Amorphous Polyethylene Research Articles

Branched polyethylenes fractionated in supercritical propane

A series of polyethylenes (PEs), were synthesized in the presence of a palladium catalyst containing bulky diimine ligands (VERSIPOL™) yielding amorphous PEs. The different PE samples were characterized by Size Exclusion Chromatography (SEC) with on line light scattering, NMR, viscometry or small angle neutron scattering (SANS) techniques to assess the molar mass, molar mass distribution (MMD), radius of gyration (or hydrodynamic radius) and degree of branching. Selected PE samples were fractionated in supercritical propane by isothermal increasing the pressure profile. In most cases, PE fractions with a sharper MMD than the original PEs were obtained. The average absolute molar masses of the resulting fractions increased steadily with increasing extraction pressure. Their dilute solution properties were compared with those of unfractionated PEs. Structural analyses confirmed the homogeneity of the degree of branching within a raw PE sample.

Comparison of pilot and industrial scale atmospheric pressure glow discharge systems including a novel electro-acoustic technique for process monitoring

A comparison of a pilot and industrial scale atmospheric pressure polymer processing plasma system has been carried out using process-monitoring diagnostic tools during treatment of amorphous polyethylene terephthalate. These systems have been compared using optical emission spectroscopy (OES), photodiode (PD) analysis and multi-variate analysis of the applied electrical and emitted electro-acoustic signals to facilitate scale up operations from the pilot to the industrial scale system. The voltage, current, electro-acoustic intensity and frequency of the plasma systems were found to change systematically with an increase in applied plasma power and addition of oxygen (O2) into a helium (He) plasma. The plasma drive frequency was pulled by the plasma reactance from approximately 26 to 16 kHz on the pilot system and from approximately 36 to 32 kHz on the industrial system, for an increase in applied plasma power and addition of O2. The OES analysis revealed a number of peaks associated with nitrogen (N2) species between 250 and 450 nm due to the presence of air within the He plasma. Temporally resolved analysis of the discharge emission carried out using a PD showed an increase in the number of discharge events per power cycle with an increase in power and a decrease in emission intensity for addition of O2 into the He plasma for both the pilot and industrial scale systems. Using these diagnostic tools both plasma stability and run to run variations were assessed. A visual analysis of the 1.2 m wide plasma was also carried out where a more homogeneous plasma was observed at higher powers.

Ethylene polymerization with hybrid nickel diimine/Cp2TiCl2 catalyst: a new method to prepare blends of linear and branched polyethylene

AbstractBlends of linear polyethylene (LPE) and branched polyethylene (BPE) display very good mechanic properties that can be beneficial for various applications such as shear thinning and melt elasticity. LPE, BPE and amorphous polyethylene can be produced using nickel diimine (DMN) catalyst under various polymerization conditions, while LPE can be obtained using metallocene catalyst. Thus, LPE/BPE blends can be achieved by in situ polymerization using a hybrid DMN/metallocene catalyst. A novel hybrid catalyst made of DMN and Cp2TiCl2 was designed and used for ethylene polymerization. A synergistic effect of the two active sites in the hybrid DMN/metallocene catalyst was observed. Blends of linear and low branched polyethylene were synthesized when polymerization was conducted at low temperature (0 °C), while blends of linear and highly branched polyethylene were obtained at high temperature (50 °C). However, the miscibility of the polymers obtained at 50 °C was dramatically reduced as compared to those obtained at 0 °C. Mesoporous particles (MCM‐41) consisting of aluminosilicate with cylindrical pores were used to support the hybrid catalyst, in which MCM‐41 provides sufficient nanoscale pores to facilitate the polymerization in well‐controlled confined spaces. Blends of LPE and BPE were synthesized by in situ polymerization without adding comonomer and characterized. The miscibility of the polymer blends can be improved by supporting the hybrid catalyst on MCM‐41. Copyright © 2009 Society of Chemical Industry

Surface modification-assisted bonding of polymer-based microfluidic devices

This paper reports on the development and application of a surface modification technique as an improved method for bonding polymer microfluidic substrates. This technique readily produced complete microfluidic chips via plasma oxidation followed by silane reagent treatment on the polymer surface. Characterization of the bonded chips was performed using scanning electron microscopy (SEM), water contact angle measurement, and tensile strength measurement. SEM images showed that the integrity of the channel features was successfully preserved. A bond strength approaching that of solvent welding was demonstrated. This technique has been successfully applied to bond dissimilar polymer substrates (polymethylmethacrylate (PMMA), amorphous polyethylene terephthalate (APET), polycarbonate (PC)), and is also applicable to bonding a hard polymer substrate to polydimethylsiloxane (PDMS) or glass.

