芯棒几何结构对旋转挤出聚乙烯管结构和性能的影响

在自行研制的新型旋转挤出加工装置的基础上,设计了凹槽型和螺纹型芯棒,对比研究了芯棒几何结构对聚乙烯(PE)管结构与性能的影响。实验结果表明,凹槽型芯棒不利于串晶的形成,而螺纹型芯棒对串晶的产生有明显促进效果。因而,在旋转挤出和螺纹型芯棒耦合作用下,可形成大量偏离轴向排列的串晶结构,从而提高PE管的环向强度。相较于常规挤出PE管,制备的PE管环向强度为29.3 MPa,提高幅度达34%。     

剪切流场中聚乙烯结晶过程的建模与模拟

基于Eder模型推导了剪切流场中球晶、串晶形态演化的数学模型,将第一法向应力差作为串晶成核的驱动,并引入两相悬浮模型描述体系,认为其由无定形相和半结晶相组成,分别用FENE-P模型和刚性哑铃模型描述。基于上述数学模型,分别构造了捕捉球晶、串晶生长的Monte Carlo法与体系控制方程求解的有限差分法,成功模拟了二维剪切流场中聚乙烯的结晶过程,给出了球晶、串晶的形态演化,分析了剪切时间、剪切速率对串晶成核密度、结晶速率、流体黏度等的影响。数值结果表明:所构造的Monte Carlo法合理有效,不仅成功捕捉了晶体的生长与碰撞,而且较为准确地预测了结晶速率。此外,提高剪切时间或剪切速率,将增加串晶成核密度、提高结晶速率、使流体黏度突增的时间点提前。     

剪切流场中聚乙烯结晶过程的建模与模拟

基于Eder模型推导了剪切流场中球晶、串晶形态演化的数学模型,将第一法向应力差作为串晶成核的驱动,并引入两相悬浮模型描述体系,认为其由无定形相和半结晶相组成,分别用FENE-P模型和刚性哑铃模型描述。基于上述数学模型,分别构造了捕捉球晶、串晶生长的Monte Carlo法与体系控制方程求解的有限差分法,成功模拟了二维剪切流场中聚乙烯的结晶过程,给出了球晶、串晶的形态演化,分析了剪切时间、剪切速率对串晶成核密度、结晶速率、流体黏度等的影响。数值结果表明:所构造的Monte Carlo法合理有效,不仅成功捕捉了晶体的生长与碰撞,而且较为准确地预测了结晶速率。此外,提高剪切时间或剪切速率,将增加串晶成核密度、提高结晶速率、使流体黏度突增的时间点提前。     

聚乳酸在剪切作用下的结晶行为研究进展

综述了剪切作用对聚乳酸（PLA）结晶动力学、晶体形态和晶型的影响;可以看出剪切对PLA的结晶动力学有显著的促进作用,如缩短结晶诱导时间,提高晶核生长速率并最终提高PLA的结晶度;施加剪切作用时,PLA得到的晶体主要是点晶、柱晶甚至串晶,说明剪切对PLA晶体形态有显著的改善作用;剪切也能促使PLA进行α′-α相转变,即对其晶体结构的完善产生积极影响。     

聚合物串晶结构制备研究进展

综述了聚合物串晶结构的制备方法,其中主要有熔融挤出流延法、熔融纺丝法和溶液结晶法等。讨论了聚合物串晶结构形成和生长机理的研究进展。研究表明,各向同性熔体的分子链会发生折叠而形成球晶,而取向熔体的分子链会被拉伸伸直而形成串晶。串晶结构（shish-kebab）形成过程中,由伸直链晶体组成的部分（shish）和由折叠链晶体组成的部分（kebab）是同时形成的。最后对串晶结构的应用前景提出了展望。     

注塑时振动力场对HDPE性能与结构的影响

利用振动注塑装置，在注塑时加入振动力场，高密度聚乙烯(HDPE)拉伸强度明显提高，特别当振动压力幅度提高后，拉伸强度提高更加明显，试验样条拉伸强度最大提高43％，此时注射温度为190℃，振动压力幅度为60MPa。通过扫描电镜(SEM)观察了HDPE的微观形态，发现HDPE的微观形态发生了较大的变化，无振动时结晶晶体为球晶形态，加入振动力场后，沿流动方向取向明显，更有明显的串晶产生。     

