Polyethylene multilayer crystals

Figure 5.6 Electron micrograph of a multilayered crystal of polyethylene displaying regular rotation of successive growth terraces. ...

High barrier plastic containers are currently made from multiple layers of a relatively thick economical polymer such as polyethylene terephthalate (PET) or polyethylene, combined with a thin layer of a barrier polymer such as ethylene vinyl alcohol (EVOH). In a similar way, multilayer barrier containers can be made with a thin layer of liquid crystal polymer (LCP), as an excellent barrier to oxygen, water vapor, and other gases. 

Recently, a novel method to prepare multilayers and nanolayers from a sequential coextrusion process has been introduced. In this method, the number of layers can be largely increased at the expense of reducing the thickness of the coextruded layers. It is also possible to obtain nanodroplet dispersions by annealing such multilayer coextruded ensemble. The following systems have been studied employing this method polyethylene/ polystyrene (PE/PS), polypropylene/polystyrene (PP/ PS), polycarbonate/poly(ethylene terephtalate) (PC/ PET), and poly(ethylene oxide)/ethylene acrilic acid copolymer (PEO/EAA). The crystallization behavior within the nanolayers and within the spheres that are obtained after heating and coalescence of the nanolayers has also been studied [18,33,66,94,104, 111, 140-143]. 

The term axialite was first coined by Bassett, Keller and Mitsuhashi to describe multilayered objects grown from relatively concentrated solutions of polyethylene in xylene. The lamellae in these objects, which are bounded by low index faces, diverge, to a first approximation, from a single central axis and, as a result, the overall appearance is either polygonal or sheaf-like depending upon the direction in which they are viewed. Although similar objects crystallized from the melt were originally termed hedrites, it is the term axialite which is now more commonly applied and it is this which will be used here to describe sheaf-like structures which have not developed a spherical envelope. 

---