Long Chain Branching Structure Research Articles

Correlating color with free radical concentration in irradiated poly(ethylene terephthalate)

When polymers are irradiated, free radicals form, often accompanied by yellowing in the material. This color gradually fades after active irradiation due to the decay of free radicals. This study analyzed the kinetics of post-irradiated PET discoloration under different temperatures using a UV–visible spectrophotometer. Meanwhile, the free radical decay was also monitored using the electron paramagnetic resonance (EPR) measurements, and then both experiments were compared. The results illustrate the potential of readily available spectrophotometry as an alternative method to track free radical evolution in irradiated polymers because free radical decay leads to the disappearance of annealable color centers over time. Furthermore, PET structure modification by irradiation was studied using extensional rheological analysis. The observed strain hardening suggests the evolution of long-chain branching structures in post-irradiated PET, especially at 50 kGy irradiation absorbed dose.

Effects of grafting and long-chain branching structures on rheological behavior, crystallization properties, foaming performance, and mechanical properties of polyamide 6

Abstract Polyamide 6 (PA6) was modified with ethylene maleic anhydride syndiotactic copolymer resin (ZeMac), and triglycidyl isocyanurate (TGIC) as modifiers to prepare a grafting structure and a long-chain branching structure, respectively. The effects of two modifiers on the rheological behavior, crystallization properties, foaming performance, and mechanical properties of PA6 were systematically studied by rotating rheometry, differential scanning calorimetry and scanning electron microscopy. The results showed that there were differences in crystallization properties between the two modification methods, but they significantly improved the rheological, foaming performance, and mechanical properties of PA6. In particular, PA6 with long-chain branching structure through TGIC modification showed better performance in various physicochemical characterizations. The introduction of ZeMac reduced the average diameter of bubbles in pure PA6 from 146.32 to 88.12 µm, and the density of bubbles increased from 1.69 × 105 to 5.35 × 105 cells·cm−3. The introduction of TGIC reduced the average diameter of bubbles in pure PA6 from 146.32 to 64.36 µm, and the density of bubbles increased to 1.31 × 106 cells·cm−3. Moreover, the mechanical properties of both nonfoamed and foamed samples were improved after modification.

In Situ Promotion of Long-Chain Branching in Polyethylene from Ziegler–Natta Catalysts

Polyethylene (PE) synthesized by Ziegler–Natta catalysts (MgCl2/TiCl4 catalysts) lack long-chain branching (LCB) structure, which to a great extent limits their end uses. In this paper, we report an in situ methodology of incorporating LCB structure into PE of Ziegler–Natta catalysts. The chemistry centers on a bifunctional reagent, ω-alkenylmethyldichlorosilane, where the reagent is first copolymerized with ethylene to anchor on some polymer chains pendant alkylmethyldichlorosilane groups, then upon treating the polymer with water adjacent polymer chain-borne alkylmethyldichlorosilane groups undergo one-on-one interchain hydrolytic condensation forming a dimeric alkylmethylsiloxane bridge interconnecting two different polymer chains as H-shape LCB structure. Control of ω-alkenylmethyldichlorosilane concentration during copolymerization is key to realize the chemistry. Gel-free LCB-PEs containing controlled amounts of LCB structure were synthesized by accompanying the copolymerization with hydrogen (H2) as chain transfer agent. The effect of alkenyl chain length of ω-alkenylmethyldichlorosilane on the formation efficiency of LCB structure was studied. The chemistry was generally applicable to different Ziegler–Natta catalysts. LCB-PEs containing increasing amounts of LCB structure exhibited proportionally increased extensional flow properties. Moreover, compared to linear plain PE, LCB-PEs were overall found to increase crystallization rates and thus increase crystallinities.

Fully biodegradable polylactide foams with ultrahigh expansion ratio and heat resistance for green packaging

Long chain branching (LCB) structures are efficiently introduced into polylactide (PLA) by employing sustainable soybean oil (SO) under the initiation of trace amount of cyclic peroxide, which displays robust foamability and heat resistance. It is discovered that with the introduction of 0.6 wt% SO, the expansion ratio and Vicat softening temperature of LCB PLA are sharply raised to 75.2-fold and 155.8 °C, respectively, which is about 17.9 and 2.6 times those of linear PLA. This is because that the amounts of LCB structures are significantly increased in LCB PLA by the addition of SO with low reactivity of internal CC bonds, which can avoid the oligomerization reaction, resulting in more dramatically improved melting strength and crystallization performance of LCB PLA. Moreover, the hydrolytic degradation of LCB PLA is largely expedited as compared to linear PLA, owing to the more rapid water permeation caused by the loose packing of LCB structures. Finally, the PLA foam tray with light weight and good heat resistance is successfully developed by using LCB PLA with 0.6 wt% SO through extrusion foaming with supercritical carbon oxide and thermoforming techniques. Hence, this research offers a green route to produce eco-friendly light-weight and high-heat-resistance LCB-PLA foam with full biodegradability, which is an ideal alternative to the non-degradable oil-based plastics in the field of disposable packaging products.

