Controlling Necking in Extrusion Film Casting Using Polymer Nanocomposites

The research described was concerned with the effect of layered-silicate-based organically modified nanoclay fillers on controlling the extent of necking in a polymer melt extrusion film casting (EFC) process. We show that a linear polythylene resin (such as a linear low-density polyethylene—LLDPE) filled with a very low percentage of well-dispersed (or intercalated) nanoclay displays an enhanced resistance to the necking phenomenon. In general, melt-compounded nanoclay-filled LLDPE resin formulations displayed a higher final film width (less necking), thus a lower final film thickness (greater draw down for the same draw ratio), and cooled down faster when compared to the base LLDPE resin. Incorporation of nanoclay filler in the mainly linear chain LLDPE resin led to significant modification of the melt rheological properties that, in turn, affected the melt processability of these formulations. Primarily, the intercalated nanoclay-filled LLDPE formulations displayed the presence of strain-hardening in unaxial extensional rheology. Additionally, the presence of well-dispersed nanoclay in the LLDPE resin led to a display of prominent extrudate swell indicating the presence of melt elasticity in such formulations. The presence of melt elasticity, as shown by shear rheology and strain-hardening, observed by uniaxial extensional rheology, contributed to the LLDPE nanoclay formulations displaying an enhanced resistance to necking for these films. It can be concluded that linear chain polymers susceptible to necking in an EFC process can be made more resistant to such necking by using nanoclay fillers at very low levels of loading.     

A new analytical method for the optimization of thermomechanical conditions in zone drawing and its application to polyethylene

An original method of zone drawing of polymers at constant load and a procedure for the optimization of thermomechanical conditions (stress, temperature) are suggested, allowing high draw ratios and favorable strength values to be obtained. The temperature (or stress) range of necking has been determined in a nonisothermal and in an isothermal regime. The advantage of the method consists in that the increasing orientation in the neck starting from the initiation point and up to fracture allows the morphology and properties to be quickly examined, depending on the varying thermomechanical conditions in different regions of the neck. At a high temperature and low load the mechanism of oriented crystallization from melt becomes operative; in opposite cases, orientation of the solid state takes place. It is shown that in the nonisothermal regime an increased rate of heating allows extreme draw ratios (up to ca. 150) to be obtained, approximately twice as high as those obtained in the isothermal regime. This is interpreted as a gradual improvement of the oriented structure by recrystallization during extension and by a quick fixation on cooling below the zone.     

Biaxial Orientation Mechanism of Drawn Natural Rubber. II. Quantitative Analysis of the Biaxial Orientation

Natural rubber film was prepared from triple centrifuged latex. The film was uniaxially drawn at room temperature and the induced crystalline orientation was quantitatively studied by wide angle X‐ray diffraction. The intensity distribution of 200 and 120 reflections confirmed that the induced crystals have a biaxial orientation with the c‐axis parallel to the draw direction. The orientation of the a‐axis was evaluated by using an orientation function of the (200) plane. The function (F ₂₀₀) indicated that the crystalline a‐axis is parallel to the film surface depending on the draw ratio and on positions in the film. The experimental results are possible to explain by shish‐kebab‐like crystallization developed from a highly oriented fibril. The secondary crystallization grows perpendicularly to the draw direction along the a‐axis. The population of the secondary crystals is controlled by an ellipsoidal free space. The shape of the ellipsoid is changed by the fibril distribution depending on width and thickness of the sample film. In this study, the quantitative biaxial orientation is consistently explained by the shish‐kebab‐like crystallization and the ellipsoidal free space.     

