Outlook and Open Questions: A Personal View

Abstract

Why has crystallization of polymers interested so many researchers for so long time and why is nonetheless our understanding of the underlying fundamental processes still at a rather rudimentary level? This is the more surprising as the subject of interest can be defined in an extremely simple way: Polymer crystallization concerns the transition from a randomly coiled to a perfectly ordered state. Probably, the key difficulty of polymer crystallization arises from the fact that the building blocks of a polymer crystal, the monomers, cannot move independently due to their connectivity to many other monomers forming a polymer chain.

The most crucial consequence of connectivity is that the formation of a perfectly ordered state would need extremely long times, sometimes much more than the duration of any feasible experiment. Thus, at finite crystal growth velocities only imperfect crystals consisting of folded molecules are formed. Because better ordered states always represent a lower free energy these imperfect crystals can only be metastable. The sometimes quite significant lifetimes of such metastable states represents a major complication in the study of polymer crystals. Moreover, such metastable states are not always well defined and their characterization is by far non trivial. Although it is possible to reproduce similar metastable states, it is clear that the thermodynamic pathway (sample history) is crucial. Typically, reaching a given crystallization or annealing temperature along two different pathways may result in two distinctly different crystalline states. In addition, the representative features of these states (and polymer crystals in general) may depend on the lengthscale of observation and the experimental techniques used. Summarizing these observations, one may conclude that there exists no single and well-defined state of a polymer crystal.