Long-chain Branching Research Articles

Conductivity Insights Into the Carbon Nanotubes Mobility Restricted by the Long‐Branched Chains During the Thermal Annealing, Fast Shear Flow and Melt to Solid Cooling Process

AbstractThe present work provides a unique insight to reveal the long chain branching (LCB) influence to the nano‐fillers, that is via analyzing the conductivity evolution during the thermal annealing, fast shear flow and melt to solid cooling process. Meanwhile the crystallization behavior is also discussed. A linear polypropylene (PPC) and a long chain branched polypropylene (PPH) are chosen as the two polymer matrices with carbon nanotubes (CNTs) as the nanofillers. During the thermal annealing process, a more obvious growth of electrical conductivity of linear PPC nanocomposites are observed compared with LCB PPH nanocomposites. The harder movement of CNTs inside LCB PPH matrix may be the main reason. This also can be reflected by the conductivity evolution during slow cooling process where a decrease‐then‐increase (DTI) phenomenon is found for PPC/CNTs systems due to the rebuilding of CNTs conductivity network after crystallization. By contrast, this does not happen for PPH/CNTs systems.

Influence of Long-Branched Chains on the Crystallization Behavior of Polyglycolide

Influence of Long-Branched Chains on the Crystallization Behavior of Polyglycolide

Rheological analysis of network structure of raw natural rubber prepared by different processing techniques

The performance of raw natural rubber (NR) is dominated by its network structure, particular the levels of long chain branching (LCB) and entanglement. Here, three type of raw rubbers were prepared using different processing methods for commercial grade NRs. Linear and nonlinear viscoelastic behaviors, as well as stress relaxation, were analyzed to access their network structures. The third relative harmonic, phase angle, and first to second quarter-period integral ratio of stress curve were utilized as indicators for the nonlinearity of samples. By combining these values with flow activation energy and characteristic times, it can be confirmed that naturally coagulated NR showed higher elasticity than acid-coagulated NR due to high levels of LCB and entanglement, and hot air drying could lead to chain degradation. These findings correlated with molecular weight parameters, gel content and Mooney viscosity results. This research establishes a method for monitoring raw NR quality and predicting its properties.

Defining quality by quantifying degradation in the mechanical recycling of polyethylene

Polyolefins have a multitude of uses across packaging, automotive and construction sectors. Their resistance to degradation during reprocessing enables recyclability, but variability in recycled polymer feedstocks renders it difficult to assure their manufacturing suitability. The lack of quality control methods has disabled circular economy pathways; product failure is costly, wasteful and time-intensive. Using rheology-simulated and extrusion-based recycling experiments, we explore the degradation pathways of high-density polyethylene (HDPE). Chain scission dominates during the initial degradation of HDPE, and increasing exposure to O2 shifts the dominant mechanism to long-chain branching. Importantly, extending this method to post-consumer recyclate (PCR), the results show potential as a methodology to assess recyclate quality to enable a circular plastics economy. In this study, we establish the validity of this rheology simulation to define a characteristic degradation parameter, relating it to the structural evolution under different environments defined for virgin HDPE and post-consumer recyclate (PCR).

A statistical approach towards determining the governing parameters in volume expansion behavior of polypropylene foams

This study delves into the complex relationship between molecular structures and properties within polypropylene (PP) foams, with a particular focus on how various material parameters influence foamability. Multiple linear regression was adopted to efficiently assess the impact of eleven material parameters on foam expansion behavior. A primary finding is the significant role of branching index (g'vis) in shaping the expansion profile (i.e., the foaming window, maximum expansion ratio (Φmax), and optimum foaming temperature (Toptimum)). Additionally, Z-average molecular weight (Mz) is identified as crucial factor influencing the foaming window. Furthermore, the Winter-Chambon parameter n, indicator of the relaxation behavior of a resin in the terminal zone, is distinguished as a key parameter in determining Φmax. The crystallization temperature (Tc) also plays a pivotal role, particularly in its relationship with Toptimum. The research further highlights the significance of long-chain branching (LCB) and suggests strategies for cost reduction by minimizing the degree of LCB without compromising foaming performance, achievable through the adjustment of Mz and n. This strategy was exemplified through the comparison of two LCB PP resins, PP-12 and PP-22, demonstrating the feasibility of achieving similar foamability with adjustments in material parameters. The findings offer a detailed assessment of these parameters' influence on foamability, presenting valuable guidelines for optimizing PP foam production and customizing material properties for targeted applications.