A dynamic rheological model for thin-film lubrication

In this study, the effects of the non-Newtonian rheological properties of the lubricant in a thin-film lubrication regime between smooth surfaces were investigated. The thin-film lubrication regime typically appears in Stribeck curves with a clearly observable minimum coefficient of friction (COF) and a low-COF region, which is desired for its lower energy dissipation. A dynamic rheology of the lubricant from the hydrodynamic lubrication regime to the thin-film lubrication regime was proposed based on the convected Maxwell constitutive equation. This rheology model includes the increased relaxation time and the yield stress of the confined lubricant thin film, as well as their dependences on the lubricant film thickness. The Deborah number (De number) was adopted to describe the liquid-solid transition of the confined lubricant thin film under shearing. Then a series of Stribeck curves were calculated based on Tichy's extended lubrication equations with a perturbation of the De number. The results show that the minimum COF points in the Stribeck curve correspond to a critical De number of 1.0, indicating a liquid-to-solid transition of the confined lubricant film. Furthermore, the two proposed parameters in the dynamic rheological model, namely negative slipping length b (indicating the lubricant interfacial effect) and the characteristic relaxation time λ 0 , were found to determine the minimum COF and the width of the low-COF region, both of which were required to optimize the shape of the Stribeck curve. The developed dynamic rheological model interprets the correlation between the rheological and interfacial properties of lubricant and its lubrication behavior in the thin-film regime.     

Chain orientation in natural rubber,Part I: The inverse yielding effect

Inhomogeneous deformations are observed in stretched natural rubber of different crosslink density; the conditions of observation, nucleation and propagation are given in the first part of the paper. In samples of low crosslink density these inhomogeneities recall necking observed in others materials and in glassy polymers when the materials are drawn above a critical draw ratio. The difference is that in natural rubbers, NR, they nucleate and propagate at constant stress during unloading. This phenomenon, called inverse yielding appears during recovery only if the samples have been drawn previously in the hardening domain. During necking propagation the stress is constant. The mechanical and crystallinity properties of samples with and without inverse yielding are studied as a function of draw ratio, crosslink density and temperature. In the second part of the paper this transition zone (neck) of thickness 2 mm is studied by WAXS at the synchrotron source. From the orientation of NR crystallites and from the orientation of the stearic acid (2%, present in this type of rubber) we conclude that the deformation in the neck follows the flow lines. From the local crystallinity of the NR crystallites one deduces the local draw ratio across this transition zone. We suggest that in all these rubbers, which present a plateau of the recovery stress strain curve, micronecking exists. This effect is discussed in the framework of the Flory theory.-1     

Biaxial Orientation Mechanism of Drawn Natural Rubber. I. Appearance of the Biaxial Orientation

Triple centrifuged natural rubber (NR) film was crystallized at room temperature by uniaxial drawing at draw ratio (λ) 6.0. Morphologies of the induced crystals were studied by wide angle X‐ray diffraction (WAXD). Using a special drawing apparatus for shifting the X‐ray beam positions, crystalline orientation was surveyed all over the drawn sample. WAXD results show the biaxial orientation (BO) with the c‐axis being parallel to the draw direction and the a‐axis parallel to the film surface. By means of the WAXD pattern with the incident beam perpendicular to the film, the drawn NR film was separated into three regions: (200+120) region indicating a low degree of BO, (200) region having a high degree of BO, and the intermediate region in between the above two regions. The degree of BO is enhanced from center to the clamping end depending on the increment of the sample width. At lower draw ratios (4.0<λ<6.0), BO was developed with increasing draw ratio. In the intermediate region, BO increases sharply with a small increment of the width. The degree of BO is lowered at the periphery of the film.     

A novel video opto-mechanical (VOM) device for studying the effect of stretching speed on the optical and structural properties of fibers

Using a novel video opto-mechanical (VOM) device, the effect of stretching speed on the optical and structural properties of polypropylene (PP) fibers during the dynamic stretching process is studied. The objective of the present study is to correlate the optical and mechanical properties of PP fibers with the speed of stretching using the multiple-beam Fizeau fringes system. The skeletonization of the multiple-beam Fizeau fringes are determined automatically by using one-dimension Fourier transform method. The stepper motor is adjusted in order to stretch the fibers continuously with constant and uniform speed until breaking of the fiber. The refractive indices, birefringence, transverse sectional area and the orientation function of PP fibers are studied as a function of the draw ratio at different speeds of stretching. An empirical formula is given to correlate the birefringence of PP fiber and both the draw ratio and the speed of stretching. Stretching process of PP with low speed (slow stretching) is recommended to overcome the necking deformation along the fiber. The VOM device could be used to measure the yield strain. The draw ratio-stretching speed superposition is discussed with some details. Microinterferograms are given for illustration.     

