茂金属聚乙烯的非等温结晶行为及其动力学研究

为探索分子量和支链含量对聚乙烯非等温结晶过程的影响,选用3组样品:(1)不同分子量的无支链线形聚乙烯;(2)低分子量的支链含量不同的试样;(3)高分子量的支链含量不同的试样.用DSC研究了这3组样品的非等温结晶动力学.结果表明:(1)与支链含量相比,分子量大小对结晶的影响是次要的,但高分子量样品的结晶度比低分子量样品低;(2)支链对聚乙烯的非等温结晶有重要影响,在支化聚乙烯中起决定作用;(3)无论是高分子量试样还是低分子量试样,支化含量增加,聚乙烯的结晶温度、结晶度、结晶动力学以及晶体的熔点等显著降低.     

Thermal and structural investigation of random ethylene/1-hexene copolymers with high 1-hexene content

Random ethylene/1-hexene copolymers with the 1-hexene content in the range from 2 to 28 mol% were produced with a novel post-metallocene catalyst and analyzed by three techniques, FTIR, ¹³C NMR, and DSC. The 1-hexene content and the sequence distribution in the copolymers were determined by means of FTIR-M and ¹³C NMR. The crystallization behavior of the copolymers was studied by DSC under dynamic and isothermal conditions; the Avrami model was used to analyze the crystallization kinetics. It was found that both the 1-hexene content and the crystallization temperature affect the relative crystallinity. The bulk crystallization rate decreases with the 1-hexene content and reduces exponentially with an increase of T _c. The melting behavior of isothermally crystallized samples was also investigated and it was found that the melting temperatures of the copolymers under equilibrium conditions were related to the composition.     

Morphology and properties of low-density (branched) polyethylene

The different types of morphology that can be developed in a large number of low-density (branched) polyethylene whole polymers, as well as in a series of fractions, have been studied for two different extreme crystallization modes. Concomitantly, thermodynamic properties of the same samples have also been determined. After isothermal crystallization at elevated temperatures, spherulitic structures are found in all the whole polymer samples. On the other hand, after rapid crystallization a variety of different types of supermolecular structures are observed which are shown to depend systematically on the concentration of side-chain branches and the relative proportion of high molecular weight species in the sample. This temperature dependence of the morphological forms is opposite to that previously reported for linear polyethylene. The studies with the fractions show that the individual species are not the cause of this behavior; rather, the total composition is the important factor. The thermodynamic properties are also quite different from those of linear polyethylene in showing virtually no molecular weight dependence and being governed primarily by the concentration of short-chain branches. The degrees of crystallinity as determined from density and enthalpy of fusion measurements do not vary much with the two extreme crystallization conditions employed, are not sensitive to the morphology, and differ from one another, even when well-developed spherulites are formed. A major influence of the branching concentration on these properties is clearly indicated.     

Characterization of linear low-density polyethylene: Cross-fractionation according to copolymer composition and molecular weight

The structure of ethylene copolymers modified by α-olefins has become an area of intense investigation since the successful commercialization of so-called linear low-density polyethylene (LLDPE) resins. The molecular structure of a series of typical commercial LLDPE copolymers was investigated and compared to LDPE and HDPE. The commercial LLDPE resins studied contained about 7% by weight of butene-1. The resins were fractionated according to short-chain branching content by a technique called temperature rising elution fractionation. Size exclusion chromatography, x-ray diffraction, ¹³C nuclear magnetic resonance, intrinsic viscosity, and differential scanning calorimetry were used to fully characterize the whole polymers as well as fractions of a selected LLDPE resin. A broad set of data was assembled in this work to investigate the short-chain branching, long-chain branching, and the molecular-weight distribution of these commercial resins. The melting behavior of the LLDPE resins was found to be strikingly different from that of LDPE and HDPE. The broad and multimodal melting envelope of the LLDPE resins was found to be due to a broad and multimodal short-chain branching distribution. No significant long-chain branching was found in the LLDPE resins. The short-chain branching was found to decrease with the increase of molecular weight in a typical commercial LLDPE resin. The unique physical properties of these resins are certainly strongly controlled by the expression of the distinctive heterogeneous comonomer incorporation in the solid-state morphological structure. The physical and mechanical properties of these materials should be ultimately understandable on the basis of the unique morphology which results from the extremely heterogeneous incorporation of modifying α-olefin in these commercial LLDPE resins.     