Correlating color with free radical concentration in irradiated poly(ethylene terephthalate)

When polymers are irradiated, free radicals form, often accompanied by yellowing in the material. This color gradually fades after active irradiation due to the decay of free radicals. This study analyzed the kinetics of post-irradiated PET discoloration under different temperatures using a UV–visible spectrophotometer. Meanwhile, the free radical decay was also monitored using the electron paramagnetic resonance (EPR) measurements, and then both experiments were compared. The results illustrate the potential of readily available spectrophotometry as an alternative method to track free radical evolution in irradiated polymers because free radical decay leads to the disappearance of annealable color centers over time. Furthermore, PET structure modification by irradiation was studied using extensional rheological analysis. The observed strain hardening suggests the evolution of long-chain branching structures in post-irradiated PET, especially at 50 kGy irradiation absorbed dose.

Role of Quinoa (Chenopodium quinoa Willd) and Chickpea (Cicer arietinum L.) Ratio in Physicochemical Stability and Microbiological Quality of Fermented Plant-Based Beverages during Storage.

Three different fermented plant-based beverages were prepared and stored for a long period (50 days) to assess the effect of the quinoa-to-chickpea ratio on physicochemical stability and microbiological quality. Physicochemical stability was evaluated based on pH, acidity, Brix degrees, water-holding capacity (WHC), viscosity, and viscoelasticity. At the end of the long-term storage period, the pH, acidity, and WHC remained stable. During the entire storage period, the beverages maintained good bacterial, fungal, and lactic acid bacteria (LAB) counts. Quinoa and chickpea flour ratios of 50% showed a higher viscosity (18 Pa.s) and WHC (65%) during short-term storage (0-30 d), indicating that the presence of chickpea flour had a positive effect on these parameters, possibly because chickpea starch contains higher amounts of amylose and long-branch chain amylopectin, which impacts the retrogradation pattern under acidic and refrigerated conditions. However, at the end of storage (50 days), the same blend had a higher acidity, lower viscosity (0.78 Pa.s), and lower LAB counts (~1 × 108 CFU/mL), indicating that the increase in chickpea flour had an adverse long-term effect on these parameters. These results suggest that although different ratios of plant sources can improve the physical aspects, they need to be incorporated in a balanced manner to avoid negative effects on both short- and long-term storage, owing to the incorporation of different types of starches and proteins affecting the stability of the system.

Effect of wheat TaDOF6 overexpression on the molecular structure and physicochemical properties of grain starch and resistant starch

This study examined the effects of overexpressing the DOF (DNA binding with one finger protein) transcription factor gene, TaDOF6, on wheat grain native starch and Ⅲ-type resistant starch (RS3). Overexpression of TaDOF6 in the endosperm resulted in a significant increase in amylopectin content, particularly in long-branched chains, as well as larger starch particle size. Additionally, there was an increase in both rapidly and slowly digestible starches, along with an elevated level of native resistant starch. The enhanced crystallinity and orderliness of native starch, along with improved pasting properties, were also observed. However, TaDOF6 overexpression negatively impacted RS3 by reducing its crystal order, thermal stability, and pasting performance. Principal component analysis highlighted the substantial role of amylopectin in determining the crystal structure and physicochemical properties of starch. Moreover, a significant correlation was found between the particle size of RS3 and its physicochemical characteristics. Overall, these findings demonstrate that TaDOF6 overexpression alters the composition of grain starch, leading to improvements in its molecular structure and physicochemical properties.

