Polymer Crystallization under Confinement by Well-Dispersed Nanoparticles

We explore the role of nanoparticle (NP) size and loading on the kinetics of polymer crystallization, in particular, the crystal growth rate (G), under conditions where the NPs and polymers are uniformly mixed. While there are significant differences with variations in NP size and loading, the measured G data fall into an apparently universal curve when we consider them on the basis of confinement. In particular, we find that the ratio of polymer volume, V-PEO, to the NP surface area, SA(NP), is the most appropriate confinement metric in the 10-100 nm range, which includes typical lamellar spacings in semicrystalline polymers (5-50 nm). Similarly, we find that the fraction of polymer crystallized also decreases with increasing confinement. We thus posit that the formation of a bound layer at the attractive NP interface is the key driver in these situations. Because the volume fraction of the bound polymer layer of thickness delta is (delta x SA(NP))/V-PEO, these results support the notion that, in addition to confining the polymer and reducing its equilibrium melting point, the NPs serve to remove a certain fraction of the PEO from the crystallization process by binding irreversibly to them. Additionally, when combined with the fact that the formation of a bound layer slows down NP dynamics, we emphasize that the bound polymer critically affects crystallization kinetics from both a structural and a dynamical perspective. These results strongly parallel our earlier work on the glass transition temperature of amorphous polymers intimately mixed with NPs where material properties are critically determined by the formation of a bound layer.