Extensional flow-induced crystallization of a polyethylene melt

Measurements of flow-induced orientation and crystallization have been made on a high-density polyethylene melt (HDPE) undergoing a planar extensional flow in a four-roll mill. The HDPE was suspended as a cylindrical droplet at the flow stagnation point in a linear low density polyethylene (LLDPE) carrier phase. Deformation and crystallization of the HDPE droplet phase were monitored using video imaging in conjunction with measurement of the birefringence and dichroism to quantify the in-situ transformation kinetics. Planar deformation rates along the symmetry axis of the molten HDPE phase were on the order of 0.03 s^?1. Measurements of the initial transformation rate following flow cessation at 131.5°C and 133.2°C show a dependence on initial amorphous phase orientation and the total Hencky strain achieved during flow. The flow-induced crystallization rate is enhanced over the quiescent transformation rate by orders of magnitude, however, the dependence on temperature is less dramatic than expected for a nucleation-controlled growth mechanism. Analysis demonstrates that the melting point elevation model cannot account either qualitatively or quantitatively for the phenomena observed, suggesting that alternative explanations for the strong orientation dependence of the transformation rate are needed.     

Extensional flow-induced crystallization in polyethylene melt spinning

Polyethylene was spun into a heated chamber, with the spin-line temperature controlled in the range of 90–120°C. Within narrow limits, the stretch rate of the fiber was also controlled. Spin-line and crystallization onset conditions were characterized. Characteristics of fibers as-spun were measured via wide-angle x-ray scattering (WAXS), density, scanning electron microscopy, and differential scanning calorimetry. Spin-line data indicate that elongational flow enhances crystallization rate and that time under spin-line conditions is an important parameter. Analysis of WAXS data shows a typical “b-axis radial” orientation, the details of which change with spin-line parameters. This orientation is consistent with growth of lamellar crystals on extended-chain fibrils (shishkebab model). Other physical data are also consistent with this microstructure. Microcamera x-ray patterns show a similarity of microstructure generation in spinning and drawing.     

Non-isothermal crystallization kinetics of graphite-reinforced crosslinked high-density polyethylene composites

Journal of Thermal Analysis and Calorimetry - Crosslinked high-density polyethylene (PEX) composites containing various amounts of graphite particles (1, 2.5 and 5 mass% or 0.479, 1.206 and 2.445...     

Reversible and irreversible crystallization in high-density polyethylene at low temperature

Reversible and irreversible crystallization and melting of high-density polyethylene at low temperature has been re-evaluated and is discussed in terms of the concept of the specific reversibility of a crystal. The concept of the specific reversibility links reversible and irreversible melting of a specific crystal such that reversible melting occurs only at slightly lower temperature than irreversible melting. In this study evidence for irreversible crystallization at low temperature in high-density polyethylene is provided, non-avoidable by primary crystallization and extended annealing at high temperature. The simultaneously observed reversible crystallization and melting at low temperature can be attributed to lateral-crystal-surface activity in addition to the well-established reversible fold-surface melting, dominant at high temperature, and evidenced by small-angle X-ray data available in the literature. This revised version was published online in August 2006 with corrections to the Cover Date.     

The kinetics of flow-induced crystallization from solution

In situ birefringence measurements of the seeded growth in a tubular flow geometry of 0.01 wt% solution of a polyethylene fraction in xylene have been used to determine the flowinduced crystallization kinetics as a function of temperature and flow rate. In contrast to earlier reports on higher molecular weight polyethylene and polypropylene systems, orientational properties of the crystallized fibers do not show a clear correlation with growth conditions (i.e., temperature and flow rate). The combined kinetic data from these experiments and our earlier studies of higher molecular weight polyethylene—xylene and polypropylene—tetralin systems are analyzed in terms of a modified from of the Avrami equation which provides a basis for separately correlating temperature and flow rate effects. The observed temperature dependency of the crystallization process can be interpreted in terms of nucleation and growth models while the flow rate dependency can be interpreted on the basis of entanglement formation arguments. Results showing liquid phase precursor formation in an atactic polystyrene system are also presented to further document the liquidphase separation which can be induced in polymers under flow.     

