Flow induced crystallization of long chain branched polypropylenes under weak shear flow

In the present work long chain branched polypropylene (LCB PP) polymers were prepared by linear polypropylene and multi-functional monomer through melt grafting reaction. A quantitative rheological method was adopted to analysis the structure parameters of LCB PP. The effects of chain branched level on the crystallization kinetics of PP were investigated by rheology, differential scanning calorimetry, polarized optical microscope and wide-angle X-ray diffraction. The dynamic viscoelastic properties of LCB PP showed that the increase in the chain branched level caused a typical deviation from the terminal behavior and a different distribution of the melt relaxation spectrum in the long relaxation time regime. It was found that the chain branched level had a significant effect on the flow induced crystallization (FIC) process of PP melts. The crystallization of LCB PP was more sensitive to shear flow than that of linear PP during induced period at low shear rates. This result also indicated that the longer relaxation time of the polymer chains played an important role in the nucleation of PP under shear flow fields. LCB PP with high chain branched level showed accelerated crystallization kinetics in comparison with that with low chain branched level.     

Influence of polypropylene topological structure evolution during melt branching reactive processing on its melt performances

Gel‐free long‐chain‐branched polypropylene (LCBPP) was prepared by the melt radical branching reaction in the presence of peroxide initiator 2,5‐dimethyl‐2,5‐di(tert‐butylperoxy) hexane peroxide, zinc dimethyldithiocarbamate, and trimethylolpropane triacrylate in a torque rheometer. It could be inferred that recombination between PP chains via radical coupled reaction took place and trimethylolpropane triacrylate was grafted onto PP backbone by the torque curves and Fourier transformed infrared spectroscopy results. The presence of long chain branches (LCB) for modified PP was verified by the gel permeation chromatography measurements and vGP plots. On the other hand, it was found that the topological structure of PP chains transformed from linear form to a long star‐like shape during the reaction progress, and the topological structure was directly determined by the radical reaction time. The topological structure of PP would further impact its melt behaviour. After complete melting of raw PP, “sparse and long” LCBPP firstly generated which possessed high melt strength owing to the increasing entanglement of long branching chains. And at the time corresponding to the summit of reaction peak on the torque curve, the modified LCBPP possess the highest melt strength owing to its long star topological structure. While as reaction time was prolonged, severe degradation of the LCBPPs would take place under too long mixing time and “dense and short” branches generated due to the residual radicals, with a sharp decline in melt strength.     

Synthesis of Long‐Chain Branched Propylene Polymers via Macromonomer Incorporation

Long‐chain branched (LCB) poly(propylene)s (PP) are synthesized by the incorporation of pre‐formed, vinyl‐terminated macromonomers using metallocene catalysts. LCB‐PP polymers made with isotactic PP macromonomers are characterized by means of multi‐angle laser light scattering and extensional viscosimetry. The LCB polymers exhibit enhanced melt properties, such as strain hardening and shear thinning. These properties are critical in many polymer fabrication processes, such as thermoforming, blow molding and foaming.     

1,6-己二醇二丙烯酸酯和苯乙烯多单体熔融接枝制备长支链聚丙烯研究

采用哈克转矩流变仪制备了1,6-己二醇二丙烯酸酯(HDDA)和苯乙烯(St)多单体熔融接枝聚丙烯(PP)体系.红外测试结果表明St的加入能够促进HDDA接枝到PP主链,提高接枝率.动态流变行为研究结果也表明采用多单体熔融接枝PP更有利于体系在熔融接枝中生成长支链(LCB).随着St单体添加量的增加,HDDA的接枝率增大...     

A novel strategy for the preparation of long chain branching polypropylene and the investigation on foamability and rheology

