Long-chain Branching Research Articles

Foaming of reactively modified polypropylene: Effects of rheology and coagent type

A series of linear controlled rheology polypropylenes were produced through reaction with dicumyl peroxide in the melt state, to investigate the effect of molecular weight and viscosity on foaming. Foaming experiments by compression molding, using a chemical blowing agent (azodicarbonamide), and by batch foaming using nitrogen-blowing agent, revealed that lower viscosity promoted higher expansion ratios and larger cells, due to reduced resistance to cell growth. Linear polypropylene was further modified using trimethylolpropane trimethacrylate and triallyl trimesate coagents. Coagent modification resulted in pronounced changes in the shear thinning and elasticity of the modified polypropylenes. However, evidence of long-chain branching was only present in polypropylene modified by triallyl trimesate. Foams based on coagent-modified polypropylenes had higher expansion ratios than their degraded counterparts. This was ascribed in part to the lower viscosities, and to a nucleating effect, arising from the presence of a finely dispersed phase of coagent-rich nanoparticles. Strain hardening in polypropylenes modified by triallyl trimesate further resulted in a finer cell structure, due to suppressed coalescence.

Peroxide induced cross‐linking by reactive melt processing of two biopolyesters: Poly(3‐hydroxybutyrate) and poly(l‐lactic acid) to improve their melting processability

ABSTRACTPoly(3‐hydroxybutyrate) (PHB) and poly(l‐lactic acid) (PLLA) were individually cross‐linked with dicumyl peroxide (DCP) (0.25–1 wt %) by reactive melt processing. The cross‐linked structures of the polymer gel were investigated by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies. The size of the polymer crystal spherulites, glass transition temperature (Tg), melting transition temperature (Tm), and crystallinity were all decreased as a result of cross‐linking. Cross‐linking density (νe) was shown to increase with DCP concentration. Based on parallel plate rheological study (dynamic and steady shear), elastic and viscous modulus (G″ and G′), complex viscosity (η*) and steady shear viscosity (η) were all shown to increase with cross‐linking. Cross‐linked PHB and PLLA showed broader molar mass distribution and formation of long chain branching (LCB) as estimated by RheoMWD. Improvements in melt strength offer bioplastic processors improved material properties and processing options, such as foaming and thermoforming, for new applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41724.

Surface segregation driven by molecular architecture asymmetry in polymer blends.

The contributions of chain ends and branch points to surface segregation of long-branched chains in blends with linear chains have been studied using neutron reflectometry and surface-enhanced Raman spectroscopy for a series of novel, well-defined polystyrenes. A linear response theory accounting for the number and type of branch points and chain ends is consistent with surface excesses and composition profile decay lengths, and allows the first determination of branch point potentials. Surface excess is determined primarily by chain ends with branch points playing a secondary role.

The effect of annealing of linear and branched polyamide 46 on the phase composition, molecular mobility and water absorption as studied by DSC, 1H and 2H solid-state NMR

The effect of annealing on the phase composition, molecular mobility and water absorption in linear and branched polyamide 46, poly(tetramethylene adipamide) – PA46, was studied by DSC, 1H and 2H solid-state NMR. A series of samples with varying amount of long-chain branches was synthesized for this study. The branches as well as a higher molecular weight of branched PA46 cause a decrease in crystallization rate as it follows from a lower crystallization temperature. 1H NMR transverse magnetization relaxation (T2 relaxation) experiments show that, at temperatures well above Tg, the amount of low mobile chain segments (rigid fraction) in all injection-molded samples equals 60–63%. This value is close to the crystallinity of linear PA46 as measured by WAXD. Although chain branches hardly affect the amount of the rigid fraction in injection-molded samples, molecular mobility in the crystalline phase of branched PA46 is lower than that in linear PA46, suggesting a more perfect crystalline structure in branched PA46 due to slower crystallization. 12.5 h annealing at 260 °C causes a large increase in the amount of the rigid fraction in linear PA46, whereas WAXD crystallinity only slightly increases upon annealing. The increase in the rigid fraction is largely caused by immobilization of the amorphous phase due to chains rearrangements resulting in stronger hydrogen bonds between amide groups in the amorphous phase. The amount of the rigid fraction in annealed samples is smaller in samples with higher amount of branches, and molecular mobility in the rigid and soft fractions is larger. This suggests that chain branches hamper chain rearrangements both in the crystalline and the amorphous phases during annealing of PA46. This results in a less ordered structure in the crystalline and the amorphous phases in annealed branched PA46. The amount of absorbed water decreases upon increasing amount of branches in injection-molded samples and upon annealing. 2H NMR experiments for PA46 saturated with D2O show that the mobility of absorbed water is strongly hindered as compared to pure water. Below ambient temperatures, mobility of water in PA46 is characterized by a very broad distribution of molecular motions. The mobility of water molecules gradually decreases upon sample cooling from ambient temperature to −40 °C. The majority of water molecules is immobilized below −40 °C. The results of the present study suggest that water uptake by PA46 is mainly determined by the strength of hydrogen bonds between amide groups in the amorphous phase, which is largely affected by the crystallization rate and annealing at elevated temperatures.

