Levels Of Long Chain Branching Research Articles

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

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

Rheological Behavior of Blends of Metallocene Catalyzed Long-Chain Branched Polyethylenes. Part I: Shear Rheological and Thermorheological Behavior.

Long-chain branched metallocene-catalyzed high-density polyethylenes (LCB-mHDPE) were solution blended to obtain blends with varying degrees of branching. A high molecular LCB-mHDPE was mixed with low molecular LCB-mHDPE at varying concentrations. The rheological behavior of those low molecular LCB-mHDPE is similar but their molar mass and molar mass distribution are significantly different. Those blends were characterized rheologically to study the effects of concentration, molar mass distribution, and long-chain branching level of the low molecular LCB-mHDPE. Owing to the ultra-long relaxation times of the high molecular LCB-mHDPE, the blends exhibited a clearly more long-chain branched behavior than the base materials. The thermorheological complexity analysis showed an apparent increase in the activation energies Ea determined from G′, G″, and especially δ. Ea(δ), which for LCB-mHDPE is a peak function, turned out to produce even more pronounced peaks than observed for LCB-mPE with narrow molar mass distribution and also LCB-mPE with broader molar mass distribution. Thus, it is possible to estimate the molar mass distribution from the details of the thermorheological complexity.

Toughening mechanism of long chain branched polyamide 6

Polyamide 6 (PA6) materials with different degrees of long chain branching (LCB) were prepared via extrusion through varying the addition of chain extender. The LCB degree was quantitatively determined by using a rheological method based on macromolecular dynamic models. The topological structures of long-chain branching and crosslinked network were effectively distinguished through rheological percolation analysis so that modified PA6 with ultra-high LCB level was obtained. The XRD analysis indicated that LCB modification favored the generation of the γ-form, and the γ-form content of LCB-PA6 increased with increasing degree of LCB, which could provide a reasonable explanation for the high Izod impact strength of ultra-high LCB-PA6. Another reason for the great enhancement of impact toughness was the strong interfacial interaction between LCB molecules and virgin PA6 matrix. A large number of fibrillar ligaments and cavities during crack propagation were observed in the SEM analysis.

The role of microstructure on melt fracture of linear low density polyethylenes

The effects of molecular characteristics on the rheology and melt fracture of several linear and branched low-density polyethylene (LLDPE and LDPE) resins in capillary extrusion were studied as functions of molecular weight, polydispersity and the level of long chain branching (LCB). The level of LCB in the resins was found qualitatively by using several rheological methods which in general agree. These are based on the zero-shear viscosity versus molecular weight relationship, the energy of activation, the linear viscoelastic properties and the characteristic shapes of the flow curves. A previously proposed criterion (critical shear stress versus plateau modulus) for the onset of sharkskin melt fracture (Allal et al., J. Non-Newtonian Fluid Mech., 134 (2006) 127–135) was tested and found to give reasonable predictions for the sharkskin instability of the polyethylenes considered in this work.

Microstructure characterization of polyethylene using thermo-rheological methods

In the present work, we use the viscoelastic moduli of a large number of industrially available polyethylenes in order to evaluate/test some of the previously proposed correlations between levels of long chain branching and polydispersity with the rheological properties. These correlations together with some new ones can be used to correct for the effects of polydispersity or long chain branching in order to assess the effect of these two molecular features on the rheological properties independently. The effects of short and long chain branching are studied providing a methodology to detect rheologically levels of short and long chain branching.

