Influence of shearing history on the properties of polymer melt. II

The flow pattern of molten polymer through capillaries was studied by using a tracing method. An incompatible polymer was added to polypropylene as a tracer, and the influence of shear in a capillary rheometer on the tracer particle size as well as on the distribution of the particles in the extrudate in relation to flow was studied. It was found that the particle size varied inversely with shear rate of extrusion, capillary aspect ratio, polymer viscosity, and extrusion temperature. The flow was found to be of the telescopic type, and the tracer particle size was independent of position along the radius of the cylindrical extrudate. It is assumed that the supermolecular structure of a polymer melt is of the cluster type suggested by Busse, and it is postulated that spherical clusters of molecules in the melt are modified by shear analogously to the spherical particles of the incompatible tracer polymer. The large spectrum of melt properties obtainable, by various shearing treatments, from a polymer of constant molecular structure suggests that a wide range of supermolecular structures must be possible in the polymer melt.     

Influence of shearing history on the properties of polymer melt. I.

Melt flow behavior was examined using “as polymerized,” mechanically intact polypropylene, and the same polymer after pelletization in a screw extruder. In addition, the polymers were extruded through different capillaries and subsequently tested for melt flow characteristics. Significant differences in shear stress at a given shear rate and shear rate critical to the onset of extrudate roughness were found and compared with the previous shearing history. Both shearing intensity and mode of flow were found to impart long-lasting and short-term effects to the melt flow properties of the polymer. These differences were not explainable on the basis of changed molecular weights, since no significant differences were observed in the molecular weights as a result of the shearing treatments used in this study.     

Effect of shear modification on the rheological behavior of two low-density polyethylene (LDPE) grades

Shear modification treatment represents a special shearing history affecting mainly the elastic behavior of polymer melts. This process has been attributed to reversible physical changes in the entanglement structure of the polymer chain network. Shear modification studies were performed for two well-characterized low-density polyethylene (LDPE) grades differing in molecular weight distribution and degree of long chain branching. The shear working of the material was carried out using a specially designed shearing unit producing definable amounts of pure shear in a continuous process. Measurements of the dynamic storage modulus, G′, steady-state shear compliance; J_e, extrudate swell, melt flow index, and the extensional behavior (Rheotens test) indicate that primarily properties associated with the elasticity of the melt are reduced in value. The observed reduction is found to correlate with the mean specific energy dissipated during sample preparation. Comparing the two LDPE grades showed that higher degrees of modification can be obtained at lower energy input levels for the more highly branched grade. Reversibility tests were performed and complete recovery of the initial material behavior was observed. Comparison of measurement results for samples prepared using the shearing unit presented here and a Brabender Plasticorder indicates that the degree of modification depends not only on the molecular structure of the polymer but also on the manner in which the shearing history is imposed upon the material.     

Effects of polymer molecular weight and filler particle size on flow behavior of wood polymer composites

The influence of polymer matrix molecular weight and filler particle size on rheological properties and extrudate distortions of metallocene polyethylene (mPE)/wood flour (WF) composites has been investigated by rotational and capillary rheometers. It was found that at low shear rates smaller filler particles provide higher shear viscosity than the larger sized filler. At high shear rates and WF loadings above 30 wt%, the effect of particle size on the melt flow properties becomes negligible. The relative increase of the storage modulus with decreasing particle size is more pronounced in the case of low molecular weight polymer matrix than that in higher molecular weight polyethylene based composites. The wood filled polyethylenes exhibit extrudate surface defects, which are complex function of the shear rate, polymer matrix molecular weight, and filler particle size. Increasing the shear rate results in pressure oscillations and spurt‐flow. It was also observed that the evolution of the extrudate surface tearing is strongly dependent on the pressure during a single pressure oscillation cycle in the spurt flow regime. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

助剂挤出法制备高强度聚四氟乙烯单丝

将聚四氟乙烯(PTFE)与润滑助剂航空煤油混合后均匀挤出,挤出样条脱除助剂,在低于PTFE熔点的温度下高倍拉伸,后经高温紧张热定型,制备高取向PTFE单纤维;通过拉曼光谱、X射线衍射、差示扫描量热分析等方法研究挤出PTFE拉伸过程中材料微细结构的变化,并对PTFE纤维的性能进行表征.结果表明:PTFE在挤出过程中并不会...     

