Structure-rheology relationships of long-chain branched polypropylene: Comparative analysis of acrylic and allylic coagent chemistry

The relationship between product structure and melt-state rheological properties is established for series of branched PP derivatives prepared by the radical-mediated grafting of tri-functional coagents. Peroxide-initiated, solvent-free additions of linear PP to triallyl trimesate (TAM), trimethylolpropane triacrylate (TMPTA) and triallyl phosphate (TAP) at high temperature are shown to produce bimodal molecular weight and branching distributions. Low-frequency dynamic shear viscosities as well as extensional viscosities are shown to be highly responsive to a hyper-branched chain population, whose abundance and molecular weight correlate with the kinetic chain length of the grafting process, and the propensity of a coagent to oligomerize.     

Chemical modification of PP architecture: Strategies for introducing long-chain branching

Two strategies for introducing long chain branching (LCB) to a polypropylene homopolymer (PP) are evaluated in terms of the product's molecular weight and branching distributions, and in terms of melt-state shear and extensional rheological properties. Single step processes involving radical-mediated addition of PP to triallyl phosphate are shown to generate bimodal products with highly differentiated chain populations, while a two step sequence involving PP addition to vinyltriethoxysilane followed by moisture-curing is shown to generate more uniform architectures. As a result, the sequential approach can improve low-frequency shear viscosity and extensional strain hardening characteristics while staying below the polyolefin's gel point. The composition and molecular weight distribution transformations that underlie sequential LCB techniques are discussed.     

Mechanical properties of ethylene/1-hexene copolymers with tailored short chain branching distributions

Recently, we have investigated a metallocene catalyst system that can produce polyethylene and ethylene/α-olefin copolymers with tailored molecular weight and short chain branching distributions (SCBD). Ethylene/α-olefin copolymers produced with this system have narrow molecular weight distributions as expected from metallocene catalysts. However, these copolymers are quite unique in that their SCBDs are broad and sometimes bimodal, similar to Ziegler-Natta LLDPE.To examine the effect of these broad SCBDs on the polymer properties, a series of poly(ethylene-co-1-hexene) resins with very distinct, and in some cases bimodal crystalline distributions, were synthesized. The attractive feature of the resins in this study is that their molecular weight distributions are similar but each possesses a different SCBD, thus effectively minimizing the effect of molecular weight on the properties investigated.It was found that the tensile properties of a copolymer could be controlled by the ratio of the crystalline species present in the sample. In this study, a balance of stiffness and toughness was exhibited by a copolymer containing a large proportion of crystalline material and a small fraction of material of lower crystallinity.     

HDPE/LLDPE reactor blends with bimodal microstructures—Part II: rheological properties

Reactor blends of polyethylene/poly(ethylene-co-1-octene) resins with bimodal molecular weight and bimodal short chain branching distributions were synthesized in a two-step polymerization process. The compositions of these blends range from low molecular weight (LMW) homopolymer to high molecular weight (HMW) copolymer and vice versa HMW homopolymer to LMW copolymer. The shear flow characteristics of these polymers in the typical processing range mostly depend on the molecular weight and MWD of the polymer and are independent of the short chain branch content. From oscillatory shear measurements, it was observed that the viscosity of HMW polymers was reduced with the addition of LMW material. For the polymers produced with this two-step polymerization process, the LMW homopolymer and HMW copolymer blends and HMW homopolymer and LMW copolymer blends were melt miscible, despite the large viscosity differences of the pure components.     

Short chain branching distribution and thermal behavior of high density polyethylene

High-density polyethylenes with unimodal and bimodal molecular weight distribution have been fractionated according to crystallizability using preparative temperature rising elution fractionation. The molecular structure and thermal properties of the fractions with their whole polymers have been characterized. The average short chain branching content of the fractions obtained ranged from 0 to 8 branches per 1000 carbon atoms while that of the whole polymers is about 2 branches per 1000 carbon atoms. The bimodal resins have a slightly higher frequency of short chain branch in higher molecular weight species than in those of the unimodal resins. The short chain branching distribution as well as the low molecular weight species in the fractions seem to be important parameters to determine thermal behavior of the fractions. The fractions with the short chain branching content above 3 branches per 1000 carbon atoms showed a significantly different thermal behavior from those with less than 3 branches per 1000 carbon atoms. © 1996 John Wiley & Sons, Inc.     

