Relaxation time spectra of linear and crosslinked polymers

In this work, an attempt has been made to define a unique relaxation time spectrum for different types of polymeric materials. Empirical models for the relaxation spectrum, proposed for linear flexible polymers in the literature, have been used. A systematic determination of the parameters defining the relaxation time spectrum has been made from dynamic mechanical data. It has been shown that the resulted expression for the relaxation modulus could then be used to calculate the stress response of the materials to various deformations. The strain rate dependence of yield behavior could also be predicted. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 679–684, 1998     

Properties of blends of linear and branched polypropylenes in film blowing

The effect of blending a long‐chain branched polypropylene (LCB‐PP) with a linear polypropylene (L‐PP) on the processability and properties of blown films was investigated. The rheological data revealed that blending an LCB‐PP with an L‐PP improved the elongational properties and the bubble stability, but a severe drop in the mechanical strength was observed for the blends. The most deteriorating effect was the reduction in the elongation at break in tensile tests carried out in the transverse direction (TD), where no yielding behaviour was observed for the blends.     

Influence of grafting formulations and extrusion conditions on properties of silane‐grafted polypropylenes

The melt grafting of unsaturated silanes onto powdered polypropylene (PP) in a Haake TW100 twin‐screw extruder and curing in hot water were studied. The influence of grafting formulations and extrusion conditions on the melt flow rates of grafted PP and the gel percentages of crosslinked PP was investigated. The gel percentages of methacryloylpropyltrimethoxysilane (VMMS)‐grafted PP were markedly higher than those of vinyltriethoxysilane (VTES)‐ and vinyltrimethoxysilane (VTMS)‐grafted PP, while significantly less degradation of PP during grafting was observed for VMMS‐grafted PP. When benzoyl peroxide (BPO) was used as an initiator, no degradation of PP during grafting was observed, and the melt flow rates of grafted PP decreased with increasing BPO concentration. In contrast, use of dicumyl peroxide (DCP) as an initiator resulted in severe degradation of PP, and the melt flow rates of grafted PP increased gradually with increasing DCP concentration. BPO resulted in higher gel percentages than those of DCP at a fixed initiator concentration. Introduction of styrene into the grafting system greatly improved the gel percentage of crosslinked PP and reduced the degradation of PP during grafting. The optimum molar ratio of styrene to monomer is at about 1.5:1. Relatively low processing temperatures and high screw speeds are favorable. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1233–1238, 2000     

Rheological behavior of controlled‐rheology polypropylenes obtained by peroxide‐promoted degradation during extrusion: Comparison between homopolymer and copolymer

In the present work, experimental studies of the free‐radical‐initiated molecular weight degradation of polypropylene in a modular self‐wiping corotating twin‐screw extruder are investigated. The control of the molecular weight distribution of polypropylene resins by peroxide degradation is widely used in the polymer industry. It allows one to adjust the viscosity of these resins to the level required for processing applications. The purpose of this work was to characterize the influence of peroxide degradation on the rheological behavior of a polypropylene homopolymer and a block polypropylene/polyethylene copolymer, which includes an addition of a low percentage of polyethylene (around 7%). The homopolymer exhibits a classical behavior: When the peroxide amount is increased, we observe a decrease in the viscosity corresponding to a decreasing molecular weight and a pronounced shift toward more Newtonian behavior. The rheological behavior of the copolymer is influenced by the presence of the polyethylene phase which greatly modifies the viscoelastic properties and increases the viscosity when the polypropylene matrix is highly degraded. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1243–1252, 2001     

反应性挤出聚丙烯接枝偶联马来酸酯的研究

在过氧化物的存在下,聚丙烯(PP)易于降解,因而进行反应性挤出接枝是很困难的。选用偶联马来酸酯(CMAE)为单体,通过反应性挤出加工使PP分子链接枝上极性官能团,从而防止了PP的降解。其产物接枝率较高,外观光滑、无气泡,具有广泛的应用价值。     

Viscoelastic characteristics of chain extended/branched and linear polyethylene terephthalate resins

