Fabrication Of Micro-Structures On Polymer Surfaces Via Physical Pressing Methods

The fabrication of micro-structures on polymer surfaces has been fulfilled via patterned compression of polyelectrolyte multilayers (PEMs) and thermal pressing method (TPM) respectively.Thin film multi-composites can be constructed by layered, sequential adsorption of solution of dispersed or dissolved species. Pressing a structured poly(dimethylsiloxane) (PDMS) stamp against the surfaces induced an irreversible compression of multilayers assembled with oppositely charged polyelectrolytes. The compression extent varies from several to hundreds of nanometers showing a positive correlation with the multilayer thickness, which can normally be tuned by the layer number or salt concentration. The effect of the desiccation degree on the ability of the compression illustrates that the structural water content plays an important role for the intrinsic properties of PEMs.Compressed multilayers have shown some distinctive properties, such as permeability, wettability, and surface morphology from those uncompressed. Small hydrophilic dye molecules are further used to study the volumetric behavior of the compressed PEMs. The diffusion coefficient of the probe decreases dramatically and loses the sensitivity to salt addition after compression. It is understandable that the compression leads to shrinkage of the vacancies in the PEMs. Here, the concept of free volume can be applied. The compression of the multilayers caused a transition of the multilayers from a rubbery state to a glassy state. An increase of electrostatic and hydrophobic interactions, can decrease the free energy of the multilayers, and stabilize the films. As a result, the closely compacted multilayers lost their sensitivity to pH changes. By compression of the multilayers with a rubber stamp having geometric patternsand post-treatment, films with spatially localized pores were produced.We have also shown the simultaneous fabrication of physical-co-chemical multi-component micro-patterns on polymer surfaces by TPM. A stamp having multi-inks is pressed into a polymer substrate at viscoelastic state to transfer the inks with high efficiency. The polymer surface is simultaneously modified when the periodic physical patterns are formed. The pattern shapes could be further tuned by factors such as ink concentration, impregnation time, and molecular nature. The ink molecules penetrate deeply into the polymer substrates. Owing to the distinctive microstructure, the created patterns were stable against long-term incubation in PBS solution. The flexibility of the stamp also enables fabrication of various structures on curved surfaces. The suspension of cells on the concave regions, modified with anti-adhesion substances, was observed on this kind of surfaces.Adopting colloidal crystals as stamps, bilayered polymer films with hierarchically topological structures on the surfaces have been fabricated. Protein molecules and cell lamellipodia showed no trend to penetrate the porous polymer layer. Interpretation of the surface wettability, using Cassie-Baxter model, suggested that the difficulty at diffusion should be attributed to the air trapped in the pores. By chemically modification, the surface wettability can be tuned from hydrophilic to superhydrophobic conveniently. As expected, protein adsorption and penetration show a positive correlation with the surface hydrophilicity.Multilevel inks were also adopted in this process. On the first level, the modified microsphere arrays are introduced as inks on structured PDMS stamps and are transferred into a polymer film by a PDMS stamp. On the second level, the ordered microspheres are removed to release the substances which are initially attached or doped in the microspheres. These released substances,biomacromolecules and quantum dots, can be then embedded into the PCL matrix or suspended in the interiors of the cellular structures. Consequently, patterned cellular structures with chemically tunable walls are formed. By coating the microspheres with multilayers previously, thin covers on the cellular structures can be obtained after removal of the templates.