基于同步辐射超小角X射线散射的高密度聚乙烯空洞化行为研究

以一系列高温结晶后自然冷却的高密度聚乙烯(HDPE)为研究对象,利用同步辐射超小角X射线散射(USAXS)和示差扫描量热技术(DSC)对样品的微观结构进行了分析,并在线研究了单轴拉伸过程中的空洞化行为.结果表明,结晶温度高于110℃后自然冷却到室温的样品中存在热稳定性不同的两组片晶,等温过程形成结构完善的厚片晶,而在冷却过程会形成有缺陷的薄片晶,两组片晶的熔点分别在133和110℃附近.在30℃拉伸时,所有样品都可观察到空洞化并伴随发白现象.并且,等温结晶中形成片晶厚度越大的样品,相应的空洞化现象越明显.在拉伸过程中,空洞出现在屈服点附近,其法向方向平行于拉伸方向,后随应变的增加发生转向,法向方向与拉伸方向垂直.样品中空穴的长度为900~1200 nm.另一方面,随着冷却过程生成薄片晶比例的增加,空洞化趋势下降.此外,提高拉伸温度,样品更倾向发生塑性形变,空洞化程度减弱.     

Structural changes and chain radius of gyration in cold-drawn polyethylene after annealing: small- and wide-angle X-ray scattering and small-angle neutron scattering studies

Samples made from linear polyethylene were drawn at room temperature and subsequently annealed at high temperatures below the melting point. The structural changes of the crystalline lamellae and lamellar superstructures as well as the single chain radius of gyration were studied by means of combined small- and wide-angle X-ray scattering and small-angle neutron scattering (SANS). After drawing, the polymeric chain segments in the crystalline phase are preferentially oriented along the drawing direction with a high degree of orientation whereas the lamellae in the samples are found to be slightly sheared exhibiting oblique surfaces as evidenced by X-ray scattering. SANS indicates that the chains are highly elongated along the drawing direction. Annealing the deformed samples at temperatures where the mechanical alpha-process of polyethylene is active leads to a thickening of both crystalline lamellae and amorphous layers. The chains in the crystalline phase retain their high degree of orientation after annealing while the lamellae are sheared to a larger extent. In addition, there is also lateral growth of the crystalline lamellae during high-temperature annealing. Despite the structural changes of the crystalline and amorphous regions, there is no evidence for global chain relaxation. The global anisotropic shape of the chains is preserved even after prolonged annealing at high temperatures. The results indicate that the mobility of polyethylene chains-as seen, e.g., by 13C NMR-is a local phenomenon. The results also yield new insight into mechanical properties of drawn PE, especially regarding stress relaxation and creep mechanisms.     

Large Plastic Deformation of Isotactic Poly(propylene) (iPP) Evaluated by WAXD Techniques

Summary: Commercial isotactic poly(propylene) (iPP), obtained in bars, was annealed and submitted to different levels of plastic deformation by uniaxial plane compression using a special device which permits well controlled temperature and strain rate. The evolution of the microstructure was followed at different degrees of deformation by wide angle x-ray diffraction (WAXD) techniques. The spherulite fragmentation process, lamellar orientation and destruction of the crystallites is argued, according to collected analytical data in the flow direction (FD), the loading direction (LD) and the lateral or constrain direction (CD). The evaluation of the WAXD patterns in terms of diffraction line position, intensity and width, permits to affirm that, while the large plastic deformation occurs, the crystalline net suffers anisotropic deformation, the crystallites become preferentially oriented along the flow direction and the crystalline phase diminish in amount indicating lesser and smaller crystallites. The gradual lamellae fragmentation occurs, starting with apparent crystalline size of approximately 30 nm for the non-deformed material and gradually decreasing to approximately 15 nm for the 70% deformed one.     

Deformation mechanisms in uniaxial‐oriented semi‐crystalline PET‐films as a function of orientation to tensile direction

Poly(ethylene terephthalate) films with oriented lamellar structure were deformed in tensile experiments and investigated in situ using small angle X‐ray scattering. The tensile direction was set parallel, normal and in an angle of 45° relative to the surface normal of the lamellae. Data were interpreted in terms of two‐dimensional autocorrelation functions. The deformation of lattice spacing and lamellar orientation can largely be explained by affine transformations. The sample, where the lamellar surface normal was normal to tensile direction, developed a chequerboard type arrangement of crystalline parts. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 159–169     

A nanoscopic view of structure and deformation of hard elastic polypropylene with scanning force microscopy

Scanning force microscopy (SFM) was used to visualize the surface of hard elastic polypropylene (HEPP) film. The surface morphology of unstrained HEPP shows crystalline and noncrystalline rows oriented parallel to the extrusion direction. The crystalline rows are composed of lamellar blocks. The dimensions of crystalline and noncrystalline regions are determined. The structural surface changes induced by stepwise elongation of the film with a home-built stretching device are documented by SFM. Stretching of HEPP perpendicular to the extrusion direction causes cracks advancing through several crystalline rows. During elongation parallel to the extrusion direction the separation of adjacent lamellae by their translatory displacement occurs. Deformation-induced structural changes of HEPP on the nanometer scale are compared with proposed deformation models. Nanostructural changes are correlated with characteristic variations in the force-elongation curve. © 1996 John Wiley & Sons, Inc.     

