Chain Microstructure Research Articles

Study on the structure and property of metallocene ethylene/1-heptene and ethylene/1-octene copolymers

Polyolefin elastomers are typically copolymers of ethylene with 1-butene, 1-hexene, or 1-octene. Other types of comonomers have not been widely applied industrially. The cost of 1-heptene obtained from Fischer-Tropsch synthesis is only half that of 1-octene, making it a viable economical substitute. This work utilized 1-heptene from Fischer-Tropsch synthesis and 1-octene from ethylene oligomerization as research subjects. Eight metallocene POEs based on 1-heptene and 1-octene were synthesized using Me2Si(C5Me4)(N t Bu)]TiCl2 (Ti-CGC) catalyst. The copolymers’ molecular structures and physical properties were characterized using HT-GPC, Temperature Rising Elution Fractionation (TREF),13C-NMR, DSC, Melt Index (MI), and Tensile tests. Compared to 1-heptene, 1-octene more effectively reduces the melting point and crystallinity, while 1-heptene’s stronger comonomer insertion capability facilitates the production of copolymers with higher comonomer content. Similar ternary sequence distributions and molecular dynamics indicate that ethylene/1-heptene and ethylene/1-octene copolymers possess comparable chain microstructures. Both ethylene/1-heptene copolymers and ethylene/1-octene copolymers exhibit excellent toughness and mechanical properties. Although the activity and insertion capacity of ethylene and 1-heptene decrease at low comonomer concentrations, they significantly improve at higher comonomer concentrations. Thus, Fischer-Tropsch synthesis of metallocene ethylene/1-heptene copolymers can effectively reduce raw material costs and be a cost-effective alternative to ethylene/1-octene copolymers.

Steric Influences on Chain Microstructure in Palladium-Catalyzed α-Olefin (Co)polymerization: Unveiling the Steric-Deficient Effect.

This study addresses the challenge of controlling branching density and branch-type distribution in late-transition-metal-catalyzed chain walking polymerizations. We explored α-diimine Pd(II) complexes with incrementally increased ortho-aryl sterics for long-chain α-olefin (co)polymerization. Pd0-Pd3 catalysts, which feature gradually increased ortho-aryl sterics and at least one small CH3 substituent, exhibited similar 2,1-insertion fractions (44-50%), polymer branching densities (55-63/1000C), and melting temperatures (26-28 °C). In contrast, Pd4 with bulky ortho-aryl sterics covering all sides demonstrated a significant increase in 2,1-insertion fractions up to 82%, leading to "PE-like" polymers with high melting temperatures (Tm > 111 °C). This abrupt change in polymerization behavior, termed the "steric-deficient effect", contrasts with the gradual changes observed in similar Ni(II) systems that we reported previously. Furthermore, due to the rapid chain walking ability of Pd(II) catalysts in long-chain α-olefin (co)polymerization, these catalysts favor the production of polyolefins with higher proportions of methyl branches compared to those produced by Ni(II) catalysts. Particularly, these Pd(II) catalysts are capable of synthesizing functionalized semicrystalline copolymers by copolymerizing 1-octene with a variety of polar comonomers, thereby significantly altering the surface properties of the materials.

A rigorous model for hydrogen response in industrial propylene polymerization loop reactors: From micro-mechanism to macro-behavior

In spite of playing a chain transfer agent role, the hydrogen activation effect in propylene polymerization with Ti-based Ziegler-Natta catalysts, so called “hydrogen response”, is usually explained by the dormant-site theory in polymer chemistry. However, how the micro-mechanism at the catalyst site scale affects the macro-behavior of industrial processes is unrevealed. This work aims at developing a rigorous model that can accurately describe hydrogen response and predict polymer chain microstructure for an industrial propylene polymerization process consisting of loop reactors in series. The model is comprised of a kinetic model, a thermodynamic model, and a macroscopic non-ideal loop reactor model. To describe hydrogen response, an elaborated kinetic model is established based on a consistent dormant site reactivation mechanism assuming a multi-site model. The model predictions are in good agreement with plant data, after taking the dormant sites reactivated by hydrogen into account. The proposed model is capable of fully assessing the effects of various process conditions, during steady-state and grade transition processes, on the macro-behavior (i.e., production, slurry density) as well as polymer properties (i.e., melt flow index, molecular weight distribution) in the industrial loop reactors. The underlying correlations are also discussed.

