Fast quantitative analysis of high density polyethylene using adiabatic-refocused INEPT spectroscopy

Phillips catalyst has been contributing to about 40% of world high‐density polyethylene production because of its ability to give products with unique microstructures like broad molecular weight distribution as well as short and long chain branches. Even after 50 years' effort, some crucial problems concerning the nature of active sites, polymerization, and branching mechanisms are still kept mysterious. In this work, ethylene and 1‐hexene copolymerization with Phillips catalyst prereduced by CO was carried out in the presence of triethyl aluminum (TEA) cocatalyst. The microstructures of polymers were investigated by 13C NMR and gel permeation chromatography (GPC) methods. A hybrid‐type kinetics was found for both homo‐ and copolymerization kinetics, which indicated that there existed two types of active sites namely site A and site B. Site A with instant activation, high activity, and fast decay was transformed from a metathesis site, namely Cr(II) site, coordinated with CO or CO2 through desorption of CO or CO2 by TEA, which contributed to the formation of short chain branches, especially methyl branches. Site B with slow activation, low activity, and slow decay was generated from reduction of residual chromate (VI) by TEA. Both 1‐hexene and TEA can decrease the molecular weight of polyethylene as well as enhance short chain branching. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4632–4641, 2005

Morphological Raman analysis of short chain branched ethylene and propylene metallocenic copolymers

Microstructural design characteristics of high-density polyethylene for thick-walled pressure pipe applications

Summary: Quantitative branch determination in polyolefins by melt‐state NMR has been investigated paying particular attention to sensitivity per unit time. Comparison of spectra obtained using spectrometers operating at 700, 500 and 300 MHz 1H Larmor frequency, with 4 and 7 mm MAS probeheads, showed that the best sensitivity was achieved at 500 MHz using a 7 mm 13C1H optimised high‐temperature probehead. For materials available in large quantities static melt‐state NMR, using large‐diameter detection coils at 300 MHz, was shown to produce comparable results to melt‐state MAS measurements in less time. Artificial line broadening, introduced by FID truncation, was reduced by the use of π pulse‐train heteronuclear dipolar‐decoupling. This decoupling method, when combined with a higher duty‐cycle, allowed for the whole FID to be acquired. Optimised methods have been applied to the characterisation of short‐chain branching (SCB) in polyethylene‐ and poly(propylene)‐co‐α‐olefins with varying comonomer incorporation. Long‐chain branch (LCB) concentrations of 8 branches per 100 000 CH2 were quantified for an industrial ‘linear’ polyethylene in 13 h, with a signal‐to‐noise ratio of 10 for the α branch site used. The use of J‐coupling mediated polarisation transfer techniques were also shown to be viable for branch quantification in the melt‐state.An example of the time efficient quantification of very low branch contents in polyethylene using optimised 13C melt‐state NMR under magic‐angle spinning. Concentrations of 7–8 branches per 100 000 CH2 groups were determined in only 13 h.magnified imageAn example of the time efficient quantification of very low branch contents in polyethylene using optimised 13C melt‐state NMR under magic‐angle spinning. Concentrations of 7–8 branches per 100 000 CH2 groups were determined in only 13 h.

We present a detailed study of the effects of short branches on the rheological behaviors of H-shaped long-chain branched polymers under shear and uniaxial elongational flows using (single “phantom” chain) bead-spring Brownian dynamics simulations. To clarify the fundamental role of short branches in both flow types, the short branches are distributed either along the chain backbone or along the four dangling long arms of the H-polymer. We observe that the fast random motions of the highly mobile short branches (in association with their very short characteristic relaxation time scales) constantly disturb chain conformation, generally leading to a more compact and less deformed chain structure against the applied flow. Accordingly, the structural and dynamical properties of the short-chain branched (SCB) H-polymers in response to the flow are strongly dependent on the location of the short branches along the chain. For instance, in comparison to the original H-polymer, the H-(SCB_backbone) polymer, where the short branches are allocated along the backbone, exhibits considerably less shear-thinning behavior resulting from the lesser degree of chain alignment and structural deformation of the SCB backbone. In contrast, the H-(SCB_arm) polymer, where the short branches are allocated along the four long arms, displays a higher degree of shear-thinning behavior arising from an effective tensile force (created by the tightly coiled “superbead” character of the arms via fast short-branch dynamics) that stretches out the backbone. Importantly, the fundamental role of the short branches in determining rheological characteristics of the SCB H-polymers remains unchanged, regardless of the flow type and flow strength.

