Shape Of Molecular Weight Distribution Research Articles

Molecular weight distribution shape approach for simultaneously enhancing the stiffness, ductility and strength of isotropic semicrystalline polymers based on linear unimodal and bimodal polyethylenes

Bimodal polyethylenes (PEs) have captured broad attention in recent years, while little is known about the contribution of bimodal molecular weight distribution (MWD) shape on their properties compared with unimodal MWD shape. In this work, a comparative investigation of MWD shape-mechanical property relationship of isotropic semicrystalline polymers based on linear unimodal and bimodal PEs has been carried out. This first systematic study indicates that MWD shape influences the chain arrangements in crystalline and amorphous phases which affect their mechanical performance. PEs with bimodal shape exhibit higher crystallinity, lower entanglements and higher fraction of tie molecules than unimodal shape, contributing to simultaneously enhanced Young's modulus, yield strength, elongation at break and tensile strength. This work demonstrates that the modulation of MWD from unimodal to bimodal shape could overcome the trade-off among stiffness, strength and ductility of semicrystalline polymers without altering chain structure, chemical composition or average molecular weight.

Molecular Weight Distribution Shape Dependence of the Crystallization Kinetics of Semicrystalline Polymers Based on Linear Unimodal and Bimodal Polyethylenes

The manipulation of the crystallization kinetics and crystallinity of polymers without altering their chemical composition, chain structure, or average molecular weight is challenging, while little attention is paid to the role of molecular weight distribution (MWD) shape. In this work, a series of well-defined linear unimodal and bimodal polyethylenes (PEs) with molecular weights ranging from 300 k to 1200 k g/mol were synthesized to serve as a model system to study the impact of bimodal MWD shape on the crystallization kinetics of semicrystalline polymers in comparison with unimodal MWD shape at the same weight-average molecular weight (Mw). It is shown that PEs with a bimodal shape exhibit a faster nucleation rate and crystallization rate with a smaller lamellar width at low isothermal temperatures. A higher crystallization enthalpy of bimodal PEs is shown in non-isothermal experiments. The mechanism behind this is elucidated, which suggests that MWD shapes mainly affect the small-scale nucleation process without altering the large-scale growth process. This first systematic comparative study on the crystallization kinetics of linear unimodal and bimodal PEs gives insight into tailoring the crystallization behavior of semicrystalline polymers from an MWD shape perspective.

Simultaneous enhancement in processability and mechanical properties of polyethylenes via tuning the molecular weight distribution from unimodal to bimodal shape

Semicrystalline polymers with bimodal molecular weight distribution (MWD) have captured broad interest in academia and industry. Current efforts to understand the effect of bimodal MWD shape compared with unimodal shape are mixed with factors such as molecular weight, chain branching or phase separation. The collective effect is difficult to decouple in order to elucidate the independent contribution of MWD shape on the properties of polymers. In this work, a chain transfer polymerization method has been utilized to prepare well-defined linear unimodal and bimodal polyethylenes (PEs), to serve as a model system for the study of the impact of the MWD shape on the crystallization, rheology and mechanical properties of semicrystalline polymers. It has been demonstrated that PEs with bimodal shape display simultaneously enhanced crystallinity, processability, Young's modulus and tensile strength while ductility is unaffected. Compared to unimodal PEs with comparable molecular weights, bimodal PEs show about 40% lower processing viscosity, while exhibiting up to 30% greater tensile strength. This represents the first systematic comparative investigation of MWD shape-property relationship at the same molecular weight over a wide molecular weight range. This work indicates that semicrystalline polymers with bimodal MWD shape have the merit to overcome the trade-off between processing and performance without altering average molecular weight, chain structure or chemical composition.

Controlling the shape of the molecular weight distribution for tailored tensile and rheological properties in thermoplastics and thermoplastic elastomers

AbstractThermoplastics and thermoplastic elastomers compose roughly 80 percent of all polymeric materials manufactured today and play an important role in numerous sectors of modern society. While the effects of molecular weight and dispersity (Ð) on the tensile and rheological properties of these materials are well‐known, only recent studies have evidenced the profound influence of the shape of the molecular weight distribution (MWD) on polymer properties. This development is largely due to the emergence of new synthetic strategies to control higher moments of the MWD. In this Perspective, we describe recent advancements by our group in understanding the effect of MWD shape on the mechanical and rheological properties of thermoplastics and thermoplastic elastomers. We highlight means to exploit MWD shape for improved processability and performance and discuss future directions in this field.