Extrusion film casting of long chain branched polypropylene

Extrusion film casting (EFC) is an important melt processing operation which is extensively used to make polypropylene (PP) films. Linear PP shows significant amount of necking and draw resonance during EFC. One of the ways to reduce necking is to introduce long chain branches (LCB) on the polymer backbone. The long branches impart extensional strain hardening behavior thereby stabilizing the melt flow. In this work, we investigate the influence of long chain branching in polypropylene on the extent of necking in the EFC process. Laboratory scale EFC experiments were performed on homopolymer PP of linear and long chain branched architectures. Simulations of the EFC process were carried out using the one-dimensional flow model of Silagy et al., Polym. Eng. Sci., 36, 2614 (1996) into which we incorporate two different multi-mode molecular constitutive equations namely, the ‘eXtended Pom-Pom’ equation (XPP, for long chain branched PP) and the ‘Rolie-Poly’ equation (RP-S, for linear PP). Our experimental data confirm that presence of long chain branching in PP reduces the extent of necking and our numerical predictions show qualitative agreement with experimental data, thereby elucidating the role of chain architecture on the extent of necking. POLYM. ENG. SCI., 55:1977–1987, 2015. © 2014 Society of Plastics Engineers

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.

Size exclusion chromatography of photo-oxidated LDPE by triple detection and its relation to rheological behavior

Sheets of low-density polyethylene (LDPE) were subjected to photo-oxidation in the presence of air using a xenon lamp to irradiate the samples for times between 1 day and 6 weeks. The formation of long-chain branching (LCB) up to one week of degradation and the competition between chain scission and crosslinking at longer periods of radiation were investigated by rheological characterization, Fourier transform infrared spectroscopy, and the solvent extraction method (Rolón-Garrido and Wagner. Polym Degrad Stabil 2014, 99:136, Rolón-Garrido and Wagner. J Rheol 2014, 58:199). In the present contribution the same samples are studied by size exclusion chromatographic characterization using triple detection (concentration, light scattering and viscosity). The gel content is determined by filtration followed by the analysis of the soluble polymer fraction. The influence of photo-oxidation time on the molecular weight distribution (molar masses and polydispersity), the mean square radius of gyration and the intrinsic viscosity contraction factors is discussed. The results are correlated with the model parameters (β and fmax2) of the molecular stress function (MSF) theory, used to describe quantitatively the rheological data in uniaxial elongation. It is verified that LCB occurs as an aside process, which up to one week of degradation dominates over chain scission, before gelation plays a critical role. It is confirmed that the model parameter β correlates with the gel content, which reflects the competition between chain scission and crosslinking, while fmax2 is found to correlate with the experimentally determined contraction factors. By comparing the data of this study with those obtained earlier for polystyrene comb melts with well defined structure, the influence of the branching frequency (i.e. the number of branch points per 1000 carbon atoms) on fmax2 becomes evident.

Rheological and foaming behavior of linear and branched polylactides

In this work, a chain extender (CE) was added to polylactide (PLA) to improve its foamability. The steady and transient rheological properties of neat PLA and CE-treated PLA revealed that the introduction of the CE profoundly affected the melt viscosity and elasticity. The linear viscoelastic properties of CE-enriched PLA suggested that a long-chain branching (LCB) structure was formed from the reaction with the CE. LCB-PLA exhibited an increased viscosity, more shear sensitivity, and longer relaxation time in comparison with the linear PLA. The LCB structure was also found to affect the transient shear stress growth and elongational flow behavior. LCB-PLA exhibited a pronounced strain hardening, whereas no strain hardening was observed for the linear PLA. Batch foaming of the linear and LCB-PLAs was also examined at foaming temperatures of 130, 140, and 155 °C. The LCB structure significantly increased the integrity of the cells, cell density, and void fraction.

