线型低密度聚乙烯的分级制备北大核心

采用制备型升温淋洗分级仪对线型低密度聚乙烯(LLDPE)进行分级和收集,得到6个不同结晶度的级分,并利用差示扫描量热仪、傅里叶变换红外光谱仪、核磁共振波谱仪和凝胶渗透色谱仪分别对原样和级分的结晶熔融行为、短链支化度、相对分子质量及其分布进行了分析。结果表明:随着淋洗温度的升高,级分的结晶度增大、支化度降低、相对分子质量增大,且LLDPE试样中存在支化不均匀性,低相对分子质量部分聚乙烯的共聚单体含量较高,高相对分子质量部分聚乙烯的共聚单体含量较少。     

线型低密度聚乙烯的分级制备

采用制备型升温淋洗分级仪对线型低密度聚乙烯(LLDPE)进行分级和收集,得到6个不同结晶度的级分,并利用差示扫描量热仪、傅里叶变换红外光谱仪、核磁共振波谱仪和凝胶渗透色谱仪分别对原样和级分的结晶熔融行为、短链支化度、相对分子质量及其分布进行了分析。结果表明:随着淋洗温度的升高,级分的结晶度增大、支化度降低、相对分子质量增大,且LLDPE试样中存在支化不均匀性,低相对分子质量部分聚乙烯的共聚单体含量较高,高相对分子质量部分聚乙烯的共聚单体含量较少。     

结晶分级技术表征PE100级与耐热级管材树脂结构的差异性

通过逐步等温结晶分级技术(SIS)、升温淋洗分级技术(TREF)和连续自成核退火分级技术(SSA)表征了聚乙烯(PE) 100级管材树脂GC100S和耐热级管材树脂4731B结构的差异性.结果表明,4731B和GC100S树脂的熔融温度、结晶温度、结晶度、片晶厚度及共聚单体在主链中的分布状况基本相同,主要区别在于含有共聚单体的聚乙烯分子链的相对含量不同,4731B不含或含有少量共聚单体的聚乙烯分子链的含量比GC100S多.     

支化PE的结构与性能

采用均相和负载α-二亚胺镍配合物催化乙烯聚合制备不同支化度的聚乙烯(PE),探讨支化度对PE密度、熔融行为、结晶度和晶型、热稳定性等的影响。随着支化度的增加,PE密度近于线性减小,熔点随之下降,结晶度呈线性下降,但热稳定性并不是单调下降。PE的差示扫描量热法升温曲线的熔融峰随着支化度增大而变低,熔融区域变宽;支化度达到48个/1 000 C时出现双峰,此时PE结晶度下降至13.0%,且衍射角为23°附近时,(200)晶面衍射峰消失,表明己转变为非结晶态的PE。     

管材专用高密度聚乙烯的热分级研究

利用差示扫描量热法和连续自成核退火分级(SSA)法研究了PE100级管材专用高密度聚乙烯(HDPE)树脂的结晶熔融行为,三个牌号HDPE树脂的1-己烯共聚单体含量按试样1、试样2、试样3逐渐降低。结果表明:两种方法得到的结晶度和熔融温度均按试样1、试样2、试样3递增,且SSA处理后试样的结晶度和熔融温度较高;短链支化度按试样1、试样2、试样3逐渐降低;试样3表现出最佳的刚性和较低的韧性,而试样1则相反。     

PET-PTMG聚醚酯熔融与结晶行为的研究

采用熔融缩聚法合成了一系列聚对苯二甲酸乙二醇酯(PET)-四氢呋喃聚醚(PTMG)聚醚酯，用DSC、偏光显微镜表征了材料的熔融与结晶性能，讨论了组成对聚醚酯的熔点、结晶温度、结晶度、结晶形态的影响。结果表明，PET—PTMG聚醚酯的熔点与组成的关系符合Baur公式；其结晶度随着聚醚含量的增加呈现先升高后下降的趋势；并为明显的结晶与非结晶的两相结构形态。     