Precise Control of Both Dispersity and Molecular Weight Distribution Shape by Polymer Blending

AbstractThe breadth and the shape of molecular weight distributions can significantly influence fundamental polymer properties that are critical for various applications. However, current approaches require the extensive synthesis of multiple polymers, are limited in dispersity precision and are typically incapable of simultaneously controlling both the dispersity and the shape of molecular weight distributions. Here we report a simplified approach, whereby on mixing two polymers (one of high Đ and one of low Đ), any intermediate dispersity value can be obtained (e.g. from 1.08 to 1.84). Unrivalled precision is achieved, with dispersity values obtained to even the nearest 0.01 (e.g. 1.37→1.38→1.39→1.40→1.41→1.42→1.43→1.44→1.45), while maintaining fairly monomodal molecular weight distributions. This approach was also employed to control the shape of molecular weight distributions and to obtain diblock copolymers with high dispersity accuracy. The straightforward nature of our methodology alongside its compatibility with a wide range of polymerisation protocols (e.g. ATRP, RAFT), significantly expands the toolbox of tailored polymeric materials and makes them accessible to all researchers.

Precise Control of Both Dispersity and Molecular Weight Distribution Shape by Polymer Blending.

The breadth and the shape of molecular weight distributions can significantly influence fundamental polymer properties that are critical for various applications. However, current approaches require the extensive synthesis of multiple polymers, are limited in dispersity precision and are typically incapable of simultaneously controlling both the dispersity and the shape of molecular weight distributions. Here we report a simplified approach, whereby on mixing two polymers (one of high Đ and one of low Đ), any intermediate dispersity value can be obtained (e.g. from 1.08 to 1.84). Unrivalled precision is achieved, with dispersity values obtained to even the nearest 0.01 (e.g. 1.37→1.38→1.39→1.40→1.41→1.42→1.43→1.44→1.45), while maintaining fairly monomodal molecular weight distributions. This approach was also employed to control the shape of molecular weight distributions and to obtain diblock copolymers with high dispersity accuracy. The straightforward nature of our methodology alongside its compatibility with a wide range of polymerisation protocols (e.g. ATRP, RAFT), significantly expands the toolbox of tailored polymeric materials and makes them accessible to all researchers.

Achieving molecular weight distribution shape control and broad dispersities using RAFT polymerizations

Metered additions of chain transfer agents are used to control molecular weight distribution (MWD) features in reversible addition-fragmentation chain-transfer polymerizations, giving polymers with tailored MWD shapes and dispersities as high as 6.2.

Shifting Boundaries: Controlling Molecular Weight Distribution Shape for Mechanically Enhanced Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) based on polystyrene (PS) are commonplace in society. The elastomeric properties of these materials are often sacrificed to increase mechanical properties such as st...

Mechanochemical tuning of molecular weight distribution of styrene homopolymers as postpolymerization modification in solvent‐free solid‐state

AbstractMechanochemical degradation by planetary ball milling (PM) is used for postpolymerization modification of styrene homopolymers (PS). A complete factorial design was chosen to study the effect of radical scavengers, milling time, initial molecular weight, and revolution radius (Rp), on the shape of molecular weight distributions (MWDs) of PS. Size‐exclusion chromatography analysis shows the feasibility of fine‐tuning MWD of PS at up to 40% conversion. Distributions ranged from unimodal to bimodal in a PM with Rp = 150 mm at different stage of milling, whereas in a PM with Rp of 60.8 mm the adjustment of unimodal distributions is achieved. Initial polydispersity is more important to develop bimodal distributions when compared with initial molecular weight. Fourier transform infrared and X‐ray electron spectrometry analysis show some suppression of PS degradation and complete oxidation inhibition of macromolecular radicals with the incorporation of radical scavengers, which we considered as additional aids when adjusting the MWDs.

Simple and secure data encryption via molecular weight distribution fingerprints

A method for encryption and safe transmission of data in the shape of molecular weight distributions (MWD) is presented.

Tailor-made thermoplastic elastomers: customisable materials via modulation of molecular weight distributions.

