectric properties. Some melt-quenched nylon 11 films were uniaxially drawn to draw ratios, ranging from 1.5 : 1 to 3.5 : 1 in increments of 0.5, at room temperature. Both undrawn and drawn films were found to exhibit an electric displacement vs. electric field hysteresis loop, characteristic of ferroelectric materials. As the draw ratio increased from 1 : 1 to 3.5 : 1, the remanent polarization increased from 25 to 53 mC/m2 and the coercive field decreased from 73 to 63 MV/m. The piezoelectric response and electromechanical coupling constant were also measured at temperatures of 25°C and 120°C under dry conditions. As the draw ratio of the films increased from 1 : 1 to 3.5 : 1, the piezoelectric strain coefficient d′31 measured at 25°C remained unchanged at ∼ 1.1 pC/N but the piezoelectric stress coefficient e′31 increased from 2.1 to 4.9 mC/m2. At 120°C, d′31 increased from 3.8 to 20 pC/N and e′31 from 0.5 to 27 mC/m2. d′31 showed a linear relationship with the remanent polarization Pr at both 25°C and 120°C for all draw ratios but e′31 showed this relationship only at 25°C. The electromechanical coupling constant k′31 increased from 0.42% to 0.61% at 25°C and from 0.42% to 1.56% at 120°C as the draw ratio increased to 3 : 1. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2737–2746, 1999

Comparison of residual stress predictions and measurements using excimer laser layer removal

AbstractResidual stresses in injection molded polymer parts can have a major effect on product performance and are therefore often undesirable. Two different models for residual stress formation in injection molded products exist in the literature. The first model assumes that stresses in molded parts are identical with free quench stresses. Deviations are attributed to molecular orientation effects. The second model assumes that no slip occurs and that stresses are related to the holding pressure. Careful measurements should be able to differentiate between these models. In this study the layer removal stress measurement technique was improved by using an excimer laser for the milling operation. In that way stress relaxation during measurements was avoided and thin layers of uniform thickness could be removed. Both resolution and measurement accuracy were greatly improved. Stress distributions in polycarbonate plates molded under low holding pressures turned out to be of the free quenching type, whereas those molded under higher pressures were of the pressure type. All stress distributions could be predicted by a recent theory that comprises the two models as limiting cases.

Surface analysis of oligothiophene films using HREELS: molecular orientation effects

The surface of evaporated films of α-oligothiophenes—quaterthiophene (4T), quinquethiophene (5T) and sexithiophene (6T)—was studied by high-resolution electron energy loss spectroscopy (HREELS), extending the common vibrational domain of energy loss to electronic excitation and secondary emission. Vibrational spectra show that the molecular orientation is not the same on graphite and gold substrates. HREELS results corroborate those obtained by reflection–absorption infrared spectroscopy (RAIRS), confirming that graphite substrates induce films where molecules lie flat on the surface, whereas gold substrates generate a molecular orientation close to perpendicularity. Using these two differently oriented samples, we demonstrate the existence of two relaxation channels for the electron surface interaction: electronic excitation and ionisation. HREELS spectra recorded with incident electron energy between 2 and 7 eV reveal that secondary electron emission originating from ionisations is always more efficient for escape directions perpendicular to the plane of the molecule. This is here associated with the π orbital origin of this emission. Copyright © 2000 John Wiley & Sons, Ltd.

Effect of molecular orientation on the biodegradability of aliphatic polyester

AbstractThe effect of morphological microstructure on the biodegradability of aliphatic polyester, poly(ϵ ‐caprolactone) (PCL) was studied in terms of crystallite size, crystallinity and amorphous and crystalline orientation factors. Microstructural changes during hydrolysis/biodegradation of the drawn PCL films were investigated by the conventional small and wide angle X‐ray scattering methods. The lower was the draw ratio, the higher the hydrolytic degradability or biodegradability. With the increase of the hydrolysis time, the long period, at earlier stage, decreased; then slightly recovered and the crystallinity increased while the lamellar thickness remained unchanged. The amorphous orientation factors start to decrease at earlier stage and gradually go down to zero before the end of hydrolysis. In the case of crystalline orientation factor, although the values decrease with increasing hydrolysis time, they do not reach zero point. After the biodegradation for 60 days, crystallinity, crystal lateral size and lamellar thickness in all drawn PCL films decreased, and then it was confirmed that even crystalline regions were degraded for long term biodegradation test.

Computer simulation of sputtering of gold targets using Sb and Sb 2 ions

It is well known that energetic impacts of Sb and Sb 2 ions on gold substrates give rise to high energy density collision cascades and potentially very high sputtering yields. A large component of the sputtering mechanism is due to the presence of a thermal spike which causes prolonged ejection of material with thermal energies. This phenomenon is investigated using molecular dynamics simulations. The energy dissipation mechanisms and the effects of molecular orientation are investigated. The overall sputtering yields are compared to experimental measurements.