Epitaxial crystallization and oriented structure of linear low‐density polyethylene/isotactic polypropylene blends obtained via dynamic packing injection molding

In this work, as a part of a long‐term project aimed at controlling of crystal structure and phase morphology for a injection molded product, we investigated the oriented structure and possible epitaxial growth of polyolefin blend (low‐density polyethylene (LLDPE)/isotatic polypropylene (iPP)), achieved by dynamic packing injection molding, which introduced strong oscillatory shear on the gradually‐cooled melt during the packing process. The crystalline and oriented structures of the prepared blends with different compositions were estimated in detail through 2D X‐ray diffraction, calorimetry, and optical microscopy. As iPP was the dominant phase (its content was more than 50 wt%), our results indicated that it could be highly oriented in the blends. In such case, it was interesting to find that LLDPE epitaxially crystallized on the oriented iPP through a crystallographic matching between (100)_LLDPE and (010)_iPP, resulting in an inclination of LLDPE chains, about 50° to the iPP chain axis. On the other hand, as iPP was the minor phase, iPP was less oriented and no epitaxial growth between iPP and LLDPE was observed; even LLDPE remained oriented. The composition‐dependent epitaxial growth of LLDPE on oriented iPP could be understood as due to: (1) the effect of crystallization sequence, it was found that iPP always crystallized before LLDPE for all compositions; (2) the dependence of oriented iPP structure on the blend composition; (3) the “mutual nucleation” between LLDPE and iPP due to their partial miscibility. Copyright © 2009 John Wiley & Sons, Ltd.     

Nonisothermal bulk crystallization studies of high density polyethylene using light depolarizing microscopy

The quiescent nonisothermal bulk crystallization kinetics of two high-density polyethylene resins were investigated by a modified light-depolarizing microscopy (LDM) technique. The technique allows studies at average cooling rates up to 2500°C/min. The polymer was found to crystallize at a pseudo-isothermal temperature even at these very high cooling rates. The overall bulk crystallization rate increased rapidly as the cooling rate and supercooling increased. Crystallization kinetics was analyzed by Avrami analysis. Avrami exponents near 3 suggested spherical growth geometry and instantaneous nucleation at predetermined sites. Observation of spherulites by optical microscopy together with a number density of spherulites that changed little with increase in cooling rate or supercooling supported this model of crystallization behavior. Analysis of the half-time of crystallization based on the Lauritzen and Hoffman secondary nucleation theory indicated that the regime II-III transition was found to occur at a degree of supercooling of approximately 22°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 681–692, 1998     

Prediction of the flow‐induced crystallization in high‐density polyethylene by a continuum model

In this work, the isothermal flow‐induced crystallization (FIC) of high‐density polyethylene (HDPE) under a simple shear flow was investigated. Two experimental modes, including steady shear and preshear treatment, were performed on the polymer melt. Based on the nonequilibrium thermodynamic theory, the FIC process of HDPE was predicted through the modification of a continuum FIC model. The theoretical predictions of the evolution of both the viscosity in steady shear flow and the complex modulus under preshear treatment were essentially related to the crystallinity of HDPE, in agreement with the experimental findings. Both experimental and predicted results showed that the applied flow field could accelerate the crystallization kinetics of HDPE significantly. However, the effect of the intensity of shear flow on the crystallization of HDPE was finite, showing a saturation phenomenon, namely, the accelerated degree of crystallization tending to level off when the shear rate was large enough. In additional, it was found that the predicted crystallinity of HDPE was very low in induction period either in steady shear flow or by preshear treatment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 531–538, 2009     

Nonisothermal crystallization kinetics and melting behavior of bimodal medium density polyethylene/low density polyethylene blends

Nonisothermal crystallization kinetics and melting behavior of bimodal-medium-density- polyethylene (BMDPE) and the blends of BMDPE/LDPE were studied using differential scanning calorimetry (DSC) at various scanning rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of BMDPE. The BMDPE DSC data were analyzed by the theory of Ozawa. Kinetic parameters such as the Avrami exponent (n), the kinetic crystallization rate constant (Z_c), the peak temperatures (T_p) and the half-time of crystallization (t_1/2) etc. were determined at various scanning rates. The appearance of double melting peaks and the double crystallization peaks in the heating and cooling DSC curves of BMDPE/LDPE blends indicated that the BMDPE and LDPE could crystallize respectively.     