Synergistic toughening effect of β-nucleating agent and long chain branching on polypropylene random copolymer

β-nucleated long chain branched (LCB) polypropylene random copolymers (PPRs) were prepared via reactive extrusion by introducing β nucleating agent (NA), dicumyl peroxide (DCP) and various contents of 1, 6-Hexanediol diacrylate (HDDA) into PPR. Results of Fourier infrared spectroscopy (FTIR) and the rheological properties demonstrate the existence of LCB polypropylene. Mechanical properties including impact strength, tensile strength and elongation at break were studied. The crystal structure, morphology and crystallization behavior were investigated via wide-angle X-ray diffraction (WAXD), polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Non-isothermal crystallization kinetics using the Jeziorny method was also studied. The results show an increased LCB degree with increasing HDDA amount. For the sample with a moderate LCB level, synergistic toughening effect of NA and LCB is achieved without deterioration of yield strength or elongation at break, partially because of its higher β-phase content and much smaller spherulite size. It exhibits the minimum values of Tc and Zc1, and the maximum values of t1/2 and n1 in the primary stage of crystallization, regardless of the cooling rate, indicating a slower crystallization rate and more complicated nucleation and crystal growth model.

The influence of long chain branches of LLDPE on processability and physical properties

Two polyethylene-based on single-site metallocene catalyst (mLLDPE) were selected to characterize the effect of long chain branching (LCB) on blown film processability, optical and mechanical properties. 13C NMR and parallel plate rheology were used to identify LCB presence on LLDPEs. Blown films were produced from 100% LLDPEs using three different machine direction (MD) stretch ratios. When the same processing conditions for the two LLDPEs grades were used, better processability was observed for LLDPE with LCB. In relation to mechanical and physical properties, Elmendorf tear and optical properties were highly influenced by the presence of LCB. Tear resistance is affected by film orientation and is inversely proportional to the level of LCB in the polymer. It was observed a reduction of 50% in the MD tear strength when comparing with the polymer without LCB. However, haze decreases significantly with the presence of LCB, about 40%.

Some reflections on the current state of Cr-based polymerization catalysts

Abstract

Effects of long chain branches on the crystallization and foaming behaviors of polypropylene-g-poly(ethylene-co-1-butene) graft copolymers with well-defined molecular structures

A series of polypropylene-g-poly(ethylene-co-1-butene) graft copolymers (PP-g-EBR) with well-defined long chain branched (LCB) molecular structures, basing on the same PP–BT precursor (PP–BT2), were used to study effects of EBR LCBs on the crystallization and foaming behaviors of PP-g-EBRs. The kinetics results of isothermal and nonisothermal crystallization verify the opposite effects of LCB structure on the crystallization process of PP backbones in PP-g-EBRs: on one hand, the indolent LCB structure can perform the function of heterogeneous nucleation to facilitate the crystallization; on the other hand, the mobility and reptation ability of PP backbones are restrained by the LCB structure, which hinders the crystallization process. Additionally, the fluctuation-assisted nucleation mechanism caused by microphase separation between the EBR rich phase and the PP rich phase may account, to some extent, for the heterogeneous nucleation effect. The PP–BT2 and PP-g-EBRs were foamed by a batch method under the same conditions, using supercritical CO2 as blowing agent. The resulting PP-g-EBR foams exhibited closed cell structure and increased cell density compared to the PP–BT2 foam, attributing to the enhanced melt strength. The cell density of PP-g-EBR foam increased first and decreased then with the LCB level increasing. The influence of LCB level on cell size was somewhat complex. Increasing LCB level, which promoted melt strength and strain hardening behavior of PP-g-EBRs, decreased the cell size and narrowed the cell size distribution. However, large cells were observed in PP-g-EBR foams with relatively high LCB level, which could be ascribed to the larger growing space introduced by the higher content of amorphous EBR LCBs. Moreover, the melting behaviors of PP–BT2 and PP-g-EBRs before and after foaming treatment were compared.

