Types Of Spherulites Research Articles

Influence of the Crosslinking on the Crystallization of Poly(tetramethylene terephthalate)

Isothermal crystallization of linear and crosslinked PTT has been studied in order to find the influence of crosslinking on the mechanism of crystallization. The samples were crystallized from the melt in the temperature range from 90 to 200°C and they were studied by roentgenographic, microscopic, and light scattering methods. It was found that the crosslinking does not change the structure of PTT crystallites. However, the influence on the type of spherulites formed is significant. In the linear polymer, depending on the crystallization temperature, spherulites of 0–90 (untwisted lamellae) and 45 (lamellae twisted at α≅45°) types of optical axis orientation were formed, while in the crosslinked system, only spherulites of 0–90 type were observed. This was explained to be a result of the presence of junctions, which make the twisting of the lamellae impossible.

Properties of Polyether-Polyester Thermoplastic Elastomers

Abstract The viscoelastic properties of polytetramethylene oxide—polytetramethylene terephthalate block polymers are strongly influenced by phase separation of the 4GT hard blocks into crystalline domains. Thermal analysis reveals a single Tg which increases with increasing 4GT content. This suggests that short sequences of hard segments form a compatible interlamellar amorphous phase with the polyether component. The Gordon-Taylor equation was found to model Tg behavior accurately, provided that the crystalline polyester component was not included in the definition of the hard segment. The melting point of the polytetramethylene terephthalate blocks depends on the average block length of crystallizable segment. Incorporating non-crystallizing polytetramethylene 1,4-cyclohexanedicarboxylate into the hard segment reduces the 4GT melting point and degree of crystallinity. The morphological features of the copolymers depend on sample composition and fabrication procedure. The basic structure is spherulitic. Three different types of spherulite were observed: positive and negative spherulites, as well as spherulites which have their optical axis 45° to their radial direction. The different spherulite types are relatively stable; annealing the samples at elevated temperatures does not alter their morphology. Annealing does increase the degree of crystallinity somewhat and produces crystallites in equilibrium at the annealing temperature. Infrared dichroism studies reveal that, at low deformations, the hard segment lamellae orient as a whole in the stretching direction. This is refleeted by the initial negative orientation of the hard segments. At this stage of elongation, the deformation of the crystallites is nearly reversible. At higher strain levels, the lamellae are disrupted and the hard segments orient positively with a high degree of orientational hysteresis. The soft segments, however, orient almost reversibly in the stress direction at all strain levels studied. It is concluded that the extensive stress softening is brought about by plastic deformation of the crystalline hard segments.

Structure of oligourethanes: 6. Fine structure of melt crystallized oligourethanes

The structure and the structure change of the melt crystallized oligourethanes, CH 3CO ▪(CH 2) 5NHCO ▪ nOCH 3, having strictly homogeneous molecular weight, were studied. The crystal system of every melt crystallized oligourethane was similar to that of the single crystal of n = 2. In the case of n = 2, crystalline rearrangement by annealing does not occur, although the number of hydrogen bonds increases. On the other hand, rearrangement occurs for n = 3 and higher oligomers. It was considered that the rearrangement for n = 3 was the transformation from the spherulite to the plate-like crystal, whilst for those of n = 6 and 8 the growth of the new spherulitic type of crystal by secondary crystallization occurs. For n = 10 and the polymer crystal the thickening was accompanied by an increase of ordered regions.

Spherulitic crystallization in biopolymers

Banded spherulites with diameters of the order of 150 μm have been grown by evaporation from solutions of the branched polysaccharide, dextran, in DMSO, formamide and water. The average size of the spherulites depends on molecular weight, evaporation temperature, and the solvent used. Activation energies for spherulitic growth were measured as a function of molecular weight and solvent environment. Two distinct types of spherulite were obtained depending only on the molecular weight: negative, banded spherulites from the high molecular weight dextran; and positive, unbanded spherulites from the low molecular weight dextran.

