超高分子量聚乙烯的结晶形态

应用光学和电子显微镜等手段研究了四种超高分子量聚乙烯初生态聚合体和熔化结晶试样各自生成的结晶形态结构。结果表明,在初生态聚合体中生成纤维状晶体,而熔化结晶试样中只能生成由折迭链片晶组成的球晶结构,熔体拉伸取向结晶时则生成类似草席晶的结构。     

拉伸速率对β-iPP薄膜微观结构和力学性能的影响

通过熔体挤出-拉伸法制备了一系列稀土类β成核剂改性等规聚丙烯(iPP)薄膜。考察了拉伸速率对β-iPP薄膜微观结构和力学性能的影响。结果表明,所有β-iPP薄膜中α晶和β晶均发生支化现象,并且β晶支化随着拉伸速率的增加而受到抑制,而α晶支化没有发生明显变化;β-iPP薄膜的分子链和片晶的取向度随着拉伸速率增加而增大,而β晶含量及其结晶度呈现相反的趋势。另外,拉伸测试结果表明,当拉伸速率为30cm/min,β-iPP薄膜的拉伸强度和韧性达到最大值。     

超高分子量聚乙烯纤维结晶行为研究进展

介绍了超高分子量聚乙烯(UHMWPE)的晶体结构,包括UHMWPE纤维在加工成型过程中微观结构、分子的结晶行为和结晶模型。重点阐述UHMWPE在稀溶液中的结晶行为,对初生纤维形成过程中的折叠链模型和串晶结构进行讨论,综述了UHMWPE纤维在后纺拉伸过程中链结构变化方面的研究进展。对纤维结晶的研究方向和研究意义进行了展望。     

熔体纺丝和凝胶纺丝UHMWPE纤维结构对比

目前,通过熔体纺丝法制备出的超高分子量聚乙烯(UHMWPE)纤维的拉伸强度不如凝胶纺丝纤维。文中利用热分析仪、广角X射线衍射仪、小角X射线散射仪研究了熔体纺丝和凝胶纺丝UHMWPE纤维的微观结构,通过对比发现,熔纺UHMWPE纤维拉伸强度不如凝胶纺丝纤维的原因在于熔体纺丝UHMWPE纤维的伸直链结晶含量低于凝纺纤维,大分子链没有获得充分的伸直,单斜晶的取向度较低,纤维的整体结构不如凝胶纺丝纤维致密。而造成这一结果的根本原因在于熔纺UHMWPE纤维的有效拉伸倍率不高。     

辐射接枝改性对UHMWPE纤维性能的影响

采用预辐射接枝聚合方法制备了丙烯酸甲酯(MA)接枝改性的超高分子量聚乙烯(UHMWPE-g-PMA)纤维。研究了接枝率对UHMWPE纤维结构和力学性能的影响。红外光谱(FT-IR)和扫描电镜(SEM)测试显示,UHMWPE纤维上接枝了PMA分子链。UHMWPE-g-PMA纤维结晶度随接枝率增大而降低;PMA接枝链对UHMWPE纤维的正交晶相无影响,但对单斜晶相和中间相影响明显。UHMWPE-g-PMA纤维的拉伸断裂强度随吸收剂量增大而减小,但不随接枝率变化;拉伸断裂伸长率随吸收剂量增大而减小,随接枝率升高而增大。接枝前后UHMWPE纤维的水接触角测试结果显示,随着接枝率的增加,UHMWPE-g-PMA纤维与水的接触角减小。     

PVF_2/PMMA共混物高取向薄膜的形态结构研究 Ⅰ.PMMA对高取向PVF_2晶相结构的影响

本文用傅里叶变换红外光谱和电子显微镜方法研究了聚甲基丙烯酸甲酯对高取向聚偏氟乙烯晶相结构的影响。在熔体拉伸的高取向聚偏氟乙烯薄膜中,聚甲基丙烯酸甲酯的存在导致了聚偏氟乙烯片晶尺寸减小,针状晶形成和β相结晶含量增加。     

