Short Chain Branching Content Research Articles

Determination of Short Chain Branching in LLDPE by Rheology

AbstractThe relationship between the structure and the final properties of materials is an important issue in polymer science. In particular, the properties of polyolefins are impacted by their branching contents. A simple rheological method is presented to estimate the degree of short chain branching (SCB) in linear low‐density polyethylene (LLDPE). A range of ethylene‐1‐hexene copolymers is synthetized using a metallocene catalyst to obtain homogenous LLDPE models with various comonomer contents. The polymers are analyzed by size exclusion chromatography, 13C NMR, and crystallization elution fractionation to determine both their molar masses and their chemical composition distributions. Subsequently, they are measured by rheology to assess the impact of SCB. This work provides calibration curves for rheology and proves that it is possible to predict the SCB content in LLDPE from the melt viscosity analysis.

On the melting behaviour of linear polyethylene single crystals in mixtures with homogeneous short chain branched polyolefins

The melting temperature depression of linear polyethylene lozenge single crystals is thoroughly examined in mixtures with a series of homogeneous high molecular weight branched polyethylene models, with special emphasis on the effect of short chain branching on the compatibility. The results indicate that the melting temperature of the lamellar single crystals is influenced both by the branching content of the surrounding matrix and by the relative volume concentration of the two polymers in the mixture. The discussion of the results is conducted around the compatibility of polymeric species as a function of the short chain branching content, invoking experimental and computational work performed by other authors. The observed depression indicates that the homogeneous mixing of the two polymers at the melting point may occur whenever the branching content is lower than 6–7 every 100 carbon atoms, a result that is reconciled with computer simulations and other experimental approaches.

Influence of electron‐beam irradiation on plasticity‐controlled and crack‐growth‐controlled failure in high‐density polyethylene

AbstractIn the present study, the influence of electron‐beam irradiation on plasticity‐controlled and crack‐growth‐controlled failure in high‐density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross‐linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross‐linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission.

The Influence of Short Chain Branch on the Crystal Characteristics and Breakdown Strength of Low-Density Polyethylene

Studying the influence of short chain branch (SCB) on the crystal properties and breakdown strength of low-density polyethylene (LDPE) plays an important role in improving the electrical and mechanical properties of LDPE by adjusting the molecular structure. In this work, the SCB and long chain branch (LCB) content of LDPE are quantitatively characterized. Grain size, crystallinity and breakdown strength of LDPE (including AC and DC) are calculated through related characterization experiments. The results show that the SCB content in LDPE is dozens of times that of LCB. Reducing the SCB content can reduce the grain size of 110 face in LDPE and improve the AC and DC breakdown strength. The trap distribution characteristics measured by the thermally stimulated depolarization current (TSDC) method show that the SCB inhibits the formation of deep cavity defects in the LDPE interface area and decreases the trap energy level of crystal defects. This inspired us to improve the breakdown strength by reducing the SCB content in LDPE.

A Short‐Chain Branching Distribution Determination Technique for Polyethylene Using IR5‐Detected GPC

AbstractIn this paper, a robust short‐chain branching (SCB) determination technique recently developed using IR5‐detected GPC (GPC‐IR5) for polyethylene resins is reported, wherein the IR5 detector is an infrared detector equipped with five filters set at different frequency ranges and a mercury‐cadmium‐telluride detector with thermoelectric cooling capability. Both intensity ratios of methyl/methylene and methyl/all C─H groups have very good linear relationship with the SCB content. Based on these relationships, an excel spreadsheet method is developed to deduce the SCB distribution across the molecular weight distribution using in‐house developed algorithms. The uncertainty of SCB determination using the GPC‐IR5 technique is found to be chromatographic signal‐to‐noise ratio dependent and can be explicitly expressed and calculated slice by slice. The SCB uncertainty is also found to be a function of experimental conditions and environment, which can affect not only the signals but also the noise levels at different IR bands. At a given polymer concentration, the higher the SCB content, the smaller the relative error bars. The SCB detection limit of this GPC‐IR5 technique is found to be at ≈0.5–1.0 SCB per 1000 TC for all samples tested. Some practical aspects for application of this technique are also discussed in this paper.

