Rheology and thermal behavior of long branching polypropylene prepared by reactive extrusion

Long‐chain branching polypropylene (LCB‐PP) was achieved by reactive extrusion in the presence of bifunctional monomer [1,6‐hexanediol diarylate (HDDA)] and peroxide of dicumyl peroxide (DCP). Influences of HDDA and DCP concentrations on the branching efficiency were comparatively evaluated. Fourier transformed infrared spectroscopy (FTIR) results indicated that the grafting reaction took place, and HDDA has been grafted on PP skeleton. In comparison with initial PP, some modified samples showed lower melt flow index because of a large number of LCB in their skeleton. Several rheology plots were used to investigate the rheological properties of the initial PP and modified PPs, and the rheological characteristics confirmed the LCB in modified PPs skeleton. DSC results showed that the crystallization temperatures of modified PPs were higher than those of initial PP and degraded PP, suggesting that the modified PPs had long‐chain branched structure. The contrastive investigation in the rheology of modified PPs suggested that proper concentrations of HDDA and DCP were more beneficial to producing LCB during reactive extrusion. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of polyfunctional monomer on melt strength and rheology of long‐chain branched polypropylene by reactive extrusion

Long chain branching (LCB) were added to linear polypropylene (PP) using reactive extrusion in the presence of selected polyfunctional monomers (PFMs) and a peroxide of dibenzoyl peroxide (BPO). Fourier Transformed Infrared spectra (FTIR) directly confirmed the grafting reaction occurred during the reactive extrusion process. Various rheological plots including viscosity curve, storage modulus, Cole‐Cole plot, and Van‐Gurp plots, confirmed that the LCB structure were introduced into modified PPs skeleton after modification. In comparison with linear PP, the branched samples exhibited higher melt strength, lower melt flow index, and the enhancement of crystallization temperature. The LCB level in modified PPs and their melt strength were affected by the type of PFM used and could be controlled by the PFM properties and structure. PFMs with lower boiling points, such as 1, 4‐butanediol diacrylate (BDDA), could not produce LCB structure in modified PP skeleton. The shorter molecular chain bifunctional monomers, such as 1,6‐hexanediol diacrylate (HDDA), favored the branching reaction if their boiling points were above the highest extrusion temperature. And some polar groups, such as hydroxyl, in the molecule of PFM were harmful to the branching reaction, which might be attributed to the harm of the polarity of groups to the dispersion of PFM in PP matrix. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Upcycling of polypropylene with various concentrations of peroxydicarbonate and dilauroyl peroxide and two processing steps

The influence of two peroxides (peroxydicarbonate/dilauroyl peroxide) with various concentrations (10–200 mmol/kg PP) and their effective opportunity to introduce long chain branched (LCB) were investigated. The dependence of a single and double extrusion step and the changes of the properties were studied. Experiments were carried out in a single screw extruder at 180°C for the first extrusion step (modification) and at 240°C for the second extrusion step (processing simulation). Melt flow rate and dynamic rheological properties were studied at a measuring temperature of 230°C. For the definitive determination of long chain branched polypropylene (LCB-PP) served the extensional rheology measurements. The mechanical properties were examined via tensile test and impact tensile test. Summarized, LCB (melt strength) could be observed via extensional rheology for all modified specimens and the mechanical properties were maintained or even improved for the modified samples. Particularly, samples containing dilauroyl peroxide display excellent mechanical properties in this study.     

苯乙烯接枝改性聚丙烯制备及其性能

以过氧化苯甲酰为引发剂,采用反应挤出法将苯乙烯(St)接枝到等规聚丙烯(iPP)上,并考察了St浓度对接枝PP结晶性能、流变性能和发泡性能的影响。由FTIR、GPC和MFR表征结果表明St成功接到PP上;DSC结果表明,接枝PP结晶峰值温度与iPP相比最大提高了15.4℃;由剪切流变测试结果,在低频区,接枝PP的G'值增大,出现明显的剪切变稀现象,tan δ-ω曲线中出现平台,说明接枝PP中有长支链结构的存在;由于接枝PP的熔融流动速率降低即熔体强度提高,使得接枝PP的泡孔更规则。     

Structure and melt rheology of long‐chain branching polypropylene/clay nanocomposites

