Interpenetration of spherulites in polymer blends

When poly(oxymethylene) is doped with increasing amounts of low molecular weight poly(oxyethylene), a stepwise disappearance of the boundaries between the spherulites is observed. At a poly(oxyethylene) content of 5 wt.-%, the replica of a bulk sample viewed in an electron microscope seems to have only centers of spherulites and slightly marked boundaries between them. A more careful examination of the replica shows that this effect is due to strong interpenetration of the lamellae of the adjacent spherulities of poly(oxymethylene). It is well known that the mechanical properties of many two-component systems are improved by adding small amounts of a second component. On the basis of our observations it is possible to correlate the interpenetration of spherulites with the improvement of some mechanical properties of bulk samples. Interpenetration is possible when two “hollow” dendritic spherulites are brought into contact. Such a change of the spherulite structure by an additive is due to a rejection of the additive into interlamellar regions.     

Coexisting Straight,Radial, and Banded Lamellae on the Six Corners of Hexagon‐Shaped Spherulites in Poly(l‐Lactide)

A novel six‐petal spherulite morphology composed of a combination of a central hexagonal core, hexagon‐shaped ring bands, and six fibrous stalks is discovered in a low‐molecular‐weight poly(l ‐lactide acid) (LM_w‐PLLA) that is melt crystallized at a specific T_c = 110 °C and confined in thin films. The structures are analyzed by using polarized optical microscopy (POM) and atomic force microscopy (AFM). Discrete lamellae, consisting of sequenced wide and small lamellae, all in flat‐on orientation with periodically up‐and‐down topology, are packed in a ring‐banded alignment, while continuous lamellae in flat‐on, tilted, or edge‐on orientations are arranged in the fibrous region. On the six corners of a symmetric hexagon‐banded spherulite, the radial fibrous lamellae overlap with ring‐band lamellae to show a mixed combination of ring‐banded and fibrous patterns. The formation of coexisting ring‐banded and fibrous patterns in a mixed morphology is associated with the lamellar packing and orientation. The radial fibrous lamellae at the six corners favorably occur in the growth fronts, which are closer to the reservoir of materials of diffusing species. Superimposed on the radial lamellae, hexagon‐shaped ring bands repeat themselves on the outskirts of the central hexagon nuclei. The geometry of the initial crystal is believed to be the influencing factors in final spherulite patterns.

     

Differences Between Stereocomplex Spherulites Obtained in Equimolar and Non‐Equimolar Poly(L‐lactide)/Poly(D‐lactide) Blends

PLLA/PDLA blends were crystallized between 120 and 195 °C. The stereocomplex spherulites acquired in equimolar and non‐equimolar blends were compared using POM, WAXD, DSC, and AFM. For equimolar blends, stereocomplex crystals show spherulites with positive birefringence, which is ascribed to the existence of domains made up of tangentially oriented lamellae. For PLLA‐rich (or PDLA‐rich) blends, the signs of the birefringence changed from a positive spherulite to a mixed spherulite and then to a negative spherulite. In negative spherulites, most lamellae orient radially. Radial and tangential cracks were observed in equimolar blends when crystallization took place above 175 °C whereas no cracks formed for non‐equimolar blends.

     

Formation of spherulites during polymerization of lactams

The mechanism of the crystal growth of nylon in the course of alkaline polymerization of lactams was studied in terms of kinetics and morphologies of the resultant crystals. It was found that in the bulk polymerization of α-pyrrolidone, the resultant polymer precipitated in the form of spherulites. Spherulites formed in the early stage of polymerization were swollen, soft balls in which thin lamellar crystallites were radiating along the spherulitic radius. It is considered that the growing chain molecules, when they attained a critical length beyond which they lost the solubility in the system, will precipitate into the form of lamellae in which molecules are aligned perpendicularly to the lamellar surface. Such spherulites matured as the polymerization proceeded. In the early stage of the polymerization, the mutual coupling of the swollen spherulites occurred. In the subsequent stage, the vacant space was occupied by the product of further polymerization, so that the coupled spherulites grew into a sphere again. On the other hand, the propagation of the chains in the individual spherulites made the spherulites compact. It is postulated that the chains grown from the surface of the lamellar nuclei, precipitate in the interstices between the lamellae and a part of these chains will form molecular ties between the lamellae giving rise to compact spherulites. The decrease in the rate of polymerization observed may be attributable to the occlusion of the growing chain ends into the precipitates. The impurity in the system, i.e., water, seems to affect the rate of the monomer addition, but does not the morphology. Spherulites were also grown in the case of nylon 6, the polymerization of ?-caprolactam.     

