Effect Of Long Chain Branching Research Articles

Molecular dynamics simulation for polyethylene crystallization: Effect of long chain branches

The influence of long branches on crystallization behavior has been studied by means of molecular dynamics simulations. Using two systems: polyethylene (PE) with long branches (LCB-PE) and PE without long branches (linear-PE) with the same molecular weight, we have examined the crystallization behavior of the two systems by molecular dynamics simulation. This paper explains the influence of long branches on the isothermal crystallization process and the non-isothermal crystallization process with similar initial interchain contact fraction (ICF) in terms of final ICF, crystal regions, crystallinity, concentration of tie chains and energy. It is found that the crystallization process is classified as two stages: the nucleation stage and the crystal growth stage. The existence of long branches is favorable for the first stage while unfavorable for the second stage. Knots that act as crystalline defects are excluded from the lamella, resulting in decreasing in regularity and crystallinity of molecular chains. From the perspective of potential energy and non-bond energy, LCB-PE has lower energy than linear-PE in the nucleation stage while the energy of linear-PE is lower than that of LCB-PE in the second stage. In short, the long branched chains inhibit the crystallization process.

Rheology of thermoplastic vulcanizates (TPVs)

Thermoplastic vulcanizates (TPVs) are a special class of polymer blends comprised of a thermoplastic polypropylene (PP) matrix and a dynamically vulcanized ethylene propylene diene monomer (EPDM) rubber. Due to the presence of crosslinked rubber particles, TPVs exhibit complex rheological fingerprints similar to those of soft elastic solids. In this study, we attempt to draw a correlation between the rheology of several TPVs to the base components, namely, PP and EPDM. The effects of long chain branching of the primary constituents (prior to crosslinking), the level of curing, and the amount of crosslinked rubber on their rheology are studied in detail and found to affect the rheology to various degrees. All TPVs studied exhibited a universal relaxation modulus of the form ( G ( t ) − G y ′ ) ∝ t − p, where G y ′ is the equilibrium modulus indicating the elastomeric nature of TPVs (increases with the rubber crosslink density) and the absence of terminal relaxation with a proportionality constant dependent on the constituent polymer combination, the level of curing, and rubber concentration. At high deformations, these complex materials yield in both shear and extensional flows, and relevant stresses are determined and compared. While yield stress in shear can be easily determined, for the first time the yield stress in extension is observed and reported for polymer melts.

Study of Rheology and Plug Assist Thermoforming of Linear and Branched PP Homopolymer and Impact Copolymer

Abstract Polypropylene (PP) is one of the fastest growing thermoplastic polymers in the world, second only to polyethylene. This is primarily due to its excellent balance of physical and chemical properties at a lower cost. PP however possesses low melt strength on account of its linear structure and hence is not easily amenable to processing techniques that involve free surface stretching deformations like thermoforming, blow molding and extrusion film casting. One way to enhance the melt strength of PP is to incorporate long chain branches in its molecular architecture. The present study focuses on the impact of rheology of linear and branched PP on their thermoforming characteristics. Two grades each of linear and long chain branched (LCB) PP homopolymer and impact copolymer (ICP) were used. It was observed that the LCB-PP homopolymer and LCB-ICP showed higher flow activation energy, reduced value of loss tangent and nearly equal frequency dependence of storage and loss moduli in shear rheology. Also, a strong strain hardening behavior was displayed in extensional rheology by the LCB grades. Plug assist thermoforming experiments were carried out to assess the effect of long chain branching on surface strain and thickness distribution for axisymmetric cups of two draw ratios. Biaxial surface strain maps of the formed cups were quantified using Grid Strain Analysis (GSA). Thermoformed cups made from LCB-PP homopolymer and LCB-impact copolymer showed lower surface strain and overall higher thickness as compared to cups made from their linear counterparts, which is in accordance with what might be expected from their rheology.

