Presence Of Long Chain Branching Research Articles

Enhancing the marine antifouling property of high-density polyethylene through melt grafting of poly (hexamethylene guanidine)

Acquiring environment-friendly, wide-spectrum antifouling materials is currently a significant challenge. Herein, we focused on covalently bonding poly (hexamethylene guanidine) (PHMG) to high-density polyethylene (HDPE) through a melt grafting reaction. The successful occurrence of this grafting reaction was confirmed by FT-IR analysis. Moreover, the presence of long-chain branching (LCB) was established through Cole-Cole plot, van Gurp-Palmen diagram, and dynamic thermomechanical property. This presence of LCB improved the impact strength of modified HDPE materials. An increased PHMG grafting content in the material led to a significant increase in the inhibition rate of Escherichia coli and Staphylococcus aureus growth, without any detectable leaching of PHMG. Furthermore, inverted fluorescence microscopy revealed a substantial inhibitory effect of the modified HDPE on diatom attachment and Chlorella sedimentation. Specifically, compared with the unmodified HDPE material, the HDPE with 2 wt% PHMG grafting showed a 79.8% decrease in the attachment rate of diatoms and a 64.5% reduction in Chlorella sedimentation. This research provides valuable insights into the development of eco-friendly antifouling materials with enhanced marine antifouling properties.

Coagent Modified Polypropylene Prepared by Reactive Extrusion: A New Look into the Structure-Property Relations of Injection Molded Parts

AbstractPeroxide-mediated reactive extrusion of linear isotactic polypropylene (L-PP) was conducted in the presence of trimethylolpropane trimethacrylate (TMPTMA) and triallyl trimesate (TAM) coagents, using a twin screw extruder. The resulting coagent-modified polypropylenes (CM-PP) had higher viscosities and elasticities, as well as increased crystallization temperature compared to PP reacted only with peroxide (DCP-PP). Additionally, deviations from terminal flow, and strain hardening were observed in PP modified with TAM, signifying the presence of long chain branching (LCB). The CM-PP formulations retained the modulus and tensile strength of the parent L-PP, in spite of their lower molar mass and viscosities, whereas their elongation at break and the impact strength were better. This was attributed to the finer spherulitic structure of these materials, and to the disappearance of the skin-core layer in the injection molded specimens.

The influence of long chain branches of LLDPE on processability and physical properties

Two polyethylene-based on single-site metallocene catalyst (mLLDPE) were selected to characterize the effect of long chain branching (LCB) on blown film processability, optical and mechanical properties. 13C NMR and parallel plate rheology were used to identify LCB presence on LLDPEs. Blown films were produced from 100% LLDPEs using three different machine direction (MD) stretch ratios. When the same processing conditions for the two LLDPEs grades were used, better processability was observed for LLDPE with LCB. In relation to mechanical and physical properties, Elmendorf tear and optical properties were highly influenced by the presence of LCB. Tear resistance is affected by film orientation and is inversely proportional to the level of LCB in the polymer. It was observed a reduction of 50% in the MD tear strength when comparing with the polymer without LCB. However, haze decreases significantly with the presence of LCB, about 40%.

Comparison between decrosslinking of crosslinked high and low density polyethylenes via ultrasonically aided extrusion

Among various polymer wastes, management of crosslinked plastics is a major environmental problem requiring a solution. Among various crosslinked plastics, recycling of crosslinked polyethylenes is of a great importance due to the presence of a three-dimensional network. To solve this problem, novel environmentally friendly technologies for decrosslinking of the crosslinked polymers are developed based on ultrasonically assisted single (SSE) and twin screw (TSE) extruders. In particular, decrosslinking of peroxide crosslinked high-density polyethylene (XHDPE) and low-density polyethylene (XLDPE) by means of an ultrasonic SSE and TSE is investigated. Barrel pressure, die pressure and ultrasonic power consumption during extrusion are recorded. Swelling, rheological, thermal analysis and tensile tests are used to elucidate the structure–property relationships of decrosslinked XHDPE and XLDPE. The frequency dependencies of the storage and loss moduli, complex viscosity and tangent loss of XHDPE, XLDPE and their decrosslinked networks are described by the post critical gel model with its parameters correlated with gel fraction and crosslink density. The dynamic, thermal and tensile properties of the decrosslinked XHDPE and XLDPE are greatly affected by the type of preferential bond breakage. It was found that the decrosslinking of XLDPE is more difficult than that of XHDPE. An analysis based on the Horikx function reveals a highly preferential breakage of crosslinks during decrosslinking of XHDPE. In contrast to decrosslinking of XHDPE, the presence of long-chain branching in XLDPE is found to lead to the breakage of its main chains during decrosslinking. An improvement and a reduction in mechanical properties of decrosslinked XHDPE and XLDPE are, respectively, observed in comparison with those of virgin XHDPE and XLDPE. Such differences in behavior are due to the occurrence of a highly preferential breakage of crosslinks in XHDPE and the breakage of main chains in XLDPE during ultrasonic decrosslinking.

