结晶分级技术在支化聚乙烯研究中的应用

介绍了近年发展起来的几种结晶分级技术及其在支化聚乙烯结构表征及性能研究方面的应用。利用升温淋洗分级技术(TREF),可根据结晶特性的不同将高分子分离成多个分布较窄的级份,通过分别表征各级份的链结构,从而可获得高分子链结构方面较为准确的信息。基于差示扫描量热技术(DSC)发展起来的两类热分级技术,主要包括逐步结晶热分级(SC)和连续自成核退火分级(SSA)技术,虽然不能从物理上对高分子进行分级,但通过选择适当的操作参数,也能得到一系列与升温淋洗分级实验类似的链结构信息,并且具有设备简单、操作方便、样品用量少、耗时短等优点。本文结合我们自己的工作,对各种分级技术的原理、实验操作及应用进行了系统综述,并展望了结晶分级技术发展的某些可能趋势。     

用TREF方法分离聚烯烃共混物

用ＴＲＥＦ方法分离聚烯烃共混物徐君庭，封麟先，杨士林（浙江大学高分子科学与工程学系，杭州，３１００２７）关键词升温淋洗分级，聚烯烃，共混物升温淋洗分级（ＴＲＥＦ）方法是８０年代发展起来的，它按结晶度将聚合物分级［１］，目前主要用于聚烯烃的分级［２，３...     

聚烯烃合金的研究进展(Ⅱ)——分级方法、结构调控与性能

聚烯烃合金是通过釜内聚合制备的多组成的聚烯烃材料。本文分别从聚烯烃合金的分级方法、组分和性能等方面对聚烯烃合金结构与性能进行了评述,重点介绍了溶剂抽提分级法、温度梯度分级法、升温淋洗分级法、等温热分级法和逐步结晶分级法等聚烯烃合金分级方法的原理,特别关注了聚丙烯/橡胶弹性体合金、聚丙烯/聚乙烯合金及聚丙烯/聚丁烯合金的组成与性能。并分析了聚烯烃合金的发展趋势。     

L-丙交酯/ε-己内酯共聚物分子链结构非均匀性的研究

利用升温淋洗分级(TREF)和连续自成核退火热分级(SSA)等方法研究了L-丙交酯(LLA)和ε-己内酯(ε-CL)共聚物(PLC)分子链结构的非均匀性.合成了2种不同摩尔比的PLC共聚物,用TREF的方法,将同一个共聚物分级出3个级份.用核磁共振氢谱(1H-NMR)和凝胶渗透色谱(GPC)等方法对每个级份的分子结构进行了表征.结果表明,随着淋洗温度的升高,共聚物级份中的ε-CL单元的含量明显降低,而且级份的数均分子量增大,分子量分布指数降低.在SSA热分级研究中,发现PLC各级份间存在明显的分子链间和分子链内结构上的差异,这种链结构差异使得各个级份表现出不同的熔融行为.     

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.     

PP/PB-1釜内合金的升温淋洗分级及级份表征

基于反应器颗粒技术制备的聚丙烯(PP)/聚丁烯-1(PB-1)釜内合金,其结构与组成,特别是嵌段共聚物级份的组成、分布、链序列结构等受两段聚合条件的影响较大.本文采用溶剂抽提法和升温淋洗分级法,对中试合成的两种PP/PB-1釜内合金进行分级,采用示差扫描量热仪(DSC)、高温核磁碳谱(13C-NMR)、高温凝胶渗透色谱...     

