Amounts Of Long-chain Branching Research Articles

Machine learning prediction of mechanical and optical properties of uniaxially oriented polymer films

AbstractImproving properties of polymers can bring about tremendous opportunities in developing new applications. However, the commonly used trial‐and‐error method cannot meet the current need for new materials. We demonstrate the utility of Machine Learning (ML) algorithms in creating structure‐process‐property models based on industrial data in polymer processing. In this study, ML algorithms were used to predict the optical and tensile strength of multi‐layer co‐extrusion polyethylene films as a function of material structures and process parameters. The input features to predict the mechanical and optical properties are the composition of five‐layer polyethylene film, polyethylene molecular properties like the amount of long chain branching , and the extrusion process conditions. Different data featuring steps are conducted to improve the quality of the input data: (1) feature importance scoring using an ensemble algorithm (XGBoost); (2) application of autoencoder to reduce the dimensionality; (3) replacing the categorical inputs with molecular characteristic properties. We then use this data to build an Artificial Neural Network. Finally, the prediction capability of the resulting model was investigated. This project demonstrates a successful end‐to‐end execution of a material data science project; from understanding material science, data engineering, algorithm development, and the model evaluation.

Oil‐Based versus Bio‐Based C8 Alkyl Chain (Meth)Acrylates in Emulsion Polymerization: Kinetics and Microstructure

Abstract2‐Octyl acrylate (2‐OA) is a very promising bio‐based monomer that is a great candidate to substitute the widely used (but oil‐based) 2‐ethylhexyl acrylate (2‐EHA). In this work, the homopolymerization of these two monomers along with their methacrylic analogues (2‐octyl methacrylate, 2‐OMA and 2‐ethylhexyl methacrylate, 2‐EHMA) is compared to analyze their performance in detail. Very similar polymerization kinetics are observed, both in conversion and particle size. However, there are important differences in the microstructure (molar mass distribution, gel content, and degree of branching). Both acrylates form high amounts of gel (2‐OA higher than 2‐EHA) while the methacrylates do not form any. It is found that the most likely reason for the higher amount of gel on 2‐OA lies on the second abstractable labile hydrogen in the side chain presented in this monomer, that is more reactive in 2‐OA than in 2‐EHA. This is supported by the molar mass distributions, that show that 2‐octyl esters have higher molar mass than their respective 2‐ethylhexyl counterparts (both in acrylates and methacrylates), as a result of a higher amount of long chain branches.

Long-Chain Hyperbranched Comb Block Copolymers: Synthesis, Microstructure, Rheology, and Thermal Behavior

A series of poly(ethylene-co-acrylic acid)-cb-atactic polypropylene (EAA-cb-aPP) comb block copolymers were synthesized by grafting aPP-OH macromonomers onto a commercial EAA copolymer made by the high-pressure free radical process. The starting EAA copolymer contains 11 wt % of EAA units and has a significant amount of long chain branches. Therefore, the EAA-cb-aPP copolymers can be classified as hyperbranched. Room temperature atomic force microscopy and X-ray scattering measurements reveal strong, finely textured, phase segregation of the amorphous aPP and semicrystalline EAA domains, which persists in the melt state. The amorphous aPP side chains have an unexpected nucleating effect that facilitates crystallization of the EAA backbone, as evidenced by an increase in crystallization temperature. Moreover, phase segregation has a strong effect on both the linear and nonlinear viscoelastic response of the copolymers. Increases in both the branching density and branch chain length result in an improvement...

Reinforcement of Cr/silica catalysts by secondary deposition of silicate oligomers

Ethyl silicate oligomers were impregnated into a generic Phillips Cr/silica catalyst used commercially for ethylene polymerization. After calcination, this treatment results in a secondary deposition of silica into the original silica matrix, which changes the porosity of the catalyst somewhat and strengthens its overall structure. The process can be used as a powerful tool to manipulate polymer properties because the molecular weight of the polymer, the MW distribution, and the amount of long-chain branching (LCB) it contains, were found to be strongly influenced, and with them all aspects of molding behavior. This result provides still more evidence that the physical structure of the Cr/silica catalyst can have major consequences for the polymerization reaction, which shapes the character of the polymer. This method of controlling polymer properties is perhaps more convenient for commercial operations than those described previously.