Texture and self-biased property of an oriented M-type barium ferrite thick film by tape casting

In this paper,the oriented M-type barium ferrite(BaM) thick films with different thicknesses are prepared by tape casting.It is found that the crystallographic alignment degree(f),the pore and the squareness ratio(Mr/Ms) are not affected by the thickness of the film.XRD and SEM results show that the thick film has hexagonal morphology with a crystal texture of c-axis grains perpendicular to film plane.The hysteresis curve indicates that the BaM thick film exhibits a self-biased property with a remanent magnetization of 3.30 T,a squareness ratio(Mr/Ms) of 0.81,and a coercivity of 0.40 T.The results show that the BaM thick film has potential for use in self-biasing microwave devices,and also proves that the tape casting technique is capable of fabricating high-quality barium ferrite films,thus providing a unique opportunity to realize the large area production of thick film.     

电场对协流式微流控装置中乳液液滴生成行为的调控机理

建立了直流电场作用下协流式微流控装置中单乳液液滴乳化生成过程的非稳态理论模型,并开展了数值模拟研究,揭示了电场对液滴乳化生成动力学行为的调控机理,阐明了流场/电场参数对液滴乳化生成特性的影响规律.研究结果表明:沿流体流动方向施加静电场可在电物性参数不同的两相流体界面法线方向上产生指向内相流体的电场力,进而强化了内相流体界面的颈缩和断裂,提升了液滴生成速率和形变程度,减小了液滴生成尺寸;在同一毛细数下,随着电毛细数的增大,乳液乳化流型由每周期仅有单一液滴生成的滴式流型转变为每周期有一个主液滴并伴随有卫星液滴生成的滴式流型;随着毛细数和电毛细数的增大,黏性拖曳力以及电场力作用增强,使内相流体颈缩过程后期更容易形成细长型液线,从而有助于诱发液线上产生Rayleigh-Plateau不稳定现象,继而促进卫星液滴的形成.     

Detection of necking deformation along polypropylene fibres axis at low draw ratios using multiple-beam microinterferometry

A modified drawing device attached with an automated system for producing multiple-beam Fizeau fringes in transmission is used to study the mechanical deformation along polypropylene (PP) fibres at low draw ratios. Two drawing processes are used for drawing PP fibres. In the first process (fast drawing), the necking deformations are formed at draw ratios from 1.2 to 1.9 along the fibre axis. While in the other process (slow/step drawing), these deformations disappeared. The refractive indices calculated at different positions along the fibre axis during the fast- and slow-drawing processes. To overcome the formation of necking deformation along the PP fibres during fast-drawing process, the slow-drawing technique is recommended. Microinterferograms are given for illustration.     

Molecular orientation evolution during low‐density polyethylene blown film extrusion using real‐time Raman spectroscopy

Real‐time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low‐density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C C symmetrical stretching vibration of polyethylene (PE) at 1132 cm⁻¹ was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c‐axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second (〈P₂(cosθ)〉) and fourth (〈P₄(cosθ)〉) moments of the orientation distribution function. The orientation parameters (P₂, P₄) were found to increase along the axial distance in the film line even past the frost‐line height (FLH). The P₂ values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I₁₁₃₂/I₁₀₆₄) obtained from a single, ZZ‐back‐scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real‐time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion. Copyright © 2008 John Wiley & Sons, Ltd.     

Crystallization Behavior of Modified Polyester with Varied Macromolecular Architecture

Several modified polyesters with varied macromolecular architecture, such as branched poly(ethylene terephthalate) (PET) based on glycerol (GL) from 0.004 to 0.05 mol ratio as a branching agent, blocked and branched poly(butylene terephthalate)‐polyether containing poly(tetramethylene oxide) (PTMO) as soft segment and GL as a branching unit, as well as segmented poly(ethylene terephthalate)‐polyether,were prepared. Their crystallization behavior was studied by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized optical microscopy (POM). It was found that a small extent of branching may enhance the crystallization of poly(ethylene terephthalate), while high degrees of branching (0.035–0.05) could block the development of crystallization. On the other hand, for even a small extent of incorporation of GL in the more flexible poly(butylene terephthalate)‐polyether chains, no enhanced crystallization was observed; blocking of crystallization from a branching defect may play the main role. The introduction of PTMO in poly(ethylene terephthalate) chains to a small degree facilitated the nucleation and speeded crystallization, but decreased the melting points of the polymers. A small number of nuclei and the greater induction time were found for branched PETs. The spherulities developed in branched PETs were larger and more perfect than those in PET due to less truncation of spherulites resulting from fewer nuclei, whereas the size of spherulities in poly(ethylene terephthalate)‐polyether became smaller with the increase of PTMO.     