Extrusion film casting of long chain branched polypropylene

Extrusion film casting (EFC) is an important melt processing operation which is extensively used to make polypropylene (PP) films. Linear PP shows significant amount of necking and draw resonance during EFC. One of the ways to reduce necking is to introduce long chain branches (LCB) on the polymer backbone. The long branches impart extensional strain hardening behavior thereby stabilizing the melt flow. In this work, we investigate the influence of long chain branching in polypropylene on the extent of necking in the EFC process. Laboratory scale EFC experiments were performed on homopolymer PP of linear and long chain branched architectures. Simulations of the EFC process were carried out using the one-dimensional flow model of Silagy et al., Polym. Eng. Sci., 36, 2614 (1996) into which we incorporate two different multi-mode molecular constitutive equations namely, the ‘eXtended Pom-Pom’ equation (XPP, for long chain branched PP) and the ‘Rolie-Poly’ equation (RP-S, for linear PP). Our experimental data confirm that presence of long chain branching in PP reduces the extent of necking and our numerical predictions show qualitative agreement with experimental data, thereby elucidating the role of chain architecture on the extent of necking. POLYM. ENG. SCI., 55:1977–1987, 2015. © 2014 Society of Plastics Engineers

Long chain branched impact copolymer of polypropylene: Microstructure and rheology

A biphasic impact copolymer of polypropylene (ICP) was modified with peroxide by reactive extrusion process resulting in reduced melt flow index, improved melt strength, and higher die swell. The polymers were for the first time subjected to systematic rheological and microstructural characterization in an effort to understand their structure‐property relations. In shear rheological tests, the modified ICP displayed higher flow activation energy, reduced values of loss tangent and nearly equal frequency dependence of storage and loss modulli. The modified ICP also showed strain hardening behaviour in uniaxial extensional rheology and higher crystallization temperature in differential scanning calorimetry (DSC). All these are definitive indications of the presence of long chain branches (LCB). Fitting the rheological data of modified ICPs with the eXtended Pom Pom (XPP) model indicated the presence of LCB on the higher molecular weight fraction in the polymer, a result which was corroborated with multi‐detector high temperature gel permeation chromatography (HT‐GPC). More importantly, the matrix and rubber phases of the ICP were separately characterized for presence of long chain branching by rheology, DSC and HT‐GPC. The results indicate that while LCB existed in the matrix phase, microgels were present in both phases indicating that the reaction with peroxide occurred in both phases. POLYM. ENG. SCI., 55:1463–1474, 2015. © 2014 Society of Plastics Engineers

Reactive extrusion of PLA, PBAT with a multi-functional epoxide: Physico-chemical and rheological properties

The aim of this work was to highlight the presence of long chain branching, LCB, due to the incorporation of a multifunctional epoxide, named Joncryl ®, as a chain extender, into both poly (lactic acid) (PLA) and poly (butylene-adipate-co-terephtalate) (PBAT) polymers. The modified polymers were prepared using a twin-screw reactive extrusion system. It has been demonstrated through this study that these modified biopolymers are thermo-rheologically complex. This complexity may be attributed to the formation of covalent bonds between both polymer and chain extender and leads to a failure of the time-temperature superposition (TTS). The linear viscoelastic properties were predicated on the use of the so-called Van-Gurp–Palmen plots, from which the topology of the modified PLA and PBAT has been studied and analyzed. Indeed, they exhibited a typical feature of a mixture of linear and randomly branched polymers. Furthermore, the resulting linear and branched chains, due to the reactive highly functionalized epoxide, are discussed using rheological investigations (relaxation spectra, flow activation energy) coupled with solution viscometry (solution viscometry properties, hydrodynamic radius) and physico-chemical properties (size-exclusion chromatography).