Effect of molecular architecture on quiescent and shear‐induced crystallization of polyethylene

The aim of this work is to investigate the effect of the molecular structure of polyethylene on the crystallization kinetics. In static conditions, the increase of the degree of branching leads to the decrease of the crystallization temperature, the melting temperature, and the crystallinity. Indeed, the crystal thickness is controlled by the length of PE segments between branching. The effect of preshear on crystallization kinetics was studied by following the dynamic modulus along the time after a treatment of constant shear rate. Particularly, the effect of the shear rate was investigated. The enhancement of crystallization kinetics appears directly linked to the relaxation time of the melt polymer. Expressed by the Weissenberg number, a “master curve” is obtained independent of the amount and length of branching, leading to the conclusion that the nucleation due to shear is conditioned by the molecular architecture mainly via its effect on the relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1597–1607, 2006     

Influence of mechanical history on the crystallization behaviour and morphology of low density polyethylene

Summary If a virginal LDPE melt is sheared a memory is frozen during crystallization inasmuch as after remelting the nucleation density and the rate of crystallization are strongly increased as compared with the original material. This is not caused by the introduction of impurities into the melt during shearing.Part of this memory is recovered by annealing or solvent treatment. This reversible contribution is discussed in terms of a shear-induced increase of trans conformations in the melt, which is partly frozen on pelletizing and which becomes active again on remelting.The non-recoverable part of the memory must be due to changes of molecular structure. Since other parameters do not change it is supposed that a change of the long-chain branching frequency during shearing is the reason for the irreversible effects.Blown films produced from LDPE pellets, which originate from a strongly sheared melt and consequently have an increased crystallization rate, are known to have improved processing and end-use properties (maximum draw-down speed, optical quality). The film blowing process itself is shown to introduce an additional mechanical history into the LDPE melt such that differences of the crystallization kinetics of the starting pellets are completely equalized. A different crystallization behaviour of pellets, therefore, cannot be responsible for the differing film properties, which are more likely influenced by the different viscoelastic response of the melt.Dedicated to Professor Dr. Werner Reif on the occasion of his 60th birthday.     

Effect of equal channel angular pressing on thermal and mechanical properties of carbon nanotube/high density polyethylene composite

High density polyethylene (HDPE) nanocomposite reinforced with 2 weight percent carbon nanotube (CNT) was fabricated using mechanical milling method. Microscopic evaluations revealed appropriate dispersion of CNTs in the matrix, and tensile tests demonstrated that the tensile strength was increased by 17%. Thermal and mechanical properties of the composite samples were investigated after equal channel angular pressing (ECAP) for up to 3 passes via route A at temperature of 80°C. Density and differential scanning calorimetry (DSC) results represented decrement in crystallinity after ECAP which was led to drop in hardness and tensile yield strength of the deformed samples. Micro Vickers and Shore D hardness results also revealed clear anisotropy in mechanical properties caused by ECAP. Dilatometry results and observation of the impact fractured surfaces of deformed samples demonstrated that oriented structures formed in amorphous and crystalline regions of the composite. This microstructure evolution also caused increase in impact strength of ECAP deformed specimens. Dynamic mechanical behavior of the processed samples was modified following ECAP. The α and γ relaxation temperatures were decreased due to the reduction of thickness of crystalline lamella obtained from DSC results, in 1 pass ECAP deformed sample. Dynamic storage and loss modulus of 3 passes ECAP deformed samples were significantly decreased due to the sharp drop in their crystallinity.     

Studies of non-isothermal crystallization and melting of ultra high molecular weight polyethylene

The non-isothermal crystallization and melting of ultra high molecular weight polyethylene (UHMWPE) were observed by means of differential scanning calorimetry and compared with those of ordinary high-density polyethylene (HDPE). The crystallization temperature (T _c ) and melting point (T _m ) of UHMWPE were found to be higher thanT _c andT _m of HDPE, and the latent heat of crystallization (δH _c ) and fusion (δH _m ) of UHMWPE are smaller thanδH _c andδH _m of HDPE. The results were explained in terms of the theory of polymer crystallization and the structure characteristics of UHMWPE. The relationships between the parameters (T _c ,T _T ,δH _c andδH _m ) and the molecular weight (M) of UHMWPE are discussed. Processing of the experimental data led to the establishment of four expressions describing the above relationships.     

