Effects of high molecular weight component on crystallization of polyethylene under shear flow

Crystallization of polyethylene (PE) blends of low and high molecular weight components under shear flow was studied using time-resolved depolarized light scattering (DPLS), focusing on effects of the high molecular weight component on the shish-kebab structure formation. Anisotropic two-dimensional scattering pattern due to shish-like structure formation was observed above a certain concentration of the high molecular weight PE. The threshold was about 2.5-3 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. On the basis of these results a gel-spinning-like mechanism for the shish-like structure formation has been proposed. The DPLS results also implied that the shish-like structure was mainly formed from the high molecular weight PE. This was confirmed by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements on an elongated PE blend of low molecular weight deuterated PE and high molecular weight hydrogenated PE (3 wt%).     

Effect of pressure on the melting and crystallization behaviour of isotactic polybutene-1

The formation, melting and phase transition of isotactic polybutene-1 under high hydrostatic pressures were studied by high-pressure d.t.a. and X-ray diffraction up to 5 kbar. The d.t.a. thermogram of melting of form I shows a single endothermic peak up to 5 kbar. Form II crystallized directly from the melt at atmospheric pressure is metastable and it transforms to form I by the application of pressure. Above 900 bar, it transforms to form I completely and the endothermic peak of melting of form II is not observed. On crystallization from the melt under high pressure, the percentage content of form I' increases with crystallization pressure and at 1.6 kbar only form I' is crystallized. Above 2 kbar form II', which shows the same X-ray diffraction pattern as form II, is crystallized from the melt. The percentage content of form II' increases with pressure above 2 kbar, and that of form I' decreases up to 5 kbar. Upon heating under high pressure above 2 kbar, a solid-solid transition from form II' to form I' is observed in d.t.a. traces and the transition is confirmed by high-pressure X-ray diffraction. The melting temperature is expressed in the form of a quadratic equation as a function of pressure for four different forms in IPB-1.     

聚丁烯-1分子结构对结晶的影响

通过差示扫描量热测试法和偏光显微镜,研究了相对分子质量和等规度分别对聚丁烯-1（i-PB）结晶度和结晶形态的影响。实验结果表明：在常温结晶时若i-PB的相对分子质量越小则结晶度越高,晶粒完善性越好;同时等规度越高则结晶度越高,晶粒完善性越好。     

The nucleation effect of linear polyethylene lamellae over the crystallization of branched polyethylene

Summary The polyamides with aromatic rings in the main chain were synthesized by the solution polymerization of 4,4-diphenylmethane diisocyanate and aliphatic dicarboxylic acid in the presence of catalyst. The thermal properties and the miscibility behaviours with polyamide-6,6 of these aromatic polyamides were studied. The aromatic polyamides synthesized with one kind of dicarboxylic acid had typical thermal properties of crystalline polymers, whereas those synthesized with the mixtures of dicarboxylic acids were not easily crystallized. The observed miscibility behaviours showed some differences from those predicted by binary interaction model.     

Effects of relaxation time and zero shear viscosity on structural evolution of linear low‐density polyethylene in shear flow

The purpose of this work is to investigate the precursor formation and crystallization of four different types of linear low‐density polyethylene in shear flow. The aggregation of string‐like structure (precursor) in micrometer scale was observed by polarized optical microscopy. Although the existence of precursors accelerates crystallization, we find that it is unnecessary for the polymer fluid to possess crystalline structures at relatively high temperatures by wide‐angle X‐ray diffraction. The rotational rheometer result suggests that relaxation time is consistent with processes of the precursor formation observed at 120 °C, while zero shear viscosity affects the boundaries of their corresponding processing windows. According to these results, we propose a mechanism that the precursor formation consists of entanglement and relaxation stages. Entanglements preserve the ordered state of polymer chains, while they return to the initially disordered state during the relaxation stage. Under shearing, the polymer chain is oriented along the flow direction, and the degree of chain motion remains limited because the oriented parts are confined by the entanglements (characterized by zero shear viscosity η₀) acting as slip‐links. However, some chain motions and relaxation (characterized by terminal relaxation time τ) can still take place during this stage. Afterwards, the polymer chain becomes disordered and some entanglements disappear. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46053.     

