Tunable Temperature-Responsive Tethered Polymer Gradients

Open Access
Author:
Hogshead, Charles Gregory
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 26, 2010
Committee Members:
  • Evangelos Manias, Dissertation Advisor
  • Evangelos Manias, Committee Chair
  • Paul C Painter, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Seong Han Kim, Committee Member
Keywords:
  • atomic force microscopy
  • switchable adhesion
  • grafting density gradient
  • temperature-responsive
  • tethered polymer
  • human serum albumin
Abstract:
Temperature-responsive tethered polymer layers, with a gradient in the polymer grafting density, were synthesized and it is demonstrated that the temperature response of the tethered chains can be systematically tuned. The tethered polymer chains were grafted from a mixed monolayer gradient by sequential exposure to difunctional oligomers of ethylene-oxide and ethylene. The relative lengths of the two alternating oligomers were varied, which altered the hydrophilic/hydrophobic balance within the polymer repeat unit, thereby allowing for tailoring the polymer’s collapse transition in aqueous solution. Contact angle measurements and dry topographic Atomic Force Microscopy (AFM) were utilized to confirm the existence of a well-defined grafting density gradient for the end-tethered polymers. The temperature response of the tethered gradient was assessed using under-water AFM force-distance spectroscopy with a custom hydrophobic colloidal probe. It was found that the adhesion between the probe and the temperature-responsive surface could be reversibly switched upon heating/cooling through the collapse/expansion of the tethered layer. Regarding the impact of polymer grafting density on the temperature-induced collapse, it was found that both the magnitude and onset temperature of the adhesion switches are grafting density dependent. Wettability experiments were conducted in the captive bubble configuration, as a function of temperature, and provided additional evidence that the temperature response of the smart surface could be systematically tuned by varying the hydrophilic/hydrophobic balance across the tethered chain’s backbone. The contact angle results also helped to verify that the adhesion switches observed in the under-water AFM experiments were a manifestation of, primarily, the switching in interfacial surface tension between the tethered chains and water. The collapse of the tethered layers occurs at similar temperatures as the cloud points of same-composition untethered chains in aqueous solution; however, the transitions for the tethered layers are broadened in temperature, which is consistent with the collapse of a planar layer being a cooperative conformational transition. Finally, to demonstrate the possible utility of the tunable temperature-responsive surfaces in biomedical applications, particularly in tissue engineering, the interactions between human serum albumin (HSA), a blood plasma protein, and the smart surface were investigated. HSA was covalently grafted to a colloidal probe and it is demonstrated that the adhesion between the protein and the surface can be reversibly switched in phosphate buffered saline (PBS) solution via temperature variation. At temperatures below the LCST collapse of the tethered layer, instances of polymer stretching, protein stretching and combined polymer/protein stretching are observed. The origin of this behavior can be attributed to the formation of bridging segments, comprised of either polymer chains, protein molecules, or entangled chains and protein molecules, between the probe and the surface, which are stretched upon the retraction of the probe.