Obituary: Professor Valery Privalko

Gold decoration was applied to elucidate the morphology of annealed and unannealed oriented structures produced by shearing of polyethylene single crystals of known thicknesses, and the morphology of drawn and undrawn “shish kebabs” obtained by stirrer-induced crystallization. Distinct Au periodicities were observed perpendicular and parallel to the unannealed oriented structure, revealing distinctly the presence of microfibrils and an alternation of crystalline and noncrystalline regions within a given microfibril of which the periodicity is, however, found to be much larger than the corresponding original crystal thickness. Annealing resulted in fibrils being completely replaced by lamellae standing edgewise with the Au particles decorating directly on top of the edge but concentrated mainly on the folds leaving the crystalline region almost entirely void of gold. Deformation studies revealed an unusual ductility associated with the shish kebab structure, with the kebab splitting first, then disappearing with increasing elongation, and finally, resulting in a uniform fiber. The kebab disappearance implies that it is molecularly integrated with the interkebab material and not a completely separate chain-folded lamella attached to it. Distinct Au periodicities were observed perpendicular and parallel to the drawn shish-kebab fibers similar to the sheared single crystals. Annealing experiments and the high ductility suggest that the interkebab material is not of a fully extended-chain morphology.     

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.     

Scanning force microscopy of the crystalline/amorphous interface of ultradrawn poly(ethylene)

We report on high-resolution experiments by Scanning Force Microscopy (SFM) indicating the tight folding of neighbouring molecular chains in Poly(Ethylene) (PE) crystals. Ultradrawn PE films were prepared in the stacked lamellar morphology exhibiting crystalline lamellae and amorphous parts. The [001] direction of the lamellae is aligned parallel to the substrate surface resulting in {hk0} planes perpendicular to the symmetry axis of the probing tip. This preparation technique allows the direct observation of the molecular arrangement in polymeric crystals as well as an investigation of the crystalline/amorphous interface by SFM: at lower magnification, crystalline and amorphous parts of the film can be distinguished clearly. High-resolution imaging on the crystalline lamellae reveals a PE pitch height of 0.26±0.02 nm while the interchain spacing measures 0.50±0.02 nm consistent with (100) lattice planes aligned parallel to the substrate surface. Finally, the molecular folding at the edges of the lamellae has been studied. Evidence is found for the adjacent reentry of individual molecules at the edge of (100) surfaces.     

Crystallization of polycarbonate from the glassy state. part I. Thin films cast from solution

The morphology of glassy amorphous thin polycarbonate film cast from solution is affected by thermal treatments. Annealing above 80° C and below T_g results in an increase in the size of the ordered regions, nodules, up to several hundred Angströms. The crystallization process from the glass, taking place at 145° C, is divided into three major steps. At first the nodules merge into patches which aggregate to form lamellar planar structures. In some cases the planar structures are well-formed single crystals. Following this, spherulitic arms develop from the planar structures as centers. These arms at first consist of aggregates of large nodules which recrystallize to form lamellae; the final morphology is spherulitic in nature. The effect of film thickness and of several substrates on the morphology has been observed. Applying stress at room temperature to the crystalline film results in a breaking up of the lamellae into small blocks.     

Role of the entangled amorphous network in tensile deformation of semicrystalline polymers

Being composed of crystalline lamellae and entangled amorphous polymeric chains in between, semicrystalline polymers always show a complicated deformation behavior under tensile deformation. In recent years, the process of tensile deformation was found to exhibit several regimes: intralamellar slipping of crystalline blocks occurs at small deformation whereas a stress-induced crystalline block disaggregation-recrystallization process occurs at a strain larger than the yield strain. The strain at this transition point is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. We report experimental evidence from true stress-strain experiments that support this argument. It is emphasized that tie molecules, which connect adjacent lamellae, are of lesser importance with respect to the deformational behavior.     

Amorphous-like Raman spectra of GaP microcrystals

Raman scattering from gas-evaporated GaP microcrystals smaller than about 400 Å has been investigated. As the crystalline size decreases from ～400 to ～170 Å, drastic changes in the Raman spectrum are observed; the TO and surface phonon peaks broaden and shift, and they strongly overlap with each other, finally transforming into a broad structure; broad bands located at around 80 and 200 cm^-1 appear and grow rapidly. The microcrystals smaller than about 250 Å show spectra very similar to those of amorphous GaP, even though the electron diffraction patterns prove that they are crystalline. The amorphous-like Raman signals seem to come from the surface layers of the microcrystals.     

