Effect of magnetic field on discotic nematic liquid crystalline polymers under simple shear flow

The effect of magnetic field on the discotic nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory, in which the molecular shape parameter (β) is defined at ?1.0. The evolution equation for the probability function of the discotic nematic LCP molecules is solved without any closure approximations. The transition among flow‐orientation modes, such as tumbling, wagging, and aligning defined similar to the rodlike LCPs, is strongly affected by the magnetic fields. The new aligning flow‐orientation mode observed for the rodlike LCPs under magnetic fields also can be investigated in the lower shear rate region. On the other hand, the effect of magnetic fields parallel to the x‐ and y‐axis on the time‐averaged first and second normal stress differences ( , ) are also studied. It can be seen that the shear rate regions of the sign changes of , are completely contrary to those conclusions achieved for the rodlike LCPs. In addition, the absolute values of increase with the magnetic field strength in the lower shear rate range owing to the new aligning flow‐orientation mode. Finally, the flow‐phase diagram versus β is also discussed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Molecular orientation in quenched channel flow of a flow aligning main chain thermotropic liquid crystalline polymer

We report measurements of molecular orientation in solid specimens of a main-chain thermotropic liquid crystalline polymer (LCP) that were quenched from mixed shear-extensional channel flows. The polymer under investigation is a random copolyether with mesogens separated by flexible hydrocarbon spacers. This polymer is known to exhibit ‘flow aligning’ dynamics under slow shear flow. Experiments were designed to preserve the molecular orientation state, representative of steady, isothermal channel flow in the solid samples, so that comparisons could be made against in situ channel flow measurements on other main chain thermotropes without flexible spacers, including a commercial fully aromatic copolyster. In the flow aligning material, little change in orientation was found in slit-contraction flows, and only modest drops in orientation were found in slit-expansion flow. This contrasts strongly with data on commercial LCPs, suggesting that these materials may be of the ‘tumbling’ type.     

Viscoelastic properties of fly ash‐filled natural rubber compounds: Effect of fly ash loading

Fly ash (FA) as a by‐product of power station plants is known to consist of silicon dioxide similar to precipitated silica. The use of FA as filler in natural rubber (NR) was of interest to reinforce and/or reduce product cost. In this article, viscoelastic properties of FA‐filled NR composites with various FA loadings were investigated with the utilization of two different modes of shear flow, namely, oscillatory and steady shear flow. It is found that the addition of FA to NR increases storage modulus (G′) and shear viscosity under both oscillatory and steady shear flow. Moreover, the oscillatory test results exhibit the unexpected increase in magnitude of viscous response with increasing FA loading in FA‐filled NR compounds. The explanation is proposed in terms of the ball‐bearing effect of FA with spherical shape associated with the occurrence of molecular degradation induced by inorganic constituents particularly manganese, iron, and copper in nonrubber component of NR as well as the small amount of heavy metals including iron, copper in FA. An isoprene rubber (IR) containing no nonrubber component was used to validate the proposed explanation. In addition, with the use of Cox‐Merz concept, the results of both complex viscosity under oscillatory shear flow and apparent shear viscosity under steady shear flow can effectively be superimposed in the case of FA‐filled compounds, supporting the promotion of viscous response by FA. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Two‐dimensional long‐flexible fiber simulation in simple shear flow

Long‐flexible fiber orientation under the influence of a flow field is an important engineering problem. One can encounter this problem in many fields. For example in fiber reinforced thermoplastics produced in both injection and compression molding, fiber orientation affect final part properties. Fiber orientation models are constructed for short fibers in a simple shear flow case and though this case is important it is not the general case. In this work we extract rotational friction coefficients from Jeffery's model, create a general case long‐flexible fiber orientation model, and apply it in a simple shear flow. POLYM. COMPOS., 37:2425–2433, 2016. © 2015 Society of Plastics Engineers     

Study of the molecular orientation heterogeneity in polypropylene injection‐molded parts by Raman spectroscopy

Polarized Raman microspectroscopy has been used to study oriented‐skin layers induced in injection‐molded isotactic polypropylene (iPP) parts. A method based on the intensity sensitivity of several Raman bands to laser light polarization was employed to estimate the degree of molecular orientation in iPP. The skin‐core molecular orientation heterogeneity in injection‐molded iPP is then evaluated via two different experimental methods. Results show that an in‐depth profile using micro‐Raman confocal technique is as valuable as an edge profile performed on a sample cross‐section because both are correlated with optical microscopy measurements. Both Raman measurements are in good agreement with optical microscopy measurements. The skin development was found to be narrowly related to the shear strain rate at the mold walls. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Real‐time ultrasonic characterization of the chain orientation of high density polyethylene melts during processing