LAMELLAR STRUCTURE OF THERMOTROPIC LIQUID CRYSTALLINE POLYMERS

The lamellar structure of a thermotropic aromatic polyester with flexible spacer has beenstudied by using transmission electron microscopy. It was found that the lamellar structure couldbe observed in the crystalline samples of this semirigid polymer crystallized from different states.The thickness of lamellae is around 10 nm, which is similar to that of the conventional polymersof flexible chain molecules. The molecular chains in the lamellae are oriented in the thicknessdirection as determined by electron diffraction. The possibility of molecular chains folding in the lamellae has been discussed.     

Molecular and Supramolecular Deformations and Disclinations in a Liquid Crystalline Copolyether Thin Films under an Electrostatic Field

An alignment study of a liquid crystalline copolyether TPP‐7/11(5/5) thin films has been carried out in a 10 kV·cm^–1 electrostatic field parallel to the thin film surface normal. This copolyether possesses a negative dielectric anisotropy. The chain molecules are homogeneously aligned in the electric field and they form two‐dimensionally ordered lamellae in a tilted columnar phase when the samples were cooled to room temperature. It is observed that the chain molecules are splayed to form bent lamellae and the chain direction is perpendicular to the tangential direction of the lamellar surfaces. These lamellae thus become replicas of the chain orientation. Due to the flexoelectric effect and density fluctuation on the thin film free surface, disclinations having topological strength s = 1, c = π/4 and defect walls form. These s = 1 disclinations possesses both left‐ and right‐handednesses. Discussion of the defect formations have been attempted.     

Structure of ultrathin polyethylene layers in multilayer films

The crystalline structures of “microlayer” and “nanolayer” polyethylene have been examined in coextruded films comprised of alternating layers of high-density polyethylene and polystyrene. Transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), and wide-angle x-ray scattering (WAXS) reveal that microlayer polyethylene, where the layer thickness is on the order of several microns, crystallizes with the normal unoriented lamellar morphology. In nanolayer films, where the film thickness of tens of nanometers is on the size scale of molecular dimensions, lamellae are oriented with the long axes perpendicular to the extrusion direction in a row-nucleated morphology similar to structures described in the literature. The lamellae are partially twisted about the long axes. The preferred twist angles of ±40° orient the lamellar surfaces normal to the layer surface. The row-nucleated morphology imparts highly anisotropic mechanical properties to the nanolayer polyethylene.     

Craze fibril extension ratio measurements in glassy block copolymers

The extension ratios of crazes in triblock copolymer films of poly(2-vinylpyridine)-polystyrene-poly(2-vinylpyridine) [PVP-PS-PVP] which had lamellar microphase domain structure were measured by transmission electron microscopy. The extension ratio when the lamellae were oriented parallel to the craze fibril direction was always greater than that when the lamellae were oriented perpendicular to this direction, reflecting the stretching of the chains of the block copolymer in a direction normal to the interfaces of the lamellar domains.     

Local lamellar structures in banded spherulites analyzed by three‐dimensional electron tomography

The lamellar morphology in banded spherulites of poly(ε‐caprolactone) blended with an amorphous polymer, poly(vinyl butyral), was investigated by three‐dimensional transmission electron tomography. It showed a local lamellar twist on a smaller scale than the band spacing by 2 orders of magnitude. It also indicated wavy lamellae and frequent variation in the direction of the lamellar plane. All these results indicated an S‐profiled lamellar structure; that is, the cross section perpendicular to the lamellar growth direction was S‐shaped. S‐profiled lamellae show these structures when they are sliced at a certain angle to the lamellar surface direction. Lamellar branching was also observed, but no screw dislocations that led to the formation of extinction rings were observed in this work. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1122–1125, 2007     

Chlorination mechanism of polyethylene crystals

A new chlorination mechanism has been suggested for polyethylene crystals. Based on electron microsocopy and infrared spectroscopy of chlorinated high-density polyethylene single crystals, it is proposed that beyond the initial chlorination of the lamellae surfaces, which accounts for only 2-3% chlorine uptake, the reaction proceeds through the lamellar side faces from the periphery inward. Thus at intermediate degrees of chlorination two major phases coexist: the undamaged interior regions of the lamellae and the surrounding chlorinated material. This mechanism differs from that of oxidative degradation, and also from the generally Accepted mechanism of chlorination. Rather than proceeding through the reacted fold surface into the crystalline core, it is proposed that the direction of chemical attack is prependicular to that suggested previously. This mechanism probably describes any chemically nondestructive attack on the crystalline regions of polymers in general.     