新型酞菁的光致发光性质研究

研究了2,3-四-(2-异丙基一5-甲基苯氧基)氢酞菁在10,77,177和300 K下石英衬底上的浇铸膜和单晶硅衬底上真空镀膜(约200 nm厚)在300 K下光致发光光谱.氢酞菁的浇铸膜光致发光光谱在上述温度下均出现荧光发射和磷光发射峰,在177和300 K下出现了1 673 nm激基缔合物峰.该峰的出现与分子抗聚集能力的强弱有关,在300 K激基缔合物峰比在177 K下的峰强,从氢酞菁分子结构特点讨论了形成激基缔合物的原因.随着温度的升高,可以观察到荧光发射峰渐渐减弱而激基缔合物峰变强.由于浇铸膜和真空镀膜的酞菁分子聚集态不同导致了斯托克司位移的差异,真空镀膜的发光峰峰值在1 140 nm左右,与酞菁浇铸膜的峰值差别较大.浇铸膜的发光峰的半高宽为300 nm,而真空镀膜发光峰的半高宽为100 nm左右.     

A numerical study of rotation effects on jets effusing from inclined holes into a cross-flow

In this work, the complexity of the flow field arising from the impact of the interaction of coolant jets with a hot cross-flow under rotation conditions was numerically simulated using large eddy simulation with artificial inflow boundary condition. The finite-volume method and the unsteady PISO (Pressure Implicit with Splitting of Operators) algorithm were applied on a non-uniform staggered grid. The simulations were performed for four different values of rotation number (Ro) of 0.0, 0.03021, 0.06042, and 0.12084, a jet Reynolds number of 4700, based on the hole width and the jet exit velocity. The air jet was injected at 30° and 90° in the streamwise direction with a density ratio of 1.04 and a velocity ratio of 0.5. The flow fields of the present study were compared with experimental data in order to validate the reliability of the LES technique. It was shown that the rotation has a strong impact on the jet trajectory behaviour and the film cooling effectiveness. The film trajectory always inclines centrifugally. Under rotating conditions, the film trajectory departs from the centreline to the left boundary. The deflection becomes greater as Ro increases. Furthermore, it was also found that the injection angle has a strong impact on separation and reattachment behaviour as well as the strength of the penetration into the cross-flow. As it increases, the distribution of the film cooling downstream the jet exit is more non-uniform and the film cooling effectiveness level slightly decreases.     

Contact ratio of rough surfaces with multiple asperities in mixed lubrication at high pressures

Relative optical intensity interference was used to measure the lubrication film thickness when four kinds of polyalphaolefin (PAO) were used as lubricants confined between a smooth sapphire disc surface and a rough steel ball surface. Maximum Hertz contact pressure up to 3 GPa was applied in the central part of the contact region in mixed lubrication. It was found that the contact ratio (the ratio of real contact region to the whole nominal contact region) is related to the film thickness, the applied pressure, the surface roughness and the rolling speed, and so on. Contact ratio evidently reduces as lubrication film thickness or rolling speed increases. Quantitative relationship between the contact ratio and the influence factors was summarized based on the nonlinear fitting of experimental measurements. A formula was put forward to calculate the contact ratio at high pressure conditions according to the current experimental results.     

Superlattices of lamellae in microporous oriented polyolefine films

The structure and mechanism of the formation of superlattices lamellae in microporous polyolefine (polyethylene and polypropylene) films obtained by polymer melt extrusion followed by annealing, uniaxial extension, and thermal fixation stages have been studied by scanning electron and atomic-force microscopy. It has been shown that oriented anisometric particles, i.e., lamellae aggregates, are formed in films as the spin draw ratio λ _f increases. At the stage of uniaxial extension (pore formation) of annealed films, a particle ensemble transforms to spatial superlattices of lamellae. Numerical processing of electron microscopy images of the film surface show that the nonmonotonic dependences of the correlation length of density fluctuations and the ratios of the alternation period of particles along extension to their thickness on the parameter λ _f correspond to a unified mechanism of lamellae ordering.     