Study on residual stresses of thin-walled injection molding

The residual stresses of the thin-walled injection molding are investigated in this study. It was realized that the behavior of residual stresses in injection molding parts was affected by different process conditions such as melt temperature, mold temperature, packing pressure and filling time. The layer removal method was used to measure the residual stresses at a thin-walled test sample by a milling machine. This simple method was demonstrated to be adequate for a thin-walled part. Moldings under different conditions were investigated to study the effects of the process conditions on the residual stresses of a thin-walled product using the elastic and viscoelastic models. The mold temperature was found to affect the size of the core region and residual stress on the surface layer of a thin-walled part in our studied range. The packing pressure was insensitive to the residual stresses in the studied high-pressure range. The residual stresses predicted by the viscoelastic model are about the same level and trend as compared to the experimental measurement.     

Strain hardening of high density polyethylene

The introduction of true stress strain measurements, at constant strain rate, has promoted the development of empirical or semiempirical models for large deformations in thermoplastics. One such theory, which proposes that the post yield deformation process can be represented by equations derived from the theories of rubber elasticity, has been successfully applied to several glassy polymers. Unexpectedly, it can also model the post yield deformation of many different grades of polyethylene, even when rubber theory is employed in the simplest Gaussian form. Strain hardening is then represented by the single strain hardening coefficient Gp. Examples are given of this equation, which can be modified to give the true engineering or nominal stress σ_n and then be differentiated to give dσ_n/dλ = Gp ? Y₀ / λ² + 2Gp / λ³, where Y₀ is the yield stress and λ the extension ratio. Negative values of this differential then predict the onset of necking in tension and positive values stabilization of the neck. The relation of Gp to molecular weight is then discussed using literature measurements for polyethylenes of differing molecular weight and similar molecular weight distributions. When these results are then plotted, a strong dependency of Gp on molecular weight is observed. Some implications of these measurements are then considered. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1090–1099, 2007     

Effect of silver nanoparticles on the dynamic crystallization behavior of nylon‐6

The effect of introducing silver nanoparticles on the rheological properties and dynamic crystallization behavior of nylon‐6 was investigated. The nanocomposites showed slightly higher viscosity than pure nylon‐6 in the low‐frequency range even at an extremely low loading level of the silver particles (0.5–1.0 wt %). The nanoparticles had a more noticeable effect on the storage modulus than on the loss modulus of a nylon‐6 melt and reduced its loss tangent. They increased the crystallization temperature of nylon‐6 by about 14 °C and produced a sharper crystalline peak. The silver nanoparticles promoted the crystallization of nylon‐6, and their effect on the dynamic crystallization of nylon‐6 at 200 °C was more notable at a lower shear rate and at 190 °C at a higher frequency. Nylon‐6 produced large spherulitic crystals, but the nanocomposites showed a grainy structure. In addition, the silver nanoparticles reduced the fraction of the α‐form crystal but increased that of the γ‐form crystal. The nanocomposites crystallized at 190 °C showed a lower melting temperature than nylon‐6 by about 3 °C, whereas the nanocomposites crystallized at 200 °C showed almost the same melting temperature. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 790–799, 2004     

Simulations of polymer crystallization under high pressure

High pressure crystallization of polypropylene was studied by means of PVT measurements and computer simulations. The isothermal crystallization behaviour were described by using a model which takes into account the effect of pressure on the temperature dependence of nucleation rate and linear growth rate. The agreement between the simulation and the experiments was seen in the tendency that the crystallization was accelerated by the high pressure. The non-isothermal crystallization behavior was also simulated by applying a generalized Avrami equation. The simulation curves well reproduced the experimental values below relative crystallinity 0.5 and below 100 MPa.     