The deformation behavior of short-chain branched, amorphous polyethylene (PE) was investigated using united atom molecular dynamics (MD) simulations under uniaxial stretching. Our research reported in this paper examined the internal mechanisms underlying the mechanical response of the PE by analyzing potential energies, dihedral angle distributions, chain orientation and entanglements. The findings indicated that the short-chain branching generally enhanced Young’s modulus, except for sample B60 with a critical ethyl branch content of 60/1000 (60 ethyl branches per 1000 backbone carbons). The Young’s modulus of B60 was the local minimum value in the various samples. Analysis of the evolution of dihedral angles shows that the short-chain branching hinders the transition from gauche to trans configurations, leading to a decrease in the trans population as the branch content increases. Regarding the sample B60 with critical branch content 60/1000, the orientation parameter and entanglement parameter of the molecular chains along the backbone were lowest in the initial structure, and the efficiency of chain disentanglement was at its lowest during deformation. The simulation results elucidated the deformation mechanism related to the conformational evolution of the molecular chains, which can provide atomic-scale insights into the mechanical properties and processability of branched polyethylene, thereby allowing for optimization in its design and application.

A complex branched polyethylene resin with excellent processing and film-forming properties is fractionated through solvent gradient fractionation (SGF) technique. Here, the good solvent is 1,2,4-trimethylbenzene (TMB) and poor solvent is ethyl cellosolve (ECS). The fractions are further analyzed using high-temperature gel permeation chromatography (GPC) coupled with triple detectors (refractive index (RI)-light scattering (LS)-viscometer (VIS)), and 13C-nuclear magnetic resonance spectroscopy (13C-NMR). The molecular weight distribution of SGF fractions is very narrow, most of them are less than 1.1. The molecular weights of SGF fractions gradually increase as the content of good solvent increases in the mixture. The fractions with different molecular weights all have branching structure, the short chain branching is major in all fractions and along with certain content of long chain branching. Branching distribution across the molecular weight distribution is discussed in detail, and branching distribution within a SGF fraction is also researched.

Successive Self-Nucleation and Annealing (SSA) in the Solvated state (STAF) is a thermal analysis technique which provides a more rapid testing alternative to temperature rising elution fractionation (TREF) for limited data analysis. Differential scanning calorimetry (DSC) is used to observe the melting behavior of the polymer after it is subjected to a series of discrete annealing steps. These annealing steps are intended to fractionate the polymer based on its short chain branching content, molecular mass, or the ‘crystallizability’. After the annealing steps are complete, the sample is cooled, and then a final melting step is performed. The resulting endotherm shows a series of peaks that correspond to each annealing step, with a decrease in short chain branching resulting in a higher peak temperature. The partial area of the highest temperature peaks above 90°C is used to calculate the fraction containing the lowest amount of short chain branching, usually assigned as the ‘homopolymer’ or ‘high density’ fraction. This method can be customized to tailor the required information for shorter run times. By eliminating some annealing steps, the shortened STAF SSA method has been shown to be able to detect and quantify minute amounts of the homopolymer fraction.

Recent investigations have shown that different topographies in polyethylene (PE) lead to either thermorheological simplicity (linear and short-chain branched PE) or two different types of thermorheologically complex behavior. Low-density polyethylene (LDPE) has a thermorheological complexity, which can be eliminated by a modulus shift, while long-chain branched metallocene PE (LCB-mPE) has a temperature dependent shape of the spectrum and thus a total failure of the time-temperature superposition principle. The reason for that behavior lies in the different relaxation times of linear and long-chain branched chains, present in LCB-mPE. The origin of the thermorheological complexity of LDPE might be the temperature dependence of the miscibility of the different molar mass fractions that differ in their content of short chain branches.