Monte Carlo study of interfacial silicon suboxide layers and oxidation kinetics

A simple simulation scheme that simultaneously describes the growth kinetics of SiO₂ films at the nanometer scale and the SiO_x/Si interface dynamics (its extent, and spatial/temporal evolution) is presented. The simulation successfully applies to experimental data in the region above and below 10 nm, reproduces the Deal and Grove linear-parabolic law and the oxide growth rate enhancement in the very thin film regime (the so-called anomalous region). According to the simulation, the oxidation is governed mainly by two processes: (a) the formation of a transition suboxide layer and (b) its subsequent drift towards the silicon bulk. We found that it is the superposition of these two processes that produces the crossover from the anomalous oxidation region behavior to the linear-parabolic law.     

Investigation on Tensile Deformation Behavior of Semi-Crystalline Polymers

The strain rate, temperature, and microstructure-dependent, tensile-yielding behavior of three semi-crystalline polymers, namely high-density polyethylene (HDPE), polyamide 6 (PA6) and low-density polyethylene (LDPE), was investigated. It is found that, depending on the strain rate and temperature, the three polymers exhibit markedly different tensile deformation behavior, especially the shape of the stress-strain curves. LDPE exhibits a uniform extension and shows no obvious geometrically unstable effect, such as necking, during the overall tensile process. HDPE and PA6, on the other hand, show clear necking and cold-drawing phenomena during the uniaxial tensile process. When considering the effect of strain temperature on necking, significant differences between HDPE and PA6 emerge. For both, the heterogeneous necking disappears and homogeneous deformation occurs with increasing temperature. For HDPE, the homogeneous deformation takes place in the vicinity of the melting temperature, while for PA6, it takes place close to the glass transition temperature instead. The conventional yield point, corresponding to the force maximum in stress-strain curves, becomes less defined as the testing temperature is increased. It is applicable, to some extent, to combine the Brereton analysis and Considère construction to predict such a point quantitatively. However, this combination can only be suitable for homogeneously deformed material. In addition, it is found that the special, double yielding behavior will take place under certain deformation conditions for all three semi-crystalline polymers. With respect to judging the appearance of the double yielding of polymers, it seems that it can be estimated qualitatively by plotting the compression residual strain-applied strain curves of the samples.     

On line interferometric investigation of the neck propagation phenomena of stretched polypropylene fibre

On line interferometric investigation of the neck propagation phenomenon of stretched fibre is carried out using an automatic multiple-beam Fizeau fringes technique. It was observed that under such a deformation condition, a neck deformation is formed and propagated steadily towards the gripped ends of the stretched sample. The neck propagation was recorded carefully during the course of the tests by means of a CCD camera. The fringe shift profile due to the neck formation was analyzed during the propagation stage. A polypropylene (PP) sample was stretched until a neck deformation is formed at room temperature. The 3D time-refractive index profile is investigated to clarify the fixed profile of propagation. The obtained microinterferograms are clarifying the fixed neck profile during the propagation of necking phenomena. The speed of neck propagation was calculated. The dependence of the propagation on the drawing speed and draw ratio was investigated.     

Flexible subwavelength gratings fabricated by reversal soft UV nanoimprint

We present a replication process, named reversal soft ultraviolet (UV) nanoimprint, to fabricate a high- aspect-ratio flexible subwavelength grating (SWG) on a polyurethane acrylate (PUA). This nanopatterning technique consists of casting, reversal UV imprint, and dry release. The UV curing process of PUA to avoid pattern collapse is investigated. Revalpha film acts as the supporting and sacrificial layer during the whole process due to its special surface energy property. The free-standing PUA structures with a period of 200 nm and a depth of 350 nm can be automatically released from the Revalpha film by heating. The PUA resist is well suited to replicate fine patterns of the mold with high aspect ratio and large area precisely and uniformly for low surface energy and low viscosity. The measured transmittance is compared with the calculation results based on rigorous coupled-wave analysis in the wavelength region ranging from 500 to 800 nm. The experimental results agree well with the theoretical calculations.