AF4/MALS/QELS/DRI characterization of regular star polymers and their "span analogs".

Asymmetric flow field-flow fractionation (AF4) coupled with multi-angle static and quasi-elastic light scattering and differential refractive index detectors, was employed for the separation and characterization of regular star-shaped polystyrenes and their linear and span analogs in tetrahydrofuran. Stars with different arm lengths were separated from each other by employing a binary slope cross-flow gradient. Cross-flow optimization enabled fast separation of polystyrenes in two- and three-component blends. Macromolecular parameters were obtained by using light-scattering and refractive index detection, and properties of polystyrenes with different molecular architectures were compared. To our knowledge, this is the first report on the separation of star polymers by AF4. Novel characterization approaches for stars are important from both applied and fundamental standpoints, as these macromolecules are valued for their tribological, drug delivery, catalytic and coating capabilities, and also serve as model compounds for the structured study of long-chain branching and its effects in polymers.

Long chain branched impact copolymer of polypropylene: Microstructure and rheology

A biphasic impact copolymer of polypropylene (ICP) was modified with peroxide by reactive extrusion process resulting in reduced melt flow index, improved melt strength, and higher die swell. The polymers were for the first time subjected to systematic rheological and microstructural characterization in an effort to understand their structure‐property relations. In shear rheological tests, the modified ICP displayed higher flow activation energy, reduced values of loss tangent and nearly equal frequency dependence of storage and loss modulli. The modified ICP also showed strain hardening behaviour in uniaxial extensional rheology and higher crystallization temperature in differential scanning calorimetry (DSC). All these are definitive indications of the presence of long chain branches (LCB). Fitting the rheological data of modified ICPs with the eXtended Pom Pom (XPP) model indicated the presence of LCB on the higher molecular weight fraction in the polymer, a result which was corroborated with multi‐detector high temperature gel permeation chromatography (HT‐GPC). More importantly, the matrix and rubber phases of the ICP were separately characterized for presence of long chain branching by rheology, DSC and HT‐GPC. The results indicate that while LCB existed in the matrix phase, microgels were present in both phases indicating that the reaction with peroxide occurred in both phases. POLYM. ENG. SCI., 55:1463–1474, 2015. © 2014 Society of Plastics Engineers

Modeling of Synthesis and Flow Properties of Propylene–Diene Copolymers

Copolymerization with nonconjugated dienes offers an attractive route for introducing long-chain branching in polypropylene. From a simplified set of rate equations for such copolymerization with a metallocene catalyst, we derive the probabilities of branch formation at different stages of the reaction in a semibatch reactor. Using these probabilities, we generate an ensemble of molecules via a Monte Carlo sampling. The knowledge of the branching topology and segment lengths allows us to compute the flow properties of the resins from computational rheology. We compare our model predictions with existing experimental data, namely the molar mass distribution and small amplitude oscillatory shear response, for a set of resins with varying diene content. The rheology data suggest that the entanglement time τe depends sensitively and in a well-defined fashion on the diene content.

Thermorheological behavior of peroxide-induced long chain branches linear low density polyethylene

Abstract In this article, the correlation between thermorheological behavior and molecular structure of linear low density polyethylene upon peroxide modification was explored. For this purpose, a commercial grade LLDPE (Exxon MobileTM LL4004EL) was reacted with different amounts of dicumyl peroxide (DCP) and their viscoelastic behavior were studied. Moreover, the samples were analyzed by size-exclusion chromatography coupled with a light scattering detector. Increasing the DCP content at roughly constant molar mass led to broadening of molecular weight distribution as well as increasing the number of long-chain branches. The latter consequently resulted in enhanced activation energy and delayed relaxation times of the LLDPE. The thermorheological behavior of peroxide modified samples was investigated and the results showed that the induced long chain branching changes the thermorheological behavior from simple to complex. The plotted results (activation energy versus phase angle) showed constant activation energy at low peroxide level (i.e., increasing the long chain branching) (up to 125 ppm) and a distinct variation from low to high phase angle with increasing the peroxide level. The theromorheological complexity threshold could be determined using these plots. The potential of thermorheological approach as an alternative powerful tool for analyzing LCB issue in peroxide modified LLDPE could be highlighted.