Long-Chain Branch Measurement in Substantially Linear Ethylene Polymers by 13C NMR with Halogenated Naphthalenes as Solvents

Substantially linear ethylene polymers (SLEPs) are important commercial products which are used in various applications such as packaging, electrical insulation, toys, pipes, footwear, roofing, automotive, fabrics, and much more. SLEPs can be produced using molecular catalysts which can lead to long chain branching (LCB). The amount of LCB has an influence on viscoelastic properties which affect film production and processing as well as mechanical and optical properties. Thus, it is important to accurately measure LCB content. 13C NMR is one of the methods that have been used to characterize LCB. It is quite challenging to measure LCB with 13C NMR in the presence of short chain branches (SCBs) longer than four carbons due to the overlap of LCB signals with SCB signals. In this paper, we describe the use of halogenated naphthalenes as suitable solvents to separate 13C signals related to LCB from SCB. The new method presented here allows for better quantification of LCB in polymer samples with a diverse arr...

Preparation and Characterization of Long Chain Branched Polypropylene through UV Irradiation and Coagent Use

Trimethylolpropane triacrylate as coagent was utilized along with benzophenone to modify polypropylene by generating long chain branches in the polypropylene molecular structure. The effects of trimethylolpropane triacrylate concentration, benzophenone concentration, and irradiation duration on viscoelastic properties and gel content were studied. The processing conditions that result in the greatest amount of long chain branching with minimum amount of gel content were identified. Gel permeation chromatography was used to confirm formation of branches and determine the branching content, whereas Fourier transform infrared spectroscopy confirmed the contribution of trimethylolpropane triacrylate in the formation of long chain branches.

Influence of Catalyst Porosity on Ethylene Polymerization

The structure and porosity of Cr/silica catalysts has a strong influence on its activity in ethylene polymerization and on the character of the polymer it produces. In this study, silicas of widely varying physical structure were chosen so that the influence of surface area, pore volume, pore diameter, and coalescence could be independently investigated by monitoring the surface activity, the polymer molecular weight (MW), MW distribution, melt flow, and the amount of long-chain branching (LCB). The results are discussed with respect to (1) fragmentation of the silica during polymerization, and (2) egress of polymer from pores inside the resulting fragments. Pores of narrow diameter were found to inhibit polymer egress, resulting in lower surface participation, which in turn raised the molecular weight. Pores of wide diameter were found to produce relatively constant surface participation and polymer molecular weight, but increased the amount of LCB in the polymer. Variations in MW are seen as a function ...

Structure and rheology of polypropylene with various architectures prepared by coagent‐assisted radical processing

AbstractThe grafting of polar monomers to polyolefin backbones carried out in the melt in the presence of peroxides is accompanied by a restructuring of the pristine macromolecule architecture depending on reaction conditions and original polyolefin structure. In the case of polypropylene (PP), side reactions, mainly degradation, are often controlled by the use of coagents which also provide a more complex scenario as far as possible reactions and final polymer structure are concerned. We have demonstrated that 2‐furyl acrylates are very active in maintaining the high molecular weight of PP during various functionalization processes carried out in the melt in the presence of peroxides. In the study reported here, PP samples modified in the melt by free radical processes initiated with peroxides in the presence of butyl‐3‐(2‐furyl) propenoate and its mixture with maleic anhydride were examined, and the effect of structural changes achieved were related to the rheology of the melt and morphological behaviour under heating. Rheological analysis is consistent with the formation of long branched macromolecules to an increasing extent with increasing content of reagents with respect to the polyolefin. Differential scanning calorimetry shows that the amount of long‐chain branching is responsible for a decrease of the melting temperature and an increase of the crystallization temperature. These results confirm that the functionalization coagent and monomer can give large macromolecule structure changes which can be driven in different directions depending on feed composition. Copyright © 2010 Society of Chemical Industry

Improved Fusing Properties and Characteristics of Polyester Toners Prepared by Chemical Milling

We developed a polyester color toner with a broad fusing latitude of from 150°C to over 220°C, using our proprietary ‘Chemical Milling Process’. In this study, we explored the relationship between the fusing latitude and the chemical milling property, on the one hand, and the chemical structure of toner resin, especially molecular weight, molecular weight distribution and the amount of long chain branching, on the other. The fusing latitude, in a hot roll fusing system (both soft and hard roll), expanded with the degree of branching while the chemical milling property was little affected by the branching. The hot offset temperature increased with the degree of branching whereas the cold offset temperature was not affected. An attempt to understand the results in terms of effects of the branching on resin physicochemical properties such as the acid value, the polymer chain dimension and thermo-rheological property is to be presented.

