Film morphology and molecular orientation in oligothiophene-fluorene films

The effects of processing conditions on film morphology and molecular orientation were studied for a novel conjugated fluorene-bithiophene oligomer, oligo(9,9-dioctylfluorene-alt-bithiophene) (OF8T2). Depending on the method of film preparation, OF8T2 molecules adopt different orientations in the films. X-ray diffraction peak at 4.9° of the OF8T2 film deposited from petroleum ether/dichloromethane mixture is attributed to a layering distance between sheets of OF8T2 chains, which are separated by the octyl side chains. Preferred orientation is clearly inferred through the absence of peaks corresponding to π-π stacking. For the spin-coated film after annealing, X-ray diffraction patterns indicate the presence of lamellar structure with the plane of the conjugated backbone normal to the substrate. The molecules were aligned with long axes along the rubbing direction when the spin-coated film was rubbed and then annealed. These results suggest a convenient approach for preparing active layers for organic optoelectronic devices by simple solution methods.     

Measurement of molecular orientation in thermotropic liquid crystalline polymers

Procedures for obtaining molecular orientational parameters from wide angle X-ray scattering patterns of samples of thermotropic liquid crystalline polymers are presented. The methods described are applied to an extrusion-aligned sample of a random copolyester of poly(ethylene terephthalate) (PET) and p-acetoxybenzoic acid. Values of the orientational parameters are obtained from both the interchain and intrachain maxima in the scattering pattern. The differences in the values so derived suggest some level of local rotational correlation     

Determination of molecular orientation using molecular dynamics for polymer melt flows in circular ducts

Molecular dynamic simulations have been used to develop a mathematical model to investigate the extension of the polymer chains and the polymer chain orientations in the circular duct flows. The effects of pressure loss and molecular weight are also investigated on the polymer chain orientation. The model's outputs indicate that with increasing pressure loss and subsequently the shear stress at wall, the trace of the conformation tensor increases. This is interpreted as an increase in the extension of polymer chains and the polymer chain orientation in the flow direction. The model also predicts that the increase of the molecular weight of polymer results in decrease of the trace of the conformation tensor and the extension of the polymer chains. POLYM. ENG. SCI., 55:1196–1202, 2015. © 2014 Society of Plastics Engineers     

Effect of molecular orientation on the fracture toughness of thermoplastics

An account is given of fracture toughness experiments on injection moulded plaques of a propylene homopolymer and two different molecular weight grades of poly(methyl methacrylate). The study was concerned essentially with the behaviour of large, centrally notched, plaques since this represents a step towards component testing. However, in order to establish this point, small specimens machined from the plaques were also tested. Analysis of behaviour is by linear elastic fracture mechanics. The study showed that the fracture toughness parameter, K_IC, was not a material constant. Variations in fracture toughness are attributed to molecular orientation produced during the injection moulding process. As expected, the propylene homopolymer was tougher than the PMMA. However, a potentially ductile material can still fail in a brittle manner if a large enough specimen is tested. Overall, this study has emphasized that tests on complete mouldings can provide a valuable means of assessing potential practical performance, and also that data obtained on small specimens can be misleading with respect to the performance of large components.     

Crystal orientation and twinning in cold-rolled ultrahigh molecular weight polypropylene

The mechanisms of plastic deformation in cold-rolled ultrahigh molecular weight polypropylene (UHMWPP) have been explored by means of wide angle X-ray (WAXD) pole figures. The melt crystallized UHMWPP slabs were rolled unidirectionally in a two-roll mill at 408 K as well as in the cross (transverse) direction. In the unidirectional rolling, all reciprocal lattice vectors of (110), (040), and (130) were found to orient preferentially in the film normal (thickness) direction. The cross-rolling of UHMWPP shows the preferential orientation of the above plane normals in the film thickness direction, but the distribution of poles broadens toward the transverse direction. Such orientation behavior is very different from that of conventional cold-rolled polypropylene. Various orientation mechanisms involving the orientation of lamellae in the film plane, twinning of the (110) plane and slippage mechanisms have been taken into consideration to account for the observed pole figures. A theoretical simulation has been carried out based on an orientation distribution function approach by introducing a slippage angle along the maximum shear stress in conjunction with intralamellar slip around the transverse direction. This model simulation conforms closely with the experimental WAXD pole figures of the (110), (040), and (130) planes.     

Analysis of molecular orientation and internal stresses in extruded plastic sheets

The development of molecular orientation and internal stresses in extruded sheet made of polypropylene was analyzed, and their correlations to operating conditions such as draw ratio, cooling rate, die temperature, melt temperature, and die gap opening were studied. Measurements of attenuated-total-reflectance infrared dichroic ratio for the surface molecular orientation, birefringence for the orientation stress distribution in the thickness direction, and free shrinkage ratio for the overall frozen-in stresses were carried out to determine the amount of orientation stresses in the extruded samples. As expected, the overall orientation stress depends strongly on draw ratio, while higher melt temperature reduces the overall orientation. It was found that faster cooling rates and lower die temperatures cause surface orientation stresses to increase as the core orientation stresses remain almost unchanged.     