Deformation in lamellar and crystalline structures: in situ simultaneous small‐angle X‐ray scattering and wide‐angle X‐ray diffraction measurements on polyethylene terephthalate fibers

Changes in the lamellar and crystalline structures were followed as a function of applied stress to understand the influence of the interactions between the crystalline and amorphous domains on the fiber properties. We observed a reversible transformation from a structure giving a four‐point small‐angle pattern to a structure giving a two‐point pattern; these structures corresponded to the lamellae with oblique and normal lamellar surfaces, respectively. The characteristics of these two structures such as the stack diameter, stack height, and tilt angle were different and were determined by the processing conditions and did not change when the fiber was elastically deformed. The structure giving a two‐point pattern was probably the load‐carrying lamellar entity in these fibers. The diameter of the lamellar stacks, tilt angle of the lamellae, and the strain in the lamellar spacing appeared to have the most influence on properties such as tenacity and dimensional stability. The long‐spacing strain, which is about the same as the fiber strain, determined the modulus at low elongation as well as ultimate elongation. These indicate that the lamellar stacks have at least as much influence as the interfibrillar chains on fiber properties. Structural features that determine the performance in semicrystalline polymers were investigated by analyzing four generations of polyethylene terephthalate fibers. Some of the fiber properties correlate with changes in the crystalline domains such as the crystalline orientation, size, and unit cell dimensions. Fibers in which the crystalline strain was large because of their strong linkages to the amorphous chains, and better load transfer, had the highest modulus and lowest ultimate elongation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1538–1553, 2003     

聚乙烯片晶辐照破坏机理的电子显微镜研究

用透射电子显微镜观察了高密度辐照聚乙烯的形态结构,并通过统计方法定量地分析了其结构与辐照剂量的关系。发现室温辐照聚乙烯的片晶形态不随辐照剂量而变化。若将室温辐照聚乙烯重新熔融,然后再于125℃下等温结晶4h后,其片晶厚度则随辐照剂量的增加而变薄,长周期亦随之变短。小角X射线散射的测试结果与上述结果符合得很好。室温辐照聚乙烯及其125℃重结晶试样的电子显微镜数据从又一直观角度验证了辐照聚乙烯“片晶内部破坏机理”的正确性。     

Preparation of Oriented Liquid‐Crystalline/Isotropic Block Copolymer Films with Parallel or Perpendicular Arrangement of Smectic Layers and Lamellar Microdomains

Films of a symmetric liquid‐crystalline/isotropic block copolymer consisting of a smectic LC side‐chain polymer and polystyrene were prepared by solvent casting from solution and from the isotropic melt. By annealing the solvent‐cast film in the S_A phase an oriented microphase‐separated film of lamellar morphology was obtained in which both the lamellae of the block copolymer and the smectic layers of the LC block were oriented parallel to the film surface. A lamellar morphology with perpendicular orientation of lamellae and smectic layers was generated by cooling the block copolymer from the melt.     

Kinetics of mesophase formation and crystallization in poly(diethylsiloxane)

Optical and calorimetric studies of the kinetics of mesophase formation and crystallization in poly(diethylsiloxane) have been conducted. The mesomorphic phase is found to grow from the isotropic melt in the form of lamellar domains about 2 μm thick in the temperature range 293–307 K. According to birefringence data, macromolecules in the mesomorphic lamellae are perpendicular to the end faces. The kinetics of mesophase formation obey the Avrami equation with the morphological parameter n close to 2 (it is equal to 1.75 ± 0.05), which corresponds to the two-dimensional growth of the mesomorphic phase from athermal nuclei. The limiting conversion of the isotropic melt was shown to be temperature-dependent. This is likely to be connected with a change in the number of nuclei with temperature. The crystallization of polymer from the mesomorphic state occurs with retention of the optical texture of the sample. The process proceeds not as a sporadic crystallization of individual mesomorphic lamellae but as a growth of the nucleated crystalline regions via a consecutive incorporation of adjacent crystallizing lamellae.     