Shear and Elongational Flow Properties of Long-Chain Branched Poly(ethylene terephthalates) and Correlations to Their Molecular Structure

Reactive processing with low-molar-mass modifiers is a well-known method to create long-chain branched (LCB) structures in a poly(ethylene terephthalate) (PET) melt. However, less is known about the elongational flow properties of LCB-PET. Therefore, the aim of this contribution is (a) to generate LCB molecules and (b) to evaluate the influence of the branching level on the transient elongational behavior. For this purpose, a commercial, linear PET and different contents (0.1–0.3 wt %) of the tetrafunctional modifier pyromellitic dianhydride (PMDA) were reactively processed. All samples were analyzed by size exclusion chromatography coupled with a light scattering device and characterized by shear and elongational rheometry. It was found that the molar mass distribution of the modified materials exhibit a high molar mass shoulder, leading to an increase of the weight-average molar mass and a broadening of the molar mass distribution. Moreover, the Mark–Houwink plot of the modified materials displays deviations from the power law toward lower intrinsic viscosities, which indicate the existence of LCB molecules. The shear viscosity shows a pronounced shear thinning behavior and a remarkable increase at low frequencies compared to the linear PET. Considering the transient elongational viscosity, a distinguished strain hardening is observed, which increases with increasing PMDA content and with increasing strain rate. From the results of the rheological and molecular characterization and by considering the chemical reaction mechanisms, it can be concluded that the PET modified with high PMDA contents has a treelike branch-on-branch architecture, which is well-known from low-density polyethylene melts.

Synthesis and Thermomechanical and Rheological Properties of Biodegradable Long-Chain Branched Poly(butylene succinate-co-butylene terephthalate) Copolyesters

Biodegradable long-chain branched (LCB) poly(butylene succinate-co-butylene terephthalate) (PBST) copolyesters were prepared via a two-step direct esterification and melt polycondensation process, using a small amount (0–2 mol %) of diglycidyl 1,2,3,6-tetrahydrophthalate (DGT) as an in situ branching agent (BA). The chemical structures of LCB PBSTs were characterized and the thermal, mechanical, and rheological properties were investigated. With increasing DGT loading, PBSTs with higher branching degree, broader molecular weight distribution, and lower melt flow index were synthesized in shorter polycondensation time. The branching of PBSTs results in a slight decrease in crystallizability, melting, and Vicat softening temperatures, but leads to an obvious decrease in elongation at break. On the other hand, the existence of LCB greatly improved the rheological properties of PBSTs. PBST with higher branching degree possesses higher storage and loss modulus, higher zero-shear viscosity, longer relaxation ti...

The effect of depth and duration of UV radiation on polypropylene modification via photoinitiation

ABSTRACTLinear polypropylene (PP) was modified using UV radiation in the presence of 0.5 wt % of benzophenone photoinitiator to introduce long chain branching (LCB) to the PP backbone. Irradiation was carried out in the solid state and the temperature level was kept below 60°C. The effects of radiation duration and sample thickness on the extent of these branching modification reactions were investigated. Viscoelastic properties, molecular weight, molecular weight distribution, and gel content were determined and compared for runs having different sample thicknesses, irradiated for different times. Comparisons were also conducted with the parent PP and the PP mixed with photoinitiator. It was found that LCB decreased by increasing the thickness of the samples. Conversely, an increase in radiation duration resulted in enhanced LCB but also led to larger gel content in the samples. Based on all these measurements and observations, a mechanism was suggested to explain formation of long chain branches (LCBs) in PP in the solid state via photoinitiation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41021.

Branching degree and rheological response correlation in peroxide-modified linear low-density polyethylene

Correlations between rheological behavior and degree of long chain branching (LCB) of linear low-density polyethylene (LLDPE) upon a peroxide (dicumyl peroxide [DCP]) modification process under various conditions are discussed in this paper. The gel content analysis revealed negligible insoluble crosslinked fraction implying that incorporation of DCP to LLDPE predominately leads to branching rather than crosslinking. The slight changes in average molecular weight and molecular weight distribution induced by peroxide modification under various conditions revealed that formation of low-molecular-weight fractions due to chain scission is also negligible. The changes in terminal, trans, and pendant double bonds concentration of the modified samples with different amounts of peroxide were well depicted by Fourier transform infrared spectroscopy. Considering insignificant changes in molecular weight and molecular weight distribution during peroxide modification, the deviation observed in zero-shear-rate viscosity (η0) values of the modified LLDPE with that of power-law equation related to the linear PEs could be reliably attributed to the presence of LCB in the peroxide modified samples. Increasing the DCP content at roughly constant molar mass led to increasing of η0 values as a result of increased degree of LCB. The increase in η0 values was ascribed to prolonged relaxation times of the polymer molecules due to the retarded reptation motion-driven relaxation mechanism. Copyright © 2014 John Wiley & Sons, Ltd.

