Scanning Probe Investigations of Nanopatterned and Thermo-responsive Polypeptoids and the Design of a Sample Stage for Force Modulation Microscopy

Abstract

Protocols for patterning nanostructures of polymers were developed to enable scanning probe microscopy (SPM) studies of surface properties at the molecular level. A chemically selective surface for patterning polymers was generated by combining particle lithography with organosilane immersion. Poly(N-allyl glycine), a biocompatible and backbone degradable polypeptoid, was grown on Si(111) at confined amine sites using surface-initiated polymerization (also known as the “graft from” approach). A thermo-responsive random copolypeptoid, poly[(N-ethyl glycine)32-r-(N-butyl glycine17)], was pre-synthesized and attached onto a patterned organosilane surface using a thiol-ene click reaction (“graft to” approach). The phase transitions of the copolypeptoid nanostructures in an aqueous environment was studied using in situ atomic force microscopy (AFM). A heated stage was used to heat the nanopatterned sample in liquid media. Force modulation microscopy (FMM), one of over 50 possible modes of SPM, is used to acquire mechanical properties of samples concurrent with topographic information. The history, instrument set-up, advantages, and applications with self-assembled monolayers, polymers, biological and inorganic samples are reviewed in this dissertation. The sample is placed directly on an xyz scanner for the most common FMM configuration. Most commercial SPM systems are designed to have the tip mounted on the piezotube scanner. To facilitate FMM with tip-mounted scanners, a sample stage was designed and constructed of machined polycarbonate. A piezoactuator in the sample stage was used to drive the vibration of the sample in the z-direction at selected frequency and amplitude. The sample stage was evaluated by studying three samples with increasing complexity. This sample-driven FMM was compared with indirect magnetic modulation (IMM), an alternate way to accomplish force modulation studies by modulating the nosecone assembly. A test platform of patterned nanoring samples was evaluated to systematically compare FMM and IMM. Both methods can be used to sensitively acquire high resolution images of sample elasticity. However, the newly designed sample stage for FMM is easier to operate compared with IMM.