The development of molecular orientation and morphological texture in thermotropic copolyesters

Liquid crystalline polymers can be processed to form high strength/modulus materials. In processing these materials, it is apparent that molecular orientation is an important factor in determining the physical strength of the processed materials. In this study a systematic investigation was carried out to determine how a thermotropic copolyester of parahydroxybenzoic acid (PHB) and polyethylene terephthalate (PET) responds to two basic types of flows: shear and extensional flow. This was accomplished by preparing sheared and extended samples under controlled conditions of temperature and flow history. Sheared disks were prepared using a disk and plate geometry of a Rheometrics Mechanical Spectrometer (RMS model 605), while extended ribbons were prepared using a slit die attached to an Instron capillary rheometer. Two copolymerer compositions of 60 mole percent and 80 mol percent PHB were investigated. The sheared disks and extended ribbons were investigated for molecular orientation and morphological textures using wide angle x-ray scattering (WAXS) and scanning electron microscopy (SEM) analysis, respectively. It was found that extensional flow has a greater capacity for orienting such materials than shear flow. Samples annealed at their softening points for 1 minute (240°C for the 60 mole percent PHB/PET copolymer and 300°C for the 80 mole percent PHB/PET copolymer) showed no significant loss of orientation, indicating that once orientation is produced it may remain in the melt for a long period of time. Sheared samples prepared by shearing the sample while cooling showed significantly higher degrees of orientation than those not cooled while being sheared. This may indicate that a minimum stress level exists for the production of orientation in shear flow.     

PAN分子取向程度与纤维结构性能的关系

研究了丙烯腈(AN),甲叉丁二酸(ITA)聚合体系中PAN分子取向与纤维结构、性能之间的关系。 结果表明:在一定范围内,PAN分子取向因子的增加有利于提高纤维的结晶度和(100)晶面法线方向上晶粒 尺寸,提高纤维的体积密度和力学性能。但同时增加了纤维内应力,使纤维的沸水收缩率增加。综合考虑取 向因子对纤维结构性能因素的影响,认为PAN原丝晶区取向因子约为0.9、总取向因子约为0.8时,纤维力 学性能较好。     

Dependence of acoustical characteristics on molecular orientation and strength of man-made fibres

Conclusions A correlative connection has been established between acoustical characteristics and the orientation of macromolecules or the strength of yarns of various chemical natures.It has been shown that any structural transformations in oriented polymers which lead to a change in molecular orientation cause a change in the absorption coefficient of ultrasonic waves.Translated from Khimicheskie Volokna, No. 5, pp. 26–28, September–October, 1984.     

Effect of molecular orientation on the dielectric properties of spin-coated polyimide films

The effect of molecular orientation on the dielectric properties of spin-coated polymide films has been studied in Situ for pyromellitic dianhydride with 4,4′-oxydianiline (PMDA/ODA) and biphenyldianhydride with p-phenylendiamine (BPDA/PPD). The degree of molecular orientation is characterized by the optical anisotropy between the in-plane and the through-plane, refractive indices. The through-plane dielectric properties are measured by fabricating parallel-plate capacitors directly onto the silicon substrate. Both the birefringence and the dielectric constant of PMDA/ODA polyimide vary with film thickness; however, these properties are independent of film thickness for BPDA/PPD films. To confirm that the measured dielectric constant obtained from the parallel-plate structures is free from a significant edge effect, finite element modeling of the electrstatic potential within the dielectric edge effect, finite element modeling of the electrostatic potential within the dielectric is performed. Models for both isotropic and anisotropic dielectric properties indicated that the fringing effects are indeed negligible for the film thicknesses examined. Thus, the changes observed in the measured dielectric constant are attributed to the variations in the molecular orientation. The optical anisotropy observed for the polyimides suggests a corresponding dielectric anisotropy in the films. An estimation using the Maxwell relation indicates that the dielectric anisotropy at 10⁵ Hz is 0.31 for PMDA/ODA and 0.85 for BPDA/PPD. © John Wiley & Sons, Inc.     

The determination of molecular orientation in uniaxially compressed PMMA by X-ray scattering

A procedure is presented for obtaining full molecular orientation information from wide angle X-ray scattering patterns of deformed non-crystalline polymers. The method is based on the analysis of experimental and calculated scattering patterns into their spherical harmonics. The results obtained for PMMA are compared with values predicted by the pseudo affine and affine deformation schemes.     

