Synthesis and Single Molecule Force Spectroscopy of Graft Copolymers of Poly(2-hydroxyethyl methacrylate-g-ethylene glycol)

Abstract

In this study, an end-functionalized graft copolymer, thiol-terminated poly(2-hydroxyethyl methacrylate-g-ethylene glycol) or SH-poly(HEMA-g-EG), was synthesized by the atom transfer radical polymerization (ATRP) method and then characterized by 1H nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and light scattering (LS). The polymer was covalently end-grafted to an Au substrate at low enough grafting densities to produce well-separated individual polymer “mushrooms” and single molecule force spectroscopy (SMFS) was then employed to measure the single polymer extensional elastic properties in phosphate buffered saline solution (PBS, ionic strength (IS) = 0.15 M, pH = 7.4) and deionized H2O (IS ∼ 6 × 10-5 M, pH = 5). In both solvents, the polymer behaved as an extensible freely jointed chain with a statistical segment length, a = 0.52 ± 0.09 nm, and a segment elasticity, ksegment = 10.5 ± 3.3 N/m, in PBS and a = 1.03 ± 0.01 nm and ksegment = 4.2 ± 0.5 N/m in H2O. Comparison to the known single molecule elasticity behavior for the PEG homopolymer suggests a smaller number and/or smaller magnitude of the noncovalent inter- and intramolecular interactions along the HEMA backbone and that even at this low PEG side chain grafting density, the side chains are still quite effective in causing local expansion of the PHEMA backbone by overcoming hydrophobic collapsing forces relative to the PHEMA homopolymer. Since the PEG chains are still well-solvated in PBS, one explanation for the small variation in nanomechanical properties with solvent is that in PBS the salt ions compete for and weaken the inter- and intramolecular H-bonding that does exist along the PHEMA backbone, causing a local contraction and an increased segmental stiffness due to the reduced noncovalent nature of the segments.