The effect of initiator, polyfunctional monomer and polybutene-1 resin on the long chain branching of random polypropylene copolymer via reactive extruder

ABSTRACT Polypropylene random copolymer (PPR) and polybutene-1(PB-1) are blended and long-chain branched (LCB) in the reactive extruder. The optimum amount of PB-1 resin, peroxide (DCP) and trimethylolpropane triacrylate (TMPTMA) for long-chain branching of PPR and probe topology formed by the branches were investigated. Fourier transformed infrared spectroscopy (FTIR) used to measure the branching degree and efficiency as well as obtaining TMPTMA grafts on PPR. Oscillatory shear rheological properties including complex viscosity, storage modulus, loss tangent, and uniaxial elongational rheometry were studied to differentiate the LCB-PPR from unmodified PPR with almost similar molecular weight samples. The LCB structure modified samples with TMPTMA and DCP, prominent higher zero-shear viscosity, longer relaxation time and pronounced strain-hardening behavior. It was found that different amount of DCP and TMPTMA can affect strain-hardening behavior and topology of branches and subsequently the foaming ability. PB-1 resin found to have a positive effect because of more sensibility to β-scission degradation and mobilizing effects. The optimization of PB-1 for the highest zero-shear viscosity, shear, and uniaxial rheological behavior is achieved at 10 wt% PB-1, while 0.08% of DCP and 3% of TMPTMA are optimum amounts of additives needed for long-chain branching process. The optimum blend could be a great alternative for foaming applications.

Determination of low degrees of long-chain branching in polybutadiene by double-bond hydrogenation followed by 13C NMR and DEPT spectroscopies

Long-chain branching (LCB) in polybutadiene (PB) was directly and accurately determined for the first time by combining the hydrogenation of PB with 13C NMR and DEPT spectroscopies. LCB in PB was converted into LCB in polyethylene (PE) by the hydrogenation of PB. No LCB in PE was observed in the 13C NMR spectrum. In contrast, LCB in PE was clearly observed in the 13C DEPT 90° spectrum. The methine carbons in LCB and short-chain branching, which is vinyl branching in PB converted into ethyl branching (EB) in PE, were quantified by 13C NMR and DEPT 90° spectroscopies. The numbers of LCB structures of commercially available PB-A and PB-B were 0.27 and 0.17/1000C, respectively. It was proved that these values were accurate with errors of ca. 10% as determined by analyses of control polymers with various ratios of hexyl branching to EB.

Preparation and characterization of high melt strength thermoplastic polyester elastomer with different topological structure using a two-step functional group reaction

Long-chain branched (LCB) thermoplastic polyester elastomer (TPEE) could be controllably prepared by the two-step reaction of the linear TPEE with triglycidyl isocyanurate (TGIC) and 2,2′-bis(2-oxazoline) (2,2′-BOZ) together in different sequences to improve their rheological properties and foaming ability. All of the modified TPEEs reacted with TGIC first and then with 2,2′-BOZ have more branching points and longer chains. These samples show a higher molecular weight, a broader molecular weight distribution. These samples also had a higher viscosity and modulus, a longer relaxation time, and a stronger strain-hardening effect. Moreover, the greater number of branching points accelerated the non-isothermal crystallization process. The foaming ability of the modified TPEEs was evaluated via a batch CO2-foaming process. The LCB structure improved the melt viscoelasticity and the foaming ability remarkably. The TPEE samples with more chain branching points and longer chains had the highest cellular densities and the largest expansion ratios, with average cell diameters of 4–13 μm, cell densities of 2.0 × 109–1.0 × 1011 cells/cm3, and expansion ratios of 2.5–14.1.

Preparation of long‐chain branched poly(ethylene terephthalate): Molecular entanglement structure and toughening mechanism

Poly(ethylene terephthalate) (PET) was long‐chain branched (LCB) by ring‐opening reaction with both pyromellitic dianhydride and tetrahydrophthalic acid diglycidyl ester as chain extenders through reactive melt processing. It was found that with the increase of chain extenders dosage, the intrinsic viscosity of PET increased and melt index decreased greatly, while both the tensile strength and impact strength of PET were remarkably improved. The elastic modulus (G′) and viscous modulus (G″) were enhanced by chain branching. Compared with PET, the complex viscosities of LCB‐PET were much higher at full frequency range, and obvious shear thinning was presented. The Cole–Cole curve deviated from the semicircular shape and the curve end was inclined to upward in high viscosity region, indicating the formation of the multiple hierarchical structures. The molecular weight of the branch (MB) was much greater than critical entanglement molecular weight (M e), which essentially confirmed the existence of LCB structure and fairly strong molecular entanglement in the LCB‐PET molecular chain. When subjected to external force, the entanglement point, acting as physical crosslinking point between the molecules, was in favor of increasing the molecular interaction, reducing the molecular slippage, and bearing a large deformation. POLYM. ENG. SCI., 59:1190–1198 2019. © 2019 Society of Plastics Engineers

