Determination of the molar mass of surface-grafted weak polyelectrolyte brushes using force spectroscopy

Abstract

The molar mass and dispersity of a polycation, poly[2-(dimethyl amino)ethyl methacrylate)] (PDMAEMA) grafted from a poly(methyl methacrylate) (PMMA) backbone, was measured by single-molecule force spectroscopy (SMFS) and shown to be consistent with results from gel permeation chromatography for the same comb polymer in aqueous solution. Comparison was then made between the comb polymer and PDMAEMA brushes that were grown from the substrate, as a function of the pH and ionic strength of the surrounding medium, and the limits of reliable characterization of the polymers are determined. A large discrepancy was observed between the responses of the comb and brush layer at low pH when the PDMAEMA molecules are extended from the supporting substrate. Here it is believed that the atomic force microscope (AFM) tip can penetrate the comb layer and selectively desorb side-chains of the comb. In the case of the well solvated PDMAEMA brushes at high pH, the tip preferentially selects larger chains, resulting in an over-estimate of the brush molar mass. The addition of salt also influenced the molar mass obtained by this technique. It is believed that salted brushes did not adhere well to the AFM tip, with subsequent desorption resulting in an underestimate of the molar mass. However, SMFS was shown to be capable of demonstrating the effect of salt on brush conformation, with greater swelling after the addition of a small amount of NaCl, but a significant decrease when 100 mM is added.

Influencing the adhesion properties and wettability of mucin protein films by variation of the environmental pH

“…stiffness, adhesion) for similar systems (polymers, gels, etc) and under similar stimuli (i.e. charge density-induced adhesion variations) 31 – 33 , its applicability to mucin films is considered as an optimal choice.…”

Surface characterization data for tethered polyacrylic acid layers synthesized on polysulfone surfaces

“…AFM force spectroscopy (AFM-FS; [12], [13], [14]) was accomplished using a Bruker MultiMode 8-HR Scanning Probe Microscope with a PicoForce Spectroscopy Control Module (Bruker, Santa Barbara, CA) under DI water and 35 g/L NaCl solution. A silicon nitride probe of a nominal tip radius of 20 nm (MLCT-D, Bruker AFM Probes, Camarillo, CA) was used, and force measurements were taken in contact mode at 1 μm ramp size and 500 nm/s tip velocity.…”

Weak polyelectrolyte brushes

“…48 These polycationic brushes (PDMAEMA) were grafted from a poly(methyl methacrylate) macroinitiator using ATRP, 38 and could be characterized by size-exclusion chromatography and so lend themselves well to a comparison of this nature. The pH dependence of PDMAEMA chain length determined by force spectroscopy, 48 gave a similar result to that for PAA. 23 This means that force spectroscopy can be used to provide a determination of polyelectrolyte molar mass (and dispersity) providing that the polymers are soluble but not too charged, which typically means near neutral pH.…”