ANALYSIS OF BRANCHING DISTRIBUTION IN POLYETHYLENES BY DIFFERENTIAL SCANNING CALORIMETRY

Short chain branching has been characterized using thermal fractionation, a stepwise isothermal crystallizationtechnique, followed by a melting analysis scan using differential scanning calorimetry. Short chain branching distributionwas also characterized by a continuous slow cooling crystallization, followed by a melting analysis scan. Four differentpolyethylenes were studied: Ziegler-Natta gas phase, Ziegler-Natta solution, metallocene, constrained-geometry single sitecatalyzed polyethylenes. The branching distribution was calculated from a calibration of branch content with meltingtemperature. The lamellar thickness was calculated based on the thermodynamic melting temperature of each polyethyleneand the surface free energy of the crystal face. The branching distribution and lamellar thickness distribution were used tocalculate weight average branch content, mean lamellar thickness, and a branch dispersity index. The results for the branchcontent were in good agreement with the known comonomer content of the polyethylenes. A limitation was that high branchcontent polyethylenes did not reach their potential crystallization at ambient temperatures. Cooling to sub-ambient wasnecessary to equilibrate the crystallization, but melting temperature versus branch content was not applicable after cooling tobelow ambient because the calibration data were not performed in this way.     

Fractionation of ethylene/1‐pentene copolymers using a combination of SEC‐FTIR and SEC‐HPer DSC

The short chain branching distribution (SCBD) and thermal properties of ethylene/1‐pentene copolymers were studied using SEC‐FTIR and SEC‐HPer DSC. The copolymers, synthesized with Cp₂ZrCl₂/MAO, were fractionated using size exclusion chromatography (SEC). The infrared analysis of the fractions showed that the copolymers had—on average—higher 1‐pentene concentration in the low molecular weight range. Furthermore, the thermal properties of the SEC deposits of these copolymers on a Germanium disc were studied using high performance differential scanning calorimetry (HPer DSC). Single SEC separations were used to accumulate fractions in the microgram range that were directly analyzed with regard to their thermal properties, thus allowing us to study SCBD as well as thermal behavior simultaneously. When these fractions (with masses ranging from 10–80 μg) were analyzed using HPer DSC, good melting and crystallization temperature distributions were obtained, proving that HPer DSC can be used as a complementary method to SEC‐FTIR. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2956–2965, 2007     

Studies of comonomer distributions and molecular segregations in metallocene-prepared polyethylenes by DSC

This paper is devoted to the study of crystallization and melting of two metallocene polyethylenes (m-PEs). A metallocene linear low density polyethylene (m-LLDPE) and a metallocene very low density polyethylene (m-VLDPE) were used consisting of 3.3 mol% butyl and 6 mol% ethyl branches, respectively. Several melt endotherms after stepwise crystallization revealed that the two m-PEs consisted of molecular fractions with different molecular weight and branch distribution. More segregation was observed for the m-VLDPE comparing with m-LLDPE. Using the relationships proposed by Hosoda, the short chain branching distribution (SCBD) and the average methylene groups in the lamella thickness were also calculated for the two polymers. These values were compared with the values obtained from theory of rubber elasticity. There was a very good correlation between the data.     

窄分子量分布茂金属短链支化聚乙烯结晶动力学

本工作用DSC方法对三种不同支化度的茂金属短链支化聚乙烯的等温、非等温结晶行为进行了研究 .样品为乙烯和己烯 1的共聚物 ,短支链主要为正丁基 ,分子量Mw =2 0 ,0 0 0 ,Mw/Mn<1 15 ,支化度 (每10 0 0C中CH3 数目 )分别为 1 6、10 4、40 .实验结果表明 ,茂金属短链支化聚乙烯结晶方式Ⅰ Ⅱ转变温度随支化度增加而降低 ,分别为 119 8℃、115 9℃、113 3℃ ;同时支链的存在降低了二次成核速率 ,增大了方式Ⅰ的结晶范围 ,总的结晶速度随支化度增大而减小     

Crystallization kinetics and melting behavior of metallocene short‐chain branched polyethylene fractions