High wear resistance of metallocene polyethylene with long-chain branch for artificial joints

Ultra-low-wear polyethylene (ULWPE) is a promising material for artificial joint implants, owing to its good biocompatibility and wear resistance. However, its molecular weight is almost an order of magnitude lower than that of Ultrahigh molecular weight polyethylene (UHMWPE). The mechanism between wear resistance and the structure of ULWPE is still unknown. This study analyzed their attributes from microstructure to macroscopic performance and revealed the wear resistance mechanisms of ULWPE by conducting an in-depth investigation of four types of polyethylene, including ULWPE, UHMWPE, and high-density polyethylene. Impact tests showed that ULWPE, with a molecular weight of 282 kg/mol, exhibited an impact energy of 110 kJ/m2, surpassing the 97 kJ/m2 recorded for UHMWPE (molecular weight over 5000 kg/mol). Furthermore, tribology tests indicated that the wear rate of the most anti-wear ULWPE was 4.479 ± 0.040 mm³/Mc, which was significantly lower than the 5.601 ± 0.456 mm³/Mc for UHMWPE. The excellent mechanical properties and wear resistance of ULWPE were derived from its unique structure. Dynamic rheological assessments and thermal classification techniques confirmed that ULWPE had long-branched chains. The long-branched chains markedly increased the proportion of the high entanglement and the tie molecules in the amorphous regions, endowing it with superior toughness. At the same time, the high crystallinity of ULWPE gives it sufficient hardness and strength. The combination of high crystallinity and high entanglement endowed ULWPE with a good balance between strength and toughness, leading to its exceptional mechanical properties and wear resistance. This study provided robust data for ULWPE's clinical use and theoretical insights for designing durable, implantable polymers.

Largely Improved Creep Resistance and Thermal-Aging Stability of Eco-Friendly Polypropylene High-Voltage Insulation by Long-Chain Branch-Induced Interfacial Constraints.

Polypropylene (PP)-based composites have attracted numerous attention as a replacement of prevailing cross-linked polyethylene (XLPE) for high-voltage insulation due to their ease of processing, recyclability, and excellent electrical performance. However, the poor resistances against high-temperature creep and thermal aging are obstacles to practical applications of PP-based thermoplastic high-voltage insulation. To address these problems, in this Letter, we synthesized an impact polypropylene copolymer (IPC) containing multifold long-chain branched (LCB) structures in phases, especially the interfaces between the PP matrix and the rubber phase. The results indicated that the structural stability of LCBIPC was significantly enhanced under extreme conditions. In comparison to IPC (without LCB structures), 24.1% less creep strain and 75.2% less unrecoverable deformation are achieved in LCBIPC at 90 °C. In addition, the thermal aging experiments were performed at 135 °C for 48 and 88 days for IPC and LCBIPC, respectively. The results show that the resistance against thermal aging was also enhanced in LCBIPC, which showed a 133% longer thermal aging life compared to IPC. Further results revealed that the interfacial layer between the PP matrix and the rubber phase was constructed in LCBIPC. The two phases are tightly linked by chemical bonds in LCB structures, leading to enforced constraints of the rubber phase at the micro level and better resistance performance against creep and thermal aging at the macro level. Evidently, the reported eco-friendly LCBIPC thermoplastic insulation shows great potential for applications in high-voltage cable insulation.

Study on the long-range chain degradation behavior of polylactic acid promoted by nanometer zinc oxide

Polylactic acid (PLA) is known for its degradative properties. Since PLA possesses a long chain macromolecular structure, its degradation behavior and characteristics have been the focus of research. In the presence of Lewis acid, the carbonyl oxygen in the ester group of PLA molecular chain can coordinate with it and be activated, so as to break the ester group and cause PLA degradation. However, there is still a lack of research strategies on the evolution of the remote structure of PLA chains, and there were few studies on the long-range degradation of PLA molecular chain in the effect of Lewis acid. In this work, a statistical method, combined with molecular weight testing, was proposed to investigate the long-range degradation behavior of PLA molecular chains. Results showed that in the presence of nano-ZnO, the long-range degradation of PLA molecular chain in solution was initially carried out in the form of terminal group biting-back and exchanging within 4 hours, and the molecular chain with new terminal-group was still long enough after degradation, gradually shortened over time. PLA began to degrade obviously after 8 h reaction. This chain scission method can provide a reaction platform for some reactions such as long-chain branching, which can be utilized to prepare PLA with special molecular chain topology and explore the mechanism of PLA chain structure transformation.

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.

A commercially viable solution process to control long-chain branching in polyethylene.