高密度聚乙烯溶液分级和结晶性能研究

采用逐步降温和控制沉淀剂用量的方法对HDPE进行溶液分级，以期制备具有特定结晶度的PE标准物质，在特定的分级工艺下，级分2结晶度的均值和扩展不确定度分别为79．34％和0．45%。同时采用GPC、FTIR和DSC对级分的摩尔质量、支化度和结晶性能进行了测定和分析，结果表明溶液分级能够将HDPE分成窄摩尔质量分布且具有特定结晶度的级分。在本研究范围内，在同样热历史和降温速率条件下，摩尔质量从高到低、支化度由大小的级分非等温结晶能力依次增强，形成晶体的有序度逐渐增大。较低摩尔质量且支化度小的窄摩尔质量分布级分具有较完善、分布更加均匀的薄片晶结构。     

高密度聚乙烯溶液分级和结晶性能研究

采用逐步降温和控制沉淀剂用量的方法对HDPE进行溶液分级，以期制备具有特定结晶度的PE标准物质。在特定的分级工艺下，级分2结晶度的均值和扩展不确定度分别为79．34％和0．45％。同时采用GPC、FTIR和DSC对级分的摩尔质量、支化度和结晶性能进行了测定和分析，结果表明溶液分级能够将HDPE分成窄摩尔质量分布且具有特定结晶度的级分。在本研究范围内，在同样热历史和降温速率条件下，摩尔质量从高到低、支化度由大到小的级分非等温结晶能力依次增强，形成晶体的有序度逐渐增大。较低摩尔质量且支化度小的窄摩尔质量分布级分具有较完善、分布更加均匀的薄片晶结构。     

LLDPE和HDPE的非等温结晶动力学

采用差示扫描量热法实现了不同降温速率条件下线型低密度聚乙烯(LLDPE)和高密度聚乙烯(HDPE)的非等温结晶,通过Jeziorny法和莫志深法研究了二者的非等温结晶动力学。结果表明:随着降温速率的增加,LLDPE和HDPE的结晶温度降低,结晶速率加快,而由于发生熔融重结晶现象导致熔融温度变化不大;LLDPE因短支链位阻效应导致晶区不完善,LLDPE结晶和熔融温度、结晶速率均低于HDPE;随着降温速率的增加,LLDPE和HDPE都从多维复杂晶体向低维简单晶体转变;分子链结构不影响二者成核机理和晶体结构;莫志深法和Jeziorny法的研究结果一致,即HDPE较LLDPE更易结晶。     

导热SiC/LLDPE复合塑料制备及性能研究

采用碳化硅(SiC)和线性低密度聚乙烯(LLDPE)粒子经粉末混合和热压成型制得导热复合塑料.研究了SiC粒子含量时LLDPE熔融温度、结晶度、热导率、介电及力学性能的影响.结果表明:SiC粒子降低了LLDPE结晶度,对熔融温度元明显影响;随SiC含量增加,热导率升高,介电常数和损耗升高,但仍然保持在较低数值,而力学性能下降.     

Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low‐density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular‐weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short‐chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short‐chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HDPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short‐chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2542–2549, 2004     

LLDPE/LDPE blends. I. Rheological,thermal, and mechanical properties

Structure and mechanical properties were studied for the binary blends of a linear low density polyethylene (LLDPE) (ethylene‐1‐hexene copolymer; density = 900 kg m⁻³) with narrow short chain branching distribution and a low density polyethylene (LDPE) which is characterized by the long chain branches. It was found by the rheological measurements that the LLDPE and the LDPE are miscible in the molten state. The steady‐state rheological properties of the blends can be predicted using oscillatory shear moduli. Furthermore, the crystallization temperature of LDPE is higher than that of the LLDPE and is found to act as a nucleating agent for the crystallization of the LLDPE. Consequently, the melting temperature, degree of crystallinity, and hardness of the blend increase rapidly with increases in the LDPE content in the blend, even though the amount of the LDPE in the blend is small. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3153–3159, 1999     