Bi12SiO20晶体的生长习性

分别沿[001],[110]及[111]3种方向用提拉法生长Bi_(12)SiO_(20)晶体,研究了生长条件对晶体形态的影响。应用PBC理论,分析了各晶面的特性:{100}和{110}为F面,{211}为S面,{111}属于K面。并依据连接能的计算,得到晶面的重要性顺序。PBC解析形态与在特定条件下生长的晶体形态相当一致。     

聚乳酸结晶的研究进展

综述了自合成以来聚乳酸（PLA）结晶行为研究的主要成果，介绍了PLA的α、β、γ三种晶型结构参数及不同晶型晶体的形成条件，以及PLA球晶、单晶、孪晶、微纤晶和串晶等结晶形态及其形成条件。此外，还介绍了PLA在不同条件下的结晶行为，着重阐述了分子结构、第二组分及温度场、应力场以及辐照等外部因素对其结晶行为的影响。     

硅橡胶断口形貌及破坏机理研究

采用扫描电子显微镜，观察了硅橡胶断口的特征形态，分析了其微观破坏机理。实验结果表明：在外加载荷作用下，硅橡胶室温下裂纹产生主要是由于分子链断裂的结果，低温下硅橡胶的微观结构破坏主要以层晶之间的破坏为主。     

聚丙烯与低密度聚乙烯的共混改性

正以塑料作为增韧材料对PP进行改性研究较早,其中较成功的例子有PP/PE体系。PP为结晶性聚合物,其生成的球晶较大,这是PP易于产生裂纹、冲击强度较低的主要原因。若能使PP的晶体细微化,则可使冲击性能得到提高。PP与PE共混体系中,PP与PE都是结晶性聚合物,它们之间没有形成共晶,而是各自结晶。但PP晶体与PE晶体之间发生相互制约作用,这种制约作用可破坏PP的球     

Mechanical properties and creep resistance in polystyrene/polyethylene blends

Recycled plastics, predominantly high‐density polyethylene (PE), are being processed in the shape of dimension lumber and marketed as “plastic lumber.” One drawback to these products is their low creep resistance or high creep speed. The objective of this study was to examine the feasibility of reducing the creep speed of PE‐based products by blending the PE with a lower‐creep plastic, in this case polystyrene (PS). Various blends of PE and PS were prepared in either a laboratory extruder or a bowl mixer and then compression‐molded. The mechanical properties, creep behavior, morphology, and thermal properties of extruded and compression‐molded samples were determined. The modulus of elasticity of the extruded blends could be estimated by a weighted average of PS and PE, even in the absence of a compatibilizer. Processing strongly affected the morphology and mechanical properties of the blends. For 50% PS : 50% PE blends, the stress–strain curves of the extruded samples showed PE‐like behavior, whereas those from compression‐molded samples were brittle, PS‐like curves. Flexural strength was 50% higher in the extruded samples than in those from compression molding. The creep experiments were performed in three‐point bending. Creep speed was lower in 50% PS : 50% PE and 75% PS : 25% PE blends than in pure PS. Creep speed of 75% PS : 25% PE was lowest of all the extruded blends. PE formed the continuous phase even when the PS content was as high as 50 wt %. For a 75% PS : 25% PE blend, cocontinuous phases were observed in the machine direction. A ribbonlike PS‐dispersed phase was observed in the 25% PS : 75% PE and 50% PS : 50% PE samples. Blending low‐creep‐speed PS with high‐creep‐speed PE appeared to successfully improve the performance of the final composite. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1100–1108, 2000     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。     

Effect of epitaxial crystallization on adhesive strength in impact-toughened isotactic polypropylene

The influence of the epitaxial crystallization of the polyethylene (PE) content on the adhesive interface strength of impact-toughened isotactic polypropylene (iPP) has been investigated. The morphologies and crystal arrangements of four different matrix/rubber combinations were observed by transmission electron microscopy (TEM) and were correlated with the interface strengths measured by a peel test. It was found that the adhesion is best at interfaces, where an epitaxial morphology of the PE was observed.     