In polyolefins, long-chain branching is introduced through an energy-intensive, high-pressure radical process to form low-density polyethylene (LDPE). In the current work, we demonstrated a ladder-like polyethylene architecture through solution polymerization of ethylene and less than 1 mole % of α,ω-dienes, using a dual-chain catalyst. The ladder-branching mechanism requires catalysts with two growing polymer chains on the same metal center, thus enchaining the diene without the requirement of a steady-state concentration of pendant vinyl groups. Molecular weight distributions lacking a high-molecular weight tail, distinctive Mark-Houwink signatures, nuclear magnetic resonance characterization, and shear and extensional rheology consistent with highly branched polyethylene architectures are described. This approach represents an industrially viable solution-polymerization process capable of producing controlled long-chain branched polyethylene with rheological properties comparable to those of LDPE or its blends with linear low-density polyethylene (LLDPE).

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.

A versatile modification strategy to enhance polyethylene properties through solution-state peroxide modifications

Utilisation of low-cost organic peroxides to introduce long-chain branching into commodity polyethylene via a solution based methodology, resulting in enhanced mechanical properties.

Low-Density Polyethylene – New insights into an old material under comparative studies using SEC-MALS, AF4-MALS, HT-HPLC and 2D-LC

LDPE can be produced by different types of autoclave or tubular reactors, which can also be cascaded. An important effort is to obtain materials with autoclave-process equivalent properties from tubular technology due to economic reasons. In this way, the molecular understanding of the materials attains a key role. Five LDPE materials were characterized, covering three applications (extrusion coating, blow molding, and film blowing) as well as three reactor technologies (type I autoclave, tubular type, and type II autoclave). SEC-MALS, SEC-IR, AF4-MALS, HPLC, and 2D-LC allowed a detailed characterization of the samples’ molar mass and branching structure while providing a unique opportunity for technique comparison. This allowed for an unprecedented understanding of the differences in molecular structure and the influence of application vs. reactor technology driven differences.Overall, SEC-IR-MALS was found to provide the most versatile information, making it an excellent choice for an initial characterization. Furthermore, the results indicate that the intended application (and thus property profile) influences LDPE structure much more than reactor technology. All five studied samples exhibit differences in molar mass distribution, short-chain as well as long-chain branching. Yet, the three extrusion coating samples (each derived from a different reactor type) appeared notably similar to each other with regard to each parameter.

A pragmatic approach to analyze the ability of ethylene‐octene copolymer in the long chain branching of polypropylene in molten and solid state

AbstractThe investigation of chain branching in polypropylene (PP) holds significant importance for comprehending the impact of structural modifications on the properties of PP. The utilization of ethylene‐octene copolymer (EOC) presents a valuable opportunity to examine the influence of branching density on the ability of electron beam irradiation or a chemical agent in a PP blend. The introducing long chain branches (LCB) into PP can enhance its mechanical properties, particularly impact strength. This characteristic is of particular significance in applications where the material is subjected to mechanical stress, such as automotive components or packaging materials. Blends of PP/EOC were produced using 20 and 30 wt% of EOC and 0.5 phr trimethylolpropane trimethacrylate (TMPTMA) monomer. To evaluate the efficiency of branching in both solid and molten states, irradiated samples and samples mixed with dicumyl peroxide (DCP) were selected. The rheological properties of the molten blends in shear and extensional modes were assessed, and their morphology was examined using scanning electron microscopy. The existence of LCB in all samples was confirmed through dynamic viscoelastic measurements. It was concluded that although the irradiation promoted chain scission within the backbone, which resulted in long chain branching, DCP created LCB on the backbone chains of PP through chemical reaction in the melt state. Additionally, the strain hardening constant (n) was calculated, and its value for the irradiated sample PP/EOC 80/20 was 0.19, whereas its value for this blend when employing DCP was 0.14. While the complex viscosity of irradiated blend (8764 Pa.s) was greater than that of melt state branched blend (8377 Pa.s) at 0.1 rad/s, in the higher frequencies it became smaller due to the more effective creation of long chain branches through peroxide grafting in the molten state. The results of the study verified the presence of LCB in the materials by observing longer relaxation time and strain hardening behavior.Highlights The chain branching of PP/EOC blends was investigated in both the melt state and solid state through the presence of TMPTMA monomer. The effectiveness of TMPTMA monomer in grafting was mostly notable in the molten state. The steady state viscosity of LCB‐(PP/EOC) was higher in the presence of dicumyl peroxide compared to irradiated PP/EOC. The samples containing 80 wt% of PP exhibited longer side branches compared to the sample with 70 wt% of PP according to shear and extensional rheometry. The induced side chain branches resulting from irradiation and the use of dicumyl peroxide help increase the miscibility of the polymers to a similar extent.