茂金属聚乙烯的短链支化结构不均匀性研究进展

茂金属支化聚乙烯技术是聚烯烃工业发展史上最重要的技术进展之一,该类产品具有优异的抗穿刺、抗撕裂、抗冲击性质,在薄膜、重包装等领域具有广泛的用途,其力学性能与共聚单体的种类、含量及其在分子链上的分布有密切关系。本文从催化剂活性中心特点和聚合反应工艺条件两方面对聚合物结构的影响出发,阐述了茂金属聚乙烯短链支化结构不均匀性产生的原因,通过核磁共振、升温淋洗分级和热分级等表征支化不均匀性的研究方法,介绍了茂金属支化聚乙烯分子主链上共聚单体的序列结构组成及分布,以及共聚物的结晶性能等方面的差异。     

低密度聚乙烯的升温淋洗分级及链结构研究

采用升温淋洗分级方法对密度相同、熔融指数不同、拉伸强度和断裂伸长率不同、膜的抗冲击强度和抗撕强度有明显差异的2种低密度聚乙烯样品进行了分级,并利用GPC、DSC、SSA、13C-NMR等多种手段对原样和级份进行了链结构表征.由13C-NMR得到样品A和B总的支化含量分别为3.93 mol%,4.82 mol%,都既含有短链支化又含有长链支化;TREF-GPC交叉分级结果可以看出,2个样品的主要分级级份集中在较高温度区域,A样品主要集中于80℃的淋洗温度,分子量集中于3.2×105区域,而B样品主要集中于75℃的淋洗温度,分子量集中于1.8×105区域,样品A具有稍高的分子量且高分子量部分含量较多.由DSC结果得知,样品A的结晶度为34.7%,级份A1~A9的结晶度在26.0%~37.4%;样品B结晶度为34.4%,其级份B1~B9的结晶度在26.2%~34.8%.另外通过热分级结果得出,不管是原样还是各个级份,都具有约10个左右的多重熔融峰,显示出分子链内具有非均匀性,2个样品级份的数均亚甲基序列长度均随着淋洗温度升高而增加,分别为34~86和32~84.此外,讨论了它们的链结构特点以及与性能之间的关系.     

负载型催化剂制备的聚丙烯等规度分布

负载型Ziesler－Natta催化剂中存在许多活性中心［'3，为了解其本质，需对其各自产生的聚合物进行分离，以往采用的溶剂抽提法［'－'j只能将聚合物大致分级．最近，升温淋洗分级法（TREF）已被运用于聚丙烯的分级卜，'，其原理是根据聚合物的结晶度分级D'，影响聚丙烯结晶性的主要因素是等规度，而分子量到达一定程度后其影响较小，故通过TREF分级可得到聚丙烯的等规度分布．TREF法的淋洗温度可控，故分级效果较好．该法在分级前需对样品进行等温结晶处理，以消除抽提法由于样品未必充分结晶而带来的误差．本文用TREF法对不同催化…     

等规聚丙烯/聚丁烯-1釜内合金的结构与性能

高分子材料的组成、 组分分布及链结构与宏观性能紧密相关. 因此, 分析多组分釜内合金材料的链结构特点与性能之间的关系至关重要. 采用升温淋洗分级的方法对两种采用序贯两段聚合原位合成的等规聚丙烯/聚丁烯-1(iPP/iPB)釜内合金在-30 ℃~140 ℃温度范围进行分级, 采用核磁共振波谱仪、 傅里叶变换红外光谱仪、 差示扫描量热仪和凝胶渗透色谱仪等表征了级分的链结构及序列分布、 热行为、 分子量(M_w)及分子量分布(M_w/M_n)等. 结果表明iPP/iPB合金主要由5种级分组成, 高等规聚丁烯(iPB)为主要组分, 同时含有少量的丁烯-丙烯嵌段共聚物(PB-b-PP)和等规聚丙烯(iPP)等. 随淋洗温度升高, PB-b-PP级分中PP嵌段长度逐渐增加, PB嵌段长度逐渐减小; 在相同的淋洗温度, 合金B的嵌段共聚物级分中PP嵌段较长且结晶较完善; 合金B中iPB组分及嵌段共聚物组分含量较高, 使得合金B具有较高的拉伸强度、 弯曲强度、 优异的抗冲击性能、 较高的维卡软化温度及较快的晶型转变速率.     

