Effect Of Short Chain Branching Research Articles

Investigate the effect of short chain branching and lamellar thickness on mechanical and optical properties of linear low-density polyethylene by successive self-nucleation/annealing (SSA) technique

In this article, various polyethylenes based on Ziegler-Natta and metallocene catalysts with different comonomer contents were investigated by successive self-nucleation and annealing (SSA) technique. Lamellar thickness, short chain branches content (SCBC) and methyl sequence lengths (MSL) were calculated by the modified Thomson-Gibbs equation and suitable calibration curves available in the literature. Moreover, linear low-density polyethylene (LLDPE) samples with different physical, mechanical and optical properties were investigated according to heat of fusion, the lamellar thickness, SCBC and methyl sequence length (MSL) of the multiple peaks. The results showed that the mechanical and optical properties such as dart drop impact strength, Elmendorf tear resistance, softening point and haze are associated with the lamellar thickness and SCBC of the resins.

On the melting behaviour of linear polyethylene single crystals in mixtures with homogeneous short chain branched polyolefins

The melting temperature depression of linear polyethylene lozenge single crystals is thoroughly examined in mixtures with a series of homogeneous high molecular weight branched polyethylene models, with special emphasis on the effect of short chain branching on the compatibility. The results indicate that the melting temperature of the lamellar single crystals is influenced both by the branching content of the surrounding matrix and by the relative volume concentration of the two polymers in the mixture. The discussion of the results is conducted around the compatibility of polymeric species as a function of the short chain branching content, invoking experimental and computational work performed by other authors. The observed depression indicates that the homogeneous mixing of the two polymers at the melting point may occur whenever the branching content is lower than 6–7 every 100 carbon atoms, a result that is reconciled with computer simulations and other experimental approaches.

Direct All-Atom Molecular Dynamics Simulations of the Effects of Short Chain Branching on Polyethylene Oligomer Crystal Nucleation

Using all-atom molecular dynamics (MD) simulations, we demonstrate that short alkyl branches can hinder the nucleation of polyethylene (PE) oligomers. Although one methyl and ethyl branch in a 50-carbon oligomer may only slow the nucleation kinetics mildly, bulkier side chains, such as propyl, butyl, and hexyl branches, disturb the arrangement of neighboring backbone atoms, preventing these atoms from joining a growing crystal, and therefore significantly suppress the nucleation of PE crystals, with no clear evidence of nucleation being observed over a 20 ns simulation run when hexyl side chains are present. The degrees of branching and the distributions of short alkyl groups on PE backbones can also affect the crystallization kinetics, with well-spaced branches having a greater impact on crystallization than branches that are grouped within a shorter distance along the backbone that is similar to or shorter than the length of the branch. We show that the linear portions of PE crystallize first and the br...

Effect of short-chain branching on the tie chains and dynamics of bimodal polyethylene: Molecular dynamics simulation

Molecular dynamics simulations were performed on bimodal polyethylene (BPE) in order to shed light on the molecular mechanism of short-chain branching on BPE nucleation process, crystallization process and especially the formation process of tie chains. The study contemplates two kinds of BPE model chains with the same branch content (BC), branch length (BL) but with different short-chain branching distribution (SCBD): short chain branches incorporated in either the long chain or the short chain. For the nucleation process, nucleation time of all models increased with the increase of BC. Short branches distributed on the long chain provoke a delay of the nucleation as compared to the case of short branches distributed on the short chain. Additionally the nucleation process is further delayed as the BL increases. Crystallization rate and crystallinity decrease as BC increases. The crystallinity of the BPE mode chains with branches placed on the long chain were lower than the case of branches placed on the short chain above a critical BC (5/1000C), and this critical BC was independent with BL. When branches distribute on the long chain, short chain is more likely to locate inside the long chain, while for the case of branches distribute on the short chain, the results are opposite. The concentration of tie chains increased with the increase of BC. For BPE model chains with branches placed on the long chains are more likely to form tie chains when compared with the case of branches placed on the short chain. The concentration of tie chains is insensitive to BL. A modified tie chains formation process mechanism is proposed to explain the SCBD effect on tie chains.

