Effect of crystallization time on the properties of melt-crystallized linear polyethylene

Linear polyethylene was crystallized isothermally from the melt. Specimens were removed at different crystallization times and quenched to room temperature. The density, static mechanical properties, and small-angle x-ray scattering (SAXS) behavior of these specimens were measured at room temperature. The density and Young's modulus increased with crystallization time, whereas the upper yield point decreased with crystallization time. SAXS data showed that a zero-angle peak gradually disappeared as crystallization time increased. Concurrently, the breadth of the SAXS peaks, the Bragg angle, and the integrated intensity decreased. Changes in the ratio of second- and first-order peak intensities were also noted. On the basis of the SAXS and density data, it was concluded that a competition exists between the thickening of existing crystals and the creation of new crystallites between the older ones. At relatively low crystallization times, numerous new crystals can form during quenching to room temperature, whereas quenching after prolonged crystallization primarily results in the additional thickening of existing crystals. No change in the density of the amorphous material is found. A model is given whereby the upper yield stress is coupled to these morphological changes through a stress concentration effect caused by a decreased population of chains connecting adjacent crystallites. The tie-chain population change occurs by their elimination as crystallites disappear.     

The influence of flow-induced crystallization on the impact toughness of high-density polyethylene

The relation between the impact toughness and flow-induced crystalline orientation of high-density polyethylene (HDPE) was investigated. Flow-induced crystalline orientation was created in the samples via injection moulding and the amount of orientation was controlled through variation of processing conditions (injection temperature) and sample thickness. The impact toughness behaviour was found to be strongly dependent on the amount of crystalline orientation, whereas the loading direction also had a strong influence, e.g. giving highest impact properties in flow direction. Subsequently, injection moulded samples of HDPE modified with calcium carbonate filler particles were tested. In this case a similar relation between crystalline orientation and loading direction was found, whereas the total amount of flow-induced crystallization was observed to be strongly influenced by the presence of the filler particles.     

The initial stages of crystallization of polyethylene from the melt

Synchrotron x-radiation has been used to follow the development of low-angle diffraction in sharp fractions of polyethylene. The polymer is shown to crystallize in very thin lamellae which rapidly thicken in a single step to twice, three times, or four times the original thickness. This dramatic refolding is more pronounced at higher crystallization temperatures. After the sudden integral jumps in fold length, the thick lamellae continue to grow thicker logarithmically with time. The significance of these findings for the most basic issues of polymer crystallization and for the experimental methodology of its study is highlighted.     

The influence of micro- and nanoparticles on model atomically flat surfaces on crystallization of polyethylene

Crystallization of high density polyethylene (PE) from the melt on model atomically flat solid surfaces decorated with micro- and nanoparticles of gold or NaCl of different size and densities is investigated. The morphology of the contact layer of PE after its detachment from the support is studied using atomic force microscopy (AFM). It is shown that the nucleating and ordering effect of the solid on PE crystallization depends to a large extend on the nanostructure of its surface, in particular on the size of the atomically flat domains and on the presence of nanoscopic obstacles. The minimum size of the flat domain which can significantly influence the PE crystallization is estimated to be of the order of 150 nm.     

Crosslinking kinetics of polyethylene with small amount of peroxide and its influence on the subsequent crystallization behaviors

Crosslinking reactions of high density polyethylene with low peroxide concentrations ranging from 0.1 wt% to 1.0 wt% at temperatures of 170, 180 and 190 ° C were monitored by rheological measurements. A critical gel forms at the peroxide concentration of 0.2 wt%, where the transition from long chain branching generation to crosslinking network formation could occur. Rheokinetics of crosslinking can be fitted well by Ding-Leonov's model. The curing rate k_2 at the earlier stage exhibits about 3 times acceleration per 10 °C with increasing temperature, while the equilibrium modulus G′ at the fully cured stage is almost independent of temperature. Influences of crosslinking on the subsequent crystallization behaviors were detected by DSC measurements. Above the critical gel concentration, crystallization is largely retarded as evidenced by the lower crystallization temperature Tc and crystallinity X_c due to the network formation. The secondary crystallization valley located at the temperature near 80 °C can be observed above the critical concentration, which becomes more evident with the increasing peroxide concentration and curing temperature. This phenomenon provides another evidence of crystallization retardation by the crosslinking network.     