Elution Temperatures of Polyethylene Copolymers in Temperature Rising Elution Fractionation (TREF): Comparison in Xylene and Trichlorobenzene Diluents

Temperature rising elution fractionation (TREF) became the preferred technique to characterize the short chain branching distribution of polyethylene copolymers. Due to technical limitations, preparative TREF (PTREF) is usually done in xylene, while trichlorobenzene is used in analytical TREF (ATREF). Attempts to correlate the TREF elution temperatures based on data published by different authors erroneously showed higher elution temperatures for xylene than for trichlorobenzene. Our study rectifies this error. The experiments were done in both solvents on the same analytical TREF instrument. For the analyzed polyethylene copolymers, we found that the average elution temperature in xylene is 3.7° ± 1°C lower than in trichlorobenzene.     

Molecular heterogeneity of ethylene‐propylene rubbers: New insights through advanced crystallization‐based and chromatographic techniques

For a long time ethylene‐propylene rubber (EPR) copolymers with high comonomer contents were believed to be amorphous materials with a random copolymer composition. This is not completely correct as has been shown by temperature rising elution fractionation (TREF) combined with differential scanning calorimetry (DSC), crystallization analysis fractionation (CRYSTAF), and high temperature–high‐performance liquid chromatography (HT‐HPLC). When using only conventional crystallization‐based fractionation methods, the comprehensive compositional analysis of EPR copolymers was impossible due to the fact that large fractions of these copolymers do not crystallize under CRYSTAF conditions. In the present work, HT‐HPLC was used for the separation of the EPR copolymers according to their ethylene and propylene distributions along the polymer chains. These investigations showed the existence of long ethylene sequences in the bulk samples which was further confirmed by DSC. The results on the bulk samples prompted us to conduct preparative fractionations of EPR copolymers having varying ethylene contents using TREF. Surprisingly, significant amounts of crystallizing materials were obtained that were analyzed using a multistep protocol. CRYSTAF and DSC analyses of the TREF fractions revealed the presence of components with large crystallizable sequences that had not been detected by the bulk samples analyses. HT‐HPLC provided a comprehensive separation and characterization of both the amorphous and the crystalline TREF fractions. The TREF fractions eluting at higher temperatures showed the presence of ethylene‐rich copolymers and PE homopolymer. In order to obtain additional structural information on the separated fractions, HT‐HPLC was coupled to Fourier transform‐infrared (FT‐IR) spectroscopy. The FT‐IR data confirmed that the TREF fractions were separated according to the ethylene contents of the eluted samples. Preparative TREF analysis together with a combination of various analytical methods proved to be useful tools in understanding the complex molecular composition of these rubber samples. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 863–874     

Characterization of polypropylene�Cpolyethylene blends by temperature rising elution and crystallization analysis fractionation

The introduction of single-site catalysts in the polyolefins industry opens new routes to design resins with improved performance through multicatalyst-multireactor processes. Physical combination of various polyolefin types in a secondary extrusion process is also a common practice to achieve new products with improved properties. The new resins have complex structures, especially in terms of composition distribution, and their characterization is not always an easy task. Techniques like temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF) are currently used to characterize the composition distribution of these resins. It has been shown that certain combinations of polyolefins may result in equivocal results if only TREF or CRYSTAF is used separately for their characterization.     

Use of FT-IR Spectrometry for On-Line Detection in Temperature Rising Elution Fractionation

Summary: Temperature rising elution fractionation (TREF) has become a popular analytical technique that is able to determine the chemical composition distribution (CCD) of an ethylene/α-olefin copolymer. An infrared (IR) detector is commonly used in TREF detection to measure the concentration of the polymer solution exiting the column as a function of elution temperature. The chemical composition of the eluting polymer at a given elution temperature can be predicted from the relationship between comonomer content and TREF elution temperature pre-established through ¹³C nuclear magnetic resonance (NMR) analysis of TREF fractions. In this article, a Fourier transform infrared (FT-IR) spectrometer has been coupled with a TREF instrument to provide a more powerful tool for characterizing complex olefin copolymers. The Partial Least Squares (PLS) technique is used when analyzing the FT-IR spectra of the eluting polymer solutions. The power of on-line FT-IR detection in TREF is demonstrated using a few complex copolymer systems, such as ethylene-octene copolymer, polystyrene grafted ethylene-vinyl acetate copolymer and ethylene-methyl acrylate copolymer.     

