Studies of molecular weight blends and interface effects on polymer crystallisation & block copolymer ordering.

摘要

We present the results of five projects, ranging in topic from polymer crystallisation to phase ordering in diblock copolymers, which form the bulk of the research for this dissertation. The first two studies focus on the topic of polymer crystal nucleation. We apply a technique of dewetting crystalline polymer from unfavorable substrates to create systems of small droplets within which nucleation can take place. Such studies are ideal for examining how factors such as confinement and length scale affect the process of nucleation. Despite the great deal of attention this topic has garnered in the literature, these earliest stages of crystallisation are still poorly understood. Furthermore, an understanding of polymer nucleation has broader applicability to nucleation in material systems in general, from small molecules to proteins. Thus, we consider studies such as these to have an impact on furthering our understanding of crystal nucleation in general. In the first study, droplets of polyethylene (PE) on an amorphous substrate are investigated while the second study focuses on droplets of poly(ethylene oxide) (PEG) on a tunable substrate. For the case of PE droplets, a surface dependent nucleation rate is directly measured for the first time. For the PEG droplets, we demonstrate our ability to direct nucleation to occur in the bulk volume, at the surface interface or at the droplet's contact line, depending on how the surface properties of the substrate are tuned. These changes in nucleation mechanism are accompanied by a large enhancement in the temperature at which crystallisation occurs.;The final two projects focus on studies of a sphere-forming block copolymer system of poly(butadiene-b-ethylene oxide) (PB-PEO) through ellispsometry. The fourth paper also takes advantage of the technique of blending different molecular weights. By blending two different PEO block lengths together, the size of the ordered PEO spheres can be tuned. The fifth study diverges from the topic of crystallisation, and investigates a morphological transition at the substrate for an ordered PB-PEO film. In recent years, a great deal of attention has been focused on ordered block copolymer systems beyond simply understanding the physics of the ordering process. The ease with which such systems self-assemble, creating monodisperse structures on a nanometre length scale make them ideally suited for technological applications. It is important that we understand the role of interfaces on such systems in order to achieve the stability and order necessary for practical applications to be feasible. In our study, we show that a reversible transition takes place in the phase-separated morphology at the substrate from a partial layer of ordered spheres to a lamellar wetting layer as temperature is varied. The detection of such a transition at the buried substrate is novel.;In the third study, spherulite growth rates have been measured in binary and ternary molecular weight blends of PEG. According to the literature, a monotonic decrease in growth rate is expected as the fraction of the long chain component in the blend increases. This is not the behavior we observe. Instead, we measure a non-monotonic change in the growth rate as a function of blend composition. While crystal growth is considered well understood, this unexpected result highlights that there are still aspects of the crystallisation process that require deeper study, in particular with respect to the impact of molecular weight. Polymers used for industrial applications usually have very broad polydispersities. Since crystalline polymers make up a large portion of these industrially important polymers, it is inherent that the effects of molecular weight blends are understood.