Polyethylene Crystals Research Articles

Hierarchical Self‐Assembly of Polyethylene Midblock Copolymers in Hot Steam: Key Role of Crystalline and Topological Structure

AbstractA series of linear‐shaped, H‐shaped, and dumbbell‐shaped semi‐crystalline triblock copolymers (E1, E2 and E3) is prepared, which contains polyethylene (PE) midblock and poly(methyl methacrylate) (PMMA) endblocks. The molecular design in this study is focused on the variation of the length and number of PMMA chains, with a fixed PE midblock (Mn = 16 000 g mol−1) and approximate composition (wtPE%:wtPMMA ≈ 1:1), upon which the hierarchical self‐assembly behavior in solvent vapor annealing (SVA) process is investigated. The dumbbell‐shaped E3, which contains the most and shortest PMMA chains, shows a drastic reduction of the Tm and crystallinity of PE midblock. It is attributed to the plasticization effects by the dumbbell architecture, which disturbed the chain folding of PE block. The variation of the topological structure of copolymers not only accelerates the transformation of morphologies from spherical to network, but also leads to the formation of new morphologies. After repeated annealing, both E1 and E2 form crossed network, columnar and flower‐like morphologies by higher‐level assembly from preassembled vermicular micelles. By comparison, the dumbbell‐shaped E3 assembled into lamellar or stacks of “lamellar” morphologies. The hierarchical self‐assembly can be governed by the combination of the driving forces of PE crystallization and the topologies structure.

Metallocene encapsulated within a hybrid silica-polystyrene support

A series of heterogeneous metallocene catalysts were prepared by encapsulating Cp2ZrCl2 within silica-polystyrene hybrid support. Immobilization was performed through a non-hydrolytic sol–gel process in which polystyrene (PS) dissolved in toluene and metallocene (1% by wt) were combined. The effect of PS (0 to 5% by wt) and silicon tetrachloride (0.5 to 1.6 [TEOS]/[SiCl4]) was evaluated in terms of catalyst activity in ethylene polymerization with methylaluminoxane as the cocatalyst. According to FTIR measurements, an increase in PS content promoted a decrease in (SiO)6 content, which indicated the formation of more compact structures. Furthermore, homogeneous distributions of polymer and silica phases were observed both in bulk and on the support surface. The results showed very small surface areas (< 20 m2 g−1), probably because PS would fill the silica pores. The catalytic activity for ethylene polymerization was shown to decrease with increasing polystyrene content within the catalyst support of low surface area. A positive correlation was found between the (SiO)6 content and catalyst activity. The increase in the PS content was followed by an increase in the degree of polyethylene crystallization, which ranged from 77 to 84%.

Molecular dynamics simulations of stretch‐induced crystallization in layered polyethylene

Molecular dynamics simulations of stretch‐induced crystallization in layered polyethylene

Temperature dependency of cavitation in impact copolymer polypropylene during stretching

Cavitation in the impact copolymer polypropylene (ICP) and its corresponding homo-isotactic polypropylene (homo-iPP) during stretching was investigated using ultra-small angle X-ray scattering technique. Different from the iPP showing a suppressed cavitation around yielding at high deformation temperature (Td), the ICP presented a more intensive cavitation at elevated Td. This unusual Td dependency of cavitation around yielding observed in ICP was suggested to be caused by the changes of the stability of the rubber particles dispersed in iPP matrix. The rubber particles exhibit a multi-phase core-shell structure which contains crystallizable polyethylene segments in the core layer, the amorphous polymer of ethylene-propylene random copolymer in the inner shell layer, and the crystallizable polypropylene segments in the outer shell layer. The softening and melting from the inner shell to the core and the outer shell took place as the Td increased. It can be envisaged that the fraction of unstable regions inside the rubber particles and the debonding between the iPP matrix and the dispersed phase gradually increased providing more nucleation sites for cavities at high Td, a more intensive cavitation was thus observed.

