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 polymer properties. III

As a continuation of the investigation of the effects of shearing history on the subsequent properties of polypropylene, the changes of crystalline structure induced by differences in shearing history were examined. It was found that even extremely high shear in a capillary does not increase the crystal orientation in the extrudate. Such orientation is increased by either extremely high rates of extension of the molten extrudate or by slight plastic deformation occurring in the solidified extrudate as a result of imposed stress. The size of crystallites was found to decrease with increasing shear rate experienced by the polymer prior to the crystallization process. A possible explanation for these changes in polymer properties related to shearing history is proposed as an extension of the cluster flow theory of Busse. This explanation takes into account the size of the ball-like clusters, their internal structure, as well as the type, number, and length of the intercluster connections. The changes induced in a polymer by shearing are of technologic importance in connection with both melt flow characteristics and solid-state properties subsequently developed.     

Influence of shearing history on the rheological properties and processability of branched polymers. I

The influence of shearing history on the viscoelastic properties of low-density polyethylene (LDPE) is investigated. Swelling of extrudates from a melt indexer is measured for monitoring the variation in viscoelasticity. Continuous shearing of molten LDPE reduces the swelling ratio. The reduction is not due to molecular degradation, as evidenced by constancy of intrinsic viscosity. The rate of the reduction in the swelling ratio depends on the shearing conditions and characteristics of LDPE, but the swelling ratio finally attain a steady value. The swelling ratio reduced by the continuous shearing is completely recovered by solvent or heat treatment. The ratio of the completely reduced swelling ratio to the completely recovered one is defined as a new index representing the viscoelastic variation, the processing index (PI), and the relationship between PI and the primary molecular parameters of LDPE is investigated. It is concluded that the variation in the viscoelastic properties becomes more remarkable with increase in the weight-average molecular weight. The cause of the viscoelastic variation is also discussed from the rheological and thermodynamic points of view.     

Influence of shearing history on the rheological properties and processability of branched polymers. III. An amorphous long-chain branched polymer

Viscoelastic properties of branched polymers vary with their shearing history; notwithstanding, their primary molecular parameters do not change. According to a recent study, such viscoelastic variation was believed to be observed only with crystalline long-chain branched polymers such as low-density polyethylene or polyacetal. That is, the origin of the viscoelastic variation was attributed to the presence of specific entanglements at the branching points, which was formed during the crystallization process. However, the viscoelastic variation of the long-chain branched polymers is the phenomenon at temperatures well above their melting points, namely, it is considered that whether the long-chain branched polymers are crystalline or not is not essential for the occurrence of the viscoelastic variation. Thus, the influence of the shearing history on the rheological properties of an amorphous long-chain branched polymer was investigated in this paper, and it was found that, irrespective of their crystallinity, the viscoelastic properties of long-chain branched polymers vary according to their shearing history.     

Effect of melt history on polymer crystallization

As discussed in this paper, melt treatment prior to crystallization, apparently proceeds by the following stages: partial melting; an insensitive region; and at higher temperatures, deactivation of nucleation sites. The deactivation-region onset temperature is quite dependent on the subsequent crystallization conditions and may not be observed in some polymers during crystallization from the melt. In addition, those polymers capable of being quenched directly from the melt to a non-crystalline glassy state and subsequently crystallized by reheating to above the glass transition, do not exhibit any more than a partial melting type melt treatment effect. The deactivation regime absence is a result of the homogeneous nucleation that occurs during crystallization from the quenched glassy state at temperatures slightly above the glass transition. Use of a metal- or glass-constraining medium does mask (at least partially) the effect of melt history upon crystallization from the melt. In addition to the masking effect of a constraining medium, some of the controversy in the literature pertaining to the existence of a melt treatment phenomenon may arise due to degredation of some polymers prior to the onset of the deactivation regime. The crystallization conditions employed are also quite influential on the possible effect melt treatment can have since the melt history phenomenon is noted by its effect on subsequent crystallization.     

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.     