In an attempt to understand the relationships between the evolution of ultrasonic signal and the change of molecular structure during processing, shear‐induced chain orientation and its subsequent disorientation of high‐density polyethylene (HDPE) melts with different melt indices were studied using a noninvasive and nondestructive ultrasonic technique. The molecular structural development during orientation and disorientation of these melts was manifested in the ultrasonic velocity. Two models were developed to describe the relaxation processes of orientation and disorientation, respectively. The effects of shear rate and temperature on the maximal degree of orientation and the relaxation time of orientation and disorientation were also explored. These results from the ultrasonic measurements were compared with ones obtained through rheological measurements, X‐ray diffraction (XRD), and infrared dichroism measurements. The experimental results indicated that the ultrasonic technique was sensitive and promising for the real‐time monitoring of the evolution of molecular structure during polymer processing. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Evaluation of molecular orientation in injection molded parts with microscale features by Raman spectroscopy

Molecular orientation of polycarbonate (PC) in injection‐molded parts with microscale features was characterized by means of polarized Raman spectroscopy, and the relationship between microstructure and replication was discussed. The microscale feature size of continuous v‐groove was 20 μm in depth and 50 μm in width. PC injection‐molded parts were molded with various molding conditions. The molecular orientation distribution along flow direction on the cross‐section of molding parts were evaluated by the intensity ratio of the bands at 635 to 703 cm^?1 (I₆₃₅/I₇₀₃) in the Raman spectra. Molecular orientation along the flow direction inside the v‐groove was higher than that of the core and the opposite surface region. In particular, the highest molecular orientation was at the surface of the v‐groove. Among the injection molding conditions, the mold temperature showed significant effect on the molecular orientation and replication. Higher mold temperature caused high replication and low molecular orientation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mixed‐mode subcritical crack growth in orthotropic polymers

A semi‐empirical model of mixed‐mode sub‐critical crack propagation is presented for orthotropic polymers subjected to continuous loading. The model considers a combination of opening and shearing propagation modes. Optical measurement of crack propagation in an orthotropic liquid crystalline polymer film for orientation angles, which define the orientation of the loading to the extrusion direction, of 45 and 90° provides the empirical constants required by the model. The model is validated by comparison of predicted and measured crack propagation at 30, 60, and 75°. The impact of orientation angle on crack propagation is significant. A critical angle is identified at which the crack propagation rates for the opening and shearing modes are equivalent. For orientation angles less than the critical angle the crack growth is dominated by the shearing mode. For orientation angles greater than the critical angle, the crack growth is dominated by the opening mode. For the liquid crystalline polymer film tested, the critical angle is 52.6°. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Rheological and structure investigation of shear‐induced crystallization of isotactic polypropylene

In the present work, the influence of well‐defined simple shear flow histories on the isothermal crystallization of an isotactic polypropylene (i‐PP) has been investigated. At first, the research of the flow conditions in terms of temperature, shear rate ( ) and shear strain (γ) has been performed by means of the rheological technique. The continuous shearing analysis enabled us to build the flow curve at 144°C showing a Newtonian region as well as a shear‐thinning zone. Indeed, for above a critical value, the molecular orientation occurring during flow provides a kinetic promotion of the crystallization process. In the rheological step‐shear flow analysis, an increase of the flow sensitivity parameter, k_S/k_Q, with increasing the shear rate at a constant strain (γ = 150) is observed. The structure of the crystallized samples has been investigated by differential scanning calorimetry (DSC) and wide angle X‐ray scattering (WAXS) methods. In agreement with the DSC, the WAXS results show that crystals with a certain bimodal distribution are generated in the samples crystallized under step‐shear flow conditions. A small orientation of the (110) plane of the i‐PP α‐phase crystals is also detected. POLYM. ENG. SCI., 45:153–162, 2005. © 2005 Society of Plastics Engineers.     

Discrete element simulation of free flowing grains in a four‐bladed mixer

Numerical simulations of granular flow in a cylindrical vessel agitated by a four‐blade impeller were performed using the discrete element method. Velocity, density, and stress profiles within the mixer displayed a periodic behavior with a fluctuation frequency equal to that of the blade rotation. Blade orientation was found to affect flow patterns and mixing kinetics. For an obtuse blade pitch orientation, a three‐dimensional recirculation zone develops in‐front of the blade due to formation of heaps where the blades are present. This flow pattern promotes vertical and radial mixing. No recirculation zone was observed when the blade orientation was changed to an acute blade pitch. The system's frictional characteristics are shown to strongly influence the granular behavior within the mixer. At low friction coefficients, the 3‐D recirculation in front of the obtuse blade is not present reducing convective mixing. Higher friction coefficients lead to an increase in granular temperature which is associated with an increase in diffusive mixing. Normal and shear stresses were found to vary with mixer height with maximum values near the bottom plate. Additionally, a strong dependence between the magnitude of the shear stresses and the friction coefficient of the particles was found. The stress tensor characteristics indicate that the granular flow in our simulations occurs in the quasi‐static regime. At the same time, the averaged pressure was found to vary linearly with bed height and could be predicted by a simple hydrostatic approximation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Crystallization behavior and molecular orientation of high density polyethylene parts prepared by gas‐assisted injection molding