A novel strategy for introducing long chain branches (LCB) on commercial polypropylene (PP) was described using an extender agent, poly (hexamethylendiamine-guanidine hydrochloride) (PHGH), to couple with glycidyl methacrylate-grafted PP (PP-GMA). The presence of LCB was confirmed by GPC and melt strength measurements in two modified PP samples, MPP-1 and MPP-2. The foamability of LCB PPs and commercially linear PP (EPS) was investigated by foam extrudate. The results showed that the foamability of LCB PPs was significantly improved. To find out the difference in foamability of EPS and LCB PPs, some rheological properties were investigated. In the dynamic shear measurement, it was found that the elastic response of LCB PPs at low frequencies was distinctly enhanced in comparison with that of EPS, implying a presence of a long relaxation time mode that was not revealed in linear PP. The elongational rheology results showed that the strain hardening behaviors of LCB PPs were still intense even at higher strain rate of 1 s^?1; moreover, MPP-2 could sustain larger elongational stress and deformation, which helps to improve the foamability. In step-shear stress relaxation measurements, the fast and slow relaxation processes were observed over the entire ranges of strains, indicating the presence of two different relaxation time modes. Also, the strain dependence of slow damping function became weaker because of the introduction of LCB on linear PP. In addition, it was also found that the rheological behaviors characterizing the long relaxation time mode were further enhanced with the increase of the concentration of PHGH in the melt grafting reactions.     

Influence of different functionality degree grafting monomers on polypropylene melt radical branching reaction and their structure

Long‐chain–branched polypropylene (LCBPP) is one of polypropylenes (PPs) with high melt strength and good melt elasticity. Recently, due to its outstanding properties, LCBPP have been attracted increasingly attention in the field of development and characterization by the researchers all over the world. In this study, LCBPP was prepared by the melt radical branching reaction in a torque rheometer. The influences of various acrylate monomers with different functionality degrees on the structure and melt performance of PP products were investigated. The results indicated that grafting monomers with different functionality degrees made diverse influences on the branching density and branching chain length of branching PP products. With the increase of the functionality degree of grafting monomers, the branching level of PP products increased gradually and the “multiplicity” of branches became increasingly obvious. Besides, a higher reactivity of pentaerythritol triacrylate with hydroxyl than the similar molecular structured pentaerythritol tetraacrylate was confirmed. Furthermore, due to the high reactivity of dipentaerythritol penta(hexa)acrylate, branching and crosslinking reaction occurred simultaneously during the reaction process. As a result, the gel content increased and finally formed highly star branching structures with a shape of “dense and short.”     

Long chain branching on linear polypropylene by solid state reactions

A method was developed for the long chain branching (LCB) of isotactic polypropylene (iPP) via modification in the solid state. PP long chains have been linked as branches to the original linear iPP chains using solid state reactions in the presence of a free radical initiator and a multifunctional monomer (co-agent). The modified samples of branched iPP were characterised by gel permeation chromatography (GPC) and rheological measurements. Several methods were applied in order to estimate indirectly the extent of branching. A ranking was made of the co-agents according to their ability in inducing LCB as opposed to cross-linking and degradation of iPP. The furfuryl sulphide (FS) showed the highest efficiency for the branching reaction, while the divinylbenzene (DVB) is not suitable for branching.     

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

The crystallization behavior of long‐chain branched (LCB) polypropylene (PP) in the supercritical carbon dioxide (scCO₂) atmosphere was investigated to show the influences of LCB and CO₂ on the formation of γ‐crystal. The crystallization experiments were performed in CO₂ atmosphere with the pressure from 1.3 to 10.4 MPa and temperature between 90 and 130 °C. The effects of LCB level, CO₂ pressure, and crystallization temperature on the content of γ‐crystal were investigated. The results showed that the influence of LCB on the formation of γ‐crystal was obvious when PP was crystallized in CO₂. The content of γ‐crystal increased with LCB level and reached a maximum of 88.2%. It could be explained that, as LCB increased the chainfolding energy of PP molecular chain and hindered it from folding back into crystal lamella, which made the formation of γ‐crystal easier. However, CO₂ was the key factor in the formation of γ‐crystal, and the influence of CO₂ on γ‐crystal was much significant than that of LCB. It was believed that the increase of free volume after dissolving of CO₂ in PP was helpful in the formation of γ‐crystal. It was found that the content of γ‐crystal increased almost linearly with CO₂ pressure (CO₂ content), and the contribution of CO₂ to γ‐crystal increased with pressure, while that of LCB increased with temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 441–451, 2008     

Simulation of gel permeation chromatography measurement for long chain branched metallocene polyethylene

The research about the polymerization reaction mechanism of long chain branched polymer provides a method to simulate the generation of LCB mPE (long chain branched metallocene polyethylene).^[1-3] In this work, after simulating the generation of one million LCB mPE molecules, we calculate the sizes (i.e. radii of gyration) of molecules in good solvents to obtain the molecular size distributions. Then we simulate the fractionation in GPC (gel permeation chromatography) measurement and the different GPC detector responses to obtain simulated GPC MWDs (molecular weight distributions). The simulated MWDs are compared to the real GPC results provided by the Dow Chemical Company.     