Long‐chain branching polypropylene (LCB‐PP)/clay nanocomposites were prepared by melt blending in a twin‐screw extruder. The microstructure and melt rheology of these nanocomposites were investigated using x‐ray diffraction, transmission electron microscopy, oscillatory shear rheology, and melt elongation testing. The results show that, the clay layers are intercalated by polymer molecular chains and exfoliate well in LCB‐PP matrix in the presence of maleic anhydride grafted PP. Rheological characteristics, such as higher storage modulus at low‐frequency and solid‐like plateau in tan‐ω curve, indicate that a compact and stable filler network structure is formed when clay is loaded at 4 phr (parts per hundred parts of) or higher. The response of the nanocomposite under melt extension reveals an initial decrease in the melt strength and elongational viscosity with increasing clay concentration up to 6 phr. Later, the melt strength and elongational viscosity show slight increases with further increasing clay concentration. These results might be caused by a reduction in the molecular weight of the LCB‐PP matrix and by the intercalation of LCB‐PP molecular chains into the clay layers. Increases in the melt strength and elongational viscosity for the nanocomposites with decreasing extrusion temperature are also observed, which is due to flow‐induced crystallization under lower extrusion temperature. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The melt grafting preparation and rheological characterization of long chain branching polypropylene

To study the rheological properties of long chain branching (LCB) polypropylene (PP), long chain branches (LCB) were grafted onto the linear PP by melt grafting reaction in the presence of a novel chain extender, poly(hexamethylendiamine-guanidine hydrochloride) (PHGH). The branching reactions between the functionalized PP and PHGH were confirmed by transient torque curves and FTIR. By differential scanning calorimetry (DSC) and polarized microscope measurements, the presence of long chain branching structures was further confirmed. Also, the viscoelastic properties of the LCB PP and linear PP under shear flow were investigated for distinguishing LCB PP from linear PP. It was found that the elastic response of LCB PP at low frequencies was significantly enhanced in comparison with that of the linear PP, implying a presence of a long relaxation time mode that was not revealed in linear PP. Moreover, the branching levels of LCB PP were quantified using a detailed method, which was in correspondence with the molar amount of PHGH grafted on PP.     

长链支化聚丙烯的反应挤出制备及其流变和热力学性能研究

在过氧化引发剂和季戊四醇三丙烯酸酯(PETA)存在下,采用反应挤出法制备了长链支化聚丙烯（LCB-PP）。用旋转流变仪和差示扫描热法系统研究了纯的和改性PP的流变性能和热力学性能,并考察了不同过氧化引发剂对改性PP的性能和支化情况的影响。研究发现,单纯使用过氧化引发剂改性时,PP以降解为主;加入多功能单体PETA后,PP以接枝反应为主。流变行为研究发现,过氧化引发剂/PETA改性的PP,其流变性能呈现如低频处储能模量增大,剪切变稀行为明显,损耗角随频率变化出现平台区,零剪切黏度增大等特点,证明改性PP存在长链支化结构,通过公式计算发现改性PP的支化度较高。差士扫描量热分析表明,过氧化引发剂/PETA改性PP的结晶温度高于纯PP,这也说明改性PP存在长链支化结构。同时发现过氧化引发剂/PETA改性PP时,过氧化引发剂结构对改性PP的流变性能、热力学性能和支化度影响较小。     

The preparation and rheology characterization of long chain branching polypropylene

In order to study the rheological behavior of long chain branching (LCB) polypropylene (PP), linear polypropylene was modified by melt grafting reaction in the presence of 2,5-dimethyl-2,5(tert-butylperoxy) hexane peroxide and pentaerythritol triacrylate (PETA) in mixer. The transient torque curves and Fourier transformed infrared spectroscopy (FTIR) results indicated that macroradical recombination reactions took place and PETA had been grafted onto PP backbone. Various rheological plots including viscosity curve, storage modulus, loss angle, Han plot, Cole-Cole plot were used to distinguish LCB PP from linear PP. On the other hand, to quantify the LCB level in modified PPs, a new method was suggested on the basis of macromolecular dynamics models. The results showed that the level of LCB was in the range of 0.025-0.38/10⁴ C . Moreover, the length of the branched chains and the content of the branched component increase with PETA concentration. Furthermore, the LCB efficiency of monomer can also be calculated, less than 20% of grafting monomers was used to form branch structure.     