Microstructural study on low viscosity poly-α-olefin oils synthesized via AlCl3/H2O cationic system in the present of xylene and heptane solvents

A molecular dynamics study of the effects of branching characteristics of LDPE on its miscibility with HDPE

Blends of high density polyethylene (HDPE) and polypropylene (PP) were prepared by the melt mixing method. The morphology and the rheological properties of extrudates were measured and related with the physical properties of the melt spun fibers, such as X-ray diffraction patterns, tensile properties, and birefringence. From the SEM photographs of the fractured surfaces, it can be seen that PP forms a continuous phase in blends of 25% and 50% of HDPE, while HDPE formed in a 75% blend. In all cases, the discrete phases were finely distributed. The viscosity and the melt elasticity of HDPE were higher than those of PP and those of HDPE/PP blends were intermediate. The first normal stress difference of the extrudates increased with the increase of HDPE content, and thus the birefingence and the initial modulus of the spun fibers did with the HDPE content. However, the crystalline orientation did not show any dependence on the composition. Tenacity showed a deviation from linearity due to the incompa...

Creeping Wild Rye (CWR), Leymus triticoides, is a salt-tolerant and perennial grass, which is planted in San Joaquin Valley, California in the USA to lower salt contents from drainage irrigation water to prevent water streams and soil from pollution and alkalization, respectively. In this study, CWR was used as raw materials to manufacture medium-density particleboard. The objective of this research was to characterize the qualities (mechanical properties and water resistance) of CWR particleboards bonded by high density polyethylene (HDPE) from recycled food carrying bags, polymeric methane diphenyl diisocyanate (PMDI) and/or rice bran adhesive (RBA). The evaluated properties of particleboard included tensile strength (TS), modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond strength (IB). Water absorption and thickness swell were used to evaluate the water resistance properties of finished CWR particleboard. The effects of the amount of HDPE, PMDI and/or RBA on the qualities of finished particleboards were determined. The partial PMDI replacement with RBA did not compromise the mechanical quality, but yield lower water resistance. Adding HDPE, however, improved the overall qualities of particleboards. In order to optimize the properties of medium-density particleboard made from CWR and HDPE, an orthogonal experimental design was employed. Results showed that up to 30% PMDI can be replaced by adding 20% HDPE with size of 10×10mm to manufacture the particleboard with higher mechanical strength and the water resistance particleboards than without HDPE.

Population balances were used to model the short chain branch distribution of polyethylene made with Ni-diimine catalysts. Branches are formed according to the chain walking mechanism without addition of comonomer. The proposed model can be used to simulate the cumulative or instantaneous distributions of short chain branches formed with these catalysts. The instantaneous distribution is particularly useful, since it can be applied to interpret experimental results with minimum effort. A simplified version of the complete instantaneous model was compared with experimental results for polyethylene branching measured with 13C NMR. The mathematical model can predict the experimental trends well and constitutes a useful data interpretation tool for the investigation of polymerization with these catalysts.

Langevin molecular dynamics (LMD) simulations have been performed to understand the role of the short chain branches (SCB) on the formation of ordered domains by cooling dilute solutions of ethylene/α‐olefins copolymer models. Three different long single‐chain models (C2000) with 0, 5, and 10 branches each 1000 carbons were selected. These models were equilibrated at high reduced temperature (T* = 13.3) and cooling in steps of 0.45 until the final temperature (T* = 6.2) by running a total of 35 × 106 LMD steps. During the cooling process, global order parameter, torsion distribution, position of the branches, and local‐bond order parameter were calculated and monitored. The peaks of crystallization for each model were calculated by differentiating the global order parameter with temperature. The Tc (crystallization temperature) decreases as the number of branches increases as has been experimentally reported. The formation of order in the copolymers is affected by the amount of the SCB in the backbone of the polymer chain. Initially, the SCB move to the folding surface. Once the SCB are located near the folding surface the order starts to grow. In all cases here shown, the C4 branches are excluded from the ordered domains. To take into account, the influence of the branch distribution, a different branch distribution model has been considered for the two‐branched systems. The crystallization fraction (α) and the density of the amorphous and ordered fractions was defined using the local‐bond order parameter. Both magnitudes decrease as the number of branches increases. These facts fairly agree with experimental literature data. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011