The dependence of hierarchy in crystalline structures and molecular orientations of high density polyethylene parts with different molecular weights molded by gas‐assisted injection molding (GAIM) was intensively examined by scanning electron microscopy, two‐dimensional wide‐angle X‐ray scattering as well as dynamic rheological measurements. The non‐isothermal crystallization kinetics of the samples were also analyzed with a differential scanning calorimeter at various scanning rates. It was found that oriented lamellar structure, shish‐kebab and common spherulites were formed in different regions of the GAIM samples. The scanning electron microscope observations were consistent with the two‐dimensional wide‐angle X‐ray scattering results and showed that the molecular chains near the mold wall had strong orientation behavior, revealing the distribution of the shear rate of the GAIM process. The differences in crystal morphologies can be attributed to molecular weight differences as well as their responses to the external fields during the GAIM process. The formation mechanism of the shish‐kebab structure under the flow field of GAIM was also explored. Copyright © 2012 Society of Chemical Industry     

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

Molten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The role of flow‐induced microstructure in rheological behavior and nonisothermal crystallization kinetics of polyethylene/organoclay nanocomposites

The evolution of polyethylene/organoclay nanocomposite microstructure via shear and extentional flow fields was studied by tracing rheological behavior and nonisothermal crystallization kinetics. Although studying microstructure formed through flow fields, two phenomena were noticed: the breaking of three‐dimensional (3D) network containing filler–filler, filler–matrix, and matrix–matrix interactions, and organoclay platelets orientation. Utilizing nonlinear viscoelastic measurements and thermal analyses, it was proven that clay alignment was present only in large enough shear flows and all elongational flows. It was observed that regardless of the type of flow field and its magnitude, due to the breaking of 3D network, the extent of crystallization can be increased. The half‐lives of the crystallization of film samples and those samples subjected to large enough shear rates for clay platelets to be aligned decreased, proving the effect of clay orientation on crystallization rate increment. Based on endotherms observed through melting behavior studies of samples, it was proven that in elongation and large amplitude shear flows, clay orientation had resulted in forming thicker crystalline lamellae, likely because of forcing the adjacent polymer chains to align with the clay platelets. POLYM. ENG. SCI., 54:1839–1847, 2014. © 2013 Society of Plastics Engineers     

Using the external magnetic field of composites to control fiber orientation and enhancement of electrical conductivity

The industrial developments have led to more applications of various composites. Since fiber orientation and distribution will influence product performance in composites, controlling said orientation and distribution is of critical importance. This study used external magnetic fields to control the fiber orientation and distribution in a polymer. The orientation of the actual fibers under magnetic field control during flowing was observed using a visualization system, which was made by PMMA and transparent epoxy as an upper cover and filling polymer. In order to clearly observe and calculate, 0.1 wt% fiber content was used, and 0.3 wt% fiber content was used to measure conductivity. Fiber distribution angles without a magnetic field concentrate parallel to the flow direction (0° ~ 30° and 151° ~ 180°) while distribution angles under magnetic field control were concentrated along the magnetic field direction, which was perpendicular to the flow direction (61° ~ 120°). The higher the magnetic flux density, the larger the torque of the electromagnetic field on the fibers and the higher the orientation of fibers was with the magnetic field. The electrical conductivity was 12.23 times higher for 1 mm fibers in an external magnetic field versus no magnetic field.     

