Three-dimensional shape of polyethylene single crystals grown from dilute solutions and from the melt

Three-dimensional morphology of polyethylene single crystals grown from dilute solutions and from the melt has been examined by atomic force microscopy. The observation of single crystals clarifies the morphology of chair-type as well as hollow pyramidal type for solution crystallization. From the melt, only chair-type was obtained. It has been confirmed that the screw dislocations in the chair-type follow a selection rule of the handedness in a manner to relieve the distortion in the chair-type. The meaning of the selection is discussed in connection with the twisting correlation in the banded spherulites grown from the melt of non-chiral polymers, such as polyethylene.     

Study on single crystals of butyl branched polyethylene in the presence of electric field

Single crystal of butyl branched polyethylene with various molecular weight formed from the melt in the presence of electric field was studied. It was found that electric field influenced morphology and structure of the butyl branched polyethylene single crystals formed. The lateral habits of the single crystals were circular shape, which was different from truncated lozenge or lenticular shape single crystals formed from the melt in the absence of electric field. The stems in the single crystals formed in the presence of electric field were perpendicular to the basal plane of the single crystals, which was different from chain tilting in single crystals formed from the melt in the absence of electric field. The electron diffraction patterns showed that the structure of the circular single crystals was a quasi-hexagonal with looser chain packing. This looser chain packing was favorable to thickening growth of single crystals through chain sliding diffusion. The thickness of the single crystals was much larger and depended on molecular weight. It indicated that the single crystal in the presence of electric field should be an extended chain type single crystal.     

Hydrothermal synthesis of giant single crystals of MFI type zeolite: Modified bulk material dissolution method

In the present study, we aimed at the development of an easy and reproducible synthesis method of giant MFI zeolite crystals. We used a new synthesis method, modified bulk material dissolution (M-BMD) method, in which a piece of quartz glass tube was inserted as a silica source in a small PTFE sleeve together with ZSM-5 powder as seed crystals. We adopted a higher HF concentration (25 mmol/25 ml) of the synthesis solution than that usually used. By using this method, only a few crystal nuclei were formed separately from each other on the surface of the quartz glass tube in the PTFE sleeve and giant zeolite crystals were reproducibly synthesized on it. Giant single crystals of MFI as large as 5.0 mm were successfully synthesized at 453 K for 90 days by using the M-BMD method.     

Annealing behavior of solution grown polyethylene single crystals

The morphology evolution of solution grown polyethylene single crystals has been studied upon annealing below their melting temperature by using atomic force microscopy (AFM). AFM investigations have been performed ex situ, which means AFM investigations at room temperature after the annealing treatment, and in situ at real temperature. Beside the well-known Swiss-cheese and picture frame appearance a novel morphological feature has been observed: the formation of saw-tooth-like patterns at the edges of the crystals, which become more and more pronounced with increasing annealing temperature. Controlled dissolution experiments have resulted in similar saw-tooth-like edge patterns of the single crystals, which indicate that the initial organization of the crystals influences the reorganization. This reorganization behavior upon annealing can be related to the presence of nano-sized fold defects, which have been monitored using the technique of n-alkane surface decoration.     

Crystal growth kinetics and the lateral habits of polyethylene crystals

The lateral growth habits of lamellar crystals of polyethylene grown from solution can be typified in terms of the aspect ratio, r, which is the ratio of the dimension of the crystals in the b-axis direction to that in the a-axis direction. The aspect ratio depends on crystallization temperature, undercooling, solvent, polymer concentration and molecular weight. At steady state growth, r can be expressed in terms of the ratio of growth rates normal to the {110} and (200) faces. Expressing the growth rates in terms of the kinetic theory of polymer crystal growth yields an expression which is used to analyse experimental results on the effect of temperature and concentration on the lateral growth habits of crystals grown from xylene. Using as two adjustable parameters the ratio of end surface free energies for the two growth surfaces and the ratio of the lateral surface free energies to fit the rversusΔT data permits the determination of these ratios with high sensitivity. The actual values obtained are dependent upon concentration, the assumed growth regime, and, most importantly, ?, the parameter in the growth rate equations apportioning the bulk free energy change to the forward and backward steps in the stem deposition process.     