Pressure and Temperature Dependence of LDPE Viscosity and Free Volume: The Effect of Molecular Structure

Abstract Temperature and pressure dependencies of shear and elongational viscosities were examined using rotational and capillary rheometers. Two different batches of the same polymer grade have shown that the molecular structure diversion significantly influences the magnitudes of pressure coefficients, which vary more than the temperature coefficients. The pressure effect on viscosity notably depends on the amount of long-chain branching in polymer. Further, the paper shows that pVT data analysis via the Simha-Somcynsky equation of state can be employed to reveal differences in temperature and pressure viscosity dependencies through the free volume fraction.

Correlation between processability and properties of a high density polyethylene by a rheological approach

The thermal-oxidative and thermal-mechanical stability of high density polyethylene (chromium based catalyst technology) was examined at many different temperatures using a rheological approach. The changes in molecular structure, which take place during processing, have been studied using a Clextral co-rotating twin-screw extruder in comparison with dynamic measurements performed with a rotational rheometer under definite conditions of temperature, strain and frequency and in presence of air. In order to evaluate the degradation response, an investigation of elastic modulus G' as a function of frequency ω on the residual sample after ageing has been carried out. The molecular weight increase, probably due to the formation of small amounts of long chain branching, is clearly observed through the growth of the elastic properties, mainly at low frequencies (i.e. high relaxation times). The stabilised polymer shows a less pronounced tendency towards degradation, even if a critical temperature (240 °C) has been found at which antioxidant has not any effect in avoiding degradation.

Long-Chain Branching in Metallocene-Catalyzed Polyethylenes Investigated by Low Oscillatory Shear and Uniaxial Extensional Rheometry

This paper explores shear and extensional rheological behavior of unimodal, metallocene-catalyzed polyethylenes with low contents of long-chain branching (LCB). The polymers were produced in semibatch slurry polymerizations with methylaluminoxane (MAO) activated metallocene catalysts bis(n-butylcyclopentadienyl)hafnium dichloride (1), rac-[ethylenebis(2-tert-butyldimethylsiloxy)indenyl)]zirconium dichloride (2), rac-[ethylenebis(1-tert-butyldimethylsiloxy)indenyl)]zirconium dichloride (3), and rac-[ethylenebis(1-triisopropylsiloxy)indenyl)]zirconium dichloride (4). Melt properties in low oscillatory shear, in contrast to molecular weight and molecular weight distribution data from gel permeation chromatography, suggested that the polymers prepared with the ethylene-bridged complexes 2, 3, and 4 contain small but different amounts of LCB. In the melt uniaxial elongation experiments, the long-chain branched polymers exhibited strain hardening at all extension rates (rate range was from 1.0 to 0.01 s-1) with continual increase in strain hardening toward low strain rates. Unexpectedly, the behavior in LVE regime low shear and uniaxial elongation in the nonlinear range arranged the polymers in dissimilar order of apparently increasing LCB. Even though both these rheological techniques are sensitive to the molecular structure, they evidently reveal different features of it. Variation in the distribution (topology) of the long-chain branching due to differences in catalyst systems offers a plausible explanation of the differences in uniaxial elongation.

Copolymerisation properties of siloxy-substituted bis(indenyl)zirconocene catalysts: modified rheological behaviour

The combination of the copolymerisation ability and vinyl end group selectivity of siloxy substitution of ethylene-bridged bis(indenyl) zirconium dichlorides suggest these catalyst as potential ones for the production of polyethylene containing small amounts of long chain branching. The role of the polymerisation conditions with these highly active catalysts can clearly be seen. Furthermore low contents of multiple branches may occur, even though the probability of attaching several macromonomers into one chain is low. The effect on melt rheological behaviour depends on both the amount of long chain branching and the length of the branch. Moreover the position of the siloxy group is very important. Polymers synthesized with catalysts, where the siloxy group is in position 1, give peculiar rheological behaviour resembling cross-linked networks although the polymers are completely soluble.