Metallocene polyethylene (mPE) fractions are recognized as being more homogeneous with respect to short‐chain branch (SCB) distribution as compared with unfractionated mPEs. Differential scanning calorimetry and polarized optical microscopy (POM) were used to study the influences of SCB content on the crystallization kinetics, melting behavior, and crystal morphology of four butyl‐branched mPE fractions. The parent mPE of the studied fractions was also investigated for comparative purposes. mPE fractions showed a much simpler crystallization behavior as compared with their parent mPE during the cooling experiments. The Ozawa equation was successfully used to analyze the nonisothermal crystallization kinetics of the fractions. The Ozawa exponent n decreased from about 3.5 to 2 as the temperature declined for each fraction, indicating the crystal‐growth geometry changed from three‐dimensional to two‐dimensional. For isothermal crystallization, the fraction with a lesser SCB content exhibited a higher crystallization temperature (T_c) window. The results from the Avrami equation analysis showed the exponent n values were around 3 (with minor variation), which implied that the crystal‐growth geometry is pseudo‐three‐dimensional. Both of the activation energies for nonisothermal and isothermal crystallization were determined for each fraction with Kissinger and Arrhenius‐type equations, respectively. Double melting peaks were observed for both nonisothermally or isothermally crystallized specimens. The high‐melting peak was confirmed induced via the annealing effect during heating scans. The Hoffman–Weeks plot was inapplicable in obtaining the equilibrium melting temperature (T_m°) for each fraction. The relationship between T_c and T_m for the fractions is approximately T_m = T_c (°C) + 8.3. The POM results indicated that the crystals of parent or fractions formed under cooling conditions did not exhibit the typical spherulitic morphology as a result of the high SCB content. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 325–337, 2002     

流化床聚合反应器两种操作模式下乙烯共聚物的链结构与力学性能

采用制备型升温淋洗分级方法,对流化床聚合反应器在持液操作模式和冷凝态操作模式下生产的A、B两种乙烯/1-丁烯/1-己烯三元共聚物进行了分级,并结合多种分析手段对样品及其各级份进行了结构表征,同时测试对比了A、B两种聚乙烯样品的力学性能.结果表明,与冷凝态操作模式生产的聚乙烯样品B相比,持液操作模式下生产的聚乙烯样品A的拉伸屈服强度、拉伸断裂强度、断裂伸长率、冲击强度和雾度都比样品B优异.样品A的低温淋洗级份相对含量低于样品B,而其高温淋洗级份相对含量高于样品B;样品A低温淋洗级份的分子量略低于样品B,而其高温淋洗级份的分子量高于样品B;样品A的薄片晶含量和厚片晶含量都比样品B多,同时样品A的片晶厚度分布比样品B宽;样品A的总支化度以及每个级份的支化度都比样品B高,且样品A的支链在分子链间的分布比样品B宽,即样品A的支链比样品B的支链更倾向于生长在高分子量部分.通过以上表征分析,发现持液操作模式下生产的样品A比冷凝态操作模式下生产的样品B的物理使用性能更加优异,适合制备高性能的拉伸缠绕膜.     

Supermolecular structure of poly(ethylene oxide) fractions

The crystalline morphology, or supermolecular structure, of poly(ethylene oxide) has been studied as a function of molecular weight and crystallization conditions. Molecular weight fractions, covering the range 6 × 10³ to 1 × 10⁷ are used over the range of accessible temperatures for isothermal crystallization as well as for a large set of controlled nonisothermal crystallization conditions. A morphological map is constructed from these studies and compared with the literature results. Prior reports were primarily confined to low molecular weights, which restricted the generalization of the findings. In the present work, as a consequence of the extended molecular weight range, conditions are established for the systematic development of several different, well-defined, organized super-molecular structures as well as for highly crystalline but disorganized systems. Strong similarities are found between the results for poly(ethylene oxide) and previous reports for linear polyethylene. A generalization for all chain molecules is suggested.     

Effect of molecular structure on rheological and crystallization properties of polyethylenes

This article describes the development of reliable techniques to measure the isothermal crystallization rates (ICR) under quiescent as well as under small amplitude, oscillatory shear conditions. Quiescent crystallization rates were obtained using a differential scanning calorimeter. Those under small amplitude shear were obtained using Rheometrics rheometers. It is shown how a small amount of long-chain branching in high-density polyethylene homopolymer (HDPE) dramatically influences rheological properties and enhances ICR. For these HDPEs, the rate increases with the increase in long-chain branching. The general application of isothermal crystallization studies, however, should be done with great caution. This is because the fundamentals of isothermal crystallization require that it be done on the basis of a fixed undercooling with respect to the equilibrium melting temperature. Such a temperature is ill-defined for the commercial polymers having broad molecular weight distribution (MWD). Nonetheless, a practical procedure is outlined wherein the melting curve of a previously isothermally crystallized sample is used as a substitute for judging the equilibrium melting point and in deciding the selection of a proper crystallization temperature. Even this new procedure may not be applicable for polymers having heterogeneous short-chain branching distribution. © 1996 John Wiley & Sons, Inc.     