Mechanical behavior and molecular orientation of polycarbonate plates prepared by press-induced deformation

Through a uniaxial hot compression method, polycarbonate (PC) plates were self-reinforced effectively by press-induced deformation. Relationships between macroscopic mechanical properties including tensile, impact as well as viscoelastic properties, and microscopic status such as morphologies and orientation were obtained in deformed PC plates. Results showed that PC plates which were compressed at appropriate temperatures (20°C–120 °C) obtained simultaneous enhancements in tensile strength (28%), elastic modulus (38%) and impact strength (700%), compared with the original plate. The impact fracture morphologies of deformed PC plates revealed evident ductile breakage. Wide-angle X-ray diffraction investigations were conducted to attribute mechanical behavior to molecular segment orientation, which was more sensitive to temperature. Self-reinforced mechanism was proposed to analyze the high correlation between various orientations of molecular segments and corresponding mechanical performance, explained by storage and loss moduli in dynamic mechanical analysis as supplemental verification. Press-induced deformation was proved to be a potentially effective method for the self-reinforcement treatment for PC transparent products.     

Craze initiation in glassy polymers: Quantifying the influence of molecular orientation

A study has been made of crazing stress in oriented glassy polystyrene. The aim was to develop a methodology for prediction of crazing stress in glassy polymers with frozen-in molecular orientation. Oriented specimens of two grades of monodisperse polystyrene (PS) and one grade of polydisperse PS were produced by uniaxial melt-drawing and subsequent quenching of compression-moulded bars. Birefringence and crazing stress parallel to the draw direction (in the presence of diethylene glycol) were measured on miniature beam specimens cut from them. The crazing stress increased substantially with orientation, and the magnitude of the increase relaxed approximately on a timescale associated with the longest Rouse time. Specifically, a linear correlation was found, to within experimental scatter, between the increase in crazing stress and the orientation expressed in terms of frozen-in conformational stress, as predicted by the theory of Maestrini and Kramer [13]. The inverse gradient (constant β in the theory) was found to be 0.059 ± 0.002, when inferring the conformational stress from the measured birefringence. Crazing was found to be suppressed in favour of yielding in the most highly oriented specimens, and this could be explained in terms of the differing sensitivities of crazing and yield to molecular orientation.     

Molecular dynamics study on the effect of molecular orientation on polymer welding

Coarse-grained molecular dynamics simulation of a bead-spring polymer model was conducted for interdiffusion of two polymer flow fronts flowing parallel to one another as would be found in a weld-line. The effect of molecular orientation of the flow fronts on the interfacial structure was simulated. It was observed that the time evolution of the interfacial thickness was suppressed by shear flow for a chain whose length was longer than entanglement length. According to the analysis of autocorrelation function of end-to-end vector, it was revealed that anisotropic motion of chain was responsible for the growth of interfacial thickness and relaxation of the chain orientation was an important mechanism to explain the effect of shear flow on the growth of interfacial thickness. From the analysis of the segment motion at the interface with taking the density profile of end beads and center beads during the diffusion, it was found that interdiffusion of the end segment was always faster than that of center segments and significantly suppressed in oriented long chain.     

The effect of molecular orientation on the mechanical properties of fiber-filled amorphous polymers

The technology of using glass fibers to increase the modulus and the strength of polymeric systems is well-developed. Much less widely exploited has been the enhancement of properties by orienting the molecules of the polymer itself. The purpose here was to look for a synergistic combination of these two strategies: using glass fiber-filled polystyrene and introducing molecular orientation into the polystyrene matrix. For rheological reasons it is not possible to introduce large stretch ratios in the rubbery state and thus the amount of molecular orientation that can be frozen into the quenched glassy composite is small. Even so, however, the rubbery elongation (which we associate with subsequent molecular orientation) has a very significant effect on the final (room temperature) mechanical properties. Analysis of these properties was made in terms of various theoretical models (those of Brody and Ward, Smallwood-Guth, and Russel and Acrivos for the Young's modulus; and that of Kelly and Tyson, as modified by Lees, for the tensile strength). These comparisons showed that the brittleness of unoriented polystyrene is such that the matrix does not effectively distribute the stress along the length of the fiber and thus the benefit of the length is not realized; whereas the higher ductility of oriented polystyrene allows such transfer and a consequent improvement of properties.     

Effect of extensional and shearing strains on molecular orientation of a polymer melt

The flow of two grades of polydimethysiloxane (Dow Corning type 200 grades 60 Pa's and 300 Pa's) was studied in a 60° wedge flow cell. The stress-optical law was validated along the center-line (extensional strains only) and at off centerline locations (extensional and shearing strains combined). Values for stressoptical coefficient were 8.01 × 10^?11 Pa^?1 and 9.61 × 10^?11 Pa^?1 respectively, the differences apparently being due to experimental error. Orientation angles along a fixed radius at 20° to the cell center-line were almost constant. Center-line orientation angles were zero, as expected. Orientation angles at a constant radius for various angular positions varied from zero at the centerline to 54° near the wall. A method for testing constitutional relations for stress is presented. The power law model and linear viscoelastic models are shown to be inadequate in describing orientation effects caused by rotation in the flow. The Goddard-Miller model was shown to express this but accuracy was not very good, perhaps because of the single Maxwell unit used.