(31d) Heterogeneous Nucleation in Polyethylene: Experiments and Molecular Simulations

The desirable mechanical, thermal, and optical properties of semicrystalline polyolefins are a consequence of the complex microstructure formed during crystallization. While the temperature of crystallization provides some degree of control over both nucleation and growth processes of crystallization, additives called nucleating agents allow the engineer to focus on the nucleation process alone, promoting the formation of more nuclei and specific polymorphs. The heterogeneous nucleation mechanisms that a particular additive promotes are still a matter of considerable empiricism.

Despite the considerable challenges of spatiotemporal resolution required to study nucleation processes, both laboratory experiments and molecular simulations have been used to study heterogeneous nucleation in chain molecules. By using these together, the nucleation of a polymer via heterogeneous nucleation with an additive in the real laboratory (experiments) can be connected to the molecular details of chain alignment and nucleation on the same additive (simulation).

However, naïve experiments that investigate nucleation during the crystallization of a bulk (macroscopic) sample of polymer are often victim to several shortcomings. Firstly, a clear separation of the nucleation and growth processes is often difficult. The formation of new critical nuclei can be concurrent with the growth of supercritical nuclei that have already formed. Secondly, difficulties with dispersion of the additive, contamination due to uncontrolled nucleation on minor impurities (e.g. residue from polymerization), and an inhomogeneous temperature profile due to the latent heat of fusion of polymer crystallization can complicate the interpretation of experimental results.

A methodology that avoids these shortcomings has been used to study homogeneous nucleation of bismuth¹ and polypropylene², and also for investigating the effects of nucleating agents in polypropylene³. In these works, differential scanning calorimetry (DSC) was used to characterize nucleation. By partitioning the material to be crystallized into many micron sized domains dispersed within an immiscible matrix, the amount of the heat flow measured by DSC due to the growth process can be limited, and is proportional rather to the nucleation of independent domains. This sample preparation also neatly deals with difficulties of nucleation due to other impurities and inhomogeneous temperature profiles.

In this work, polyethylene and potassium hydrogen phthalate (PHP) as nucleating agent were dispersed by melt blending within an immiscible matrix of polystyrene, and subsequently examined with DSC. After identifying the peak in crystallization that was present only in samples containing nucleating agent, isothermal nucleation experiments were conducted at several temperatures near this peak to measure the heterogeneous nucleation rate and activation energy as functions of temperature. The sizes of the PE domains were characterized by dynamic light scattering and scanning electron microscopy.

Following the work of Bourque ^4,5, the heterogeneous nucleation of n-pentacontane (C50) on PHP was also investigated with molecular dynamic simulations of C50 sandwiched between crystalline layers of PHP. By tracking the growth front of the C50 crystal nucleated from the melt on the PHP surface, induction times for nucleation were characterized as a function of crystallization temperature. Epitaxy between the C50 and PHP crystals was observed.

The results of the experiments and molecular simulations discussed above will be presented, as well as a comparison between the temperature dependence of nucleation measured by both methods.

References

1. Rasmussen, D.H.; Loper, Jr, C.R. DSC: a rapid method for isothermal nucleation rate measurement. Acta Metallurgica 1976, 24, 117-123.
2. Santana, O.O.; Müller, A.J. Homogeneous nucleation of the dispersed crystallisable component of immiscible polymer blends. Polymer Bulletin 1994, 32, 471-477.
3. Langhe, D.S.; Keum, J.K.; Hiltner, A.; Baer, E. Fractionated crystallization of α- and β-nucleated polypropylene droplets. Polymer Physics 2010, 49, 159-171.
4. Bourque, A.J.; Locker, C.R.; Rutledge, G.C. Heterogeneous nucleation of an n-alkane on tetrahedrally coordinated crystals. Journal of Physical Chemistry B 2017, 121, 904-911.
5. Bourque, A.J.; Rutledge, G.C. Heterogeneous nucleation of an n-alkane on graphene-like materials. European Polymer Journal 2018, 104, 64-71.