Ring‐Banded Spherulitic Crystals of Poly(3‐butylthiophene) via Controlled Solvent Evaporation

The preparation of ring‐banded spherulites in poly(3‐butylthiophene) via controlled solvent evaporation of solution‐cast films is reported. The spherulites display unusual concentric ring‐banded structures under both polarized and unpolarized lights. The size of the ring‐banded spherulites is 300 ± 100 µm in diameter and the periodicity of the bands is 15 ± 2 µm. The periodic bands of the spherulite consist of alternating ridge and valley surface patterns and the crystalline lamellae in the bands are more or less parallel to the radial growth direction of the spherulites. Local lamellar bending and branching are observed analogous to that of classical non‐conjugated polymers. A possible diffusion‐induced rhythmic growth mechanism is proposed to interpret the formation of periodic banding of the spherulite.     

Micronecking in thin films of isotactic polypropylene

Thin melt crystallized spherulitic films of isotactic polypropylene have been deformed in uniaxial tension at different temperatures and examined by transmission electron microscopy. As the temperature was increased above approximately ?80°C, a transition from high aspect ratio craze-like features to highly voided shear deformation zones was observed, these latter initiating both at spherulite boundaries and along the spherulite diameters perpendicular to the tensile axis. In rapidly crystallized films, the draw ratio in the deformation zones was estimated to be close to the natural draw ratio of the melt entanglement network, and was independent of test temperature in the range investigated. However, for crystallization temperatures above approximately 125°C, the mean draw ratio within the deformation zones was found to increase with crystallization temperature. It was also observed that, although the draw ratios in deformation zones in ß spherulites were similar to those in α spherulites in films containing both modifications, the former showed more homogeneous textures. This suggests that the particularly pronounced textures associated with α spherulites in polypropylene thin films are linked to their crosshatched lath-like lamellar morphology.     

Precipitation-crystallization of poly(1,4-phenyleneterephthalamide) from nematic solution

Precipitation-crystallization of poly(1,4-phenyleneterephthalamide) (PPTA) from sheared nematic solutions was studied by polarized light microscopy. Three kinds of morphological structures, viz. spherulite, shish-kebab, and band structures were observed in films prepared from sheared anisotropic solutions. An exhaustive study of the band structure by using various techniques to prepare the samples for electron microscopy was made. The results obtained indicated that the morphology of the band structure is composed of fibrils bent in a zigzag manner and highly oriented along the shearing direction of the molecular chains in the neighbouring bands with the angle ±14° as measured from selected-area electron diffraction. A finer periodical structure of 300 Å arranged along the zigzag fibrils was observed.     

Determination of surface morphology of diblock copolymers of styrene and butadiene by atomic force microscopy

A detailed study by atomic force microscopy of the surfaces of diblock copolymers which exhibit lamellar and spherical microphase domains in the bulk is presented. The surface morphology of the lamellar samples consists of corrugations. Transmission electron microscopy of cross sections which include the surface and near surface demonstrate the connection between the corrugations and the bulk lamellae. The surface of a sample with spherical bulk domains was found to consist of a series of pits that can be linked with centers on the faces of the cubic cell arrangement of the bulk. Disclinations and intersections of lamellae could be found. The latter result in lamellar break-up in the contact area of the two lamellar orientations. Lamellar tilt can lead to variation in periodicity and depth of the surface morphology. Damage due to scanning was observed, but only for the sample with spherical morphology.     

