Subsidiary Lamellae Research Articles

On morphology and polymer blends: polystyrene and polyethylene

This paper compares the morphologies observed in two blend systems, namely blends of isotactic and atactic polystyrene, and blends of linear and branched polyethylene. Crystallization of isotactic polystyrene at high temperatures ( ≳ 200°C) produces immature sheaf-like aggregates of lamellae. Addition of atactic polymer results in modified morphologies that are based upon bundles of lamellae, separated from one another by non-crystalline regions. At high atactic contents sheaf-like objects are no longer observed, crystallization occurring to give structures that are better described as multilayered crystals. At temperatures below ∼ 200°C, mature spherulites develop. Addition of atactic polystyrene again results in morphologies in which lamellae are grouped into stacks separated from one another by non-crystalline regions. Again, at high atactic contents a less complex (sheaf-like) overall morphology is observed. In all the above microstructures, lamellae exhibit an inherently hexagonal habit. However, at lower crystallization temperatures observations suggest that lateral lamellar growth may be restricted. Isothermal crystallization of linear polyethylene at temperatures above ∼ 125°C results in morphologies in which a complex lamellar hierarchy exists, as a consequence of molecular fractionation during crystallization. Addition of branched polyethylene, which is unable to crystallize at these temperatures, suppresses fractionation such that the resulting morphologies may be described in terms of only one dominant and one subsidiary lamellar population. When viewed along the crystallographic b axis, dominant lamellae appear planar or mildly S-shaped in profile, whilst subsidiary lamellae are planar and inclined at an angle of ∼25° to the local dominant population. In blends with a high branched content, apparently non-crystalline regions can be seen between individual lamellae. Thus, in polyethylene blends, the non-crystallizing component is located between individual lamellae rather than between bundles of lamellae, as is the case in polystyrene. This difference arises from the parallel growth of dominant and subsidiary lamellae in polystyrene as opposed to the inclined development of subsidiaries with respect to the dominant lamellae in the polyethylene blends. In all the samples studied, spherulite growth occurs via a dominant/ subsidiary mechanism.

Morphology of linear PE blends crystallized at the transition temperature between regime I and regime II growth

Binary blends of linear PE sharp fractions [Mw=2 500 (L2.5) and 66 000 (L66)], all crystallized at the temperature of the transition between regime I and regime II growth, have been studied by TEM after Kanig treatment. It is shown that the dominant lamellae are always long and straight and form stacks. The number of lamellae in these stacks decreases with increasing content of L2.5. The lamellar width perpendicular to [010] is 1–3 μm for L66 and the 50/50 blend, but anomalously high for the 80/20 (L2.5/L66) blend. Thin subsidiary lamellae are observed only in the 50/50 and 80/20 blends. A significant fraction of these samples contains domains of segregated L2.5 showing no lamellar contrast.

On the development of polypropylene spherulites

The organization of lamellae in developing spherulites of i-polypropylene growing from the melt at 155 and 150°C has been investigated in detail using permanganic etching and transmission electron microscopy (TEM). The study forms a link between the simplest development at 160°C, when there is no twinning, and common cross-hatched textures occurring for crystallization down to at least 110°C. Whereas at 160°C growth starts with fan-like splaying of lath-like lamellae, at 155°C and below early objects are quadrites which develop as two families of laths with a common b-axis and a mutual orientation of 80°. This orientation relaxes with distance but is still recognizable with microbeam X-rays to sizes >0.2mm. Randomization in space of the orientation of the growth direction a∗, and also around this axis, occurs at smaller distances at lower crystallization temperatures. It is inferred that so-called axialites and spherulites are objects which differ in their degree of randomization rather than in kind. The characteristic cross-hatched texture of α-polypropylene develops differently according to the location in a growing spherulite. Whereas cross-hatching is symmetrical at the centre of a quadrite, in the outer regions radial lamellae dominate. Cross-hatching lamellae form later and are restricted in length to the unfilled space available. As such they are in competition with radial subsidiary lamellae. There is an increasing ratio of transverse to parallel subsidiary lamellae with lower crystallization temperatures. Epitaxy between the two cross-hatching components is observed to occur on the prism faces of lamellae in agreement with molecular mechanisms proposed by Binsbergen and de Lange 5 and Lotz and Wittman 21.

On the lamellar morphology of melt-crystallized isotactic polystyrene

The lamellar morphology of melt-crystallized isotactic polystyrene has been investigated by extending the technique of permanganic etching for electron microscopy. This paper reports on objects grown at 220°C which are comparatively uncomplicated, being aggregates of hexagonal lamellae, with smooth facets and distinguishable internal sectors, organized into axialites. To a first approximation only, hexagonal lamellae splay apart about a common diagonal presenting three characteristic projections in orthogonal directions. These are hexagonal, sheaflike and an array of approximately parallel lamellae. In reality the term axialite is an oversimplification. Splaying is not restricted to a single axis but occurs in three dimensions making these objects incipient spherulites. Polystyrene spherulites grown as low as 180°C the temperature of maximum growth rate are also constructed on the same principles as are spherulites of polyethylene and isotactic polypropylene. These are that a framework is established by individual dominant lamellae which branch and splay apart leaving interstices to be filled by later-crystallizing subsidiary lamellae. There is evidence that subsidiary lamellae contain shorter molecules on average than do dominant lamellae. The dominant/subsidiary construction is not what had been assumed in the Keith and Padden theory of spherulitic growth nor is there evidence of cellulation or local diffusion influencing development even though crystal sizes exceed the Keith and Padden parameter · by two orders of magnitude. We conclude that polymer spherulites can form independently of the mechanism proposed by Keith and Padden 14. It is suggested that the cause of lamellae splaying apart is a mutual repulsion due to uncrystallized portions of molecules between dominant lamellae.

