Soft matter under confinement is a growing, interdisciplinary research field with yet unknown basic principles. Nanoporous hard templates provide a two-dimensionally confined space in which self-organization processes such as crystallization, protein secondary structure formation, mesophase formation and phase separation can be manipulated giving rise to unprecedented confinement-induced morphologies with new and exciting properties. A principal focus of the current work is finding the basic underlying principles that give rise to directed self-organization and controlled phase state in a range of soft materials under confinement. This approach includes the synthesis of the hard templates based on nanoporous anodic aluminum oxide (AAO) (in collaboration with M. Steinhart, University of Osnabrück), as well as structural, thermodynamic and dynamical characterization in a number of soft materials with different types of interactions. These include crystallizable polymers, amphiphilic molecules, liquid crystals and biopolymers with important potential applications.

In a first topic, we explored the effect of size limitation on the energetics involved in crystal nucleation, growth and orientation of highly isotactic polypropylene (iPP) confined in AAO by differential scanning calorimetry.¹ A transformation from a predominantly heterogeneous to predominantly homogeneous nucleation takes place if the pore diameter is smaller than 65 nm. Crystallization was suppressed with decreasing pore size and the absence of nucleation bellow 20 nm pores indicated the critical nucleus size. In another topic, liquid crystal (LC) mesophase orientation induced by surface anchoring and wall-induced density modulations have length scales that compete with the length scales set by elasticity as well as by the bulk correlations. Hence, the equilibrium director field morphology and final optical and dielectric properties depend on the elastic constants, surface coupling, the size of the system and its density as well as on applied external fields. The nematic-to-isotropic and the crystal-to-nematic transition temperatures in 5CB/AAO were reduced linearly with the inverse pore diameter.² The crystalline phase was completely suppressed in pores having diameters of 35 nm and below providing an estimate of the critical nucleus size. Furthermore, the liquid-to-glass temperature was reduced in confinement as anticipated by the model of rotational diffusion within a cavity. In a third topic poly(γ-benzyl-L-glutamate) (PBLG) peptide nanorods were synthesized inside AAO hard templates and the dynamics were investigated by DS.³ It was shown that PBLG confined to AAO with pore diameters below 65 nm exhibit altered segmental dynamics characterized by significantly weaker temperature dependence and a reduced glass temperature. Overall, understanding the self-assembly, thermodynamics and dynamics of soft materials under confinement will allow for their rational design as functional devices with tunable mechanical strength, processability, electronic and optical properties.

1. H. Duran, M. Steinhart, H.-J. Butt, G. Floudas: From heterogeneous to homogeneous nucleation of isotactic poly(propylene) confined to nanoporous Alumina. Nano Letters, 11, 1671 (2011).
2. C. Grigoriadis, H. Duran, M. Steinhart, M. Kappl, H.-J. Butt, G. Floudas: Suppression of phase transitions in a confined liquid crystal. ACS Nano 11, 9208 (2011).
3. H. Duran, A. Gitsas, G. Floudas, M. Mondeshki, M. Steinhart, W. Knoll: Poly(γ-benzyl-L-glutamate) peptides confined in nanoporous alumina: pore diameter dependence of self-assembly and segmental dynamics. Macromolecules (Commun.), 42, 2881