Effect of long chain branching on the rheological behavior, crystallization and mechanical properties of polypropylene random copolymer

Long chain branched polypropylene random copolymers (LCB-PPRs) were prepared via reactive extrusion with the addition of dicumyl peroxide (DCP) and various amounts of 1,6-hexanediol diacrylate (HDDA) into PPR. Fourier transform infrared spectrometer (FTIR) was applied to confirm the existence of branching and investigate the grafting degree for the modified PPRs. Melt flow index (MFI) and oscillatory shear rheological properties including complex viscosity, storage modulus, loss tangent and the Cole-Cole plots were studied to differentiate the LCB-PPRs from linear PPR. Differential scanning calorimetry (DSC) and polarized light microscopy (PLM) were used to study the melting and crystallization behavior and the spherulite morphology, respectively. Qualitative and quantitative analyses of rheological curves demonstrated the existence of LCB. The effect of the LCB on crystalline morphology, crystallization behavior and molecular mobility, and, thereby, the mechanical properties were studied and analyzed. Due to the entanglements between molecular chains and the nucleating effect of LCB, LCB-PPRs showed higher crystallization temperature and crystallinity, higher crystallization rate, more uniformly dispersed and much smaller crystallite compared with virgin PPR, thus giving rise to significantly improve impact strength. Moreover, the LCB-PPRs exhibited the improved yield strength. The mobility of the molecular chain segments, as demonstrated by dynamic mechanical analysis (DMA), was improved for the modified PPRs, which also contributed to the improvement of their mechanical properties.

Shear-induced enhancements of crystallization kinetics and morphological transformation for long chain branched polylactides with different branching degrees.

The effects of long chain branching (LCB) degree on the shear-induced isothermal crystallization kinetics of a series of LCB polylactides (LCB PLAs) have been investigated by using rotational rheometer, polarized optical microscopy (POM) and scanning electron microscopy (SEM). Dynamic viscoelastic properties obtained by small-amplitude oscillatory shear (SAOS) tests indicate that LCB PLAs show more broadened relaxation time spectra with increasing LCB degree. Upon a pre-shear at the shear rate of 1 s−1 LCB PLAs show much faster crystallization kinetics than linear PLA and the crystallization kinetics is enhanced with increasing LCB degree. By modeling the system as a suspension the quantitative evaluation of nucleation density can be derived from rheological experiments. The nucleation density is greatly enhanced with increasing LCB degree and a saturation in shear time is observed. Crystalline morphologies for LCB PLAs observed by POM and SEM demonstrate the enhancement of nucleation density with increasing LCB degree and a transformation from spherulitic to orientated crystalline morphologies. The observation can be ascribed to longer relaxation time of the longest macromolecular chains and broadened, complex relaxation behaviors due to the introduction of LCB into PLA, which is essential in stabilizing the orientated crystal nuclei after pre-shear.

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%.

A new strategy for preparation of long-chain branched polypropylene via reactive extrusion with supercritical CO2 designed for an improved foaming approach

A novel strategy was designed for the preparation of a long-chain branched polypropylene (PP) with improved foamability via reactive extrusion in the presence of supercritical CO2 (scCO2). Benzoyl peroxide was used as a radical initiator and trimethylolpropane triacrylate (TMPTA) was applied as a polyfunctional reactive monomer during extrusion. Fourier transform infrared spectroscopy and high temperature GPC confirmed that TMPTA was grafted onto PP chains, and the presence of scCO2 promoted the grafting and branching reactions, and hindered polymer degradation. A possible mechanism was proposed to explain the effect of scCO2 on the branching reactions. In addition, rheological behavior of pure PP and modified PP samples was studied to investigate the effect of long chain branching of PP on the melt viscosity and strength, and foaming behavior was studied to confirm the subsequent effect on its foamability. It was found that the long chain branching increased the melt viscosity and strength of modified PP samples, which favored the foamability, and that the foaming windows were expanded in the presence of scCO2. Thus, it provided an advanced foaming approach via preparation of long-chain branched PP through reactive extrusion with scCO2 both working as the reactive medium and the foaming agent.