Rheology of metallocene-catalyzed monomodal and bimodal polyethylenes

The 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 (η_o) at the same values of weight average molecular weight (M_w) 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 (ω_o) 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.     

HDPE/LLDPE reactor blends with bimodal microstructures—part I: mechanical properties

Reactor blends of polyethylene/poly(ethylene-co-1-octene) resins with bimodal molecular weight and bimodal short chain branching distributions were synthesized in a two-step polymerization process. The compositions of these blends range from low molecular weight (LMW) homopolymer to high molecular weight (HMW) copolymer and, vice versa, HMW homopolymer to LMW copolymer. The physical properties of the blends were found to be consistent with the nature of the individual components. For the tensile properties, the stiffness decreases with increasing the fraction of the copolymer, regardless of the molecular weight of the homopolymer fraction. For these blends with bimodal microstructures, it was confirmed that the degree of crystallinity governs the stiffness of the polymer. However, the energy dampening properties of the polymers benefit from the presence of the copolymer. A balance of stiffness and toughness can be obtained by altering the composition of the blends. For some blends, the presence of HMW homopolymer can dominate the tensile properties, showing little variation in the stiffness with increased addition of copolymer. It was also demonstrated that the testing conditions and thermal treatment of the polymer greatly influence the resulting elastic and energy dampening properties. Depending on the desired application, annealing these polymers (especially very low density copolymers) not only increases the crystallinity and stiffness, but also changes the frequency response of the dynamic mechanical properties.     

Determination of the molecular characteristics of commercial polyethylenes with different architectures and the relation with the melt flow index

The molecular weight distribution curves of several commercial polyethylene samples were evaluated by high‐temperature gel permeation chromatography with two detectors (a refractive‐index detector and a viscometer) to determine the molecular sizes and architectures (branching). The polymer samples included high‐ and low‐density polyethylenes with different molecular weight distributions (wide, medium, unimodal, and bimodal) from nine producers. The results were tested against the melt flow index and zero‐shear melt viscosity to find correlations. The data for high‐density polyethylene correlated well with the molecular weight, whereas the data for low‐density polyethylene did not correlate. However, when the weight‐average molecular weight was corrected by the branching parameter and a factor form, all the polyethylene samples fit a single equation. These results indicate that the melt flow index is dependent not only on the molecular weight but also on the molecular shape, including branching. The relation accounted for samples of different resin producers, molecular weights (65,000–638,000), and polydispersities (2.9–20). The use of the branching parameter for the correction of the molecular weight allowed the correlation of these parameters despite differences in the technologies, molecular weights, and molecular architectures. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1572–1578, 2007     

Examination of the effects of computationally determined network topology on an analytical constitutive model for bimodal elastomers

Bimodal elastomer networks, so called due to their nominal bimodal molecular weight distribution of starting oligomers, are of continued interest due to the enhanced strength and toughness seen in certain mixtures. Researchers have suggested that the enhanced properties stem from the particular micromechanics of the networks formed by these systems at these optimal compositions. This work extends an existing analytical constitutive model for bimodal elastomer networks by incorporating aspects of network topology, including network connectivity patterns and realistic chain length distributions, determined through computational simulations of the formation of the network structure. These factors are included as functions of bimodal composition and are shown to affect the predicted mechanical, optical, and orientation responses of the network. The extended model elucidates how the naturally occurring doubled connection topology creates a micro-mechanism that lowers overall chain orientation in the lower molecular weight component and recreates experimentally observed optical response phenomena. Specifically, the model predicts that the presence of the stiff, contractile, doubled connections forces the rest of the network to conform more to the macroscopic stretch ratio while reducing the measured average orientation of the lower molecular weight component in the system; the effect diminishes as the composition-dependent population of doubled connections in the system decreases.     