Influence of shearing history on the rheological properties and processability of branched polymers. II. Optical properties of low-density polyethylene blown films

An investigation was performed to determine how optical properties of LDPE blown films changed when the material was subjected to extrusion shearing. In this study, shearing histories were given to the materials by designed extrusion shearing. Recognizable variations take place in haze and gloss of the blown film during the extrusion shearing. Such variations were expressed as a function of the processing index (PI), which was introduced in a preceding paper as a measure of the memory effect of shearing histories of LDPE. This means that the variations originate in a certain change in the cohesive state of the polymer molecules attributable to the shearing.     

Dynamic melt properties of polymer blends

The dynamic mechanical properties of blends of polymer melts were measured using the orthogonal rheometer. Two-phase blends, polyethylene-polystyrene, polyethylene-poly-(methylmethacrylate), and polystyrene-polymethylmethacrylate, were studied. The in-phase and out-of-phase moduli were measured over the range of composition and at frequencies between 10^?4 and 10 revolutions/sec. The out-of-phase modulus increases in a monotonic manner with composition. The in-phase modulus, however, shows a maximum with composition in two cases. Examination of the relaxation spectra of these blends shows that when no maximum occurs it can be written as an additive function of the spectra of the components. In the case where a maximum is observed in the modulus the measured spectrum of the blend is shifted in frequency relative to the calculated one. This is tentatively attributed to slight interpretation and solubility of one phase in the other in these cases.     

Glass‐polymer melt hybrids. I: Viscoelastic properties of novel affordable organic‐inorganic polymer hybrids

A novel class of organic‐inorganic polymer hybrids was developed by melt‐blending up to 50 (v/v) % [about 83 (w/w) %] tin‐based polyphosphate glass (Pglass) and low‐density polyethylene (LDPE) in conventional plastics processing equipment. The liquid‐ and solid‐state rheology of the polymer hybrids was studied under oscillatory shear flow and deformation to understand the behavior of these materials and to accelerate efforts to melt process the Pglass with organic polymers. All the materials were found to be linearly viscoelastic in the range of temperature and frequencies examined and their viscoelastic functions increased with increasing Pglass concentration. The Pglass significantly enhanced the shear‐thinning characteristics of the Pglass‐LDPE hybrid, indicating the presence of nonlinear chemical and physical interactions between the hybrid components. Morphological examination of the materials by scanning electron microscopy revealed interesting evolution of microstructure of the Pglass phase from droplets (or round beads) to elongated and interpenetrating network structures as the glass concentration was increased in the Pglass‐LDPE hybrids. Melt viscosities of the materials were well described by a simple power‐law equation and a Maxwellian (Hookean) model with three relaxation times. Time‐temperature superpositioning (TTS) of the complex viscosity versus frequency data was excellent at 170°C < T < 220°C and the temperature dependencies of the shift factors conformed excellently well to predictions from an Arrhenius‐type relation, enabling calculation of the flow‐activation energies (25–285 kj/mol) for the materials. The beneficial function of the Pglass in the hybrid system was significantly enhanced by pre‐treating the glass with coupling agents prior to incorporating them into the Pglass‐LDPE hybrids.     

Influence of shearing history on the rheological properties and processability of branched polymers. IV. Capillary flow and die swell of low-density polyethylene

Low-density polyethylene (LDPE) melts show anomalous rheological behavior; their viscoelastic properties vary with their shearing histories although their molecular structural parameters do not change. Capillary flow and die swell behavior were dependent not only on the experimental conditions such as temperature or shear stress but also on the processing index (PI), which was introduced in a preceding article in order to quantify the anomalous rheological behavior of LDPE melts. In addition, it was found that the flow activation energy at constant shear stress also varied with the shearing histories. The experimental findings are discussed in terms of the rheological flow units of LDPE melts.     