Influence of shear in the crystallization of polyethylene in the presence of SWCNTs

It is well accepted that due to epitaxy matching, carbon nanotubes are good nucleating agent for linear polyethylene. We demonstrate that not only in the quiescent conditions but also at the relatively low shear rates the presence of single-walled carbon nanotube (SWCNT) accelerates the crystallization kinetics of polyethylene (PE). The influence of SWCNTs on the crystallization kinetics in the quiescent condition is followed with the help of rheological and differential scanning calorimetry studies. The influence of flow on the stretch of the polymer chain is probed using time-resolved small-angle X-ray scattering (SAXS) and is verified with the Deborah number. SAXS data indicates that the strong shearing conditions (shear rate > 50/s for 1 s) are requisite to form shish-kebab structure in the neat polymer. However, for the low shear (shear rate < 50/s for 1 s), the shish-kebab structure that arises due to chain orientation is enhanced in the presence of SWCNTs. The development of oriented structures in SWCNT/PE composites and their absence in the neat polymer under low shear rate indicates that the presence of SWCNTs plays a significant role in the chain orientation. Overall, the results manifest the influence of SWCNTs on chain relaxation of the polymer.     

Flow-induced crystallization of linear polyethylene above its normal melting point

The phenomenon of flow-induced crystallization was investigated using a linear polyethylene above its normal melting point flowing continuously in a Biconical Rheometer. It was found that the resin crystallized in the superheated state at rates which increased with increasing shear rate and decreasing temperature. A method of analysis of the temperature dependence of the various stages of flow induced crystallization is proposed. It deals with and attempts to explain the experimental fact that a higher viscosity enhances the rate of flow-induced crystallization in contrast to the effect of viscosity on the rate of quiescent crystallization. Some of the flow-induced crystallization samples were transparent and exhibited a high DSC thermogram “tail”.     

Spherulitic crystallization behavior of linear low‐density polyethylene

In this study the crystallization behavior of linear low‐density polyethylenes (LLDPEs) (ethylene‐α‐olefin copolymers) was studied by polarized light microscopy. A modified Hoffman‐Lauritzen (MHL) expression is proposed whereby the equilibrium melting temperature, T (T), is replaced with the melting temperature of the crystal stem is replaced with the maximum possible stem length, T. It successfully describes the crystalline spherulitic growth kinetics for both homogeneous and heterogeneous LLDPEs. In addition to regimes III and II, another regime (IM) was found in the high crystallization temperature range. Linear growth behavior of crystalline spherulites was observed in regime III, and nonlinear growth behavior was found in regimes II and IM. The basal surface free energy can be estimated from the short chain branching polydispersity (SCBP) for LLDPEs with excluded comonomers. Polym. Eng. Sci. 45:74–83, 2005. © 2004 Society of Plastics Engineers.     

Effects of carbon nanofibers on the crystallization kinetics of polyethylene oxide

Differential scanning calorimetry (DSC) was used to investigate the crystallization behavior of polyethylene oxide (PEO) and carbon nanofiber (CNF) filled PEO systems under non-isothermal experimental conditions. The dispersion and distribution of CNF of the composites were studied using scanning electron microscopy. Studies showed the uniform segregation of CNFs in PEO. Different crystallization kinetic models were used to study the dependence of crystal nucleation on the filler content. Modified Avrami analysis showed that PEO undergoes change of crystallization from 3-D to 1-D crystal while going from primary to secondary crystallization. The crystallization kinetic of PEO reversed at CNF loading higher than 1 wt% of PEO. Based on modified Avrami and the combined approach of Avrami and Ozawa, it is concluded that the CNF retards the crystallization of PEO at all CNF loading under study.     

Effects of additions of high-density polyethylene on the processability of linear low-density polyethylene

In considering the processing characteristics of typical linear low-density polyethylenes, it is very likely that the nature and amount of the “high-density” portion of the short chain branching distribution has a strong effect on rheological behavior. In this study, highdensity resins of varying molecular weight were blended into a linear low-density polyethylene base resin to determine the effect on viscosity, elasticity, and onset of haze. It was determined that it is not only the high-molecular-weight characteristics of the high-density portion but also the linear nature of the molecules that has a negative effect on processability. © 1993 John Wiley & Sons, Inc.     

II-I phase transformation of melt-crystallized oriented lamellae of polybutene-1 by shear deformation

Summary Modification II-I transformation of polybutene-1 was investigated on oriented lamellae, which were melt-crystallized in the temperature gradient. WAXS photograph of as-grown specimen showed prefered orientation of [110] to the lamellar axis, and after transformation it showed twelve point pattern due to dual orientation of hexagonal lattice. By shear deformation parallel or perpendicular to lamellar axis, WAXS photograph showed the appearance of six point pattern due to single orientation of hexagonal lattice, and also the enhancement of transformation rate. These were explained by the growth of hexagonal nuclei formed by the slip in the tetragonal crystal.     