Modeling of branching density and branching distribution in low-density polyethylene polymerization

Low-density polyethylene (ldPE) is a general purpose polymer with various applications. By this reason, many publications can be found on the ldPE polymerization modeling. However, scission reaction and branching distribution are only recently considered in the modeling studies due to difficulties in measurement and computation of scission effect and branchings of polymer. Our previous papers [Kim, D.M., et al., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of CSTR. Chemical Engineering Science 59, 699-718; Kim, D.M., Iedema, P.D., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of a tubular reactor. Chemical Engineering Science, submitted for publication] are concerned with the scission reaction during ldPE polymerization and its effect on molecular weight distribution (MWD) of ldPE for various reactor types. Here we consider branching distributions as a function of chain length for CSTR and tubular reactor processes. To simultaneously deal with chain length and branching distributions, the concept of pseudo-distributions is used, meaning that branching distributions are described by their main moments. The computation results are compared with properties of ldPE samples from a CSTR and a tubular reactor. Number and weight average branchings and branching density increase as chain length increases until the longest chain length. The concentrations of long chain branching (LCB) are close to those of first branching moment in both CSTR and tubular reactor systems. The branching dispersity, a measure for the width of the branching distribution at a certain chain length, has the highest value at shorter chain length and then monotonously decreases approaching to 1.0 as chain length increases. Excellent agreements in branching dispersities between calculation with branching moments and prediction with assumption of binomial distribution for a tubular reactor and CSTR processes show that the branching distribution follows a binomial distribution for both processes.     

Poly[bis(m-chlorophenoxy)phosphazene]. Macromolecular characterization and degradation studies

The macromolecular structures of five poly[bis(m-chlorophenoxy)phosphazene] samples are critically analyzed. There are significant variations in the solubility behavior and physical properties of the polymers. Property differences are attributed mainly to the incomplete nucleophilic substitution of the dichlorophosphazene polymer precursor. All the polymers are found to have high molecular weights and broad, bimodal molecular weight distributions. However, differences in branching are noted and the presence of thermally labile “weak links” on the polymer chain backbones is suspected. At 165°C in static air, the polyphosphazene degrades by a random degradation mechanism and for long exposure times is considerably more stable than polystyrene.     

Study of molecular weight and chain branching architectures of natural rubber

The origin of the unique bimodal molecular weight distribution (MWD) of natural rubber (NR) has been controversial up to now. Studying the connection between particle size and molecular weight (MW) might be a key approach to revealing the mystery of NR architectures. In this study, through constructing NR models as objectives and employing gel‐permeation chromatography coupled to a viscosity detector as well as a multiangle laser light scattering detector (GPC‐DP‐MALLS), we have acquired branching parameters for NR from solid experiments and data fitting. It is found that small rubber particles (SRPs) and large rubber particles (LRPs) jointly construct the unique bimodal MWD of NR. SRPs with low branching numbers (Bn) and branching frequency (λ) are believed to be composed of almost linear rubber molecules having no chain‐end groups to be branched. In contrast, LRPs transform their MWD curve into a clear bimodal peak after transesterification and possess high Bn. Meanwhile, the formation of branch points in LRP by hydrogen bonding and ionic linkages has been clearly confirmed. Thus, a clear and exact structure of NR has been revealed for the first time. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci, 2016.     

Structure and Physical Properties of Polyethylenes obtained from Dual Catalysis Process

Polyethylenes with bimodal molecular weight distribution were synthesized by using dual catalyst systems. It is found that the molecular weight and its distribution is highly influenced by the molar ratio between the two catalytic centres, and that a synergistic effect exists, being the molecular weight of the products higher than that corresponding to the materials obtained from the isolated catalysts. What it is interesting is that the molecular weight distribution shape, the microstructure and the final polymer properties could be regulated by the selection of this ratio. Results from this study show that materials with a broad range of microstructures, crystallinities and mechanical properties are obtained. The correlations found show that although a double population of macromolecular species exists, both are able to co-crystallize to form a unique crystalline structure from dilute solution. Rheological testing point towards a pronounced shear thinning behaviour, very high relaxation times, and thermorheological complexity, suggesting the incorporation of long chain branching during the polymerisation process. In addition, simple additive models applied to the linear viscoelastic properties of the polymers, by using the rheological response of the pure components, are unable to explain the experimental results obtained, suggesting a tandem effect rather than a dual action between the two active centres.     

