Surface modification of polymers Our work in this field has focused on two polymer substrates, poly(methylmethacrylate) or PMMA and poly(dimethylsiloxane), PDMS. Both find widespread application in soft lithography, microfluidic device fabrication and semiconductor photoresist coatings, amongst others. Our interest is in using AFM, and particularly chemical force titration methods in order to observe directly the effects surface modification have not only on the on the interesting morphology (Figure 7) produced, but on the chemical species present at the surface.
Figure 7 AFM image showing an oxidised polydimethylsiloxane (PDMS) substrate upon which aminopropyltriethoxysilane has been deposited using a siloxane cross-linking technique.
Many interesting compounds may be grafted onto oxidized PDMS, via condensation of an appropriate alkoxysilane compound. In Figure 8 below, we see results from the deposition of a perfluorinated chain onto PDMS. Note that upon plasma oxidation (red spectra) formation of Si-OH species renders the PDMS surface much more hydrophilic and capable of supporting a large zeta potential (surface charge) over a wide pH range. Upon termination with the perfluorinated group that the surface charge on these species remains quite large, but the surface remains hydrophobic and furthermore selectively adsorbs proteins and peptides tagged with perfluoro groups. These experiments have been used to design better surface preparation methods for materials used in microfluidic devices.
Figure 8 Perfluorination of PDMS. The left hand graph shows the C 1s spectrum of unmodified PDMS (black), oxidized PDMS (red) and the perfluoro-terminated PDMS. Note the new C 1s lines that arise due to contributions from C atoms surrounded by the electronegative F atoms. The right hand curve shows the variation of zeta potential, and hence surface charge, for these surfaces as a function of pH.