Deformation of extremely thin polyethylene films

In order to establish a better correlation between the drawing mechanism of bulk polyethylene (PE) and the observation of plastic deformation of single crystals and the ensuing microfibril formation extremely thin PF films floating on hot water were deformed and studied by electron microscopy. The drawn film consists of about 300 Å wide folded chain blocks arranged in microfibrils. The chains are oriented in the microfibril axis. The lateral coalescence of the blocks into lamellae produces a surface structure of the same type as observed with bulk samples. One hence concludes that also in the latter case the destruction of the original spherulitic structure in the necking zone first produces microfibrils composed of blocks broken off a single lamella. The bundle of microfibrils from a single stack of lamellae with similar draw ratio and draw rate laterally coalesces forming a fibril, i.e., a stack of mosaic type lamellae with rather irregular fold containing surfaces and extending over more or less the whole cross section of the fibril. In contrast with bulk samples, but in agreement with the single crystal deformation, the long period of the drawn film agrees with that of the starting material. The same situation exists with annealing which increases the long period and the order in lamellar orientation and long period, but does not produce any crystal rotation about an axis in the sample surface and perpendicular to the draw direction which is so conspicuous with drawn bulk PE.     

Method for ultrastructural studies of the intact tissue-metal interface

Samples were prepared for ultrastructural studies of the intact interface between metallic implants and tissue by transmission electron microscopy. The method is based on plastic embedding of implant and tissue and subsequent removal of the bulk metal by electrochemical dissolution (electropolishing), to facilitate preparation of ultrathin sections for transmission electron microscopy. Surface sensitive spectroscopy (Auger electron microscopy and X-ray photoemission spectroscopy) and transmission electron microscopy EDX results show that the method produces samples with an intact interface, containing the implant surface oxide and the adjacent tissue. Examples of application of the method on titanium, zirconium and aluminium implants in soft tissue are given.     

Influence des conditions de fusion et de cristallisation sur la morphologie superstructurale du polypropylène isotactique cristallisé au dessus de 130°C

The morphology of polypropylene samples cooled from the melt and then isothermally crystallized above 130°C, was studied by optical polarization microscopy. If the temperature of the melt is raised above 175°C, the spherulitic superstructure is fully destroyed. Isothermal crystallization involves negative or positive birefringence which mainly depends on the temperature of crystallization (above or below 130°C). If the temperature of the melt is raised to 172°C, that is to say at a temperature at which no birefringence is detectable, isothermal crystallization leads to negative spherulites above 155°C and to positive ones below 155°C. If the melt is heated between 172°C and 175°C, crystalline lamellae are partially destroyed and they further lead to crystallization of small positive spherulites, which are located inside the initial spherulitic superstructure. These observations are explained in terms of a quadritic arrangement of the lamellae of crystallized polypropylene. Moreover, the crystallization temperature of 155°C appears as a true transition temperature beyond which tangential lamellae cannot exist; this always implies negative birefringence of the spherulites, whatever the melting conditions are.     

Crystallization of a copolyester in microlayers and blends with polycarbonate

The crystallization behavior of coextruded microlayered sheets comprised of 657 alternating layers of polycarbonate (PC) and a miscible copolyester of mainly 1,4-cyclohexanedimethanol and terephthalic acid (KODAR) was investigated as a function of annealing time when the KODAR was crystallized isothermally from the glass at 195°C. Comparisons were made with crystallization of KODAR alone, and with crystallization of KODAR from melt blends with PC. The kinetics of crystallization and the morphology of the crystallized KODAR were characterized with differential scanning calorimetry, and by examination of thin sections microtomed from annealed specimens in the polarized light microscope and the transmission electron microscope. The growth rate of small, birefringent KODAR spherulites was non-linear, and was strongly affected by diffusion of PC into the KODAR layers. Diffusion of amorphous PC into the KODAR layers retarded nucleation and spherulite growth and decreased spherulite density. The effect became more pronounced as the KODAR layer thickness was reduced. Spherulities nucleated randomly throughout the KODAR layers in the PC/KODAR 20/80 (w/w) microlayer and grew rapidly to form a continuous layer of impinged spherulites. In contrast, spherulites in the PC/KODAR 40/60 and 60/40 microlayers nucleated and grew along the center of the KODAR layers where the KODAR concentration was highest.     