Using vibrational mode analysis for predicting the coefficient of thermal expansion of amorphous polymers

AbstractMolecular dynamics simulations and vibrational analysis on model amorphous polyethylene structures demonstrate that the main contribution to the coefficient of thermal expansion (CTE) is due to structural relaxations even at temperatures below the glass transition. The contribution due to vibrational relaxation, calculated within the Grüneissen theory of solids, accounts only for a small fraction of the CTE. Based on our findings we argue that tailoring of polymers with low CTE should focus on reduction of the relaxation processes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2114–2121, 2009

Molecular Dynamic Studies on Deformation of Polymer Resist During Thermal Nano Imprint Lithographic Process

Molecular dynamic simulation of nano imprint lithography (NIL) in which nanoscale patterned stamp is pressed onto amorphous polyethylene (PE) surface is performed to study the deformation behavior of polymer resist. Force fields including bond, angle, torsion, and Lennard Jones potential are used to describe the inter-molecular and intra-molecular forces of PE molecules and stamp. Periodic boundary condition is used in horizontal direction and canonical NVT ensemble is employed to control the system temperature. Using the simulation results, the behavior of polymer resist is investigated during the imprinting process. The mechanism of resist deformation is analyzed by considering various parameters including the surface geometry, atom distribution, and density. Especially, the density in the bottom (emboss) region is found to be larger than the top (cavity) region due to compression of polymer molecules. The result indicates that small scale patterning of polymer resist largely depends on compression rather than the flow of molecules. The numerical results are compared with the local density measurement data using the atomic force microscope (AFM). They exhibit a similar behavior at least in a qualitative sense. Additional simulations, where the stamp geometry and molecular mobility are varied and are performed to investigate the deformation characteristics of polymer resist. From these simulations, important elements for understanding of NIL process are obtained.

Dynamics of Molecular Impacts on Soft Materials: From Fullerenes to Organic Nanodrops

The present theoretical study explores the interaction of various energetic molecular projectiles and clusters with a model polymeric surface, with direct implications for surface analysis by mass spectrometry. The projectile sizes (up to 23 kDa) are intermediate between the polyatomic ions (SF(5), C(60)) used in secondary ion mass spectrometry and the large organic microdroplets generated, for example, in desorption electrospray ionization. The target is a model of amorphous polyethylene, already used in a previous study [Delcorte, A.; Garrison, B. J. J. Phys. Chem. C 2007, 111, 15312]. The chosen method relies on classical molecular dynamics (MD) simulations, using a coarse-grained description of polymeric samples for high energy or long time calculations (20-50 ps) and a full atomistic description for low energy or short time calculations (<1 ps). Two regions of sputtering or desorption are observed depending on the projectile energy per nucleon (i.e., effectively the velocity). The transition, occurring around 1 eV/nucleon, is identified by a change of slope in the curve of the sputtering yield per nucleon vs energy per nucleon. Beyond 1 eV/nucleon, the sputtering yield depends only on the total projectile energy and not on the projectile nuclearity. Below 1 eV/nucleon, i.e., around the sputtering threshold for small projectiles, yields are influenced by both the projectile energy and nuclearity. Deposition of intact molecular clusters is also observed at the lowest energies per nucleon. The transition in the sputtering curve is connected to a change of energy deposition mechanisms, from atomistic and mesoscopic processes to hydrodynamic flow. It also corresponds to a change in terms of fragmentation. Below 1 eV/nucleon, the projectiles are not able to induce bond scissions in the sample. This region of molecular emission with minimal fragmentation offers new analytical perspectives, out of reach of smaller molecular clusters such as fullerenes.