Two chemically modified chain extended/branched polyethylene terephthalate (PET) resins and one unmodified resin, considered to be linear, were characterized in terms of their melt flow, die swell, and viscoelastic properties. The three resins had reportedly similar nominal intrinsic viscosities but exhibited different viscoelastic behavior. The modified resins had lower melt flow index, higher die swell, higher complex viscosity and higher storage modulus than the unmodified one. The Cole–Cole plots of the resins were independent of temperature, and the data for modified resins formed a group that lay below the data group for the unmodified PET. The distribution of relaxation times was determined. The modified resins had higher relaxation strength, G_i, especially at high relaxation times, λ_i. The mean relaxation times of the chain extended/branched resins were approximately an order of magnitude higher than that of the unmodified resin, implying pronounced elastic character. The modified resins had better foaming characteristics in extrusion foam processing than the unmodified one owing to their elastic nature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1371–1377, 2000     

Conformation transition and crystalline phase variation of long chain branched isotactic polypropylenes (LCB-iPP)

The changes of conformation and crystalline structure of long chain branched isotactic polypropylene (LCB-iPP) under different crystallization temperatures and the effects of their special molecular architecture on the crystallization behavior were investigated by a combination of Fourier transform infrared spectroscopy (FT-IR), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). In these polymers, long chain branching was introduced via in situ polymerization of polypropylene and an asymmetric diene monomer using the metallocene catalyst technology. Through the characterization of the specific IR band variation, it was proved that the conformational orders of helical sequences of LCB-iPP show great changes in different crystallization temperature ranges. In lower crystallization temperature range (100-130 °C), the intensities of all regular helical conformation bands of LCB-iPP increase with the increasing crystallization temperature and the regular helical conformation bands with more monomer units increase faster than that with less monomer units. In higher crystallization temperature range (130-150 °C), the intensities of all regular helical conformation bands of LCB-iPP decrease with the increasing crystallization temperature and the regular helical conformation bands with more monomer units decrease faster than that with less monomer units. The results of WAXD and DSC showed that LCB-iPP crystallizes from the melt as a mixture of α and γ forms. The content of the γ form increases with the increasing crystallization temperature, reaches a maximum value at 130 °C, and then decreases with a further increase of the temperature. At the same time, the crystallization of γ form is favored by the presence of the LCB structure of iPP. Moreover, the transitional temperatures of different helical conformations and crystallization structures of LCB-iPP show obvious correlations.     

Rheological properties of blends of linear and long‐chain branched polypropylenes

Blends of a long‐chain branched polypropylene (LCB‐PP) and a linear polypropylene (L‐PP) were prepared using a twin‐screw extruder. Linear viscoelastic properties such as complex viscosity, storage modulus, and weighted relaxation spectrum were determined as functions of LCB‐PP content. Shear data obtained from commercial rheometers as well as from a slit‐die rheometer were used to verify the Cox‐Merz relation for the neat components as well as for a blend. Elongational properties were obtained using a Sentmanat Extensional Rheometer (SER) unit mounted on an Advanced Rheometric Expansion System (ARES) rheometer and the converging die. A significant strain hardening was observed for the neat LCB‐PP as well as for all the blends, but the strain hardening decreased with increasing strain rate. The apparent steady elongational viscosity values evaluated using the converging die were observed to be comparable at high deformation rates to those obtained from the SER unit, but the differences increased as the strain rate decreased. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

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     

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     

Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes

Blends of a long-chain branched polypropylene (LCB-PP) and four linear polypropylenes (L-PP) having different molecular weights were prepared using a twin screw extruder. The linear viscoelastic properties suggested the immiscibility of the high molecular weight L-PP based blends, and the miscibility of the low molecular weight L-PP based blends. In addition, the Palierne emulsion model showed good predictions of the linear viscoelastic properties for both miscible and immiscible PP blends. However, as expected, the low-frequency results showed a clear effect of the interfacial tension on the elastic modulus of the blends for the high molecular weight L-PP based blends. A successful application of time-temperature superposition (TTS) was found for the blends and neat components. Uniaxial elongational properties were obtained using a SER unit mounted on an ARES rheometer. A significant strain hardening was observed for the neat LCB-PP as well as for all the blends. The influence of adding LCB-PP on the crystallinity, crystallization temperature, melting point, and rate of crystallization were studied using differential scanning calorimetry (DSC). It was found that the melting point and degree of crystallinity of the blends first increased by adding up to 20 wt% of the branched component but decreased by further addition. Adding a small amount of LCB-PP caused significant increase of the crystallization temperature while no dramatic changes were observed for blends containing 10 wt% LCB-PP and more. Furthermore, the crystalline morphology during and after crystallization of the various samples was monitored using polarized optical microscopy (POM). Compared to the neat linear polymers, finer and numerous spherulites were observed for the blends and LCB-PP. Dynamic mechanical (DMA) data of the blends and pure components were also analyzed and positive deviations from the Fox equation for the glass transition temperature, T_g, were observed for the blends.     