动态保压成型下中等分子量聚乙烯串晶形成的影响

采用自制的动态保压注射成型(DPIM)装置,向中等分子量的HDPE中添加高、低分子量聚乙烯(HMWPE、LMWPE)双重诱导成型自增强串晶的组织与性能影响进行研究。实验表明,在双重诱导和采用DPIM装置下,试样产生了大量的串晶结构,在160℃动态保压注射试样的拉伸强度为111.6 MPa,是其HDPE纯料静态试样拉伸强度24.5 MPa的4.56倍,提高了355.5%;通过WAXD、DSC和SEM测试可知,在DPIM下,试样产生互锁紧密的串晶结构;高分子量聚乙烯的加入有利于串晶脊纤维晶体的生成,低分子量聚乙烯的加入有利于片晶生长增厚,从而使得串晶结构更完善。     

淬火致内应力对聚羟基丁酸戊酸酯薄膜结晶行为影响EI北大核心CSCD

通过对聚羟基丁酸戊酸酯(PHBV)薄膜进行低温淬火使薄膜形成不均匀内应力场,研究淬火致内应力对薄膜结晶行为的影响。用偏光显微镜对淬火样品结晶行为的研究结果表明,PHBV在90℃等温结晶形成环带球晶,但经低温淬火后再在90℃等温结晶却能形成取向结晶结构:当淬火温度(Tq)为-196℃时,形成横晶;当Tq为-80℃时,形成2种不同成核密度结晶结构;而Tq高于-20℃,只形成球晶。对横晶结构进行扫描电镜和X射线衍射表征,结果都显示片晶垂直于薄膜表面生长,表明结晶时分子链与薄膜平行,也进一步说明内应力是形成横晶结构的主要因素。     

热拉伸对尼龙6薄膜微观结构与力学性能的影响

本工作研究了尼龙6(PA6)薄膜的热拉伸工艺对其微观结构和力学性能的影响规律.在玻璃化转变温度(Tg)到熔点(Tm)之间的加工温度范围内对PA6薄膜进行了不同程度的单向热拉伸,并对其微观结构与力学性能进行表征测试.结果表明:随着拉伸温度和拉伸比的提高,PA6中的β晶型向α晶型的转变程度增大,促进了PA6中分子链沿拉伸方向的结晶和取向,从而显著提高了PA6的结晶度,并发现其无定形区减少,玻璃化转变温度(Tg)提高.拉伸温度的提高有利于PA6中α(002)晶面的生长,高温下拉伸形成的α晶体更完整;随着拉伸比的提高,形成的α晶体完整程度先升高后降低.热拉伸后的PA6薄膜的拉伸强度和储能模量增加,断裂伸长率降低;与未拉伸的PA6薄膜相比,拉伸温度为160℃、拉伸比为3的PA6薄膜拉伸强度增加371％,断裂伸长率降低235％.     

淬火致内应力对聚羟基丁酸戊酸酯薄膜结晶行为影响

通过对聚羟基丁酸戊酸酯(PHBV)薄膜进行低温淬火使薄膜形成不均匀内应力场,研究淬火致内应力对薄膜结晶行为的影响。用偏光显微镜对淬火样品结晶行为的研究结果表明,PHBV在90℃等温结晶形成环带球晶,但经低温淬火后再在90℃等温结晶却能形成取向结晶结构:当淬火温度(Tq)为-196℃时,形成横晶;当Tq为-80℃时,形成2种不同成核密度结晶结构;而Tq高于-20℃,只形成球晶。对横晶结构进行扫描电镜和X射线衍射表征,结果都显示片晶垂直于薄膜表面生长,表明结晶时分子链与薄膜平行,也进一步说明内应力是形成横晶结构的主要因素。     

High modulus filaments of polyethylene with lamellar structure by melt processing; the role of the high molecular weight component