Novel fluorosilicone thermoplastic vulcanizates prepared via core‐shell dynamic vulcanization: Effect of fluororubber/silicone rubber ratio on morphology,crystallization behavior,and mechanical properties

Recently, we have reported a novel core‐shell dynamic vulcanization method to prepare poly(vinylidene fluoride) (PVDF)/fluororubber (FKM)/silicone rubber (SR) thermoplastic vulcanizates (TPVs) with cross‐linked rubber core‐shell particles. However, the shell thickness on the properties has not been studied in detail. Herein, these PVDF‐based TPVs different FKM‐shell thickness were prepared by changing FKM/SR ratios. The effect of FKM‐shell/SR‐core ratio on morphology, crystallization, and mechanical properties of the ternary TPVs was studied. The results showed that the FKM shell had more positive effect on interfacial‐induced crystallization behavior than the SR core due to its better compatibility with PVDF. When the FKM/SR ratio was <1, FKM was not enough to encapsulate SR completely, which resulted in the formation of imperfect core‐shell structure. However, when the FKM/SR ratio was >1, perfect core‐shell structure was formed. Therefore, the mechanical properties improved with increasing FKM content; especially, a remarkable improvement was observed when FKM/SR ratio was >1. This study could provide more information for the design of TPVs with core‐shell structure.     

Ductile–brittle‐transition phenomenon in polypropylene/ethylene‐propylene‐diene rubber blends obtained by dynamic packing injection molding: A new understanding of the rubber‐toughening mechanism

For a more complete understanding of the toughening mechanism of polypropylene (PP)/ethylene‐propylene‐diene rubber (EPDM) blends, dynamic packing injection molding was used to control the phase morphology and rubber particle orientation in the matrix. The relative impact strength of the blends increased at low EPDM contents, and then a definite ductile–brittle (D–B) transition was observed when the EPDM content reached 25 wt %, at which point blends should fail in the ductile mode with conventional molding. Wide‐angle X‐ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) were used to investigate the shear‐induced crystal structure, morphology, orientation, and phase separation of the blends. WAXD results showed that the observed D–B transition took place mainly for a constant crystal structure (α form). Also, no remarkable changes in the crystallinity and melting point of PP were observed by DSC. The highly oriented and elongated rubber particles were seen via SEM at high EPDM contents. Our results suggest that Wu's criterion is no longer valid when dispersed rubber particles are elongated and oriented. The possible fracture mechanism is discussed on the basis of the stress concentration in a filler‐dispersed matrix. It can be concluded that not only the interparticle distance but also the stress fields around individual particles play an important role in polymer toughening. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2086–2097, 2002     

Scanning electron microscopy of oriented high density polyethylene

Summary The microfibrillar and lamellar morphologies in cold-drawn and cold-drawn/annealed high-density polyethylene sheets were observed by means of scanning electron microscopy. Differences in contrast on fracture surfaces for cold-drawn sheet are interpreted in terms of a preferential orientation of inter-microfibrillar tie molecules in the plane of the sheet brought about by the drawing mechanism. In annealed, cold-drawn sheet, stacks of lamellae were observed which showed twinned orientations of inclined lamellae. This roof-top structure is interpreted in terms of shear within the individual microfibrils during micronecking, and corresponds to the well-known 4-point small-angle X-ray pattern for this type of specimen. Light etching with fuming nitric acid was necessary in order to resolve the individual lamellar texture.With 9 figures     

Flash DSC crystallization study of blown film grade bimodal high density polyethylene (HDPE) resins. Part 2. Non‐isothermal kinetics

Non‐isothermal ultra‐fast cooling crystallization tests were conducted on three blown film grade bimodal high density polyethylene (HDPE) resins using a fast differential scanning calorimeter, the Flash DSC. Non‐isothermal tests were performed at cooling rates between 50 and 4000°K/s, and the data were analyzed using the modified Avrami model by Jeziorny (Polymer, 1978 , 19, 1142). Non‐isothermal data were used to propose a new method named crystallization–time–temperature–superposition, and the two activation energies were obtained for each of the resins. This is very useful for obtaining theoretical crystallization kinetics data at different cooling rates, allowing its use in ultra‐fast cooling polymer processes such as blown film. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1822–1827     

