Multimodal Molecular Weight Distributions Research Articles

Modeling the spatial characteristics of extrusion flow instabilities for styrene-butadiene rubbers: Investigating the influence of molecular weight distribution, molecular architecture, and temperature

The extrusion flow instabilities of three commercial styrene-butadiene rubbers (SBR) are investigated as a function of molecular weight distribution (MWD); molecular architecture (linear, branched); and temperature. The samples have multimodal MWD, with the main component being SBR and a low amount, less than 10 wt. %, of low-molecular weight hydrocarbons. Deviation from the Cox–Merz rule at high angular frequencies/shear rates becomes intense as the amount of medium-molecular weight component increases. Optical analysis is used to identify and quantify spatial surface distortions, specifically wavelength (λ) and height (h), of the different types of extrusion flow instabilities. Qualitative constitutive models are reviewed and used to fit the experimental data for the spatial characteristics of extrusion flow instability. The fitting parameters as obtained by the models are correlated with molecular properties of the materials. It is found that the characteristic spatial wavelength (λ) increases as the extrusion temperature decreases. Hence, the influence of temperature on the spatial characteristic wavelength is investigated and an Arrhenius behavior is observed.

Preparation of multimodal high and ultra-high molecular weight polyethylene

The multimodal molecular weight distribution of high molecular weight polyethylene combines enhanced mechanical properties and good processing properties. The conventional slurry process produces a high-density polyethylene with mono or bimodal molecular weight distribution, using the classical Ziegler type catalysts supported on inorganic materials. The production of multimodal molecular weight distribution high molecular weight polyethylene is very intricate and requires parallel reactor cascades with different residence times or the use of a supported catalyst with different active sites. This study shows that ethylene polymerization reactions using an unsupported Ziegler type catalyst, in a batch reactor, can produce polymers with a density higher than 0.93 g mL−1, a melting temperature of about 140 °C, and a degree of crystallization of about 79%. The molecular weight distributions of the produced polymer have a broad dispersion of molecular weight (log M) 2 to 7.2 and there are four peaks, showing multimodal distribution and large molar weight. The polyethylene produced using this single-step method is a blend of high and very high molecular weight polyethylene.

Dissecting the cohesiveness among aromatics, saturates and structural features of aromatics towards needle coke generation in DCU from clarified oil by analytical techniques

A comprehensive study relating structural parameters of carbonaceousfeedstock with quality of produced needle coke in delayed coker unit (DCU) has been carried out. The used feedstock, clarified oil (CLO), was studied by various analytical techniques; the results of which was then correlated with needle coke properties. The average structural parameters of CLO as determined by q-1H and q-13C NMR, class-wise distribution of aromatics by HPLC-PDA-ELSD and molecular weight distribution (MWD)-cum-estimation of individual components possessing high needle coke potential from HRMS provided in-depth understanding about the science of maintaining balance between reactivity and fluidity during needle coke production. It was observed that presence of an optimal amount of saturates and all of mono-, di-, tri-, tetra-, penta- aromatics in an optimum ratio help producing good quality needle coke. The study also reveals that while the presence of methyl-substituted aromatics and multimodal MWD are desirable, presence of long chain-substituted aromatics, heteroatoms, non-linear polyaromatics with high bay regions and uniformity in MWD are detrimental to a better mesophase formation and uni-axial rearrangement of molecules during delayed coking process. The quality of needle coke was determined, in addition to CTE values, by crystallinity index using XRD and SEM analysis. Few compositional features of CLO that appears to be critical in determining needle coke character are listed to rate them for their needle coke potential.

Intramolecular cyclization in hyperbranched star copolymers via one-pot Am+Bn+C1 step-growth polymerization resulting in decreased cyclic defect