Carbon nanotubes periodically decorated by high-density polyethylene crystalline using solution crystallization

The carbon nanotubes (CNTs) periodically decorated by high-density polyethylene (HDPE) composites with nanohybrid shish kebabs (NHSK) structures were prepared by CNTs-initiated solution crystallization. The disc-shaped HDPE crystalline lamellae were periodically located on the surface of CNTs in the direction perpendicular to the nanotube axis. Observations from scanning electron microscopy and transmission electron microscopy showed that with the increasing of crystallization temperature, the lateral dimension of the lamellae was decreased and the distance between two neighboring lamellae was increased. However, the thickness of the lamellae did not vary with the crystallization temperature. The formation mechanism of the NHSK structures was also explained. The one-dimensional structure and the ultra-high curved surface of CNTs lead to strong geometry confinement, which plays a main role in the formation of the NHSKs. Supported by the National Natural Science Foundation of China (Grant No. 50772031), the Chinese Program for New Century Excellent Talents in University (Grant No. NCET-05-0678), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Ministry of Education, Hubei Provincial Department of Education (Grant No. Q200610005), and Hubei Provincial Science & Technology Department (Grant No. 2006ABA020)     

Mechanical properties and crystallization of high-density polyethylene composites with mesostructured cellular silica foam

In this study, composites of high-density polyethylene (HDPE) with mesostructured cellular foam (MCF) silicas have been prepared by melt mixing and studied for the first time. Two different MCF silica analogues having different pore size were used, i.e., 12 nm (MCF-12) and 50 nm (MCF-50). The MCF content in the mesocomposites was 1, 2.5, 5, and 10 mass%. All HDPE/MCF-50 mesocomposites exhibited improved mechanical properties compared with neat HDPE, indicating that the mesocellular silica foam particles with the large mesopore size can act as efficient reinforcing agents. On the other hand, the MCF-12 silica with the smaller size mesopores induced inferior mechanical properties, mainly due to the poorer dispersion of the silica particles and the formation of large aggregates. The mesocellular silica foam particles also affected the thermal properties and the crystallization characteristics of HDPE. Crystallization of mesocomposites was faster than that of neat HDPE. Crystallization kinetics was analyzed with the Avrami equation for both isothermal and non-isothermal conditions. For isothermal crystallization, the Avrami exponent increased with increasing crystallization temperature from 2 to 3. In non-isothermal crystallization, the values of the Avrami exponent increased from 3 to 6.3 with decreasing cooling rate. Lower activation energy values of non-isothermal crystallization were calculated using the isoconversional method of Friedman, as well as using the Kissinger’s equation. Finally, the nucleation efficiency of the mesocellular silica foam particles was estimated from data associated with non-isothermal crystallization, according to the method of Dobreva.     

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.     

A simple model of flow-induced crystallization memory

Flow induced crystallization of polymer systems exhibits strong memory effects. Crystalline structures gradually change when the flow is switched off and the polymer is relaxed prior to crystallization. A simple model based on the multidimensional theory of crystal nucleation^[1] is proposed. Steady, potential flow applied to a polymer fluid above melting temperature (T_p > T_m) results in molecular orientation of crystallizing units. The flow controls formation of molecular clusters which convert into athermal nuclei when the system is cooled down to crystallization temperature, T_c < T_m. Orientation effects gradually disappear when the melt is relaxed above T_m in the absence of flow or stress.     

A continuum model for the dynamics of flow-induced crystallization

A model for flow-induced crystallization is developed which is based on ideas from the theory of strain-induced crystallization, coupled with an irreversible thermodynamic formalism based on the continuum Hamiltonian Poisson Brackets. The latter allows accounting for the changing energetics during simultaneous flow deformation and extended-chain crystallization. Input parameters to the model include the molecular relaxation time, a crystallization parameter, and the molecular weight. Calculations of the crystallization rate, chain elongation, stress, and birefringence are presented for a variety of flow kinematics and flow histories, including transient processes following cessation of flow. Induction times based on a reasonable choice for the induction crystallinity follow experimentally observed trends reported in the literature. © 1996 John Wiley & Sons, Inc.     

The effect of prior crystallinity on the flow-induced crystallization of poly(ethylene oxide)

After flowing in a dilatometer bulb for a small fraction of the duration of the transformation, a relaxed melt of poly(ethylene oxide) (M?_n = (5.9 ± 0.1) × 10³) showed marked increases in isothermal crystallization rate. The extent of increase was greater when flow was imposed at modestly later stages rather than at the earliest stage of a crystallization. Kinetic parameters for the flow-induced crystallizations were obtained via modification of the conventional mathematical treatment of the kinetics of phase change, thereby allowing the analytical resolution of the overall process into flowinduced and quiescent components. Determination of the flow-induced crystallization parameters required independent determination of the kinetic parameters for quiescent crystallizations at that temperature. The Avrami exponents n_f which characterized the flow-induced portions of the crystallizations were larger for those instances in which flow was imposed at the more advanced stages of the crystallizations, thus indicating a transition in crystallization mechanism. It is suggested that prior crystallinity present at the time of flow contributed to the crystallization by serving as a source of nucleation sites. However, in light of the experimental procedure employed, values of n_f approximating 4 that were obtained are not susceptible to mechanistic interpretations now extant.     

