The effect of molecular weight on the structure and thermal properties of polyethylene

The morphology of polyethylene single crystals prepared isothermally in solution was found to be independent of molecular weight. The enthalpy of fusion, lamellar fold period, and optical appearance were invariant for samples grown from fractions ranging from 20,000 to 2,000,000 in molecular weight. The mass fraction of lamellae which thicken during heating decreased linearly with increasing log molecular weight. The melting temperature of the crystals was also nearly independent of molecular weight.

The superheating of polyethylene crystals was observed to be a function of molecular weight and morphology. At a comparatively high molecular weight the heating rate of the calorimeter exceeded the crystal melting rate, which shifted the observed melting temperature to an anomalously high value. The incorporation of defects within the crystals by irradiation-induced cross-links or chain entanglements increased the melting rate of the high molecular weight samples and thereby minimized the effects of superheating.

The apparent heat of fusion of melt crystallized polyethylene decreased linearly with increasing log molecular weight. In contrast to this behavior the crystallinity of single crystals from dilute solution was independent of molecular weight.

In previous papers we have shown that reorganization of polymer single crystals is suppressed by cross-linking [1—3]. With the appropriate selection of heating rate and irradiation dose, the melting temperatures of solution grown crystals of various morphologies were determined in the absence of lamellar thickening. The observed melting temperatures of polyethylene single crystals with different X-ray fold periods were found to fit the following expression:

T_m = T_m0[1—2σ_e/H_f?] with an equilibrium melting temperature (T_m0) of 145.8 ± 1.0°C and a surface free energy (σ_e) of 89 ± 5 ergs cm^?2 for a polyethylene crystal of infinite dimensions. In addition, at a constant heating rate it was observed that the fraction of crystals which thickened prior to melting decreased with increasing fold period.

Since cross-linking polyethylene increases the molecular weight of the material, it is instructive to investigate the reorganization characteristics of single crystals prepared from polyethylene fractions. Single crystals were prepared in xylene from molecular weight fractions of polyethylene and the effect of molecular weight upon the structure and thermal properties of the crystals was determined.     

On the formation of lamellae during annealing of extended chain crystals of radiation-polymerized trioxane

The structure changes of radiation-polymerized trioxane taking place during annealing have been studied by means of electron microscopy, X-ray small- and wide-angle scattering, and differential thermal analysis. The original fibrillar crystals, supposedly consisting of extended chains, change into lamellar crystals due to annealing at temperatures between 150° and 190°C. Lamella formation can be connected with the appearance of a long period of about 200A which is not observed in the unannealed sample.

During annealing within the same temperature range the X-ray reflections due to the twin structure of the original polytrioxane disappear, whereas the orientation of the fraction with its c-axis parallel to the c-axis of the parent trioxane remains unaltered.

The melting point of the lamellar crystals obtained by annealing is 186°-187°C and, therefore, considerably higher than the melting point (175°C) of crystals grown during cooling of a polytrioxane melt. The equilibrium melting point of an undisturbed extended-chain poly-oxymethylene single crystal must be still higher and may even approach 200°C.

On the basis of the electron-microscopical observations and the X-ray results, it is supposed that the process of lamella formation takes place continuously by reorganization of the whole structure including the already grown lamellae. The rate of this process is the faster the higher the annealing temperature. Some possible mechanisms are discussed but the final reasons for the observed structure changes are not known.     

高压下尼龙1010-单壁碳纳米管复合材料的结晶行为

 采用XKY-6×1200MN型六面顶压机,在不同温度、压力条件下处理30 min后制备了尼龙1010(PA1010)-单壁碳纳米管（SWCNT）复合材料的高压结晶样品,通过X射线衍射（XRD）、差热分析仪（DSC）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）,研究了高压处理样品的结晶行为、结构变化及形貌特征。结果表明：在1.0~2.5 GPa压力下,属于高压熔体结晶;在3.0和4.5 GPa压力下属于高压退火处理;高压结晶或高压退火均有助于聚合物片层晶体的增厚,并且高压熔体结晶的增厚效果优于高压退火处理。XRD结果表明,PA1010的三斜晶型在高压处理后保持不变,高压熔体结晶或高压退火都可以使(100)晶面和(010)晶面间距减小,即高压处理致使聚合物分子链紧密堆积。DSC结果表明：在高压熔体结晶过程中,升高压力和温度可以得到片层厚度较大的PA1010晶体;在2.0 GPa、350 ℃下获得的高压结晶样品的熔点和结晶度最高,分别达到208.5 ℃和64.6%。SEM和TEM结果表明：与常压结晶样品相比,高压结晶样品内部出现c轴厚度超过150 μm的大尺寸晶体;SWCNT与PA1010基体之间形成相互穿插的网络结构,刚性的SWCNT作为高压成核剂促进PA1010晶体生长和增厚。     

