Linear Low-density Polyethylene Chains Research Articles

Investigation of the effect of electron beam irradiation on tensile, thermal and gas barrier properties of graphene nanoplatelet loaded thermoplastic nanocomposite films

Packaging materials based on lightweight and flexible thermoplastic/graphene composite films are gaining popularity in both the academic and industrial sectors. In this research work, graphene nanoplatelet (GNP) of 1–5 wt% was integrated into linear low density polyethylene (LLDPE) by employing melt compounding technique at 160 °C to produce flexible composite films. The LLDPE/GNP nanocomposite films were irradiated with doses up to 50 kGy using a 2.5 MeV electron beam accelerator in air at ambient temperature. The uniform distribution of GNP in the thermoplastic matrix obtained from FESEM analysis improved the samples' mechanical characteristics. As the GNP concentration in the LLDPE matrix increases, both the thermal stability and impermeability qualities (oxygen and water vapor) get significantly improved. Electron beam irradiation of film samples further enhances the physico-mechanical, thermal and gas barrier properties compared to non-irradiated composite films and neat LLDPE film. The tighter connections between graphene and LLDPE chains as well as the development of cross-linked networks by E-beam irradiation are responsible for the improved performance properties. Therefore, GNP incorporation as well as electron beam irradiation of thermoplastic nanocomposite films could be a viable alternative for producing high-performance and high-barrier flexible LLDPE/GNP composite films for packaging applications.

Plasticization and cocrystallization in LLDPE/wax blends

ABSTRACTThe structure and thermal properties of linear low‐density polyethylene (LLDPE)/medium soft paraffin wax blends, prepared by melt mixing, were investigated by differential scanning calorimetry (DSC) and small‐ and wide‐angle X‐ray scattering (SAXS and WAXS). The blends form a single phase in the melt as determined by SAXS. Upon cooling from the melt, two crystalline phases develop for blends with more than 10 wt % wax characterized by widely different melting points. The wax acts as an effective plasticizer for LLDPE, decreasing both its crystallization and melting temperature. The higher melting point crystalline phase is formed by less branched LLDPE fractions. On the other hand, the lower melting point crystalline phase is a wax‐rich phase constituted by cocrystals of extended chain wax and short linear sequences of highly branched LLDPE chains. The presence of cocrystals was evidenced by standard DSC results, successive self‐nucleation and annealing (SSA) thermal fractionation and by the detection of a new SAXS signal attributed to the lamellar long period of the cocrystals. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1469–1482

Crystallization of linear low density polyethylene on an in situ oriented isotactic polypropylene substrate manipulated by an extensional flow field

In this work, we demonstrate that utilization of extensional flow with different intensities can regulate the flow-induced crystallization and epitaxially surface-induced crystallization simultaneously in crystalline–crystalline immiscible blends, leading to improved interfacial adhesion and thus enhanced mechanical properties, which provides a versatile methodology to industrially achieve polymer blends with advanced performance. An accessible methodology, i.e., “extrusion–hot stretching–quenching”, was applied to fulfill the scalable achievement of an epitaxial interface for a linear low density polyethylene (LLDPE)/isotactic polypropylene (iPP) blend, where LLDPE could epitaxially grow on an oriented iPP substrate but greatly influenced by the flow field, with its chains and lamellae aligned abnormally off the flow direction revealed by wide angle X-ray diffraction and small angle X-ray scattering, respectively. Depending on the intensity of flow, the above effect of flow can be divided into two types: under a strong flow field, the LLDPE chains prefer to align along the flow direction, inducing the formation of a shish-kebab structure. For another type, i.e., under a weak flow field, the pre-oriented LLDPE chains can relax quickly and epitaxially nucleate on the surface of the oriented iPP substrate. During further growth, the epitaxial LLDPE lamellae deform and reorient along the flow direction under the mechanism of flow-induced block slips, fragmentation and reorientation. Moreover, it is believed that incomplete lamellar twist also occurs under flow. Mechanical property tests demonstrate that an epitaxial structure significantly improves the interfacial adhesion between LLDPE and iPP, showing remarkable enhancements in both strength and toughness.

Thermal Properties of Linear Low Density Polyethylene/Thermoplastic Starch/Banana Fiber Composites

The effect of thermoplastic starch (TPS) and banana fiber contents on thermal characteristics of linear low-density polyethylene (LLDPE) matrix were investigated. The measurements from differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA), proved the effectiveness of TPS and banana fiber in improving the blend degradation. On the other hand the LLDPE/TPS/banana fiber composites showed better thermal stability than the LLDPE/TPS blend, which is reflected to the LLDPE chains movement restriction. The incorporation of banana fiber into the LLDPE/TPS blends was found to interfere with the chains movement and resulting in more thermally stable and improving the blends stiffness.