The effects of long chain branching of polypropylene and chain extension of poly(ethylene terephthalate) on the thermal behavior, rheology and morphology of their blends

Polypropylene is long chain branched, PET is chain extended/branched and their blends are prepared and studied.

Effect of long chain branching on the properties of polyethylene synthesized via metallocene catalysis

In this research, the effect of long chain branching (LCB) and polymerization conditions on thermal, mechanical, and rheological properties of polyethylene synthesized via a metallocene polymerization was studied. The LCB was varied in the range of 0.64–1.14 per 104 atoms of C. 13C NMR spectra showed that the distributions of both short as well as long chain branches in the polymer backbone chain are influenced by polymerization conditions. The increase in ethylene pressure leads to rise in polymer yield, catalyst activity, molecular weight, and narrowing of molecular weight distribution. In contrast, the increase of polymerization duration results in broadening of MWD and a decrease in catalyst activity. In addition, the influence of frequency and LCB on dynamic shear and extensional melt rheology has been reported. The polymer crystallization was discussed in light of Avrami model and modified Hoffman-Lauritzen theory. LCB promoted the transport of chain segments but retarded the nucleation in polyethylene crystallization. The tensile strength decreased with the increase in LCB content. The different macroscopic properties were correlated to LCB content.

Studies on crystallization kinetics of bimodal long chain branched polylactides

To investigate the effects of long chain branching (LCB) on the crystallization kinetics of polylactides (PLA), a series of long chain branched PLA samples have been prepared, which have been proven to be bimodal of linear PLA and long chain branched PLA with different branching degrees. The crystallization kinetics of these LCB PLA samples have been investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The POM results show that the spherulitic growth rates of the LCB PLA samples are lower than that of the linear PLA precursor and the PLA spherulitic growth rate decreases with increasing branching degree at each isothermal crystallization temperature. In contrast, the nucleation density increases with increasing branching degree. The crystallization kinetics from the POM observation are analyzed using the Hoffman–Lauritzen theory. In the studied temperature range, both linear PLA and LCB PLA samples crystallize according to the Regime II mechanism. The nucleation constant (Kg) and fold surface free energy (σe) decrease with increasing branching degree, suggesting that the LCB PLA samples have lower free energy barriers for nucleation than the linear PLA precursor. Analysis of the DSC data by the Avrami equation indicates that the crystallization of both the linear PLA and LCB PLA samples follows a three-dimensional crystal growth. The half crystallization time reduces with increasing branching degree and the overall crystallization rates for the LCB PLA samples are higher than that of linear PLA.

Shear-Induced Nucleation and Morphological Evolution for Bimodal Long Chain Branched Polylactide

The effects of long chain branching on the nucleation density enhancements and morphological evolution for polylactide (PLA) materials during shear-induced isothermal crystallization process were thoroughly investigated by using rotational rheometer and polarized optical microscopy (POM). Shear-induced nucleation density enhancements for the long chain branched PLA (LCB PLA) were studied on the basis of the determination of the critical shear rate, for which the stretch of the longest chains of the linear component is expected. The results of shear-induced isothermal crystallization kinetics show that the crystallization process under shear is greatly enhanced compared to the quiescent conditions and the crystallization kinetics is accelerated with the increases in shear rate and/or shear time. LCB PLA crystallizes much faster than linear PLA under the same shear condition. A saturation effect of shear time on crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations d...