Synthesis and Structure–Property Relationships of Polypropylene-g-poly(ethylene-co-1-butene) Graft Copolymers with Well-Defined Long Chain Branched Molecular Structures

A series of polypropylene-g-poly(ethylene-co-1-butene) (PP-g-EBR) graft copolymers with well-defined long chain branched (LCB) molecular structures were synthesized via the combination of coordination polymerization and anionic polymerization. The structure–property relationships of PP-g-EBR were systematically investigated. The structural information was clarified by characterization of 1H NMR and SEC equipped with triple detectors. The DMA measurements demonstrated that the amorphous EBR branches were miscible with the PP backbones. The melt behavior of the PP-g-EBRs was studied with small-amplitude dynamic rheological measurements. Increasing branch length and branch density both led to increased zero-shear viscosity, more pronounced shear-thinning behavior, elevated value of storage modulus at low shear frequencies, more significant upshift deviation from linear behavior in the Han plot, and reduced loss angle. Comparatively, branch length contributed more to the improvement of rheological properties than branch density did. Interestingly, DSC results showed that the crystallization temperature of PP-g-EBR increased with the enhancement of branch length and branch density; however, when the LCB level exceeded a certain degree, further increasing the LCB level would reduce the crystallization temperature.

Long-chain branching in PE from Cr/aluminophosphate catalysts

Phillips Cr/silica catalysts have long been known to produce long-chain branching (LCB) in polyethylene (PE) through macromer incorporation. In fact, the manipulation of LCB levels through catalyst variables is a very useful way of controlling properties and molding behavior in commercial PE resins. Using alumina as the support, instead of silica, is one way of achieving very low levels of LCB. Adding small amounts of phosphate to the alumina base as a modifier causes a large rise in LCB levels. Phosphate also promotes the activity and melt index potential of the catalyst by contributing a large new low-MW population of PE chains. In this study LCB levels were measured and found to correlate nicely with the amount of phosphate added. The polymer from such a catalyst was then solvent-fractionated to determine the location of the LCB. It was found to be concentrated in this same low-MW polymer, to which the phosphate contributes, which suggests that the phosphate-associated Cr sites have a tendency to produce LCB. However, the incorporation of 1-hexene resulted in a flat short-chain branch (SCB) profile, indicating that there was no parallel behavior among the different sites between SCB and LCB incorporation. These results indicate a slightly different mechanistic pathway between the two monomers (macromer and 1-hexene). Furthermore the Cr sites must have incorporated macromers that were produced only at the same site, and not (as is usually portrayed) randomly from a general pool of vinyls contributed by all sites. To explain this finding, alternatives to the widely accepted random-incorporation mechanism are proposed, including a “tethered” vinyl end-group, and intra-, rather than the traditional inter-, molecular insertion of macromer.

Thermal oxidation‐induced long chain branching and its effect on phase separation kinetics of a polyethylene blend

AbstractThermal oxidation‐induced long chain branching (LCB) during the molding processes for polyolefin copolymer poly(ethylene‐co‐butene) (PEB) and its blend with another polyolefin copolymer poly(ethylene‐co‐hexene) (PEH/PEB 50/50 blend, denoted as H50) was investigated mainly by rheological measurements. LCB with different levels could be introduced on PEB backbones by changing the molding temperature and/or molding time, which could be sensitively characterized by changes of rheological parameters, that is, storage modulus G′ and complex viscosity η*. Thermal oxidation‐induced LCB of PEB in H50 samples could largely influence the phase separation kinetics. Rheological measurements and phase‐contrast optical microscope observations coherently indicated that thermal oxidation‐induced LCB of PEB more or less retarded the development of phase separation and once it reached a certain level, the reduced chain diffusion even arrested phase separation. The decrease of mass‐averaged molecular mass in H50 with high LCB level was ascribed to the reduced hydrodynamic volume. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Rheological control in foaming polymeric materials: I. Amorphous polymers

The influence of rheological properties, especially melt strength, on foam structures, such as cell size, cell density and cell size distribution, of amorphous polymer was investigated. The rheology of polystyrene (PS) was controlled by molecular modification with free radical reaction, and PS with long chain branching (LCB) level ranging from 0.15 to 1.6 branching point per 10 4 carbon atom was gotten. The shear and elongational rheology were found to be dependent on the LCB structure, and the strain hardening behavior of modified samples in transient elongational viscosity confirmed the existence of long branched chain. The effects of chain structure and foaming conditions such as temperature and pressure were studied by the analysis on the foam structures obtained by supercritical CO 2. The experimental results revealed that increasing LCB level would decrease cell size, make cell size distribution narrower and slightly increase cell density. The effects of chain topology on the foam structures were also investigated by numerical simulation, where Pom–Pom model was used to describe the effect of backbone length and arm length. The dependence of cell size on the arm length was consistently observed in experiments and simulation. It suggested that the arm length had greater influence on the cell radius than the backbone length. Therefore, the relationship among foam structures, rheological properties and molecular structures can be established from both experiments and simulation, which can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.