Investigation of conditions of spherulite formation in mixtures of crystalline and amorphous polymers

AbstractThe supermolecular structure of mixtures of crystalline polymers (low pressure and high pressure polyethylene, isotactic polypropylene) with an amorphous polymer (atactic polypropylene) from the point of view of the influence of the amorphous component on the morphology of the crystalline component has been investigated. The criterion of changes in the morphological state of larger supermolecular formations was the microscopic image of the samples obtained with an optical microscope, both between crossed nicols and in phase contrast. It has been established that the formation of typical spherulites depends on the amount of admixture of the amorphous polymer and also on the treatment of the samples. Upon crystallization in presence of a small amount of solvent (p‐xylene), formation of typical spherulites of the crystalline component of the mixture can be observed while the same mixture in case of evaporation of the solvent presents a granulated refractive structure without typical spherulites, regardless of the rate of cooling. The probable role of the solvent is to facilitate mutual segregation of the microphases of both polymers in consequence of decrease in viscosity of the mixture. It has been further shown that even after briefly heating the mixture to a temperature of 220°C., before crystallization, spherulites do not form; on the contrary, spherulites originally present disappear and a structure consisting of smaller refractive formations of crystalline polymer is obtained. The formation of this structure, emerging after destruction of spherulites, might be attributed to an increase of interpenetration of both polymers and to an increase of the contact surfaces between components of the mixture.

Formation of spherulites in polyamide melts: Part III. Even‐even polyamides

AbstractA preliminary study of the isothermal crystallization of “even‐even” polyamides reveals striking similarities in spherulitic morphology. The observed variations in textural features of spherulites of nylon 210, nylon 66, nylon 610, and nylon 1010 show parallel changes during growth conditions and conform to a definite sequence of behavior. At least four different types of spherulites exist in each polymer. Optical and x‐ray techniques were used to examine some of these spherulites. Crystalline platelets possessing single‐crystal properties have been grown in thin films of these polymers near their respective melting points.

Light scattering from polymer films

AbstractA variety of films were examined by low angle laser light scattering to yield information on the size, shape, and orientation of superstructure. Films of poly(ethylene terephthalate) and polypropylene give distinctive scattering patterns in the form of two‐ and four‐leaf clovers which change with polarization direction, stretching, heat setting, and heat relaxation. Changes in pattern geometry vary systematically with film properties, x‐ray, and polarized infrared structural parameters. In one‐way oriented poly(ethylene terephthalate) films, the quadrant, leaves tend toward the equator and elongate with increased draw, taking on a fiberlike appearance, whereas with two‐way drawn films the leaves shift away from the equator and patterns become more circular with increased second direction draw. Melt‐pressed, quenched, unnucleated, polypropylene films show a characteristic spherulitic type pattern. The higher the quench temperature, the larger the spherulites become. Calculated spherulite diameters increase with quench temperature up to 15m for temperatures of 60°C. Upon drawing, the patterns elongate owing to Spherulite deformation and internal reorganization of crystallites, taking on a fiberlike geometry. Nucleated and slowly cooled resins show very little scattering and no spherulitic quadrant leaves. Heat setting usually led to the appearance of new pattern features characteristic of new structures. Heat relaxation tended to smear out the Scatterning patterns as a result of the disappearance of superstructure.

Some anomalous viscosities of poly‐γ‐methyl‐L‐glutamate (PMLG) solutions and relations between the aging of the solutions and the structure of spherulites grown from them