Lamellar changes during heat treatment in isotactic polystyrene crystals

In the present investigation, the melting and thickening processes in lamellar crystals of isotactic polystyrene have been studied by transmission electron microscopy. It is shown that under properly chosen experimental conditions for the polymer, one can continuously follow the physical changes involved during the thickening as well as melting of lamellar crystals on heat treatment. The study of crystals grown at different temperatures reveals that melting of a single lamella starts at various areas. A commonly observed feature is the preferential melting of elastically bent parts of a lamella. It is indicated that the occurrence of melting in the various parts is due to a structural variation along the surface of lamellae resulting in a hindrance of the lamellar thickening process. At particular temperatures, melting of lamellar crystals is followed by recrystallization. The occurrence of a solid-stage thickening process is the major process so far observed during slow heat treatments. Considerable change in surface structure of the crystals grown at different temperatures is clearly reflected during the heat treatment. The rates of heating have marked influence on the resulting morphology of the crystalline superstructures.     

Stacked-Lamellar Structure in High-Speed Spun PET Fibers as Revealed by TEM

Morphological study of high-speed spun poly(ethylene terephthalate) (HSS-PET) fibers spun at 6 km/min was performed by a combination of alkaline etching, surface replica method, and transmission electron microscopy. In the two-stage replica of the alkaline-etched fiber, the stacked-lamella-like structure was observed, in which lamella-like striations were stacked in a direction leaning at about 40° away from the fiber axis. The spacing between the adjacent striations, namely the lamellar periodicity, was measured to be 10–30 nm. We have proposed that the stacked-lamellar structure, which is mainly composed of mosaic lamellar crystals and has the extended-chain crystals connecting the adjacently stacked lamellae, is an appropriate model for the fiber structure of HSS-PET fibers.     

Deformation and stress-induced transformation of polybutene-1

The deformation at room temperature of thick (ca. 2 mm) and thin (ca. 1000 Å) films of polybutene was characterized by electron microscopy and electron x-ray diffraction. The thin films, as studied, draw by micronecking whereas the thick films draw uniformly. In both types of samples the details of the deformation process varied with the orientation of the lamellae in the original spherulitic structure; in all cases, however, the lamellae broke up into “mosaic” blocks prior to uniform fibril formation. The tetragonal-hexagonal phase transformation accompanying the deformation of fresh polybutene films generally appeared to occur within the blocks except in those situations in which the lamellae are being drawn perpendicular to the molecular axes, that is, parallel to the lamella surface; in the latter case the tetragonal form can be at least partially retained until unfolding and fibril formation occurs.     

Structural Interpretation of Eyring Activation Parameters for Tensile Yielding Behavior of Isotactic Polypropylene Solids

The temperature and strain rate dependence of the stress-strain curves of isotactic polypropylene were analyzed on the basis of Eyring kinetic theory. The molecular and structural basis underlying the parameters, such as activation volume and activation energy, used by the Eyring analysis were studied using the lamellar cluster theory, where a stacked lamella acts as a basic structural unit under yielding and necking deformation. It was suggested that, at lower temperatures, the activation volume corresponds to chain slippage within crystalline lamellae in the lamellar clusters whereas at higher temperature the increase in activation volume results in intralamellar slip corresponding to the α₂ relaxation.     

Molecular Packing in Crystalline Domains of Unsymmetrical Poly(benzoxazole-imide): Investigation using Modeling of a Model Compound