Hyperbranched star copolymers are important soft materials that have been employed for aqueous self-assembly and bioapplication, but their one-pot one-batch synthesis strategy and relevant topology are rarely discussed. In this contribution, we produce hyperbranched star poly(vinyl ether ester)s (mPEG-hb-PVEEs) amphiphiles with multimodal molecular weight distribution via one-pot one-batch Am+Bn+C1 (m ≥ 2, n ≥ 3) step-growth polymerization. Based on the topological analysis of these hyperbranched star copolymers, a convenient expression of the number ratio of monomeric structural units (NA/NB) is deduced to describe the cyclic defect of intramolecular cyclization only by using proton nuclear magnetic resonance spectroscopy. The introduction of long-chain terminators and the change in the molar feed ratio of A2:B3:C1 considerably affect the NA/NB so as to give rise to increased influence of number of macromolecules and decreased influence of intramolecular cyclization, which are then responsible for an aqueous self-assembly behavior of mPEG-hb-PVEEs amphiphiles. Overall, this study opens new possibilities for the precise description of intramolecular cyclization and controllable synthesis of hyperbranched star copolymers via one-pot Am+Bn+C1 step-growth polymerization.

Mechano‐Optical Characterization of Extrusion Flow Instabilities in Styrene‐Butadiene Rubbers: Investigating the Influence of Molecular Properties and Die Geometry

AbstractThe extrusion flow instabilities of two commercial styrene‐butadiene rubbers are investigated as they vary in isomer content (1,4‐cis, 1,4‐trans, and 1,2 conformation) of the butadiene monomer and the molecular architecture (linear, branched). The investigated samples have similar multimodal molecular weight distribution. Two geometries of extrusion dies, slit and round capillary, are compared in terms of the type and the spatial characteristics of the flow instabilities. The latter are quantified using three methods: a highly pressure sensitive slit die, online and offline optical analysis. The highly pressure‐sensitive slit die has three piezoelectric pressure transducers (Δt ≈ 10−3 s and Δp ≈ 10−5 bar) placed along the die length. The characteristic frequency (fChar.) of the flow instabilities follows a power law behavior as a function of shear rate to a 0.5 power for both materials, . A qualitative model is used to predict the spatial characteristic wavelength (λ) of the flow instabilities from round capillary to slit dies and vice versa. Slip velocities (Vs) are used to quantify the slippage at slit and round capillary dies as well.

Biodegradation of 3D‐printed polylactic acid milliprojections under physiological conditions

AbstractThe degradation of 3D‐printed polylactic acid (PLA) milliprojections of different molecular weights and crystallinity was studied under physiological conditions for a few months. The motivation of the work was to investigate the use of such printed materials as a potential release device in a transdermal patch. Upon being subjecting to a degradation environment, the remnant milliprojections were investigated in terms of weight loss and change in molecular weight. The critical molecular weights of the polymers were found to be between 8,000 g/mol and 13,000 g/mol. Among the polymers examined, the amorphous PLA had the fastest rate of degradation, losing almost half of its weight at the end of the study. Differential scanning calorimetry analysis further showed that the onset of degradation was favored in the amorphous regions of the polymers while scanning electron microscopy (SEM) findings suggest enhanced hydrolysis due to greater exposure caused by the increased porosity and crack formation upon reaching the critical molecular weight. Gel permeation chromatography (GPC) showed the emergence of multimodal molecular weight distribution during degradation indicating the fragmentation in the polymer bulk. The results of SEM and GPC analysis suggest that bulk erosion was the main mechanism of degradation of the 3D‐printed models.

Synthesis of isoprenic polybutadiene macromonomers for the preparation of branched polybutadiene

Polybutadiene macromonomers with a high 1,4-microstructure content and a polymerizable isoprene end group were synthesized by termination of living polybutadienyllithium chains. While different isoprene derivatives were investigated as terminating agents for the preparation of the macromonomers, 2-bromomethyl-1,3-butadiene provided the best results. The termination reaction was optimized in terms of solvent composition, reaction temperature and reactivity-modifying additives used. The addition of 10 eq of terminating agent in equal volumes of hexane and tetrahydrofuran at −110 °C provided the macromonomer in 95% yield. Anionic copolymerization of the macromonomer with 1,3-butadiene in hexane yielded branched polybutadiene with a narrow molecular weight distribution (Mw/Mn = 1.07), but seeded initiation was necessary to avoid side reactions leading to multimodal molecular weight distributions.