Determination of the Composition of LDPE/LLDPE Blends via 13C NMR

AbstractPolyethylene film properties can be manipulated by blending linear low‐density polyethylene (LLDPE) with low density polyethylene (LDPE) resin. The accurate determination of LDPE content in those blends, however, is challenging. Differential scanning calorimetry analysis has been traditionally used to determine the LDPE content, but the result can have significant error due to several limitations. Due to co‐crystallization, significant error can also result using temperature rising elution fractionation technique. In principle, the LDPE content would be determined by 13C NMR analysis if the short‐chain branch (SCB) content of the LDPE could be determined accurately. But unambiguous assignment of all peaks in the 13C NMR spectrum of a LDPE is difficult, if possible. Consequently, the SCB content of LDPE cannot be determined accurately. Therefore, a new approach is required to analyze the LDPE/LLDPE blend composition by 13C NMR on a routine basis. In this paper, a robust 13C NMR method to determine the LDPE content in LDPE/LLDPE blends is presented. By identifying unique LDPE peaks in the 13C NMR spectrum of a LDPE/LLDPE blend compared to that of LLDPE, the blend composition can be determined. The accuracy and precision of this method will also be discussed.

Further investigation of the relationship between polymer structure and HDPE post yield properties

Using typical characterization methods (hyphenated chromatography and calorimetry), the effects of molecular weight and short chain branching (SCB) on relative rates of crystallization and the resulting effects on the post yield tensile properties were quantified for a robust set of high-density polyethylene samples. A semi-empirical chain folding equation coupled with the new structure parameters PSP4 (tie molecule content) and the average number of lamella segments per chain (ALSC) were proposed. These parameters are based on a random polymer walk, including passes through the crystalline lamella, the number of times a chain folds in each lamella and the subsequent effects on the end to end distance of the polymer chain, <R>. Intuitively, the current approach is expected to yield enhanced correlations to mechanical performance since, in addition to maintaining the improvements in lamella size calculation, persistence length dependence on SCB content, and multi-ties per chain, it also accounts for polymer chain folding and lamella orientation. Results showed that homopolymers had a significantly higher level of chain folding compared to copolymers. Moreover, as the length of the SCB increased from ethyl to butyl, less folding was predicted thereby leaving more of the polymer chain to form tie molecules. The quantity PSP43/ALSC, exhibited a linear relationship with strain hardening modulus (SHM) with excellent correlation for a diverse set of unimodal and bimodal resins made using different catalyst systems, which included homopolymers, copolymers, and blends of different catalyst type and modality. Given the simplicity of calculation, use of actual polymer data, and the good predictability offered for SHM for a diverse set of resins, these primary structure parameters (PSP4 and ALSC) have high potential to contribute to the in-depth understanding of polymer structure-property relationships.

Molecular Dynamics Simulation of Ethylene/Hexene Copolymer Adsorption onto Graphene: New Insight into Thermal Gradient Interaction Chromatography

AbstractCrystallization‐based fractionation techniques are powerful methods for the analysis of short‐chain branching (SCB) in linear low‐density polyethylene. Recently, thermal gradient interaction chromatography (TGIC) has been developed for SCB determination of semi‐crystalline and amorphous polyolefins. In TGIC, the fractionation mechanism relies on the interaction of polyolefin chains with a graphite surface upon temperature change for a given solvent strength. Using molecular dynamics simulations, both the enthalpic and entropic contributions responsible for the separation of short‐chain branched polyethylene are estimated. A new thermodynamic framework for understanding the fractionation mechanism of TGIC is proposed. It is confirmed that at high SCB content, the decrease of the adsorption enthalpy is mostly due to the increased steric hindrance of short branches. However, for low chain branching density, strong increase of the persistence length observed during adsorption decreases the conformational entropy of the polymer and thus counter‐balances the favorable adsorption enthalpy. This entropic contribution may also explain the narrowing of the peak observed experimentally with low short‐chain branching density at high elution temperature.

Microstructure characterization of one high-speed extrusion coating polyethylene resin fractionated by solvent gradient fractionation