Polymer defect engineering – conductive 2D organic platelets from precise thiophene-doped polyethylene

Synthesis and crystallization of polyethylene with precisely positioned thiophene groups were used to produce polymer crystals with a conductive surface.

Graphite-LDH hybrid supported zirconocene for ethylene polymerization: Influence of the support on the crystallization kinetics and thermal stability of polyethylene

A hybrid nanomaterial, low crystallinity graphite-layered double hydroxide, was used as a support for zirconocene catalyst. The hybrid nanomaterial graphite-NiAl layered double hydroxides (G/LDH) were synthesized by the co-precipitation method. The synthesized nanomaterial was used as a support for zirconocene catalyst. The polymerization reactions were carried out for ethylene polymerization. The synthesized polyethylene (PE) was analyzed by using differential scanning calorimetry (DSC), Crystallization analysis & fractionation (CRYSTAF), and thermogravimetric analysis for its thermal and microstructural characteristics. The crystallization kinetics were studied by the Ozawa and combined Ozawa and Avrami models. It was found that the presence of G/LDH from the catalyst support nucleated the PE crystallization and shifted the crystallization onset temperature to a higher value. However, the overall crystallization rate was slowed by the presence of the nanomaterial due to growth impingement. Moreover, the PE synthesized by G/LDH supported catalyst possessed higher thermal stability than PE synthesized by unsupported zirconocene catalyst. The integral isoconversional method was used to evaluate the activation energy of thermal degradation and crystallization kinetics. The degradation mechanism was validated by the application of the integral master plot technique. The degradation mechanism of neat PE resembled phase boundary controlled mechanism second and third-order, i.e. (R2, R3), while PE synthesized by G/LDH supported catalyst had a shift in degradation mechanism from (R2, R3) to a diffusion-limited mechanism at the later stages of degradation.

Flow-Induced Crystallization in Polyethylene: Effect of Flow Time on Development of Shish-Kebab.

The flow-induced formation and relaxation of the representative oriented shish-kebab structure were studied with synchrotron small-angle X-ray scattering (SAXS) method. The flow duration was varied from 2 to 6 s at an identical strain rate to reveal the effect of flow time on stability and dimension of formed shish. It was found that the short flow time of 2 s was able to generate shish during flow, which, however, relaxed during the isothermal process after cessation of flow. An increase in flow time can improve the shish stability and the long flow time of 6 s can generate the stable shish that nucleate the growth of kebab lamellae. In addition, the quantitative analysis of SAXS results showed that with increasing flow time from 2 to 6 s, the shish length increased from 242 to 574 nm, while the shish diameter remained around 34 nm. This detailed information of the formed shish-kebab structure can be used to shed light on their evolution that occurred during flow from 2 to 6 s, where shish grew at a longitudinal speed of around 80 nm/s, and there was an improvement in the stability and nucleation capability for kebab lamellae.

Bulk rheology and surface tribo-rheometry toward the investigation of polyisobutylene migration in model and recycled multilayer agricultural films

This paper is dealing with the polyisobutylene (PIB) migration phenomenon through the post-consumer agricultural waste multilayer films based on four linear low-density polyethylene (LLDPE) matrices. Through this work, a bridge between shear, elongational rheology, and tack surface properties was revealed on both model and recycled blends. Hereto, effects of aging time and temperature were investigated and rationalized depending on the short- and long-chain branching in LLDPE matrices as well their polyethylene (PE) crystallization. The linear viscoelastic properties were influenced despite the small PIB amounts. This migration influenced also the slippage in steady flow regime. It was also demonstrated that transient uniaxial extensional properties are very sensitive in the presence of PIB, which seems to hold back the PE strain hardening properties. Therefore, the axial force and the surface friction coefficient were determined and discussed in correlation with bulk rheological findings. Hence, the obtained results contributed to unveiling new insights on the physical mechanisms governing the PIB migration with or without fillers to inhibit its migration in their recycled films.