The effect of chain architecture on “sharkskin” of metallocene polyethylenes

Dynamic viscoelastic and extrusion capillary results of metallocene based polyethylenes are analyzed. Three samples show very high viscosities at low frequencies and large relaxation times, which is a symptom of the presence of small amounts of long chain branching (LCB). A linear correlation is found between the sharkskin dynamics (periodicity) and a characteristic entanglement-disentanglement time. It is found that this correlation does not hold for samples suspected of LCB.

Rheology of metallocene‐catalyzed monomodal and bimodal polyethylenes

AbstractThe effect of molecular architecture on the dynamic viscoelastic properties of new metallocene high density polyethylenes has been analyzed. Bimodal molecular weight distribution metallocene polyethylenes show features different from conventional polydisperse and bimodal polyethylenes. Higher values of Newtonian viscosity (ηo) at the same values of weight average molecular weight (Mw) and stronger frequency dependence of dynamic viscosity (η′) than in conventional HDPE‐s have been observed; this leads to lower values of the characteristic frequency for the onset of non‐Newtonian behavior (ωo) and higher values of the power law index (α). These features are probably due to the presence of very small amounts of long chain branching (LCB). The implications of these results in polymer processing are analyzed comparing extrusion rheometer data, which leads to the conclusion that extrusion difficulties in metallocene catalyzed polyethylenes can be overcome with bimodal molecular weight distributions.

Small-Amplitude Oscillatory Shear Flow Measurements as a Tool To Detect Very Low Amounts of Long Chain Branching in Polyethylenes

Dynamic viscoelastic results of 23 noncommercial metallocene-catalyzed polyethylenes and poly(ethylene/1-hexene) copolymers, in the range 130−190 °C, are presented. The effects of well-determined structural parameters such as molecular weight, polydispersity, and degree of short chain branching (SCB), are analyzed. The molecular weight varies between 60 000 and 325 000, the polydispersity between 1.8 and 7.3, and SCB between 0 and 48.5 branches/1000 C atoms. It is observed that a group of 11 polymers displays rheological specific features which can be summarized as follows: (a) higher dynamic viscosities at low frequencies than other polyethylenes and ethylene/1-hexene copolymers of similar molecular weight, polydispersity and SCB degree; (b) higher relaxation times than narrow molecular weight distribution polyethylenes of similar dynamic viscosities at low frequencies but similar relaxation times to those of broad molecular weight distribution; (c) higher values of elastic modulus, in comparison with polyethylenes of similar molecular weight, polydispersity, and SCB but of the same order of magnitude as those of broader molecular weight distribution; (d) higher activation energy of flow than linear polyethylenes of the same molecular weight, polydispersity, and SCB level. An analysis of the literature results leads us to suspect that the polymers which show a “dissident” behavior possess a certain very low degree of long chain branching (LCB). The analysis of the samples by SEC coupled with intrinsic viscosimetry reveals that some of these 11 polymers are long chain branched. However, this technique does not appear to be enough sensitive to detect very small amounts of LCB, and an alternative single rheological method, based on the effect of temperature on dynamic viscosity, is proposed to evaluate the possible presence of LCB.

Modell zur Beschreibung der rheologischen Eigenschaften von Blends aus HDPE und langkettenverzweigtem linearem Polyethylen niederer Dichte