Comprehensive branching analysis of polyethylene by combined fractionation and thermal analysis

Linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) differ significantly in their branching types and branching distributions. For a comprehensive analysis, preparative temperature rising elution fractionation and/or preparative molar mass fractionation are used to fractionate typical LLDPE and LDPE bulk resins into narrowly distributed fractions. The chain structures of the bulk resins and their fractions are further analysed using SEC, crystallization analysis fractionation, DSC and high‐temperature HPLC to provide detailed information on short chain branching in LLDPE and long chain branching in LDPE. For LDPE it is shown that the multiple fractionation approach is a powerful source of sample libraries that may have similar molar masses and different branching structures or alternatively similar branching but different molar masses. The analysis of these library samples by thermal analysis provides a much deeper insight into the molecular heterogeneity of the samples compared to bulk sample analysis. © 2018 Society of Chemical Industry     

Physical gelation of crystallizing metallocene and Ziegler-Natta ethylene-hexene copolymers

The effect of molecular architecture on the evolution of viscoelastic properties during crystallization was investigated using ethylene-hexene copolymers manufactured via metallocene (M-LLDPE) and Ziegler-Natta (ZN-LLDPE) processes. Differences in branching distribution were shown to have a drastic effect on the viscoelastic properties near the gel point. It is shown that the branching distribution rather than branch content is the determining parameter for the evolution of the rheological properties during isothermal and non-isothermal crystallization, and for the width of the solidification interval. We developed a partial melting technique for the preparation of stable critical gels of LLDPE whose viscoelastic properties correspond to the intermediate state between melt and solid. Local molecular conformation and crystallinity in these gels were characterized by Raman spectroscopy, which shows that the transition from melt-like to solid-like rheological behavior (physical gelation) in LLDPE occurs at a very low overall crystallinity of less than 5%.     

升温淋洗分级研究聚丙烯流延膜专用料的微观结构及热学表征

采用升温淋洗分级技术分离了2种聚丙烯流延膜(CPP)专用料(CPP1、CPP2),通过红外光谱仪、差示扫描量热仪和高温凝胶色谱对2种样品的链结构、熔融和结晶行为进行了表征,确定了产品各级分的分子结构信息.结果表明,对于低温(<100℃)级分,CPP1样品中其含量为9.29％(质量分数,下同),CPP2样品中其含量为26...     

Study on molecular chain heterogeneity of linear low‐density polyethylene by cross‐fractionation of temperature rising elution fractionation and successive self‐nucleation/annealing thermal fractionation

The molecular chain heterogeneity of commercial linear low‐density polyethylene (LLDPE) was investigated by cross‐fractionation of temperature rising elution fractionation (TREF) and successive self‐nucleation/annealing (SSA) thermal fractionation by use of DSC. The results indicate that the linear relationships between crystallinity or melting temperature and the elution temperature are confirmed by TREF fractions. Intermolecular heterogeneity exists in the original LLDPE, whereas there is less intermolecular heterogeneity in the TREF fractions. After SSA thermal fractionation, the multiple endothermic peaks for both LLDPE and their TREF fractions are mainly attributed to the heterogeneities of ethylene sequence length (ESL) and lamellar thickness. The statistical terms, including weighted mean L _w, arithmetic mean L _n, and broad index L _w/L _n, were introduced to evaluate the heterogeneities of ESL and lamellar thickness of polyethylene. The difference of broadness index indicates that TREF fractions of LLDPE have less inter‐ and intramolecular heterogeneities of both ESL and lamellar thickness than those of the original LLDPE. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1710–1718, 2004     

Thermal and morphological characterization of binary blends of fractions of 1-octene LLDPE

An ethylene/1-octene linear low-density polyethylene (LLDPE B) has been fractionated with respect to the short-chain branching content of the molecules, using the preparative temperature-rising elution fractionation (PTREF) technique. The LLDPE B studied, which is characterized by a high degree of heterogeneity on the level of the molecular weight and the comonomer content, was fractionated into six fractions having a more homogeneous intermolecular comonomer distribution. As a result of the differences in the chain microstructure of the fractions, a strong change in the thermal characteristics such as onset temperature of crystallization from the melt and melting temperature has been found. The morphology of the fractions, i.e., spherulitic texture and semicrystalline lamellar ordering, also strongly depends on the chain microstructure. In an attempt to elucidate the mutual influence of molecules having a different chain microstructure, as occurring within the unfractionated copolymer, blends of preparative TREF fractions were prepared and investigated with respect to their thermal behavior and morphology. © 1993 John Wiley & Sons, Inc.     

绝缘导热Si_3N_4/LLDPE复合材料的研制

用粉末法制备了Si3N4增强线型低密度聚乙烯(LLDPE)导热材料,研究了填料Si3N4对LLDPE熔融温度、结晶度、热导率、电绝缘性能及力学性能的影响.结果表明,加入Si3N4粒子,提高了LLDPE的熔融温度和结晶度;当w(Si3N4)为30%时,体系热导率达基体的6～7倍,热导率随Si3N4粒径的减小而增大,此时体系仍然具有较高电阻率及良好的介电性能,但力学性能明显下降.     

Rheological,physical, and thermal characterization of single‐site catalyst based polyethylenes for seal layer applications

Four single‐site metallocene catalyst based polyethylenes (mPEs), one ultra low density polyethylene, one conventional linear low density polyethylene (LLDPE), and one low density polyethylene (LDPE) were selected to characterize the effect of side chain branches on physical and mechanical properties. Rheological experiments were carried out to extract complex viscosity and elasticity as a function of frequency. Elongational viscosity tests were also performed to assess long chain branching. For some mPEs, sparse long chain branching improved shear thinning and elasticity of the chains in melt state. During elongation, mPEs with a narrow linear chain distribution showed initially greater melt strength whereas for longer elongation, the mPEs with long chain branching lead over in strength. Cast films were produced from the mPEs and their physical (such as crystallinity, crystal size) and mechanical properties were tested. A double melting peak was observed in the differential scanning calorimetry thermograms of the mPE films. A relatively sharp strain hardening behavior in tensile tests was observed for the mPEs films when compared to LLDPE. Fourier transform infrared was used as an effective and fast method to investigate side chain length. It was found that the positioning of side chain, co‐monomer length, and content influence the melting behavior of mPE films. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

A molecular dynamics study of the effects of branching characteristics of LDPE on its miscibility with HDPE

The effects of branching characteristics of low-density polyethylene (LDPE) on its melt miscibility with high-density polyethylene (HDPE) were studied using molecular simulation. In particular, molecular dynamics (MD) was applied to compute Hildebrand solubility parameters (δ) of models of HDPE and LDPE with different branch contents at five temperatures that are well above their melting temperatures. Values computed for δ agreed very well with experiment. The Flory-Huggins interaction parameters (χ) for blends of HDPE and different LDPE models were then calculated using the computed δ values. The level of branch content for LDPE above which the blends are immiscible and segregate in the melt was found to be around 30 branches/1000 long chain carbons at the chosen simulation temperatures. This value is significantly lower than that of butene-based linear low-density polyethylene (LLDPE) (40 branches/1000 carbons) in the blends with HDPE computed by one of the authors (polymer 2000; 41:8741). The major difference between LDPE and LLDPE models is that each modeled LDPE molecule has three long chains while each modeled LLDPE molecule had only one long chain. The present results together with those of the LLDPE/HDPE blends suggest that the long chain branching may have significant influence on the miscibility of polyethylene blends at elevated temperatures.