Synthesis of polyolefin elastomers from amine-imine nickel-catalyzed ethylene polymerization

The catalytic synthesis of polyolefin elastomers by using ethylene as a sole feedstock is fascinating, while currently reported chain walking catalysts are only limited to α-diimine type. In this contribution, we developed alternative amine-imine nickel catalysts for ethylene polymerization to prepare polyolefin elastomers. The cooperative effect of steric hindrance between the less bulky amine moiety and the bulky imine moiety significantly enhanced the performance of amine-imine nickel-catalyzed ethylene polymerization. The amine-imine nickel catalysts also showed excellent controlled behavior for ethylene polymerization, and living ethylene polymerization was also realized to generate narrowly distributed polyethylenes. The microstructure of the resultant polyethylenes could be tuned by catalyst structure, reaction temperature, and ethylene pressure. The polyethylenes produced by Ni2 with the cyclic amine have higher long-chain branching contents, therefore showing better elastic properties. The strain-at-break value of 888.0 % and the strain recovery value of > 79.0 % are super to most values of polyolefin elastomers generated from α-diimine nickel catalysts.

Development of a model for granule-bound starch synthase I activity using free-energy calculations

Starch is a branched polymer of glucose with two components, both of which have (1 → 4)-α linear links and (1 → 6)-α branch points: amylopectin, of high molecular weight with many short branches, and amylose, of lower molecular weight and only a few long-chain branches. Granule-bound starch synthase I (GBSSI) is one of the main enzymes controlling amylose synthesis and chain-length distribution. As production of different GBSSI mutants is time-consuming and laborious, molecular dynamics (MD) simulations are used here to predict the binding of different GBSSI mutants to a representative amylose fragment. The simulations were atomistic, with explicit solvent and docking, a method successfully used to understand the binding of wild-type GBSSI to amylose fragments. The binding of GBSSI to G5 (a pentasaccharide amylose fragment) is combined with free-energy calculations employing a thermodynamic integration method to predict the effects of mutations on enzyme activity. Ten GBSSI mutants with different enzyme activities were analyzed to find the structural and energy changes among different single amino-acid mutants and their possible relationship to starch characteristics. Comparing the structural changes and the relative binding free energy of G5 to the wild type GBSSI and GBSSI mutants, it was found that mutants with negative binding energy (lower than −2.0 kcal/mol) are more likely to have higher enzyme activity and amylose content compared to the wild type. This theoretical paper used simulations and robust free energy calculations to interpret in planta data with potential predictions as to what mutants might be generated to give desired properties. This study can be used to help develop grains with improved functional properties.

Eliminating the thermal degradation of polylactic acid by the modification of a macromolecular epoxy‐type chain extender

AbstractA novel macromolecular epoxy chain extender was synthesized and used to modify the PLA resin for developing conventional melt extrusion technology. Fourier‐transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and 1H nuclear magnetic resonance (1H NMR) characterized the successful synthesis. For the first time, the molecular weight plays a key role in preventing thermal degradation and chain extension were discussed experimentally. Macromolecular chain extender showed superior advantages in that the chain extender with higher molecular weight was easier to get the long chain branched (LCB) structure in the modification of PLA. The characterization by rotational rheometer, GPC, melt index, chemical titration, and differential scanning calorimeter (DSC), results demonstrate that the macromolecular epoxy chain extender could decrease the concentration of terminal carboxyl groups and increase with only 0.5% dosage. At a concentration ≥2%, there were a large number of long chain branched structures in the chain extension system for modified PLA. It showed better melt strength and would influence the crystallinity of PLA which would significantly improve the processability of PLA, especially in high‐temperature extrusion, blow molding, spinning, and other processing technologies and applications.