Melting of polymer single crystals studied by dynamic Monte Carlo simulations

We report dynamic Monte Carlo simulations of lattice polymers melting from a metastable chain-folded lamellar single crystal. The single crystal was raised and then melted in an ultrathin film of polymers wetting on a solid substrate, mimicking the melting observations made by using Atomic Force Microscopy. We observed that the thickness distribution of the single crystal appears quite inhomogeneous and the thickness increases gradually from facetted edges to the center. Therefore, at low melting temperatures, melting stops at a certain crystal thickness, and melting-recrystallization occurs when allowing crystal thickening; at intermediate temperatures, melting maintains the crystal shape and exhibits different speeds in two stages; at high temperatures, fast melting makes a melting hole in the thinnest region, as well as a saw-tooth-like pattern at the crystal edges. In addition, the linear melting rates at low temperatures align on the curve extrapolated from the linear crystal growth rates. The temperature dependence of the melting rates exhibits a regime transition similar to crystal growth. Such kinetic symmetry persists in the melting rates with variable frictional barriers for c -slip diffusion in the crystal as well as with variable chain lengths. Visual inspections revealed highly frequent reversals upon melting of single chains at the wedge-shaped lateral front of the lamellar crystal. We concluded that the melting kinetics is dominated by the reverse process of intramolecular secondary crystal nucleation of polymers.     

Frequency and temperature amplitude dependence of complex heat capacity in the melting region of polymers

Amplitude and frequency dependence of reversible melting of polycaprolactone (PCL) and an ethylene octene copolymer (EOM) were studied using temperature-modulated differential scanning calorimetry (TMDSC) (2?10^?1 Hz <f < 0.05 Hz) and shear spectroscopy (dynamic mechanical analysis, DMA) (5?10^?4 Hz < f < 100 Hz). It was found that the excess heat capacity of PCL is constant for temperature amplitudes in the range 5 mK < A_T < 2 K. The excess heat capacity decreases with frequency of temperature perturbation and tends to zero at about 0.1 Hz and 100 Hz for PCL and EOM, respectively. The constant excess heat capacity and the frequency dependence support the idea that reversible melting is related to a relaxation process on the surface of the polymer crystals. The occurrence of such a relaxation process was shown by shear modulus measurements in the same frequency and temperature region. The relaxation process is, in the melting region, much slower than main relaxation (glass transition). At low temperatures, a crossover can be seen, indicating the independence of both processes because of spatial separation. The main relaxation is related to the melt, while the other is related to the crystal surface.     

Self-poisoning of crystal nuclei in hard-rod liquids

We report a Monte Carlo study of the pathway for crystal nucleation in a fluid of hard, colloidal rods. In the earliest stages of nucleation, a lamellar crystallite forms. Subsequent thickening of this lamella is hampered by the fact that the top and bottom surfaces of this crystallite are preferentially covered by rods that align parallel to the surface. As a consequence, subsequent growth of individual crystals is stunted. Experimental evidence for such stunted crystal growth has recently been reported by Maeda and Maeda in experiments on suspensions of colloidal rods [Phys. Rev. Lett. 90, 018303 (2003)]]. The simulations suggest that, in experiments, the growth of multilayer colloidal crystals can be selectively enhanced by the application of an external aligning field.     

Multimolecule glasses and crystals of isotactic polystyrene

When spreading dilute solutions of isotactic polystyrene (iPS) on water, it was observed that different concentrations produce different morphologies in the resulting amorphous films. From 10^?1 to 10^?2 wt% solutions, ribbonlike films and circular platelets of multimolecule aggregates were obtained. After isothermal crystallization of these amorphous films, lamellae, incipient spherulites, and single-molecule crystals were observed. The formation of lamellar crystals lying flat on the substrate is an indication of the absence of epitaxy by the substrate, which would require the molecular chains to be in the surface plane and thus yield lamellae normal to the substrate. The dense packing in the center of sheaflike spherulites is caused by screw-dislocation growth, and it produces structures similar to shish kebabs. It was also observed that the lamellae grown from the center develop secondary growth spirals, causing the branching and splaying of the structure typical for spherulitic growth.     

Melting kinetics in polymers

In polymers, it is possible to obtain single chain forming single crystals. It is feasible to melt these crystals by simple consecutive detachment of chain segments from the crystalline substrate and its diffusion into the melt. However, complication in the melting process occurs when the chain in the process of detachment from the surface is shared between different crystals. Experimentally, a clear distinction in different melting processes is observed, by the differences in the activation energies required for the consecutive detachment of chain segments or of segments having topological constraints. The consecutive detachment of free chain segments starts at the melting temperature predicted from the Gibbs-Thomson equation, whereas higher temperature or time is required if the chain has to overcome the constraints.     