Preparative temperature rising elution fractionation of polyolefin elastomer and its chain microstructure

AbstractPolyolefin elastomer (POE) has very weak crystalline ability, consequently, applying the conventional preparative temperature‐rising elution fractionation (P‐TREF) to separate is challenging. Here, a unique, home‐built P‐TREF apparatus with an extensive range of temperatures from −80°C to 150°C is applied to first fractionate POE depending on its crystallizability. The main fractions are eluted at 0°C, 8°C, 15°C, 20°C, 25°C, 30°C, and 35°C. The corresponding weight percentages of fractions are 8.31, 13.38, 15.59, 12.05, 13.39, 17.30, and 10.53 wt%, respectively. The chain structures of fractions are further analyzed by high‐temperature gel permeation chromatography (HT‐GPC), 13C‐nuclear magnetic resonance spectroscopy (13C‐NMR), differential scanning calorimetry (DSC), and successive self‐nucleation and annealing (SSA). The crystallinity of the fraction grows continually as the elution temperature rises. The 1‐octene comonomer concentrations within the fractions decreases from 13.8 to 7.9 mol% when the elution temperature rises from −10°C to 40°C. These findings enable for the detailed recognition of the chain microstructure of POE resin and the extension of the TREF approach to POE resins. This lays the groundwork for fundamental studies and practical uses in industry.

Simultaneously enhancing the flow properties and mechanical strength of polyphenylene sulfide via in‐mold ultrasonic‐assisted injection molding

AbstractInjection molding is widely used for making intricate plastic products due to its precision, cost efficiency, and fast production. Industries like consumer electronics, aerospace, and medical, demand high mechanical properties from these parts. However, high‐performance polymers face challenges in flowability. Ultrasonic‐assisted injection molding offers a solution, but its impact on flowability and mechanical properties is not fully understood. We propose a novel injection mold design with in‐mold ultrasonic vibration to address this. By applying high‐frequency shear forces to the melt, it alters polymer chain alignment, enhancing flow and microstructure. Employing polyphenylene sulfide (PPS) resin as the primary material, this study systematically explores the impact of varying ultrasonic vibration durations and stages on melt filling behavior and product performance. Compared with conventional molds, ultrasonic‐assisted molds reduce melt viscosity by 38% and improve mechanical properties by 10.21%, with a 17.1% increase in crystallinity. This technology shows promise for improving flowability and mechanical properties in injection molding, with potential for wide industrial application.

Chain heterogeneity of poly(lactic acid) and its influence on crystallization kinetics

Temperature rising elution fractionation (TREF) approach was used to separate a biodegradable poly(lactic acid) (PLA) resin into ten fractions and completely establish the relationship between chain microstructure and properties. The main fractions were mainly eluted at 100, 110, 114, and 118 °C, and their mass percentages were 7.98 wt%, 44.83 wt%, 19.64 wt%, and 11.90 wt%, respectively. Through the use of successive self-nucleation/annealing (SSA) thermal fractionation, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and 13C-nuclear magnetic resonance spectroscopy (13C NMR), the intermolecular and intramolecular differences of PLA were further explored. Fractions eluted at 90, 110, 118, and 126 °C were also chosen to research the non-isothermal cold crystallization kinetics, and fractions eluted at 110, 118, and 126 °C were chosen to explore the non-isothermal crystallization kinetics in order to simulate the real process. The findings demonstrated that the Liu-Mo approach were more suited the non-isothermal crystallization and non-isothermal cold crystallization kinetics of PLA. As the elution temperature increased, so did the stereoregularity of the fractions, the crystallization rate, the crystallization capacity, and the lamellar thickness. These will lay a foundation for its basic research and industrial application.