Magnetocrystalline anisotropy study of Co-substituted M-type strontium hexaferrite single crystals

The study on the magnetocrystalline anisotropy (MA) of La–Co co-substituted strontium hexaferrite (La–Co SrM) shows a joint effect of Fe²⁺ and Co²⁺ ions in the enhancement of MA. Since the role of Fe²⁺ single ion has been studied with La-substituted strontium ferrite, in this work, Co-substituted strontium hexaferrite SrFe_12-xCo_xO₁₉ (Co-SrM) single crystals were successfully grown for 0 ≤ x ≤ 0.31 by the Na₂CO₃ flux method to elucidate the anisotropy of Co²⁺ single ions. Co-substitution in this preparation condition has a limit solubility of 0.31 and enhances uniaxial magnetic anisotropy field H_A by 19% for 0.03 = x ≤ 0.11, with a mere loss of 7% of saturation magnetization M_S at 5 K. The enhanced H_A of Co-SrM is reported for the first time, even higher than that of La–Co SrM, which is suitable to be used as permanent magnets in this concentration range. But with the further substitution of Co, the planar anisotropy of x = 0.31 was observed at 5 K. The potential nonlinear magnetic structure of Co-SrM remains to be discovered for magnetoelectric effects. This work is also of great significance as a complement to the magnetocrystalline anisotropy study of La–Co SrM.     

Catalytic combustion of methane on palladium single crystals

Catalytic methane combustion was studied over the palladium single crystals Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0). Under lean reaction conditions at 600 K (O₂:CH₄ = 10:1), stoichiometric palladium oxide was formed with an increase in surface area by a factor of approximately two. The oxide phase formed a “cauliflower-like” surface structure composed of approximately 4 nm sized semispherical oxide agglomerates. This oxide structure was independent of the original metal single crystal orientation. The turnover rates over the oxide structure starting with metal single crystals were 0.7 s⁻¹ on Pd(1 1 1), 0.9 s⁻¹ on Pd(1 0 0) and 0.9 s⁻¹ on Pd(1 1 0) at 600 K, 160 Torr O₂, 16 Torr CH₄, 1 Torr H₂O and N₂ balance to 800 Torr, suggesting that the methane combustion reaction is independent of the initial structure of the catalyst. Methane combustion on palladium single crystals experienced an activation period in which the initial turnover rates based on the initial Pd surface area were about 1/8–1/4 of the steady-state rates determined based on the oxide surface area. This activation period was caused by the slow oxidation of palladium single crystals and concomitant surface area increase during reaction. The increase in surface area happened mostly in the first 10 min of reaction. Carbon dissolution into the crystal was not found during methane combustion under reaction conditions in excess oxygen.     

Indiscriminate revelation of dislocations in single crystal SiC by inductively coupled plasma etching

To reveal dislocations in SiC wafers, conventionally, molten KOH etching method has been widely used. However, when highly doped sites exist on the wafer, the molten KOH etching method is not applicable owing to the enhanced isotropic electrochemical etching phenomenon. In this study, plasma etching is first applied to reveal dislocations in a 4H-SiC wafer with both highly doped and lightly doped areas. The mechanisms of dislocation revelation by dry etching have been theoretically analyzed and it has been revealed that the dislocation revelation ability of dry etching is highly related to the temperature of the etching process. The results demonstrate that inductively coupled plasma (ICP) etching can maintain its effectiveness for dislocation revelation of SiC wafers regardless of the doping concentrations. This work offers an alternative approach to indiscriminately and accurately reveal dislocations in SiC wafers.     

Annealing of polyethylene crystals in the hexagonal phase

The kinetics of thickening of polyethylene crystals in the hexagonal phase is studied. Distributions of thickness, L, of bands and/or lamellae are obtained by electron microscopy for a specimen annealed at p=6.1 kbar, T=242°C for 30, 300, 3600 and 86400 seconds. The dependence of the average band thickness $\text{L}\text{?}\text{(t)}$ on the annealing time t can be expressed as $\text{L}\text{?}\text{(t)=46·}\text{log}\text{(}\text{t}\text{t}{}_0\text{)+}\text{L}\text{?}\text{(t}{}_0\text{) [}\text{nm}\text{]}$. Moreover, these distributions can be superposed on each other if we scale L by $\text{L}\text{?}\text{(t)}$. A model for the thickening process of a lamellae is proposed; coalescence of two bands occurs by disappearance of the boundary with the rate proportional to exp(?αL). The value of α depends on the mobility of defects in the boundary and is discussed in detail.     

Plastic deformation of polyethylene crystals as a function of crystal thickness and compression rate

Plastic deformation of polyethylene (PE) samples with crystals of various thickness was studied during uniaxial compression with initial compressive strain rates of 5.5×10⁻⁵, 1.1×10⁻³ and 5.5×10⁻³ s⁻¹. Samples with a broad range of crystals thickness, from usual 20 up to 170 nm, were obtained by crystallization under high pressure. The samples underwent recoverable compression below the compression ratio of 1.05-1.07. Following yield, plastic flow sets in above a compression ratio of 1.12. At a compression rate of 5.5×10⁻⁵ s⁻¹ the yield stress increases with the increase of crystal thickness up to 40 nm. For crystals thicker than 40 nm the yield stress levels off and remains constant. This experimental dependence was compared with the model developed on the basis of classical crystal plasticity and the monolithic nucleation of screw dislocations from polymer crystals. In that model contrary to the experimental evidence, the yield stress does not saturate with increase of crystal thickness. The activation volumes determined from strain rate jump experiments and from stress relaxation for crystals thicker than 40 nm are nearly constant at a level of 8.1 nm³. This activation length agrees very well with 40 nm for crystal thickness above which the yield stress levels off. It is proposed, as shown in a companion communication, that for PE crystals thicker than 40 nm two other modes of dislocation emission in the form of half loops of edge and screw dislocations begin to govern the strain rate, which no longer depend on lamella thickness.     

