(732b) Tuning the Wall Thickness of Templated Polystyrene Nanotubes Produced from Melt Infiltration

In this work, we demonstrate methods of varying the wall thickness of polystyrene nanotubes produced from template synthesis by utilizing classical theories of liquids spreading on flat and curved substrates. We find that factors which affect the surface energy of the polymer and substrate such as the annealing temperature, annealing atmosphere and polymer molecular weight influence the thickness of the nanotubes which are achieved. Results from this study are in qualitative agreement with what is predicted from classical theories. Nanotubes with wall thicknesses ranging from 18 to 40 nm were achieved by infiltrating different molecular weight polystyrene. Higher molecular weight samples possessed larger wall thicknesses compared to lower molecular weight samples. Thermal characterization with differential scanning calorimetry measurements revealed significant Tg-confinement behavior as a function of wall thickness. Post infiltration processing such as annealing was also found to affect the thickness. Supported tubes annealed at high temperatures possessed thinner wall diameters compared to those annealed at lower temperatures. Additional results suggest that atmosphere can have an effect on the wall thickness. Alteration of the surface energy through changes to factors such as the temperature, polymer molecular weight and atmosphere affects the equilibrium tube thickness and may allow for dimensional control over the nanostructures produced from template synthesis.  

The ability to tune the dimensions of nanomaterials is critical for applications which rely upon the unique properties which arise due to the greater effect of free surfaces, interfacial interactions and small diffusion lengths at the nanoscale. Confinement in one or more dimensions can lead to enhancements in the mechanical strength, electrical, optical, thermal properties of materials and benefit areas involving drug delivery, medical devices, interconnects, sensing, catalysis, filtration and nanocomposites. The ability to adjust material properties using the length scale of these structures imparts additional requirements e.g. the ability to achieve uniform size and shapes and the means by which one can tune their dimensions.

Template synthesis is often employed because it allows for dimensional tunability and can be used to infiltrate a wide range of organic and inorganic materials. Anodic aluminum oxide templates (AAO) can be produced using a variety of different electrolytes and conditions and yield pores tens to several hundred nanometers in size. Infiltration into the templates results in negative replication of the size, shape and length of the pore morphology. This is useful for creating nanostructures with custom exterior dimensions and shapes; however, the ability to tune the characteristics of internal features such as the wall thickness of nanotubes is very limited. Control over the wall thickness of nanotubes is typically carried out using solution based techniques to infiltrate polymer into the membrane. The nanotube wall thickness can be varied by changing the molecular weight of the polymer, solution concentration, type of solvent and size of the template used. However, the uniformity and the reproducibility of nanostructures produced using solution based approaches can be very unpredictable. Holes or undulations are often observed as a result of the solvent evaporation process or the evolution of Rayleigh instabilities. Furthermore, extensive drying of the samples must be carried out because residual solvent may influence the morphology of the nanostructures or influence their properties. Melt infiltration generally yields more uniform and reproducible nanostructures and is simpler because it is solvent-less but little is known or has been done regarding the wall thickness tunability of nanotubes produced in this manner. Classical theories regarding the spreading of low energy fluids on flat and curved substrates and recent experimental work suggest that the thickness of polymer films or tubes supported in AAO templates can be modified by annealing under different conditions.

According to classical wetting theories the propensity for a low energy liquid (e.g. a polymer melt) to spread across a high energy surface is characterized by the spreading coefficient which accounts for the difference in energy between an exposed substrate versus one that has been wetted by a liquid film. A polymer liquid will wet the substrate when the total system energy can be lowered be minimized. This process can occur very rapidly and is often described as a "precursor film". When exposed to a template membrane under complete wetting conditions, e.g. high temperatures, the polymer coats the interior and forms nanotubes. The thickness of the precursor film reaches an equilibrium value which is determined by the balance between long range Van der Waals forces and surface energies. This prevents the film from spreading without bounds into an infinitely thin layer of material. Changes to the surface energy can shift the equilibrium value and result in changes to the equilibrium thickness. Factors such as the temperature and atmosphere are known to influence the surface energy of a polymer melt and can be used to modify the wall thickness of template nanotubes. Likewise, the molecular weight of the polymer or the functionality of the substrate can also affect the surface energy of the system.

 Various molecular weights of polystyrene were melt infiltrated into AAO templates under the same thermal conditions. Nanotubes with wall thicknesses ranging from 18 to 40 nm were achieved. Thicker nanotubes were created by infiltrating higher molecular weight samples whereas thinner tubes were obtained from lower molecular weights. This is in agreement with classical theory; the surface energy of a polymer increases with molecular weight and increases the equilibrium thickness. The infiltrated templates were also measured using differential scanning calorimetry to determine the change in Tg-confinement behavior as a function of thickness. Tg reductions as large as 20 °C from the bulk Tg were observed for the thinnest tubes whereas bulk-like Tg behavior was obtained for the thickest tubes. It was also discovered that wall thickness can be modified by annealing at different temperatures after infiltration. For the same molecular weight polymer, nanotubes annealed at 160 °C possessed a lower wall thickness than those annealed at 130 °C. These results suggest that it is possible to tailor the wall thickness of a polymer sample by modifying the annealing conditions without having to use different molecular weight polymers or modifying the surface energies of the substrate. These results further suggest that other factors such as the atmosphere and the surface energy of the substrate may affect the thickness and that factors such as the amount of polymer available for infiltration will not based upon what is predicted from classical spreading behavior.