Influence of extrusion temperature on molecular architecture and crystallization behavior of peroxide‐induced slightly crosslinked poly(L‐lactide) by reactive extrusion

The influence of temperature during reactive extrusion of poly(L ‐lactide) (PLLA) on the molecular architecture and crystallization behavior was investigated for OO‐(t‐butyl) O‐(2‐ethylhexyl) peroxycarbonate‐modified polymer. The long chain–branched PLLA (LCB‐PLLA) content and its structure in the resulting slightly crosslinked PLLA (χ‐PLLA) containing linear and LCB‐PLLA were characterized by both analyses, size exclusion chromatography equipped with multiangle laser light scattering and rheological measurements. A reduction of LCB‐PLLA content in χ‐PLLA and an increase of number of branches in LCB‐PLLA were found with increasing the extrusion temperature. An increase of extrusion temperature induces different process in the polymer: decrease of the lifetime of peroxide, increase of the radical concentration due to rapid peroxide decomposition rate, and increase of the chain diffusion to the amorphous phase. Among these indices, the lifetime of peroxide is a good index for crosslinking behavior of PLLA during extrusion. As for the isothermal crystallization behavior from the melt, the Avrami crystallization rate constant of χ‐PLLA increases as an increase of LCB‐PLLA content in χ‐PLLA. This implies that LCB‐PLLA acts as a nucleating agent for PLLA. Furthermore, regime analysis and the free energy of nucleus of χ‐PLLA were investigated using Hoffman–Lauritzen theory. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The preparation and crystallization of long chain branching polylactide made by melt radicals reaction

Long chain branching (LCB) of polylactic acid (PLA) was successfully prepared by melt radicals reaction with pentaerythritol triacrylate (PETA) and bis (1‐methyl‐1‐phenylethyl) peroxide (DCP). The topological structure of the LCB was investigated by rheology and branch‐on‐branch (BOB) model was used to estimate the exact chain structures of the products, where comb‐like LCB structures were generated due to the complex coupling between different macro‐radicals. LCB structure was found to affect the crystallization of PLA products. In the temperature range of 110–130°C, the crystallization rate parameter (k) was improved sharply and the half crystallization time was decreased significantly after the grafting of PETA, which was ascribed to the enhanced hydrogen bonding in PETA‐grafted long chain branching PLA. By comparing with the LCB PLA made from chain extension using multifunctional monomer, it shows that the crystallization becomes slower in a highly branched material with extremely long relaxation time if the effect of hydrogen bonding is similar. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Preparation and characterization of high melt strength polypropylene with long chain branched structure by the reactive extrusion process

The reactive extrusion of maleic anhydride grafted polypropylene (PP‐g‐MAH) with ethylenediamine (EDA) as coupling agent is carried out in a corotating twin‐screw extruder to produce long chain branched polypropylene (LCBPP). Part of PP‐g‐MAH is replaced by maleic anhydride grafted high‐density polyethylene (HDPE‐g‐MAH) or linear low‐density polyethylene (LLDPE‐g‐MAH) to obtain hybrid long chain branched (LCB) polyolefins. Compared with the PP‐g‐MAH, PE‐g‐MAH, and their blends, the LCB polyolefins exhibit excellent dynamic shear and transient extensional rheological characteristics such as increased dynamic modulus, higher low‐frequency complex viscosity, broader relaxation spectra, significantly enhanced melt strength and strain‐hardening behaviors. The LCB polyolefins also have higher tensile strength, tensile modulus, impact strength and lower elongation at break than their blends. Furthermore, supercritical carbon dioxide (scCO₂) is constructively introduced in the reactive extrusion process. In the presence of scCO₂, the motor current of the twin extruder is decreased and LCB polyolefins with lower melt flow rate (MFR), higher complex viscosity and increased tensile strength and modulus can be obtained. This indicates that the application of scCO₂ can reduce the viscosity of melt in extruder, enhance the diffusion of reactive species, and then facilitate the long chain branching reaction between anhydride group and primary amine group. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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     

高熔体强度聚丙烯中长支链结构的分析与表征方法新进展

高熔体强度聚丙烯(high melt strength polypropylene,HMSPP)是具有较高熔体强度和弹性、在熔融拉伸时存在应变硬化现象的一种新型聚丙烯材料。近年来,对长支链(LCB)型HMSPP (LCB-HMSPP)的研发引起了学术界和工业界的广泛关注。在利用各种方法和工艺制备LCB-HMSPP之后,所得聚合物是否具有长支链结构,长支链的化学组成、密度及链长度等拓扑结构信息,都需要通过各种仪器分析与表征方法对其进行定性或定量验证。本文着重对傅里叶变换红外光谱(FTIR)法、核磁共振碳谱(¹³C NMR)法、凝胶渗透色谱(GPC)及其联用方法、流变学表征法和结晶行为表征法在LCB-HMSPP中LCB链结构分析与表征中的研究新进展进行总结和评述,介绍了这些方法各自的优缺点和适用性并对这些方法进行了对比,最后对用于LCB-HMSPP中LCB链结构的分析与表征方法研究的未来发展及应用前景进行了展望。     

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 10⁴ 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 CO₂. 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.     