Effect of the vibration shear flow field in capillary dynamic rheometer on the crystallization behavior of polypropylene

The self-made capillary dynamic rheometer was adopted to study the relationship between the crystallization behavior of isotactic polypropylene (iPP) and the vibration shear conditions, namely, vibration amplitude and vibration frequency. The crystalline structure of iPP under different vibration conditions was characterized by using differential scanning calorimeter (DSC) and wide-angle X-ray diffractometer (WAXD) techniques. The samples extruded under vibration shear conditions had a higher melting temperature (from DSC). A new shoulder-shape peak appeared at ca. 162 °C under low frequency or low amplitude conditions, which was engulfed by the main melting peak with the increase of the vibration amplitude or frequency. This was probably an indication that more perfect crystals had formed [Polym Eng Sci 38 (1998) 1-20]. The WAXD demonstrated that crystalline form of iPP extruded was not changed but the average crystalline size decreased, according to the Scherrer formula [Analytical methods of polymer materials, China Petrochemical Press, Beijing, 1997]. This proved a large increase in the number of small crystals.     

Effect of molecular structure on rheological and crystallization properties of polyethylenes

This article describes the development of reliable techniques to measure the isothermal crystallization rates (ICR) under quiescent as well as under small amplitude, oscillatory shear conditions. Quiescent crystallization rates were obtained using a differential scanning calorimeter. Those under small amplitude shear were obtained using Rheometrics rheometers. It is shown how a small amount of long-chain branching in high-density polyethylene homopolymer (HDPE) dramatically influences rheological properties and enhances ICR. For these HDPEs, the rate increases with the increase in long-chain branching. The general application of isothermal crystallization studies, however, should be done with great caution. This is because the fundamentals of isothermal crystallization require that it be done on the basis of a fixed undercooling with respect to the equilibrium melting temperature. Such a temperature is ill-defined for the commercial polymers having broad molecular weight distribution (MWD). Nonetheless, a practical procedure is outlined wherein the melting curve of a previously isothermally crystallized sample is used as a substitute for judging the equilibrium melting point and in deciding the selection of a proper crystallization temperature. Even this new procedure may not be applicable for polymers having heterogeneous short-chain branching distribution. © 1996 John Wiley & Sons, Inc.     

聚乙烯在石墨表面结晶的分子动力学模拟

采用分子动力学方法模拟了聚乙烯在石墨(001)表面的吸附和结晶过程;直观的给出了聚乙烯链被石墨(001)面吸附并诱导形成有序的片层晶体的过程;发现结晶温度对得到的有序结构中的聚乙烯链相对石墨表面的特定取向有影响(300 K和600 K时的取向方向不同);表面覆盖率影响聚乙烯吸附层的厚度,对取向的方向无影响.     

异戊橡胶微观序列结构与结晶性能研究

天然橡胶特有的应变诱导结晶能力赋予其优异的力学性能.异戊橡胶作为唯一能够替代天然橡胶的合成橡胶品种,其应变诱导结晶能力受到关注.本文对3种异戊橡胶的微观序列结构进行了分析,并研究了其结晶性能.核磁分析结果表明:3种异戊橡胶的顺-1,4-结构含量差别不大;从1,4-结构单元的键接方式(序列结构)看,SKI-5和YS-IR分子链中顺-1,4-结构单元均以头-尾相接的方式沿分子链排列,不存在头-头键和尾-尾键接方式;SKI-3中约有0.4%~0.5%的1,4-单元采取头-头键接方式,约有0.3%~0.6%的1,4-单元采取尾-尾键接方式;根据定量计算结果,从分子链上1,4-结构单元的序列分布来看,SKI-3的规整性与SKI-5、YS-IR相近或略高.XRD研究结果表明:炭黑填充的天然橡胶硫化胶拉伸至400%以上时发生取向结晶;而炭黑填充的异戊橡胶硫化胶需拉伸至500%以上时才发生取向结晶.基本物理机械性能研究表明:3种异戊橡胶的性能相当,拉伸强度和撕裂强度明显低于天然橡胶;由于结构和组成上的差异,异戊橡胶的结晶能力较天然橡胶差.