Effects of plasticizer on shear modification of polystyrene

The elastic and viscous properties of polymer melts may be affected by the shear history of the polymer. The extrudate swell of a polymer melt is primarily a manifestation of the elasticity of the polymer melt. In this study, a single screw extruder was used to impose different shear histories on a polystyrene polymer which was processed with and without added plasticizer. The extrudate swell and apparent viscosity of these melts were measured with a capillary rheometer. These characteristics of unplasticized polystyrene are almost not affected by the various preshearing processes. However, the extrudate swell and viscosity of polystyrene containing plasticizer are affected by plasticizer level, shear history and thermal history. After most of the plasticizer in the presheared plasticized polystyrene was extracted, the extrudate swell was still lower than that of the parent sheared polystyrene with the same shear history and the same plasticizer content. These results were obtained without significant changes in molecular weight. Shear modification by conventional process equipment may become impractical if the shear field intensity or dwell time of the material in the apparatus is limited. In such cases, shear refinability by standard process equipment may be observed if the coupling density in the polymer is reduced by some additional means, such as blending with a plasticizer.     

Concept of secondary heterogeneous structure of long-chain branched polyethylene

The mechanism of formation of surface roughness and extrusion swelling of the extrudate and the steady-shear viscous flow behavior in the region of high shear rate for branched polymers were investigated using two low-density polyethylenes and their sheared samples. These two polyethylenes varied in their degree of branching, molecular weight, and molecular weight distribution but were similar in their melt flow index. The effect of molecular parameters, especially long-chain branching, on viscoelastic properties in the molten state was also considered. Samples of various degree of shearing level were prepared by passing them repeatedly through an extruder. Results of intrinsic viscometry, gel permeation chromatography, and infrared spectroscopy of the original and the sheared samples indicate that no appreciable variation between them takes place in the molecular parameters during the process of extrusion shearing. Both surface roughness and extrusion swelling of the extrudate diminish with increase in the extent of shear. The extrusion shearing affects the surface roughness and extrusion swelling of the extrudate as well as the capillary entrance effect more markedly for the highly branched polymers with considerably higher molecular weight than for the less branched species with bell-type molecular weight distribution. These results demonstrate that heterogeneity becomes more conspicuous with the degree of long-chain branching level, and therefore the role of long-chain branching in the development of the heterogeneity is particularly important. It is suggested that the secondary heterogeneous structure arises through phase separation or from the heterogeneous formation of strongly entangled network at the branching point of the long-chain branching in the manufacturing process of the low-density polyethylene and that its presence causes the distinctive viscoelastic properties of long-chain branched polymer melts.     

Shear‐Induced Skin‐Core Structure of Molten Isotactic Polypropylene and the Formation of β‐Crystal

The study of crystallization behavior and crystalline morphology of polymer melt under shear flow is of great interest due to the strong effect of flow field on the final properties of polymer products in the practical processing. In this respect, the shearing hot stage provides a unique tool which monitors sensitively the changes in crystalline structure induced by precise experimental conditions. Herein, the impacts of both melting temperature and shear rate on the crystallization behavior of isotactic polypropylene (iPP) melt are investigated. Under static conditions, there are only random spherulite structures. Once shear is involved, the cylindrite‐layers appear near both surfaces of the sample, which is consistent with the skin‐core structure in the injection molded parts. Meanwhile, the β‐crystals can be developed and are related to the molecular orientation, depending on the applied melting temperatures and shear rates. More interestingly, the crystallinity of β‐crystal in the pure iPP can reach 15%. The above results indicate that the melting temperature and shear rate are important factors in determining the β‐form crystal development of iPP matrix.     