Synthesis and rheological investigation of model symmetric 3-arm star polyethylene

A variety of linear and 3-arm star polyethylene (PE) model polymers covering a wide range of molecular weight are synthesized by the living polymerization of butadiene and the subsequent hydrogenation. Several rheological parameters of these model linear and 3-arm star PE samples are analyzed for detecting the long chain branching (LCB) structure. It is found that the analyses based on zero shear viscosity, vGP plot and flow activation energy are very sensitive to the 3-arm star PEs. The information on the presence of LCB can be obtained with these methods even for low molecular weight samples, which can not be determined by GPC-MALLS. However the information about the LCB structure can not be obtained by the rheological methods alone.

Effect of high molar mass species on linear viscoelastic properties of polyethylene melts

The molecular features and the linear viscoelastic melt properties of a series of polydisperse high density polyethylene are investigated. The main molecular characteristic of the materials studied is the presence of high molar mass species, which give rise to unusual values of the Mz/Mw ratio. This molecular particularity strongly affects to the linear viscoelastic melt properties as the Newtonian viscosity and the steady-state shear recoverable compliance. Both the third and the fourth moments of the molecular weight distribution affect to the values of these properties, which follow the trend expected by the reptation model. The evidences clearly prove the effect not only of the polydispersity index, but more interestingly of the shape of the molecular weight distribution on the dynamics of the systems. The behavior shown by the experimental samples studied in this work is interesting as the linear viscoelastic properties have become widely used, not only to test the molecular weight distribution dependence of the viscoelastic fingerprint and the processability, but also to assess the possible presence of long chain branching.

Effect of molecular weight distribution on Newtonian viscosity of linear polyethylene

A strong effect of the third moment of the molecular weight distribution on the melt linear viscoelastic response of linear polyethylene samples is reported. The Newtonian viscosity of samples with broad molecular weight distributions shows a noticeable increase with respect to model linear samples with the same weight-average molecular weight but symmetric and narrower molecular weight distributions. These experimental results are in agreement with previous empirical and theoretical studies based on reptation concepts that predict a dependence of the viscosity in terms of the M z/M w ratio. In addition, it is possible to obtain a description of the whole linear viscoelastic fingerprint of the samples by mapping the entanglement parameters obtained from atomistic simulations together with the molecular features into the reptation model. The results shown here are of great importance, as the Newtonian viscosity–molecular weight relationship has become a widely used tool, not only to test the molecular weight dependence, but also to assess the possible presence of long-chain branching in polyolefins.

Structural determination of ethylene‐propylene‐diene rubber (EPDM) containing high degree of controlled long‐chain branching

AbstractThis work highlights an attempt to characterize the degree and nature of long‐chain branching (LCB) in an unknown sample of ethylene‐propylene‐diene rubber (EPDM). Two EPDM rubbers selected for this study were comparable in comonomer compositions but significantly different with respect to molar mass and the presence of LCB. Both rubbers contained 5‐ethylidene‐2‐norbornene (ENB) as diene. Solution cast films of pure EPDM samples were used for different characterization techniques. 1H‐NMR, and 13C‐NMR were used for assessing the comonomer ratios and LCB. Size exclusion chromatography (SEC) equipped with triple detector system was used to determine the molar mass (both absolute and relative) and polydispersity index (PDI). Presence of branching was also detected using sec‐viscometry. Rheological analysis has also been used for characterizing LCB. Finally, on the basis of the experimental findings and the available theories, an attempt was made to identify the chemical nature and degree of LCB. This study reveals the possibility of detailed characterization of molecular architecture of EPDM containing LCB by comparing with an essentially linear EPDM in light of an existing theory. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Detection and quantification of branching in polyacrylates by size-exclusion chromatography (SEC) and melt-state 13C NMR spectroscopy