Solution-grown single crystals of random terpolymer liquid crystal polymers: Chain folding,morphology, and effect of annealing

Thin (100–150 Å thick) single crystals of large lateral dimensions have been grown from dilute solution in xylene of random terpolymer, thermotropic, liquid crystal polymers containing approximately equimolar amounts of C₆ or C₇ flexible segments, oxybenzoate, and dioxyphenyl. With both polymers having molecular lengths of 800 Å or longer, electron diffraction patterns indicate chain folding. The crystal structure, hexagonal, is less perfect than found previously for thin films of the C₇ (and C₅) polymer crystallized from the nematic state by slow cooling (orthorhombic). Annealing of the crystals below the crystal-liquid crystal transition results in merging of overgrowths and rough crystal edges into the main crystal and a surface roughening, followed, at longer times and/or higher temperatures, by lamella thickening. Annealing in the nematic state results in the development of rosettes of lamellae. Implications of the results for crystallization mechanisms and morphology of the nematic state are discussed.     

从昇华的NaCl蒸汽生长单晶体

半个世纪以来,从NaCl的蒸汽生长其单晶体的企图并没有得到正面的结果,解理面上的取向性结晶也尚未有超过数个分子层者。对于从溶液结晶出来时NaCl晶面发展的顺序,或者相对的重要性的研究已有肯定的结果。关于晶核或生长驼峰发端的位置从几率上看应在晶面何处的时尚理论未能从NaCl自溶液中结晶出来的实验加以证实或否定。为探索从蒸汽相生长NaCl单晶体的规律,本实验初步获得较大尺度的单晶体,并同时使取向性晶面上结晶达到1毫米以上的厚度。升华蒸汽来源于高纯度的从熔融体结晶出来的单晶体,并使母晶和结晶基面保持在近于NaC     

Stirring-induced crystallization of polyethylene from Decalin

Stirring-induced solution grown fibrillar crystals of polyethylene have been obtained from solution in Decalin. The crystals were examined by electron microscopy, and their melting and annealing behavior as well as their response to attack by fuming nitric acid were investigated. Crystals formed at lower temperatures exhibit the morphology and properties of conventional shish kebabs. Crystals formed at higher temperatures have a ribbonlike morphology and exhibit different properties from those of conventional shish kebabs.     

Influence of ion bombardment induced radiation damages in the crystal lattice on the angular regularlties in ion scattering

The paper examines the angular, spatial and energy distribution of ions scattered by various faces of silicon and germanium single crystals. It is shown that, due to the orderly arrangement of atoms in the crystal lattice, the orientation effects in the angular, spatial and energy distributions are observed at temperatures exceeding the annealing temperature of ion bombardment induced radiation damage. At high temperatures of the sample a reduction of the anisotropy in the angular, spatial and energy distributions of scattered ions was observed which is due to the changing transparency of the crystal resulting from the thermal vibrations of atoms in the lattice.

Results of investigations testify to the possibility of using the orientation dependences of phenomena occurring during the interaction of ions with crystals for determining the annealing temperature of ion bombardment damage and for studying the kinetics of the process of ion bombardment. It is shown that further studies along the same lines as conducted by the authors of this paper could develop a technique for the quantitative estimation of ion bombardment damage in crystals and also in thin epitaxial films.     

Photoluminescence of ZnTe and CdTe single crystals grown with halogen-containing transport gases

Bulk ZnTe and CdTe single crystals were grown using Cl-, Br-, and I-containing transporting gas agents. Photoluminescence spectra consisting of a free-exciton band and a low-energy band of extrinsic origin were studied at various temperatures, excitation levels, and time delays. It is shown that the low-energy band can be assigned to the emission of donor-acceptor pairs. No noticeable differences were observed between the emission spectra of crystals grown with Cl, Br, and I.     

Growth, structural, spectral, mechanical and dielectric characterization of RbCl-doped l-alanine hydrogen chloride monohydrate single crystals

Pure (undoped) and RbCl-doped LAHC single crystals were grown successfully by the solution method with the slow evaporation technique at room temperature. The grown crystals were colourless and transparent. The solubility of the grown samples were found out at various temperatures. The lattice parameters of the grown crystals were determined by the single crystal X-ray diffraction technique and the diffracting planes were indentified by recording the powder X-ray diffraction pattern. UV-visible transmittance studies were carried out for the grown samples. Chemical analysis and atomic absorption studies indicate the presence of rubidium in the doped LAHC crystals. Nonlinear optical studies reveal that the SHG efficiency increases when the LAHC crystal is doped with rubidium chloride (RbCl). From microhardness studies, it is observed that the RbCl-doped LAHC crystal is harder than the pure sample. It is observed that the dielectric properties of the LAHC crystal are altered when it is doped with rubidium chloride.     