Correction: Effect of short-chain branching on interfacial polymer structure and dynamics under shear flow.

Correction for 'Effect of short-chain branching on interfacial polymer structure and dynamics under shear flow' by Sohdam Jeong et al., Soft Matter, 2017, 13, 8644-8650.

Effect of Short Chain Branching on the Interlamellar Structure of Semicrystalline Polyethylene

We use molecular simulations with a united atom force field to examine the effect of short chain branching (SCB) on the noncrystalline, interlamellar structure typical of linear low density polyethylene (LLDPE). The model is predicated on a metastable thermodynamic equilibrium within the interlamellar space of the crystal stack and accounts explicitly for the various chain topologies (loops, tails, and bridges) therein. We examine three branched systems containing methyl, ethyl, and butyl side branches and compare our results to high density polyethylene (HDPE), without branches. We also compare results for two united atom force fields, PYS and TraPPE-UA, within the context of these simulations. In contrast to conventional wisdom, our simulations indicate that the thicknesses of the interfacial regions in systems with SCB are smaller than those observed for a linear polyethylene without branches and that branches are uniformly distributed throughout the interlamellar region. We find a prevalence of gauche...

Effect of short-chain branching on interfacial polymer structure and dynamics under shear flow.

We present a detailed analysis on the effect of short-chain branches on the structure and dynamics of interfacial chains using atomistic nonequilibrium molecular dynamics simulations of confined polyethylene melts in a wide range of shear rates. The intrinsically fast random motions of the short branches constantly disturb the overall chain conformation, leading to a more compact and less deformed chain structure of the short-chain branched (SCB) polymer against the imposed flow field in comparison with the corresponding linear polymer. Moreover, such highly mobile short branches along the backbone of the SCB polymer lead to relatively weaker out-of-plane wagging dynamics of interfacial chains, with highly curvy backbone structures in the intermediate flow regime. In conjunction with the contribution of short branches (as opposed to that of the backbone) to the total interfacial friction between the chains and the wall, the SCB polymer shows a nearly constant behavior in the degree of slip (ds) with respect to shear rate in the weak-to-intermediate flow regimes. On the contrary, in the strong flow regime where irregular chain rotation and tumbling dynamics occur via intensive dynamical collisions between interfacial chains and the wall, an enhancement effect on the chain detachment from the wall, caused by short branches, leads to a steeper increase in ds for the SCB polymer than for the linear polymer. Remarkably, the SCB chains at the interface exhibit two distinct types of rolling mechanisms along the backbone, with a half-dumbbell mesoscopic structure at strong flow fields, in addition to the typical hairpin-like tumbling behavior displayed by the linear chains.

Effect of short chain branching in molecular dimensions and Newtonian viscosity of ethylene/1-hexene copolymers: matching conformational and rheological experimental properties and atomistic simulations

The molecular dimensions in dilute solution and the linear viscoelastic melt properties of a series of linear ethylene/1-hexene random copolymers with variable short chain branching content are reported. The results obtained from size exclusion chromatography and viscosimetry in dilute solution show a molecular contraction as the branching level increases. Additionally, a clear dependence of the Newtonian viscosity with the short chain branching content at T = 463 K is obtained. Both experimental observations are in agreement with previous experimental results found in ethylene/α-olefin copolymers, but more interestingly with recent full atomistic simulations made in our group in this type of macromolecular systems. The dependences observed can be related to the changes observed in the macromolecular conformational features and also in the equilibration entanglement time and the molecular weight between entanglements as the number of short chain branches increases.

Persistence Length of Short-Chain Branched Polyethylene

The effect of short-chain branching (SCB) on the persistence length lp of polyethylene (PE) was studied using small-angle neutron scattering (SANS). In thermodynamically good solvents, lp can be measured directly from the scattering vector qtr at the crossover from good solvent mass-fractal scaling to the rodlike persistent scaling, using the unified equation described in the text. The method was used to study lp of both linear and branched PE in deuterated p-xylene, which is a good solvent for PE at 125 °C. The results indicate an increase in lp of the backbone chain with increasing SCB content, independently measured using Fourier transform infrared spectroscopy (FTIR). These results corroborate the behavior previously reported in Monte Carlo simulations of short-chain branched polymers. A functional relationship for lp in terms of the number of SCBs is proposed.