聚丙烯的流变性能及发泡性能研究

采用凝胶渗透色谱仪(GPC),差示扫描量热仪(DSC)、高级流变扩展系统(ARES),熔体拉伸流变仪对等规聚丙烯(PP1)、接枝改性聚丙烯(PP2)及Borealis的高熔体强度聚丙烯(PP3)进行了测试和表征,并利用自行研制的超临界流体挤出发泡实验装置,对上述3种PP进行了超临界二氧化碳挤出发泡研究,初步探讨了聚丙烯的流变性能对聚丙烯发泡性能的影响。结果表明,带有支化结构的PP2和PP3具有较高的熔体强度和熔体弹性,能够得到泡孔均匀的挤出发泡样品,具有较好的可发泡性。     

Rheological control in foaming polymeric materials: II. Semi-crystalline polymers

The influence of rheological properties and crystallization on foam structures, such as cell diameter, cell density and cell size distribution, of semi-crystalline polymer was investigated. The rheological properties of polypropylene (PP) were controlled by long chain branching (LCB) modification with free radical reaction and its crystallinity. The foaming behavior could be well correlated with the crystal structure and the rheological properties of polymers. The results showed that the long chain branching modification changed the crystallization speed, the diameter and the number of crystal and the rheological behavior as well. The interplay between the crystallization and the rheology of polymers with different chain structures can cause different nucleation mechanism in foaming. Both the cell size of linear PP and LCB PP decrease with crystallization time, and the cell density increases with crystallization time. The crystals in PPs acted as heterogeneous nucleation cites for bubbles, but the cell density of LCB PP is much higher than that of linear PP because of it higher spherulites density. The higher viscosity of branched PP further made its cell diameter smaller than that of linear one. Therefore, the foam structure can be well controlled by tuning the chain structure and crystal structures.     

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

Incorporating 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.     

Rheological assessment of variable molecular chain structures of linear low‐density polyethylene under reactive modification

The aim of this study was to investigate how changes in the molecular structure of linear low‐density polyethylene (LLDPE) during peroxide modification can be detected by a simple rheological method. For this purpose, a commercial‐grade LLDPE (Exxon Mobile LL4004EL) was reacted with different doses of dicumyl peroxide (DCP). The samples were analyzed by size exclusion chromatography coupled with a light‐scattering detector. With increasing DCP dose, at a roughly constant molar mass, an increasing number of long‐chain branches were found. The dynamic shear oscillatory measurements showed a deviation of the phase angle–complex shear modulus curve from that of the linear LLDPE, which was attributed to the presence of long‐chain branching. By the use of a simple rheological method that used melt rheology, transformations in the molecular architecture induced on the original LLDPE during the early stages of reactive modification were indicated. Reasonable and consistent estimates of the degree of long‐chain branching (x) and the volume fraction of the various molecular species produced in the peroxide modification of LLDPE were obtained. Various three‐dimensional plots were constructed to exhibit the correlation between the process parameters and x. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39617.     

Trace levels of mechanochemical effects in pulverized polyolefins

This research investigated the structural changes that occur on different polyethylene polymer systems as a result of a novel pulverization process called solid‐state shear pulverization (S³P). High‐density polyethylene, low‐density polyethylene, and two forms of linear low‐density polyethylene were run through a pulverizer under high shear conditions as well as low shear conditions. The physical properties were examined before and after the pulverization via melt index, melt rheology, GPC, and DSC, techniques. The low shear pulverization did not noticeably alter the physical properties of the polymers. In contrast, high shear pulverization did result in an increase in viscosity as observed by melt index and oscillatory shear experiments, although solid‐state and bulk properties as observed by DSC and GPC were not affected. These results indicate that a small amount of mechanochemically induced changes occur as a result of the pulverization process, including incorporation of a small amount of long‐chain branches randomly placed on a few of the polymer chains. No evidence of short‐chain branching resulting from S³P processing was found in these systems. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 671–679, 2001     

长链支化聚丙烯的制备及其熔体流变性能的研究

在过氧化物引发剂和季戊四醇三丙烯酸酯存在下,利用反应挤出法制备了长链支化聚丙烯(LCB-PP)。采用熔体流动速率(MFR)仪、旋转流变仪和熔体强度测试仪对纯聚丙烯(PP)及其改性PP进行测试与表征。讨论了不同的过氧化物引发剂对改性PP流变性能的影响。结果表明,采用过氧化苯甲酰时,改性PP具有较高的熔体强度、较低的MFR,并且在低频处储能模量增大。同时发现,随温度的升高,改性PP的熔体强度逐渐降低,但升高到一定温度后,熔体强度的变化不明显。