Study of the relationship between shear modification and melt fracture in extrusion of LDPE

Successive passes through an extruder can modify the melt morphology of low-density polyethylene (LDPE) by applying a shearing process. The major effects of shear modification are to decrease the elastic properties, as evidenced by the decrease in extrudate swell at the exit of a capillary and in melt viscosity at a low-frequency region. The effect of shear modification is also shown by the delay in the onset of melt fracture upon extrusion. The critical shear stress resulted from extrudate appearance, and apparent discontinuity in the flow curve of LDPE shows a similar value with polypropylene rather than those of LLDPE and HDPE. The shearing histories experienced by these materials did not result in any measurable change in molecular weight, so that the chemical modification process such as degradation and crosslinking may be ruled out. These behaviors were also confirmed to the fact that the extrudate swell was fully reversible by annealing in a molten state. The effects of shear modification on rheological properties could be explained by the changes in melt morphology owing to the disentanglement of temporary couplings between long branches. Also, a reduction in melt elasticity by shear modification of LDPE can be used as an effective tool to improve the surface roughness of extrudates in the cable-making process. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2187–2195, 1998     

Effect of complex flow kinematics on the molecular orientation distribution in injection molding of liquid crystalline copolyesters

Wide-angle X-ray scattering (WAXS) is used to probe the molecular orientation in steady isothermal complex channel flows (in situ) and in injection molded plaques (ex situ) of a new, low-cost aromatic copolyester based on the mesogen 4,4′-dihydroxy-α-methylstilbene (DHαMS). Complex orientation states arise from the competition of inhomogeneous mixed shear and extension in isothermal flows. Slit-contraction flows lead to a significant but temporary increase in the average degree of molecular orientation, suggesting that this polymer is of the ‘shear-tumbling’ type. Conversely, bimodal orientation states are observed in slit-expansion flows, where transverse extension leads to a strong reduction in the average degree of molecular orientation along the flow direction. Similar bimodal orientation states are observed in injection molded plaques, suggesting that these kinematic concepts translate rather directly to the more complex transient non-isothermal case of injection molding. Variations in orientation state induced by changes in plaque thickness may be rationalized by systematic changes in the relative importance of shear and extension. These results suggest a complementary perspective on ‘skin-core’ morphologies in liquid crystalline polymer moldings, and provide a clear conceptual link between more fundamental studies in isothermal flows and structure development during processing.     

支化结构对树枝状聚合物溶液剪切前后流变性能的影响

采用吴茵混调器模拟聚合物在其配制、输送、注入中的机械剪切作用,研究了不同支化程度对剪切前后的驱油用树枝状聚合物溶液的流变性能的影响。首先制备了3种不同支化程度的树枝状聚合物,研究了剪切前后的树枝状聚合物的分子链粒径分布和分子量大小,然后研究了不同因素对树枝状聚合物流变性能的影响并考察了树枝状聚合物溶液的黏弹性能,最后结合环境扫描电镜分析支化结构对剪切前后树枝状聚合物溶液的流变性能的影响。结果表明：支化程度高的树枝状聚合物具有更大的流体力学半径和分子量,受环境影响较小;树枝状聚合物溶液呈现假塑性流体特征,支化程度越高,剪切前后的聚合物溶液幂律指数n越小、稠度系数K越大;支化程度高的树枝状聚合物溶液支链间越容易发生缠结,形成致密、多层的空间网状结构,致使剪切前后的聚合物溶液的流变性能越好。     

Experimental studies on extrudate swell behavior of PS and LLDPE melts in single and dual capillary dies

Extrudate swell behavior of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts was investigated using a constant shear rate capillary rheometer. Two capillary dies with different design configurations were used, one being a single flow channel and the other being a dual flow channel. A number of extrudate swell related parameters were examined, and used to explain the discrepancies in the extrudate swell results obtained from the single and dual flow channel dies, the parameters including output rate and output rate ratio, power law index, wall shear rate, wall shear stress, melt residence time, pressure drop induced temperature rise, flow channel position relative to the barrel centerline, and the flow patterns. It was found in this work that the power law index (n value) was the main parameter to determine the output rate ratio and the extrudate swell between the large and small holes for the dual flow channel die: the greater the n value the lower the output rate ratio and thus decreased extrudate swell ratio. The differences in the extrudate swell ratio and flow properties for PS and LLDPE melts resulted from the output rate ratio and the molecular chain structure, respectively. The extrudate swell was observed to increase with wall shear rate. The discrepancies in the extrudate swell results from single and dual dies for a given shear rate were caused by differences in the flow patterns in the barrel and die, and the change in the melt velocities flowing from the barrel and in the die to the die exit. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1713–1722, 2003     

