Crystallization Behavior Of Polyethylene Research Articles

Investigation of Crystallization and Relaxation Effects in Coarse-Grained Polyethylene Systems after Uniaxial Stretching.

In this study, we investigate the thermo-mechanical relaxation and crystallization behavior of polyethylene using mesoscale molecular dynamics simulations. Our models specifically mimic constraints that occur in real-life polymer processing: After strong uniaxial stretching of the melt, we quench and release the polymer chains at different loading conditions. These conditions allow for free or hindered shrinkage, respectively. We present the shrinkage and swelling behavior as well as the crystallization kinetics over up to 600 ns simulation time. We are able to precisely evaluate how the interplay of chain length, temperature, local entanglements and orientation of chain segments influences crystallization and relaxation behavior. From our models, we determine the temperature dependent crystallization rate of polyethylene, including crystallization onset temperature.

Influence of hydrogenated petroleum resin on the crystallization behavior of polyethylene for biaxially stretched films

Influence of hydrogenated petroleum resin on the crystallization behavior of polyethylene for biaxially stretched films

Effect of Ag nanowires on crystallization behavior of polyethylene glycol/expanded vermiculite composite phase change material

The thermal conductivity enhancement fillers usually have significant effect on the crystallization behavior of composite phase change materials (CPCMs), thereby affecting heat release performance. In this work, the non-isothermal crystallization of polyethylene glycol (PEG)-Ag nanowires/expanded vermiculite CPCMs (PAE CPCMs) was studied in detail to clarify the effect of Ag nanowires on crystallization behavior of PAE CPCMs. The results indicated that the Ag nanowires inhibited crystal growth of PEG because PAE CPCMs showed longer half crystallization time, lower crystallization rate and higher value of F(T) than PEG during non-isothermal crystallization, which could quantitatively explain the degradation of heat release performance of PAE CPCMs caused by surfaces interaction of Ag nanowires-induced decline of crystallinity.

Study on the crystallization behaviour of polyethylene grafted onto carbon nanotubes using molecular dynamics simulation

ABSTRACT The mechanical property of polymer can be effectively changed by grafting polymer onto the surface of nano-fillers. In this paper, the crystallization behaviour of polyethylene grafted on carbon nanotubes with different grafting densities have been explored in detail by using molecular dynamics simulation. During the crystallization process, the crystallinity, the site order parameter, the mean square displacement, the radius of gyration and the bond-orientational order parameter were recorded. The results show that higher grafting density results in stronger crowding effect and weaker mobility of molecular chain. The ultimate crystallinity of the polymer increases with the increase of grafting density. The orientation of crystal bond indicates that the polymer with higher grafting density tends to be perpendicular to the surface of nano-fillers. Those findings in this work are of great values for further exploring reinforcement mechanism and also the microscopic mechanism of crystallization behaviours of polymer nanocomposites.

Non-Isothermal Crystallization Behavior and Thermal Properties of Polyethylene Tuned by Polypropylene and Reinforced with Reduced Graphene Oxide

This research work is the first to report thermal stability, heat deformation resistance, and crystallization behavior of a Polyethylene (PE)-based biphasic polyolefin system reinforced with Reduced Graphene Oxide (RGO), which was obtained through Graphene Oxide (GO) chemical reduction. Polypropylene (PP) represented the polymeric dispersed phase. A strategic PE/PP/RGO manufacturing procedure was employed to thermodynamically localize RGO at the PE/PP interface, as confirmed by Transmission Electron Microscopy (TEM), bringing a uniform micro phase dispersion into the macro phase. In addition, studies of PE non-isothermal crystallization kinetics indicated that the morphology tunable micro phase and the nanolayered RGO promoted a nucleation-controlled PE crystallization, which was supported by Polarized Light Optical Microscopy (PLOM). This, together with fine morphology, justified the remarkable enhancement registered for the ternary system’s thermal stability and heat deformation resistance. Different filler loads were employed, with weight fractions of 2% and 4%. It was observed that the former, being better exfoliated and more homogeneously distributed at the PE/PP interface than the latter, led to a more improved PE crystallization, alongside a greater ternary system’s thermal properties. Moreover, the thermal stability of PE/PP reinforced with 2% of RGO was even higher than that of virgin PP, while their heat deformation resistance values were found to be similar. Therefore, this unique outcome provides industries, such as the energy and automotive sectors, with the opportunity to substitute PP-rich products with those mostly comprised of a cheaper, more abundant, yet performant PE.