Large amplitude oscillatory shear and uniaxial extensional rheology of blends from linear and long-chain branched polyethylene and polypropylene

In this article, normal stresses and shear stresses in oscillatory shear flow are measured at small and large deformation amplitudes. New material parameters are then introduced based on Fourier transform rheology and stress decomposition analysis of normal and shear stress measurements. Furthermore, uniaxial extensional measurements are performed and compared to simulation results using the molecular stress function model. Different behaviors were observed for the polyethylene and polypropylene type blends, which are believed to arise from the different types of long-chain branching (LCB) topology present in each of the systems. The use of the new material parameters proposed and described within this article has the potential to allow for a better understanding of structure-property relationships in industrial LCB materials.

Modification of rice grain starch for lump-free cooked rice using thermostable disproportionating enzymes

Thermostable disproportionation enzymes active at around the gelatinization temperature of rice starch (70–80°C) were used to improve the quality of cooked rice with lump-free properties. Cyclodextrin glucanotransferase from Pyrococcus furiosus (CGTase) and a starch binding domain-attached alpha-glucanotransferase from Thermus aquaticus (αGTase) were prepared from an Escherichia coli transformant carrying a recombinant plasmid. The stickiness determined by a texture analyzer showed that both enzymes had the effect of reducing the lumping properties of cooked rice grains and rice porridge. The enzyme treatment also provided a finer, creamier, and more uniform texture of rice porridge. An HPAEC analysis of the starch structure in cooked rice and rice porridge revealed that the proportion of both short (DP<6) and long-branch chains (DP>20) increased in the enzyme-treated samples. Additionally, the amylose content of the cooked rice grain treated with αGTase and CGTase decreased by 36% and 30%, whereas that of porridge was reduced by 36% and 35%, respectively. In particular, the amylose content on the surface of the enzyme-treated cooked rice grains was reduced by 50% compared with that of untreated cooked rice. These results indicated that the glucan segment of amylose was transferred to amylopectin branch chains by a transferring reaction with both enzymes, reducing the amylose content of grains during cooking. Mechanistically, the decrease in the amylose content filmed on the surface is likely to be responsible for reducing the stickiness of cooked rice grains and rice porridge. Furthermore, the retrogradation rate determined by a differential scanning calorimeter was significantly reduced in the enzyme-treated samples after storage for 10h at 4°C. The decrease in long-chain amyloses, may lead to the retardation of re-crystallization. These physicochemical properties of the modified cooked rice and rice porridge can be applied to various rice-based products in the food industry.

Effects of processing variables on polypropylene degradation and long chain branching with UV irradiation

A technique has been developed to modify the melt properties of polypropylene (PP). Photoinitiators along with UV irradiation were employed to introduce long chain branching (LCB) and/or crosslinking (CL). Statistically designed experiments were carried out to study the effect of processing conditions, such as photoinitiator concentration, duration of irradiation, UV lamp intensity, cooling air pressure, and photoinitiator type, on rheological properties, molecular weight characteristics and branching level. Samples were evaluated through linear viscoelastic (LVE) measurements, extensional rheometry, gel content, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Results clearly indicated that PP can be successfully modified in order to enhance strain hardening behaviour without significant gel formation.

Effect of thermal modification on rheological properties of polyethylene blends

We examined the effects of thermal modification under flow field on the rheological properties of linear low-density polyethylene (LLDPE) with high molecular weight, low-density polyethylene (LDPE), and their blends, without thermal stabilizer. Although structural changes during processing are not detected by size extrusion chromatography or nuclear magnetic resonance spectroscopy, linear viscoelastic properties changed greatly, especially for the LLDPE. A cross-linking reaction took place, leading to, presumably, star-shaped long-chain branches. Consequently, the modified LLDPE, having high zero-shear viscosity, became a thermorheologically complex melt. Moreover, it should be noted that the drawdown force, defined as the uniaxial elongational force at a constant draw ratio, was significantly enhanced for the blends. Enhancement of elongational viscosity was also detected. The drawdown force and elongational viscosity are marked for the thermally modified blend as compared with those for the blend of thermally modified pure components. Intermolecular cross-linking reactions between LDPE and LLDPE, yielding polymers with more than two branch points per chain, result in marked strain-hardening in the elongational viscosity behavior even at small strain. The recovery curve of the oscillatory modulus after the shear modification is further evidence of a branched structure.