A simulation study on the effects of shear flow on the microstructure and electrical properties of carbon nanotube/polymer composites

We employ a fiber-level simulation technique to simulate carbon nanotube (CNT)/polymer composites in simple shear flow. This model incorporates CNT flexibility, irregular CNT equilibrium shapes and CNT interactions. Electrical conductivity of the composites is determined using a resistor network algorithm. Tunneling resistance of the insulating matrix film between nanotubes is also considered. We show that the rate of imposed shear flow influences the composite conductivity by facilitating the formation or destruction of the conductive aggregates. In addition, the conductivity evolution during shearing for different concentrations is investigated. At low concentration, percolating clusters form and break simultaneously which causes large conductivity fluctuations during the simulations. When sufficiently large concentrations are reached, percolating clusters persist during shearing and the conductivity fluctuations decrease. In agreement with previous research we determine that increasing the shear rate causes alignment of the nanotubes in the flow direction. We show that upon shearing at constant shear rate, the system attains a state with substantially constant electrical conductivity, nanotube orientation and agglomerate size that is a function of the applied shear rate. The state reached for a given shear rate is independent of the initial state of orientation and aggregation.     

A novel microstream injection molding method for thermotropic liquid crystalline polymers to promote mechanical isotropy: A matrixing microbeam X-ray study

The effects of flow-altering inserts and mold cavity geometry on the mechanical properties of an injection molded liquid crystalline polymer were studied to produce parts with properties approaching macroscopically isotropic state. By inserting fine metal mesh barriers to the gates of the mold cavities, a large number of highly oriented microstreams are produced. After their creation these highly oriented streams of differing flow vectors intertwine and this texture remains reasonably intact even after substantial shearing and extension history imparted on them during ensuing flow into the cavity. This method is effective in the interior away from the skin regions formed under the shearing flow during injection. The local molecular orientation was determined using a matrixing microbeam WAXS technique that allows precision movement of the sample in the microbeam X-ray. Samples produced with the 1.0 mm² mesh showed large variations in the local symmetry axis with respect to the machine as measured by microbeam X-ray diffraction incrementally from the edge to the core of the parts. In comparison, samples with no mesh insert showed only gradual changes in the tilt angle (angle between local symmetry axis and flow direction). The modulus and tensile strengths of all samples with the 1.0 mm² mesh inserts were found to approach virtual global mechanical isotropy.     

Effect of die temperature on the flow of polymer melts. Part II: Extrudate swell

The effect of die wall temperature on the extrudate swell of polymer melts flowing through dies with single and dual circular channels was studied. Extrudate swell was measured at constant flow rates using an Instron capillary rheometer with a modified die section. It was found that under isothermal conditions, extrudate swell plotted against the average wall shear stress gave rise to a temperature independent correlation for polystyrene. Under non-isothermal conditions, such a correlation did not exist, which might be due to the change of wall shear stress in the axial direction. The extrudate swell in the non-isothermal cases can be better correlated with the wall shear stress at die exit. For the two-hole die, changes of die wall temperature varied both the flow rate ratio and the extru date swell ratio. The latter is, however, much less sensitive to the die wall temperature than the former.     

Shear effects on crystallization behavior of poly(ethylene-co-octene) copolymers

Crystallization behavior of sheared polymer melts of a series of poly(ethylene-co-octene)s with different octene content was investigated by different scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and synchrotron small-angle X-ray scattering (SR-SAXS) techniques. The DSC results indicated that the hexyl branches content had dramatic effects on the thermal properties and crystallinity of polyethylene. It was also found that shear had no obvious effects on the size of either crystallite or lamellae. However, both crystallite and lamellae were oriented by shearing, especially for the lamellae. All obtained results indicate that the initial states of the polymer melt play an important role in affecting the crystallization behaviors. The difference of the shear-induced crystalline structure evolution and the orientation between crystallite and lamellae support the preordered mesomorphic phase of flexible polymer crystallization process proposed by Strobl.     

