Morphology design in 2D confinement
Abstract
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We are investigating crystallization of different crystallizable materials inside nanoporous and microporous materials. Examples include semiconductors such as cadmium selenide and semi-crystalline polymers. Semi-crystalline polymers are an important class of materials. They consist of sheet-like lamella crystals, which are separated by amorphous regions. Crystallization in semi-crystalline polymers is commonly a non-equilibrium process. The crystalline morphology, which largely determines the mechanical, electronic, optical and piezoelectric properties of polymeric specimens, depends on the applied thermal history. It is obvious that the two-dimensional confinement of nanopores and micropores influences the crystallization. Aiming at rational texture engineering, we are investigating how crystal size and crystal orientation in polymeric nanofibers and microfibers can be adjusted. For example, macroscopic ensembles of non-isothermally crystallized polymer nanofibers exhibit uniform crystal orientation if they were crystallized when connected with a bulk polymer film on the surface of the templates. In this case, heterogeneous nucleation occurs within the bulk reservoir. When the growth fronts of the crystallites arrive at the template surface, a kinetic selection process at the pore openings takes place. Only crystals having their direction of fastest growth aligned with the pore axes proceed into the pores. As example in case, we selected poly(vinylidene difluoride) (PVDF), a semi-crystalline high-performance polymer, some copolymers of which show pronounced ferroelectricity. The <020> direction of PVDF has the highest growth rate. In the bulk, PVDF crystals form spherulitic superstructures. The <020> direction points radially outwards. If a spherulite hits a pore, lamella crystals having their <020> direction oriented parallel to the pore axis grow straight along the pores (Fig. 1).
Figure 1. Schematic diagram illustrating the selection of a specific growth direction, when PVDF crystals formed in the bulk proceed into the pores of a porous template. The crystallographic direction exhibiting the highest growth rate aligns with the pore axes.
If the polymer nanofibers are separated within the pores during non-isothermal crystallization by removing any material from the surface of the templates (Fig. 2), crystallization is initiated by homogeneous nucleation within each crystallizing entity. All crystal orientations allowing the lamella crystals growing along the pores show up in statistical frequency, whereas all crystal orientations resulting in impingement of the growing crystals on the pore walls are suppressed. We are investigating the crystal orientation in ensembles of polymer nanofibers aligned inside the pores of porous templates by wide angle X-ray diffraction (WAXS) (Fig. 3).
Figure 2. Scanning electron micrograph of the surface of a self-ordered porous alumina template (pore diameter 35 nm) containing separated PVDF nanowires, after the cleaning of the template surface.
Figure 3. Wide angle X-ray patterns of non-isothermally crystallized PVDF nanowires aligned within ordered porous alumina (pore diameter 35 nm), measured in reflection mode. a) Nanowires crystallized when connected to a bulk PVDF film on the surface of the template. The exclusive appearance of the (020) peak indicates that the <020> direction of the crystallites is uniformly parallel to the pore axes. b) Nanowires non-isothermally crystallized in the separated state. All crystal orientations enabling the growth of the lamella crystals along the pore, which are represented by (hk0) indices, appear, whereas crystal orientations that result in impingement of the growing crystals on the pore walls, represented by (hkl) indices with non-zero l index, are missing. c) Powder pattern of PVDF.