Functionally tuned nanolayered graphene as reinforcement of polyethylene nanocomposites for lightweight transportation industry

System integration and multifunctionality requirements of high-performance materials in the transportation sector are pushing manufacturers to search for advanced functional polymers that are easy to recycle, cost-effective and not energy intensive. This study reports, for the first time, an advanced and lightweight functional nanocomposite material with higher thermal deformation resistance and thermo-mechanical properties than those of conventional polyolefins used in the aforesaid sector. This unique material consists of a biphasic polyolefin system, with Polyethylene (PE) being the macro phase and Polypropylene (PP) being the micro phase, and a novel surface-active nanostructured functional graphene, which thermodynamically co-locate itself at the interface of the PE/PP binary system, thanks to our proposed strategic mixing mechanism. It is proven that the targeted interfacial localization of the novel functional nanolayered graphene promoted a nucleation-controlled PE crystallization, as demonstrated by optical microscopy and studies on macro phase non-isothermal crystallization kinetics. This justifies the remarkable enhancement of nanocomposite’s thermo-mechanical properties, leading to the potential use of this new functional material as a lightweight thermoplastic olefinic nanocomposite in the above-mentioned industrial sector. Lastly, the outcome of this study opens new avenues in the applications of advanced, lightweight, and functional polyolefin-based materials for diverse applications, increasing the environmental footprint.

Effect of Chain Configuration on Thermal Conductivity of Polyethylene—A Molecular Dynamic Simulation Study

Stretched polyethylene (PE) fibers are found to have super high thermal conductivity, while the bulk of polyethylene is usually thermal insulating even for those with high crystalline degree. A molecular dynamic simulation is deliberately carried out to examine the relationship between chain configuration and thermal conductivity of polyethylene. In this simulation study, independent and interacting PE chains being stretched are compared with the aim to find out the effect of stretching on thermal conductivity of PE. Various crystallization conditions for PE bulk are considered. It is found that heat transports predominately along the covalent chain rather than across chains in PE crystals. Our simulation study helps to understand experimental findings on thermal conductivity of PE at different states. We also predict that amorphous PE may be super thermally conductive if chains are strictly stretched along heat flux.

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.

Effect of Hexagonal Boron Nitride Nanoplatelet on Crystal Nucleation, Mechanical Behavior, and Thermal Stability of High‐Density Polyethylene‐Based Nanocomposites

AbstractHexagonal boron nitride nanoplatelets (BNNP) significantly enhance the mechanical and thermal properties of high‐density polyethylene. In order to ensure superior dispersibility of BNNP, two different techniques, colloidal solution, and solvent blending, are used for fabricating nanocomposites. Compared to solvent blending, the colloidal solution‐based fabrication technique has superior dispersion capabilities for nanofillers in the PE matrix, which helps in improving the thermal stability and crystallinity in synthesized nanocomposites. A significant increase in the rate of crystallization of polyethylene (PE) is reported at 1% weight fraction of BNNP, which further increases with the increase in weight percentage of BNNP. BNNP acts as a new source of nucleating sites for crystallization in PE that also reduces the size of secondary structures spherulites. Enhanced tensile strength and Young’s modulus of BNNP/PE nanocomposites are reported with the increase in weight contribution of BNNP in the range of 1% to 5%.

DFT study of n-alkyl carboxylic acids on oxidized aluminum surfaces: From standalone molecules to self-assembled-monolayers