Preparation and characterization of grafting styrene onto LLDPE by cyclohexane compatibilization in suspension polymerization

ABSTRACTA novel method of grafting styrene onto linear low‐density polyethylene (LLDPE) by suspension polymerization was systematically evaluated. Cyclohexane as a compatibilizer was introduced to swell and activate the surface of LLDPE molecular chain for amplifying the contact point of styrene monomer with LLDPE. A series of copolymer of grafting polystyrene (PS) onto LLDPE, known as LLDPE‐g‐PS, were prepared with different ratios of cyclohexane/styrene monomer and various LLDPE dosages. FTIR and 1H NMR techniques both confirmed successful PS grafting onto the LLDPE chains. In addition, SEM images of LLDPE‐g‐PS particles showed that the cross‐section morphology becomes smooth and dense with suitable cyclohexane dosages, indicating a better compatibility between LLDPE and PS. The highest grafting efficiency was 28.4% at 10 mL/g cyclohexane and styrene monomer when 8% LLDPE was added. In these conditions, the LLDPE‐g‐PS elongation at break increased by about 30 times compared with PS. Moreover, thermal gravimetric analysis (TGA) demonstrated that LLDPE‐g‐PS possesses much higher thermal stability than pure PS. Therefore, the optimal amount of cyclohexane as compatibilizer could increase the grafting efficiency and improve the toughness of PS. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41671.

Polymer nanoparticles: their effect on rheological, thermal, and mechanical properties of linear low-density polyethylene (LLDPE)

Rheological, thermal, and mechanical properties of polymer particle/LLDPE blends were studied in this paper. The blends were prepared individually by incorporating nanoparticles of polystyrene (nPS) of ∼60 nm and polymethyl methacrylate (nPMMA) of ∼50 nm with different wt% loading (i.e., 0.10–0.5%). It was shown from the experimental results that rheological, thermal and mechanical properties were increased as polymer particles blended with LLDPE. Blends with 0.25 wt% loading of nPS and 0.5 wt% loading of nPMMA exhibited better rheological, thermal, and mechanical properties compared with that of other wt% loadings. The improvements in properties were due to the close packing of LLDPE chains as recorded by improvement in crystallinity of LLDPE with addition of nPS and nPMMA as shown by SEM. Copyright © 2010 John Wiley & Sons, Ltd.

MD Simulation Study of the Influence of Branch Content on Crystallization of Branched Polyethylene Chains with Uniform Branch Distribution

Linear low-density polyethylene (LLDPE) chains with different levels of branch content (BC), ranging from 10 to 80 branches/1000 C, distributed uniformly along the chain were simulated in vacuum at a temperature of 350 K. The influence of BC on the relaxation and crystallization of LLDPE chains was studied. The collapse of the branched chains was found to occur via a local followed by a global collapse mechanism with branches acting as nucleation points for the collapse of the molecule leading to a faster collapse of chains with higher BC. The trans population was found to be dominant at all branch contents; however, it decreases with increasing BC. Increasing BC was found to decrease order and to strongly influence chain conformation. Chain conformation undergoes a transition from lamellar to a more random coil-like structure near a critical BC of 50 branches/1000 C. Branches were observed to be excluded from the lamella and to self assemble at high BC. This work also provides insight into the conformation adopted during the coil-globule transition experienced by a single chain in an infinitely dilute solution much below the Θ temperature.

Crystallization of partially miscible linear low-density polyethylene/poly(ethylene-co-vinylacetate) blends

Miscibility and crystallization of linear low-density polyethylene (LLDPE)/poly(ethylene-co-vinylacetate) (EVA) blends were investigated by optical microscopy (OM) and differential scanning calorimetry (DSC). It was found that there existed liquid–liquid phase separation (LLPS) below 220 °C over the whole composition. However, the depression in the crystallization temperature, melting temperature and equilibrium melting temperature of LLDPE all indicated that this polymer pair was partially miscible. The crystallization and melting behavior of LLDPE were determined by the dilute effect of non-crystalline EVA and the probable co-crystallization of parts of EVA chains with LLDPE chains. The crystallization and melting behavior of EVA was determined by the competence between a nucleation effect of LLDPE crystals and partial miscibility between this polymer pair, which was different from that of LLDPE.

Creating layers of concentrated inorganic particles by interdiffusion of polyethylenes in microlayers

Interdiffusion of a polymer pair in microlayers was exploited to increase the concentration of inorganic particles in one of the components. When microlayers of linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) were taken into the melt, greater mobility of linear LLDPE chains compared to branched LDPE chains caused the layer boundary to move in the direction of the more slowly diffusing chains in a manner similar to the Kirkendall effect in metals. This resulted in substantial shrinkage of the LLDPE layers and corresponding thickening of the LDPE layers. Adding a particulate in the LLDPE did not impede the process of interdiffusion in the melt, and the resultant shrinkage served to increase the particle concentration. For example, resistivity of initially nonconductive LLDPE layers containing nickel platelets decreased by 6 orders of magnitude into the semiconductor range after shrinkage concentrated the particles. The concentrating effect was also demonstrated with TiO2 particles and talc platelets. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2877–2885, 1999