Conformational Transition Behavior of Amorphous Polyethylene across the Glass Transition Temperature

Molecular dynamics simulations have been used to investigate the conformational transition behavior in amorphous polyethylene with different chain lengths across the glass transition temperature (T(g)). In the present study, we examined the barrier height of conformational transition rates in different states. It was found that two lines of the logarithmic rates versus inverse temperature in the melt state and in the glass state are evidently different. The two lines have an intersection, which indicates T(g) well. The barrier height in the glass state was unexpectedly observed lower than that in the melt state. For gaining better understanding of the transition barrier reduction, we analyzed motion heterogeneity of the systems and found the torsional transition rate distribution becoming gradually heterogeneous when the temperature went down to the glass state. The result indicates that the motion heterogeneity was caused by the torsion transition being "frozen". The frozen torsions made the system into a nonequilibrium state and possess a novel transition behavior, which accounted for most of the transitions that started at a location close to top of barrier, and also the enhancement of a small magnitude of transition jumps.

Influence of the torsional potential on the glass transition temperature and the structure of amorphous polyethylene

The effect of the torsional potential on several thermodynamic and structural properties of a system of polyethylene chains has been analyzed. To this end, molecular dynamics simulations of a coarse-grained model, whose sites interact through a force field with bending, torsional, and nonbonded terms, have been considered. The torsional potential has three stable configurations: gauche-, trans, and gauche+ . It has been modeled using a simple functional form with only two parameters: the trans-gauche and the gauche-gauche energy barriers. In order to analyze the influence of these parameters on the properties considered in this work, five models with different values of the torsional barriers have been considered. We have observed that the glass transition temperature, the intrachain radial distribution function, the radius of gyration, and the end to end distribution functions are very sensitive to the changes in the trans-gauche torsional barrier. Moreover, at low temperatures, the interchain radial distribution function, the orientational correlation function, and the volume distribution functions of the Voronoi polyhedra, that surround every site of the polymeric chains, also depend on the trans-gauche torsional barrier. On the contrary, the gauche-gauche energy barrier has a minor influence in the properties considered in this work.

Kapitza conductance of silicon–amorphous polyethylene interfaces by molecular dynamics simulations

We use nonequilibrium molecular dynamics simulation to elucidate the interfacial thermal conductance between single-crystal silicon and amorphous polyethylene. In particular, we investigate the role of solid stiffness and the bonding strength across the interface on the interfacial thermal conductance. Simulations of interfacial scattering of individual phonon wave packets indicate that neither diffuse mismatch model nor acoustic mismatch model describes the interfacial scattering process quantitatively. In general, transmission coefficients for longitudinal phonons are significantly higher than those for transverse phonons. We also observe that anharmonic processes can be important for interfacial conductance.

Shape-Memory Properties of Radiopaque Micro-Composites from Amorphous Polyether Urethanes Designed for Medical Application

AbstractBiocompatible shape-memory polymers are of high significance for application in medical devices or instruments for minimally invasive surgery. To follow the medical device placement or changes in shape of the device in vivo by imaging methods like X-ray techniques, radiopacity of the polymer is required. In this work, we explored the shape-memory properties of radiopaque polymer composites prepared by incorporation of barium sulphate micro-particles in a biomedical grade polyether urethane (PEU) by co-extrusion technique. The filler content was varied from 5 wt% to 40 wt%, which was confirmed by thermal gravimetric analysis (TGA) measurements, while the particle distribution was visualized by scanning electron microscopy (SEM). The thermal and mechanical properties of the composites were investigated by means of dynamic mechanical analysis at varied temperature (DMTA) and tensile tests. The shape-memory properties of PEU composites were quantified in cyclic, thermomechanical experiments. A significant increase in Young’s modulus and a decrease in elongation at break were observed for PEU composites with increasing content of BaSO4, while the DMTA results were not affected by incorporation of the fillers. All samples exhibited excellent shape-memory properties with shape fixity rates (Rf) above 98% and values for shape recovery rate (Rr) in the range of 81% to 93%. The maximum stress (σmax) obtained under constant strain recovery conditions increased from 0.6 MPa to 1.4 MPa with raising amount of BaSO4, while the corresponding temperature (Tσ,max) as well as the switching temperature (Tsw) determined under stress-free conditions remained constant for all polymer composites.