Metallocene ethylene-1-octene copolymers: Influence of comonomer content on thermo-mechanical, rheological, and thermo-oxidative behaviours before and after melt processing in an internal mixer

The influence of the comonomer content in a series of metallocene-based ethylene-1-octene copolymers (m-LLDPE) on thermo-mechanical, rheological, and thermo-oxidative behaviours during melt processing were examined using a range of characterisation techniques. The amount of branching was calculated from 13C NMR and studies using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to determine the effect of short chain branching (SCB, comonomer content) on thermal and mechanical characteristics of the polymer. The effect of melt processing at different temperatures on the thermo-oxidative behaviour of the polymers was investigated by examining the changes in rheological properties, using both melt flow and capillary rheometry, and the evolution of oxidation products during processing using infrared spectroscopy. The results show that the comonomer content and catalyst type greatly affect thermal, mechanical and oxidative behaviour of the polymers. For the metallocene polymer series, it was shown from both DSC and DMA that (i) crystallinity and melting temperatures decreased linearly with comonomer content, (ii) the intensity of the β-transition increased, and (iii) the position of the tan δ max peak corresponding to the α-transition shifted to lower temperatures, with higher comonomer content. In contrast, a corresponding Ziegler polymer containing the same level of SCB as in one of the m-LLDPE polymers, showed different characteristics due to its more heterogeneous nature: higher elongational viscosity, and a double melting peak with broader intensity that occurred at higher temperature (from DSC endotherm) indicating a much broader short chain branch distribution. The thermo-oxidative behaviour of the polymers after melt processing was similarly influenced by the comonomer content. Rheological characteristics and changes in concentrations of carbonyl and the different unsaturated groups, particularly vinyl, vinylidene and trans-vinylene, during processing of m-LLDPE polymers, showed that polymers with lower levels of SCB gave rise to predominantly crosslinking reactions at all processing temperatures. By contrast, chain scission reactions at higher processing temperatures became more favoured in the higher comonomer-containing polymers. Compared to its metallocene analogue, the Ziegler polymer showed a much higher degree of crosslinking at all temperatures because of the high levels of vinyl unsaturation initially present.

Effect of Short-Chain Branching on the Rheology of Polyolefins

A series of carefully synthesized ethylene/butene copolymers (0.2−0.85 mole fraction butene) were synthesized using metallocene catalysts to probe the relationship between the chemical architecture of polyolefins and their rheology. The dependence of , the plateau modulus, on mb, the molecular weight per backbone bond, which we have seen previously for polyolefins is obeyed by these copolymers. Moreover, the modulus at the frequency at which G‘ = G‘ ‘ also scales with copolymer composition. We also show that the van Gurp−Palmen plots (|G*| vs δ) for these copolymers show a universal behavior, which could allow for the characterization of copolymer composition by rheology. We demonstrate that combining small-amplitude oscillatory shear data with stress relaxation experiments the theory of linear viscoelasticity allows us to determine low-frequency behavior much more rapidly. The zero shear viscosity of these ethylene copolymers also shows a regular dependence on the copolymer composition, and we propose a new scaling relationship for zero shear viscosity in terms of mb. We show that this applies to a wide range of polyolefins and show similar relations for the equilibration time, τe, and monomeric friction factor, ζ.

Effect of short chain branching upon the crystallization of model polyamides-11

In this paper, we study the effect of short (C7) branches on the crystallization behavior of model polyamides-11. The crystallization kinetics of samples with different amounts of branches are followed by differential scanning calorimetry, turbidity, and small angle light scattering measurements under both quiescent and shear-stimulated conditions. It is found that the presence of small amounts of C7-branches (<3 branches per 1000 atoms in the main chain) sensibly slows down the overall crystallization kinetics and the spherulitic linear growth rate. The application of relatively strong shear conditions (up to shear rates of 10 1/s and 60 strain units) does not provide any sensible increase of the crystallization kinetics and any modification of the spherulitic morphology.

