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

Lookup NU author(s): Professor Mark GeogheganORCiD


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© 2015 Elsevier Ltd. 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.

Publication metadata

Author(s): Al-Baradi A, Tomlinson MR, Zhang ZJ, Geoghegan M

Publication type: Article

Publication status: Published

Journal: Polymer

Year: 2015

Volume: 67

Pages: 111-117

Print publication date: 12/06/2015

Online publication date: 29/04/2015

Acceptance date: 08/04/2015

ISSN (print): 0032-3861

ISSN (electronic): 1873-2291

Publisher: Elsevier Ltd


DOI: 10.1016/j.polymer.2015.04.021


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