Mathematical Modeling of the Long‐Chain Branch Structure of Polyolefins Made with Two Metallocene Catalysts: An Algebraic Solution

A mathematical model was developed to describe the populations of polymer chains containing different numbers of long‐chain branches (LCBs) made with a combination of two single‐site catalysts. One of the catalysts produces only linear chains (linear‐catalyst) and the other produces linear and long‐branched chains (LCB‐catalyst). The model shows that when the selectivity for macromer formation of the linear‐catalyst is the same as that of the LCB‐catalyst, it is not possible to maximize the number of LCB per chain, even though the number of LCB per 1 000 carbon atoms (C) can be maximized. On the other hand, if the selectivity for macromer formation of the linear‐catalyst is higher than that of the LCB‐catalyst, both LCB/1 000 C and LCB/chain pass through maxima when varying the fraction of the linear‐catalyst in the reactor. More importantly, polymer populations with different numbers of LCB per chain will reach their maximum values at different ratios of linear‐catalyst to LCB‐catalyst, thus permitting the maximization of individual polymer populations in the mixture.     

Obtaining aliphatic branched polycarbonates via simple copolymerization of trimethylene carbonate with cyclic carbonate containing pendant ester groups

Different possibilities for obtaining branched, functional carbonate copolymers are presented in this study. Copolymers were synthesized according to the ring‐opening polymerization (ROP) of the cyclic carbonate monomers, containing pendant ester groups. As an example, we chose copolymerization of ethyl 5‐methyl‐2‐oxo‐1,3‐dioxane‐5‐carboxylate (MTC‐Et) with trimethylene carbonate (TMC), using zinc (II) and lanthanum (III) acetylacetonates as ROP initiators. The transesterification processes of ester groups in pendant, short chains, appearing during conducted copolymerization, led to the establishment of two different fractions: first‐branched and high molecular weight fraction and second‐linear and low molecular weight. The content of this high‐molecular‐weight fraction increased with both: the amount of MTC‐Et in started reaction mixture and the time of conducted copolymerization. Reactivity constants in studied reaction were determined. It was possible to obtain the copolymer fraction (ca. 30%) with molecular weight of up to a million g/mol, with a highly branched chain microstructure using lanthanum (III) acetylacetonate as initiator. Conclusions were based on detailed NMR analysis, determining microstructure of the copolymer chains and additionally on GPC and DSC measurement. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 808–819     

Investigation of polypropylene degradation during melt processing using a profluorescent nitroxide probe: A laboratory-scale study

Degradation of polypropylene (PP) during melt processing was studied using a novel profluorescence technique. The profluorescent nitroxide probe, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was added to PP during melt processing to act as a sensor for carbon-centred radicals. Trapping of carbon-centred radicals, formed during degradation of PP, led to an increase in fluorescence emission from TMDBIO adducts. Through analysis of viscosity changes during processing cumulative chain scission degradation was estimated. At processing temperatures of 210 °C or below, fluorescence emission from TMDBIO adducts could be correlated with cumulative chain scissions when the number of chain scissions was small. At higher temperatures, a correlation was not observed most probably due to radical-trap instability rather than decomposition of the TMDBIO. Thus, TMDBIO may be used as a profluorescent sensor for degradation of PP during melt processing when the processing temperature is 210 °C or below.     

