Modification of surfaces with adsorption of amphiphilic polymers

Publication year: 2014
Authors: Katri Kontturi
Affiliations:

Department of Forest Products Technology, Aalto University, Helsinki, Finland

Published in: Doctoral Dissertation
doi: ISBN: 978-952-60-5443-8

In this work the adsorption of amphiphilic polymers and their utilization in the modification of planar surfaces were examined. The approach was to systematically explore adsorption of very different types of polymers, including derivatives of cationic starch (CS), both charged and neutral synthetic block-structured polymers with amphiphilic character and different levels of solubility, on different well-defined model surfaces and the consequential changes in the properties of the surfaces. Cellulosic substrates were in the main role, complemented with other surfaces of hydrophilic or hydrophobic nature.

Independent of the molecular architecture of an amphiphilic polyelectrolyte, a high degree of hydrophobic modification and/or sufficient screening of repulsive charges by added simple electrolyte significantly increased the thickness of the layer adsorbed on anionic surfaces. When it came to hydrophobic surfaces, the adsorption of different polymers from aqueous solution was favored regardless of the actual affinity, due to the tendency of the system to minimize the contact between the hydrophobic substrate and water. Although the degree of hydrophobic modification of amphiphilic polyelectrolytes correlated with the resulting surface coverage, formation of a uniform polymer layer was rarely obtained.

The intrinsic amphiphilic nature of cellulose became efficient in a system where adsorption was not ruled by strong electrostatic forces. Due to its amphiphilicity, cellulose allowed hydrophobically end-modified poly(ethylene glycol) to attach onto the surface in a less strict conformation than on merely hydrophilic or hydrophobic surfaces, enabling formation of a network-structured layer. As a substrate for electrostatically driven adsorption, the cellulose substrate diverges from other studied hydrophilic substrates (silica and mica) due to its low charge density and porosity. Low charge of regenerated cellulose hinders highly charged molecules from adapting flat conformations on the surface. On the other hand, the porous structure allows penetration of the molecules into cellulosic films.

The wettability and swelling properties of cellulosic materials were strongly affected by adsorption of cationic (amphiphilic) substances. A thick and rough layer of highly hydrophobic, kinetically trapped micelles decreased the hydrophilicity of the surface most efficiently. The hydrophobizing potential of the micelles was strongly strengthened by annealing above the glass transition temperature of the micellar core.


MP-SPR keywords: cellulose films, dry film, polymer adsorbtion, refractive index, swelling, thickness, wet film