Effect of Short Chain Branching of LDPE on its Miscibility with Linear HDPE

AbstractSummary: The influences of short chain branching (SBC) on the melt miscibility of low‐density polyethylene (LDPE) with linear high‐density polyethylene (HDPE) were investigated by rheological methods. Two LDPE resins with different branch contents were blended with the same linear HDPE. Dynamic and steady shear measurements were carried out using a Rheometrics ARES rheometer at 190 °C. The rheology of the low‐SCB LDPE (9 CH3/1 000 C) blends with HDPE can be predicted by the linear additivity rule. Hence, blends were suggested to be completely miscible at all compositions. However, blends of the high branch content LDPE (SCB = 19 CH3/1 000 C) were completely immiscible. Also, the different viscous and elastic properties of all the immiscible blends were much higher than the corresponding values for the more viscous and elastic blend component. The ratio of interfacial tension to droplet radius was estimated from Scholz et al. model as ≈1 500 N · m−2. The level of SCB in LDPE was found to have a strong influence on its miscibility with linear HDPE.Cole‐Cole plot for blends of LDPE1 with HDPE.magnified imageCole‐Cole plot for blends of LDPE1 with HDPE.

The effect of short chain branching on local chain organisation in isotopically labelled blends of polyethylene

The complementary techniques of small angle neutron scattering (SANS) and infra-red spectroscopy have been used to determine features of molecular trajectory for isotopic blends incorporating linear polyethylene and copolymers containing butyl or hexyl branches. SANS data show both a more compact conformation for a copolymer guest molecule than for a linear guest and also a smaller molecular expansion with increasing molecular weight when both guest and host are copolymers. These blends also show the smallest proportion of [lcub ]110[rcub ] isolated guest stems, while wholly linear blends show the largest proportion, on the evidence of the infra-red CD 2 bending band profiles. Estimates are made of the sizes of ‘groups’ of adjacent stems, and these also show a corresponding dependence on the sample type, indicating a higher proportion of adjacent re-entry for copolymer blends than for linear blends.

Effect of Short Chain Branching on the Coil Dimensions of Polyolefins in Dilute Solution

This paper deals with the short branch effect in α-olefin copolymers on radius of gyration Rg and intrinsic viscosity [η] in trichlorobenzene at 135 °C. A wide range of structurally uniform copolymers of ethylene with propylene, 1-butene, 1-hexene, and 1-octene were analyzed by size exclusion chromatography with on-line refractive index, low-angle and multiangle light-scattering and viscometry detectors. Variation of refractive index increment with composition was determined, and corrections for finite concentration were applied. Both Rg and [η] are accurately described by power laws Rg = KsMαs and [η] = KvMαv. The coefficients Ks and Kv vary with comonomer type and amount, but the same exponents, αs = 0.58 and αv = 0.695, apply within the measurement uncertainty to all polymers studied. The ratios gSCB = Rg2/(Rg2)PE and g‘SCB = [η]/([η])PE vary linearly with comonomer weight fraction, with slopes that depend on the comonomer type. Various explanations for the observations are considered.

The effect of short-chain branching on the thermolysis/reactive distillation of polyethylene

Thermolysis, coupled with reactive distillation, was investigated as a process to convert waste olefin-based polymers into value-added products. The degradation of two types of polyethylene, linear low and high density, was investigated. The initial molecular structure of the polymer was found to have a large effect on the rate of molecular weight reduction. The linear low-density polyethylene started to produce volatile products earlier and at a faster rate than the high-density polyethylene. Preferential scission of the side branches from the linear low-density polymer backbone was shown to be one of the first steps of the degradation mechanism. Once the branches were stripped from the linear low-density polyethylene, the degradation products were similar to those produced from high-density polyethylene. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1415–1421, 1999