Effect of branching and molecular weight on the viscoelastic properties of poly(butyl acrylate)

A series of poly(butyl acrylate) samples were prepared by emulsion polymerization with a range of molecular weights and degrees of chain branching. Characterization was performed with NMR (giving the fraction of branching, ranging from approximately 0 to 7%), gel permeation chromatography, viscometry, and determination of the gel fraction. The dynamic mechanical response, that is, the frequency dependence of the storage and loss moduli G′(ω) and G″(ω) was measured from 0.02 to 200 Hz. The occurrence of a significant insoluble fraction in the sample meant that full characterization of the molecular weight distribution was not possible, and so an unambiguous separation of the dependencies of the mechanical response on the degree of long‐chain branching (LCB) and short‐chain branching (SCB) and the molecular weight could not be made; however, trends dependent on the molecular weight alone were insufficient to model the results. At high frequencies, all trends in G′(ω) and G″(ω) could be ascribed to molecular weight dependencies; at low frequencies, the effects of both the molecular weight and total degree of branching could be inferred, with more highly branched samples showing lower storage and loss moduli. Although the relative amounts of SCB and LCB could not be determined, no dynamic features attributable to LCB were observed. The low‐frequency trends could be semiquantitatively fitted with reptation and retraction theory if it was assumed that an increased degree of SCB led to an increased tube size. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3335–3349, 2002     

Challenges in polypropylene by chemical modification

Radical reactions of i-PP are a well known technical process for the chemical degradation to increase the flowability of the i-PP melt. By decreasing the temperature and increasing the life time of the PP-radicals, a process to synthesize long chain branched i-PP, was developed. The long chain branched-i-PP allows to introduce the i-PP in processing technologies as blow moulding film technology or foaming technology. The mechanism will be discussed. The radical grafting of Polypropylene (PP) becomes more important for the developing of PP-alloys with extended properties. Methylmethacrylate and styrene were polymerized and grafted in PP at low temperatures in solid state. Grafting and polymerizing in solid state means solving the monomers in the PP-powder directly from the reactor without contacting with oxygen (air). The reaction is started by the thermal decomposition of a peroxide. The reactivity of the primary radicals from the peroxide and the transfer reactions of the polymer radical of the PMMA or PS influences the amount of the grafted polymer. The solubility of the monomers and the peroxide in the amorphous i-PP-phase was measured. The grafting yield and the dispersity of the second polymer depends on the solubility and dispersity of peroxide and monomers in the PP-powder particles.     

4种低密度聚乙烯树脂的链结构及其性能

选用4种商品化的具有不同熔体流动速率的低密度聚乙烯(LDPE),利用高温凝胶渗透色谱仪(HT-GPC)、碳核磁共振谱仪(¹³C NMR)、差示扫描量热仪(DSC)和流变仪研究其链结构特点及其流变性能。 按照相对分子质量的差异分成两组,D-1和Q-1,D-3和Y-1,每组的两个样品具有相近的平均相对分子质量。 ¹³C NMR的结果表明,4种LDPE都既含有短链支化又含有长链支化,且短链支化含量均高于长链支化含量;而短链支化中丁基含量最多。 连续自成核退火热分级(SSA)结果表明,树脂中均含有不同长度的可结晶的亚甲基序列,即每种树脂分子链内的短链支化分布不均匀。 探讨了相对分子质量及其分布、亚甲基序列长度及其分布、支化含量、结晶度等因素对树脂熔融行为、流变行为和薄膜力学性能的影响,发现Q-1的低相对分子质量尾端和Y-1的长链支化含量均影响熔体流动速率,平均亚甲基序列长度决定熔融峰的位置,结晶度直接影响薄膜的力学性能。 基于上述结果,建立结构与性能的关联。     

高熔体强度聚丙烯的制备与表征*

高熔体强度聚丙烯（HMSPP）相比于普通聚丙烯具有优越的综合性能,其良好的加工性能使其具有广阔的应用前景,并成为许多国家近年来竞相开发的热点。本文从HMSPP的结构特点、制备方法、性能测试等方面总结了近年来的研究进展,着重介绍了反应器合成长链支化型高熔体强度聚丙烯（LCB-HMSPP）的方法以及其结构表征,并在总结国内外现状的基础上展望了高熔体强度聚丙烯的发展前景。     

Long‐chain‐branched polyethene by the copolymerization of ethene and nonconjugated α,ω‐dienes