To probe the molecular packing in crystalline domains of an unsymmetrical poly(benzoxazole-imide) (BPDA-BOA) introduced alternating phenylbenzoxazole and bisphthalimide units via the two-step polymerization of 5-amino-2-(4- aminophenyl)benzoxazole (BOA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), the crystal structure of its model compound, 5-phthalimido-2-(4-phthalimidophenyl)benzoxazole (PA-BOA), was investigated using powder wide-angle X-ray diffraction (WAXD) combined with molecular modeling using a Materials Studio program. Powder WAXD pattern of the model compound can be well indexed in terms of amonoclinic unit cell with parameters of a = 12.005 Å, b = 3.837 Å, c = 23.562 Å, β = 96.711°. There are two molecules in the unit cell with the space group of P2 (3). Based on the crystal structure of the model compound powders and the WAXD analysis of poly(benzoxazole-imide) film, it can be concluded that the projection of the monomer repeat length of the poly(benzoxazole-imide) chains in the chain directionis around 19.0 Å, and the interchain side-by-side distance and face-to-face distance between the centroids of two neighboring BPDA-BOA chains are around 6.10 and 4.01 Å, respectively. The difference of molecular packing between the poly(benzoxazole-imide) chains in their crystalline domains and the model compounds in their crystals are also presented. Both the side-by-side and face-to-face π-π stacking distances between two neighboring polymer chains in the poly(benzoxazole-imide) crystalline domains are significantly larger than the corresponding values between two neighboring molecules in the model compound crystals due to a more kinked and twisted conformation.     

Laser Light and X-Ray Diffraction Studies of Oriented Isotactic Polypropylene (iPP) Prepared by Temperature Slope Crystallization

Isotactic polypropylene (iPP) film was melt-crystallized in a temperature gradient. The iPP film showed well oriented α- and β-crystalline textures along the gradient. The crystalline structure, phase transition boundary and lamellar twisting were examined by X-ray diffraction and laser light diffraction (LLD). On the α-β boundary, LLD shows a sharp streak perpendicular to the boundary, where the a-axis of the β-crystal is oriented perpendicular to the temperature gradient. Apart from the boundary, the a-axis of the β-crystal becomes parallel to the gradient. The β-crystal shows lamellar twisting with a pitch of 200 μm at room temperature. When heated the β-crystal, the lamellar distance of 295Å at room temperature decreases to 285Å at 80–100°C and then increases to more than 300Å above 120°C. During the heating, the value of the twist period increases from 200 to 210 μm at 90–100°C, and then to above 224 μm at 140°C. The increase of the twist period is related to the increasing crystalline thickness of the β-lamellae.     

CRITICAL STRAINS DETERMINING THE YIELD BEHAVIOR OF s-PP*

Video-controlled tensile deformation experiments giving true stressstrain curves were carried out on samples of s-PP. Dividing the total tensile deformation into elastic and plastic parts shows three critical strains (A, B, C) for which the differential compliance and the recovery properties change. It was found that all critical strains remain constant on varying the crystallite thickness or the testing temperature. Point A marks the end of the linearelastic range. The yield point, as given by the maximum on the engineering stress-strain curve for a necking sample, essentially corresponds to the second critical point, although the yield point is shifted relative to B to higher strains and varies with temperature. It is associated with the collective onset of inter- and intralamellar slip processes. At the third critical strain (C), the lamellae become destroyed, and fibrils are formed. The texture changes accompanying the drawing determined by a simultaneous measurement of wide-angle X-ray scattering (WAXS) patterns indicate a common activity of intra- and interlamellar slip processes setting in at A and a dominance of intralamellar block slips at moderate deformations above B. The Young's modulus and the yield stress show a large decrease with temperature, whereas there are only minor changes in the crystallinity. We understand it as resulting from a change of coupling, both between the crystalline lamellae and of the blocks within the lamellae.

     

RELATIONSHIPS BETWEEN MECHANICAL TENSILE BEHAVIOR AND MICRO-MECHANISMS IN POLY(VINYLIDENE FLUORIDE) AT HIGH TEMPERATURES: INFLUENCE OF THE MOLECULAR WEIGHT DISTRIBUTION

Polyvinylidene fluoride (PVDF) is a crystalline polymer known for its multiple crystalline phases. When elongated at room temperature before necking, the nucleation and growth of micro-voids is a major process. No phase transition was observed. In this paper, micromechanisms of deformation are studied and linked to the macroscopic mechanical tensile behavior at temperatures higher than 100°C and after necking. Cavitation, crystalline phase transitions and orientation process are investigated by small and wide angle x-ray experiments. Two PVDF with different molecular weight distributions are considered. These microstructure differences mainly affect cavitation.     

Lamellar orientation on the surface of a polymer determined by ToF-SIMS and AFM

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH₂ and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.     