挤出共混中的高剪切应力对PP-HDPE-滑石粉共混材料力学性能的影响

采用提高双螺杆挤出机螺杆转速的方法,研究了双螺杆挤出机的高剪切应力作用对聚丙烯(PP)、PP-滑石粉和PP-高密度聚乙烯(HDPE)-滑石粉共混材料力学性能的影响.结果表明:双螺杆挤出机的高螺杆转速、高剪切应力作用,可促进材料中滑石粉颗粒的均匀分散、促进HDPE均匀分散于PP基体的乙丙共聚物的相态之中,可引起PP或HDPE的断链反应、引起HDPE大分子自由基与PP基体中的EPR相以及与滑石粉表面偶联助剂之间的结合反应,引起共混材料缺口冲击强度的显著增大;少量极性烯类单体的加入和较低的挤出反应温度条件(180℃)有利于形成的大分子自由基与极性烯类单体的接枝、嵌段反应、界面间的偶联结合反应及其原位增粘作用,并引起PP-HDPE-滑石粉共混材料缺口冲击强度的进一步增大.     

Influence of melt deformation history on orientation in vitrified polymers

An experimental study of the development of orientation in polystyrene melts during flow and its retention in vitrified parts is described. It is shown on the basis of elongational and shear flow experiments that orientation in vitrified polystyrenes may be predicted from a knowledge of the stress field at the time of vitrification and application of stress-optical laws. More generally a relationship between birefringence and principal stress difference is found which correlates (1) on-line isothermal shear flow, (2) on-line non-isothermal elongational flow (melt spinning), and (3) vitrified samples formed in both shear and elongational flow. It is further proposed that orientation in polymer chains in deforming melts is uniquely dependent on stress—specifically, that the ratio of the stress-optical constant C to the intrinsic birefringence Δ° is approximately a constant.     

Study on morphology of ternary polymer blends. I. Effects of melt viscosity and interfacial interaction

The morphology of some ternary blends was investigated. In all of the blends polypropylene, as the major phase, was blended with two different minor phases, ethylene–propylene–diene terpolymer (EPDM) or ethylene–propylene–rubber (EPR) as the first minor phase and high‐density polyethylene (HDPE) or polystyrene (PS) as the second minor phase. All the blends were investigated in a constant composition of 70/15/15 wt %. Theoretical models predict that the dispersed phase of a multiphase polymer blend will either form an encapsulation‐type phase morphology or phases will remain separately dispersed, depending on which morphology has the lower free energy or positive spreading coefficient. Interfacial interaction between phases was found to play a significant role in determining the type of morphology of these blend systems. A core–shell‐type morphology for HDPE encapsulated by rubber was obtained for PP/rubber/PE ternary blends, whereas PP/rubber/PS blends showed a separately dispersed type of morphology. These results were found to be in good agreement with the theoretical predictions. Steady‐state torque for each component was used to study the effect of melt viscosity ratio on the morphology of the blends. It was found that the torque ratios affect only the size of the dispersed phases and have no appreciable influence on the type of morphology. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1129–1137, 2001     

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。     

熔体层厚对聚合物共挤出胀大的影响

基于流变学理论和Phan-Thien-Tanne本构方程,建立了三维等温黏弹共挤成型流动过程的数值模型。运用有限元方法对数值模型进行了模拟计算,研究了熔体层厚对共挤成型的影响,分析了熔体层厚对挤出胀大率、偏转变形及界面形貌的影响。结果表明:熔体层厚对挤出胀大率和界面形貌的影响较大,随着熔体层厚的增加,挤出胀大率减小;随着共挤出熔体层厚差值的增大,界面形状趋于不稳定;熔体离开口模后产生低黏度熔体向高黏度熔体一侧偏转变形,但变形程度受熔体层厚的影响不大。     

A fundamental study of polymer melt devolatilization. Part I: Some experiments on foam-enhanced devolatilization

A systematic, experimental study was conducted on the elementary process of devolatilization (DV) and the various parameters affecting it, using a specially designed apparatus, which is devoid of the flow complexities present in industrial equipment and which permits independent control of all pertinent parameters. The device simulates a flow configuration present in all rotating machinery used in DV, viz. the rotating melt pool. Parameters studied include exposure time, agitation rate, surface-to-volume ratio, melt viscosity, vapor pressure of volatile contaminant, applied vacuum level, concentration level of contaminant, and addition of inert substances for DV enhancement. The results of most parametric studies are presented here, but some will be published in a subsequent paper. All feed and product contaminant levels were analyzed using gas chromatography.