One low density polyethylene (LDPE) resin with high-speed extrusion coating property is fractionated through solvent gradient fractionation (SGF) technique using 1,2,4-trimethylbenzene (TMB) and ethyl cellosolve (ECS) as good/poor solvent pair at 115 °C. The pristine sample and its fractions are characterized by high-temperature gel permeation chromatography (HT-GPC) coupled with triple detectors (refractive index RI, light scattering LS, viscometer VIS), 13C–nuclear magnetic resonance spectroscopy (13C–NMR), differential scanning calorimetry (DSC) and successive self-nucleation/annealing (SSA) thermal fractionation. By adjusting the ratio of good/poor solvent, the obtained fractions show their molecular weight from 1.58 × 103 g/mol to 4.76 × 105 g/mol. It is found that the fractions with high molecular weight (fractions 10–13) occupy about 55.85% in resin. Particularly, the molecular weight distribution (MWD) of most fractions is in the range of 1.1–1.2. Each fraction contains more short chain branch (SCB) and less long chain branch (LCB) simultaneously. With increasing the molecular weight, the branching content shows no regular change. The lowest SCB and total branch content regions correspond to molecular weight 1.97 × 104 to 4.10 × 104 g/mol. The melting and crystallization temperatures of fractions firstly increase and then decrease with the molecular weight. The crystallinity decreases gradually from 51.7% to 31.1%. In the SSA thermal fractionation, each fraction shows a broad range of endotherm with multiple melting peaks in DSC curve corresponding to the different methylene sequence length (MSL) (L n and L w ). The longest L n (L w ) region occurs in the molecular weight of 8.95 × 103 to 3.14 × 104 g/mol. The relationship between chain microstructure and properties is also discussed.

Effect of Microstructure of High Density Polyethylene on Catalytic Degradation: A Comparison Between Nano Clay and FCC

The effect of microstructure of high density polyethylene (HDPE) on catalytic degradation over fluid catalytic cracking (FCC) catalyst and Nano clay is investigated. An ethylene/1-hexene copolymer (HDPE) was fractionated based on 1-hexene content to different fractions using preparative temperature rising elution fractionation (P-TREF) method. The short chain branch (SCB) content of each fraction calculated based on 1H NMR results. The homogenous SCB content fractions were subsequently analyzed using TGA and thermal degradation behavior of samples were compared with catalytic degradation. The results showed that FCC, in contrast to nano clay, is more sensitive to microstructure of HDPE during catalytic degradation. Therefore, linear chains can start degradation sooner on FCC than the branched chains. In comparison of nano clay and FCC it was found that the nano clay, reduces temperature at maximum degradation rate (Tmax), but FCC reduces the T5% and Tmax significantly.

Coarse-grained simulations on the crystallization, melting and annealing processes of short chain branched polyolefins

Coarse-grained molecular dynamics simulations studies on supercooled linear and short chain branched polyethylene melts are reported. This is the first time that a coarse-grained simulation study is carried out on the crystallization process of short chain branched polyethylenes. Crystallization and subsequent melting temperatures are related linearly to the inverse of the lamellar thickness. Different melting lines, depending on short chain branching content are obtained, but a tendency to a single crystallization line is observed. These results qualitatively match to the experimental lines obtained in model homogeneous ethylene/α-olefin copolymers. Additionally, a strong lamellar thickening process is reported for the linear model at temperatures below the melting point, which follows the classical interpretation reported in polyethylene, with a logarithmic increase of the crystal thickness with time. However, for the branched samples the thickening is impeded at the same degree of undercooling. In the branched models the short chain branches are excluded from the crystal remaining at the surface and amorphous regions. This causes the pinning of these regions, minimizing crystal chain diffusion and hindering crystal stem thickening. Again the simulated results qualitatively agree with the experimental data.

The influence of short-chain branching on the morphology and structure of polyethylene single crystals

The influence of short-chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed high-molecular-weight polyethylene samples. A strong effect of short-chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short-chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high-molecular-weight polyethylene. The curvature can be explained by a continuous increase in the step initiationstep propagation rates ratio with short-chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro-diffraction and WAXS results clearly indicate that all samples crystallize in the orthorhombic form. An increase of the unit cell parameter a(0) is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1751-1762

Molecular chain heterogeneity of a branched polyethylene resin using cross-fractionation techniques