Influence of chemical structure on enzymatic degradation of single crystals of PCL-b-PEO amphiphilic block copolymer

Solution-grown lamellar single crystals of PCL homopolymer and amphiphilic block copolymers of PCL-b-PEO-b-PCL and PCL-b-PEO-FG (functional groups FG = NH₂, OCH₃) were prepared by self-seeding procedures. The crystal structure and morphology of these single crystals were mainly studied by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results indicated that the shorter PEO blocks as well as the functional groups bonded to the end of PEO block are in an amorphous state and located in the surface of single crystals. The enzymatic degradation of single crystals prepared from these homopolymer and amphiphilic block copolymers has been well-studied to demonstrate the effects of chemical structure on degradation behavior. The single crystals showed similar morphologies before enzymatic degradation but very different surface character after enzymatic degradation. Such differences resulted from the PEO block and functional end groups. The results of this work indicated the important role of chemical structure in determining biodegradation behavior.     

Real time observation of crystallization in polyethylene oxide with video rate atomic force microscopy

Video rate atomic force microscopy (VideoAFM), with a frame rate of 14 frames/s and a tip velocity of up to 15 cms⁻¹, is used to image polyethylene oxide films during crystal growth. The capabilities of VideoAFM when applied to semicrystalline polymer surfaces are explored. Image quality comparable to that found with conventional contact AFM is achieved but with a nearly 1000 times improvement in time resolution. By applying the technique to the real-time observation of crystal growth, different modes of rapid crystallization are followed in real time. Observation of the spherulite growth front allows measurement of growth rates at the lamellar scale, from which a factor of two difference in the rate of radial growth to the rate of tangential growth is observed, confirming that the elongated nature of spherulite lamellae is due to geometric constraints rather than an inherent fibrillar character. Measurements on screw dislocation growth, when large amounts of crystallizable material is trapped at the surface show that the terrace height does not influence the rate of crystal growth, confirming that under these conditions processes at the lamellar growth front control the rate of growth. When only a thin film of molten material is left on the surface of the already crystallized film dendritic growth is observed, implying a diffusion controlled process under these far from equilibrium conditions.     

Suppression of crystallization in a plastic crystal electrolyte (SN/LiClO4) by a polymeric additive (polyethylene oxide) for battery applications

A basic problem with many promising solid electrolyte materials for battery applications is that crystallization in these materials at room temperature makes ionic mobilities plummet, thus compromising battery function. In the present work, we consider the use of a polymer additive (polyethylene oxide, PEO) to inhibit the crystallization of a promising battery electrolyte material, the organic crystal forming molecule succinonitrile (SN) mixed with a salt (LiClO₄). While SN spherulite formation still occurs at low PEO concentrations, the SN spherulites become progressively irregular and smaller with an increasing PEO concentration until a ‘critical’ PEO concentration (20% molar fraction PEO) is reached where SN crystallization is no longer observable by optical microscopy at room temperature. Increasing the PEO concentration further to 70% (molar fraction PEO) leads to a high PEO concentration regime where PEO spherulites become readily apparent by optical microscopy. Additional diffraction and thermodynamic measurements establish the predominantly amorphous nature of our electrolyte-polymer mixtures at intermediate PEO concentrations (20-60% molar fraction PEO) and electrical conductivity measurements confirm that these complex mixtures exhibit the phenomenology of glass-forming liquids. Importantly, the intermediate PEO concentration electrolyte-polymer mixtures retain a relatively high conductivity at room temperature in comparison to the semicrystalline materials that are obtained at low and high PEO concentrations. We have thus demonstrated an effective strategy for creating highly conductive and stable conductive polymer-electrolyte materials at room temperature that are promising for battery applications.     

Effects of sintering aid and atmosphere powder on the growth of (K0.5Na0.5)NbO3 single crystals fabricated by solid-state crystal growth method

(K_0.5Na_0.5)NbO₃ (KNN) + x (= 1, 0.5, 0.05, and 0) wt%Co₃O₄ single crystals were fabricated by a solid-state crystal growth method with a KTaO₃ seed crystal and a KNN atmosphere powder, and the effects of the sintering aid content x and the addition of Co₃O₄ to the atmosphere powder on the growth of the single crystals were investigated. The formation of pores in the single crystals was suppressed by a decrease of x, which, however, decreased the crystal growth length. On the other hand, dense and large KNN single crystals could be fabricated by sintering with a KNN + 5 wt%Co₃O₄ atmosphere at x = 0. The dielectric, ferroelectric, and piezoelectric properties of the KNN single crystals were comparable to those of reported (K,Na)NbO₃ single crystals. These results would be useful for fabricating dense and large single crystals by the solid-state crystal growth method.     

