Nanoscale Polyethylene Crystal Dispersions

Abstract

Polyethylene (PE) and its bulk morphology have been studied intensely over the past 50 years. Micron-sized single crystals, carefully grown from very diluted organic solutions, have contributed to the fundamental understanding of polymer crystallization. However, non-aggregated and freely moving nanoscale polyethylene crystals have become accessible only very recently. The structure and properties of this novel system were explored.Aqueous dispersions of PE nanoparticles with various microstructures and crystallinities were synthesized via catalytic polymerization of ethylene with water-soluble salicylaldiminato Ni(II)-complexes, employing a procedure previously reported by our group. These colloidally stable dispersions consist of particles of semicrystalline linear, or branched polyethylene, dependant on the catalyst employed, with a volume average size of ca. 10 nm. This unprecedentedly small size is attributed to the effective particle nucleation and further growth in individual compartments.The single crystal nature and unique single lamellar structure of nanoparticles of linear PE of ca. 60% bulk crystallinity (PE I) was illustrated by cryo-TEM and SAXS analysis, carried out by M. Ballauff et al. In contrast, a more spherical shape is observed for nanoparticles of branched PE of much lower crystallinity (PE II), which probably consist of micelle-like crystals rather than defined lamellae. Interestingly, for ca. 80 nm size particles of PE I formed at a lower surfactant concentration, random aggregation of primary particles, rather than expanding of the single lamella in its lateral dimension, probably occurs at the early stage of particle growth, which results in a more irregular particle shape as observed by cryo-TEM and AFM. Furthermore, it was strongly indicated by the studies with different methods that the crystalline structure in such nascent nanoparticles is subject to reorganization upon thermal treatment at temperatures both below and above the melting temperature (determined by standard DSC on the dispersions). For the less crystalline particles composed of branched polymer, a structure composed of very small crystallites embedded in an amorphous matrix is in agreement with experimental data and agrees with the observed more spherical nature. Further investigations on the relationship between the lamellar thickness in the tempered particles and the corresponding recrystallization temperature may shed light on the equilibrium structure of polyethylene crystallized in confinement.Melting/crystallization in such nanoscale entities were studied for the first time as a means of switching environmental polarity sensed by guest molecules up-taken by the nanoparticles. Studies with lipophilic guest molecules, that is the fluorescent dyes pyrene and Nile Red, show that the guest molecules were taken up by the PE nanocrystals and located in the amorphous portion. This results in a much more apolar environment experienced by the guest molecules by comparison to the hydrophobic core of SDS micelles. The polarity experienced by pyrene located in the particles is higher in more crystalline particles. This is attributed to a sensing of the aqueous phase by pyrene forced to the water-particle interface by the lamellar crystallites. Upon melting/crystallization, the polarity experienced by the pyrene probe changes significantly. The temperature at which this polarity switch occurs was controlled by the degree of branching of the polymer.An uptake of lipophilic guest molecules by PE nanocrystals was also observed for nitroxide spin probes. In agreement with the fluorescence studies, CW-ESR of the spin probes in the PE dispersions suggests again partition of the slightly hydrophilic nitroxide radicals between the polymer phase and the aqueous phase, as concluded from the observed lineshape, which differs from the spectra taken in pure aqueous solutions or in polymer films. However quantitative analysis could not be retrieved due to the complex nature of the spectra, which are the sum of several contributions. In addition, both CW-ESR and ESEEM studies show that restructuring, probably via lamellar thickening, occurs upon crystallization from once melted PE I nanocrystals. In line with this interpretation, no significant difference was observed for less crystalline PE II nanoparticles.Starting from the pre-formed nanocrystals, submicron semicrystalline polyethylene films can be prepared under much more benign conditions, by comparison to the traditional route starting from a diluted polymer solution in hot organic solvents. Aside the aforementioned advantages, the crystallinity of thus obtained films can be conveniently controlled by the crystallinity present in the polymer crystals. Homogenous films without defects were observed over micrometer areas. Sufficient interactions between the amorphous portions of the nanocrystals provided free-standing films. In the nascent films the primary PE nanocrystals orient preferentially with their lamellar axis perpendicular to the substrate surface, as shown by electron diffraction studies. Correspondingly, flat-on assembly of PE nanocrystals in discontinuous monolayers is observed by AFM.Upon melting/recrystallization the continuity of the submicron films is maintained without significant dewetting. Detailed morphologies formed upon recrystallization, dependant on the film thickness, are in general similar to organic solution-cast films. However, upon annealing of the monolayer-like assembly of PE nanocrystals, lamellar thickening was observed, without changes of crystal orientation. Surfactant adsorbed to the nanocrystals in the aqueous dispersion desorbs at least partially during formation of the nascent films, and upon annealing below the melting point surfactant migrates to the film-air interface to form aggregates, which can be removed by rinsing, during which the film stays intact and structurally unaltered as revealed by AFM and water contact angles.Surface functionalization of PE nanocrystals by copolymerization with various types of amphiphilic comonomers was demonstrated. Despite favored incorporation for ethylene by the catalysts employed, copolymer particles with moderate incorporations of the studied comonomers were obtained. Incorporation of a comonomer containing an anionic sulfate group was observed to afford better colloidal stability by comparison to nonionic PEG and saccharide groups. The crystallinity of the copolymer particles with respect to the ethylene derived portion is similar as in homopolyethylene particles.