Nanocrystalization: Crystal Growth and Polymorphism of Confined Polymers

Abstract

Nanoporous hard templates provide a two-dimensionally confined space in which self-organization processes such as crystallization, protein secondary structure formation, and phase separation can be fundamentally different from those obtained in thin films and in the bulk. Understanding the crystallization, thermodynamics and dynamics of polymers under confinement allow for their rational design as functional devices with tunable mechanical strength, processability, electronic and optical properties. The principal focus of this lecture is finding the basic underlying principles that give rise to nucleation and crystal growth in a range of soft materials (crystallizable polymers, amphiphilic molecules, liquid crystals, low-molecular weight liquids and biopolymers) under hard-confinement. Bulk crystallization of polymers proceeds usually via heterogeneous nucleation and gives rise to structure over multiple length scales: from the crystalline unit cell (1-2 nm), to alternating crystalline/amorphous lamellae (50-100 nm) and to spherulitic superstructures (>500 nm and up to cm). This is the dominant nucleation mechanism in bulk polymers. Important studies of polymer crystallization confined to droplets and within the spherical nanodomains of block copolymers emphasized the interplay between heterogeneous and homogeneous nucleation and explored the kinetics of homogeneous nucleation observed at high supercooling. Some authors have even reported only homogeneous nucleation of polymers confined to AAO nanopores. This finding is surprising and leads to several questions about the type of nucleation under hard confinement typical for AAO: Is the crystallization process always homogeneous within the small AAO nanopores? Do surface effects always dominate? And even more, can hard confinement completely suppress polymer crystallization and if so what is the required size? Providing answers to these questions is of technological relevance for the understanding of nanocomposites containing semi-crystalline polymers. We will try to explore these issues both comparing the recent studies and research activities of our group.