不同温度生长DKDP晶体的性质

用点籽晶快速生长法在不同温度区间生长了四块氘含量60%DKDP晶体,观察了不同温度下晶体生长过程,同时测量了晶体的透过光谱、摇摆曲线和1053nm激光损伤阈值。结果表明,在高温区间晶体生长外形优于低温生长的晶体。不同温区晶体的透过光谱并无明显变化,但是随着生长温度的降低,晶体的结构完整性变差,1053nm激光损伤阈值降低。     

-1不同温度生长DKDP晶体的性质

用点籽晶快速生长法在不同温度区间生长了四块氘含量60% DKDP晶体,观察了不同温度下晶体生长过程,同时测量了晶体的透过光谱、摇摆曲线和1053 nm激光损伤阈值。结果表明,在高温区间晶体生长外形优于低温生长的晶体。不同温区晶体的透过光谱并无明显变化,但是随着生长温度的降低,晶体的结构完整性变差,1053 nm激光损伤阈值降低。     

Metallic films forming on the surface of alkali-halide crystals during thermal diffusion of intracrystalline immpurities

It was detected for the first time that films consisting of a transition-metal-based structure form (via thermal diffusion of intracrystalline impurities) on the surface of alkali-halide (LiF, NaF) crystals activated by transition metals Co, Ni, or Mn. The thickness, density, and composition of the films are shown to be different, depending on the heat treatment conditions. The crystals were annealed at temperatures varying from 473 to 1073 K in vacuum and air. The surface structures forming upon annealing in vacuum exhibit magnetic properties. The films were studied by optical, x-ray fluorescence, and electron spectroscopy to reveal the mechanisms of transition-metal film formation during thermal annealing.     

Exploring the melting of a semirigid-chain polymer with temperature-resolved small-angle <Emphasis Type="Italic">X</Emphasis>-ray scattering

The thermal behavior of semirigid semicrystalline polymers differs significantly from that of flexible-chain polymers. The origin of the differences is believed to lie in the higher energy expenditure associated with the formation of adjacent re-entry folds at the crystalline surface in the case of semirigid chains. The effect of constraints imposed by the interlamellar amorphous regions on the neighboring crystals was studied with temperature-resolved synchrotron radiation small-angle X-ray scattering (SAXS). The analysis of SAXS patterns with a generalized paracrystalline lamellar stack model indicates that melting of a semirigid-chain polymer is not a random process but that the crystals grown in the smallest amorphous gaps melt first. This suggests that the hitherto largely neglected geometrical confinement effects may play an important role in determining the thermodynamic stability of semirigid-chain polymer crystals.Received: 5 March 2004, Published online: 4 May 2004PACS: 61.41. + e Polymers, elastomers, and plastics - 64.70.Dv Solid-liquid transitions - 81.10.Aj Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation     

Structural-phase state of chromium-fullerite-chromium films subjected to heat treatment in vacuum

The patterns of change in the structure and the phase composition of chromium-fullerite-chromium films subjected to heat treatment in vacuum at different temperatures (470, 570, and 620 K) are studied via scanning electron and atomic force microscopies, X-ray diffraction, reflection-electron diffraction, and X-ray spectral microanalysis. It is found that sample annealing at 470 and 570 K leads to recrystallization of the fullerite and the emergence and growth of a new Cr _x C₆₀ phase, accompanied by volumetric changes that generate internal mechanical stresses. During the process of stress relaxation, the film starts to crack, and lamellar crystallites of the fullerite phase start to grow. Film destruction and the growth of C₆₀ crystallites on the surface are not observed in chromium-fullerite-chromium films annealed at 620 K.     

Solution-grown crystals of poly(vinyl alcohol)

The growth mechanism and crystalline texture of solution-grown crystals of poly(vinyl alcohol) have been studied by electron microscopy. From morphological data it is shown that single crystals of poly(vinyl alcohol) (PVA) have two growth faces, i.e., the (100) and (101) fold planes. It is suggested that the parallelogrammic single crystals grow from the center by molecular folds. The rate of addition on the (100) plane is about three times larger than that on the (101) plane. Twinning of the PVA crystals takes place at the (100), (001), (101), and (101) planes at the time of the nucleation or during the growth. The single crystal and twins of PVA are corrugated lamellae. Granulated structure is observed on the surface in a figure similar to the external form of the crystals. Dark-field micrographs show that the PVA single crystals possess a mosaic structure with the arrangement of the blocks deviating slightly from parallel alignment in the lamella. The reactivities of single crystals to various aldehydes indicate that the single crystals contain crystal lattice defects.