Using Nanofiller Assemblies to Control the Crystallization Kinetics of High-Density Polyethylene

Polyethylene-grafted nanoparticles (NPs) are organized into a variety of assemblies in a polydisperse polyethylene melt by tailoring the graft density and molecular weights of the graft chains. Under these conditions, we systematically vary the NP assemble state and study its consequences on crystal nucleation and growth rates. We find that the nucleation rate is suppressed below that of the unfilled polymer for good NP spatial dispersion. However, poorer dispersion, which leads to the formation of NP assemblies, can accelerate nucleation likely by providing multiple heterogeneous sites due to topographical features. We find that a key parameter, the chain overcrowding in the brush, can predict these nucleation trends; in this language, the most enhanced nucleation rate is found when the grafts are the most overcrowded and, hence, the least interpenetrated with the matrix chains. This result is consistent with one other literature result that utilized short crystallizable polyethylene glycol grafts in short polyethylene oxide (PEO) matrices. The growth kinetics were retarded for all nanocomposites, and their temperature dependences were essentially equal to that for the pure polymer (in the absence of NPs); the NPs thus do not affect secondary nucleation, indicating that the transport of the matrix to the growth front is the rate-determining step. Thus, evidently, the increase in matrix viscosity, or reduction in growth rates, is directly determined by the agglomeration state of the NPs. These results are consistent with past works with bare silica NPs in PEO and with silica NPs grafted with amorphous chains in a PEO matrix, suggesting that growth kinetics in these systems apparently follow “universal” behavior. Additionally, there are initial hints that the ratio of the effective surface area of the NP clusters per unit matrix volume provides a unified description of the NP-induced confinement that slows growth kinetics. Our work thus shows that there is an evolving understanding of the role of NPs in crystallization kinetics, in particular, crystal growth where trends appear to be independent of the grafts’ ability to crystallize.