The typical standard long chain branching (LCB) polyethylene (PE) SRM1476 is issued by the American National Bureau of Standards, which is widely used as a standard reference material in chromatographic experiments. Although some works have been reported in literatures, they are not consistent, such as the data about its molecular weight and molecular weight distribution. Therefore, it is necessary to further investigate the microstructure features of SRM1476 in order to better serve as a reference object in comparison with other unknown resins. In this study, the molecular chain heterogeneity of SRM1476 is investigated extensively and compared with the linear PE SRM1475. Based on the characterization of the original sample, SRM1476 actually has both LCB and short chain branching (SCB) structures, as well as the SCB content is more than LCB content in 13C-NMR results. After successive self-nucleation and annealing (SSA) thermal fractionation, SRM1476 shows a broad-range endotherm with multiple melting peaks (more than eight peaks), which proves the molecular chain heterogeneity in SRM1476. SRM1476 is fractionated into nine fractions by preparative temperature rising elution fractionation (TREF). At the high elution temperature region, the amounts of fractions are more than 90 %, and their molecular weights are higher. At the low elution temperature region below 50 °C, molecular weights of fractions are lower and their amounts are less than 5 %. Differential scanning calorimetry (DSC) melting curves of TREF fractions with higher elution temperatures shift toward higher temperature with sharper melting peaks. In the results of TREF-SSA cross-fractionation, each fraction shows a broad-range endotherm with multiple melting peaks that shift toward the high elution temperature region with the elution temperature. The arithmetic mean methylene sequence lengths of TREF fractions gradually increase from 33 to 105 as elution temperature increases. Results from TREF-DSC and TREF-SSA cross-fractionations indicate that SRM1476 and its TREF fractions have both intramolecular and intermolecular heterogeneity.

The interrelationships between microstructure and melting, crystallization and thermal degradation behaviors of fractionated ethylene/1-butene copolymer

The interrelationships between microstructure and melting, crystallization and thermal degradation behaviors of a commercial Ziegler–Natta (Z–N)-based ethylene/1-butene copolymer were investigated. The copolymer was fractionated based on short chain branch (SCB) content by preparative temperature-rising elution fractionation (P-TREF) method. A broad multipeak chemical composition distribution was obtained for both molecular weight distribution and short chain branch distribution. A difference of about 48 °C in melting temperature (Tm) has been observed for fractions with 50 branches of different SCB contents. A logarithmic relationship was obtained between the methylene sequence length calculated based on proton-1 nuclear magnetic resonance results and the P-TREF elution temperature (ET). A relationship between the SCB content and the inverse of lamellae thickness (Lc) was established. Two linear functionalities were found for Tm versus ET and crystallization temperature (Tc) versus the SCB content. To the best of our knowledge, as a first effort, surprisingly a linear relationship between the temperature at the maximum rate of degradation (Tmax) and the SCB content was acquired, which showed less sensitivity to the SCB content than Tm. Also, it was found that the degradation initiation temperature (T5%) and activation energy (Ea) are increased by the decreasing SCB content. Moreover, it was demonstrated that Hosoda equation is not applicable for different ethylene/α-olefin copolymers based on different catalyst types with dissimilar fractionations and experimental conditions.

4-Dimensional Modeling Strategy for an Improved Understanding of Miniemulsion NMP of Acrylates Initiated by SG1-Macroinitiator

For the first time, a kinetic model considering four-dimensional Smith–Ewart equations is presented to simultaneously calculate the time evolution of the conversion, number-average chain length, dispersity, end-group functionality (EGF), and short chain branching (SCB) content for the miniemulsion NMP of n-butyl acrylate (nBuA), initiated by poly(nBuA)-(N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) at 393 K ([nBuA]0:[poly(nBuA)-SG1]0 = 300). On the basis of literature kinetic and diffusion parameters, model analysis reveals that backbiting cannot be neglected for an accurate description of the NMP characteristics, despite the low number of SCBs formed per chain (ca. 2) and that the small loss of EGF at low conversions is mainly caused by chain transfer to monomer. SG1 partitioning (partitioning coefficient Γ = 50) between the organic and aqueous phase increases the dispersity and polymerization rate at low particle diameters (dp < ca. 50 nm) with a limited effect on the EGF profile. However, the ...

Structural and rheological property relationship of bimodal polyethylene with improved environment stress cracking resistance

Five bimodal polyethylene samples with different molecular parameters were studied in order to understand the incidence of molecular architecture on the environment stress cracking resistance (ESCR). The results showed that the tensile strain hardening stiffness at low strain rate was shown to be a fast and reliable indicator for evaluating ESCR of polyethylene. The reduction of chain dimension from phase separation would lead to the decrease of the possibility for a molecular in melt to form a tie molecular but an increasing number of short chain branch content was shown to increases strain hardening stiffness or tie chains. Physical chain entanglements were established as another source of inter-lamellar linkages that contribute to ESCR of polyethylene. Within the similar branch content and no phase separation, due to an increasing in plateau modulus was shown to increases hardening stiffness or ESCR of resins.