Robust cobalt catalysts with N,N-bidentate aldimine imidazolidine-2-imine/guanidine ancillary ligand for isoprene polymerization

A new family of well-defined cobalt complexes (Co1–Co7) bearing N,N-bidentate chelate aldimine imidazolidine-2-imine/guanidine ancillary ligands were designed and successfully synthesized. All of the target metal complexes were structurally characterized by single-crystal X-ray diffraction analysis. It revealed that the Co atom adopted distorted tetrahedral coordination geometry and formed a unique six-membered chelate ring which exhibited structural distinctions with traditional N^N-based α-diimine and β-diimine Co complexes. In combination with a very low amount of Et2AlCl (as low as 50 equiv.), these Co complexes exhibited high activity (up to 99 % yield within 6 hrs) and good thermal stability (up to 80 °C) in the coordination polymerizations of isoprene to afford cis-1,4/3,4-polyisoprenes with adjustable molecular weights Mn (25.9–138.4 kg/mol) and cis-1,4 contents (64.7–73.3 %). The different substituents on these Co catalysts also exerted influence on catalytic behaviors, chain microstructures, and polymer properties. This study represents the first example of imidazolidine-imine/guanidine analog ligated Cobalt complexes for the polymerization of isoprene.

From Chemical Drawing to Electronic Properties of Semiconducting Polymers in Bulk: A Tool for Chemical Discovery.

A quantum chemistry (QC)/molecular dynamics (MD) scheme is developed to calculate electronic properties of semiconducting polymers in three steps: (i) constructing the polymer force field through a unified workflow, (ii) equilibrating polymer models, and (iii) calculating electronic structure properties (e.g., density of states and localization length) from the equilibrated models by QC approaches. Notably, as the second step of this scheme is generally the most time-consuming one, we introduce an alternative method to compute thermally averaged electronic properties in bulk, based on the simulation of a polymer chain in the solution of its repeat units, which is shown to reproduce the microstructure of polymer chains and their electrostatic effect (successfully tested for five benchmark polymers) 10 times faster than state-of-the-art methods. In fact, this scheme offers a consistent and speedy way of estimating electronic properties of polymers from their chemical drawings, thus ensuring the availability of a homogeneous set of simulations to derive structure-property relationships and material design principles. As an example, we show how the electrostatic effect of the polymer chain environment can disturb the localized electronic states at the band tails and how this effect is more significant in the case of diketopyrrolopyrrole polymers as compared to indacenodithiophene and dithiopheneindenofluorene ones.

Shear relaxation behavior for degraded polypropylene melt in torque rheometer

AbstractShear relaxation behavior for polymer melt exists in different processing techniques and influences the dimensional accuracy and production efficiency of polymer products. A micro physical and mathematical model based on microstructure of polymer chains and processing parameters was established and verified by experimental results obtained from a torque rheometer. The shear relaxation time of polypropylene degraded by di‐tert‐butyl peroxide was characterized with torque rheometer. The effects of microstructure and processing parameters on the shear relaxation time was investigated. The results showed that a wider molecular weight distribution could result in a decrease in shear relaxation time and polypropylene chains with higher molecular weight were more sensitive to molecular weight distribution. The shear relaxation time declined exponentially with shear rate and increased with internal pressure. The temperature could enhance the movement of polymer chains and reduce the shear relaxation time linearly, but polypropylene with lower molecular weight was less sensitive to temperature. The insight obtained from the mathematical and experimental results can be profoundly applied in practical process control to manufacture product with better dimensional stability and production efficiency for viscoelastic polymer.Highlights A physical and mathematical model based on microstructure and processing parameters of molecular chains was established to explore the shear relaxation mechanism. Shear relaxation time of polypropylene degraded by di‐tert‐butyl peroxide was characterized with the help of torque rheometer. The effects of microstructure and processing parameters on shear relaxation time was investigated to optimize the manufacturing of polypropylene products.

Relationship between the poly(glycerol adipate) prepolymers chain microstructure and linear viscoelastic properties of the elastomers