Fast Analytical Temperature Rising Elution Fractionation (TREF) of Polypropylenes

Analytical temperature rising elution fractionation (TREF) is a complementary technique of gel permeation chromatography (GPC) for the analysis of polyolefin structure. By connecting a high-temperature GPC with a gas chromatograph (GC) oven it is possible to build a fast analytical TREF, which permits a dramatic reduction in analysis time by directly injecting the polymer solution onto the cold column, as compared with the traditional TREF in which the slow cooling step usually takes more than 40 h. The method was successfully applied on six commercial random and homo polypropylenes for which similar thermograms were obtained for fast- and slow-cooling TREF. The obtained results show subtle differences in the behavior of polypropylene melting in solution as compared with previously analyzed polyethylene. The most important is the difference of 12.6°C between the melting temperatures in the presence of trichlorobenzene and xylene, which is much higher for polypropylene than the 3.7°C measured for polyethylene.     

Polyolefin analysis by single‐step crystallization fractionation

Temperature rising elution fractionation (TREF) fractionates polymer chains with respect to their crystallizability, independently of molecular weight effects. In order to achieve a good fractionation, TREF requires a time‐consuming polymer deposition step over an inert support before the elution step. A single‐step crystallization fractionation method has been developed recently,^1,2 Crystallization Analysis Fractionation (CRYSTAF), in which the chemical composition (or short chain branching) distribution of olefin copolymers can be measured by monitoring on‐line polymer concentration in solution at decreasing temperatures. For the present experimental investigation, a CRYSTAF‐prototype has been assembled and used to fractionate several linear low‐density polyethylene (LLDPE) samples. These results were compared to the ones measured by the commercial CRYSTAF apparatus from Polymer ChAR. Additionally, CRYSTAF results from Polymer ChAR were compared to analytical TREF results. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 539–552, 1999     

Modelling the fractionation process in TREF systems: Thermodynamic simple approach

A simple thermodynamic model, based on an extension of Flory-Huggins theory, is applied to temperature rising elution fractionation (TREF). Dependence of the fractionation process on melting temperature, melting enthalpy, average crystallinity, average crystallizable sequence length, and polymer-solvent interaction parameter is predicted. Results from the model fit experimental TREF data, and correctly predict number-average branch points for TREF fractions. © 1995 John Wiley & Sons, Inc.     

Short Chain Branches Distribution Characterization of Ethylene/1-Hexene Copolymers by Using TREF + 13C-NMR and TREF + SC Methods

Summary: Short chain branches distribution (SCBD) is the key factor for high density polyethylene (HDPE) pipe materials to achieve their excellent performance for long term (50 years) applications. However, the precise SCBD characterization of these HDPE materials with relatively low content of comonomer incorporation still remained as a challenge in this field. In this work, two characterization methods, namely temperature rising elution fractionation (TREF) cross step crystallization (SC) (TREF + SC) and TREF cross ¹³C-NMR (TREF + ¹³C-NMR), have been respectively used to qualitatively and quantitatively investigate the SCBD for two HDPE pipe materials (PE-1 and PE-2 with different long term performances) with small amount of 1-hexene incorporation prepared from SiO₂-supported silyl chromate catalyst system (S-2 catalyst) during UNIPOL gas phase polymerization. The comparison of SCBD between the two samples showed that: although short chain branches of PE-2 with good performance were less than those of PE-1 with bad performance, PE-2 showed less comonomer incorporation on the low crystallinity and low molecular weight (MW) fractions keeping even higher comonomer incorporation on the high crystallinity and high MW parts compared with PE-1. This difference on the SCBD for PE-1 and PE-2 was thought to be the key factor which is responsible for their great difference on environment slow crack resistance (ESCR). Moreover, TREF + SC method further reflected the intra- and inter-molecular heterogeneities of each fraction from the two HDPE samples through the lamella thickness distribution compared with TREF + ¹³C-NMR.