Rheological properties of molten polymers. I. Homopolymer systems

Experimental work has been carried out to investigate the influence of molecular weight distribution and long chain branching on both viscous and elastic properties of molten polymers, using a capillary rheometer, as described in a recent paper by Han. The materials used for the study are three high-density polyethylene samples of widely different molecular weight distributions and a low-density polyethylene containing much long-chain branching. For the analysis of the experimental data, and to obtain the information on the melt elasticity, the concept of the exit pressure recently advanced by Han is used. The study shows that the sample containing long-chain branching is much more elastic than the samples containing little or no long-chain branching, and that the broader the molecular weight distribution of the material, the more elastic the material is. These findings are in conformity with those reported in the literature. Also studied were blends of two high-density polyethylenes having widely different molecular weight distributions. The results of the blends systems show a maximum in melt viscosity as well as in elasticity for a certain blending ratio. The results of the present study may be of considerable interest to those who are concerned with modifying the structure of polymer and also with determining optimum processing conditions.     

Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low‐density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular‐weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short‐chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short‐chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HDPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short‐chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2542–2549, 2004     

Modality of molecular weight distributions

Even restricting attention to weight distributions, it is ambiguous to merely say that a polymer is “not bimodal.” A simple example is shown wherein the weight distribution of log (molecular weight) is bimodal, but the weight distribution of molecular weight is not bimodal.     

Prediction of the molecular and polymer solution properties of LDPE in a high-pressure tubular reactor using a novel Monte Carlo approach

In the present work, a novel kinetic/topology Monte Carlo algorithm is developed for the prediction of molecular, topological and solution properties of highly branched low-density polyethylene (LDPE), produced in a high-pressure multi-zonal tubular reactor. It is shown that the combined kinetic/topology MC algorithm can provide comprehensive information regarding the distributed molecular and topological properties of LDPE (i.e., molecular weight distribution, short- and long-chain branching distributions, joint molecular weight-long chain branching distribution, branching order distribution, seniority/priority distributions, etc.) The molecular/topological results obtained from the MC algorithm are then introduced into a random-walk molecular simulator to calculate the solution properties of LDPE (i.e., the mean radius of gyration, R_g, and the branching factor, g) in terms of the chain length of the branched polyethylene. The validity of the commonly applied approximation regarding the random scission of highly branched polymer chains is assessed by a direct comparison of the average molecular properties of LDPE (i.e., number and weight average molecular weights), calculated by the combined kinetic/topology MC algorithm, with the respective predictions obtained by the commonly applied method of moments (MOM). Through this comparison it is demonstrated that the ambiguous implementation of the random scission reaction in the MOM formulation can result in erroneous predictions of the weight average molecular weight and MWD of LDPE. Finally, the effects of two key process parameters, namely, the polymerization temperature profile and the solvent concentration, on the molecular, topological and polymer solution properties of LDPE produced in a multi-zonal tubular reactor are investigated.     

Optimierende reaktionsführung lebender polymerer. Darstellung von produkten mit bimodalen molekulargewichtsverteilungsfunktionen im rohrreaktor

Polymers with bimodal molecular weight distributions are remarkable for advantageous cold flow properties and good manufacturing. Using polyreaction of isoprene with n-butyllithium in n-heptane as an example for polyreactions of living polymers, for a tubular flow reactor with double initiator infusion a procedure is described which permits to precalculate optimal values of control variables used for the production of polymers which have required bimodal molecular weight distributions. The analysis of thus produced polymers by ultracentrifugal measurements shows an excellent agreement of theoretical and practical molecular weight distributions.     

通过物性分析对电缆绝缘料LDPE 2210H进行质量优化

运用差示扫描量热仪(DSC)、凝胶渗透色谱仪(GPC)及傅里叶红外光谱仪(FTIR)对低密度聚乙烯(LDPE)树脂的分子链结构、分子量及其分布和熔融结晶行为进行了对比分析。根据测定结果,调整树脂生产工艺参数,使LDPE 2210H电缆料的分子量稳定在8.9×104~10.3×104,分子量分布控制在4.7~5.6,短链支化度为16.1%~18.9%;其交联性能得到了改善,耐热性得到了提高。通过微观结构分析和质量跟踪监控,对LDPE 2210H树脂的质量优化起到一定的指导作用。