Transient effects in the crystallization of polyethylene

A specimen of linear polyethylene was subjected to isothermal secondary crystallization at a series of temperatures below the primary isothermal crystallization temperature, the melting and primary crystallization stages being held constant throughout the investigation. Dilatometric measurements exhibit an S-character at low values of undercooling T_p – T_s, where T_p and T_s are, respectively, the primary and secondary crystallization temperatures, whereas at larger undercooling, an initial very rapid crystallization is followed by a very slow stage. When corrected for thermal contraction of the polymer, the net degree of secondary transformation is seen to peak at a temperature about 5°C below T_p. The S-character of the isotherms and the peaked temperature variation of degree of transformation lead to the conclusion that a large portion of the secondary crystallization consists of the nucleation and growth of the new crystallites. Johnson-Mehl-Avrami analysis leads to a model of heterogeneous nucleation within the remaining amorphous zones, followed by one-dimensional, diffusion-controlled growth.     

Time-resolved small-angle neutron scattering study of polyethylene crystallization from solution

With time-resolved small-angle neutron scattering (TR-SANS), the crystallization kinetics of polyethylene from deuterated o-xylene solutions upon a temperature jump have been investigated. On the basis of a morphological model of coexisting lamellar stacks and coil chains in solution, experimental data have been quantitatively analyzed to provide structural information, such as the lamellar long period, the lamellar crystal thickness, the thickness of the amorphous layers between lamellae, the degree of crystallinity, and the crystal growth rate at various degrees of undercooling. The viability of TR-SANS for studying polymer crystallization is demonstrated through the consistency of these measurements and well-established knowledge of polyethylene crystallization from xylene solutions. One unique feature of this experimentation is that both the growth of lamellar crystals and the condensation of coil chains from solution are monitored simultaneously. The ratio of the crystal growth to the chain consumption rate decreases rapidly with a decreasing degree of undercooling. The Avrami analysis suggests that the growth mechanism approaches two-dimensional behavior at higher temperatures, and this is consistent with the observation of an increasing ratio of the sharp-surface area to the bulk crystal growth rate with temperature. The limitations, possible remedies, and potentials of TR-SANS for studying polymer crystallization are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3133–3147, 2004     

不同条件对硫酸钙结晶影响的实验研究

近年来由于微波化学的快速发展使人们认识到微波作为一种手段对化学反应的过程有着深刻的影响,在材料领域更是得到了日益广泛的应用,因此把微波应用于结晶过程也成为人们关注的热点。早在1966年磁场就被应用于晶体的生长。近年来,已有很多文献报道电场、磁场对晶体生长的影响。     

Study on the improvement of crystallization of high density polyethylene through dynamic packing injection molding

To better understand the effect of high‐density polyethylene on crystal morphology under the shear stress field, the dynamic packing injection molding was used to prepare the oriented pure polyethylene. In this present work, dynamic packing injection molding can exert a successive shear field on the melt in the mold cavity during packing stage. The structure of the samples was characterized through differential scanning calorimetry, wide‐angle X‐ray diffraction, and scanning electron microscopy. The oriented lamellae perpendicular to the flow direction can be formed. Oscillating stress between the lower and upper critical values can induce shish forming. The shish kebabs can be generated through sufficient relaxation led by slow cooling and suitable shear. We roughly discuss the relationship between the shear rate and crystal shape. Our results set up a method to reinforce polymer parts by regulating morphology and structure which can be used to do practical processing condition. Copyright © 2016 John Wiley & Sons, Ltd.     

The influence of molecular structure and crystallization time on the efficiency of diastereoisomeric salt forming resolutions

Reciprocal resolutions between compounds (racemates and enantiomers) with similar structures have been examined. Amongst structurally similar compounds (so called relative structures) several N-acyl amino acids and amino acid esters were investigated. A part of the resolving agent or the racemic compound could be replaced by an achiral compound with a relative structure and an additive could occasionally improve significantly the efficiency of the resolution. Both the kinetic and the thermodynamic controls were observed as governing factors of the reciprocal resolutions.     

高性能甲苯烷基化合成二甲苯催化剂研究

通过对比不同孔结构分子筛的甲苯甲醇烷基化催化性能,发现分子筛孔道尺寸与目标芳烃分子动力学尺寸的有效匹配以及孔道空间限制效应对反应路径的约束管理,对实现高性能烷基化至关重要。并结合XRD、BET、NH₃-TPD和SEM表征分析,通过先后负载La₂O₃和P₂O₅对硅铝比为60的ZSM-5进行复合改性修饰,提升其骨架水热稳定性的同时,选择性地消除内外表面大部分强酸中心,保留弱+中强酸作为烷基化催化活性位,所得MAT-HZSM-5催化该反应表现出很高的甲醇烷基化效率和良好的反应稳定性,在氮气反应气氛下,连续运行500 h无明显失活迹象,甲苯转化率维持在35%-38%,二甲苯选择性60%-77%,甲醇烷基化效率大于90%。     

Characterization of polyethylene crystallization from an oriented melt by molecular dynamics simulation