Liquid–liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with an octane-chain branch density of 120/1000 carbon was prepared at different concentrations. The two components of this set of blends possessed isorefractive indices, thus, making it difficult to detect their liquid–liquid phase separation via scattering techniques. Above the experimentally observed melting temperature of HDPE, Tm=133°C, this series of blends can be considered to be in the liquid state. The LLDPE crystallization temperature was below 50°C; therefore, above 80°C and below the melting temperature of HDPE, a series of crystalline–amorphous PE blends could be created. A specifically designed two-step isothermal experimental procedure was utilized to monitor the liquid–liquid phase separation of this set of blends. The first step was to quench the system from temperatures of known miscibility and isothermally anneal them at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from liquid–liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. The time for developing 50% of the total crystallinity (t1/2) was used to monitor the crystallization kinetics. Because phase separation results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the liquid–liquid phase separation for the system indicated by faster t1/2. The annealing temperature in the first step that exhibits an onset of the decrease in t1/2 is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to form spherulites which decorate the phase-separated morphology. Therefore, the crystal dispersion indicates whether the phase separation followed a nucleation-and-growth process or a spinodal decomposition process. These crystal-decorated morphologies enabled the spinodal curve to be experimentally determined for the first time in this set of blends.

Structure, Dimensions, and Entanglement Statistics of Long Linear Polyethylene Chains

This work elucidates the effect of both temperature and molecular length on the conformational and structural properties as well as on the entanglement statistics of long amorphous, polydisperse, and molten linear polyethylene (PE). A large number of PE samples are modeled in atomistic detail, with average molecular lengths ranging from C24 up to C1,000 over a wide range of temperatures in the interval of 300 <or= T <or= 600 K under constant pressure (P ) 1 atm). By employing enhanced chain-connectivity-altering moves, full-scale equilibration is achieved within modest computational time even for the longest molecules at ambient conditions.At a second stage, direct geometrical analysis is applied on all equilibrated polymer configurations providing the corresponding primitive paths and intermolecular entanglements. Simulation findings on the characteristic ratio, density, and atomic packing are in excellent agreement with available experimental data. The same holds for the calculated plateau modulus; simulation predicts 1.8 ( 0.1 MPa. Regarding the primitive path statistics, the average contour length and the number of entanglements are found to exhibit a simple exponential type dependency on temperature. For the polydisperse samples studied here, a superposition of Poissonians(often represented by a negative binomial) describes best the distribution of entanglements of the primitive paths.

Synthesis and Characterization of Well-Defined 12,12-Ammonium Ionenes: Evaluating Mechanical Properties as a Function of Molecular Weight

Water-soluble 12,12-ammonium ionenes were prepared via the Menschutkin reaction from 1,12-dibromododecane and 1,12-bis(N,N-dimethylamino)dodecane. A stoichiometric imbalance of monomers controlled the final molecular weights of the polymers. The absolute molecular weights were determined for the first time using an online multiangle laser light scattering (MALLS) detector in aqueous size exclusion chromatography (SEC). Weight-average molecular weights ranged from 4300 to 20 900 g/mol. Relationships between weight-average molecular weight and mechanical properties were established for a series of 12,12-ammonium ionenes using both tensile testing and dynamic mechanical analysis (DMA). Tensile analysis of the higher molecular weight ionenes revealed an average tensile strength of 20 MPa and elongations ranging from 230 to 440%. Dissociation of ionic aggregates was observed at 85−88 °C in DMA experiments, and the glass transition temperatures increased with increasing molecular weight (61−88 °C). X-ray scattering revealed an amorphous polyethylene peak at ≈14 nm−1 and a sharp ionic group correlation peak at 4.38 nm−1. These correlations agreed well with the proposed macromolecular structure.

Modélisation par éléments finis du chauffage infrarouge des membranes thermoplastiques semi-transparentes

Resume Dans cette etude, nous avons etabli par la methode des elements finis en trois dimensions une approche simplifiee pour analyser l'evolution de la temperature au sein d'une membrane semi-transparente de type polyethylene terephtalate (PET) amorphe, exposees a une source radiative. En se basant sur le comportement optique du PET, un transfert radiatif monodirectionnel, suivant l'epaisseur de la membrane est considere. Des simulations numeriques ont permis de confronter et de valider les resultats obtenus vis-a-vis des resultats analytiques et experimentaux. Comme application, nous avons etudie le chauffage d'une feuille thermoplastique en PET exposee a une source radiative infrarouge.

Two-stage polymer embossing of co-planar microfluidic features for microfluidic devices

A two-stage embossing technique for fabricating microchannels for microfluidic devices is presented. A micromachined aluminum mold is used to emboss a polyetherimide (PEI) substrate with a relatively high glass transition temperature (Tg). The embossed PEI is then used as a mold for embossing an amorphous polyethylene terephthalate (APET) substrate with a lower Tg. The resulting APET substrate has the same features as those of the aluminum mold. Successful transfer of features from the aluminum mold to the APET substrate was verified by profilometry, and an application of this method in production of a microfluidic device is presented.