Necking in extrusion film casting: The role of macromolecular architecture

Extrusion film casting (EFC) is used on an industrial scale to produce several thousand tons of polymer films and coatings. While significant research has been carried out on necking of films of viscoelastic melts in EFC, the influence of macromolecular chain architecture on the necking behavior is not yet fully understood. In the present research, we have explored experimentally and theoretically the effects of long chain branching and molecular weight distribution on the extent of necking during EFC. Polyethylenes of essentially linear architecture but having narrow and broad molecular weight distributions, and polyethylenes having long chain branching were used for experimental studies. The EFC process was analyzed using the one-dimensional flow model of Silagy et al. [Polym. Eng. Sci. 36(21), 2614–2625 (1996)] in which multimode molecular constitutive equations namely the “extended pom-pom” equation (for long chain branched polymer melts) and the “Rolie–Poly (Rouse linear entangled polymers)” equation (for linear polymer melts) were incorporated. We show that the model qualitatively captures the salient features of the experimental data thereby elucidating the role of chain architecture on the extent of necking.

Effects of long chain branching on melt fractures in capillary extrusion of metallocene‐catalyzed linear low‐density polyethylene melts

AbstractTo investigate the effect of long chain branching (LCB) on melt fractures of metallocene‐catalyzed linear low‐density polyethylene (mLLDPE), we prepared a series of sparsely long‐chain‐branched mLLDPEs with well‐defined degrees of LCB. Gross melt fractures were observed to decrease as the degree of LCB increases. This is in accordance with a prediction based on the observation that LCB enhances chain entanglement and consequently increases the melt strength of a polymer. However, sharkskin melt fracture (SMF) was observed to be more severe with the degree of LCB. There have been debates over the effect of LCB on SMF. According to a well‐known mechanism of SMF, SMF is expected to decrease with the degree of LCB. Therefore, the majority of research groups believe that SMF decreases with the degree of LCB. This study clearly shows that the SMF becomes more severe with an increase of the degree of LCB and suggests another possible mechanism for the SMF. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Quantifying Extrusion Modification of LDPE with a Microcompounder

Abstract Processibility of LDPE is known to be sensitive to processing history. Significant changes in processibility and product properties can occur through repeated extrusion without detectable changes in molecular weight and molecular weight distribution. Such changes of LDPE rheology are simulated with DSM micro-compounder and characterized with the small angle oscillatory tests and the SER uni-axial extensional flow with a K-BKZ analysis. Both the linear and the nonlinear rheology data suggest the major mechanism in the extrusion with the DSM micro-compounder is the reduction of the effects of long chain branching. The linear rheology analysis, along with GPC and NMR characterization confirms that the changes in the extrusion are not due to chemical crosslinking, though an increase in backbone molecular weight at extreme conditions is perceivable through the minor increase in molecular weight distribution, the increase in zero-shear viscosity and the decrease of melt index. The extensional rheology results, on the other hand, reveal the depression of the strain hardening with prolonged extrusion at 190 degree C or higher extrusion temperature. The stress growth data were evaluated with a K-BKZ constitutive with exponential damping function. The depression of strain hardening is well characterized with the strain hardening factor and the non-linear exponent beta as function of the extrusion conditions. The damping factor β is observed to be especially effective as the parameter to follow the depression of the strain hardening due to microcompounder extrusion. The microcompounder extrusion and the SER analysis with the K-BKZ analysis prove to be a new useful tool for simulating the shear modification of the LDPE.

Effect of long chain branching on nonisothermal crystallization behavior of polyethylenes synthesized with constrained geometry catalyst

AbstractNonisothermal crystallization behavior of linear and long chain branched (LCB) polyethylene (PE) samples having similar molecular weight but different long‐chain branching densities (LCBD) up to 0.44 C per 1000 carbons was investigated using differential scanning calorimetry (DSC) at various scanning rates. The LCB PE samples were prepared in our high‐temperature, high‐pressure continuous stirred‐tank reactor (CSTR) system using the constrained geometry catalyst. The existence of LCB was found to affect the PE crystallization behavior considerably. The enthalpy of crystallization and the ultimate degree of crystallinity decreased with the increase of LCBD. At the relatively low cooling rates, the small amount of LCB promoted nucleation but restrained chain movement and reduced the crystal growth rate. There was ∼ 17% of crystallinity generated from a secondary crystallization. The energy barrier became significant with the LCB structure, resulting in chain diffusion limitations and lower LCB PEs overall crystallization rates than their linear counterpart. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers

Interfacial tension of linear and branched PP in supercritical carbon dioxide

In this study, sessile drops are imaged in a high-pressure and high-temperature view chamber to determine the density and interfacial tension of linear polypropylene (LPP) and branched polypropylene (BPP) melts in supercritical carbon dioxide (CO 2). The pressure–volume–temperature (PVT) data of polyprophylene (PP)–CO 2 is investigated by monitoring the swelling changes of the polymer melt in supercritical CO 2. The density difference between the polymer/CO 2 mixture and the CO 2 is determined by combining the swelling results with the CO 2 solubility information in the polymer melt. Both the Sanchez–Lacombe (SL) and the Simha–Somcynsky (SS) equations-of-state (EOS) are applied to predict the density of the PP–CO 2 mixture, which is then compared to the density data obtained experimentally. The dependence of interfacial tension on the temperature and pressure of PP in supercritical CO 2 is investigated at temperatures from 180 °C to 220 °C and pressures up to 31 MPa. Effects of long-chain branching on the density and interfacial tension of PP–CO 2 mixtures are discussed.

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.

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

Thermorheological properties of LLDPE/LDPE blends

The thermorheological behavior of a number of linear low-density polyethylene and low-density polyethylene (LLDPE/LDPE) blends was studied with emphasis on the effects of long chain branching. A Ziegler–Natta, LLDPE (LL3001.32) was blended with four LDPEs having distinctly different molecular weights. The weight fractions of the LDPEs used in the blends were 1, 5, 10, 20, 50, and 75%. Differential scanning calorimetry (DSC) analysis has shown that all blends exhibited more than one crystal type. At high LDPE weight fractions, 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 co-crystallization of the two components. The linear viscoelastic characterization was performed, and mastercurves at 150 °C were constructed for all blends to check miscibility. In addition, Van Gurp Palmen, zero-shear viscosity vs composition, Cole–Cole, and the weighted relaxation spectra plots were constructed to check the thermorheological behavior of all blends. In general, good agreement is found among these various methods. The elongational behavior of the blends was studied using a uniaxial extensional rheometer, the SER universal testing platform from Xpansion Instruments. The blends exhibit strain-hardening behavior at high rates of deformation even at LDPE concentrations as low as 1%, which suggests the strong effect of branching added by the LDPE component.

Linear Rheological Properties of the Semifluorinated Copolymer Tetrafluoroethylene-Hexafluoropropylene-Vinylidenfluoride (THV) with Controlled Amounts of Long-Chain Branching

Various semifluorinated polymers of narrow molar mass distributions and definitely changed molecular structure are employed as model substances to investigate the impact of the polymer architecture on key rheological properties in the linear range of shear flow. These model samples originate from free radical terpolymerizations of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and vinylidenfluoride (VDF), referred to as THV. They were tailored to cover a broad range of mass-average molar masses of a straight linear and of a long-chain branched topography. By the controlled incorporation of long-chain branches (LCB) into the fluoropolymers, a complex thermorheological behavior is observed. The effect of long-chain branching also becomes evident by dynamic-mechanical measurements represented by a plot of the angle δ versus the complex modulus |G*(ω)|. Compared to the linear reference materials for which η0 ∼ Mw3.8 was found, the zero-shear rate viscosity η0 of some of the branched polymers is significantly higher than that of a linear product with equivalent Mw, for others it comes to lie below the curve for linear samples. The experimental η0 (Mw) data of the branched THV are in good agreement with an approach proposed by Janzen and Colby using the structural information from the molecular characterization. Moreover, it was found that plotting η0 as a function of the intrinsic viscosity [η] more sensitively discriminates between the various long-chain branched samples than η0 (Mw). Comparing the sensitivity of methods based on rheological properties in solution and in the molten state, it is demonstrated that the linear behavior of the melts can very favorably be used to get an insight into the molecular architecture of THV polymers.