Crystal structure changes during isothermal crystallization, cooling and heating of linear polyethylene

The dynamic process of crystal structure change during isothermal crystallization, cooling and heating of linear polyethylene with different molecular weight and polydispersity was followed by wide-angle X-ray diffraction (WARD) measurement. From the WARD data, variations of unit cell parameters a and b and changes in crystallinity were estimated. During isothermal crystallization, both cell parameters were found to decrease with time, suggesting that the crystal structure was becoming more perfect. With an increase in molecular weight or crystallization temperature, the rate of crystal perfection and the attainable crystallinity were found to decrease. This behavior can be explained by the formation of thicker lamellae, which probably have a lower degree of defects and a reduced surface-to-volume ratio in the crystals. Upon cooling and heating, the cell parameters appeared to contract and expand accordingly. The thermal contraction and expansion of parameter a is considerably larger than that of b, which probably results from the weaker intra-chain interactions along the a-axis, which is perpendicular to the spherulite growth direction.     

Influence of the method of mixing on homogeneity and crystallization kinetics of blends of linear and branched polyethylene

The influence of mixing method—solution and melt mixing—on the homogeneity and crystallization kinetics of a series of blends of single‐site materials of linear polyethylene and ethyl‐branched polyethylene was studied by differential scanning calorimetry. Data obtained for heats of melting and crystallization, melting and crystallization peak temperatures, and melting and crystallization temperature profiles were essentially the same for the samples obtained by the two mixing methods. The results obtained can be interpreted as indicating that melt mixing is capable of producing homogeneous melts of these relatively low molar mass polymers, given that solution mixing is considered to give perfectly homogeneous blends. The heat associated with the high temperature melting peak after crystallization at 125°C of the blend samples, obtained by the two preparation methods, was higher than that of the linear polyethylene included in the blends, suggesting that a part of the branched polyethylene crystallized at 125°C. The unblended branched polyethylene showed no crystallization at 125°C. Samples obtained by powder mixing showed independent crystallization and melting of the linear and branched polyethylene components. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1730–1736, 2004     

Molecular weight distribution and environmental stress cracking of linear polyethylene

Molecular weight distributions of both Phillips and Zieglertype high density polyethylenes were determined following fractionation of the polymer samples using a column elution technique. The Wesslau log normal distribution function was used to describe the distributions of the resins investigated. Resistance to environmental stress rupture of speciments cut from compression molded plaques of these samples was measured by the constant tensile loading procedure. Data are presented showing annealed resins with “broad” molecular weight distributions, characterized particularly by a quantity of low molecular weigt material and a high molecular weight “tail,” to have poorer stress crack resistance than samples having a “narrow” molecular weight distribution. Stress crack resistance of specimens quenched from the melt, however, tends to improve for “broad” distribution resins, while decrasing for those polyethylenes having a “narrow” molecualar weigt distribution. Differences in crystal structure are used to explain the physical bassis for these effects.     

Effects of thermal history on the peel strength of linear polyethylene

Peel strength of linear polyethylene–aluminum bonds is strongly affected by thermal history. Slow cooling from the melt resulted in 10 lb/in. peel strength; quenching, in 50–90 lb/in.; annealing of quenched bonds reduced the peel strength to 30 lb/in. Adhesion to aluminum was obtained by treating the substrates in a solution of vinyl triethoxysilane. Cohesive failure occurred regardless of thermal treatment. The changes in peel strength were not due to changes in strength of the bulk polymer. Rather, the volume of polymer deformed in peeling seemed to correlate with strength. No correlation was found between peel strength and degree of crystallinity or of transcrystalline orientation. Segregation resulting in a weak boundary layer seems to be the probable cause of the dependence of peel strength on cooling rate, and changes in the tie molecule configuration, the cause of loss of strength on annealing.     

Effects of the processing conditions and blending with linear low-density polyethylene on the properties of low-density polyethylene films

Effects of blending low-density polyethylene (LDPE) with linear low-density polyethylene (LLDPE) were studied on extrusion blown films. The tensile strength, the tear strength, the elongation at break, as well as haze showed more or less additivity between the properties of LDPE and LLDPE except in the range of 20–40% where synergistic effects were observed. The LLDPE had higher tensile strength and elongation at break than did the LDPE in both test directions, as well as higher tear strength in the transverse direction. The impact energies of the LLDPE and the LDPE were approximately the same, but the tear strength of the LLDPE was lower than that of LDPE in the machine direction. The comparative mechanical properties strongly depend on the processing conditions and structural parameters such as the molecular weight and the molecular weight distribution of both classes of materials. The LLDPE in this study had a higher molecular weight in comparison to the LDPE of the study, as implied from its lower melt flow index (MFI) in comparison to that of the LDPE. The effects of processing conditions such as the blow-up ratio (BUR) and the draw-down ratio (DDR) were also studied at 20/80 (LLDPE/LDPE) ratio. Tensile strength, elongation at break, and tear strength in both directions became equalized, and the impact energy decreased as the BUR and the DDR approached each other.