Structural factors in deformation of crystalline polymers

The multiple α absorption of bulk-crystallized polyethylene (PE) was separated into the α₁ and the α₂ absorptions on the assumption that this α₂ absorption is associated with shear deformation of lamellar crystals, i.e., has the same characteristics as in single crystal mats. The separated α₁ mechanism is related to the molecular motions in the intermosaic block region. The α₁ process is very sensitive to static and dynamic deformation, whereas the α₂ process is not affected. Plastic deformation of bulk crystallized PE was analyzed in terms of true stress and true strain. The temperature dependence of the critical yield stress below 60°C showed the same magnitude of activation energy (26 kcal/mole) as that of α₁. The leading mechanism of deformation at lower temperatures is the breakdown of lamellar crystals into mosaic blocks. Compressive deformation of solid-state extrudates along the molecular axis, giving rise to kink bands, was analyzed with X-ray goniometry and in terms of the strain-rate dependence of the yield stress. The deformation of the crystals in the kink bands occurred by superposition of intercrystallite slip (α₁) and uniform shear deformation (α₂). It was concluded that consideration of intermosaic slip mechanisms (α₁), in addition to the shear deformation (α₂) and the interlamellar deformation (β), is effective and helpful to understand the deformation process of crystalline polymers.     

Morphology of cold-drawn polyethylene

Small-angle studies suggest the presence of either a 1000- or a 100-Å periodicity in cold-drawn polyethylene. A 1000-Å periodicity is shown in cold-drawn polyethylene thin films to be due to regular placements of portions of two or three lamellae which stayed together during deformation by interlamellar ties. The 100-Å periodicity is shown by dark-field electron microscopy and, in particular, by bright-field diffraction-contrast microscopy to be due to alternating crystalline and noncrystalline units aligned in 100-Å diameter microfibrils. At lower draw ratios, a lateral alignment of the 100-Å units is observed, resulting in the formation of “wavy lamellae.” Although the wavy lamellae are destroyed at higher draw ratios, the 100-Å units remain present throughout the sample. These observations suggest the possible presence of a grain structure of the order of 100 Å in the original crystal.     

TIME-RESOLVED SAXS/WAXS STUDY OF PHASE BEHAVIOR AND CRYSTALLIZATION IN POLYMER BLENDS

Real-time SAXS and WAXS patterns have been simultaneously obtained during isothermal melt-crystallization of blends of low-molecular-weight poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA). The analysis of results shows that the originally homogeneous, single-phase polymer blend separates into two phases. The PMMA molecules diffuse from the blend and form completely segregated regions while PEO starts to crystallize. The first and dominating effect at the beginning of crystallization is the formation of unstable lamellae of nonintegrally folded chains (NIF). The real-time crystallinity and density of the PEO crystalline phase in absolute units were obtained from the time-resolved SAXS/WAXS results. The structure development proceeds in two steps. A very fast evolution of PEO crystals from the melt starts to crystallize in disordered NIF lamellae with thick amorphous interlayers and with a lower density of crystalline phase. The steep growth of crystallinity and crystalline density mean quick thickening of crystalline part of lamellae and improvement of their crystalline structure. In the second step, the structure of the crystalline phase gradually improves and crystallinity grows very slowly. The recrystallization of NIF lamellae into extended chain lamellae (EC) and lamellae with once folded chains (1F) proceeds during both stages of crystallization.     

Selective cleaning of ions of heavy metals from water solutions using the H-form of todorokite synthesized by the hydrothermal method

A phase of Mg-todorokite with a size of structural tunnels of ～10 Å has been synthesized by the hydrothermal method in the form of lamellar crystalline aggregates 50–100 nm in size. Topotactic interaction with concentrated nitric acid results in the formation of the H-form of todorokite, having a high ability to adsorb Cs⁺, Pb²⁺, and Ba²⁺ ions from water solutions. H-todorokite can be used as an ion filter for cleaning ions of heavy metals from water solutions.     

Photoconductivity of potassium colloids in KCl single crystals

Detailed photoconductivity spectra of potassium colloids in KCl single crystals are presented. A broad band peaked at ～2 eV, together with a small shoulder at ～1.3 eV are observed for crystals containing colloids with radii in the 20–500 Å range.The spectrum shape is essentially independent of temperature and colloid radius. On the other hand, the maximum photoconductivity yield depends on the colloid radius reaching a maximum value at about 60 Å.