Rheological properties of long chain branched polyethylene melts at high shear rate

Capillary extrusion experiments involving a number of polyethylenes with emphasis on assessing the effect of long chain branching (LCB) are performed. None of the metallocene catalyzed linear low density polyethylenes (mLLDPE) produced by Dow Chemicals, which are believed to have some level of LCB, show temperature dependence on the viscosity at the gross melt fracture regime. Furthermore, these materials do not show spurt or stick-slip flow, in contrast with most linear polyethylenes. LDPE and blends of LDPE with LLDPE having LCB also show the absence of stick-slip flow, but show temperature dependence on the viscosity. From these observations, we conclude that the stick-slip flow is very sensitive to the existence of LCB.

Rheology and thermal properties of polypropylene modified by reactive extrusion with dicumyl peroxide and trimethylol propane triacrylate

AbstractTrimethylol propane triacrylate (TMPTA) and dicumyl peroxide (DCP) were used to modify polypropylene (PP) by reactive extrusion in a twin‐screw extruder. The effects of TMPTA concentration on oscillatory shear rheology, melt elongational rheology, and thermal properties were comparatively evaluated. Fourier transform infrared spectroscopy indicated that the grafting reaction took place and TMPTA had been grafted onto the PP backbone. Differential scanning calorimetric results showed that the crystallization temperatures of modified PPs were higher than those of the initial and degraded PPs. The rheological characteristics such as higher storage modulus (G′) at low frequency, increased degree of shear thinning, a plateau in tan δ–ω plot, and upturning at high viscosity in the Cole–Cole plots proved that the long‐chain branches have been added to the linear PP molecule. The melt elongational rheology showed that the modified PPs exhibit improved melt strength and increased elongational viscosity in the presence of TMPTA and DCP, which further confirmed the existence of long‐chain branching (LCB) in their backbone. According to the analytical results from oscillatory shear rheology and elongational rheology, it can be inferred that the LCB level in modified samples increases with an increase in TMPTA concentration. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 28:16–25, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20146

The influence of porosity on the Phillips Cr/silica catalyst 2. Polyethylene elasticity

AbstractThe Phillips Cr/silica catalyst produces low levels of long chain branching (LCB) in polyethylene, which have a powerful influence on industrial molding behavior. Although many catalyst and reactor variables determine the degree of LCB, perhaps the most significant of these is the morphology of the silica support. In this study many different types of silicas were converted into Cr/silica catalysts, which were tested in ethylene polymerization, and the resultant polymer elasticity was then determined. In some experiments, the surface area of the catalyst seemed to correlate quite well with polymer elasticity. In other tests, however, no connection with surface area was evident but the pore volume was quite influential. Together, all these studies suggest that it is the degree of structural reinforcement of the silica matrix, rather than any one physical measurement of porosity, that influences elasticity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 845–865, 2009

Control on the topological structure of polyolefin elastomer by reactive processing

The feasibility of preliminary tailoring of the long chain branched (LCB) polymer through complex flow field was evaluated in the torque rheometer, for the reaction of melt polyolefin elastomer (POE) with peroxides at elevated temperatures. With the compensation of temperature, the strength of complex shear flow could be the only factor affecting the reaction kinetics and mechanism. The results of sample characterization by the rheological and dilute polymer solution methods indicated that the degradation mainly made the length of LCB arm shorter and shorter as the rotational speed increases. Extremely, a certain amount of LCB degraded to be linear chains again due to the scission approaching the branching point at intense mixing condition. One new LCB index (DLCB) was defined from nonlinear oscillatory shear, and a nearly linear relationship between it and long chain branching index (LCBI) was found, which can be a map to quantify LCB level by Fourier Transform Rheology (FTR).