We report on a systematic density-functional-theory (DFT) study of adsorption of n-alkyl carboxylic acids (CA) on oxidized aluminum surfaces, where we address the roles of the adsorption mode, molecular coverage, tilt angle of alkyl chain, and alkyl chain size—from CA-2 to CA-18, where the suffix represents the number of C atoms in the molecule—on the stability of the formed CA monolayers. Adsorption was modeled on two models of the oxidized aluminum surface: both consist of a hydroxylated ultrathin-oxide film on top of Al(111). The two models differ in OH coverage, types of adsorption sites, and their lateral distribution. Two different adsorption modes were considered: (i) plain adsorption mode, where the CA binds to the surface with H-bonds, and (ii) acid-base condensation adsorption mode, where CA replaces a surface OH group by forming a water molecule as a co-product. Carboxylates bind considerably stronger to the surface than intact CAs, but due to bond-breaking energy cost, the adsorption free energy of the condensation mode is by about 0.15 eV less exergonic than that of the plain mode. While on the fully hydroxylated surface we only identified monodentate bonded carboxylates, bidentates are by about 0.7 eV more stable than monodentates near OH vacancies. Molecular packing within the monolayer is driven by lateral interchain interactions and the optimum tilt angle is achieved when the interchain distances are similar to those in the polyethylene crystal. This implies that the tilt angle depends on the coverage, i.e., it decreases with increasing coverage. At full monolayer coverage, the tilt angles are found to be about 40°–50° vs. the surface-normal, depending on the oxide/hydroxide composition on Al, and do not depend on chain size for longer chains (e.g., for CA-8 and beyond). The adsorption energy can be decomposed into the headgroup–surface and lateral interchain components, the former being largely independent on the chain size and the latter being proportional to the number of C atoms in the chain. Consequently, the adsorption is stabilized by about 1 eV/molecule at full monolayer coverage as passing from CA-2 to CA-18. Finally, we propose a simplified way to calculate the vibrational contribution to the free energy of the adsorbed monolayers.

Thermally reduced graphene/polyethylene nanocomposites: effects of graphene on isothermal and nonisothermal crystallization of polyethylene

The crystallization behavior of polyethylene/thermally reduced graphene (PE/TRG) nanocomposites prepared via solvent blending is investigated using a differential scanning calorimeter, and results are compared with PE/carbon black (CB) composites. The effects of TRG and CB concentrations on the crystallization process are studied under isothermal and dynamic conditions. The Avrami and modified Avrami equations provided excellent fits to isothermal and dynamic crystallization kinetics data, respectively. The TRG nanosheets acted as nucleating agents during crystallization attributed to substantial decrease in crystallization half time at higher TRG concentrations. The reduced surface energy of the nanocomposites with incorporation of TRG further confirmed its nucleating behavior.

Crystallization rates of moderate and ultrahigh molecular weight polyethylene characterized by Flash DSC measurement

AbstractWe compared crystallization rates in three bulk commercial polyethylene samples separately consisting of two ultrahigh molecular weights and one moderate molecular weight. The lower‐end temperature of isothermal crystallization was expanded by means of Flash DSC measurement in order to approach the practically fast crystallization region. The results showed that, in the whole temperature range of our observations, ultrahigh molecular weights make polyethylene crystallization faster than moderate molecular weights. This behaviour is opposite to the general trend in linear polymers. In addition, ultrahigh molecular weights make polyethylene crystals resist fast heating before melting more than moderate molecular weights, due to high entanglement around the crystals. We discuss the possible mechanism of fast crystallization in ultrahigh molecular weight polyethylene, which is beneficial for prosthesis uses. © 2019 Society of Chemical Industry

Revisiting flow-induced crystallization of polyethylene inversely: An in situ swelling SANS study

The inherent mechanisms of flow-induced crystallization (FIC), especially the amorphous formation, have been revisited inversely, via in situ swelling small angle neutron scattering (SANS). The neutron contrast between crystalline and amorphous phases of crosslinked high-density polyethylene was strengthened by deuterated o-xylene-d10. Thereby, the morphological evolutions and kinetics of various FIC samples (imposed initial strain: 0.6, 0.9, 1.4, and 2.3) during the swelling process were obtained simultaneously. The half time for swelling to reach equilibrium decreases from ca.39100 s–4500 s for S0.0 to S2.3. Moreover, the evolutions of the expanded long period (LP) are similar to the swelling kinetics. Interestingly, the increment LPt-LP1 increases monotonically from 1.55 nm to 4.32 nm. However, the expanding ratio (LPt/LP1) first increases from 1.05 to 1.13 for S0.0 to S1.4, then decreases to 1.11 for S2.3. Combining ex situ small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) results, current work demonstrates that both crystalline degree and different morphology contribute to the influences of swelling. It suggests that the generated amorphous networks triggered by different strain were endued with various inherent structure and characters, and the nature of amorphous phase can severely affect the swelling behavior.