Chain branching has been investigated in a homologous series of poly( n-alkyl acrylates) (methyl, ethyl, n-butyl, n-hexyl) obtained by radical polymerization. The total amount of chain branching was quantified using melt-state 13C nuclear magnetic resonance (NMR) spectroscopy. It gave access to low degrees of branching in both soluble and insoluble polyacrylates, homopolymers and copolymers. The lowest degree of branching was found for the ethyl member of the series with quantification by conventional solution-state NMR found to take a prohibitively long time. The method proposed here is compared to the ones published previously, and previous literature results are critically reviewed. The presence of long-chain branching (LCB) was selectively detected using multiple-detection size-exclusion chromatography (SEC), with LCB being found for all soluble homopolymers but the poly( n-butyl acrylate). This finding was confirmed by close examination of the Mark–Houwink parameters for the various polyacrylates studied in this work or those previously published.

Viscoelasticity and macromolecular topology in single-site catalyzed polyethylene

The relationship between polymer chain topology, i.e. long chain branching (LCB), and rheological properties in polyolefins obtained using single-site catalysts has been the topic of major scientific activity since the late 1990s [1–14]. Indeed, melt-state rheological measurements are being considered as one of the most sensitive methods for detecting sparse LCB in polyethylene (PE) [1, 15]. For example the flow activation energy, Ea, related to the temperature dependence of viscoelastic properties, is altered by the presence of LCB, and the cross-point frequency between storage and loss moduli, xx, and the crosspoint modulus, Gx, are also very sensitive features to the molecular architecture [16–18]. In this context, the study of these properties can give us information about the relationship between polymer chain topology and catalyst structure. In this work, a set of PE samples was synthesized using different metallocene catalysts. The main objective is to find relationships between chemical structure of the catalysts and polymer chain architecture by rheological testing. This is an interesting task as there is still a lack of information in the literature concerning how the catalyst structure affects the creation of LCB during polymerization [15]. The PE samples were synthesized using the catalysts shown in Table 1, together with MAO as co-catalyst. The synthesis of the samples was carried out under the same experimental conditions: a reactor temperature of 20 C, an ethylene pressure of 2 bar, 3 9 10 mol L of metallocene concentration, and a molar ratio MAO (methyl aluminoxane)/metallocene of 3,000 [17–21]. The metallocene catalysts were prepared using synthetic protocols that we have previously reported [20, 21]. The molecular properties of the polymer samples obtained by size exclusion chromatography analysis are listed in Table 1. The details about the experimental procedures are described elsewhere [18, 19]. It has been very difficult to estimate the molecular features in the case of C4 sample, probably due to extremely high molecular weight. In this case we have estimated molecular weight and polydispersity index by rheological methods. The materials were stabilized with 1.0 wt.% Irganox 1010 to avoid degradation at high temperatures. The rheological measurements were carried out using parallel plate geometry (15 mm diameter). Frequency sweeps in the linear viscoelastic regime were performed with a Bohlin CVO rheometer in the angular frequency range between 0.006 and 60 rad s. The linear viscoelastic regime has been located by previous strain sweeps. The measurements have been performed in the temperature range between 145 and 190 C and the time-temperature superposition principle (TTSP) applied to a reference temperature of T0 = 190 C. From these results, the characteristic features described above (Ea, xx, and Gx) have been obtained for the materials under study (see Table 1). The material labeled as A0 has been synthesized by using the [Zr(g-C5H5)2Cl2] catalyst (A), which is the simplest structural fragment used in this study. This catalyst has been previously reported to produce LCB [2, 14]. The sample shows a very high Ea value and a significant elastic character, in spite of the very narrow molecular J. F. Vega (&) J. Martinez-Salazar Departamento de Fisica Macromolecular, Instituto de Estructura de la Materia, CSIC, Serrano 113 bis, 28006 Madrid, Spain e-mail: imtv477@iem.cfmac.csic.es