Rheological Features and Flow‐Induced Crystallization of Branched Poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] Copolyesters

AbstractA set of amorphous poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] (PECT) copolymers containing 25 and 30% of 1,4‐cyclohexane dimethylene (CHDM) units and small amounts of branching agent pentaerythritol (PER) is investigated. The level of long chain branching was estimated by analyzing the positive deviation from $\eta _0 \sim\overline M _{\rm w}^{3.54}$ law. Branching also produced melt elasticity enhancement which is desirable for certain processing methods. Capillary extrusion experiments at 180 °C generated flow‐induced crystallization in PECT containing 25% of CHDM. Crystallization increased with the amount of PER added, which was explained by the favorable effect of branching to increase elongational rate at the entrance of the capillary. Linear and branched PECTs containing 30% of CHDM did not crystallize.magnified image

Thermorheological properties of LLDPE/LDPE blends: Effects of production technology of LLDPE

AbstractThe thermorheological behavior of a number of LLDPE/LDPE blends was studied with emphasis on the effects of the production technology of the linear low‐density polyethylene (LLDPE) and the effects of long chain branching (LCB). Two Ziegler‐Natta LLDPE's (LL3001.32 and Dowlex2045G) and two metallocene LLDPEs (AffinityPL1840 and Exact 3128) were blended with a single low‐density polyethylene (LDPE), with all LLDPEs having distinctly different molecular weight. The weight fractions of the LDPEs used in the blends were 1, 5, 10, 20, 50, and 75%. DSC analysis has shown that the blends with metallocence LLDPEs are miscible in the crystal state, whereas for the Ziegler‐Natta, apart from the two distinct peaks of the individual components, a third peak appears which indicates the existence of a third phase that is created from the cocrystallization of components from the two blended polymers. The linear viscoelastic characterization was performed and mastercurves at 150 °C were constructed for all blends to check miscibility using the time temperature superposition principle. In addition, Van Gurp Palmen and zero‐shear viscosity versus composition were constructed to check the thermorheological behavior of all blends. In general, good agreement is found among these various methods. It was concluded that metallocene LLDPEs are more compatible with LDPE at all LDPE compositions when compared with their Ziegler‐Natta counterparts. Finally, the extensional properties of all blends were studied to examine the effects of different levels of LCB on their extensional rheological properties. It was concluded that extensional rheology is a sensitive tool capable of detecting subtle changes in the polyethylene macrostructure, that is, low levels of LCB. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1669–1683, 2008

The formation of γ-crystal in long-chain branched polypropylene under supercritical carbon dioxide

The crystallization behavior of long-chain branched (LCB) polypropylene (PP) in the supercritical carbon dioxide (scCO2) atmosphere was investigated to show the influences of LCB and CO2 on the formation of y-crystal. The crystallization experiments were performed in CO2 atmosphere with the pressure from 1.3 to 10.4 MPa and temperature between 90 and 130 °C. The effects of LCB level, CO2 pressure, and crystallization temperature on the content of y-crystal were investigated. The results showed that the influence of LCB on the formation of y-crystal was obvious when PP was crystallized in CO2. The content of y-crystal increased with LCB level and reached a maximum of 88.2%. It could be explained that, as LCB increased the chainfolding energy of PP molecular chain and hindered it from folding back into crystal lamella, which made the formation of y-crystal easier. However, CO2 was the key factor in the formation of y-crystal, and the influence of CO2 on y-crystal was much significant than that of LCB. It was believed that the increase of free volume after dissolving of CO2 in PP was helpful in the formation of y-crystal. It was found that the content of y-crystal increased almost linearly with CO2 pressure (CO2 content), and the contribution of CO2 to y-crystal increased with pressure, while that of LCB increased with temperature.