Die rheologischen Eigenschaften von Blends aus einem Polyethylen hoher Dichte (HDPE) und einem mittels Metallocen-Katalysatoren hergestellten linearen Polyethylen niederer Dichte mit einem Anteil an Langkettenverzweigungen (HBPE) wurden bezüglich der Abhängigkeit der Scherviskosität von der Schergeschwindigkeit über den gesamten Zusammensetzungsbereich der Polymerblends untersucht. Während das reine HDPE einen starken Abfall der Viskosität mit steigender Schergeschwindigkeit aufweist, ist dieses Abfallen beim HBPE wesentlich schwächer ausgeprägt. Bei einer Schergeschwindigkeit von ca. 300 s–1 weisen beide Produkte gleiche Viskositäten auf. Die Viskositäten der Blends zeigen in Abhängigkeit von der Schergeschwindigkeit einen nichtlinearen Übergang zwischen dem Verhalten der reinen Komponenten HDPE und HBPE. Dieses Verhalten konnte mit einem mathematischen Modell beschrieben werden, wobei sich insbesondere rationale Gleichungen neben häufig verwendeten rheologischen Modellgleichungen als geeignet erwiesen. Schließlich wurde im Hinblick auf eine weitere Diskussion als Maß für die Strukturviskosität neben dem Abfall der Viskosität mit steigender Schergeschwindigkeit auch der Exponent im Ostwald-de Waeleschen Gesetz betrachtet. Der Schwellwert, d.h. die Strangaufweitung bei der Extrusion, wurde als Ausdruck für die elastischen Anteile der Polymerschmelze berechnet und ihre Abhängigkeit von der Schergeschwindigkeit und der Zusammensetzung mathematisch dargestellt und diskutiert. Dabei wurde beobachtet, daß der Schwellwert eines Blends eine geringere Veränderung mit der Schergeschwindigkeit erfährt, als ihn die Einzelkomponenten aufweisen. The rheological properties of blends of high-density polyethylene (HDPE) and linear polyethylene of low density with an amount of long-chain branching (HBPE) synthesized with metallocene catalysts were investigated regarding the dependence of the shear viscosity from the shear rate over the whole composition range. While the pure HDPE shows a strong decrease in viscosity with increasing shear rate, the decrease for HBPE is much less distinctive. For a shear rate of about 300 s–1 both products show similar viscosities. The viscosities of the blends, in dependence from the shear rate, show a nonlinear change between the characteristics of the pure components HDPE and HBPE. This behaviour could be described with a mathematical model, for which especially rational functions together with often used rheological model terms gave good results. Finally, besides the decrease of the viscosity with increasing shear rate, the exponent in the Ostwald-de Waele law was discussed as measure for the intrinsic viscosity. The threshold value, i. e. the Merrington effect during extrusion, was calculated as measure for the elastic amounts in the polymer melt, and their dependence from the shear rate and from the composition was expressed mathematically and discussed. It was observed that the treshold value shows a less significant change in dependence from the shear rate than the pure components do.

Processability of polyethylene homopolymers and copolymers with respect to their molecular structure

The processability of commercial polyethylene homopolymers and copolymers, including both high and low density polyethylenes, was evaluated with respect to their molecular structure by measuring their melt rheological and thermodynamic properties. Short chain branching (SCB) mainly controls the density and thermodynamic properties, but it has little effect on the melt rheological properties. Long chain branching (LCB) has little effect on the density and the thennodynamic porperties, but it has drastic effects on the melt rheological properties. LCB increases the pseudoplasticity and the flow activation energy, reducing the viscosity in processing and thus improving the processability. Very small amounts of LCB in metallocene type low density polyethylenes very effectively reduce the viscosity and improve the flow stability in processing.

Improving Polymer Processability Utilizing Constrained Geometry Single Sue Catalyst Technology

Constrained geometry single site catalyst technology has enabled Dow Chemical to produce a new family of homogeneous copolymers which expand the horizons of polyolefins. This INSITE'0 technology makes it possible to control the amounts of long chain branching (LCB) and short chain branching (SCB) to produce a polymer that has the good physical properties obtained with a narrow molecular weight distribution (MWD, where Mw/M. 2) and yet has good processability-as described in this paper. For example, in a typical melt extrusion process, these new polymers are less susceptible to melt fracture than the other homogeneous (narrow MWD) polyolefins and even the broader MWD heterogeneous polyolefins such as linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE).

Tensile stress measurements on linear and branches low-density polyethylene melts. Interpretation of results with a generalized Maxwell model

Dynamic shear experiments in the linear range of deformation and extensional tests at constant strain rate have been carried out on a linear low-density polyethylene (LLDPE) melt and on two branched low-density polyethylene (LDPE) melts with different amounts of long-chain branching. Both the dynamic shear moduli and the tensile stress obey the time–temperature superposition principle. A simple model based on a nonaffine generalized Maxwell model with two relaxation times is proposed to describe the rheological behavior in elongation of these melts. Close agreement between the model and the experimental data can be obtained by adjusting the two relaxation times and the “slip parameter” of entanglements. The variations of these parameters with strain rate and their relationship with molecular structure are discussed.