Study of a skin-core type of crystallinity distribution within polyethylene specimen crystallized under high pressure

The regularity of ‘skin-core’ type of crystallinity distribution within the polyethylene cylindrical specimen crystallized under high pressure was investigated via modulated differential scanning calorimeter (MDSC), wide angle X-ray diffraction meter (WAXD), dynamic mechanical analyzer (DMA) and scanning electron microscope (SEM). The relationship between the gradient of high pressure stress field and the ‘skin-core’ structure of polyethylene was also established. The results indicate that there exists a distinguished difference of crystallinity, morphology and dynamical mechanical properties between skin layer and core region within the cylindrical specimen crystallized under 650 MPa pressure. The measurement data of the non-reversible heat flow of MDSC and WAXD for both skin layer and core region are in accordance with that marked difference. According to the law of stress decay of high pressure stress field and the high pressure crystallization theory of polymers, it was found that the uneven distribution of high pressure stress field, especially, the high pressure decay effect at the edge of plunger and cylindrical channel of the mold is proposed as the principal factor to the formation of the ‘skin-core’ type of crystallinity distribution within the polyethylene cylindrical specimen crystallized under high pressure.     

The crystallization of low-density polyethylene: a molecular dynamics simulation

Three models (star-shaped, H-shaped, and comb-shaped polyethylenes) are used to study the crystallization behavior of low-density polyethylene at the molecular level by means of molecular dynamics simulation. It is shown that, for the three types of polyethylene corresponding to the models, the neighboring sequences of trans bonds firstly aggregate together to form local ordered domains, and then they coalesce to a lamellar structure. In the process, the branching sites are rejected to the fold surface gradually. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the number of the branch sites and the length of the branch play an important role in determining the formation of the lamellar structure. The longer the length of the branch and the fewer the number of the branch sites, the more perfect lamellar structure can be formed.     

Characteristics of slug flow in a fluidized bed of polyethylene particles

The slug flow behavior of polyethylene particles was examined in a fluidized bed of 7 cm ID and 50 cm in height. The employed polymer particles were high density polyethylene (HDPE) with the average particle size of 603 μm. The slugging flow of polyethylene particles was analyzed from the measured pressure drop signals by classical statistical methods such as absolute average deviation, probability density function, power spectrum, auto-cor-relation, and cross-correlation. The results show that in spite of high dielectric constant of polymer particles, the slugging phenomena such as incipient slugging velocity, slugging frequency and slugging rise velocity were very similar to the Geldart B type non-polymeric particles. It was observed that slug frequencies decreased with gas velocity and the limiting slug frequency was observed for the gas velocities in this study.     

Vibrations of crystallographic defects associated with a single chain in polyethylene

The vibrational behaviour of crystallographic defects associated with a single chain were investigated for a dispiration, disclination, and dislocation in polyethylene. An approximate longitudinal modulus for the defects was determined by using conformational calculations to estimate the energy changes associated with changes in the length of a defect. This modulus, combined with the mass per unit length of the defect, was used to estimate the lowest longitudinal frequency of the defect, which was found to be around 100 cm^?1 for all the defects considered. Normal mode vibrational calculations for oligomers containing defects showed that the predicted lowest longitudinal modes could be identified by examination of the displacements associated with modes occurring in the estimated frequency range. It was shown that the defects could be considered as localized oscillators embedded in the crystal and coupled to the vibrational modes of the crystal. The presence of defects provides special mechanisms for coupling light waves and lattice vibrations in the crystal which may affect the Raman spectrum.     

Studying the length of trans conformational sequences in polyethylene using Raman spectroscopy: a computational study

The long standing question of how long a trans sequence in a polyethylene needs to be in order to contribute to the all-trans Raman bands at 1060 and 1130 cm⁻¹ is addressed. Various hexadecane, C₁₆H₃₄, conformers were studied using ab initio type calculations revealing vibrational frequencies and intensities. We found that only trans sequences longer than 10 trans bonds reveal Raman intensity primarily selectively at the same frequencies as the C-C stretching modes arising from crystalline polyethylene. Since it is statistically highly unlikely that such long trans sequences occur in the amorphous phase, intensity arising from all-trans sequences not belonging to crystalline structure may only be expected as a consequence of strong non-equilibrium conditions (e.g. shear), or the presence of tie-molecules.