The ability to change polymer properties has in the past largely been a factor of modulating the molecular weight, molecular weight distribution breadth, crosslinking, or branching. The use of controlled MWD shape has recently emerged as a promising avenue towards modifying polymer properties. Taking advantage of molecular weight distribution shape, we report a simple and efficient approach for tuning material properties in polystyrene-block-polyisoprene-block-polystyrene (SIS) thermoplastic elastomers (TPEs). We find that the skew of the MWD function governs tensile properties and can be used as a handle to predictably vary polymer toughness while reducing energy dissipation.

Controlling the Shape of Molecular Weight Distributions in Coordination Polymerization and Its Impact on Physical Properties

High-density polyethylene (HDPE) is utilized in a multitude of commercial products worldwide due to its broad spectrum of physical properties and low production costs. Molecular weight and dispersity (Đ) are known to affect the tensile and rheological properties of HDPE, but little is known about the influence of the molecular weight distribution (MWD) shape on these properties. In this work, we investigate this matter through the temporal regulation of initiation in a living coordination-insertion polymerization of ethylene. This method provides precise control over the MWD shape which, in turn, offers a systematic study on the influence of MWD shape on the physical properties of HDPE. Through rheological testing, we observe a difference in complex viscosity and shear thinning with opposite MWD skew. However, tensile testing reveals that the MWD skew does not impact the strain at break, signifying the ability to influence HDPE processing without compromising material strength.

Controlling polymer properties through the shape of the molecular-weight distribution

The manipulation of a polymer’s properties without altering its chemical composition is a major challenge in polymer chemistry, materials science and engineering. Although variables such as chemical structure, branching, molecular weight and dispersity are routinely used to control the architecture and physical properties of polymers, little attention is given to the often profound effect of the breadth and shape of the molecular-weight distribution (MWD) on the properties of polymers. Synthetic strategies now make it possible to explore the importance of parameters such as skew and the higher moments of the MWD function beyond the average and standard deviation. In this Review, we describe early accounts of the effect of MWD shape on polymer properties; discuss synthetic strategies for controlling MWD shape; describe current endeavours to understand the influence of MWD shape on rheological and mechanical properties and phase behaviour; and provide insight into the future of using MWDs in the design of polymeric materials. Controlling polymerization is required to fully understand and tune the function of a polymer. Alongside variables such as dispersity, molecular weight and chemical composition, the shape of the molecular-weight distribution has substantial influence on polymer properties. In this Review, we discuss methods to systematically control the shape of a polymer’s molecular-weight distribution, as well as explore the profound effects of shape on polymer properties.

Catalyst: Size Distribution in Self-Assembly Matters

Dafni Moatsou completed her PhD in chemistry at the University of Warwick in 2015, was a Marie Sklodowska-Curie fellow at the Adolphe Merkle Institute until 2017, and is a junior research group leader at the Karlsruhe Institute of Technology, where she focuses on sequence control and self-assembly of polymers. Rachel O’Reilly completed her PhD in 2003 at Imperial College London. She currently holds a chair in chemistry within the College of Engineering and Physical Sciences at the University of Birmingham. In August 2018 she became the head of the School of Chemistry. Her research targets the design, synthesis, and application of polymeric materials, where control over architecture, functionality, and reactivity are central to their application.

Tailoring polymer dispersity and shape of molecular weight distributions: methods and applications.

The width and shape of molecular weight distributions can significantly affect the properties of polymeric materials and thus are key parameters to control. This mini-review aims to critically summarise recent approaches developed to tailor molecular weight distributions and highlights the strengths and limitations of each technique. Special emphasis will also be given to applications where tuning the molecular weight distribution has been used as a strategy to not only enhance polymer properties but also to increase the fundamental understanding behind complex mechanisms and phenomena.

Molecular Weight Distribution Shape as a Versatile Approach to Tailoring Block Copolymer Phase Behavior.

The molecular weight distributions (MWDs) of block copolymers significantly impact their morphological phase behavior, but exploiting these features as a means to tune material properties has been limited to the MWD breadth, or dispersity (Đ). Manipulation of the entire MWD has promising potential to address this challenge by providing a convenient and versatile route toward tailoring polymer nanostructure. Herein, we describe the self-assembly of poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) where the PS blocks have systematically deviating compositions of molecular weights. We find that controlling the MWD shape, breadth and skew, afforded access to different morphologies in samples with the same molecular characteristics, including Đ. As such, we illustrate the generality and effectiveness of this strategy and anticipate that it will facilitate the increased deployment of disperse polymer compositions in advanced materials applications.