Step‐like melting mechanisms of isothermally crystallized isotactic polypropylene

The complex melting behavior of isotactic polypropylene, after isothermal crystallization, was studied within the context of step‐like melting mechanisms which were previously proposed for high temperature polymers. The morphological characteristics of the melting process were also studied as a function of molecular weight, and close similarities were observed with respect to high temperature polymers. Positive birefringence crystals of low molecular weight samples developed double melting behavior in three steps. The first melting step was assigned to continuous melting of secondary crosshatch reversing lamellae, together with recrystallization of the remaining isothermal crystals. In the second melting step (first melting endotherm), crystals tended to lose their original coarse negative birefringence due to melting of secondary reversing branching. This effect rendered new, finer texture, but still negative birefringence crystals. In the third melting step (second melting endotherm), there was a combination of melting of two crystal populations, one consisting of the remaining fraction of reversing primary crystals, and the other consisting of nonreversing primary crystals. A crosshatch secondary branching model was therefore proposed to explain the overall results. Mixed birefringence spherulites of high molecular weight samples displayed similar, although proportional, behavior under identical crystallization and melting conditions corroborating the proposed melting mechanism. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2188–2200, 2008     

Light scattering studies of mixtures and fractions of linear polyethylene

Previous work on the small-angle light scattering of polyethylene films, to determine the supermolecular structure, has been continued. One of the main efforts has been the study of a binary mixture whose low molecular weight component forms well defined spherulites and whose high molecular weight component yields a poorly defined rod-like morphology. The addition of the high molecular weight fraction causes a progressive deterioration of the initial spherulitic morphology; a relatively small amount of the high molecular weight species causes a major decrease in the spherulitic size. However, there are no indications of any spherulitic structures when the weight fraction of the high molecular weight species is 0.5 or greater. The isothermal crystallization of a fraction M = 6.6 × 10⁵ was also studied. Spherulites were formed at low crystallization temperatures while at the higher crystallization temperatures the morphology became nondistinct. Preliminary studies with solvents indicate that high molecular fractions, which do not form spherulites when crystallized in the pure state, do so when crystallized from highly swollen solutions.     

The triple melting behavior of poly(ethylene terephthalate): Molecular weight effects

The melting behavior of isothermally crystallized PET has been studied using linear heating in a differential scanning calorimeter (DSC). Variables such as crystallization temperature, crystallization time, heating rate, and average molecular weight are the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET is a function of programmable DSC variables such as crystallization temperature, crystallization time, and heating rate. However, in testing the hypothesis that there was a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC-independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involves secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1757–1774, 1997     

STRUCTURAL CHARACTERIZATION OF POLYETHYLENES BY DSC ANALYSIS AFTER CRYSTALLIZATION SEGREGATION*

The molecular structure of polyethylene (PE) samples with various comonomers including propylene, 1-buteneand 1-hexene was investigated by DSC and ~(13)C-NMR techniques. The density of the samples varies from 0.948 g/cm~3 to0.917 g/cm~3, and the molecular weight determined by the GPC method is in the range of 1～2×10~5. The branch point contentof the samples was determined by ~(13)C-NMR measurements and was found to be less than 20 per 1000 C atoms along themain chain. Crystallization segregation DSC technique (CSDSC) was used to characterize the branch point distribution or thesegment length distribution of PEs. The crystallization segregation was performed in a successive annealing process atdecreasing temperatures. The interval of two successive annealing temperatures was 6 K, and the time length of eachannealing step was 2.5 h. The CSDSC results clearly indicate that all the PE samples used, including some metallocene PEs,more or less exhibit their non-uniformity in segment length distribution, and bimodal or multimodal CSDSC curves wereusually observed. For quantitative characterization of the CSDSC curves and the segment length distribution two parameters,the average melting point, T_(mAV), and the root-mean-square deviation of melting temperature, (ΔT_(mAV)~2)~(1/2), were proposed.T_(mAV) is corresponding to the average segment length due to branching and (ΔT_(mAV)~2)~(1/2) gives information about the width ofthe segment length distribution. Experimental results show that both the degree of average melting temperature depressionand the width of the distribution seem to increase with increasing the branching content and are dependent on the type ofcomonomers. Very good reproducibility and additivity of the CSDSC method were evidenced experimentally. It wasconcluded that the CSDSC technique is a sensitive and convenient method for characterizing the segment length distribution of branched polyethylenes and will be of great interest in structure-property relationship studies of crystalline polymers.     

