D. C. Bassett

An improved permanganic etchant for polyolefines

Abstract The technique of permanganic etching reveals lamellar detail in polyethylene and other polyolefines allowing representative melt-crystallized morphologies to be studied with the electron microscope. When etching with the original recipe is prolonged artefacts on a scale of ∼ 10 μ m can develop and have probably been misinterpreted as genuine features in some instances. It is emphasized that the conditions of etching should be adjusted to suit the needs of individual specimens. The morphology of artefacts is demonstrated so that they may be recognized as such should they occur. A simple modification of the etchant by incorporating orthophosphoric acid has been found which avoids formation of artefacts on polyethylene.

On crystallization phenomena in PEEK

Abstract The crystallization and lamellar morphology of PEEK have been investigated by thermal analysis and electron microscopy following permanganic etching. It is shown that the two peaks in typical melting endotherms (of samples which have been crystallized either from the melt or by heating from the glassy state) represent different components of the morphology, formed in two stages of crystallization. The earlier stage is represented by a broad upper melting peak; the lower melting peak develops later. In crystallization from the glass at higher values of T a , the lower peak represents crystallites which have been reorganized at T a but which originated from the lower part of the primary endotherm extending below T a . Additional crystallinity may also develop from a portion of the rubbery phase which did not crystallize initially. In crystallization from the melt at T c , the first product shows a broad melting endotherm extending down to T c . After a time, the secondary peak develops; it lies just above T c , and thus represents crystallites only just stable at T c . Electron microscopy shows that spherulites of PEEK are constructed from a framework of branching and splaying individual dominant lamellae with subsequent infilling. The dominant and other primary lamellae melt in the upper peak while material melting in the lower peak lies between them. Internal evidence suggests that this correlation of melting point and location within the spherulite architecture is an inherent function of that location, reflecting changing constraints in the sequence of crystal growth, rather than fractional crystallization due to variations in molecular properties. Accordingly, the two melting peaks would be a consequence of spherulitic growth which would be most prominent when later crystallization is difficult to achieve, i.e. in polymers of low crystallinity.

The hot compaction of high modulus melt-spun polyethylene fibres

The production of solid section highly oriented polyethylene by compaction of melt-spun polyethylene fibres is described. Differential scanning calorimetry, X-ray diffraction and electron microscopy have been used to determine the structure of the compacted polymer. The essential feature of the process is shown to be selective surface melting of the fibres to form a polyethylene/polyethylene composite of very high integrity, yet maintaining a very high proportion of the strength and stiffness of the fibres.

On the lamellar morphology of isotactic polypropylene spherulites

Abstract 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 development of polypropylene spherulites

Abstract 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 .

The hot compaction behaviour of woven oriented polypropylene fibres and tapes. I. Mechanical properties

Abstract The aim of this work was to establish the important parameters that control the hot compaction behaviour of woven oriented polypropylene. Five commercial woven cloths, based on four different polypropylene polymers, were selected so that the perceived important variables could be studied. These include the mechanical properties of the original oriented tapes or fibres, the geometry of the oriented reinforcement (fibres or tapes), the mechanical properties of the base polymer (which are crucially dependant on the molecular weight and morphology), and the weave style. The five cloths were chosen so as to explore the boundaries of these various parameters, i.e. low and high molecular weight: circular or rectangular reinforcement (fibres or tapes): low or high tape initial orientation: coarse or fine weave. A vital aspect of this study was the realisation that hot compacted polypropylene could be envisaged as a composite, comprising an oriented ‘reinforcement’ bound together by a matrix phase, formed by melting and recrystallisation of the original oriented material. We have established the crucial importance of the properties of the melted and recrystallised matrix phase, especially the level of ductility, in controlling the properties of the hot compacted composite.

On isolated lamellae of melt-crystallized polyethylene☆

The growth, isolation and examination of individual melt-grown crystals of linear polyethylene is reported. Monolayers grown at 130°C have continuously curved elliptic prism faces (despite being grown under ‘regime I’ conditions) and fold surfaces close to {2 0 1}. Such monolayers, showing a sector boundary along their length, and distinct sectors differing very slightly in orientation across this boundary, have been revealed in dark-field electron microscopy. This is evidence for regularity in chain folding. A more frequent form is when a central screw dislocation is present near the centre of a lamella. The spiral terrace developing from this shows a remarkable habit in which adjacent layers splay apart, but growth soon stops, leaving the upper and lowermost layers plano-convex in outline. In every case, the planar outline is of a prism face making an acute angle with the previous layer. This is a consequence of growth having only occurred around screw dislocations of the appropriate hand: right-handed for a (201) surface. It is suggested that this habit, illustrating how spiral development selects screw dislocations of a consistent hand, may be the missing element in understanding the growth of banded spherulites in polyethylenes via systematic arrays of similar screw dislocations.

Permanganic etching of PEEK

Abstract The technique of permanganic etching as already published reveals spherulitic structure and lamellar detail in polyolefines, and has recently been modified for application to isotactic polystyrene. For application to poly(aryl-ether-ether-ketone), or PEEK, neither variant is suitable, but a new modification of the etchant, based on phosphoric acid, allows the morphology of PEEK to be studied under the electron microscope, revealing differences between specimens crystallized at different temperatures.

The hot compaction of polyethylene terephthalate

A process is described for the successful compaction of polyethylene terephthalate fibres. The measurement of mechanical properties shows that a very high proportion of the original fibre properties are retained and that the compacted samples have a good degree of coherence. Electron microscopy studies of suitably etched samples reveals the effect of the compaction temperature on the structure of the compacted samples.

On the lamellar morphology of melt-crystallized isotactic polystyrene

Abstract 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 Padden14. It is suggested that the cause of lamellae splaying apart is a mutual repulsion due to uncrystallized portions of molecules between dominant lamellae.