Reversible stimulus-responsive polymers for the control of the surface interfacial and nanomechanical properties

Abstract

Surfaces with reversible stimulus-responsive properties have great potential for a wide variety of applications, such as transport, separation, and detection of biomolecules, controlled adhesion, friction, and lubrication in microfluidic systems, and force or displacement generation in micro- and nanoscale devices. Surface bound stimulusresponsive polymers are ideal candidates for above applications due to their conformational sensitivity to many stimuli with controlled molecular weight, composition, architecture and topology. In this thesis, one particular class of stimulus-responsive polymers, pH-sensitive comb-type graft copolymers with ionizable main chain segments was investigated. Mono(end)-functional thiol-terminated poly(methacrylic acid-gethylene glycol) (HS-poly(MAA-g-EG)) with three different macromolecular architectures (number average molecular weight, Mn = 27K, PEG graft density, PEG(%) = 7.7%, backbone contour length, Lcontour= 41.1 nm; Mn= 15K, PEG(%) = 8.8%, Lcontour = 22.1 nm; Mn = 17K, PEG(%)= 1.9%, Lcontour = 39.8 nm) have been synthesized via atomic transfer radical polymerization and characterized by 'H NMR, GPC and FT-IR. Stimulus responsive surfaces were prepared via chemically end-attached "brush-brushes" formed by chemisorption of the copolymers on Au substrates. Chemically specific high resolution force spectroscopy (HRFS) was carried out with probe tips (end radius-50 nm) functionalized with HS(CH2)10COOH (a carboxy-terminated selfassembling monolayer or COOH-SAM) to measure the normal nanoscale interaction forces, F, as a function of probe-tip sample separation distance, D,in a series of aqueous buffer solutions of varied pH (=4-9) and constant ionic strength (IS=0.005M NaC1).

(cont.) The higher PEG grafting density surfaces (27K, 15K) exhibited the unique property of "nanomechanical switching" with pH, i.e. the normal intersurface force inverted from net repulsive (high pH, ionized uncomplexed side chains) to net attractive (low pH, sidechain/main-chain hydrogen bonding complexation). The 17K polymer brushes did not exhibit nanomechanical switching and maintained a slightly repulsive intersurface force at low pH. Surface plasmon resonance (SPR) was employed to assess the adsorption of human serum albumin (HSA) to these poly(MAA-g-EG) brushes in aqueous buffer solutions of varying pH. Polymers with a higher grafting density of hydrophilic PEG side chains and longer polymer backbones showed much less HSA adsorption at high pH and more protein adsorption at low pH. Surprisingly, HSA adsorption was found to be greatly amplified at intermediate pH6 (~1.4-1.8 x greater than that of the hydrophobic state of polymer layers at pH4). Higher PEG grafting density and a longer polymer backbone demonstrated larger protein adsorption amplification at pH6, which may be due to increased molecular mobility/disorder at a metastable state of the conformational transition. For the lateral force interaction between the end-grafted polymer layers and a probe tip (nominal radius - 50 nm) functionalized with OH-SAM (HS(CH2)11OH), as pH decreased, both the 27K and 17K polymer layers exhibit an abrupt change in lateral proportionality coefficient (ratio of lateral force to normal force) between pH7.1 and pH6 with larger lateral proportionality coefficients, [mu] ~ 0.63-0.89 at pH 4-6 and decreased [mu] ~ 0.12-0.34 at pH 7.1-9.

(cont.) The 27K polymer had relatively higher p values at pH < 6 (0.89±0.19) but smaller [mu] at pH > 7.1 (0.21±0.04) than the 17K polymer, indicating that a more dramatic change in lateral force coefficient is expected for stimulus-responsive graft copolymers with higher side chain grafting densities.