Crystallization behavior of polylactide on highly oriented polyethylene thin films

The crystalline structure and morphology of the PLA crystallized isothermally from the glassy state on highly oriented PE substrates at 130℃were investigated by means of optical microscopy,AFM and X-ray diffraction.The results indicate that the PE substrate influences the crystallization behavior of PLA remarkably,which leads to the growth of PLA crystals on PE substrate always in edge-on form rather than the twisted lamellar crystals from edge-on to flat-on when crystallizing the PLA on glass surface under the same condition.The edge-on PLA lamellae on the PE substrate are preferentially arranged with their long axes in the chain direction of the PE substrate crystals.It is further demonstrated that except for the different crystal orientation,the PE does not influence the crystalline modification and crystallinity of the PLA.     

LLDPE/IPP共混物高取向薄膜的附生结晶

本文用透射电子显微术、电子衍射等方法研究了线性低密度聚乙烯(LLDPE)和等规聚丙烯(IPP)共混物高取向薄膜的形态结构.在熔体拉伸薄膜中统组分的LLDPE与IPP均以高取向的片晶形式存在,片晶生长方向垂直手拉伸方向.当共混物中LLDPE含量较低(小于40％)时,作为分散相的LLDPE在IPP上附生结晶.两种片晶的c轴成45°交角,附生结晶的接触面为LLDPE的(100)和IPP的(010).而在LLDPE含量大于50％时,LLDPE形成独立的相区,则不存在附生结晶现象,结果两种片晶的生长方向均垂直于拉伸方向.在135℃热处理15min,然后自然冷却的LLDPE/IPP共混物薄膜中,当LLDPE含量≤50％时,LLDPE仍然在IPP上附生生长,二者的结构关系与热处理前的相同.     

A study of melting of low density polyethylene by in situ WAXD

Analysis of low density polyethylene using in situ wide-angle X-ray diffraction shows that the melting process takes place over a broad temperature range with continuous destruction of lamellar laterals. This process exhibits decreasing rate as the temperature increases, indicating that in general each lamellae has less order in outer regions than in central ones. The shifts in angular position of the amorphous halo as a function of the temperature show that the amorphous phase in a semicrystalline sample is less disordered compared to that in an overall melted sample. As the temperature increases the unit cell parameter a increases, generating a crystalline phase density reduction.     

Deformation of oriented polyethylene

The deformation of polyethylene in terms of structural processes has been investigated by low-and wide-angle x-ray diffraction in the case of low-density and, to a lesser extent, high-density polyethylene. The samples possessed a range of simple textures which enabled the deformation processes to be identified. The results are interpreted in terms of a model of stacks of lamellae which have axes along the original draw direction and which deform by lamellar slip, chain slip, and lamellar separation. In most cases these processes accounted for the macroscopic strain but in some cases discrepancies were observed which could be accounted for by inhomogeneous deformation or by the effects of a distribution of lamellar thicknesses. Attempts were made to identify fibrillar slip, without success. The relative contributions of the various deformation processes are examined as a function of temperature and sample treatment by defining a compliance constant for each process. Below room temperature, the results are consistent with expectations based on the α and β mechanical relaxations, whereas the unusual effects at high temperatures are attributed to gradual melting. The compliance constants are also found to depend on the annealing temperature of the sample, and are used to predict the mechanical anisotropy. The volume changes accompanying lamellar separation are examined. They were less than expected in low-density polyethylene, but satisfactory agreement was obtained in high-density polyethylene. A general relation is suggested between volume changes and the lateral development of the lamellae. Hence in narrow lamellae the interlamellar layer can contract laterally whereas the greater constraints imposed by wide lamellae lead to void formation. Other effects examined include the reversibility of the processes which is most marked in the case of chain slip and which is explained by the presence of restoring forces in the amorphous regions including the fold surface. Finally, the differences between low- and highdensity polyethylene are highlighted, emphasizing the part played in the deformation by the amorphous component.     

Hierarchical crystalline structures induced by temperature profile in HDPE bars during melt penetration process

The hierarchical crystalline morphologies and orientation structures across the thickness direction in high-density polyethylene(HDPE) molded bars were investigated via a novel melt-penetrating processing method named multi-melt multi-injection molding(M3IM). The samples with various mold temperatures(20, 40 and 60 °C) were prepared, and the effects of the external temperature profile on the evolution of crystalline microstructures were studied. With scanning electron microscopy(SEM), the transition of crystalline morphology from ring-banded structure to oriented lamellae was observed with decreasing mold temperature, and the oriented lamellae were formed at the sub-skin layer of the samples at the lowest mold temperature, which was further testified by differential scanning calorimetry(DSC). With the decline of mold temperature, the degree of orientation, obtained from two-dimensional small angle X-ray scattering(2D-SAXS), was increased and long periods rose a little. Thus, decreasing mold temperature was beneficial to the formation of orientation structures because the relaxation of chains was weakened.