Nanoparticle Organization by Growing Polyethylene Crystal Fronts.

We investigate the crystallization-induced ordering of C18 grafted 14 nm diameter spherical silica nanoparticles (NPs) in a short chain (Mw = 4 kDa, ĐM ≈ 2.3) polyethylene and a commercial high-density polyethylene (Mw = 152 kDa, ĐM ≈ 3.2) matrix. For slow isothermal crystallization of the low molecular weight matrix, the NPs segregate into the interlamellar regions. This result establishes the generality of our earlier work on poly(ethylene oxide) based materials and suggests that crystallization can be used to control NP dispersion across different polymer classes. The incompatibility between the particles and the matrix in the Mw = 152 kDa results in a competition between filler organization and filler agglomeration. The mechanical properties improve due to the addition of NPs and are further enhanced by particle organization, even for the case of the macrophase-separated mixtures in the Mw = 152 kDa matrix. In contrast, dielectric behavior is strongly affected by the scale of NP organization, with the lower molecular weight matrix showing more significant increases in permittivity due to the local scale of NP ordering.

The crystallization and rheological behaviors of the ultrahigh molecular weight polyethylene swollen by petrolatum

In this paper, petrolatum was utilized as a swelling agent to improve the processability of ultrahigh molecular weight polyethylene (UHMWPE), and the microstructure, crystallization behaviors and rheological behaviors of these swollen UHMWPE were investigated. Petrolatum exhibits strong swelling ability and remarkably improves the chain mobility and processability of UHMWPE. With the increase of petrolatum concentration, more and more petrolatum molecules penetrate into the entangled chain networks of UHMWPE, and the enhanced chain disentanglements accelerate the crystallization of UHMWPE, strengthen the deformation breakage resistance of UHMWPE and make the chain segments of UHMWPE relax more quickly.

Structural evolution of stretch deformed HDPE/RGO nanocomposites: An in-situ synchrotron SAXS and WAXD study

We have reported that the incorporation of reduced graphene oxide (RGO) can induce epitaxial crystallization of high density polyethylene (HDPE) on RGO surfaces and then enhance mechanical properties of HDPE/RGO nanocomposites, but the underline intensifying mechanism remains unclear. Herein, to further unveil the influence of epitaxial crystallization imposing on the improvement of mechanical properties, the structural evolution of HDPE/RGO nanocomposites during the stretching process is explored as a function of RGO content by in-situ synchrotron wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. It has shown that the introducing of RGO apparently retards the structural evolution in the tensile deformation and the hysteresis is accentuated dramatically with increasing RGO content. The epitaxial growth of HDPE upon RGO surfaces forms large sized crystals and the amount of epitaxial crystals enlarges with RGO content. The coupling of the remained existence of large sized crystals and the smaller density after stretching demonstrates that the interplay between RGO and large amount of epitaxial crystals can undergo the strong stress and thus hampers and delays the structure change and crystal fragmentation during the tensile deformation of HDPE/RGO nanocomposites. The present results not only makes clear that it is the epitaxial crystallization stemming from interfacial interaction between HDPE and RGO that plays the major role to delay the structural evolution and damage and thus lead to the enhancement of mechanical properties of HDPE/RGO nanocomposites, but reveal the underline stress-induced fragmentation-recrystallization mechanism for this tensile deformation.

Molecular Dynamics Study of Polyethylene: Anomalous Chain Mobility in the Condis Phase

A sharp (several orders of magnitude) increase of chain mobility was observed at a certain temperature in molecular dynamics simulation of the polyethylene crystal. Chain mobility as a function of ...