Phase structure and crystallization behavior of polyethylene in its blends with cis-1,4-butadiene rubber

Phase structure and crystallization behavior of polyethylene (PE) in its blends with cis-1,4-butadiene rubber (BR) at different blend ratios and sample preparation conditions were studied. The PE is finely dispersed in the BR matrix. For samples hot pressed at 145 °C, circular PE microdomains with randomly oriented PE lamellar aggregates were produced. The domain size and number increase with increasing PE content. When the PE content is over 10 wt%, most of the PE domains impinged each other. The separated PE domains are connected by PE stripes with parallel arranged lamellar aggregates. For samples hot pressed at 140 °C, elongated PE microdomains with oriented PE lamellar aggregates were obtained due to the shear flow. The crystallization of PE in the blends depends on the phase structure. Confined crystallization of PE occurs in small microdomains at relatively low temperature. With the increase of domain size, the crystallization ability of PE increases while the confined crystallization decreases.

Effect of Rapid Compression on the Crystallization Behaviour of Polyethylene

The crystallization behaviour of PE samples with different molecular weight was investigated through two different processes, concerning rapid compression (RC) and quench cooling (QC). The scanning electron micrographs, wide angle X-ray diffraction and Raman results showed that all the recovered samples were composed of spherulites. For the QC process, the effect of molecular weight on the spherulites morphologies was slight, but for the RC process, with increased molecular weight (MW), the growth rate as well as the final average diameter of the spherulites decreased remarkably. Additionally, for samples with the same MW, the mass fraction of the rigid amorphous phase of RC samples was lower. Furthermore, the crystallinity distributions in the longitudinal and latitude directions of the RC samples were both more uniform than those of the QC samples, implying that in the RC process, the pressure distribution was quite uniform and there was no pressure decay.

Influence of Branch Incorporation into the Lamella Crystal on the Crystallization Behavior of Polyethylene with Precisely Spaced Branches

Depending on the degree of short chain branch (SCB) incorporation, the crystallization behavior and resultant crystalline structure drastically change in polyethylene with precisely spaced branches. In polyethylene with hexyl branches precisely spaced on every 21st carbon (HB21), only crystallization mediated by a transient hexagonal phase without incorporation of the SCB was observed. On the other hand, in polyethylene with ethyl branches precisely spaced on every 21st carbon (EB21), crystallization behavior was strongly dependent on the crystallization temperature. A thin lamella was formed through crystallization mediated by a hexagonal phase and no thickening occurred at 5–8 °C, while thickening of the transient hexagonal lamellae occurred at 10–15 °C, and one SCB seemed to be incorporated into a crystal stem. At 17 °C, no thickening of the hexagonal phase occurred and a hexagonal phase with sufficient lamella thickness was directly formed from the melt. At 21–28 °C, crystallization mediated by hexago...

Different influences of nucleating agents on crystallization behavior of polyethylene and polypropylene

The crystallization behavior of polyethylene (PE) and polypropylene (PP), including the neat ones and the ones nucleated with the same nucleating agent (NA), was studied by DSC. It was found that the nucleating agent decelerated the PE nonisothermal crystallization process. NA did enhance the nucleating rates for both PE and PP, but the linear growth rate dominated the volumetric growth rate for PE, and the volumetric growth rate dominated the overall crystallization rate. That is why PE nucleated with NA had a slower overall crystallization rate than the neat one.

Comparison study of PE epitaxy on carbon nanotubes and graphene oxide and PE/graphene oxide as amphiphilic molecular structure for solvent separation

Carbon nanotubes (CNTs) and graphene nanosheets, as one-dimensional and two-dimensional carbon-based nanomaterials respectively, have different abilities to induce the polymer crystallization. In this study, hybrid materials, polyethylene (PE) decorating on CNTs and graphene oxide (GO), were prepared by a facile and efficient method using supercritical carbon dioxide (SC CO2) as anti-solvent. And the morphology and crystallization behavior of PE on CNTs and GO were investigated by transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectra, wide angle X-ray diffraction, and differential scanning calorimetry. Although both CNTs and GO could act as nucleating agents to induce PE epitaxial growth, CNTs were decorated by PE lamellar crystals forming nanohybrid “shish-kebab” (NHSK) structure, whereas GO sheets were only decorated with petal-like PE crystals. The varying morphologies of the nanohybrids depend on the PE epitaxy and the interactions between polymer chains and substrates. High surface curvature and the perfect ordered crystal structure of CNTs make PE crystals periodically grow on CNTs. While PE crystals grow and form multiple orientation-lamellae on GO due to the lattice matching and complex interactions between PE chains and GO. In addition, our experimental results show an interesting and evident stratification phenomenon for the PE/GO hybrid material, implying that GO decorated by PE have a screening function for the solvents. We anticipate that this work can widen the area of functionalization of carbon-based nanomaterials with a controlled means by an environmentally benign method, which are important for the functional design in nanodevice applications.