On the lamellar morphology of isotactic polypropylene spherulites

The lamellar structures within melt-grown spherulites of the monoclinic form of isotactic polypropylene have been studied by transmission electron microscopy following permanganic etching. Spherulites grown at 160°C are composed solely of α laths which develop in the classical radiating branching manner from axialitic precursors. The branching units are not, however, fibres but individual dominant lamellae as previously found for polyethylene and isotactic polystyrene. The theory of instability of planar interfaces of Keith and Padden is not applicable to this textural situation, although local diffusion fields may add an additional structural dimension to spherulites which form quickly at, say, 130°C. Instead, one may possibly look to pressure from compressed cilia as the cause of splaying between adjacent lamellae. Cross-hatching is present in spherulites grown at 155°C and below in the early stages, and thereafter is concentrated in specific locations and subsidiary lamellae. The cross-hatching members are less stable, probably because they are composed of shorter and/or less tactic molecules.

On the morphology of melt-crystallized polyethylene - I. Lamellar profiles

The morphology of melt-crystallized polyethylene has been surveyed, and it has been revealed that all specimens are highly lamellar. Classifi­cation of the textures is possible in terms of the lamellar profiles in b -projection, i. e. down the radii of spherulites. For polyethylene crystal­lized as the orthorhombic phase only three types of lamellar profile have been observed for readily accessible growth conditions. There are ridged sheets of alternating {201} facets, planar {201} sheets and S-shaped sheets which are, to a first approximation, derivable from {201} platelets by shear of the lateral regions displacing {100} parallel to c . It is possible, moreover, to distinguish between the first-forming or dominant lamellae which lay down the framework of the texture and the later-forming or subsidiary lamellae which fill the framework. Dominant lamellae may have ridged, planar or S-shaped profiles but subsidiary lamellae appear only ridged or planar. Ridged dominant sheets form under régime I conditions as a consequence of distinct facets on the growth front. The association of ridges and growth facets indicates regularity in folding. S-shaped sheets are usually dominant in régime II growth. They appear to have been deformed from planar {201} sheets by forces associated with the forma­tion of banded spherulites.

On the morphology of melt-crystallized polyethylene - II. Lamellae and their crystallization conditions

The microstructure of melt-crystallized linear polyethylene has been correlated with the variables of crystallization for most readily attainable conditions. All samples are filled with well defined lamellae with an aver­age chain inclination of about 35° to lamellar normals. The lamellar thickness depends upon supercooling rather than directly on crystalliza­tion temperature, which indicates that it is a kinetically determined quantity. The simple assumption that it is a constant multiple of the height given by secondary nucleation is, however, incorrect. Lamellar profiles depend only upon the crystallization temperature and molecular mass of the polyethylene concerned. They are independent of the extent of spherulitic development and are not determined solely by the kinetic régime in which crystals grow. Dominant S-shaped lamellae (Ss) and their associated subsidiary platelets are, nevertheless, the prevalent form for crystallization within régime II, i. e. in most cases of practical import­ance. The distinction between dominant and subsidiary lamellae is linked to fractional crystallization. At low supercoolings it is shown that shorter molecules are concentrated within subsidiary lamellae, and the trend to separate later-crystallizing species is likely to persist, to a lesser degree, even to quenched samples. With the use of added branched molecules this has been demonstrated to occur. The consequences of spatial segre­gation are likely to include increased vulnerability to mechanical and environmentally induced failure.

Lamellar morphologies in melt-crystallized polyethylene

The lamellar morphologies of linear polyethylene crystallized from the melt in vacuo have been surveyed. It is shown that spherulites form by the outward growth of dominant lamellae along b, with low angle branching, with the intervening spaces subsequently filled by subsidiary lamellae. There are three principal habits of dominant lamellae: ridged sheets of alternating {201} facets, planar {201} sheets and S- or C-shaped sheets which can be derived from {201} sheets by suitable shear. These occur in increasing order of growth rate, caused either by increasing molecular mass or decreasing crystallization temperature. The most widespread morphology is one of S-shaped dominant lamellae with narrow {201} subsidiary platelets; these are found, e.g., in banded spherulites. It can be shown, at least in certain circumstances, that longer molecules tend to form the dominant lamellae and shorter ones the subsidiary. Even in quenched samples the lamellar texture is not homogeneous. Here and elsewhere it has a lateral scale which can be measured by the widths of dominant lamellae, but this varies much less than the Keith and Padden parameter δ to which it has been thought to be related.

Mineralogic and Petrologic Study of Lunar Anorthosite Slide 15415,18

The anorthosite slide 15415,18 contains 98 percent subhedral plagioclase (97 mole percent anorthite), two pyroxenes: diopsidic augite (46 percent wollastonite, 39 percent enstatite, 16 percent ferrosilite) with subsidiary (100) lamellae and grains of hypersthene (2.5 percent wollastonite, 58 percent enstatite, 39.5 percent ferrosilite), and traces of ilmenite. The pyroxene occurs interstitial to, and as small grains enclosed within, plagioclase. The textures and compositions of the phases appear compatible with an origin by concentration and adcumulus growth of plagioclase from a gabbroic anorthosite (or hyperaluminous) magma in a "plutonic" environment.