Processability of LLDPE/LDPE blends: Capillary extrusion studies

AbstractThe processing behavior of a number of linear low‐density polyethylenes/low density polyethylene (LLDPE/ LDPE) blends with emphasis on the effects of long chain branches is presented. A Ziegler‐Natta linear low‐density polyethylene was blended with four low‐density polyethylene LDPE's having distinctly different molecular weights. The weight fractions of the LDPEs used in the blends were 1, 5, 10, 20, 50, and 75 wt%. Capillary extrusion reveals that the onset of sharkskin and gross melt fracture are slightly influenced with the addition of LDPE into LLDPE. However, the amplitude of the oscillations in the stick‐slip flow regime was found to scale well with the weight fraction of LDPE. Amounts as low as 1 wt% LDPE have a significant effect on the amplitude of pressure oscillations. These effects are clearly due to the presence of long chain branching (LCB); furthermore, it was observed that the onset of this flow regime was shifted to higher shear rates with increase of LDPE content. On the other hand, shear rheology is not sensitive to detect addition of small levels of LDPE up to 20 wt%. Extensional rheology can detect levels of LDPE as small as 1 wt% only at high Hencky strain rates (typically greater than 5s−1) and only for certain blends, typically those that contain LDPE of high molecular weight. It is suggested that the magnitude of oscillations in the oscillating melt fracture flow regime is a sensitive method capable of detecting low levels of LCB. POLYM. ENG. SCI., 47:1317–1326, 2007. © 2007 Society of Plastics Engineers

Molecular Characterization of Semi-Fluorinated Copolymers with a Controlled Amount of Long-Chain Branching

The molecular structure of various semi-fluorinated polymers derived from tetrafluoroethylene, hexafluoropropylene, and vinylidenefluoride is examined by different characterization methods in dilute solution. The polymer synthesis was tailored to set up samples with a straight linear topography as well as model polymers with a controlled amount of long chain branching. As anticipated from the reaction kinetics of the polymerization process, the absolute molar mass distributions of the linear samples as obtained by size exclusion chromatography coupled with multiangle light scattering are well described by a Schulz−Zimm distribution. With [η] ∼ M0.73 and 〈s2〉1/2 ∼ M0.53 in THF, scaling laws typical of coil dimensions of linear macromolecules in good solvents were observed. In the second set of samples, the presence of long-chain branching (LCB) becomes evident from a reduced radius of gyration in the high molar mass region. Solution viscometry on the polydisperse samples reveals a decreasing contraction fa...

Surface characterisation of ethylene–propylene–diene rubber upon exposure to aqueous acidic solution

Two types of pure ethylene propylene diene rubbers were exposed to two different acids for varying period of time. Surface characterisation was carried out using X-ray photoelectron spectroscopy (XPS). Two EPDM rubbers selected for this study were comparable in co-monomer compositions but significantly different with respect to molar mass and the presence of long chain branching. Both rubbers contained 5-ethylidene-2-norbornene (ENB) as diene. Solution cast films of pure EPDM samples were exposed in two different acidic solutions, viz. chromosulphuric (Cr (VI)/H 2SO 4) and sulphuric acid (H 2SO 4) (20%, v/v) at ambient temperature from 1 to 12 weeks. XPS analysis indicated that several oxygenated species were formed on the surface of both rubbers after exposure. It was postulated from the XPS analyses that both aqueous acidic solutions attacked the olefinic double bonds (C C) of ENB. Furthermore, 20% Cr (VI)/H 2SO 4 also attacked the allylic carbon–hydrogen (C H) bonds of ENB resulting in more oxygenated species on the surface compared to 20% H 2SO 4 under identical conditions. Cr (VI) in the 20% Cr (VI)/H 2SO 4 was found to play an important role in alteration of surface chemistry. Studies using a model system consisting of EPDM mixed with Cr (VI) and Cr (III) salts revealed that the change of oxidation state from Cr (VI) to Cr (III) as a consequence of direct involvement of Cr (VI) in the chemical alteration of EPDM surfaces. Interestingly, the presence of long chain branching and molar mass did not significantly influence the chemical processes owing to the acid treatment.