Comparison Study of Morphology and Crystallization Behavior of Polyethylene and Poly(ethylene oxide) on Single-Walled Carbon Nanotubes

In this work, we provided a comparison study of morphology and crystallization behavior of polyethylene (PE) and poly(ethylene oxide) (PEO) on single-walled carbon nanotubes (SWNTs) with assistance of supercritical CO(2). The resulting polymer/SWNT nanohybrids were characterized by transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectra, wide-angle X-ray diffraction, and differential scanning calorimetry. SWNT small bundles were decorated by PE lamellar crystals, forming nanohybrid "shish-kebab" (NHSK) structure, whereas SWNTs were only wrapped by a thin amorphous polymer coating in the case of PEO. The varying morphologies of the nanohybrids were found to depend on the molecular conformation and the interactions between polymer chains and SWNTs. Nonisothermal experiments showed that SWNTs provided heterogeneous nucleation sites for PE crystallization, while the NHSK structure hindered polymer chain diffusion and crystal growth. Also, SWNTs played antinucleation effect on PEO. In addition, the formation mechanism analysis indicated that PE chains preferred to form a homogeneous coating along the tube axis before proceeding to kebab crystal growth. The purpose of this work is to enlarge the area of theoretical understanding of introducing precisely hierarchical structures on carbon nanotubes, which are important for functional design in nanodevice applications.

Crystallization behavior of polyethylene on silicon wafers in solution casting processes traced by time-resolved measurements of synchrotron grazing-incidence small-angle and wide-angle X-ray scattering

Crystallization behavior of polyethylene (PE) on silicon wafers in solution casting processes has been successfully traced by time-resolved grazing-incidence small-angle and wide-angle X-ray scattering (GISWAXS) measurements utilizing synchrotron radiation. A p-xylene solution of PE kept at ca. 343 K was dropped on a silicon wafer at ca. 298 K. While the p-xylene evaporated naturally from the dropped solution sample, PE chains crystallized to be a thin film. Raman spectral measurements were performed simultaneously with the GISWAXS measurements to evaluate quantitatively the p-xylene the dropped solution contained. Grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns indicated nucleation and crystal growth in the dropped solution and the following as-cast film. GIWAXS and Raman spectral data revealed that crystallization of PE was enhanced after complete evaporation of the p-xylene from the dropped solution. The [110] and [200] directions of the orthorhombic PE crystal became relatively parallel to the wafer surface with time, which implied that the flat-on lamellae with respect to the wafer surface were mainly formed in the as-cast film. On the other hand, grazing-incidence small-angle X-ray scattering (GISAXS) patterns implied formation of isolated lamellae in the dropped solution. The lamellae and amorphous might alternatively be stacked in the preferred direction perpendicular to the wafer surface. The synchrotron GISWAXS experimental method could be applied for kinetic study on hierarchical structure of polymer thin films.

On the time for fold surfaces to order during the crystallization of polyethylene from the melt and its dependence on molecular parameters

New experiments underpin the interpretation of the basic division in crystallization behaviour of polyethylene in terms of whether or not there is time for the fold surface to order before the next molecular layer is added at the growth front. For typical growth rates, in Régime II, polyethylene lamellae form with disordered {001} fold surfaces then transform, with lamellar thickening and twisting, towards the more-ordered condition found for slower crystallization in Régime I, in which lamellae form with and retain {201} fold surfaces. Several linear and linear-low-density polyethylenes have been used to show that, for the same polymer crystallized alone or in a blend, the growth rate at which the change in initial lamellar condition occurs is reasonably constant thereby supporting the concept of a specific time for surfaces to attain the ordered {201} state. This specific time, in the range from milliseconds to seconds, increases with molecular length, and in linear-low-density polymer, for higher branch contents.

Simultaneous wide‐angle X‐ray diffraction and differential scanning calorimetry analysis of the melting and recrystallization behavior of polyethylene and isotactic polypropylene containing cyclopentane units in the main chain

AbstractThe melting and crystallization behavior of polyethylene and isotactic polypropylene containing 1,2‐ or 1,3‐disubstituted cyclopentane units in the main chain has been studied with simultaneous wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry. For the ethylene‐based copolymers, the position of a reflection peak in the WAXD patterns shifts to a low angle with the increasing acquired temperature. The temperature dependence on the axial length of the crystal lattice is more marked in the copolymers forming orthorhombic crystals (containing 1,2‐cyclopentane or 5.6 mol % 1,3‐cyclopentane units) than in those forming hexagonal crystals (containing 8.1 mol % 1,3‐cyclopentane units). For the isotactic propylene‐based copolymers, the position of the reflection peaks in the WAXD patterns is independent of the acquired temperature. The proportion of the γ form in the copolymer containing the 1,2‐cyclopentane units is higher than that in the copolymers containing the 1,3‐cyclopentane units. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1457–1465, 2004