Thermal Stability of High Density Polyethylene-Clay Nanocomposites Produced by in situ Solvent Polymerization

High density polyethylene and high density polyethylene-clay nanocomposites were produced using direct solvent polymerization and a Ziegler catalyst system (TiCl4 and triethylaluminum in hexane). The produced polymer has a high average molecular weight and a multimodal molecular weight distribution composed of four distributions including a very high molecular weight component. The laboratory polymer has a thermal stability in inert atmosphere similar to the commercial high density polyethylene produced by Braskem. In oxidant atmosphere the produced polymer presents three thermal oxidation events above 400ºC due to the combustion of low, medium and high molecular weight molecules. The thermal oxidation of the nanocomposites is shifted and reduced for high temperatures indicating an improvement in the thermal stability of the polymeric matrix due to the clay barrier effect for gases and volatile compounds.

Tailoring Polymer Molecular Weight Distribution and Multimodality in RAFT Polymerization Using Tube Reactor with Recycle

AbstractSolution reversible addition fragmentation chain transfer (RAFT) polymerization of butyl acrylate in 50 wt% toluene, initiated with 2,2′‐azobisisobutyronitrile and mediated with 3‐benzyltrithiocarbonyl propionic acid, is carried out in a tube reactor of 1.65 mm inner diameter. The tube reactor is operated in three modes: batch tube reactor (inlet and outlet closed, recycle open), continuous tube reactor (inlet and outlet open, recycle closed), and loop tube reactor (inlet, outlet, and recycle all open). The effects of inlet and outlet flow rates, residence time, and recycle ratio on the polymerization rate and polymer molecular weight distribution (MWD) are systematically investigated. The dynamic and steady state kinetics of the three modes of operation are analyzed and compared. Polymer samples having multimodal MWD are generated using the loop reactor. It is found that the MWD and multimodality can be readily controlled by residence time (τ) and recycle ratio.

Automatic Synthesis of Multimodal Polymers

An automatic molecular weight controller is used in semi‐batch reactions to produce final polymers that contain distinct subpopulations with different molecular weight distributions (MWD). While blending polymers with different MWD is frequently used to make multimodal polymer products, a method is introduced here allowing the multimodal product to be made in successive, automatically controlled stages in the same reactor. The method is demonstrated using free radical polymerization of acrylamide to produce widely separated multimodal MWD. Automatic Continuous Online Monitoring of Polymerization reactions with a Control Interface (ACOMP/CI) was used. Weight average molecular weight (Mw) in the ACOMP/CI was continuously measured with multi‐angle light scattering and ultra‐violet absorption. The controller used two principles: both polymerization rate and instantaneous molecular weight are proportional to monomer concentration. By controlling monomer flow rate into the reactor, the controller produced a targeted amount of polymer with a desired first Mw and then automatically introduced chain transfer agent (CTA) into the reactor to produce a second population of polymers with much lower Mw. Subsequently, reactions were performed to produce trimodal populations. This approach is kindred to multi‐stage synthesis of polymers where distinctly different processes are carried out in succeeding stages to produce polymers with highly specific properties. image

Simulating dynamic modes of polymer synthesis, based on the method of moments for multimodal distributions

Aspects of using the method of moments to simulate processes of the synthesis of polymers by means of gel chromatography are considered. Problems of modeling multimodal molecular weight distributions and their evolution as a function of conversion are discussed.

Di- and trinuclear zirconium complexes as catalysts for ethylene polymerization

Bimodal and multimodal molecular weight distributions of polyethylenes can be achieved with di- or trinuclear zirconium catalysts in homogeneous solution. In order to prepare such complexes, bis(phenoxyimine) zirconium complexes and zirconocene complexes were coupled in Sonogashira reactions. Hydrosilylation reactions were applied for the combination of metallocene complexes with silane substituted N-heterocyclic compounds, such as 3,5-dimethylpyrazoles. The resulting di- and trinuclear complexes were activated with methylaluminoxane (MAO) and applied as catalysts for ethylene polymerization. The catalytic performances of these complexes and their applications in olefin polymerization reactions are discussed.

