Polymer Physics Prize (2022): Controlling Nanoparticle Ordering by Directional Polymer Crystallization*

We have previously shown that nanoparticles (NPs), which are initially miscible with a polymer melt, can be ordered by polymer crystallization. The key variable here is a Peclet number (Pe) that compares the time scale of NP motion to the rate of polymer crystallization. For large Pe the NPs are frozen in place during polymer crystallization. However, for Pe values less than 1 the NPs are moved by the growing crystals and selectively placed in the interlamellar, interfibrillar and interspherulitic regions. These results are in line with the Keith and Padden’s findings, expanded on by Russell et al., on the spatial dispersion of amorphous polymer defects in a semicrystalline morphology. The resulting samples have Young’s moduli that are increased by a factor of ~2 relative to ones where the NPs are randomly ordered.

 

While these morphological results are of interest on their own, we believe that additional synergies in properties can be obtained if we could move from spherulitic polymer morphologies (which are locally anisotropic but spatially isotropic) to more directionally crystallized samples with a preferential orientation. To this end we employ the zone annealing methodology, a directional crystallization protocol popular in metallurgy and first applied to semicrystalline polymers by Lovinger and Gryte nearly 50 years ago. Using this method, we show that we can order and orient the NP assemblies into desired directions. In addition to these novel experimental findings, we propose a new theoretical framework that allows us to model the directional crystallization of polymers and the key variables that control them.