The effects of short-chain branching and comonomer type on the interfacial tension of polypropylene-polyolefin elastomer blends

The imbedded fiber retraction method was used to assess the effect of increasing octene content and comonomer type on the compatibility of polypropylene-polyolefin elastomer (PP-POE) blends via direct measure of the interfacial tension. The interfacial tension was found to decrease monotonically with increasing octene content from a starting value of 1.5 ± 0.16 dyn cm at an initial octene level of 9% down to 0.56 ± 0.07 dyn cm at an octene content of 24%. These effects can be interpreted in terms of the effective decrease in the molecular weight between chain ends for the branched POE materials. The experimental data were found to be described well by a modification of the empirical relationship used to describe the effect of molecular weight on the interfacial tension for linear materials. The power-law parameter was found to be numerically equivalent for that obtained for the molecular weight dependence of linear materials. The measured interfacial tension was also found to be dependent on the type of comonomer used in the PP-POE systems. The interfacial tension ranged from 1.07 ± 0.09 dyn cm for a PP-POE system made using ethylene-propylene down to 0.56 ± 0.07 dyn cm for a PP-POE made using ethylene-octene (24% octene). © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1175–1181, 1997

Effect of short chain branching on the blown film properties of linear low density polyethylene

Three linear low density polyethylene (LLDPE) resins of similar melt index and density were synthesized with different comonomers in the Unipol pilot-plant scale reactor. The molecular structure, blown film morphology, and film strength properties of the resins have been comprehensively characterized. The film dart drop impact strength of the LLDPEs increases in the order of ethylene/1-butene, ethylene/1-octene, and ethylene/1-hexene copolymers; whereas the Elmendorf tear strength of them increases in the order of ethylene/1-butene, ethylene/1-hexene, and ethylene/1-octene copolymers. The mechanical properties seem to be highly associated with the length and distribution of short chain branches and, consequently, the lamellar thickness distribution of the resins. © 1996 John Wiley & Sons, Inc.

Effect of short chain branching on the blown film properties of linear low density polyethylene

Three linear low density polyethylene (LLDPE) resins of similar melt index and density were synthesized with different comonomers in the Unipol pilot-plant scale reactor. The molecular structure, blown film morphology, and film strength properties of the resins have been comprehensively characterized. The film dart drop impact strength of the LLDPEs increases in the order of ethylene/1-butene, ethylene/1-octene, and ethylene/1-hexene copolymers; whereas the Elmendorf tear strength of them increases in the order of ethylene/1-butene, ethylene/1-hexene, and ethylene/1-octene copolymers. The mechanical properties seem to be highly associated with the length and distribution of short chain branches and, consequently, the lamellar thickness distribution of the resins. © 1996 John Wiley & Sons, Inc.

Effect of short chain branching on the viscoelastic behavior during fatigue fracture of medium density ethylene copolymers

AbstractA method to determine viscoelastic changes in medium density polyethylene (MDPE) pipe specimens associated with the crack tip during fatigue crack initiation (FCI) and propagation (FCP) experiments is described. The load‐displacement curves are analyzed to obtain the phase angle, δ. Changes in δ are related to the number of cycles of crack initiation of three different MDPE copolymers: hexene (H), butene (B), and methyl pentene (MP) copolymers. These changes are related to craze formation and growth at the notch tip, leading to crack initiation and to the irreversible work, Wi, expended on them. Within a given material, step wise increments in δ distinguish the onset of crack initiation and the brittle‐to‐ductile transition in crack growth. The magnitudes of tan δ and Wi are noted to be in quantitative agreement with the resistance of the three copolymers to FCI and brittle propagation that rank in the order: isobutyl (MP) &gt; ethyl (B) &gt; butyl (H). Similar crystallinity of the three copolymers insinuates a hypothesis that variance in the nature of chain entanglements associated with the respective branch type might be accountable for the observed differences in viscoelastic character. The final stage of failure by ductile tearing is dominated by large scale plastic flow that seemingly overshadows the material differences governing time dependent brittle fracture.