In a previous work it was shown that, by appropriate melt processing, oriented filament polyethylene plugs could be produced such as possessed ultra-high modulus in spite of containing predominantly lamellar structures, the advantageous properties being due to the parallel and interlocking arrangement of the lamellae. In the present work it is demonstrated how the extreme high end of the molecular weight distribution is instrumental in the attainment of such structures. The longest chains, even if present in very small amounts, produce fibrous crystals during extrusion which are inadequate to influence the properties in themselves but by serving as nuclei for lamellar crystallization determine the detailed arrangements and hence the mechanical effectiveness of the lamellar texture. We show examples of how sensitively these structures and the resulting properties can be influenced by slight variations in the high molecular weight content of the material. In fact, materials which otherwise would not produce the desired effect can be made to do so through prior blending with a few percent of ultra high molecular weight material. By judicious creation of bimodal distributions, however, the previous batch production of high modulus plugs with lamellar structures can be turned into a continuous spinning process, thus for the first time achieving the production of oriented, high modulus filaments in the course of a single-step continuous extrusion.     

Compliances and failure modes of oriented chain-extended polyethylene

The quantitative dependence of Young's modulus and fracture behaviour on the angle between the tensile axis and the orientation direction has been determined for oriented polyethylene with lamellae of chain-extended dimensions. The form of the modulus dependence was similar to that for the majority of oriented polymers which have been studied, but the fracture behaviour was atypical, ductility only being obtained when the test axis was parallel to the orientation direction, with brittle fracture occurring in all other directions for all except very high molecular mass materials.The behaviour of this material in compression alongc has also been studied; in this case kink bands andc-axis cleavage were observed. It is shown that kink band formation may be explained solely by uniformc axis shear deformation of the lamellar crystals.     

Micellar morphology and its plastic deformation behaviour in ultra-high molecular weight polyethylene

Highly oriented samples from ultra-high molecular weight and normal high-density polyethylene (UHMWPE, HDPE) were prepared under the same experimental conditions. The morphology of the UHMWPE, investigated by transmission electron microscopy, was found to be oriented micellar, while the HDPE samples had a shishkebab morphology. Tensile test experiments under cyclic loading conditions exhibit a strain-hardening effect (increase in stress for the onset of plastic deformation) for HDPE, while the UHMWPE shows a decrease in Young's modulus, which is attributed to similar molecular mechanisms as for the Mullins effect in particle-filled elastomers.     

Epitaxial crystallization of high-density polyethylene on polypropylene in solution-cast films

Epitaxial crystallization of high-density polyethylene (HDPE) on isotactic polypropylene (iPP) in solution-cast films has been investigated by electron microscopy. The specimen-tilt technique of electron microscopy has been used to study the structural relationship between HDPE and iPP crystals. HDPE exhibits different crystalline morphologies in the two basic types of iPP spherulite textures, cross-hatched and lathlike regions. In the former, the crystallographic c axis of HDPE lamellae is in the film plane, while in the latter, the c axis of HDPE crystallites is at an angle of about 50 with the normal of the film. In both structural regions of iPP, however, the contact planes of epitaxial growth are (010) for iPP and (100) for HDPE.     

Reinforcement of polyethylene with polypropylene by a blending and deformation process

The blending of polymers to achieve either unique or intermediate properties has become a rather common practice. High density polyethylene (HDPE) and isotactic polypropylene (PP) are immiscible in the melt state and phase segregate. This behaviour and their difference in melting point (132 against 165° C) has been exploited to produce a uniaxial reinforcement of HDPE with PP fibres by a process of melt blending, and tensile drawing followed by annealing. Tensile drawing of the blends results in the transformation of each phase to a fibrous structure having an increased modulus and tensile strength. The annealing of this material to melt and recrystallize the HDPE converts it to a lower modulus ductile lamellar structure which is reinforced with the fibrous PP regions. Both the modulus and tensile strength in the fibre direction fit simple composite theory for isotropic HDPE filled with higher modulus PP fibres over the entire composition range.     