New aspects on the rheological behaviour of metallocene catalysed polyethylenes

In this article, the linear viscoelastic properties of a family of metallocene-catalysed ethylene homopolymers and ethylene/1-hexene copolymers near the ‘plateau’ and in the terminal regions have been measured. The zero shear viscosity and relaxation time both scale to 3.41 power of the molecular weight, as is observed for most of the linear polymers. At the same time, no rheological signs associated with the presence of long chain branching are detected. However, an unexpected low value of the plateau modulus has been found. This result is discussed in terms of a possible decrease in the entanglement density and/or an increase in chain flexibility with respect to other model polymers. A simple method, previously tested in the literature, based on simplified molecular dynamics, enables a correct description of the dynamic modulus from the knowledge of the form of the molecular weight distribution and the material rheological constants G N o and η o.

Rheological characterization of molten ethylene–α-olefin copolymers synthesized with Et[Ind] 2ZrCl 2/MAO catalyst

The linear viscoelastic behavior of copolymers of ethylene with 1-hexene, 1-octene, 1-decene and 1-octadecene prepared with Et[Ind] 2 ZrCl 2/MAO is analyzed. The copolymers with α-olefin concentration greater than approximately 14 mol% have the simple rheological behavior with temperature that may be associated to linear polyethylenes with short chain branches. On the other hand, the polymers having comonomer concentration lower than approximately 14 mol% are thermo-rheologically complex materials. The rheological behavior of these copolymers may not be explained solely by the presence of long chain branching. A dendritic-like molecular structure with branches-on-branches has to be considered to explain the complexity observed in the case of the ethylene copolymers with low comonomer concentration and ethylene homopolymer. The complex chain structure would be the result of the incorporation of branched macromonomers in the polymers produced by single-site catalysts. The level of branching and complexity of molecular structure increases as the comonomer concentration decreases. No dependence with the length of the α-olefin was found in any of the analyzed parameters.

Long chain branching and polydispersity effects on the rheological properties of polyethylenes

AbstractThe rheological behavior of linear, and branched polyethylenes is studied as a function of the weight average molecular weight (Mw) and its distribution (MWD) as well as the level of long chain branching in an attempt to identify correlations between long chain branching, polydispersity and rheological properties. It is found that a need for vertical shift of the viscoelastic moduli data to obtain the master curves using the time‐temperature superposition principle is associated with the existence of long chain branching in the structure of the polymer. The degree of vertical shift is found to correlate with the level of long chain branching. This correlation corroborates with the observation that long chain branching correlates with the horizontal flow energy of activation. Plots of atan(G″/G′) vs. G* (known as Van Gurp plots) also reveal some important features that can be used as signs of specific features in the structure of polymers. More specifically, the area included below the Van Gurp curves correlates with the level of long chain branching and polydispersity index. The correlations are presented in graphical form and they can be used to associate rheological properties with the presence of long chain branching and/or polydispersity.

Shear and extensional rheology of sparsely branched metallocene-catalyzed polyethylenes

The purpose of this study was to identify any rheological effects that are consistent with the presence of sparse levels of long chain branching (LCB) in three commercial metallocene-catalyzed polyethylenes (MCPE) all of the same melt flow index of 1.0. Two Dow INSITE MCPEs with apparently varying levels of LCB of approximately 0.17 and 0.57/10 000 carbon atoms and one Exxon EXXPOL MCPE with no LCB were studied. The breadth of distribution as determined by Mw/Mn of the three samples was 2.11 for the Exxon and one of the Dow samples, and 2.42 for the other Dow sample that had the highest degree of LCB. The MCPE with the highest branching seemed to have a slightly higher molecular weight tail in the distribution. Both the Dow samples had significantly higher flow activation energies than the Exxon sample, consistent with the presence of LCB, but this method could not distinguish between the two branched polymers. The differences in Mw could also not account for the appreciably higher zero-shear viscosities of the branched samples relative to the linear sample. Despite the differences in Mw and LCB content in the two Dow samples, they exhibited almost identical shear flow curves at temperatures between 120 and 170 °C. They also exhibited very similar shear stress growth behavior. Under constant extension rate deformation, the two samples with LCB showed a significant degree of strain hardening relative to the linear sample. Comparison between the two Dow samples revealed that the sample with the higher degree of LCB showed a greater degree of strain-hardening behavior. The extensional behavior is consistent with the LCB content determined by means of solution light scattering.