Effect of nonhomogeneous shear rates on the flow-induced crystallization of flexible macromolecules

Several observations are made regarding the role of nonhomogeneous shearing rates in the process of flow-induced fibrillar crystallization from solution. The flow geometry chosen in the present analysis is that occurring in the annular region between two concentric cylinders. Computational evaluation and discussion of the effect of stress-induced diffusion is presented in terms of the elastic dumbbell model. Significant variations in concentrations in the annular region are predicted at full development. Estimates of the dynamics of development of these profiles indicate that the length to capillary radii for full development depends upon the molecular weight of the polymer and the relative dimensions of the concentric cylinders. These results indicate the important role of stress-induced diffusion in the process of thickening of fibrillar crystals.     

Effect of polyethylene glycol on the crystallization and impact properties of polylactide‐based blends

Polylactide (PLA) was plasticized by polyethylene glycols (PEGs) with five different molecular weights (M_w = 200–20,000 g/mol). The effects of content and molecular weight of PEG on the crystallization and impact properties of PLA were studied by wide‐angle X‐ray diffraction, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, and V‐notched impact tests, respectively. The results revealed that PEG‐10,000 could significantly improve the crystallization capacity and impact toughness of PLA. When the PEG‐10,000 content ranged from 0 to 20 wt%, the increases in both V‐notched Izod and Charpy impact strengths of PLA/PEG‐10,000 blends were 206.10% and 137.25%, respectively. Meanwhile, the crystallinity of PLA/PEG‐10,000 blends increased from 3.95% to 43.42%. For 10 wt% PEG content, the crystallization and impact properties of PLA/PEG blends mainly depended upon PEG molecular weight. With increasing the M_w of PEG, the crystallinity and impact strength of PLA/PEG blends first decreased and then increased. The introduction of PEG reduced the intermolecular force and enhanced the mobility of PLA chains, thus improving the crystallization capacity and flexibility of PLA. Copyright © 2015 John Wiley & Sons, Ltd.     

Confined-growth crystallization of polyethylene

A new typical orientation pattern of polyethylene has been observed in extruded, melt-drawn composites containing 10% polyethylene and 90% polystyrene. In these composites, the polyethylene phase is dispersed in the polystyrene matrix as thin, long ribbons (width 1000 Å, thickness 500 Å). The b axis of the crystallites is found oriented preferentially along the long dimension of the ribbons, i.e., in the extrusion direction. The a and c axes of the crystallites show no preferred orientation. This texture pattern is attributed to the fact that, in view of the small cross section of the polyethylene phase, crystallization can proceed only along the long axis of the ribbons. Since the b axis is the direction of fastest growth in polyethylene (and the radial direction in a spherulite), most polyethylene unit cells are oriented with their b axes in the long dimension of the ribbons.     

Some fundamental aspects of the kinetics of flow-induced crystallization of polymers

Some indispensable fundamental aspects of the kinetics of polymer crystallization are summed up. Those aspects are of particular interest in the development of crystalline structures during polymer processing. In this connection also a monograph (Janeschitz-Kriegl 2009) must be mentioned, which has been written last year by the first author of this article. The present paper contains a selection of particularly interesting subjects. It turns out that many of these subjects have eluded a mathematical treatment. The pertinent list is given in the introduction. And those people, who feel pressed to produce computer programs, are exhorted to mind the content of the present article and also of the mentioned monograph in avoiding arbitrary assumptions.     

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.     

Study of the influence of concentration on the time for first appearance of crystallization of polyethylene from xylene

An experiment is reported in which a simple laser light-scattering technique is used to measure the time for a critical turbidity to appear during the crystallization of polyethylene in xylene. The effect is examined over the entire range of solution concentration, and it is found that the relation between this concentration and temperature is linear for all solutions, provided that the time for turbidity to develop is arranged to be the same in all cases. In the appendix it is reasoned that similar results could have been obtained if, instead, times had been measured to a given degree of crystallinity rather than of turbidity. Departures from this linearity at low concentrations are taken as indicating multimolecular nucleation for all concentrations greater than about 1%.     

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.