Mononuclear complexes with heterocyclic ligands as ethylene polymerization catalysts for single-reactor bimodal polyethylene technology

27 complexes of titanium, zirconium and vanadium with 1,2-bis(benzimidazolyl)benzene, 1,2-bis(benzimidazolyl)ethane and 1,1-bis(benzimidazolyl)methane ligands have been synthesized and characterized. After activation with methylalumoxane (MAO) these complexes proved as good catalysts for ethylene polymerization inspite of the hetero atoms in the ligands and the oxophilicity of the metals. Nearly all produced polyethylenes showed bimodal or multimodal molecular weight distributions indicating more than one active site of the catalysts in the various polymerization steps. Obviously, after activation with MAO, a mononuclear catalyst molecule can interact differently with the cocatalyst. This is an easy and elegant approach to single-reactor bimodal/multimodal polyethylene technology. The polymerization activity of the catalyst system 1,2-bis(benzimidazolyl) benzene zirconium tetrachloride (29/MAO) was investigated at different polymerization temperatures and structure–property-relationships were studied. The effect of the polymerization temperature on both molecular weights and polydispersities of the polymers produced with catalyst 29/MAO was investigated at 20, 40, and 60°C. The molecular weights Mw were 1.3⋅106, 1.2⋅106 and 9.7⋅105g/mol and the polydispersity indices PDI 693, 248 and 223 respectively.

Bis(benzothiazolyl) titanium, zirconium and vanadium complexes as catalysts for ethylene polymerization

13 titanium, zirconium and vanadium complexes with 1,1-bis(benzothiazolyl)methane, 1,2-bis(benzothiazolyl)-ethane, 1,2-bis(benzothiazolyl)benzene and 2,6-bis(benzothiazolyl) pyridine ligands were synthesized and characterized. After activation with methylalumoxane (MAO), these complexes proved as good catalysts for ethylene polymerization in spite of the hetero atoms in the catalyst molecules and the oxophilicity of the metals. Structure–property-relationship studies indicated that the bridging unit connecting the two heterocyclic moieties has a strong influence on the performance of the catalysts. Different interactions of the catalyst cation and the bulky MAO anion, due to the various hetero atoms, could be responsible for this behavior. The produced polyethylenes show bimodal or multimodal molecular weight distributions indicating more than one active site in the polymerization process. Therefore such mononuclear catalysts are good candidates for single-reactor bimodal or multimodal polyethylene technology. The polymerization activity of the catalyst system [1,2-bis(benzothiazolyl)benzene zirconium tetrachloride] 13/MAO was investigated under different polymerization conditions.

Ethylene polymerization with novel phenoxy-imine catalysts bearing 4-vinylphenyl group

This contribution reports ethylene polymerization behavior of titanium complexes incorporating bis(phenoxyimine) ligands. Six phenoxy-imine Ti(IV) complexes {6-R1-2-[CH=N(2,6-difluoro-3,5-diR2-4-R3Ph)]C6H3O}2TiCl2 (1: R1 = H, R2 = H, R3 = H; 2: R1 = H, R2 = H, R3 = 4-vinylphenyl; 3: R1 = CH3, R2 = H, R3 = H; 4: R1 = CH3, R2 = H, R3 = 4-vinylphenyl; 5: R1 = CH3, R2 = F, R3 = H; 6: R1 = CH3, R2 = F, R3 = 4-vinylphenyl) have been synthesized and evaluated for ethylene polymerization using dried MAO (simplified as DMAO) as cocatalyst. An obvious catalytic heterogeneity of Cat 2 (Complex 2/DMAO) towards ethylene polymerization was observed, which was illustrated by decreased activity, multimodal molecular weight distribution and partially improved particle morphology comparing with Cat 1. Moreover, Cat 3 exhibits “living” characteristics in the process under certain conditions (25 °C, less than 20 min). Otherwise, the moderate to high ethylene polymerization activity of ca. 105–106 g PE/(mol Ti·h) and high molecular weight (Mw = 105–106) of polyethylene can be obtained by changing the skeleton structure of these complexes.

Poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) Four-arm Star Functional Copolymers by Quasiliving ATRP: Equivalent Synthetic Routes by Protected and Nonprotected HEMA Comonomers