Molecular dynamics is used to characterize the process of crystallization for a united atom model of polyethylene. An oriented melt is produced by uniaxial deformation under constant load, followed by quenching below the melting temperature at zero load. The development of crystallinity is monitored simultaneously using molecular-based order parameters for density, energy, and orientation. For crystallization temperatures ranging from 325 to 375 K, these simulations clearly show the hallmarks of crystal nucleation and growth. We can identify multiple nucleation events, lamellar growth up to the limit imposed by periodic boundaries of the simulation cell, and lamellar thickening. We observe a competition between the rate of nucleation, which results in multiple crystallites, the rate of chain extension, which results in thicker lamellae, and the rate of chain conformational relaxation, which is manifested in lower degrees of residual order in the noncrystalline portion of the simulation. The temperature dependence of lamellar thickness is in accord with experimental data. At the higher temperatures, tilted chain lamellae are observed to form with lamellar interfaces corresponding approximately to the [201] facet, indicative of the influence of interfacial energy.     

Molecular energetics of the epitaxial crystallization of polyethylene on alkali halide substrates

A series of energy calculations have been carried out in order to describe the epitaxial crystallization of polyethylene from solution in terms of molecular interactions. The total energy has been computed as the sum of electrostatic, dispersion, and repulsion contributions. The potential energy associated with a section of a planar-zigzag polyethylene chain has been determined for various orientations and positions above three different planar alkali halide substrates. Inspection of the energy surfaces as a function of the spatial degrees of freedom reveals that the minimum energy orientation of the chain is that for which it is parallel to the substrate [(001) cleavage plane] and aligned in the 〈110〉 direction on the surface, a result in accordance with available experimental evidence. Specifically, the chain preference is to position itself along rows of positive ions, whereas alignment along rows of negative charge appears energetically unfavorable. The mode of chain orientation is virtually independent of lattice matching criteria. Dispersionrepulsive forces have been found to be most sensitive to orientation and greater in magnitude than the electrostatic forces.     

Absolute integrated SAXS measurements during the crystallization of linear polyethylene

Absolute intensity measurements of a dynamic small-angle x-ray scattering from a linear polyethylene were carried out during polymer crystallization from melt in a temperature range of 113.5° to 124.5°C. The mean-square modulation of the electron density over the irradiated volume was evaluated and the feasibility of dynamic experimentation for crystallization kinetic analysis was established. The results provide an absolute value of mass density of the amorphous phase of a semicrystalline polymer at the crystallization temperature.     

High-resolution analysis of the influence of reactant concentration on nucleation time and growth of polyethyleneimine-trimethoxymethylsilane particles

High-resolution dynamic light scattering (DLS), scanning electron microscopy (SEM), time-lapse photography, and attenuated total reflectance Fourier transform infrared spectroscopy were used to analyze the growth kinetics of polyethyleneimine (PEI)-silica particles fabricated from the condensation of hydrolyzed trimethoxymethylsilane (TMOMS) and PEI/phosphate buffer (PEI/PB). Depending on the concentration of hydrolyzed TMOMS and PEI/PB, three stages were identified. We observed the existence of a nucleation time that has never been reported in the literature when TMOMS has been used. During this nucleation time, particles of less than 25 nm were detected using in situ DLS measurements taken every 15 s (high resolution), a DLS time-scale resolution not previously reported. In addition, the length of the nucleation time depended mainly on the PEI/PB concentration, but also TMOMS concentration. The growth stage was evident from the rapid increase of particle size with time. Due to the high resolution of the DLS measurements, a peak could be observed in the particle diameter during particle growth, which corresponds to a secondary population of particles required for the larger particles to further increase in size. Finally, during the equilibrium region, particles reached their maximum diameter that was independent of the concentration of PEI/PB and only changed with concentration of hydrolyzed TMOMS.     

Effect of melt viscosity on the crystallization kinetics of pre-sheared metallocene polyethylene melts

This work aims to verify the impossibility mentioned in the literature of saturating the crystallization kinetics of pre-sheared metallocene polyethylene melts. Similarly to results reported for other materials, and contrary to other published works, an acceleration of crystallization kinetics with the increase of shear strain and its saturation at large strain values was found. Similar strain values, with the same temperature variation, were evaluated with independent experiments using different devices, which allowed us to identify the steady state as the melt state responsible for the saturation of crystallization kinetics. Since this is a partially disentangled melt state, with viscosity lower than that of fully entangled (relaxed) melts, we assign the acceleration of crystallization kinetics by application of shear, and its saturation, mainly to the facilitated diffusion of chain segments to the lamellae growth front. This conclusion is supported additionally with the experimental results of other authors.