用球形Ziegler-Natta催化剂制备PE/PP原位合金的相态结构研究

通过偏光显微镜、扫描电镜观察了用球形Ziegler-Natta催化剂制备的PE/PP原位合金的结晶形貌和相态结构,随着无规乙丙共聚物和多嵌段乙丙共聚物含量的减少,合金中的球晶的粒径逐渐变大,合金中的两相之间的界面也更加清晰。随着合金中无规乙丙共聚物和多嵌段乙丙共聚物含量的增加,其屈服断裂程度越来越明显,抗冲强度也越好。     

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the ‘use temperature’ of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments were studied. Microstructural stability experiments were conducted in air from 1200 to 1500°C and creep tests at temperatures of 1400 to 1700°C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400°C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior are discussed.     

Orientation and property correlations for LLDPE blown films

The orientation and property correlations of biaxially oriented polyethylene (PE) blown films have been studied. A linear low density polyethylene (LLDPE) (DOWLEX ? 2045A) was used to fabricate films at different conditions with blow up ratio, die gap, and frost line height as the variables. The White‐Spruiell orientation factors of crystal unit cells, amorphous chains, and Herman's orientation factors of lamellae were determined from wide‐angle X‐ray diffraction pole figure, birefringence, and small angle X‐ray scattering (SAXS). A general orientation pattern with the crystal unit cell a‐axis preferentially oriented to MD, b‐axis to TD, lamellae stacking along the MD, and amorphous chains preferentially to the MD has been found for all films in this study. A correlation between the orientation of each element of the morphology hierarchy has been revealed. Key mechanical properties including dart impact and Elmendorf tear strength in both MD and TD have been determined. Good correlation has been found among these properties. Most importantly, these properties have excellent correlation to the orientation. These correlations have been linked to underlying morphology and microdeformation mechanisms. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 898–907, 2006     

高熔体强度聚丙烯对短玻纤增强聚丙烯性能的影响

考察了高熔体强度聚丙烯树脂对玻纤增强聚丙烯材料高温力学性能和拉伸蠕变性能等的影响,并进行平板流变分析及材料微观形貌测试.结果表明:随着高熔体强度聚丙烯质量分数增加,材料弯曲强度和高温拉伸强度降低,高温拉伸的断裂伸长率增加;高熔体强度聚丙烯树脂的添加可以改善材料的高温耐蠕变性能,添加质量分数为20％的高熔体强度聚丙烯树脂...     

聚乙烯基木塑复合材料蠕变量影响因素分析

在室温恒定的情况下,主要研究了原材料配比、材料制备工艺参数在不同载荷条件下对木塑复合材料24 h蠕变性能的影响。结果表明,在24 h蠕变试验中各载荷下影响蠕变量的优势因素各不相同,但木塑比都为第一显著因素;载荷为30 % 弯曲强度时,木塑比与螺杆转速为显著因素;载荷为50 %弯曲强度时,木塑比为显著因素;载荷为70 %弯曲强度时,只有木塑比为极显著因素。     

PE100燃气管材的非线性蠕变行为及其本构模型研究

对PE100燃气管材开展不同应力水平（2.4~9.6 MPa）下的常温蠕变测试,结果显示应力不超过5.4 MPa时,材料的蠕变柔量与应力水平无关,呈现出线性黏弹性行为,当应力高于5.4 MPa时,材料发生非线性黏弹性蠕变。基于单积分型非线性黏弹性本构理论,采用Findley模型对蠕变行为进行分析,并与Struik经验模型进行比较。结果表明,Findley模型和Struik模型均能较好地描述PE100管材的蠕变行为,但对高应力下的非线性蠕变行为,Findley模型比Struik模型描述得更准确。     

Creep fatigue in engineered wood fiber and plastic compositions

Creep behaviour of unmodified and functionally modified thermoplastic‐wood fibre composites was studied. For PVC, PE and PP‐based composites creep is strongly dependent on the amount of load, time and temperature. A small rise in the temperature above ambient temperature increased creep significantly for PVC‐woodfiber composites. Instantaneous creep resistance of woodfibre‐filled PP is higher than that of PE‐based composites. PP and PE‐based wood composites were modified with maleic and maleimide compounds. Maleic or maleimide modification of woodfibre improved transient creep behaviour of PP‐woodfibre composite but it did not show practically any effect on instantaneous creep. A mathematical model has been proposed to predict creep behaviour of PVC, PP an PE‐based wood fiber composites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 260–268, 2000