End-Bridging Monte Carlo Simulation of Bulk and Grafted Amorphous Polyethylene Above and Below the Glass Transition

The very efficient end-bridging Monte Carlo (EBMC) method has been employed in order to simulate an amorphous, polydisperse 80-chain large C156 polyethylene (PE) system in atomistic detail over a wide range of temperatures (from 600 down to 150 K) and determine its glass transition temperature (Tg). Two sets of simulations have been performed: one with a bulk, isotropic sample and the other with a thin film in which all the 80 PE chains were grafted on a hard substrate on one side (corresponding to a high grafting density equal to σ = 1.75 nm-2) and exposed to vacuum on the other side. In the simulations, a united-atom model was employed for PE ensuring that only the purely amorphous phase of PE was simulated at all temperatures. In all cases, very long simulations were carried out in order to give enough time for the system to relax at all length scales. For all temperatures studied, the longest relaxation time was found to be present by descriptors associated with the system's long-range conformational characteristics. In contrast, more local, internal structural features were always faster in equilibrating. As a result, the time autocorrelation function for the chain end-to-end unit vector, fu(t), was found to drop to zero and then clearly fluctuate around this value only for temperatures higher than about 220 K for both systems. For lower temperatures, fu(t) did not relax completely, even after 2 × 107 CPU seconds. Additional volumetric simulation data demonstrated a sharp change in the density and potential energy of both systems in the neighborhood of the 230 K, which are considered as features of the glass transition for amorphous PE. The Tg value suggested by the present EBMC simulations for amorphous (bulk or grafted) PE is (230 ± 10) K, which is consistent with the value of 237 K measured experimentally by Wunderlich [J. of Chem. Phys. 1962, 37, 1203] and Loufakis and Wunderlich [J. Phys. Chem. 1988, 92, 4205] for PE in the limit of zero crystallinity. Further, the predicted change in the heat capacity at constant pressure at the glass transition is Δcp = 1.2 × 10-4 kcal g-1 K-1, which is very close to the value of 1.5 × 10-4 kcal g-1 K-1 measured experimentally by Wunderlich [J. of Chem. Phys. 1962, 37, 1203]. Additional results on the temperature dependence of the conformational and structural properties in the two PE systems are also reported and discussed in detail.

Yield Mode Transition in Amorphous PET

In the previous paper, we showed in a constant strain rate shear test using an amorphous polyethylene terephthalate (PET) thin plate that 1. there were two kinds of yield mode, diffuse slip line (DSL) and localized shear band (LSB) modes and 2. the DSL mode appeared in a range of relatively high temperature and low strain rate and the LSB mode occurred in a range of relatively low temperature and high strain rate. In this paper, we investigated the condition of the yield mode transition in both tensile and shear tests and showed that 1. the yield mode transition occurred in tensile test as well as in shear test depending on test condition and 2. an example of a critical condition for the transition was given as a function of temperature T and strain rate S by T Sm = C where m and C were the constants.

Molecular dynamics simulation of interfacial thermal conductance between silicon and amorphous polyethylene

Using nonequilibrium molecular dynamics simulations, we study the interfacial thermal conductance between single crystal silicon and amorphous polyethylene (PE). We estimate that the silicon-PE interfacial thermal conductance is about 20MW∕m2K at room temperature, which is equivalent to the resistance of 16nm thick layer of bulk amorphous polyethylene. We also study the role of solid stiffness and the bonding strength across the interface on the interfacial thermal resistance. With strong interfacial bonding, our results are in agreement with the diffuse mismatch model and phonon radiation limit predictions, suggesting that in this case, heat carrying acoustic phonons in solids have transmission coefficients to polymer equal almost to unity.

Highly Amorphous Solid Polymer Electrolytes for Lithium-Ion Batteries

Two new amorphous polyethers have been synthesized and characterized by functionalization of a commercially available epichlorohydrin-ethylene oxide copolymer. The functionalized polyethers, ECH-EO-Phth and ECH-EO-Iodide, after purification and vacuum drying, displayed both excellent mechanical strength and compatibility with lithium salts. Thermal analysis by differential scanning calorimetry showed the complete amorphous character of the electrolytes in the temperature range of -100{degree sign} C to 120{degree sign}C and ionic conductivities of ~1.32x10-6 S/cm at room temperature.