电子束辐照对HDPE结构与亲水性能影响的研究

研究了在空气中电子束辐照所引起的ＨＤＰＥ分子结构变化以及辐照对ＨＤＰＥ亲水性的影响”。结果表明,通过电子束辐照可在ＨＤＰＥ分子链上引入含氧极性基团,使的表面自由能增大,极性增强,与水的接触角减小。辐照ＨＤＰＥ的分子量降低,分子量分布变窄。     

Morphology oftrans-1, 4-polyisoprene crystallized in thin films

The development of melt-grown crystalline morphology in thin films of trans-1,4-polyisoprene (TPI) was studied by optical and electron microscopy. The crystalline morphologies observed are explained in terms of thin chainfolded lamellar crystals which would grow (if unrestricted) to a diamond shape. The electron diffraction patterns obtained can be indexed using the unit cells proposed by Fisher [3]. The preferred growth faces for the low melting form crystals (LMF) are the (120) planes and probably the (110) planes for the high melting form crystals (HMF). LMF crystals are exclusive to LMF spherulites and HMF crystals are exclusive to HMF spherulites. At large supercoolings both LMF and HMF spherulites nucleate as bundles of lamellar crystals and grow by extensive, twisting, branching and spawning. LMF spherulites grown at small supercoolings develop as hedrites/axialites, or as splayed groups of large crystals differing in orientation with respect to the electron beam. The frequency of twisting, branching and spawning being minimal at these supercoolings. Row nucleated structures are observed in strained films. They consist of long wavy backbone crystals (5 to 25 nm thick) lying in the direction of strain, overgrown by lamellar crystals (5 to 10 nm thick) oriented at right angles to the strain direction. At some temperatures both LMF and HMF crystals nucleate in the same row. The morphologies observed in thin films are compared with and discussed in terms of the observations made of bulk-grown and solution-grown crystals.     

Deformation mechanisms during uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene

The crystal deformation mechanisms during solid-state uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene (UHMW-PE) film have been studied as a function of draw ratio. At higher draw ratios (3) the fine slip processes during uniaxial drawing of melt-crystallized UHMW-PE result in a single-erystal-like (1 0 0) [0 0 1] texture, whereas the normals to the lamellae are inclined by more than 45° with respect to the applied force. It is postulated that in melt-crystallized UHMW-PE the coarse slip process is predominantly restricted due to the fold plane restraints, preventing lamellae from breaking up and rotating with their normals towards the draw direction. The inclination of lamellar normals with respect to the draw direction prohibits further drawing because shear stresses act perpendicular instead of parallel to the lamellar normals.     

On the tensile behaviour of oriented polyethylene

Tensile tests of oriented, high-density, polyethylene films were carried out in order to assess how far the intrinsic molecular and textural features of the samples affected their mechanical behaviour, and to see whether this behaviour bore any resemblance to that of metals. Brittle fracture, ductile fracture or necking followed by drawing occurred, depending on the pretreatment used to orient the material, the angle between the tensile axis and the c axis, and the speed of testing. The feature of the crystalline texture (common to all the films) which was important in determining the geometry of deformation was the alignment of the c axis; the additional orientations present in some of the films did not significantly affect the deformation geometry. The results suggested that ductile deformation approximated to slip in the direction of the c axis, while brittle fracture was observed to occur on planes parallel to the c axis. There were some noticeable differences in behaviour between those films which had received a pre-anneal and those which had not; these differences appeared not to be due to differences in the crystalline texture. Several striking similarities to the mechanical behaviour of hexagonal single crystals were observed.     

Bonding of highly oriented polypropylene sheets by epitaxial crystallization of polyethylene

The bonding of highly oriented polypropylene (PP) by means of a thin epitaxial polyethylene (PE) interlayer, was investigated. Epitaxy occurred in HDPE over a range of molecular weight, including ultra-high molecular weights. It was also observed for LDPE, although the epitaxy was less well-developed than for HDPE. Laminates of highly oriented PP thinly coated with PE and treated to induce cross-hatchng of PE showed much improved adhesion, compared with uncoated PP treated in a similar fashion. The PE epitaxy is believed to be responsible for this behaviour. The adhesive performance was also shown to be a function of the molecular weight and its distribution of PE, as well as of the annealing treatment of the PE coat.