(161av) Heterogeneous Nucleation Mechanisms in Polyolefins: Experiments Linked with Molecular Simulations

Polyolefins are an important global commodity used for many applications because of their toughness and strength. These attractive mechanical properties are linked to the complex interconnected microstructure of the crystalline and amorphous regions in these semicrystalline materials, which largely develops during solidification under the influence of processing. The microstructure can be manipulated through changes in stress, temperature, and pressure, to name a few. Of particular interest are nucleating agents, a class of additives that permit manipulation of microstructure through their effects on crystallite grain sizes, realization of different polymorphs, and lamellar alignment through the phenomenon of heterogeneous nucleation. However, the mechanisms by which the microstructure is altered is still a matter of art, as the effectiveness of a nucleating agent cannot be predicted a priori.

Experimentally, nucleating agents have been studied for many years. Some general heuristics have been proposed^1,2,3, but there are several shortcomings with regard to elucidating the mechanisms occurring. Most nucleation experiments begin with milled nucleation agents dispersed in a bulk polymer melt that crystallizes when quenched below the melting point. The effects of the nucleating agent on crystallization kinetics are captured using an Avrami analysis, which quantifies the kinetics through a few composite parameters. However, the Avrami analysis offers little insight into the microstructural or molecular origins of the observed kinetics. Examples of such shortcomings include uncertainties about which facet(s) of the additive are responsible for heterogeneous nucleation, whether local curvature of the additive has any influence, and how well the additive is dispersed in the melt. In many cases, other impurities (e.g. catalyst residue from polymerization) can obscure the underlying action of the nucleating agent. Additionally, the Avrami analysis does not clearly distinguish between nucleation and growth processes. Finally, as most Avrami analyses are applied to experiments using macroscopic amounts of polymer, an inhomogeneous temperature profile exists, both due to the slow cooling of the polymer from the surface in contact with the environment, and due to the latent heat of fusion released as the polymer crystallizes.

Following the work of Cormia et al.⁴ and Carvalho et al.⁵ we employ an alternative methodology for the quantitative characterization of heterogeneous nucleation that deals with these issues. This methodology entails the dispersion of micrometer-sized polymer droplets onto a well-characterized surface of the nucleating agent, and observation of crystallization under a polarized light microscope. Each droplet then becomes an independent crystallization experiment, from which information about heterogeneous nucleation can be learned from the temperature- and droplet size-dependence of crystallization within each droplet in contact with the nucleation surface. The small sizes of the droplets ensure that the rate of growth plays little role in the observed crystallization time of any individual droplet, so that the kinetics of nucleation are clearly distinguished. Correlation of the probability of nucleation with the size of the droplets sheds light on the origin of nucleation, whether it occurs within the bulk of the droplet (homogeneous or impurity-induced nucleation), at an interface of the droplet with its surroundings (heterogeneous nucleation), or at the 3-phase contact line. Additionally, we can calculate a nucleation rate and induction times separate from a growth rate to more accurately describe the nucleation kinetics.

However, several shortcomings still remain. The resolution of this method is limited to the micron size and millisecond time scale observable by light microscopy. To examine the molecular level origins of the nucleation mechanisms, we employ molecular dynamics (MD) simulations, which allows observation on very short spatiotemporal scales. Molecular simulations have previously been implemented to quantify crystal nucleation of oligoethylenes via homogeneous nucleation⁶ and via surface-induced heterogeneous nucleation.^7,8,9

In this work, we apply the methods of Carvalho et al. and of Bourque et al. to the study of heterogeneous nucleation of polyethylene on common substrates, for the first time. Induction times for crystal nucleation of the polymer on each substrate are determined by both methods, and their relative magnitudes compared, for purposes of validation. For those cases where nucleation is determined to occur heterogeneously, we furthermore examine the ordering of the polymer molecules at the interface in order to infer mechanisms such as epitaxial matching between the nucleating agent and the polymer crystal.

References

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