Single Lamella Nanoparticles of Polyethylene

Abstract

We present a complete analysis of the structure of polyethylene (PE) nanoparticles synthesized and stabilized in water under very mild conditions (15°C, 40 atm) by a nickel-catalyzed polymerization in aqueous solution. Combining cryogenic transmission electron microscopy (cryo-TEM) with X-ray scattering, we demonstrate that this new synthetic route leads to a stable dispersion of individual PE nanoparticles with a narrow size distribution. Most of the semicrystalline particles have a hexagonal shape (lateral size 25 nm, thickness 9 nm) and exhibit the habit of a truncated lozenge. The combination of cryo-TEM and small-angle X-ray scattering demonstrates that the particles consist of a single crystalline lamella sandwiched between two thin amorphous polymer layers ("nanohamburgers"). Hence, these nanocrystals that comprise only ca. 14 chains present the smallest single crystals of PE ever reported. The very small thickness of the crystalline lamella (6.3 nm) is related to the extreme undercooling (more than 100°C) that is due to the low temperature at which the polymerization takes place. This strong undercooling cannot be achieved by any other method so far. Dispersions of polyethylene nanocrystals may have a high potential for a further understanding of polymer crystallization as well as for materials science as, e.g., for the fabrication of extremely thin crystalline layers.Polyethylene (PE) is a commodity polymer that has become ubiquitous over the past several decades because of its low price and good mechanical properties. 1 Hence, the number of applications of the material is huge and many millions of tons are produced worldwide annually. However, PE has hardly played any role in the field of nanotechnology. This is due to the problem that PE is produced either by free radical polymerization under high pressure and temperature or with metal-organic catalysts working exclusively under strictly water-free conditions. Polymer nanoparticles and their composites with inorganic compounds, however, are very often produced in aqueous systems. 2 Recently, it was demonstrated that ethylene can be polymerized in aqueous systems in a catalytic fashion by Ni(II) complexes. [3][4][5][6] By virtue of this novel synthesis, long chains of polyethylene can be generated in a well-controlled environment and at ambient temperature. Thus, it could be shown that aqueous PE dispersions can be produced. This novel way of polymerization hence opens the way for the creation of nanostructures made from PE. Up to now, the particles synthesized in this way were semicrystalline and for the largest part consisted of stacks of several crystalline lamellae. 6

Annealing of Single Lamella Nanoparticles of Polyethylene

“…As already discussed in previous work, the faceted nanocrystals can be treated in good approximation as circular platelets for the SAXS analysis. 17 The height of the amorphous PE regions including the hydrocarbon SDS tails is given by L a /2, while L denotes the total height of the platelet (see Figure 1). The electron contrasts are defined as ∆b The effect of size polydispersity of the PE particles is taken into account by an appropriate average using a distribution function characterizing the degree of polydispersity.…”

Critical Casimir interaction of ellipsoidal colloids with a planar wall

“…Rodlike [51] or disklike [52] architectures, dumbbell-shaped particles [53,54], and particles with ellipsoidal shape [55,56] have been synthesized and characterized. The size of these particles ranges between 10 nm and 10 µm.…”

Two‐Dimensional Oligo(phenylene‐ethynylene‐butadiynylene)s: All‐Covalent Nanoscale Spoked Wheels

“…[13][14] However, for these macromolecules defined structure does not necessarily mean defined size and shape, some of them behave rather like flexible membrane sheets than rigid disks. [15] There have also been reports on single-lamella polyethylene crystallites, which are shape-persistent due to intramolecular interactions, [16] but the problem of size control remains. By contrast, rigid, defined 2D oligomers offer several evident advantages over the inorganic counterparts.…”

“…[11b, 16,47,49] Small-angle neutron scattering was performed on several samples of 1 in CDCl 3 and in a contrast series of THF/ [D 8 ]THF mixtures. As a result of the limited solubility of 1, the investigations were restricted to low concentrations.…”