Molecular orientation distributions during injection molding of liquid crystalline polymers: Ex situ investigation of partially filled moldings

The development of molecular orientation in thermotropic liquid crystalline polymers (TLCPs) during injection molding has been investigated using two‐dimensional wide‐angle X‐ray scattering coordinated with numerical computations employing the Larson–Doi polydomain model. Orientation distributions were measured in “short shot” moldings to characterize structural evolution prior to completion of mold filling, in both thin and thick rectangular plaques. Distinct orientation patterns are observed near the filling front. In particular, strong extension at the melt front results in nearly transverse molecular alignment. Far away from the flow front shear competes with extension to produce complex spatial distributions of orientation. The relative influence of shear is stronger in the thin plaque, producing orientation along the filling direction. Exploiting an analogy between the Larson–Doi model and a fiber orientation model, we test the ability of process simulation tools to predict TLCP orientation distributions during molding. Substantial discrepancies between model predictions and experimental measurements are found near the flow front in partially filled short shots, attributed to the limits of the Hele–Shaw approximation used in the computations. Much of the flow front effect is however “washed out” by subsequent shear flow as mold filling progresses, leading to improved agreement between experiment and corresponding numerical predictions. POLYM. ENG. SCI.,, 2011. © 2011 Society of Plastics Engineers     

Numerical simulation of aggregate dispersion in different flow fields using discrete element method

An attempt was made to study the aggregate dispersion process in three different flow fields namely; steady shear, elongation flow, and combined shear and elongational flows using the discrete element method. The simulation was performed on two aggregate structures characterized by their fractal dimensions. The predicted results showed that the aggregate break‐up process evaluated in terms of weighted average fragment size 〈w〉 follows a power–law type relation as 〈w〉 = kt^?m in all the three flow fields. The dispersion performance of different flow fields evaluated by dispersing rate and a final steady‐state fragment size was found to be dependent upon the extent of applied stress and flow fields such that at low applied stress levels much smaller steady state values of 〈w〉 could be obtained for the elongational flow. The aggregate structure, characterized by its fractal dimension, was found to have different effects on the aggregate dispersion process depending on the flow field and applied stress level. The results predicted from this simulation could be explained in terms of ability of flow fields in rotating the aggregates and fragments in appropriate position to be broken up and the fractal dimensions of aggregates. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermal and shear flow effects on microstructure of a thermotropic liquid crystalline polymer

The effects of annealing time and temperature on the texture and transient shear rheological behavior of a thermotropic liquid crystalline polymer were investigated. The texture evolution during annealing included two processes: the reduction of the defect density and the increase of the domain size but decrease of the domain number. We confirmed that the threaded texture caused the first shear stress peak and the shear stress minimum during shear flow startup. More importantly, using the wide‐angle X‐ray diffraction technique, we determined that the second shear stress peak during flow startup can be attributed to the evolution of the molecular orientation. POLYM. ENG. SCI., 46:1215–1222, 2006. © 2006 Society of Plastics Engineers     

Molecular orientation in injection molding

A semiquantitative model is proposed to explain the complex molecular orientation distribution, observed in injection moldings of amorphous polymers. The model incorporates flow and heat transfer mechanisms coupled with molecular theories. The orientation in the surface skin is related to steady elongational flow in the advancing front, whereas the orientation in the core is related to the shear flow, behind the front, between two solidyfying layers. Coupled with the elongational and shear-induced orientations, a molecular relaxation process takes place which is determined by the rate of heat transfer. The bead-and-spring macromolecular theory was used to calculate root mean end-to-end distances of macromolecules in the various flow fields, as well as the relaxation process.     

Effect of molecular weight of epoxidized natural rubber on shear strength of adhesives

The dependence of shear strength of epoxidized natural rubber (ENR)‐based adhesives on molecular weight of the rubber is studied using coumarone–indene resin, gum rosin, and petro resin as tackifiers. The adhesive was coated on polyethylene terephthalate (PET) film substrate using a SHEEN hand coater at various coating thickness. The shear strength of adhesives was determined by a Texture Analyzer. Results show a maximum at 6.63 × 10⁴ and 4.14 × 10⁴ for ENR 25 and ENR 50, respectively, after which the shear strength decreases with further increases in molecular weight for all the coating thickness. This observation is attributed to varying degree of cohesiveness which culminates at the respective optimum molecular weight of ENR. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of dynamic shear on the microcellular foaming of polypropylene/high‐density polyethylene blends

Dynamic shear in the axial direction of a rotor was vertically superposed on the melt flow direction, and its effects on the shear rate and melt strength were investigated theoretically. Polypropylene/high‐density polyethylene blends were microcellularly foamed with different vibration parameters. The experimental results were compared with those of a theoretical analysis, and the effects of dynamic shear on the foamability and ultimate cell structure were analyzed in detail. The theoretical results showed that the shear rate and melt strength increased with an increase in the vibration amplitude and frequency. The enhanced melt strength could effectively restrict cell growth, prevent cell rupture, and improve foamability. The experimental results showed that the cell orientation decreased and the cell structure was improved when axial dynamic shear induced by rotor vibrations was superposed on the melt flow direction. Furthermore, the cell diameter decreased and the cell density increased with increases in the vibration amplitude and frequency. The experimental results were very consistent with the theoretical analysis. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009