The rheological and physical properties of linear and branched polypropylene blends

The rheological, thermal, and mechanical properties of blends consisting of a linear high melt flow rate polypropylene (PP) and two branched PPs are characterized in detail. Blends containing branched PPs display evidence of miscibility in the melt state and exhibit high melt elasticity together with significant strain hardening in extensional deformation while retaining good flow properties. Out of the two blend systems examined the blends containing linear and branched PPs with similar melt flow rates have better mechanical properties, higher crystallization temperatures, and higher crystallinities. POLYM. ENG. SCI., 47:1133–1140, 2007. © 2007 Society of Plastics Engineers     

Grafting of maleic anhydride onto polypropylene by reactive extrusion

Maleic anhydride grafting onto polypropylene was conducted in a twin‐screw extruder according to an experimental design in which the maleic anhydride and peroxide concentrations were varied. The modified polypropylene was characterized by FTIR spectroscopy, melt‐flow index measurements, size‐exclusion chromatography, differential scanning calorimetry, and nuclear magnetic resonance. The results showed that only the independent variable peroxide concentration influenced the amount of reacted maleic anhydride, whereas the two variables studied influenced the molecular weight of the grafted polypropylene. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2706–2717, 2002     

Study of mechanical and rheological behaviors of linear and branched polycarbonates blends

A study of the mechanical and rheological properties of linear and branched polycarbonates blends is presented. Phase separations of the blends were checked through DSC and SEM, and, subsequently, mechanical and rheological properties were investigated. Phase separations were not observed in the blends. The mechanical properties were examined through tensile, flexural, and impact tests. All the mechanical properties of the blends were relatively independent of the compositions. For study of the rheological properties, melt viscosity, storage and loss moduli, and melt tension of the blends with various compositions were examined at various temperatures. The dependence of the viscosity on the molecular weight was also studied. As the content of branched polycarbonate increases, the dependence of the viscosity on the molecular weight and the shear thinning behavior became more marked. Melt tensions were also increased as the branched polycarbonate content increased in the blends for all tested temperatures. In this study, the blend systems which have same mechanical properties but different rheological properties can be obtained through blending of linear and branched polycarbonates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1814–1824, 2001     

改性聚丙烯反应挤出可控降解研究

利用反应挤出技术，研究了聚丙烯（PP）、PP及改性剂和无规共聚聚丙烯（PPR）在过氧化物作用下在反应挤出机中进行可控降解的规律。试验发现，产物的熔体流动速率（MFR）随过氧化二异丙苯引发剂（DCP）用量的增加而呈成倍增长趋势；封闭反应挤出机排气口更利于可控降解反应；可控降解产物在满足材料力学性能需要的前提下其外观质感得到了显著提高；加入改性剂EVA（乙烯／醋酸乙烯共聚物）或POE（乙烯／辛烯共聚物）后有利于提高产物的韧性和表观质量；PPR在反应挤出机中表现出与纯PP相同的降解规律，但产物的性能在降解后得到了提高。     

Enhancement of graft yield and control of degradation during polypropylene maleation in the presence of polyfunctional monomer

Free‐radical initiated grafting of maleic anhydride (MAH)/polyfunctional acrylate (PFA) multimonomer system onto polypropylene (PP) via reactive extrusion was studied. The effects of PFA and initiator concentration on the grafting reaction were investigated. It was shown that PFA as a comonomer could greatly enhance MAH grafting degree, which increased monotonically as the molar ratio of PFA to MAH increased. The rheology test demonstrated that the viscosity of grafted PP was also promoted as more PFA was used. The formation of branched structure during the grafting process was proved by oscillatory shear rheological analysis. The mechanism of grafting in the presence of PFA was discussed, suggesting PFA had higher reactivity with PP macroradicals than MAH and therefore forming stabilized macroradicals, thus resulting in depression of β‐scission and favoring the formation of branched structure. Higher initiator concentration gave higher MAH grafting degree but more severe degradation. The mechanical properties of the grafted PP were comparable with those of unmodified PP. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

