Single Macromolecule Nanomechanical Design: Poly(2-hydroxyethyl methacrylate-g-ethylene glycol) Graft Copolymers of Varying Architecture

Abstract

With the advent of nanotechnology, the behavior of individual polymer chains is increasingly being studied due to their potential to act as molecular springs, switches, sensors, shock absorbers, and motors. Single molecule force spectroscopy (SMFS) or “tensile testing” is a powerful technique that is fast enabling rational nanomechanical design of synthetic polymers for such applications.1-3 Recently, we reported the synthesis and characterization of a series of poly(2-hydroxyethyl methacrylate-g-ethylene glycol) or poly(HEMA-g-EG) neutral graft copolymers of varying macromolecular architecture with such functions in mind.3,4 For accurate SMFS experiments, a number of experimental factors were addressed,3,4 including (1) end-functionalization for strong and specific end-tethering to a planar substrate, (2) high enough molecular weight (preferably ∼100 kg/ mol) to enable long enough extensions beyond the nonspecific surface forces regime and clear observation of nanomechanical profiles, and (3) verification of low enough grafting densities after chemisorption to ensure probing of single molecules. The graft copolymers studied here were also designed to be water-soluble since aqueous solutions are typically much easier to work with than organic solvents in current nanomechanical devices and also, in the longer term, to enable the use of such polymers for biomedical applications. SMFS on poly(HEMA-g-EG)120K in aqueous solution was reported by us previously (Figure 1).3 The numerical subscript in the abbreviated polymer name label (referred to in the previous sentence) is the numberaverage molecular weight, Mn, of the graft copolymer in g/mol (as determined by 1H nuclear magnetic resonance (NMR)) “K” is an abbreviation for 1000. This particular graft copolymer had a poly(ethylene glycol) or PEG side-chain molecular weight, MWPEG, of 2080K, an average number of PEG chains per PHEMA (poly(2-hydroxyethyl methacrylate)) chain, NPEG, of ∼8, corresponding to a 1% molar ratio of PEG to HEMA (graft density), and a number-average degree of polymerization ratio of EG (ethylene glycol) to HEMA (2hydroxyethyl methacrylate), DPn,EG/DPn,HEMA, of 0.4. Poly(HEMA-g-EG)120K behaved similar to an extensible freely jointed chain5 with a statistical segment length, a, between 0.5 and 1.0 nm and segment elasticity, ksegment, between 5 and 15 N/m, both of which were largely insensitive to solution ionic strength. This relatively low value of the resisting force to extension suggested minimal noncovalent intramolecular interactions and that the PEG side chains were quite effective in causing local expansion of the PHEMA backbone and overcoming hydrophobic collapsing forces between the HEMA methyl groups. In this paper, we report the single molecule nanomechanical properties of a number of poly(HEMA-g-EG) graft copolymers with varying macromolecular architecture, in particular with DPn,EG/DPn,HEMA between 0 (PHEMA) and 1.8. Characterization data and corresponding schematics of the architecture of these polymers are given in Figure 2 of ref 4.