This paper investigates the influence of the time of enzymatic polymerization of glycerol and divinyl adipate on the chain microstructure and the thermal and linear viscoelastic properties (LVE) of the poly(glycerol adipate) prepolymer (pPGA). The immobilized Candida antarctica lipase B (CALB) was used as a regioselective catalyst in the polytransesterification reaction. A series of pPGA prepolymers were synthesized at different duration times of 8, 24, 48 and 72 h and the degree of polymerization (DP) and degree of branching (DB) were determined based on a quantitative NMR analysis. Due to lack in the literature of the described relationship between the properties of the pPGA prepolymers and the PGA elastomers, the main goal of the research was to correlate the microstructure of the prepolymers with linear viscoelastic properties of the elastomers obtained by thermal crosslinking of the respective prepolymers. The respective pPGA prepolymers were cured at 110 °C for 5 days to obtain PGA elastomers. The LVE properties of the PGA elastomers were investigated based on the master curves of the storage (G') and loss (G") modulus. The differences in DP and BD for the pPGA prepolymers translated into the PGA elastomer crosslinking densities and the rubbery plateau modulus (G0). It was shown for the first time that the minor differences in the structure of the pPGA prepolymers (DP of 13 and 21, and DB of 8.4% and 9.7% for the pPGA prepolymers synthesized for 8 and 24 h, respectively) translate into significant differences in the LVE of the PGA elastomers (G0 of 116 kPa and 50 kPa for the PGA elastomers derived from pPGA8 and pPGA24 prepolymers, respectively).

Recent Advances in Controlled Production of Long-Chain Branched Polyolefins.

Polyolefins, composed of carbon and hydrogen atoms, dominate global polymer production. This stems from the wide range of physical and mechanical properties that various polyolefins can cover. Their versatile properties are largely tuned by chain microstructure, including molar mass distribution, comonomer content, and long-chain branching (LCB). Specifically, LCB imparts unique characteristics, notably enhances processability crucial for downstream applications. Tailoring LCB structural features has encouraged academic and industrial efforts, chronicle in this review from a chemistry standpoint. While encompassing post-reaction modification based traditional methods like peroxide grafting, ionizing beam irradiation, and coupling reactions, the main focus is given to catalyst-centric strategies and innovative polymerization schemes. The advent of single-site catalysts-metallocenes and late transition metals catalysts-amplifies interest in tailored chemical methods, but the progress in LCB formation flourishes via tandem catalytic systems and bimetallic catalysts under controlled reaction conditions. Specifically, the breakthrough in coordinative chain transfer polymerization unveils a novel avenue for controlled LCB synthesis by sequential chain propagation, transfer, liberation, and enchainment. This short review highlights recent approaches for the production of LCB polyolefins that can provide a roadmap crucial for researchers in academia and industry, steering their efforts toward further advancements in the production of tailored polyolefin.

The Effect of Feeding Sequence on the Structure and Properties of the Ethylene/1-Octene Copolymer in the Semi-Continuous Polymerization Reaction System.

The performance of ethylene/1-octene copolymer primarily depends on the microstructure of the polymer chain. This study employed a new method to control the inter-distribution of hexyl chain branches directly on the backbone of the ethylene/1-octene copolymer. Three ethylene/1-octene copolymers with different inter-distributions of hexyl chain branches were synthesized using [Me2Si(C5Me4) (NtBu)] TiCl2 (Ti-CGC) by different feeding sequences in the semi-continuous polymerization reaction system. The three copolymers were named according to the feeding sequence of the materials: ethylene/1-octene/Ti-CGC (EOC), 1-octene/Ti-CGC/ethylene (OCE), and ethylene/Ti-CGC/1-octene (ECO), respectively. The structure and properties of the copolymers were characterized using HT-GPC, 13C-NMR, DSC, WAXD, DMA, MI, and Uniaxial Tension Test. The results showed that the feeding sequence greatly affected the comonomer distribution of the molecular chains, molecular weight, molecular weight distribution, and chemical composition of the copolymers, consequently influencing their thermal performance and mechanical properties. Thus, it is probable that one could obtain an ethylene/1-octene copolymer with designed properties by controlling the feeding sequence during the ethylene/1-octene semi-continuous copolymerization process.

Molecular chain orientation and mechanical properties of hydrogenated nitrile-butadiene rubbers with different acrylonitrile content and crosslink density

A study of the thermal and mechanical properties, as well as the molecular chain orientation of sulfur-vulcanized hydrogenated nitrile-butadiene rubber (HNBR) with acrylonitrile (ACN) content of 34 and 36 wt% and different crosslink density is presented. The objective is to establish the relationships between structural changes during stretching and the tensile and elastic behavior of HNBR having different chain microstructure. The (vulcanized) HNBR samples are amorphous at room temperature with glass transition temperatures that increase with increasing ACN content and crosslink density. The samples do not exhibit strain-induced crystallization. Stretching at room temperature only induces orientation of the amorphous segments along the stretching direction, which contributes to improved tensile strength, strain-hardening, and toughness, while preserving excellent elastic recovery of the rubbers. Precise correlations are established between the degree of orientation of the amorphous chain and the deformation in the framework of the affine deformation model of rubber elasticity.