The source of degradation during extrusion of polystyrene

The nature and magnitude of mechanical reactions of polystyrene in capillary flow has been examined in a model extrusion process. Studies on polystyrene quantify the sensitive increase in shear degradation tendency with increasing polymer molecular weight. A molecular weight spectrum caused by the shear stress profile was measured across the extrudate radius by the new technique of solvent coring. It was further determined that an appreciable fraction of the mechanical reaction is shear induced in the capillary reservoir. This is confirmed by precision determinations of molecular weights and distributions by gel permeation chromatography on samples taken from concentric layers in the capillary reservoir after 50% sample extrusion. These results, involving traces of oxygen as a chemical probe, describe the stress profile in the reservoir and in the capillary during the pressure extrusion of high molecular weight polystyrene. Thus, changes in molecular weight and distribution may be attributable to changes in different portions of the shear geometry rather than the uniform changes generally considered. Clear evidence is also presented showing the dramatic effects of oxygen on these shear-induced changes in molecular weight and distribution.     

剪切对PP/HDPE共混体系微孔发泡成型的影响

在熔体流动方向垂直叠加一个轴向脉动剪切,研究转子转动产生的剪切和轴向脉动剪切对PP/HDPE共混体系微孔发泡成型的影响。研究结果表明,随着转子转速增加,泡孔尺寸减小,气泡成核密度增大。但是转子转速过快,泡孔沿剪切的方向被拉长,泡孔取向严重,泡体质量变差。在熔体流动方向垂直叠加一个轴向脉动剪切以后,泡孔的取向现象减小,泡孔逐渐趋于圆形,泡孔结构得到改善。而且随着振动振幅和频率的增加,泡孔直径减小,泡孔密度增加。     

Combined effects of nanotube aspect ratio and shear rate on the carbon nanotube/polymer composites

We use fiber-level simulations to investigate the combined effects of carbon nanotube (CNT) aspect ratio and shear rate on the microstructure and electrical properties of CNT/polymer composites. In our previous studies, we studied the effects of aspect ratio at a constant shear rate as well as the effects of shear rate for a constant aspect ratio. In this study electrical properties and microstructure changes (e.g. agglomeration/deagglomeration, network strength, nanotube orientation) of CNT/polymer composites are investigated for varying aspect ratios at different shear rates. When shear rate is increased, we observe a decrease in the electrical conductivity and an increase in the anisotropy factor due to the deagglomeration and flow induced orientation. Increasing aspect ratio shifts the conductivity vs. shear rate curve to larger values and anisotropy vs. shear rate curve to lower values showing that there is a tendency for tube agglomeration when high aspect ratio nanotubes are used. On the other hand when low aspect ratio nanotubes are used, conductive networks can be more easily destroyed by the shear forces because networks formed by low aspect ratio nanotubes have lower strength than those formed by high aspect ratio nanotubes. Our results show that electrical conductivity is anisotropic with a larger component in the flow direction. The critical shear rate defined as the shear rate where the conductive network is destroyed and all components of the composite conductivity decrease to the matrix conductivity, shifts to higher values when the aspect ratio is increased. Reduced alignment and increased entanglement are the reasons of this decrease.     

Particle size and molecular weight effects on the melt flow of emulsion PVC

The melt flow behavior of straight emulsion-polymerized PVC in a capillary extrusion rheometer has been found to depend upon both the molecular weight and the particle size of the sample. Observations of flow-rate, post-extrusion swell, and extrudate appearance, as functions of extrusion temperature and pressure, suggest that both molecular deformation and particle slippage are involved in the flow mechanism. The relative importance of these two modes of flow varies with extrusion conditions and with the PVC molecular weight and particle size. Particle slippage is favored by large particle size, high molecular weight, and low temperature and by a shear stress above a critical yield value. Apparent melt viscosity, swelling, and roughness are minimized under conditions corresponding to the maximum contribution of particle slippage. In the proper range of temperature and shear rate, straight emulsion PVC yields smooth, low-swell extrudates of excellent physical properties.