Session H2: Polymer Physics Prize

Challenges for Polymer Theory and Simulation

I will discuss some contemporary topics in polymer physics that represent challenges for theorists and computer simulators in the coming decade. These are: \begin{itemize} \item The challenge of multiple scales -- bridging the atomistic to the continuum \item Structure-property relations for nanocomposites \item Supramolecular polymer assembly \item Science and engineering of conjugated polymer interfaces -- electronic structure meets polymer physics \item Rheology and structure of inhomogeneous polymers \item Ultimate mechanical properties of everything polymeric \end{itemize} There are common aspects to these challenging topics; for example, multiple scales and the curse of dimensionality are pervasive. My presentation will touch on the theoretical tools that are needed to conduct research in these areas, and will highlight a few contributions from my own group.   

Self-Assembly of Monolayer and Multilayer Films of Spherical-Domain Diblock Copolymers

Self-assembly of block copolymers in thin films can yield templates for nanolithographic patterning of substrates on very small length scales as well as ordered multilayer structures for membrane and electronic applications. On a fundamental level these thin block copolymer melts raise many interesting questions about self-assembly in 2D and how the transition from 2D to 3D occurs as the film is increased in thickness. To answer these, scattering techniques such as grazing incidence small angle X-ray scattering (GISAXS) can be a very useful complement to the normal imaging techniques of AFM, SEM and TEM, but in combination with self-consistent field theoretic (SCFT) simulations, these become even more powerful. I highlight one such set of questions, how the packing of spherical block copolymer domains confined to a thin film changes as the thickness of the film is increased layer by layer of spheres, as a concrete example of the usefulness to this combined approach and how the SCFT methods pioneered by Glenn Fredrickson and his colleagues have profoundly influenced experimental polymer physics. Using GISAXS we find hexagonal symmetry in films 1-4 layers thick. Stacking in films 1-4 layers thick is close-packed \textit{AB}, \textit{ABA and ABAB}. At 5 layers, the hexagonal symmetry breaks to form an orthorhombic phase, characterized by second/first nearest-neighbors $a_{1}/a_{2}$ and lattice angle \textit{$\varphi $}. As the number of layers is increased, $a_{1}$ and \textit{$\varphi $} increase continuously to approach that for BCC (110) planes. From measurements collected above and below the critical angle of the polymer, the structure is uniform throughout the depth of the film. SCFT calculations provide a semi-quantitative description of the transition and the insight that it is a consequence of competition between the optimal HEX packing at the film surfaces with the preferred BCC (110) inner layer packing in the bulk.   

On the consequences of interacting with Glenn Fredrickson

Since joining Bell Labs in 1984, and subsequently UC Santa Barbara in 1990, Glenn Fredrickson has contributed many timely and inspiring theories to the polymer physics community. These developments have had a significant impact on the speaker, often resulting in years of experimental research. This lecture will trace forward representative examples of Fredrickson's theoretical insights that have spawned innovative and scholarly research.   

Supramolecular concepts in self-assembly of complex polymer systems

We discuss the complexation, the self-assembly behaviour and nanostructures obtained in comb-like liquid crystalline polymers formed by ionic complexation of cationic dendronized polymers and anionic lipids. The resulting self-assembled materials exhibit thermotropic liquid crystalline behaviour and a rich state diagram. The topology of the LC phases resulting from the self-assembly process, their lattice parameter and the distribution of polymer and lipid domains are discussed via birefringency analysis, small angle x-ray scattering, differential scanning calorimetry and transmission electron microscope. Depending on the generation of the dendronized polymer and the length of the alkyl chains, amorphous, lamellar, columnar hexagonal and a rarely observed columnar tetragonal phase can be obtained, where the long-range ordering of the structures is a function of the generation of the dendronized polymer considered and the lattice space is of the order of 3-6 nm. The selective staining of polymer/lipid domains allows establishing unambiguously the composition of each domain in the observed nanostructures and a structural model is proposed which accounts for the systematic variations of structure in terms of alkyl chain length as well as polymer generation. Furthermore, we discuss our recent efforts towards enhancing long-range order via external applied fields. Owing to the reversible nature of the ionic complexation this process proves high relevance for nanoporous channels, biomimetic, transport and nanotemplating applications. References: Canilho, N.; Kasemi, E.; Mezzenga, R.; Schluter, A.D. \textit{J. Am. Chem. Soc}. \textbf{128}, 13998 (2006).   

PE Crystallization and Rotator Phases.

Recent work on crystallization of polyethylene (PE) implicates a metastable rotator phase as the nucleating phase. This claim invites the questions: what is the structure of this phase, what is the free energy difference $\Delta F$ driving nucleation, and what is the surface free energy $\Sigma$ of the phase in contact with the melt? Related proposals for critical nuclei in polymer crystallization (``fringed micelles'') were dispensed with long ago, with estimates of $\Delta F$ and $\Sigma$ that were unduly pessimistic. With more recent theoretical tools, we can revisit PE nucleation, comparing crystalline and rotator phase nuclei. To do so requires a model that can describe the bulk and surface free energy of both phases. To compare bulk free energies, we use a 6-state generalized Potts model, in which the disordered phase represents the rotator phase. Using a multiscale approach, coupling constants are obtained from solid-state simulations of domain walls between six degenerate crystalline orderings. The surface free energies, dominated by entropic penalties of melt segments near the nucleus surface (the ``fringe''), are calculated using methods developed in the context of polymer brushes. Combining these ingredients, we can make a more enlightened comparison of the nucleation barrier for crystalline and rotator phase nuclei.