Systematic investigations of ortho-aryl substitution effect on chain microstructure in nickel-catalyzed long chain α-olefin polymerization and copolymerization with methyl undecenoate

The tuning of branching density and branch-type distribution of the polyolefin through the ligand modification in “chain-walking”-type late transition metal catalysts is highly desired but remains a significant challenge in olefin polymerization. Herein, we performed a systematic investigation on the (co)polymerization of long chain α-olefin (1-octene and 1-decene) by a series of α-diimine Ni(II) catalysts with the varied ligand sterics arising from the number of the ortho-aryl (4-methylphenyl) substituents. The effects of structural variations, mainly including the steric tuning on the ortho-position of N-aryl ring and ligand backbone, steric distribution (ligand symmetry) tuning and electronic tuning, and polymerization conditions on catalytic activities, polymer molecular weight, polymer branching density, and branch-type distribution were described. The steric effect of ortho-methylphenyl substituents played a major role rather than the electronic effect in determining the preference of insertion fashion (1,2-insertion or 2,1-insertion) and chain walking in long chain α-olefin polymerization. As the bulk and number of ortho-methylphenyl substituents increased, the preferred 2,1-insertion and 1, ω-enchainment (chain straightening) were observed even at high monomer concentrations, thus leading to the decrease of branching density as well as the percentage of methyl branches, and eventually affording the semicrystalline “polyethylene-like” polymer with high Tm up to 121 °C and good mechanical properties. Moreover, the tuning of chain microstructure can be achieved over a very wide range through the modification of ligand sterics. Most importantly, these Ni(II) catalysts could copolymerize long chain α-olefin and polar monomer methyl undecenoate (MU) to generate the various functionalized semicrystalline copolymers with high incorporation ratios of MU up to 11% and Tm in a wide range of 5.6–93 °C.

Enhanced viscoelastic magneto-mechanical properties for off-axis anisotropic magnetorheological elastomers: Experiment and modelling

In this study, the microstructure of tilt angle magnetic chains is adopted to construct off-axis anisotropic magnetorheological elastomers (MREs) for tailoring viscoelastic magneto-mechanical properties. The magnetorheological behaviour of anisotropic MREs is investigated using magneto-controlled smart devices to achieve dynamic mechanical response. The viscoelastic behaviour is modelled by fractional derivative (FDM) and extended Kelvin-Voigt model (EKVM), the viscoelastic properties of anisotropic MREs are characterized by experimental and theoretical methods. Meanwhile, a magneto-induced mechanical model is established to reveal the influence of the tilt angle of the magnetic chain, the prestrain, the mass fraction of carbonyl iron particles (CIPs), and the magnetic flux on the magneto-induced mechanical properties. First, the complex moduli of anisotropic MREs with 25% and 40% mass fractions containing 0°, 15°, and 25° magnetic chains were experimentally characterized under shear mode using excitation frequency (1–100 Hz) and flux densities (300mT). The nonlinear robustness method is used to identify the parameters of the storage modulus and loss modulus functions from the experimental results in the viscoelastic theoretical models. The error analysis of anisotropic MREs for complex moduli between the theoretical and experimental solutions is evaluated at different tilt angles and CIPs contents. Finally, magnetorheological tests on anisotropic MREs verified the theoretical results on the magneto-induced elastic modulus influenced by tilt angle, magnetic particle content and flux density. The description of the storage modulus in EKVM, with an error of less than 3%, is superior to that in FDM, with an error of less than 8.1% in the frequency domain. The loss modulus of anisotropic MREs in FDM for different tilt angles and mass fractions, was accurately evaluated at low frequency compared to the EKVM solution.

Manipulating Cis-Trans Copolymer Chain Conformation to Simultaneously Improve Permittivity and DC Breakdown Strength in Polythiourea.