AbstractPoly‐γ‐methyl‐L‐glutamate was obtained by the NCA method in chloroform with the use of triethylamine, diethylamine, and n‐hexylamine as initiators. After polymerization, the solution was allowed to stand at 20 ± 2°C., and thin films cast from solvent, with short aging periods (∼1 month), no spherulites formed in the films. Aging periods of 2–6 months yielded positive spherulites; after much longer aging periods (6–10 months), negative spherulites were observed. In the positive spherulites, the helical axes oriented along the radius, but in negative spherulites the helical axes oriented along the tangential direction. These positive and negative spherulites were constructed of microfibrils about 45–67 A. in diameter. The relative viscosity of the aged solutions was also measured. The longer the aging time, the lower the viscosities of these aged solutions; the viscosities decrease sharply to 3 months of aging, and with aging times of 4–20 months the changes in viscosity are very small. However, the viscosity changes in every measurement and always increases during the measurement. Details of changes in viscosity are different depending upon the initiator used and the aging time. These results indicate that the decrease in viscosity is caused by intermolecular aggregation, while the increase during measurement is due to a breakup of the aggregates. Thus, the changes in the soluble state of the polymer in the aged solution seem to be closely related to the growth of both types of spherulites. The initiator has a significant influence on the structure of these aggregates. Polymer prepared with nHA initiator formed no spherulites, regardless of the aging time. No evidence was observed of the presence of the folding orientation of these α‐helical molecules or microfibrils, which indicates that the crystallization habits of helical molecules, such as polypeptides, are different from those of polyethylene. The crystallization mechanism of polypeptides is not by the so‐called folding mechanism, but by the side‐by‐side crystallization mechanism, as proposed by Ishiara or Flory. A difference in the mechanism of growth of positive and negative spherulite is also proposed.

Microscope investigations of extension and fracture in crystalline polymers

AbstractThin films of polyethylene, polyhexamethyleneadipamide, polypropylene, and polyoxymethylene have been examined under the microscope during the application of a tensile stress. The response to stress in each case varies with the thermal history of the sample, and indeed this variation correlates closely with the size and type of spherulites formed during sample preparation. The particular behavior of each polymer has been described. In general, it is found that extension of the polymer under stress is usually possible if the spherulites are sufficiently small. In these four cases, long annealing brings about spherulitic growth and a brittle structure which fractures without extension when tension is applied.

The morphology and internal structure of poly(ethylene oxide) spherulites

AbstractThe development of spherulites in thin films has been observed. At low supercoolings they develop scalloped edges, with each scallop sector having a branched structure reminiscent of a fern frond. At high supercoolings they develop smooth circular boundaries. An x‐ray diffraction study has been made both of the fronded structures and of the more perfect spherulites formed at higher supercoolings. Only one of the latter types of spherulites was examined. Here it appears that the crystallites have a rotation axis perpendicular to the spherulite radius. This axis is the 4, 0, 1 direction in the crystal which has a monoclinic unit cell. The fronded spherulites, when examined along the radial frond spine, have the same rotation axis of the crystallites, both ±45° to the radius. This double orientation of rotation axis also occurs in material crystallized in a fine capillary, where the spherulite radial direction is presumably along the capillary axis. Low‐angle x‐ray diffraction shows spacings of about 200 A. (sometimes to sixth order) parallel to the spherulite radius for both fronded and perfect spherulites.

Spherulites in nylon 610 and nylon 66

AbstractThe two types of spherulites in nylon 610 differ in birefringence, melting point, and rate of growth. The rates of growth depend strongly upon the degree of supercooling, and—at least for the positive spherulites—to a small extent on the molecular weight. The rate curve for the positive spherulites has a singular point at 188°C. and appears to have a maximum in the neighborhood of 120°C. Concentric rings were observed in positive spherulites only after crystallization in the temperature range between 188° and 195°C. Positive spherulites formed near 180°C. have maximum birefringence. Negative spherulites were observed only in specimens allowed to crystallize between 205° and 225°C. Three types of spherulites were observed in nylon 66. The large, positive, and highly birefringent spherulites have the same melting point, 273—274°C., as the negative spherulites. The small positive spherulite with normal birefringence melt at 269.5—270°C. Simultaneous growth of all three types of spherulites was observed only between 250 and 255°C.

