nucleation rate

crystal size

crystallization behavior

x-ray scattering

crystallization kinetics

Accounts of chemical research

ConspectusCrystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called "fractionated crystallization", i.e., multiple crystallization peaks during cooling.Because the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material.Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the "chain perpendicular to" to the "chain parallel to" the AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including c...

Confined Crystallization of Polymers within Nanopores