Ethene was copolymerized (1) with 1,5‐hexadiene with rac‐ethylenebis(indenyl)zirconium dichloride/methylaluminoxane (MAO) used as a catalyst and (2) with 1,7‐octadiene with bis(n‐butylcyclopentadienyl)zirconium dichloride/MAO and rac‐ethylenebis(indenyl)hafnium dichloride (Et[Ind]₂HfCl₂)/MAO used as catalysts at 80 °C in toluene. The copolymer microstructure and the influence of diene incorporation on the rheological properties were examined. Ethene and 1,5‐hexadiene formed a copolymer in which a major fraction of the 1,5‐hexadiene was incorporated into rings and a small fraction formed 1‐butenyl branches. The copolymerization of ethene with 1,7‐octadiene resulted in a higher selectivity toward branch formation. Some of the branches formed long‐chain‐branching (LCB) structures. The ring formation selectivity increased with decreasing ethene concentration in the polymerization reactor. Melt rheological properties of the diene copolymers resembled those of metallocene‐catalyzed LCB homopolyethenes and depended on the vinyl content, the catalyst, and the polymerization conditions. At high diene contents, all three catalysts produced crosslinked polyethene. This was especially pronounced with Et[Ind]₂HfCl₂, where only 0.2 mol % 1,7‐octadiene in the copolymer was required to achieve significantly modified rheological properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3805–3817, 2001     

多官能团单体对马来酸酐接枝聚丙烯流变行为的影响

研究了多官能团辅助单体偏苯三酸三丙烯酯(TATM)对马来酸酐接枝聚丙烯的熔体流动速率(MFR)和流变行为的影响.当以不含抗氧剂的聚丙烯粉料为原料时,TATM的加入对稳定接枝产物MFR的效果并不理想.但将聚丙烯粉料添加抗氧剂并造粒后,TATM的加入则可有效稳定体系的MFR,各种流变曲线显示出接枝产物的熔体弹性明显提高,说明有枝化或交联结构出现.针对PP粉料和粒料间接枝产物流变行为存在的重大差异,结合熔体自由基接枝反应的机理进行了解释.TATM能够起到稳定MFR的效果是由于其提高了接枝物的熔体弹性,从而抵消了聚丙烯熔体接枝反应中所不可避免的β断链所造成的剪切黏度下降.聚丙烯粉料和粒料中抗氧剂的差异对聚丙烯的加工降解有严重影响,造成了TATM在不同聚丙烯体系中效果的差异.     

Branched structure formation in free radical polymerization of vinyl acetate

The branched structure formation during free radical polymerization of vinyl acetate is investigated in detail by application of the computer simulations on the basis of the Monte Carlo sampling technique. Simulations are made for the whole molecular weight distribution (MWD), the MWDs for polymer molecules containing 0, 1, 2, 3, etc., branch points, the branching density as functions of both size and the number of branch points, the spatial distribution of the branched chains, etc. It was found that the effect of polyradicals on the formed MWD could be neglected for batch polymerizations of the present reaction system. A large number of relatively small branch chains are formed due to both chain transfer to polymer (CTP) and the terminal double-bond polymerization (TDBP). The radius of gyration at a Θ state is found to agree satisfactorily with the Zimm-Stockmayer equation for random branching in spite of the heterogeneous branched structure formed in the polymerization. The present investigation reveals important characteristics of the complex molecular structure formation during free radical polymerization that involves both CTP and TDBP. © 1996 John Wiley & Sons, Inc.     

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

β-nucleated long chain branched (LCB) polypropylene random copolymers (PPRs) were prepared via reactive extrusion by introducing β nucleating agent (NA), dicumyl peroxide (DCP) and various contents of 1, 6-Hexanediol diacrylate (HDDA) into PPR. Results of Fourier infrared spectroscopy (FTIR) and the rheological properties demonstrate the existence of LCB polypropylene. Mechanical properties including impact strength, tensile strength and elongation at break were studied. The crystal structure, morphology and crystallization behavior were investigated via wide-angle X-ray diffraction (WAXD), polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Non-isothermal crystallization kinetics using the Jeziorny method was also studied. The results show an increased LCB degree with increasing HDDA amount. For the sample with a moderate LCB level, synergistic toughening effect of NA and LCB is achieved without deterioration of yield strength or elongation at break, partially because of its higher β-phase content and much smaller spherulite size. It exhibits the minimum values of T_c and Z_c1, and the maximum values of t_1/2 and n₁ in the primary stage of crystallization, regardless of the cooling rate, indicating a slower crystallization rate and more complicated nucleation and crystal growth model.