DSC analysis of isothermal melt-crystallization, glass transition and melting behavior of poly(l-lactide) with different molecular weights

Isothermal melt-crystallization, glass transition and melting behavior of poly(l-lactide) (PLLA) with different molecular weights were investigated by using differential scanning calorimetry. Analysis by Avrami equation showed that crystallization was initiated by heterogeneous nucleation, followed by 3-dimensional growth. The maximum reciprocal half-time of crystallization (1/t_1/2) was detected at 105 °C. Double endothermic peaks were observed around the glass transition for PLLA with intermediate crystallinities, indicating the coexistence of bulk-like and confined amorphous regions. Double-melting behavior was analyzed and combined with the equilibrium melting temperature evaluation by non-linear Hoffman-Weeks extrapolation, from which a value of 207.6 °C was deduced for PLLA of infinite molecular weight. Lauritzen-Hoffman theory was employed to analyze the crystallization kinetics. Regime II-III transition was found to occur at 120 °C for PLLA of lower molecular weight. The crystal morphology was also examined by scanning electron microscopy through chemical etching method.     

Defects distribution of metallocene and MgCl2-supported Ziegler-Natta isotactic poly(propylenes) as revealed by fractionation and crystallization behaviors

The inter and intramolecular distribution of defects of poly(propylenes) of the Ziegler-Natta (ZN) and metallocene (M) types, with matched molar masses and overall defect concentrations, are inferred from the crystallization and polymorphic behavior of their narrow molecular mass fractions. The fractions obtained from the M-iPP display a range in molecular masses but the same concentration of defects and provide direct evidence of the uniform intermolecular defect distribution and the “single site” nature of the catalyst. The stereodefects of the ZN-iPP fractions are more concentrated in the low molecular mass fractions, corroborating a broad interchain distribution of the nonisotactic content. In addition, the invariance of the linear growth rates among the ZN fractions and very low contents of the gamma polymorph, developed even by the most defected ZN fraction, are consistent with a stereo blocky intramolecular distribution of defects in the ZN-iPP molecules. In contrast to the linear growth rates, which are sensitive to the defect microstructure, the overall crystallization rates correlate with nucleation density and not necessarily with the iPP chain microstructure.     

Effect of molecular weight on the crystalline structure of polytetrafluoroethylene as-polymerized

Melting and crystallization behavior of polytetrafluoroethylene as polymerized in emulsion and suspension is shown to depend on molecular weight. DSC heating curves for virgin PTFE with low molecular weight below 3 × 10⁵ have a single peak, whereas curves for higher molecular weight samples have double peaks. With increasing heating rate the areas of higher melting peaks become larger than the lower melting peaks. The morphology of polymer exhibiting double melting peaks is mainly folded ribbons or granular particles. The phenomenon of double melting is explained on the basis of two different crystalline states which correspond to the “fold regions” and the “linear segments” in a folded ribbon. The melting temperature of virgin PTFE is almost constant at ca. 330°C for molecular weights below 1 × 10⁶, and rises as the molecular weight increases above 1 × 10⁶. The heat of melting of virgin PTFE is nearly independent of molecular weight. On the basis of these results, we propose a model for melting and crystallization of low and high molecular weight PTFE and for the crystal structure.     

Calibration curve establishment and fractionation temperature selection of polyethylene for preparative temperature rising elution fractionation

A series of copolymers of ethylene with 1-hexene synthesized using a metallocene catalyst are selected and mixed. The blend is fractionated via preparative temperature rising elution fractionation(P-TREF). All fractions are characterized via high-temperature gel permeation chromatography(GPC), 13 C nuclear magnetic resonance spectroscopy(13C-NMR), and differential scanning calorimetry(DSC). The changes in the DSC melting peak temperatures of the fractions from P-TREF as a function of elution temperature are almost linear, thereby providing a reference through which the elution temperature of TREF experiments could be selected. Moreover, the standard calibration curve(ethylene/1-hexene) of P-TREF is established, which relates to the degree of short-chain branching of the fractions. The standard calibration curve of P-TREF is beneficial to study on the complicated branching structure of polyethylene. A convenient method for selecting the fractionation temperature for TREF experiments is elaborated. The polyethylene sample is fractionated via successive self-nucleation and annealing(SSA) thermal fractionation. A multiple-melting endotherm is obtained through the final DSC heating scan for the sample after SSA thermal fractionation. A series of fractionation temperatures are then selected through the relationship between the DSC melting peak temperature and TREF elution temperature.