The autoorientation mechanism of crystallization

AbstractAttempts have been made to explain a few details of the sometimes misunderstood autoorientation mechanism of crystallization. The autoorientation mechanism is not a model for a particular type of spherulite, but one of the fundamental factors which influence the structure of substances with linear molecules or linear aggregates of molecules or atoms. It is possible to explain on this basis why spherulitic structures are formed or not formed, depending on circumstances. Another argument for autoorientation is that under certain conditions the molecules in adjacent regions are perpendicular to each other. From that point of view the structure of “herringbone” spherulites and ringed spherulities is pertinent to autoorientation. These structures have been investigated and models for these spherulites are proposed. Herringbone spherulites are easily reproducible in gutta‐percha. The ringed spherulites have a rhythmic layer structure. The molecules in adjacent layers are perpendicular to the radius and perpendicular to each other. They grow probably by a process of twinning at regular intervals. The visibility of the rings, especially when the rings are out of focus in unpolarized light, is caused by Becke lines. Clear evidence for the layer structure is the fact that borderlines between adjacent spherulites sometimes follow a zigzag path. The structure of the center of spherulites is also in agreement with autoorientation. Popoff's double leaves in gutta‐percha spherulites are formed if regions at both sides of the sheaflike structure, in the center of spherulites, crystallize in a different manner from that of the sheaf itself. Spherulites formed at low temperatures usually have only an apparent fibrosity, whereas spherulites formed at high temperatures can consist of real needle‐shaped crystals, sometimes separated by less crystalline interstitial material. These and other aspects of the crystallization of polyolefins and other polymers are shown to be in general agreement with the conception of autoorientation.

Spherulitic Crystallization in Polypropylene

A survey has been made of spherulites formed by the isothermal crystallization of polypropylene from the melt in the temperature range 110°–148°C. Four types of spherulite have been distinguished and their structural morphology, optical properties, melting behavior, and growth rates have been examined.

Evidence for a Second Crystal Form of Polypropylene

An attempt is made to account for the observed birefringence of the various types of polypropylene spherulite in terms of preferred crystalline orientation. Evidence is also presented showing that some of these spherulites crystallize with a new metastable crystal structure, not previously described.

An electron microscope study of the growth and structure of spherulites in polyethylene

AbstractA surface‐etching technique has been devised which reveals the spherulites in molded Polyethylene for study with the electron microscope. By means of this technique it is possible to observe the effect of various heating cycles and polymer structures on the size and perfection of the spherulites. The etching technique shows that two different types of large spherulites may be formed in polyethylene. One, which may be formed on molding, contains a relatively low amount of internal order. The other, which forms on long heat aging or annealing, contains a high degree of internal order. Heat aging at temperatures well below the melting point will induce the poorly ordered type of spherulite to break up and regrow in the highly ordered form. Spherulite structure was observed to depend not only on heat treatment, but also on chain branching and molecular weight. Increased linearity induces the formation of a more regular, sheetlike or laminated structure within the spherulites, while lower molecular weight causes the lamellae and spherulites as well to be larger in size. Extremely high molecular weight linear resins showed no spherulitic structure as such, but were fibrillar in appearance. Annealing or long heat aging of these resins induced the formation of a row type aggregate structure rather than the typical spherulites observed in normal molecular weight resins. Electron microscope studies of stress‐cracked surfaces indicate that a high degree of internal spherulitic order such as is formed on long heat aging may well be the primary cause of stress‐cracking in polyethylene. This is further indicated by resins such as the ethylene/propylene copolymers, in which highly ordered spherulites do not form with heat aging. These resins are very resistant to stress cracking. Structural studies show that the ethylene/propylene copolymers are very nonrandom, and it is believed that this interferes with the formation of perfect, and consequently crack‐prone, spherulites.

The fine structure of polytetrafluoroethylene

AbstractElectron micrographs of highly crystalline PTFE crystallized by slow cooling from temperatures not far above the melting point show well‐marked bands, often with striations perpendicular to the bands; optical evidence shows that the chain molecules are parallel to the striations. The structure is in marked contrast to the spherulitic structure of most polymers; it appears that in PTFE the molecules are straight and parallel for much greater distances than in other polymers, and it is suggested that the width of the bands is a measure of molecular length. The unusual structure is attributed to the unusual stiffness of the fluorocarbon chain; the molecules in the liquid are straighter and less tangled than in other polymer melts. Heating to 500°C., followed by slow cooling, gives a modified structure approaching the spherulitic type found in other polymers; it is suggested that this is due to increased molecular tangling in the melt at higher temperatures. One type of polymer shows bands of remarkably uniform thickness, suggesting unexpected uniformity of molecular length. PTFE wax made by thermal or radiation degradation of the high polymer shows well‐defined spiral growth steps due to dislocations; the step heights suggest an unexpected uniformity of molecular length.