Studies on blends of LLDPE and polar polymers compatibilized by a random copolymer

Compatibilization of blends of Linear low-density polyethylene (LLDPE)-poly(methyl methacrylate) (PMMA) and LLDPE-copolymer of methyl methacrylate (MMA) and 4-vinylpyridine (poly(MMA-co-4VP) with poly(ethylene-co-methacrylic acid) (EMAA) have been studied. Mechanical properties of the LLDPE-PMMA blends increase upon addition of EMAA. In order to further improve interfacial adhesion of LLDPE and PMMA, 4-vinyl pyridine units are introduced into PMMA chains, or poly(MMA-co-4VP) is used as the polar polymer. In LLDPE-poly(MMA-co-4VP)-EMAA blends, interaction of MAA in EMAA with 4VP of poly(MMA-co-4VP) causes a band shift in the infrared (IR) spectra. Chemical shifts of N-1s binding energy in X-ray photoelectronic spectroscopy (XPS) experiments indicate a transfer of proton from MAA to 4VP. Scanning electron microscopy (SEM) pictures show that the morphology of the blends were improved upon addition of EMAA. Nonradiative energy transfer (NRET) fluorescence results attest that there exists interdiffusion of chromophore-labeled LLDPE chains and chromophore-labeled poly(MMA-co-4VP) chains in the interface. Based on experimental results, the mechanism of compatibilization is studied in detail. Compatibilization is realized through the interaction between MAA in EMAA with 4VP in poly(MMA-co-4VP). (C) 1999 John Wiley & Sons, Inc.

Reactions of amorphous PE radical-pairs in vacuo and in acetylene: A comparison of gel fraction data with Flory-Stockmayer and atomistic modelling analyses

The radiation-induced crosslinking of linear low density polyethylene chains has been analysed theoretically using an atomistic analysis and a Flory-Stockmayer (FS) analysis. Experimentally, gel fractions greater than the theoretical maximum obtainable, assuming every radical reacts to form a crosslink (namely, D.G(R) 2 ), indicate the occurrence of chain-reactions both in the presence of acetylene and in vacuo. Based on previous studies, chain-reaction mechanisms are postulated, involving unterminated polyenes in acetylene and scission-generated vinyls in vacuo. Accordingly, numerical comparisons of the theoretical and experimental gel-fraction results, as a function of dose (D), have been used to calculate the number of ‘gel-effective’ chain-steps per initial alkyl radical generated ( N CS, D) for both the atomistic and FS analyses. Both systems of analysis show that, as dose increases, N CS, D for the acetylene and in vacuo results decrease from maximum values at or just after the gel points. The decay is interpreted as resulting from the increased probability of radical-radical termination reactions towards higher levels of dose. Although there are differences between the two analyses, both give essentially the same interpretation of the experimental results. The atomistic analysis predicts maximum values of N CS, D of 9.0 in the presence of acetylene and 2.9 in vacuo. The corresponding values from the FS analysis are 9.4 and 2.8 respectively. As the dose is increased further, the atomistic analysis gives 2.1th- and 2.6th-order decays of N CS, D as functions of the dose, for the acetylene and in vacuo results respectively. The FS analysis gives, correspondingly, 2.6th- and 2.8th-order decays. General equations are developed for irradiated polymers, which relate gel fraction to the dose and the number of initial radicals generated in a pre-irradiated number-average degree of polymerization, via N CS, D.

Basic functionalization of molten linear low‐density polyethylene with 2‐(dimethylamino)ethyl methacrylate in an intermeshing corotating twin‐screw extruder

AbstractFunctionalization of molten linear low‐density polyethylene (LLDPE) with 2‐(dimethylamino)ethyl methacrylate (DMAEMA) was studied in an intermeshing co‐rotating twin‐screw extruder using a peroxide initiator. The influence of monomer concentration, initiator concentration, reaction temperature, the screw speed, and the mean reaction residence time on the grafting reaction was investigated in order to determine the reaction conditions necessary to obtain a controlled degree of grafting, DG, while minimizing homopolymerization and crosslinking. Relatively high grafting levels can be obtained without excessive polyethylene crosslinking by using high monomer concentrations, wm, with low initiator concentrations, wI. DG increased with increasing monomer concentration when wm exceeded a certain value. Processing temperatures between 130 and 160°C are optimal to achieve high grafting efficiency, GE. Too high a processing temperature resulted in both low DG and GE. An optimal reaction residence time of about 5 min was found for wm = 23 wt % and wI = 0.56 wt % to obtain relatively high DG and GE. The melt flow index of the reaction product, MFI, increased with increasing wm at constant wI. This suggests that the DMAEMA monomer has the ability to suppress crosslinking of the LLDPE chains. The ability of the DMAEMA monomer to compete for initiator and polymer radicals and reduce LLDPE crosslinking was further demonstrated by the study of sequential addition of monomer and initiator along the extruder. The competition among the three reactions (i.e., the desired grafting, homopolymerization, and crosslinking) is discussed.