通过反应挤出提高聚丙烯的熔体强度

利用反应挤出技术研究了不同反应物对聚丙烯(PP)熔体强度的影响。考察了不同用量的低密度聚乙烯(LDPE)、乙烯-乙酸乙烯酯共聚物(EVA)、季戊四醇三丙烯酸酯(PETA)、二乙烯基苯(DVB)以及上述物质的混合物在过氧化二异丙苯的引发下对PP熔体强度、熔体流动速率、熔垂的影响。结果表明,LDPE、EVA的加入对产物熔体强度的影响有限,PFTA也只能使其提高1倍左右;而DVB的加入可使产物的熔体强度显著提高,仅加入1％就可使熔体强度提高20倍,熔垂实验也证明了这一点;几种反应物混合使用效果不如单独使用好。     

Effect of long‐chain branches of polypropylene on rheological properties and foam‐extrusion performances

The effect of modifying polypropylene by the addition of long‐chain branches on the rheological properties and performance of foam extrusion was studied. Three polypropylenes, two long‐chain‐branched polypropylenes and a linear polypropylene, were compared in this study. The modification was performed with a reactive‐extrusion process with the addition of a multifunctional monomer and peroxide. The rheological properties were measured with a parallel‐plate and elongational rheometer to characterize the branching degree. The change from a linear structure to a long‐chain‐branched nonlinear structure increased the melt strength and elasticity of polypropylene. Also, there was a significant improvement in the melt tension and sag resistance for branched polypropylenes. Foaming extrusion was performed, and the effect of the process variables on the foam density was analyzed with Taguchi's experimental design method. For this study, an L₁₈(2¹3⁵) orthogonal array was used on six parameters at two or three levels of variation. The considered parameters were the polypropylene type, the blowing agent type, the blowing agent content, the die temperature, the screw speed (rpm), and the capillary die length/diameter ratio. As a result, the most significant factor that influenced the foam density was the degree of long‐chain branching of polypropylene. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1793–1800, 2005     

聚丙烯与二乙烯基苯的熔融反应及其产物性能

使用哈克密炼机和反应挤出机研究了聚丙烯（PP）和二乙烯基苯（DVB）在过氧化二异丙苯（DOP）引发作用下的熔融反应规律。试验表明该反应的时间短,可以在反应挤出机料膛中完成,适合于挤出法生产。对产物的红外光谱研究表明,反应挤出前后物料的分子结构无明显变化;差示扫描量热仪（DSC）等的分析表明,反应挤出产物中生成了部分β晶型PP,使产物的熔体强度、冲击强度和维卡软化点都得到了显著提高;经反应挤出后产物的熔体流动速率明显下降,熔体的表观黏度比PP的有显著提高。     

Comparison of carboxylated and maleated polypropylene as reactive compatibilizers in polypropylene/polyamide‐6,6 blends

Using reactive extrusion, polypropylene is functionalized with maleic anhydride and compared on an equimolar basis to polypropylene that is functionalized with an asymmetric, carboxylic acid containing peroxide. The grafting efficiency for the asymmetric peroxide is double that obtained for the maleic anhydride system. Moreover, the asymmetric peroxide yields a functionalized material with minimal molecular weight degradation and desirable mechanical properties, relative to maleic anhydride‐grafted polypropylene. In compatibilized blends of polypropylene and nylon 6,6, the polypropylene that was functionalized with the asymmetric peroxide is found to be an improved compatibilizer compared to that of maleic anhydride‐grafted polypropylene. The differences in mechanical properties of the two different functionalized polypropylene materials and their respective blends are rationalized on the basis of the grafting efficiency, molecular weight degradation during reactive extrusion, and effect of free functional species on the ability to form graft copolymers in compatibilized blends. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2398–2407, 2001