Relationship between Peroxide Initiators and Properties of Styrene Grafted Polypropylene via Reactive Extrusion

ABSTRACTTo understand the relationship between the initiators and the properties of grafted polypropylene (PP), and provide guidance for designing polymers with different performance through selecting appropriate initiators, a series of styrene (St) grafted PP was prepared by modifying commercial linear PP via reactive extrusion using two different peroxide initiators, dicumyl peroxide (DCP) and benzoyl peroxide (BPO). Fourier transform infrared spectra indicated that the use of DCP led to a higher St grafting degree compared to the system using BPO. The melt flow index and rheological characteristics suggested the existence of short chain branching (SCB) structures in the St grafted PP using DCP, and long chain branching (LCB) structures in the St grafted PP using BPO. Differential scanning calorimetry and polarized optical microscopy results showed that the degradation of the PP chains and the introduction of SCB structures hindered the crystallization process of the St grafted PP using DCP, and the existence of the LCB structures accelerated the crystallization process of the St grafted PP using BPO. We suggest this research can contribute to the understanding of methods to prepare grafted PP with special properties via reactive extrusion by using proper initiators.

In situ shrinking fibers enhance strain hardening and foamability of linear polymers

The strain hardening behavior of polymers has important roles in processing such as foaming, film formation, and fiber spinning. The most common method to enhance strain hardening is to introduce a long-chain branching structure on the backbone of a linear polymer, but this method is costly and challenging to tailor the behavior. We hypothesized that in situ shrinking fibers can increase the strain hardening of linear polymers, and the degree can be efficiently controlled. In this study, we show that heat-activated shrinking fibers compounded in linear polypropylene enhance strain hardening and foamability. Moreover, changing processing conditions, such as temperature, can amplify the degree of enhancement. Rheological measurements and physical foaming tests are shown to support our hypothesis.

The morphological evolution and β-crystal distribution of isotactic polypropylene with the assistance of a long chain branched structure at micro-injection molding condition

Long chain branching polypropylene (LCBPP) has good application prospect but the synergistic effects of the long chain branching (LCB) structure and complex flow field on the crystallization behavior of LCB polymers are still elusive. In the present work, microparts of isotactic polypropylene (iPP)/LCBPP blends were prepared by microinjection molding (MIM). The effect of the LCB structure on the morphology evolution and the β-crystal distribution in microparts were investigated. Interestingly, adding LCBPP facilitates the formation of oriented β-crystals along the shear direction in the core layer of the microparts but unexpectedly, decreases the relative content of β-crystals due to the heterogeneous nucleation effect of LCBPP. Moreover, the introduction of LCBPP can also improve the orientation degree of the molecular chains and shrink the thickness of the core layer thereby inevitably suppressing the “skin–core” structure which might optimize the properties of MIM parts.

Introduction of a long-chain branching structure by ultraviolet-induced reactive extrusion to improve cell morphology and processing properties of polylactide foam

Long-chain branching structure introduced by UV-induced reactive extrusion could widen its foaming processing window.

Long chain branched poly(butylene succinate‐co‐terephthalate) copolyesters using pentaerythritol as branching agent: Synthesis, thermo‐mechanical, and rheological properties

ABSTRACTIncorporating long chain branching (LCB) structure into biodegradable copolyesters can effectively improve their melt strength and film blowing processability. However, branching also results in deterioration of crystallizability which is also important for copolyester properties and processing. In this study, pentaerythritol (PER) was used as branching agent (BA) instead of previous used in‐situ BA, diglycidyl 1,2,3,6‐tetrahydrophthalate (DGT), to synthesize LCB poly(butylene succinate‐co‐terephthalate) (PBST) copolyesters. The chain structure was characterized and the effects of branching on thermal transition, mechanical, and rheological properties were investigated. Similar to DGT, copolymerizing small amount of PER (0.1–0.4 mol %) generates LCB structure and, therefore, improves the melt elasticity or strength and tensile modulus but reduces the elongation at break. Differing from DGT, PER showed higher branching efficiency, and PER‐branched PBSTs exhibited unchanged or even improved crystallization ability compared with linear PBST. The improved melt strength coupled with good crystallizability will endow PER‐branched PBSTs with better film blowing processability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2017,134, 44544.