The morphology of semicrystalline segmented poly(ether ester) thermoplastic elastomers

The supermolecular structure of melt-crystallized segmented copoly(ether ester)s based on poly(tetramethylene terephthalate) and poly(tetramethylene oxide) is investigated by polarizing microscopy and electron microscopic techniques. Spherulites are formed on isothermal crystallization at supercoolings ΔT_u < 30°C. The spherulites recrystallize to form dendritic structures on further annealing at higher temperatures. The spherulites and dendrites are builtup from lamellae which exhibit little variation concerning the thickness of their crystalline core. The internal phase boundaries in these systems are made visible with a newly developed staining method. A network of interlocked individual lamellae is formed on rapid quenching of the melt films. Row crystallization (shish-kebabs) is observed, if the melt is sheared during the crystallization. It is concluded from these observations that the chemical structure of the segmented copolymers bears little relevance for the supermolecular structure of these materials except for the thickness of the crystalline core of the lamellae which seems to be related to the molecular length of the hard segment sequences occurring with the highest concentration in the sample.     

Unconventional Non‐birefringent or Birefringent Concentric Ring‐Banded Spherulites in Poly(L‐lactic acid) Thin Films

Unconventional concentric up‐and‐down bands, differing from the conventional bright‐ extinction or blue/orange bands in other polymers, in poly(L ‐lactic acid) are found via controlled thin films. Non‐birefringent and birefringent concentric banded spherulites are obtained from 350–400 nm and 1800–2000 nm film thicknesses, respectively. Non‐birefringent concentric ring‐banded spherulites are arranged only by flat‐on lamellae; in contrast, complex combinations of flat‐on, edge‐on, and round‐shaped lamellae in four transitional zones are present within a single band in the thicker films of birefringent, concentric, ring‐banded spherulites. Interband is an optically extinction crevice that segregates two neighboring bands. Unique lamellar arrangements at specific T_c and thickness confinement are two factors for the unconventional concentric bands.     

Phage-like structures (SE-particles) in cells of the unstable and stable spheroplast type L-form of Proteus mirabilis D 52

In cells of the stable and unstable spheroplast type L-form of Proteus mirabilis D 52 small ellipsoidal particles (SE-particles) could be found after investigation of ultrathin sections. The SE-particles have diameters of 220–260 and 280–320 Å. They are composed of an electron lucent centre, an internal electron deanse layer, an electron lucent line and an outer electron dense layer. There is a porus of about 25 Å in the internal electron dense layer. The SE-particles may be arranged in chains up to 10 particles. It seems that the particle formation starts near the cytoplasmic membrane at one or more sites in a cell. Comparing electron microscopic observations with data from literature it seems certain that SE-particles are bacteriophage-like structures. However, it is not clear until now, whether they are intact tail-less phages or defect phage structures.     

The Comparison of the Ringed Spherulite Morphology of PCL Blends with Poly(vinyl chloride), Poly(bisphenol A carbonate) and Poly(hydroxyether of bisphenol A)

The crystallization and ringed spherulite morphology of poly(ε‐caprolactone) (PCL) in the miscible blends of PCL/poly(vinyl chloride) (PVC), PCL/poly(hydroxyether of bisphenol A) (phenoxy resin) and PCL/poly(bisphenol A carbonate) (PC) were investigated by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). Through the comparison of the PCL crystalline morphology with the interaction energy density B between the miscible components in these blends, it was found that the addition of the non‐crystallizable component had great effect on the regularity of the ringed spherulite, which was coincided with the change of the interaction energy density B. To grow regular ringed spherulites in the PCL miscible blends, it was most important that the crystallization rate of PCL in the blends must be matched with the diffusion rate of the non‐crystallizable component. Such a matching relation in the process of the ringed spherulite growth was a most important condition for the regular twisting of PCL lamellae.     