Exploiting Molecular Weight Distribution Shape to Tune Domain Spacing in Block Copolymer Thin Films.

We report a method for tuning the domain spacing ( Dsp) of self-assembled block copolymer thin films of poly(styrene- block-methyl methacrylate) (PS- b-PMMA) over a large range of lamellar periods. By modifying the molecular weight distribution (MWD) shape (including both the breadth and skew) of the PS block via temporal control of polymer chain initiation in anionic polymerization, we observe increases of up to 41% in Dsp for polymers with the same overall molecular weight ( Mn ≈ 125 kg mol-1) without significantly changing the overall morphology or chemical composition of the final material. In conjunction with our experimental efforts, we have utilized concepts from population statistics and least-squares analysis to develop a model for predicting Dsp based on the first three moments of the MWDs. This statistical model reproduces experimental Dsp values with high fidelity (with mean absolute errors of 1.2 nm or 1.8%) and provides novel physical insight into the individual and collective roles played by the MWD moments in determining this property of interest. This work demonstrates that both MWD breadth and skew have a profound influence over Dsp, thereby providing an experimental and conceptual platform for exploiting MWD shape as a simple and modular handle for fine-tuning Dsp in block copolymer thin films.

Controlling Molecular Weight Distributions through Photoinduced Flow Polymerization

Molecular weight distribution plays an important role by giving specific properties to polymeric materials. Despite the recent progress in controlled/“living” radical polymerization, polymer chemists do not have the ability to tune and manipulate the shape of molecular weight distributions (MWDs) (except for a few recent examples). In this article, new synthetic procedures have been developed for controlling MWDs using photoinduced electron/energy transfer–reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, in conjunction with flow processes. By adjusting the pump flow rates, the chemical concentrations and residence times of the reactant streams were tailored throughout the polymerization, leading to control over the MWD. Changes to the intensity and wavelength of the light source also induced changes in the polymerization, allowing alteration of the MWD. The protocols described here should be amenable to a range of polymerization techniques and reaction setups.

Manipulation of Molecular Weight Distribution Shape as a New Strategy to Control Processing Parameters.

Molecular weight and dispersity (Ð) influence physical and rheological properties of polymers, which are of significant importance in polymer processing technologies. However, these parameters provide only partial information about the precise composition of polymers, which is reflected by the shape and symmetry of molecular weight distribution (MWD). In this work, the effect of MWD symmetry on thermal and rheological properties of polymers with identical molecular weights and Ð is demonstrated. Remarkably, when the MWD is skewed to higher molecular weight, a higher glass transition temperature (Tg ), increased stiffness, increased thermal stability, and higher apparent viscosities are observed. These observed differences are attributed to the chain length composition of the polymers, easily controlled by the synthetic strategy. This work demonstrates a versatile approach to engineer the properties of polymers using controlled synthesis to skew the shape of MWD.

Molecular Weight Distributions and Average Molecular Weights of Pyrolysis Oils from Oil Shales: Literature Data and Measurements by Size Exclusion Chromatography (SEC) and Atmospheric Solids Analysis Probe Mass Spectroscopy (ASAP MS) for Oils from Four Different Deposits

Molecular weight (MW) data for shale oils, including molecular weight distributions (MWDs) as well as average MWs, is difficult to find. A thorough literature review of MW data for shale oils from different pyrolysis processes, found scattered across a wide range of topics (or papers on various topics) and analyzed here, was carried out as part of the study. However, because the data are for oils produced under different conditions, and the exact pyrolysis conditions are sometimes not fully described, then the MW values generally are not directly comparable. Therefore, in the experimental part of the current study, average MWs and MWDs were measured for shale oils from four different oil shales (Green River in the western United States, Attarat Umm Ghudran in Jordan, Kukersite in Estonia, and Dictyonema in Estonia) obtained by retorting under identical pyrolysis conditions, using a standardized Fischer assay method. The main goal of the study, to measure the MWDs of the Fischer assay oils, was accomplished using size exclusion chromatography (SEC). The MWD data obtained was also compared to that from atmospheric solids analysis probe mass spectroscopy (ASAP MS). In addition, MWDs of industrial Kukersite shale oil from Kiviter and Galoter processes were evaluated for comparison. The MWDs of Fischer assay shale oils obtained in the current study ranged from 100 g mol–1 to 600 g mol–1. While the oils show comparable average MW values, the parameters describing the width and shape of MWDs were more dependent on the parent oil shale.