Rotationally molded polyethylene: Structural characterization by x‐ray and microhardness measurements

AbstractRotationally molded polyethylene (PE) blended in two ways (turbo blending and extrusion) with nucleating and nonnucleating pigments is structurally characterized by wide‐ and small‐angle x‐ray scattering (WAXS and SAXS, respectively), DSC and microhardness measurements. Morphological observations are performed by polarized light microscopy. The melting temperature and the degree of crystallinity (from both DSC and WAXS) remain essentially constant regardless of sample preparation and type of pigment. The same holds for the crystal sizes from WAXS and the lamella thickness from SAXS. Only the values of microhardness depend on the type of pigment, increasing about 10% when a nucleating type is used. The almost constant values of these properties, contrasting to the spherulitic morphology, are explained by the fact that the processing conditions in rotational molding are very favorable for crystallization. As a consequence, optimal crystalline structure is achieved, which masks significantly the effect of pigments and blending conditions on the crystallization behavior of polyethylene. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 116–124, 2001

Morphological studies of PE crystallized between iPP and PTFE oriented films

The crystallization behavior of polyethylene (PE) between oriented isotactic polypropylene (iPP) and poly(tetrafluoroethylene) (PTFE) substrates in sandwich triple layered samples at various crystallization conditions was studied by means of transmission electron microscopy and electron diffraction. It is well known that both iPP and PTFE oriented substrates are active nucleation agents for PE, but result in different crystallization manners of PE. While the iPP substrate leads to an epitaxial crystallization of PE in a way, with the molecular chain directions of both polymers ±50° apart, an oriented overgrowth of PE on the PTFE substrate with parallel chains of both polymers is identified. By using sandwich samples with the PE in between the two different substrates, i.e. iPP and PTFE, the nucleation efficiency of both polymers for PE can be compared. The results indicate that at high undercoolings, the iPP and PTFE oriented crystals have about the same nucleation ability onto PE, but at lower undercoolings, the PE crystallizes all on the PTFE side. This indicates unambiguously that the nucleation of PE on the PTFE substrate starts at a higher temperature than that on the iPP oriented substrate.

Morphology of polyethylene blended with thermotropic hydroxyethyl cellulose acetate

Morphology of low-density polyethylene (PE)/thermotropic hydroxyethyl cellulose acetate (HECA) blends, and the melting and crystallization behavior of PE in the blends were studied. The “sea island” morphology was observed in PE/HECA blends. The process of melting and crystallization of PE in blends was independent of the HECA fraction when the PE fraction was larger than 50 wt %. When the PE fraction is smaller than 20 wt %, however, multiple crystallization was observed in the low temperature region. HECA was incompatible with PE crystals in the blends, but partially compatible with the amorphous part of PE. HECA could exist between the PE lamellae in PE spherulites, and concentric ring morphology was observed in spherulites. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1285–1291, 1998

Crystallization behaviour of polyethylene and i-polybutene-1 blends

The mutual influence of the components on the crystallization behaviour of polyblends, namely, isotactic polybutene-1 (PB) with low-density and high-density polyethylene (LDPE and HDPE), has been studied using techniques such as differential scanning calorimetry, infra-red spectroscopy, wide-angle X-ray diffraction, scanning electron microscopy, etc. Each component in the blend is observed to crystallize independently. There is phase separation and incompatibility, as shown from tensile properties and scanning electron microscopic observation of the fracture surface of the blend. For HDPE-PE blends (<30% HDPE), unusual form I′ crystals of PB are observed along with the usual form II.

Effects of Blending on the Melting and Crystallization of Polyethylene ahd Polypropylene under High Pressure

The melting and crystallization behavior of polyethylene (PE) and polypropylene (PP) blends under high hydrostatic pressures up to 500 MPa was studied by differential thermal analysis (DTA). These high pressure crystallized blends were also investigated at 1 atm by DTA, X-ray diffraction, and electron microscopy. The melting temperature Tm of PE in the blend under high pressure was determined from the peak temperature of a DTA curve of melting. The Tm versus pressure curve for PE in the blend shifts to the high pressure side with decreasing PE content. In the DTA curve of melting at 1 atm for a blend crystallized under 450 MPa, an endothermic peak that could not be assigned to the melting of folded PE chain crystals (FCC) appeared on the low temperature side of the melting peak for extended chain crystals (ECC), indicating that small PE crystals different from FCC were formed in the molten PP during crystallization. The X-ray diffraction pattern of this blend showed that no eutectic crystallization occurs even in the isothermal crystallization under high pressure. Electron microscopic observations of the fracture surface of the high pressure crystallized blend showed a clear boundary between the PE and PP domains, suggesting that no interdiffusion of the two polymers occurs.