Thermal Behavior of Ring‐Band versus Maltese‐Cross Spherulites: Case of Monomorphic Poly(ethylene adipate)

Summary: Two types of spherulites, extinction‐ring and Maltese‐cross, in poly(ethylene adipate) (PEA) were identified, isolated, and separately characterized using DSC and polarized‐light optical microscopy (POM). Ring‐band spherulites in PEA occurred only in a very narrow temperature range roughly between 25 and 30 °C, while Maltese‐cross spherulites at 35 °C and above or at 20 °C and below. Thermal behavior of ring‐spherulites is interpreted, analyzed, and compared to that of Maltese‐cross spherulites. The thermal behavior, like the morphology, was found to be significantly different between these two types of spherulites. Among the three multiple melting peaks in PEA, the highest P₃ is proven to be related to melting of lamellae in the ring‐band spherulites; P₁ and P₂, whose relative extent of overlapping is dependent on the temperature of crystallization, are related to melting of lamellae in the regular ringless spherulites. This study has provided urgent and timely evidence for one major step further in the interpretation of relationships between the thermal behavior, melting, and extinction‐ring versus Maltese‐cross spherulites in semi‐crystalline polymers.

DSC curves (10 °C · min^?1, scanned from where the trace begins) for: (a) ring‐spherulites in PEA crystallized at 30 °C for 90 min, (b) sample‐(a) heated to 50 °C for 1 min, and (c) sample‐(a) heated to 50 °C, then quenched to 20 °C.     

Negative pressure effects during the isothermal crystallization of isotactic poly(propylene) studied by light and atomic force microscopy

The formation of holes during the late stage of the isothermal crystallization in thin films of isotactic poly(propylene) between two cover glasses was observed by light microscopy and atomic force microscopy. This behavior can be described consistently by the well-known negative pressure effect. Light microscopy reveals the simultaneous and sudden occurrence of a large number of small holes at the liquid-solid interface after the liquid in front of the spherulites is completely confined by other spherulites for a certain time interval. In exceptional cases only a few holes appear and finally large cavities are formed. Atomic force microscopy measurements carried out in the height mode are able to prove the hole formation in front of the spherulites. Furthermore, a substantial thinning of the two-dimensional spherulites in thin films can be observed prior to the hole formation.     

Acetylation as a means of selectively blocking vicinal diols of carbohydrates in electron microscopy

Acetylation for selectively blocking the vicinal diols of carbohydrates has successfully been applied to ultrathin sections in electron microscopy. From buffered glutaraldehyde fixed rat colon and jejunum tissues embedded in LR-White, ultrathin sections were cut, treated with hot acetic anhydride in pyridine and then reacted for periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP). The PA-TCH-SP reaction of all the glycoproteins and glycogen involved in the colonic and jejunal epithelia was selectively blocked by prior acetylation, providing a means of substantiating the presence of vicinal diols of carbohydrates in electron microscopy.     

Crystallization of cellulose from dilute solutions

Cellulose samples having a degree of polymerization (P) of 400, 200, 100, and 50 were crystallized isothermally from dilute solutions of aqueous sodium hydroxide and from cellulose xanthate solutions. Observation of the suspension under an optical microscope revealed that in most cases various shapes of huge lamellae in the order of 10 to 50μ were formed. These lamellae could easily be disintegrated by ultrasonic cavitation and under an electron microscope they were seen as fibrous sheets. In all the cases studied, the lengths of the fibrils were equivalent to or longer than the fully extended molecular length. The huge lamellae viewed under an optical microscope are considered to be loosely packed aggregates of the fibrils. In view of the low crystallinity found in these fibrils, the fringed micelle model seems rather preferable to the extended chain crystals. Viscometric measurements showed that the induction period, where reduced viscosity decreased or did not change, was followed by the pre-crystallization stage which was manifested by a sudden increase, and then crystallization started as revealed by a rapid decrease in the reduced viscosity. This implies that cellulose chains were stiffened by the formation of intramolecular hydrogen bonds before the intermolecular crystallization started. If the formation of the intramolecular hydrogen bonds prior to a regular ordering of the chains in a lattice was avoided, therefore, it is anticipated that cellulose crystallizes in the form of chain folded lamellae. This was actually achieved by using an aqueous solution of equal moles of sodium hydroxide and methanol as the solvent. In this case, the reduced viscosity did not increase appreciably and dropped as the crystallization started. Namely, stiffening of the cellulose chains did not occur in the pre-crystallization stage. The electron microscopy revealed that the crystals formed were lamellar in this case. The size of the lamellae was in the range of 0.5 to 5μ, and their thickness was in the order of 80 to 120 Å. Molecular length of cellulose used being ca. 2000 Å, the chains must be folded in these crystals.