Comprehensive investigations were carried out on the synthesis of well-defined poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) P(MMA-co-HEMA) copolymers, i.e., PMMA with predetermined average number of pendant hydroxyl functionalities, by using HEMA and trimethylsilyl-protected HEMA (TMS-EMA) under quasiliving ATRP conditions by a tetrafunctional initiator and CuCl/2,2′-bipyridyl (bpy) catalyst system in methanol at 10°C. It was found that these two synthetic routes, that is the direct and protected approaches, are equivalent in terms of yields, hydroxyl functionality and molecular weight distribution (MWD). Bulk copolymerization of MMA and HEMA by ATRP led to broad multimodal MWD in contrast to copolymers with narrow MWD in the presence of methanol. A two-step purification method applying silica-alumina chromatography followed by treatment with an acidic ion-exchange resin, used also as the deprotecting agent, was found to provide copolymers free of catalyst contamination. At higher monomer conversions, GPC analyses indicate the occurrence of chain-chain coupling by recombination of the macroradicals resulting in lower than theoretical apparent initiating efficiencies. According to the results on the effect of reaction time on monomer conversion, relatively short reaction times, less than 2 h, are sufficient to obtain high yields and PMMA with desired functionalities and MW under the applied quasiliving ATRP conditions. The resulting four-arm star hydroxyl-functional P(MMA-co-HEMA) copolymers offer a variety of new possibilities for the preparation of a variety of novel PMMA-based macromolecular architectures.

Rheological characterization of okra pectins

Polysaccharides from okra pods (Abelmoschus esculentus) were extracted using a sequential extraction protocol and compared with a simple extraction at pH 6. Rheological properties of three okra extracts were then investigated by means of molecular weight determination, dilute solution rheology, steady shear and oscillatory rheological measurements. The extraction protocols resulted in extracts of relatively high purity and multimodal molecular weight distribution. Furthermore, molecular parameters of the isolated biopolymers such as intrinsic viscosity, Huggins constant, critical concentration and coil overlap parameter were calculated from dilute solution viscometry. Investigation of the generalized flow behaviour using a modified Cross equation and Cox–Merz plots showed evidence that as concentration increases specific interactions start taking place among the polymeric chains that modify the rheological behaviour of the extracts. The change in the rheological behaviour could not only be explained by differences in the molecular weight of the samples but also should be attributed to the fine structure of the chains that are obtained under the different extraction protocols. Present investigation shows that further optimization of such protocols may result in polysaccharide fractions with specific rheological properties.

Preparation of polyethylene with controlled bimodal molecular weight distribution using zirconium complexes

Four zirconium complexes containing fully deprotonated 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di- tert-pentylphenol were used as catalysts for the polymerization of ethylene. In the presence of methylalumoxane (MAO) as a cocatalyst, the precursors were highly active for polyethylene with bimodal or multimodal molecular weight distribution.

Analysis of branched polymers by high resolution multidetector size exclusion chromatography: Separation of the effects of branching and molecular weight distribution

AbstractThis article presents the SEC analysis of branched polyisobutylene PIB and polystyrene PS with high molecular weight and broad multimodal molecular weight distribution. Both polymers were synthesized using an inimer technique, which results in long‐chain branched polymers with statistical branching and broad multimodal distributions. Using high resolution multidetector Size Exclusion Chromatography SEC the polymers were analyzed based on three branching factors: g = (Rz,br/Rz,lin)Mw; h = (〈Rh〉z,br/〈Rh〉z,lin)Mw ; and ρ = (R 1/2/〈Rh〉z). It is generally accepted that for monodisperse branched polymers g and h < 1. In the case of our polydisperse PIB and PS, it was seen that g and h > 1, and ρ increases with molar mass and the number of chain ends as predicted earlier. The multidetector SEC system allowed for the separation of branching and polydispersity, reported here for the first time experimentally. The g parameter as a function of DPi was compared to the theory developed by Zimm and Stockmayer. The plots followed a similar trend, but were shifted by a factor related to the average chain length between branching points. The ρ parameter decreased with increasing DPi, as predicted theoretically by Kajiwara. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Towards the Sustainable Production of Higher‐Molecular‐Weight Polyglycerol

AbstractThe step‐growth polymerization of glycerol to relatively high molecular weight polyglycerol is investigated. Glycerol is a renewable material that can be derived from the transesterification of vegetable oils to biodiesel. Several soluble catalysts and their reaction mechanisms are compared and their effect on polymer molecular weight and microstructure is measured. High‐molecular‐weight PG with multimodal molecular weight distributions is observed using GPC, and partial branching is identified using 13C NMR spectroscopy. Theoretical models are used to predict the gel point and to calculate monomer functionality. magnified image