Polythioureas (PTUs) show great potentials for applications in the new generation of film capacitors due to their excellent dielectric properties. Herein, the cis-trans copolymer chain of PTU is successfully tailored by employing cis and trans cyclohexyl spacers. The relationship between the copolymer chain conformation, microstructure, and dielectric properties is carefully explored by a series of analysis. Compared with cis conformation, the trans with less steric hindrance can promote the formation of H-bonds. The enhanced H-bonding interactions not only reduce the molecular inter-chain spacing, but also drive the self-assembly of molecular chains to form cylindrical and droplet nano-morphologies. The phase separation between cis and trans PTUs is confirmed by combining the experimental results of TEM and DSC, and the CT64-PTU with the most two-phase interface thus obtains the highest permittivity of 5.5 (@10Hz). The reduced molecular inter-chain spacing is accompanied by a decreased hopping distance of charges, which improves breakdown strength by 17% from 498 MV/m to 580 MV/m. Therefore, the cis-trans copolymer chain conformation in PTU provides a simultaneous high permittivity and breakdown strength. This research offers a strategy to further design high-performance dielectrics via regulation of copolymer chain conformation.

Influence of Cooling Rate on the Temperature Rising Elution Fractionation of One Poly(1‐butene‐co‐ethylene) Resin and Microstructure of Its Fractions

AbstractTREF (temperature rising elution fractionation) is a method to separate resins based on crystallizability. The influence of TREF cooling rate (V0) on the fractionation process is still unclear. Herein, a poly(1‐butene‐co‐ethylene) resin is fractionated by the TREF method at three different cooling rates (1, 2, and 4 °C h−1), and then the fractions are analyzed by high‐temperature gel permeation chromatography (HT‐GPC), differential scanning calorimetry (DSC), and 13C‐nuclear magnetic resonance spectroscopy (13C‐NMR). The cooling rate V0 affects the crystallization process of the PB‐1 resin, resulting in different fraction contents at the same elution temperature. Furthermore, the cooling rate also affects the chain microstructure of fractions obtained from the same elution temperatures, i.e., the content of 1‐butene and ethylene monomer units, average sequence length and isotacticity. The lower cooling rate is beneficial to the effective separation of the fractions with weak crystallizability. These results have important guidance for optimizing fractionation conditions and obtaining different narrow distribution fractions of poly(1‐butene) (PB‐1) resin.

Thermoplastic elastomers based on lactide and caprolactone: The influence of chain microstructure on surface topography and subsequent interaction with cells

Surface biophysical properties of biomaterials, including surface topography and roughness, determine the interaction of a biomaterial with the surrounding cells, tissues and organs once implanted in the human body. Herein, the surface topography of thermoplastic copolymers based on lactide and caprolactone showing elastomeric behaviour was modulated by precisely controlling the chain microstructure (i.e., distribution and length of the repetitive units within the polymeric chain). The synthesized copolymers were subjected to different thermal treatments from the melt, leading to polymeric films with various surface textures and roughness values. Copolymers synthesized with triphenyl bismuth as a catalyst, with a more random distribution of the repetitive units, showed limited crystallization capability. Accordingly, only the copolymer with higher amount of l-lactide (i.e., 80 wt%) subjected to an isothermal treatment from the melt at 70 °C was able to crystallize, and spherulites of around 7 μm were discernible by atomic force microscopy. In contrast, the copolymers synthesized with stannous octoate, which had a more blocky nature, showed axialitic crystalline domains at the submicron-to nanoscale when subjected to an isothermal treatment from the melt at 50 °C, whereas well-defined spherulites of sizes up to 14 μm were obtained at 70 °C. Human fibroblast showed a more elongated morphology when seeded on those samples having higher roughness values and larger spherulites, whereas they had a more spread morphology when seeded on the amorphous, smooth surfaces. As concluded from the present study, by precisely controlling the chain microstructure of the synthesized copolymers, a wide variety of surface topographies can be obtained, which has a clear impact on the way the biomaterial interacts with cells. This opens the possibility to study the influence of surface biophysical properties on more complex cell processes in the future, including inflammatory or foreign body response processes.

Chain microstructure of two in-reactor alloy impact polypropylene resins with same stiffness and different toughness

Chain microstructure of two in-reactor alloy impact polypropylene resins with same stiffness and different toughness

Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation

Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation