Slow stable crack growth in high density polyethylenes

The phenomenon of slow stable crack growth in polyethylene is investigated using notched specimens subject to constant load and the concepts of fracture mechanics. The effect of specimen geometry and dimension, the loading and the mode of loading on the applied stress intensity factor versus crack speed ($\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$) curves has been studied to demonstrate that K_c is the controlling stress parameter for crack growth under suitable conditions. $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves are obtained for a high density polyethylene homopolymer in distilled water and in a diluted detergent solution at four different temperatures. Results are also obtained for a much tougher medium density polyethylene copolymer whenever possible. Several mechanisms can be identified from the form of the $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves. Two, in particular, have been observed but not explained before: (i) crack growth with a time dependence of 0.25, and (ii) the high $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ slopes for crack growth in a tough copolymer. With the help of scanning electron microscopic studies of the fracture surfaces, (i) is postulated to be due to diffusion controlled void growth process and (ii) is considered to be the result of crack tip blunting effects. From the temperature dependence of crack growth, the activation energy of the diffusion controlled crack growth process is found to coincide with that of the x-relaxation process in polyethylene implying that diffusion controlled crack growth may be related to the motion of main chains in the polymer.     

碳钢贮热水箱的模拟腐蚀试验及涂层耐蚀性能研究

碳钢贮热水箱长期在30～80°C温变的热水环境中工作,易受到热水及水中杂质的腐蚀。研究了不同温度下Q235钢在自来水及含100μg/mLNaCl的自来水中的腐蚀速率。对裸钢及其涂覆相同厚度的不同涂层──酚醛环氧(自制)、钛纳米聚合物和改性聚乙烯后在70°C自来水中的极化曲线进行了比较。采用动电位扫描极化曲线测量和电化学交流阻抗谱法(EIS)分别对涂覆不同涂层的Q235钢进行了耐蚀性能研究。结果表明,3种防腐涂层均具有较佳的抗热水性,耐蚀能力较强,适宜对Q235钢贮热水箱进行防护。     

丙烯酸接枝线形低密度聚乙烯的热学行为及结晶形态

利用差示扫描量热仪研究了丙烯酸接枝线形低密度聚乙烯（PE-LLD-g-AA）的热学行为，结果表明，与纯线形低密度聚乙烯（PE—LLD）相比，PE-LLD-g-AA的熔融温度（Tm）略有增加，结晶温度（Tc）增加大约4℃，熔融焓（AHm）随AA含量的增加而降低。还利用差示扫描量热仪研究了PE—LLD和PE—LLD-g—AA的等温结晶动力学，用扫描电子显微镜观察了PE—LLD-g—AA等温结晶形态。结果表明，PE-LLD-g-AA的结晶速率大于纯PE—LLD的，随着接枝率的增加，PE-LLD的球晶半径减小，接枝到PE—LLD分子链上的AA分子起到了成核剂的作用。     

水基Fe3O4磁流体的制备工艺研究

用共沉淀法制备Fe3O4磁流体,总结出用十二烷基磺酸钠与聚乙二醇作为表面活性剂制备水基磁流体的合适条件:(1)反应温度为室温或不高于35℃;(2)表面活性剂十二烷基磺酸钠的最佳用量为0.0030～0.0040 g/80 mL;(3)第一次包裹的最佳pH为9～10;(4)聚乙二醇作为第二次包裹的表面活剂时,体系最佳温度为40℃左右;(5)表面活性剂聚乙二醇的最佳用量为0.0050～0.0060 g/80 mL。通过实验制得了能稳定存在180 d的水基磁流体。并用透射电镜、红外光谱、分光光度计、古埃磁天平等进行了初步表征。     

The influence of branch length on the deformation and microstructure of polyethylene

The length and number of side chain branches have a profound influence on the microstructure and physical properties of polyethylene (PE). For a series of linear PE copolymers: environmental stress cracking resistance (ESCR), melting points, creep resistance and modulus, and equilibrium spherulite size were all found to increase with increasing branch length (methyl to hexyl) at a given density and molecular weight. It is proposed that (at a fixed molecular weight) branch length and branch concentration determine spherulite size and, consequently, spherulitic boundary areas, in which the dry crazing/voiding occurs during the incubation period of environmental stress cracking (ESC). At a fixed density, decreased spherulite size contributes to greater spherulite boundary slip and increased creep at low (less than 2 MPa) stresses.     

Dispirations,disclinations, dislocations,and chain twist in polyethylene crystals

It is proposed that the twist in polyethylene chains that can result from crystallization and subsequent deformation aggregates at boundaries and becomes a template for further reorganization that results in the long period observed in polyethylene fibres. The observed lower density at the boundaries requires the transport of free volume to the twist boundaries. Dispirations, disclinations and dislocations are crystallographic defects that are involved in the necessary transport mechanism. Twist and bend, derived from the Eulerian angles which are computed from the sets of chain internal coordinates, relate the orientation of different segments of a chain. Twist and bend are useful for the characterization of both crystallographic defects and arbitrary conformations of polymer chains. Defects, along with folds, chain ends, and ordinary edge and screw dislocations provide a basis for interpretation of structure-property relationships in solid polyethylene.     

Preparation and characterization of linear low‐density polyethylene/low‐density polyethylene/TiO2 membranes

Because of their special functions, the application of nanoscale powders has recently attracted both industrial and theoretical interest. In this study, nanoscale TiO₂, which exhibited a special UV absorption and consequent antibacterial function, was added to a low‐density polyethylene/linear low‐density polyethylene hybrid by melt compounding to yield functional composite membranes. TiO₂ exhibited an apparent induced nucleation effect on the crystallization of polyethylene, and the size of the crystallites decreased while the number increaed with the introduction of TiO₂; however, the crystallinity of polyethylene changed little. Also, TiO₂ exhibited an ideal dispersion in the membrane with an average size less than 100 nm, and this excellent dispersion provided the membranes extra UV absorption; moreover, the transparency of the membranes was maintained to satisfy common requirements. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 216–221, 2005     

Effects of silicone oil and polymeric modifiers on the mechanical properties of highly filled LLDPE

The effects of coupling agents, silicone oil, and three types of polymeric modifiers on the mechanical properties of linear low density polyethylene (LLDPE) composites highly filled with aluminium hydroxide [Al(OH)₃] were studied. Polymeric modifiers that contain polar groups, such as silane‐grafted polyethylene (Si‐g‐PE) and acrylic‐acid‐grafted ethylene‐vinyl acetate copolymer (AA‐g‐EVA), improve the mechanical properties dramatically, while nonpolar modifiers improve them to some extent. When Al(OH)₃ was treated using a titanate coupling agent, the silicone oil increased the impact strength and elongation at break of the LLDPE/Al(OH)₃ composites. Introduction of a polymeric modifier containing polar groups destroys the beneficial effects of silicone oil on film mechanical properties, while the introduction of a nonpolar elastomeric polymeric modifier retains the high impact strength and elongation at break. SEM analyses provide the indirect evidence of the encapsulation of silicone oil around the filler. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 121–128, 2002     

低逾渗MWCNTs-CB/UHMWPE导电复合材料的制备及性能

采用溶液法及机械共混法分别制备了均匀结构的炭黑(CB)/超高分子量聚乙烯(UHMWPE)及隔离结构的多壁碳纳米管(MWCNTs)-CB/UHMWPE复合薄膜。扫描电镜分析显示,虽然大部分CB均匀分散于UHMWPE基体中,但依然存在明显的局部团聚,而隔离结构中的MWCNTs-CB分布于UHMWPE界面间,更易形成导电通道。导电测试结果表明,复合材料的导电逾渗值由均匀分布的4.91%(体积分数)下降到隔离结构的0.42%。MWCNTs的加入完善了CB间的导电网络,使复合材料的逾渗值进一步下降,当CB∶MWCNTs=15∶1时,复合薄膜的逾渗值由0.42%(体积分数)下降到0.24%,然而混合填料中MWCNTs含量的进一步增加几乎对逾渗值没有影响。力学性能研究表明,隔离型复合材料的拉伸强度和断裂伸长率随填充剂含量的增加呈现出先上升后下降的趋势。     

水杨醛亚胺前过渡金属烯烃聚合催化剂研究进展

水杨醛亚胺前过渡金属催化剂是由水杨醛亚胺配体和前过渡金属配位生成的一种性能优越、用途广泛的烯烃聚合催化剂。水杨醛亚胺配体具有易于通过有机合成手段引入不同取代基的特点,因此通过改变催化活性中心附近的空间位阻效应、电子效应及金属活性中心数量,可以实现对聚合活性、聚合产物及产物分布的调控。文中综述了水杨醛亚胺前过渡金属催化剂的开发现状,重点阐述了水杨醛亚胺前过渡金属催化剂的配体结构对催化性能的影响,涉及到高选择性乙烯三聚、乙烯/α-烯烃、乙烯/极性单体共聚、乙烯聚合生产超高分子量聚乙烯等众多烯烃聚合领域,为这类催化剂的深入研究及工业化应用提供参考。