Relationship between molecular structure, crystallization behavior, and mechanical properties of long chain branching polypropylene

The crystallization behaviors and mechanical properties of both linear isotactic polypropylene (iPP) and long chain branching polypropylene (LCBPP) were investigated with differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. The results indicated that the long chain branching (LCB) structure could act as nucleating agent accelerating the nucleation but impeding the crystal growth of LCBPP, and eventually increased the crystallization peak temperature and decreased the spherulite size of LCBPP. Moreover, with the presence of LCB structure, β-form polypropylene was generated in LCBPPs under injection molding conditions, and the content of β-form increased with the increasing of LCB structure content. Benefiting from the combined effect of the LCB structure and β-form crystal, the LCBPPs revealed not only outstanding toughness but also desired stiffness. The impact strength, flexural modulus and tensile strength of HMSPP3 were increased by, respectively, 260.0, 54.7, and 17.2 % than those of linear iPP. These results are helpful to develop new types of high performance polypropylene.

Shear-induced β -form polypropylene in long chain branching isotactic polypropylene

Long chain branching polypropylene (LCBPP) with different long chain branching (LCB) contents were prepared by reactive extrusion in the presence of styrene and benzoyl peroxide, and their shear-induced crystallization behaviors were investigated. The results indicated that the LCB structure extended the relaxation time of LCBPP in the molten state, which led to the formation of β-form polypropylene under shear and high cooling rate. The content of β-form (Kβ) increased with the increase of LCB content, shearing rate and cooling rate. The Kβ value of LCBPP3 whose weight average molecular weight was 920,000 g mol−1 could be up to 52.0% with a shear rate of 60 s−1 associated with a cooling rate of 280°C min−1. This study is expected not only to have a deeper understanding of the shear-induced crystallization behavior of LCBPP, but also provide a new strategy to obtain high level β-form polypropylene. POLYM. ENG. SCI., 56:240–247, 2016. © 2015 Society of Plastics Engineers

Mechanical and morphological properties of injection molded linear and branched-polylactide (PLA) nanocomposite foams

Linear and long chain branched (LCB) polylactide (PLA)–clay nanocomposites were prepared via melt compounding using a twin-screw extruder. An organo-modified clay, Cloisite 30B, and a chain extender, Joncryl, were employed in this study. The resulting systems were also foamed in a conventional injection molding machine using a chemical blowing agent, namely activated azodicarbonamide. The PLA crystallinity, clay dispersion, and foam cellular morphology were characterized. Unlike the linear PLA, the addition of nanoclay markedly reduced the crystallinity of the LCB-PLA. The clay inclusion as well as the LCB structure generally decreased the cell size and foam density, while increasing the cell density. The mechanical properties of the unfoamed and foamed linear and LCB-PLA samples and their corresponding nanocomposites were also characterized by tensile and Izod impact testing. The corresponding results revealed that the LCB-PLA nanocomposite with 0.5wt% clay loading exhibited a significantly improved cellular structure in comparison with other foamed samples, leading to increased values of the specific modulus, tensile and impact strength.

Quiescent and shear-induced crystallization of linear and branched polylactides

The quiescent and shear-induced isothermal crystallization behavior of linear and long-chain branched (LCB) polylactides (PLAs) was investigated at a temperature of 130 °C. LCB-PLAs were produced by the reaction with a multifunctional chain extender, Joncryl©. In quiescent crystallization, the presence of the LCB structure accelerated the nucleation process and reduced the induction time, depending on the level of branching. The impact of shear strain and shear rate on crystallization was also examined. The shear-induced crystallization of the linear and LCB-PLAs was affected by both the total shear strain and shear rate. The crystallization kinetics of the LCB-PLAs was more affected by shear than that of the linear PLA. The crystalline morphology of the linear and LCB-PLAs under quiescent and step shear rate conditions was examined using a Linkam optical shearing system. An increase in the spherulite density was observed in the strained melt of both linear (33 %) and LCB-PLAs (15 %), in comparison with those of unstrained counterparts. Optical micrographs confirmed that the crystal nucleation was affected by the shear flow. Long-chain branching significantly promoted the nucleation density (6.7 times), although it diminished the crystal growth rate from 4.4 to 2.0 μm/min.

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

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

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.

Preparation and rheological characterization of long chain branching polylactide

Long chain branching (LCB) of polylactide (PLA) was successfully prepared by the successive reactions of PLA with pyromellitic dianhydride (PMDA) and 1,4-phenylene-bis-oxazoline (PBOZ) together. The topological structures of the LCB generated from functional group reactions were investigated thoroughly by gel permeation chromatography (GPC) and rheology. Qualitative information about the branching structures could be readily obtained from linear viscoelasticity, nonlinear oscillatory shear experiments and strain hardening in elongational experiments. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict probable compositions and chain topologies of the products, which were reasonably explained by the suggested mechanism of functional group reactions. It was found out that the star-like LCB structure generated in these reactions contributed remarkably to the enhancement of strain hardening under elongational flow.