Structure analysis of ethylene/1-octene copolymers synthesized from living coordination polymerization

A series of ethylene/1-octene random and block copolymer samples were synthesized via a living coordination polymerization catalyzed by bis[N-(3-methylsalicylidene)-2,3,4,5,6-pentafluoroanilinato] titanium(IV) dichloride/dMAO. The chain microstructures of the copolymers were elucidated by analytic TREF and DSC thermal fractionation techniques in this work. It was found that the living random copolymers possessed narrower intrachain composition distributions than those prepared from conventional metallocene catalysts. With the same short chain branching content, the melting temperature of living random copolymer was about 16°C lower than metallocene-catalyzed random copolymer. The living block copolymers were also studied in comparison with the olefin multiblock copolymer (OBC) produced from chain shuttling polymerization. The interchain composition distribution of OBC was found broader than living block copolymers. However, the crystal thickness distribution of OBC was narrower. The diblock copolymer had a bimodal distribution in the crystal thickness and the step triblock copolymer showed a trimodal distribution, in contrast to OBC’s single modal distribution.

Effect of short chain branching in molecular dimensions and Newtonian viscosity of ethylene/1-hexene copolymers: matching conformational and rheological experimental properties and atomistic simulations

The molecular dimensions in dilute solution and the linear viscoelastic melt properties of a series of linear ethylene/1-hexene random copolymers with variable short chain branching content are reported. The results obtained from size exclusion chromatography and viscosimetry in dilute solution show a molecular contraction as the branching level increases. Additionally, a clear dependence of the Newtonian viscosity with the short chain branching content at T = 463 K is obtained. Both experimental observations are in agreement with previous experimental results found in ethylene/α-olefin copolymers, but more interestingly with recent full atomistic simulations made in our group in this type of macromolecular systems. The dependences observed can be related to the changes observed in the macromolecular conformational features and also in the equilibration entanglement time and the molecular weight between entanglements as the number of short chain branches increases.

Effect of Molecular Structure on the Short-Term and Long-Term Mechanical Behavior of High-Density Polyethylene

Environmental stress cracking (ESC) of polyethylene (PE) in structural applications can result in catastrophic failure without any visible warning. Polyethylene with excellent short-term mechanical strength can have poor long-term performance because of ESC. Because PE structures can have an expected service life of 50 years or more, the lack of understanding regarding their long-term mechanical performance is a major concern. For practical reasons, it is not possible to test PE for years before use. Currently, most mechanical tests for PE only run for a few hours. Therefore, it is important to understand the connections between short-term and long-term mechanical properties. Short-term mechanical behavior, such as creep over 8 h, is studied in conjunction with ESC for six high-density PE resins. In addition, the molecular structure influences on creep of PE are extensively investigated. Complex creep behavior is explained from the point of combined effects of molecular properties. It was found that high-density polyethylene (HDPE) resins with large creep strain usually exhibit high environmental stress cracking resistance. However, the short-term creep behavior of HDPE was affected by molecular weight, molecular weight distribution, and short-chain branching content of the material.

Improvement of Hardening Stiffness Test as an Indicator of Environmental Stress Cracking Resistance of Polyethylene

Long term mechanical behavior of polyethylene (PE) is of great importance especially in cases where structural integrity is required. In order to predict characteristics of the mechanical behavior of PE, it is necessary to fully understand the molecular structure of the employed resins. In this study, evaluation of several micromolecular properties of PE is conducted. These properties influence an important performance indicator of PE for structural applications, namely, the environmental stress cracking resistance (ESCR). ESCR in PE resins occurs through a slow crack growth mechanism under low applied stresses and long periods of time. This property is usually assessed by unreliable and time consuming testing methods such as the notch constant load test (NCLT) on notched PE specimens in the presence of an aggressive fluid at elevated temperatures. In the work presented herein, relationships between molecular structure and material response characteristics, mainly between molecular weight properties and short chain branching content in relation to strain hardening behavior of PE resins, were investigated based on results from tensile experiments. Inter-lamellar entanglements are believed to be the main feature controlling slow crack growth of PE. Extent of entanglements and entanglement efficiency has been investigated by monitoring the strain hardening behavior of PE resins in solid state through a uniaxial tensile test. The hardening stiffness (HS) test for prediction of ESCR was refined and improved to cover a broader range of PE resins, along with easier sample preparation, and faster testing. The improved test offers a more reliable and consistent ESCR picture without the drawbacks of the subjective notching process and ad-hoc presence of aggressive fluids.